Kantipur Engineering College
A course manual on Concrete Technology
Chapter - 1 Introduction to concrete and concrete
materials
1.1 Use of concrete in structure and types of concrete
Concrete is a composite material composed of coarse granular material (the aggregate or
filler) embedded in a hard matrix of material (the cement or binder) that fills the space
between the aggregate particles and glues them together. It can also be considered as a
mixture consisting of binding medium and aggregate particles. The binding medium is
formed by the hydraulic reaction between cement and water so the main constituents of
concrete are cement, aggregates and water.
According to the type of binder used, there are many different kinds of concrete. For
instance, Portland cement concrete, asphalt concrete, and epoxy concrete. In concrete
construction, the Portland cement concrete is utilized the most. Thus, in our course, the
term concrete usually refers to Portland cement concrete.
The relationship between these ingredients of concrete can be viewed in two ways. One can
regard coarse aggregate as mini-masonry which is joined together by cement paste or
concrete can be regarded as two phase material consisting of cement phase and aggregate
phase.
1.1.1 Structural use of concrete
Concrete is the most widely used construction material in the world. It is used in many
different structures such as dam, pavement, building frame or bridge. Also, it is the most
widely used material in the world, far exceeding other materials. Its worldwide production
exceeds that of steel by a factor of 10 in tonnage and by more than a factor of 30 in volume.
The present consumption of concrete is over 10 billion tons a year, that is, each person on
earth consumes more than 1.7 ton of concrete per year. It is more than 10 times of the
consumption by weight of steel. Concrete is neither as strong nor as tough as steel, so why
is concrete so popular? The major reasons behind this fact are briefly described as under:
A. Economical:
Concrete is the most inexpensive and the most readily available material. The cost of
production of concrete is low compared with other engineered construction materials. The
three major components of concrete viz. water, aggregate and cement are the most
inexpensive materials and available in every corner of the world as compared with steel,
plastic and polymer. This enables concrete to be locally produced anywhere in the world,
thus avoiding the transportation costs necessary for most other materials.
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
B. Ambient temperature hardened material:
Because cement is a low temperature bonded inorganic material and its reaction occurs at
room temperature, concrete can gain its strength at ambient temperature.
C. Ability to be cast:
It can be formed into different desired shape and sizes right at the construction site.
D. Energy efficiency:
Low energy consumption for production, compare with steel especially. The energy content
of plain concrete is 450-750 kWh / ton and that of reinforced concrete is 800-3200 kWh/ton,
compared with 8000 kWh/ton for structural steel.
E. Excellent resistance to water:
Unlike wood and steel, concrete can harden in water and can withstand the action of water
without serious deterioration. This makes concrete an ideal material for building structures
to control, store, and transport water. Contrary to popular belief, pure water is not
deleterious to concrete, even to reinforced concrete: it is the chemicals dissolved in water,
such as chlorides, sulfates, and carbon dioxide, which cause deterioration of concrete
structures.
F. High temperature resistance:
Concrete conducts heat slowly and is able to store considerable quantities of heat from the
environment (can stand 6-8 hours in fire) and thus can be used as protective coating for
steel structure.
G. Ability to consume waste:
Many industrial wastes can be recycled as a substitute for cement or aggregate. Examples
are fly ash, ground tire and slag.
H. Ability to work with reinforcing steel:
Concrete and steel possess similar coefficient of thermal expansion (steel 1.2 x 10 -5;
concrete 1.0-1.5 x 10-5). Concrete also provides good protection to steel. Therefore, while
steel bars provide the necessary tensile strength, concrete provides a perfect environment
for the steel, acting as a physical barrier to the ingress of aggressive species and preventing
steel corrosion by providing a highly alkaline environment with pH about 13.5, passivating
the steel.
I.
Less maintenance required:
No coating or painting is needed as for steel structures.
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
1.1.2 Types of concrete
A. Plain cement concrete
 It is the concrete which do not possess any reinforcement.
 It strong in compression but weak in tension.
B. Reinforced cement concrete
 It is the concrete where reinforcement is provided.
 It strong both in compression and in tension as compared to plain cement concrete.
C. Pre-stressed concrete
 It is the concrete in which stresses are induced artificially before its actual use.
 This type of concrete can take high compressive and tensile load.
Special types of concrete
(a) Light weight concrete
 Prepared by using cellular, porous or light weight aggregate having specific gravity
less than 2.67
 Also manufactured by introducing air in the concrete (air entrained concrete) or
omitting sand in concrete (no fines concrete )
 Reduction in dead load, haulage and handling cost are its chief advantages.
(b) Fiber reinforced concrete
Prepared by addition of small, closely spaced and uniformly dispersed fibres to concrete
which acts as crack arrestor and also increase tensile strength. Examples are steel fibres,
nylon, asbestos etc
(c) Concrete containing polymer
Here polymer is used as a binder instead of cement
Minimum voids, hydrophobic, resistance to chemical attack are its benefits
(d) Ferro cement concrete
Reinforcement consists of closely spaced, multiple layers of mesh or fine rods sample
surrounded by cement mortar
(e) Very high strength concrete
Strength of concrete is in the range of 60 – 100 MPa
Low porosity and low water cement ratio are its characteristics
Colum section and reinforcement can be reduced for the same amount of load.
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
(f) High density concrete
Density is in the range of 3360 – 3840 kg/m3 whereas density of conventional concrete is
around 2400 kg/m3
Density can be as high as 5280 kg/m3 by using iron as both coarse and fine aggregate
Used in construction of radiation shield
Also concrete can be classified as (Based on unit weight)
 Ultra light concrete <1,200 kg/m3
 Lightweight concrete 1200- 1,800 kg/m3
 Normal-weight concrete ~ 2,400 kg/m3
 Heavyweight concrete > 3,200 kg/m3
Furthermore, (Based on strength of cylindrical sample)
 Low-strength concrete < 20 MPa compressive strength
 Moderate-strength concrete 20 -50 MPa compressive strength
 High-strength concrete 50 - 200 MPa compressive strength
 Ultra high-strength concrete > 200 MPa compressive strength
Likewise there are several others types such as
 Non cracking concrete
 Chemical resistance concrete
 Temperature resistance concrete
 Heavy weight concrete
 Fire resistance concrete
These special types of concrete are made by
 New manufacturing process
 Use of various types of ingredients
 Use of admixtures
 Use of polymer
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
1.2 Concrete materials (Role of different materials, aggregate
cement, water and admixture)
1.2.1 Aggregate (properties of aggregate and their gradation)
Aggregates are the important constituents of the concrete which give body to the concrete,
provides strength, reduce shrinkage and affect economy. It was earlier considered that the
aggregates were chemically inert but now it is recognized that some of the aggregates are
chemically active and also form chemical bond with the cement paste. The fact that they
occupy about 70 – 80 percent volume of the concrete clearly shows that they have
considerable effects on the overall properties of the concrete.
A. Role of aggregates
 It provides mass of the particles which resists the action of applied loads (abrasion,
impact, moisture and others environmental effects)
 It reduces the volume changes and shrinkage resulting from the setting and
hardening process and from the moisture changes in the cement paste. Same goes
for the creep.
 It provides relatively cheap filler for the cementing materials
B. Types of aggregate
(a) According to source
I . Natural aggregates
(i) Aggregates from the igneous rocs
Aggregates from the igneous rocks are most desirable because they are
normally hard, tough and dense. These aggregates from the igneous rocks are the most
chemically active concrete aggregates and show a tendency to react with alkalies in cement.
As the igneous rock is one of the widely occurring types of rocks on the face of the earth,
bulk of the concrete aggregates that are derived, are of igneous origin.
(ii) Aggregates from the sedimentary rocks
The quality of aggregates derived from sedimentary rocks may vary in quality depending
upon the cementing material and the pressure under which these rocks are compacted.
Some siliceous sandstone as well as limestone can yield good concrete aggregate.
(iii) Aggregates from the metamorphic rocks
Metamorphic rock shows foliated structure and if the thickness of the foliation is less, then
individual aggregate may exhibit foliation which is not a desirable characteristic in
aggregate. However, metamorphic rocks particularly quartzite and gneiss has been used for
the production of good concrete aggregates.
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
II . Artificial aggregate
Aggregates created artificially to fulfill some specific purpose are called artificial aggregates.
Broken brick, Air-cooled slag, Sintered fly ash, bloated clay are some examples of commonly
used artificial aggregates.
(b) According to the size
I . Coarse aggregate
Aggregates having size greater than 4.75 mm are called coarse aggregates
II . Fine aggregates
Aggregates having size less than 4.75 mm are called fine aggregates
C. Properties of aggregates
(a) Physical properties
I . Shape





Rounded (fully water worn or completely shaped by attrition )
Irregular (naturally irregular or partly shaped by attrition )
Flaky (least dimension is less than 0.6 times mean dimension )
Elongated (Largest dimension is 1.8 times greater than the mean dimension)
Angular particle (Particle possessing well defined edge )
From the view point of workability and economy of cement, round aggregate are considered
better than angular but the concrete using only round aggregate is poor in strength due to
lack of bonding between the smooth surface of the aggregate and cement paste.
From the view point of strength, angular aggregates are better than rounded aggregates
because:
 They have better interlocking effects in concrete so making them better for road and
pavement.
 Surface area of rough textured angular aggregates is more than rounded aggregates
which results in higher bond strength between cement paste and aggregates.
II . Texture
Texture is the property of aggregate the measure of which depends upon how smooth,
rough, dull and polished particle surface is. It depends upon particle hardness, grain size,
pore structures, structures of the rock and degree of forces acting.
As the smoothness of the surface increases, contact area decreases which means bonding
area decreases. Therefore, smooth particles have lesser bonding area resulting in weaker
bond strength as compared to rough textured particles which have large bonding area.
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
On the other hand, greater smoothness means, it will require less cement paste to lubricate
its movement among other particles which means greater workability for the same water
and cement content. As such, quantity of cement paste required is less.
III . Specific gravity
Specific gravity or relative density, according to ASTM is the ratio of mass in air of unit
volume of materials to the mass of same volume of water at the stated temperature.
Absolute specific gravity refers to the volume of solid excluding all pores whereas apparent
specific gravity refers to the volume of solid material including impermeable pores but
excluding capillary pores. It is the apparent specific gravity which is commonly used in
concrete technology, the actual definition being ratio of oven dry mass of aggregate to mass
of same volume of water.
IV . Bulk density
Absolute density refers to the volume of the individual particles only but in practice, it is not
physically possible to pack aggregate particles without any voids between them so bulk
density is commonly used as a means to define the compactness of the aggregate sample
which is the actual mass of aggregate that would fill a container of unit volume. As such,
bulk density depends on how densely the aggregates are compacted consequently on
particle size distribution and their shape.
V . Porosity
Porosity of the aggregate directly influences absorption and moisture content of the
aggregate. Water absorption refers to the water contained by aggregates in saturated
surface dry condition whereas the moisture content is water in excess of saturated surface
dry condition. As such, total water content is the sum of moisture content and absorbed
water. Porosity also affects bond between aggregate and cement paste, resistance of
concrete to freezing and thawing, chemical stability, resistance to abrasion and specific
gravity.
VI . Bulking of aggregate
Bulking is defined as the increase in volume of aggregates due to the presence of water film
around the surface. Free moisture forms a film of water around the particle surface which
creates the surface tension and that keeps the neighboring particles away from one
another. As such, separation of particle gets increased resulting in increase in volume.
When we plot a graph between % increase in volume and % of moisture added in a sample
of aggregate, it is found that bulking of aggregate increases with increase in moisture
content up to a certain limit but beyond that, volume decreases with further increase of
moisture content. Also it is found, bulking of fine aggregates is more than bulking of coarse
aggregates. Bulking of sand can be as high as 40 percent.
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
Figure 1-1 : Bulking of aggregate
Bulking of aggregate do not possess considerable problem in case of weight batching but it
can certainly be a problem if batching is done by volume. In case of volume batching, if
bulking occurs, it will increase the apparent volume of the aggregate but actually the
increase in volume would be occupied by moisture and voids. As such, actual volume of the
aggregate will decrease but the moisture content will increase beyond the specified value
which will definitely affect the concrete in every possible way.
(b) Mechanical Properties
I . Bond and Bond Strength
Bond is formed by interlocking capacity of the aggregate and adhesion between aggregates
and cement paste. Bond strength is the resistance developed against the splitting of particle
of aggregates from hardened cement paste. As discussed earlier, rough textured and
angular aggregates have more bond strength compared to round aggregates.
II . Hardness
Resistance of aggregate to wear is called hardness. It is an important parameter especially if
the concrete is to be used for road constructions. It can be determined by Los Angeles
abrasion test or Dorry Abrasion test or Deval attrition test. Among them, Los Angeles test is
more popular and also used in Nepal. This test involves taking specified quantity of standard
size material along with the specified number of abrasive charge in a standard cylinder and
revolving it for certain specified revolutions. The particles smaller than 1.7 mm size are
removed. The loss in weight expressed as a percentage of the original weight taken gives
the abrasion value of the aggregate or the hardness of the aggregate. For wearing surface,
its value should not be greater than 30 percent and not more than 50 percent for other
surface.
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
III . Toughness
It is defined as the resistance of aggregate against failure by impact or ability of aggregate to
withstand repeated blows. It is measured in terms of aggregate impact value which is ratio
of the weight of the fines (finer than 2.36 mm ) formed to the weight of the total sample
expressed as a percentage when the sample is subjected to impact loading of 14 kg falling
from a height of 38 cm for 15 times. The aggregate impact value (IS 283-1970) should not
exceed 45 percent by weight of the aggregate used for concrete other than wearing surface
and 30 percent by weight for concrete used in wearing surface.
IV . Crushing strength
It is the strength or the resistance of aggregates to crushing against gradually applied
compressive load. Crushing strength of the aggregates is measured in terms of aggregates
crushing value which is the ratio of weight of aggregates crushed finer than 2.36 mm to the
original sample passing through 12.5 mm sieve and retained on 10 mm sieve. The aggregate
is placed in a cylindrical mould and a load of 40 ton is applied through a plunger. Aggregates
crushing value is limited to 30 percent for road and pavements and 45 percent for other
structures.
(c) Chemical properties
I . Alkali aggregate reaction
For a long time, aggregate were thought to be inert material but later after 1940, it was
clear that they were not fully inert. Some aggregate contains reactive silica which reacts
with alkalis (sodium oxide and potassium oxide) present in the cement. This reaction starts
with attack on reactive siliceous minerals on aggregates by alkaline hydroxide derived from
cement. As a result, alkali silicate gel of unlimited swelling type is formed. The progressive
swelling of this gel results in disruption of the concrete with spreading of pattern cracks and
eventually failure of structures. The rate of deterioration may be slow or fast depending
upon the conditions. It is also to be noted that only such aggregates which contains reactive
silica in particular proportions and in particular fineness are found to exhibit tendencies for
alkali aggregate reaction. It is possible to reduce such tendencies by altering either the
proportions of reactive silica or its fineness.
Certain factors which promote this reaction are




High alkali content
Reactive type of aggregates
Availability of moisture
Optimum temperature and conditions
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
D. Gradation of aggregate
Gradation of aggregate is the process of producing that sample of aggregates which
contains all standard size of aggregates in required proportion such that the sample
contains minimum voids. Well graded sample containing minimum voids requires minimum
paste to fill which means less quantity of cement and water which will further means
increased economy, higher strength, lower shrinkage and greater durability. Having fully
understood the importance of good grading in making quality concrete, many researchers
have directed their studies to achieve good grading of aggregate at the construction site.
Various methods have been suggested for arriving at an optimum grading. All these
methods, procedures and formulae point to the fact that, none is reliable and satisfactory
for field application. At the site, a reliable satisfactory grading can only be obtained by
actual trial and error which takes into considerations the characteristics of the local
materials with respect to the size fraction, shape, surface texture, flakiness index and
elongation index.
(a) Sieve analysis
The process of dividing a sample of aggregates into fraction of same particle size is known as
sieve analysis and its purpose is to determine the grading or particle size distribution of the
aggregates. Grading pattern of sample of coarse aggregate and fine aggregate is assessed by
sieving a sample successively through all the sieves mounted one above another with large
sieve on top. Usually, the consecutive sieve size are constantly doubled such as 10 mm, 20
mm, 40 mm etc. As per IS standard, the sieve size for aggregates used in making concrete
are 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 600 micron, 300 micron and 150
micron. The aggregate fraction from 80 mm to 4.75 mm is termed as coarse aggregates and
those fractions from 4.75 mm to 150 micron are termed as fine aggregates. Sieving can be
done either manually or mechanically. In the manual operation, the sieve is shaken giving
movements in all possible directions to give chances to all the particles for passing through
the sieve.
(b) Fineness modulus
Fineness modulus is a parameter introduced by Abrams for arriving at satisfactory grading of
the aggregate. He found out that any sieve analysis curve of aggregate that will give the
same fineness modulus will require a same quantity of water to produce a mix of same
plasticity and gives concrete of same strength as long as it is not too coarse for the quantity
of cement used.
Fineness modulus is an index number which indicates fineness or coarseness of the material
but because different grading can give the same fineness modulus, it does not define the
grading of aggregate. It is an empirical factor obtained by adding the cumulative percentage
of the aggregate retained on each sieve ranging from 80 mm to 150 um and dividing the
sum by 100. The larger the figure, the coarser the material is.
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
An example of sieve analysis conducted on a sample of coarse as well as fine aggregate to
find out the fineness modulus is given as below:
Coarse Aggregate
IS Sieve Size
Cum. Wt. Retained
80 mm
Wt. Retained
(Kg)
0
0
0
100
40 mm
0
0
0
100
20 mm
6
6
40
60
10 mm
5
11
73.3
26.7
4.75 mm
4
15
100
0
2.36 mm
-
-
100
0
1.18 mm
-
-
100
0
600 micron
-
-
100
0
300 micron
-
-
100
0
150 micron
-
-
100
0
Pan
-
-
-
0
∑ = 15 kg
% cum Retained
% passing
∑ sum = 713.3
Fineness Modulus (F.M) = 713.3/100 = 7.133
Fine Aggregate
IS Sieve Size Wt. Retained (Kg)
Cum. Wt. Retained
% cum Retained
% passing
80 mm
-
-
-
-
40 mm
-
-
-
-
20 mm
-
-
-
-
10 mm
0
0
0
100
4.75 mm
10
10
2
98
2.36 mm
50
60
12
88
1.18 mm
50
110
22
78
600 micron
95
205
41
59
300 micron
175
380
76
24
150 micron
85
465
93
7
Pan
35
500
-
-
∑ sum = 246
(F.M) = 2.46
∑ = 500 gm
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
(c) Grading curve
The curve obtained by plotting cumulative percentage passing of aggregate through some
standard sieves as ordinate and the size or number of those standard sieve in logarithmic
scale as abscissa is called grading curve. Grading curve is a way of expressing grading pattern
of the aggregate such that it gives clear pictorial view of the grading. Also the comparison of
grading pattern of number of samples can be made at one glace from such curve. In a
grading pattern of aggregate, if all particles size is present in certain proportions, then such
pattern of particle size distribution is called continuous grading. Whereas, if certain particle
size are absent, then sub type of grading is called gap grading of aggregate. The gap grading
is shown by a horizontal line in a grading curve.
One of the most commonly referred practical grading curves is those produced by Road
Research Laboratory. On the basis of large number of experiments in connection with
bringing out mix design procedure, road research laboratory has prepared a set of type of
grading for all in aggregate graded down from 20 mm and 40 mm.
In practice, it is difficult to get the aggregate to conform to any particular standard curve
exactly. For convenience, grading limits are laid down in various specifications rather than to
conform exactly to a particular grading curve. A table below shows grading limits for fine
aggregates.
IS
Sieve
Size
10 mm
Percentage Passing by Weight for
Grading
Zone
100 I
Grading
Zone
100 II
Grading
Zone
100 III
Grading
Zone
100 IV
4.75
mm
2.36
90-100
90-100
90-100
95-100
60-95
75-100
85-100
95-100
mm
1.18
mm
600
30-70
55-90
75-100
90-100
15-34
35-59
60-79
80-100
micron
300
micron
150
5-20
8-30
12-40
15-50
0-10
0-10
0-10
0-15
micron
A course manual on Concrete Technolgoy
12
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
1.2.2 Cement (Manufacturing of cement, compound composition of Portland
cement, structure and reactivity of compounds)
Cement is a binding material manufactured from combination of calcareous materials like
limestone and chalk, argillaceous materials such as clay or shale, siliceous materials etc.
A. Manufacture of Portland cement
Mixing
Burning
Grinding
(1400-1500 0C)
(Ball or Tube Mill)
Lime + Silica + Alumina
(Rotary Kiln)
(Ball Mill)
Cement
Clinker Formation
It can be seen that cement is manufactured from the combination of calcareous materials
(limestone and chalk), silica and alumina (clay or shale). The process of manufacturing of
cement consists of grinding the raw materials into very fine powder form, mixing them in a
predetermined proportions and burning them in large rotary kiln at a temperature of about
1400 degree centigrade. When these materials partially fuse to form a ball, it is known as
clinker. The clinker is cooled, grinded to a fine powder, some amount of gypsum is added
and the resulting product is the commercial Portland cement. Gypsum here is added to
retard the initial setting time of the cement.
Chemical reaction in the rotary kiln
Limestone (CaO + CO2) + Clay (SiO2 + Al2O3 + Fe2O3 + H2O)
1450 0C
Rotary Kiln
Clinker
Which consists of
 Tricalcium Silicate (3 CaO. SiO2)
 Dicalcium Silicate (2 CaO. SiO2)
 Tricalcium Aluminate (3 CaO. Al2O3)
 Tetracalcium Aluminoferrite (4 CaO. Al2O3.Fe2O3)
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
IS standard has specify the following requirements to be fulfilled during the manufacturing
of cement for 33 grade Portland cement.
 Ratio of lime to silica, alumina and ferric oxide combined also called the lime
saturation factor should be between 1.02 and 0.66.
 Ratio of percentage of alumina to iron oxide should not be less than 0.66.
 Weight of insoluble residue should not be greater than 4 percent.
 Weight of magnesia should not be greater than 6 percent.
 Loss on ignition should not be greater than 5 percent.
B. Compound composition of Portland cement
The raw materials used for the manufacture of cement consist mainly of lime, silica,
alumina, clay and iron oxide. The relative proportions of these oxides compound are
responsible for influencing various properties of cement in addition to rate of cooling and
fineness of grinding. These oxides react with one another in the kiln at higher temperature
to form more complex compounds. These oxides and their composition in percentage are
shown in tabular form as below.
Oxides
Percentage Content
Lime (CaO)
60-67
Silica (SiO2)
17-25
Alumina (Al2O3)
3-8
Iron Oxide (Fe2O3)
0.5-0.6
Magnesium Oxide (MgO)
0.1-4
Alkalies (K2O, Na2O)
0.4-1.3
Sulphur Trioxide (SO3)
1.3-3
Bogue’s compound
As suggested earlier, those oxide compounds present in the raw materials of the cement
when subjected to high clinkering temperature combine with each other to form complex
compounds known as Bogue’s compounds. The four major compounds also known as
bogue’s compounds are listed as below.
Name
of
compound
Tricalcium silicate
Formula
3 CaO. SiO2
Abbreviated
Formula
C3S
Dicalcium silicate
2 CaO. SiO2
C2S
Tricalcium
aluminate
Tetracalcium
3 CaO. Al2O3
C3A
4 CaO. Al2O3.Fe2O3
C4AF
aluminoferrite
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
Bogue has also suggested equations for calculating the percentage of these above
mentioned four compounds which are as follows:
C3S = 4.07 (CaO) - 7.60 (SiO2) - 6.72 (Al2O3) - 1.43 (Fe2O3) - 2.85 (SO3)
C2S = 2.87 (SiO2) - 0.754 (C3S)
C3A = 2.65 (Al2O3) - 1.69 (Fe2O3)
C4AF = 3.04 (Fe2O3)
C. Structure and reactivity of compounds
Advancement in science and technology has greatly helped to recognize the microstructure
of cement compounds before and after the hydration of cement. X-ray diffraction, X-ray
fluorescence method, electron microscope etc has helped to reveal the crystalline and
amorphous structure of hydrated and anhydrate cement paste.
Both Le-chatelier and Tornebohm observed 4 different kinds of crystals in the sections of
cement clinkers. Tornebohm called these 4 kinds of crystal as Alite, Belite, Celite and Felite.
The descriptions of these crystals were found similar to the Bogue’s compounds so
sometimes in literature these 4 Bogue’s compounds i.e. C3S, C2S, C3A, and C4AF are called
as Alite, Belite, Celite and Felite respectively.
C3S and C2S
 C3S and C2S are the two major compounds responsible for the strength of hydrated
cement paste. Together, they constitute about 70 – 80 % of the total volume of the
cement. Average C3S content in modern cement is about 45 % and that of C2S is
about 25 %.
 C2S react slowly as compared to C3S with water but it is the one which contribute
most to the strength of the cement. C2S is responsible to ultimate strength of the
cement whereas C3S contribute most to early strength of the cement.
 Naturally, C3S is more reactive than C2S and releases more heat than C3S during
hydration ( almost double than C2S )
C3A and C4AF
 C3A is the most reactive among all and also releases highest amount of heat during
hydration. It contributes very little to strength and mostly acts as a flux and also
contributes to the initial setting of the cement.
 C4AF is present in cement in very small quantity and it does not affect the behavior
of cement significantly as compared to other compounds but its presence
accelerates the hydration of silicates.
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Figure 1-2 Development of strength of Compounds
D. Hydration of cement
Anhydrous cement doesn’t bind coarse and fine aggregate. It acquires adhesive property
only when mixed with water. The chemical reaction that takes place between cement and
water when they are mixed together is called hydration of the cement. The reaction is as
below:
Adhesive property of cement paste is due to formation of calcium silicate hydrates (C3S2H3)
Approximately same amount of water is required for hydration but C3S produce more Ca
(OH) 2. Due to lesser amount of Ca (OH) 2, C2S is more suitable in acidic environment.
It was doubtful before that hydration product of both C3S and C2S results in the formation of
same hydrated compound but later on, it was found out that ultimate products are actually
the same. The approximate reactions of C3S and C2S with water are shown below where the
numbers in the brackets indicate corresponding masses.
2C3S + 6H
(100)
C3S2H3 + 3 Ca (OH) 2
(24)
(75)
2 C2S + 4 H
(100)
(49)
C3S2H3 + Ca (OH) 2
(21)
(99)
(22)
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For C3A
C3A + 6H
C3AH6
(100)
(140)
(40)
The reaction of C3A with water is rapid and would lead to the flash set which is prevented by
the addition of gypsum to the cement clinker. Even so, the rate of reaction is faster than
that of calcium silicates.
E. Heat of hydration
The reaction between cement and water is exothermic and releases large amount of heat.
That heat is called heat of hydration. Different compounds hydrates at different rates and so
liberates different amount of heat. The highest amount of heat is produced by C 3A, followed
by C3S and C2S. Fineness also influences the rate of heat generation but not the total heat.
The total quantity of heat generated in complete hydration will depend upon the relative
quantities of major compounds present in the cement. Study and control of heat of
hydration becomes important in the construction of dams and mass concrete works where
the temperature of interior portion can be as high as 500C above the original temperature of
the concrete mass at the time of placing. In the usual range of Portland cement, about one
half of the total heat is liberated between 1 and 3 days, about three quarters in 7 days and
nearly 90 percent in 6 months. The total quantity of heat liberated by the four compounds
of the cement in 3 days is as below:
Compound
Heat of hydration ( 3 days )
C3S
58 cal/gm
C2S
12 cal/gm
C3A
212 cal/gm
C4AF
69 cal/gm
F. Introduction to special types of cement
(a) Rapid hardening Portland cement
 Prepared by increasing degree of fineness and lowering the content of C2S
 Used if formwork has to be removed early, in road repair, prefabricated concrete
works and in cold weather.
(b) Low heat Portland cement
 This type of cement has low heat of hydration due to lower content of C3A and C3S.
 The development of strength is slow but ultimate strength remains unaffected.
 Used especially in mass concrete works such as dams.
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(c) Sulphate resistance cement
 Sulphate reacts with free calcium hydroxide to form calcium sulphate. Then C3A
reacts with calcium sulphate to form calcium sulphoaluminate which is very
expansive in nature. This expansion causes results in cracks and disintegration of
concrete. This attack is called sulphate attack.
 This cement has low C3A and low C4AF content to resist sulphate attack.
 Used especially in marine works, foundation and basement, construction of sewage
treatment plant etc
(d) Portland pozzolanic cement
 Prepared by intergrading OPC cement clinker with pozzolanic material (10 -25 %).
 Pozzolana does not have cementing property in itself but in presence of water and
calcium hydroxide, it forms compounds having cementitious properties.
 Less heat of hydration, great resistance to chemical attack, cheaper in cost etc are its
chief advantages.
 Useful particularly in marine, hydraulic and mass concrete works.
(e) Portland blast furnace slag cement
 Prepared by mixing OPC cement clinker, gypsum and granulated blast furnace in
suitable proportions.
 Hydrophobic cement
 Prepared by mixing OPC cement clinker with water repellent film such as oleic acid
and stearic acid.
 Used if cement has to be stored for long time before use.
(f) Quick cement
 It is prepared by reducing gypsum in the cement. The initial and final setting time of
cement is 5 minutes and 30 minutes.
(g) White cement
 Almost same as OPC but has high purity limestone (96 % CaCO3 and less than 0.07 %
iron oxide.
(h) Masonry cement
Cement mortar is superior to lime mortar in terms of strength and setting properties but
inferior to in terms of workability, water retentivity, shrinkage and extensibility. Masonry
cement has all the advantages of cement and lime mortar.
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1.2.3 Use of water in concrete:
Water is one of the important ingredients of the concrete mix. Quantity of water has direct
effect on the strength of the cement but when we are talking here about the use of water,
we are focusing on the quality of water to be used in the concrete rather than quantity.
Water is used in concreter for the following purposes.
A. Water for mixing concrete and washing aggregate
Generally, same water is used for mixing and washing aggregates. A general saying is that if
water is fit for drinking, then it is fit for mixing and washing but it may not be true for all
conditions. Drinking water may be unsuitable if it has high concentration of potassium and
sodium due to risk of alkali aggregate reaction. Also, the water containing small amounts of
sugar may be suitable for drinking but not for drinking.
In general, any water with pH between 6.0 and 8.0 which doesn’t taste saline or brackish is
suitable for mixing.
Presence of algae is also undesirable as it leads to air entrainment which ultimately leads to
reduction in strength.
Occasionally, sea water has also to be considered but it increases the risk of corrosion in
reinforcement.
B. Water for curing concrete
Generally water satisfactory for mixing is also suitable for curing purposes. However, iron
and organic matter may cause staining if water flows slowly and evaporates quickly.
Also the water should be free from substance that attacks the hardened concrete like free
CO2.
Curing with sea water may lead to corrosion of reinforcement.
1.2.4 Admixtures (classification of admixture, introduction to commonly
used admixture (super-plasticizer, water proofing agent, retarders), use of
mineral admixtures in concrete)
Admixture is defined as a material other than cement, water and aggregate that is used as a
ingredient of the concrete and is added to the batch immediately before or during mixing in
order to modify the properties of ordinary concrete so as to make it more suitable to any
situation.
Classification of admixtures
Admixtures are classified as below:
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A. Chemical Admixture
(a) Accelerating admixtures
Accelerating admixtures are added to the concrete to increase the rate of early strength
development which allows us to:




Permit earlier removal of formwork
Reduce the required period of curing
Advance the time that a structure can be placed in service
Partially compensate for the retarding effect of low temperature during cold
weather concreting
 In the emergency repair works.
Some of the recently developed accelerators are so powerful that is possible to make the
cement set into stone hard in a matter of five minutes or less. With the availability of such
powerful accelerators, the works such as underwater concreting, emergency repair works,
and basement waterproofing operations have become very easy. The commonly used
accelerator in the past was calcium chloride but nowadays some of the soluble carbonates,
silicates fluosilicates and some of the organic compounds such as triethanolamine are used.
(b) Retarding admixtures
An admixture which slows down the chemical process of hydration so that the concrete
remains plastic and workable for a longer time than a normal concrete is called retarder or
retarding admixture. Retarders are used to overcome the accelerating effect of high
temperature on setting properties of concrete in hot weather concreting.
Also, sometimes the concrete may have to be placed in difficult situations and delay may
occur in transporting and placing. In ready mixed concrete practices, concrete are
manufactured in central batching plant and transported over a long distance which may
take considerable amount of time. In the above cases, setting of the concrete have to be
retarded which is the primary function of retarders. Calcium sulphate (gypsum) and
common sugar, for examples, are the most commonly used retarders.
(c) Plasticizers (water reducer)
Right amount of workability is the requirements of good concrete. Also the degree of
workability differs according to the situations. A high degree of workability is required
especially in deep beams, thin walls of water retaining structures with high percentage of
reinforcement, column and beam junction etc. one way to increase the workability is to
increase the water content but increase in water beyond the specified limit have harmful
effect on its strength. As such, plasticizers are used in such situation application of whose
will increase the workability of the concrete without increasing the water content.
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Plasticizers are used in amount from 0.1 % to 0.4 % by weight of the cement and this will
reduce the mixing water by 5 % to 15 %, workability remaining the same. The use of
plasticizers improves the durability as well as the strength of the concrete. Sometimes, they
are used to reduce the cement content and heat of hydration as well. Calcium, Sodium and
ammonium lignosulphonates are the most commonly used plasticizers.
(d) Super plasticizers (high range water reducers)
Super plasticizers are the improved version of plasticizers. With the workability remaining
constant, plasticizers can reduce the mixing water up to 15 % whereas super plasticizers will
reduce the water up to 30 %. It is the use of super plasticizers which has made it possible to
use w/c ratio as low as 0.25 or even lower and yet make a flowing concrete with the
strength up to 120 MPa or even more. Super plasticizers are used in production of self
leveling, self compacting concrete and in production of high strength and high performance
concrete.
(e) Air entraining admixtures
Air entraining admixtures are used to make air entrained concrete. Air entrained concrete is
made by mixing a small quantity of air entraining agent or by using air entraining cement.
These air entraining agents consists of millions of air bubbles which will acts as a flexible ball
bearing and will modify the properties of plastic concrete regarding workability,
segregation, bleeding and finishing quality of the concrete. It also modifies the properties of
hardened concrete regarding its resistance to frost action and permeability.
(f) Water proofing admixtures
Concrete is susceptible to water under two conditions; first is due to direct water pressure
on one side and second is due to absorption of surface water by capillary action. These are
the admixtures which renders the concrete waterproof or impervious. Water proofing
admixtures may be obtained in powder, paste and liquid forms and contain either pore
filling or water repellent materials.
The chief materials in pore filling category are silicate of soda, aluminum and zinc sulphate,
aluminum and calcium chloride. These are chemically active pore fillers and they also
accelerate the setting time of concrete rendering the concrete more impervious at early
age. The chemically inert pore filler such as chalk and talc improves the workability which
helps in reduction of water making a dense concrete which is basically impervious. Materials
like soda, potash soaps, resin, vegetable oil, fats, waxes etc acts as water repelling materials
in this type of admixture.
(g) Pigment admixtures
Pigments added to produce color in the finished concrete are called pigment admixture or
coloring agents. These pigments are thoroughly mixed with dry cement or with the dry
concrete mixtures before the addition of mixing water.
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(h) Corrosion inhibitors
Corrosion of reinforcement in concrete is a problem worldwide but the situation is more
critical if the concrete is exposed to saline or brackish water or to the industrial fumes.
These are the admixture which helps to protect the steel reinforcement used in concrete
from corrosion. Some of the corrosion inhibitors used today are sodium benzoate, calcium
lingnosulphonate (corrosion from ac or dc), sodium and calcium nitrate etc.
(i) Bonding agent
Bonding admixtures are water emulsions of several organic materials that are mixed with
cement or mortar grout for application to an old concrete surface just prior to patching with
new mortar or concrete. Their function is to increase the bond strength between old and
new concrete. The commonly used bonding admixtures are made from natural rubber,
synthetic rubber or from any organic polymer.
B. Pozzolanic admixtures (Mineral admixtures)
Pozzolanic materials are the silicious and aluminious materials which in themselves possess
little or no cementitious value, but will, in finely divided form and in the presence of
moisture, chemically react with calcium hydroxide liberated on hydration to form
compounds containing cementitious properties.
The history of the use of pozzolanic materials is as old as ancient Greeks and Rome. It was
recognized long time ago that suitable pozzolans used in the appropriate amount will
modify the certain properties of fresh and hardened concrete and mortars. After the
development of natural cement and Portland cement, the use of pozzolans declined but in
recent times, pozzolans have been extensively used in Europe, USA and Japan as an
ingredient of Portland cement concrete particularly for marine and hydraulic structure.
It has been proved that the best pozzolans in optimum proportions mixed with Portland
cement improves the many quality of concrete such as:








Lowering the heat of hydration and thermal shrinkage
Increase the water tightness
Reduce the alkali aggregate reaction
Improve the resistance to sulphate and sea water
Improve the extensibility
Lowers the susceptibility to dissolution and leaching
Improves workability
Lowers the production cost
Some of the commonly used pozzolanic materials are clay and shales, diatomaceous earth,
fly ash, blast furnace slag, silica fume, metakaoline, surkhi, rice husk ash etc.
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Chapter - 2 Structure of concrete
2.1 Concrete as a three phase system
The type, amount, size, shape, and distribution of phases present in a solid constitute the
structures of the concrete. Generally, structures of the concrete can be divided into two
parts known as micro and macro structures.
Macrostructures refers to gross elements of the structures which is visible to the human eye
and can be readily seen from the cross section of the materials. Microstructures refer to the
microscopically magnified portion of the macrostructures visible under the electron
microscope whose magnifying capacity is of the order of 105.
From examination of a cross section of concrete, the two phases that can easily be
distinguished are aggregate particles of varying size and shape, and the binding medium
composed of an incoherent mass of the hydrated cement paste. At the microscopic level,
the complexities of the concrete microstructure are evident. It becomes obvious that the
two phases of the microstructure are neither homogeneously distributed with respect to
each other, nor are they themselves homogeneous. In the presence of aggregate, the
microstructure of hydrated cement paste in the vicinity of large aggregate particles is
usually very different from the microstructure of bulk paste or mortar in the system. In fact,
many aspects of concrete behavior under stress can be explained only when the cement
paste aggregate interface is treated as a third phase of the concrete microstructure
commonly known as transition phase.
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2.2 Structures of the aggregate phase
The aggregate phase is predominantly responsible for the unit weight, elastic modulus, and
dimensional stability of concrete. These properties of concrete depend to a large extent on
the bulk density and strength of the aggregate which, in turn, are determined by physical
rather than chemical characteristics of the aggregate. In other words, the chemical or the
mineralogical composition of the solid phases in aggregate is usually less important than the
physical characteristics, such as volume, size, and distribution of pores.
Being stronger than the other two phases of concrete, the aggregate phase has usually no
direct influence on the strength of concrete except in the case of some highly porous and
weak aggregates, such as pumice. The size and the shape of coarse aggregate can, however,
affect the strength of concrete in an indirect way. It is obvious that the larger the size of
aggregate in concrete and the higher the proportion of elongated and flat particles, the
greater will be the tendency for water films to accumulate next to the aggregate surface,
thus weakening the interfacial transition zone.
2.3 Structure of the hydrated cement phase
Hydrated cement phase in concrete refers to the cement paste made from the Portland
cement. It is the paste formed by the chemical reaction between cement and water. To
understand about hydrated cement phase of the concrete, let us remember some points
regarding anhydrous Portland cement.
Anhydrous cement is a gray powder composed of angular particles ranging from 1 to 50 um
produced by pulverizing clinker with small amount of gypsum. Clinker is a heterogeneous
mixture of several compounds formed by the high temperature reaction between calcium
oxide, silica, alumina and iron oxide. The complex compounds formed in the clinker are C 3A,
C2S, C3AS and C4AF.
When Portland cement is dispersed in water, calcium sulphate and high temperature
components of calcium begins to go into solution. As a result of interaction between calcium
sulphate, aluminates and hydroxyl ions, within a few minutes of hydration, the needle
shaped crystal of calcium trisulfoaluminte hydrate called ettringite is formed. A few hours
later, large prismatic crystals of aluminium hydroxide and very small fibrous crystals of
calcium silicates hydrates begin to fill the empty space formerly occupied by water and
dissolving cement particles. After some days, depending upon alumina- sulfate ratio of
Portland cement, ettringite may become unstable and will decompose to form
monosulfonate aluminium hydrate.
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From the micro structural model of hydrated cement paste, it is noted that various phases
are neither uniformly distributed nor are they uniform in size and morphology. In solids,
microstructural inhomogenity can lead to serious effects on strength and other related
mechanical properties because these properties are controlled by the microstructural
extremes and not the average microstructure. Thus in addition to evolution of
microstructures as a result of chemical changes, which occurs after cement comes in contact
with water, attention has to be made in certain rheological properties of freshly mixed
concrete which influences microstructure of hardened paste.
For example, anhydrous part of cement have tendency to attract each other and form
flocks, which traps large quantity of mixing water. Obviously, local variation in water cement
ratio would be primary source of evolution of heterogeneous microstructure.
2.3.1 Solids in hydrated cement phase.
Types, amount and characteristics of four principal solid phases in hydrated cement phase
that can be resolved by electron microscope are as follows.
A. Calcium silicate hydrates (C-S-H)
 It occupies 50 – 60 % of volume in hydrated cement paste and is most important in
determining properties of paste.
 C-S-H is hyphenated which means it is not a well defined compound.
 Morphology of the C-S-H varies from poorly crystalline fiber to reticular network.
 Due to colloidal dimensions and tendency to cluster, C-S-H crystal could only be
resolved with advent of electron microscope.
 In older literature, the material is often referred as C-S-H gel. The internal structure
of C-S-H was previously assumed to resemble the natural mineral tobermorite and
that’s why C-S-H was sometimes called tobermorite gel.
 Exacts structure of C-S-H is unknown. Several models have been proposed to explain
the properties. According to powers brunauer model, the material has layered
structure with very high surface area of the order of 100- 700 m2/gm. and the
strength of material is due to van der waals force. The size of gel pores (solid to solid
distance) is about 18 A.
B. Calcium hydroxide
 Also called portlandite and consists of 20 -25 % of solid volume in hydrated cement
paste.
 In contrast to C-S-H, it is composed with definite stiochiometry which forms large
crystals with definite hexagonal prism morphology and is affected by available space,
temperature of hydration and impurities present.
 Strength contributing potential is limited as result of considerably lower surface area
compared to C-S-H.
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C. Calcium sulfoaluminate hydrates
 It occupies 15 – 20 percent of the volume of solids in hydrated cement paste.
 It plays a minor role in microstructure relationships.
 At early stage of hydration, sulphate alumina ratio favors the formation of
trisulphate hydrate ( C6AS3H32 ) also called ettringite which forms needle shaped
prismatic crystal. Ettringite eventually transforms to monosulphate hydrate
(C4ASH18 ) which forms hexagonal plate crystal presence of which in cement makes it
vulnerable to sulphate attacks.
D. Unhydrated clinker grain
Depending upon particle size distribution of anhydrous cement and degree of hydration,
some unhydrated clinker grain may be found in microstructure of hydrated cement paste
even long after hydration. Because of limited available space between the particles,
hydration products tend to crystallize in close proximity to hydrating clinker particles which
gives appearance of coating formation around them.
2.3.2 Voids in hydrated cement paste
In addition to solids, hydrated cement paste contains several types of voids which have
important influence on the properties of the concrete.
A. Interlayered space in C-S-H
Powers assumed that interlayer space within the C-S-H structure to be 18 A and determined
that it accounts for the 28 percent of the total porosity in solid C-S-H. However, Feldman
and Sereda suggest that space may vary from 5 to 25 A.
This size of void is too small to have an adverse effect on the strength and permeability of
the hydrated cement paste. However, as discussed below, water in these small voids can be
held together by hydrogen bonding and its removal under certain conditions may contribute
to drying shrinkage and creep.
B. Capillary voids
Capillary voids represent the space not filled by the solid components of the hydrated
cement paste. Size of the capillary voids is determined by the distance between original
anhydrous cement particles in the freshly mixed concrete and degree of hydration. In well
hydrated and low water cement ratio paste, capillary voids may range from 10 to 50 nm
whereas in high water cement ratio paste, void may be as large as 3 to 5 um. Capillary voids
larger than 50 nm referred to as macropores in modern literature are more influential in
determining the strength and permeability characteristics whereas voids smaller than 50 nm
referred to micropores play an important part in drying shrinkage and creep.
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C. Air voids
Capillary voids are irregular but air voids are generally spherical. A small amount of air gets
trapped in the cement paste during concrete mixing and for various reasons, some
admixtures are added to entrain air voids. Entrapped air voids may be as large as 3 mm
whereas entrained air voids usually range from 50 to 200 um. In both cases, they are bigger
than capillary voids and are capable of adversely affecting the strength of the concrete.
2.3.3 Water in the hydrated cement paste
Depending upon the environmental humidity and porosity of the paste, untreated cement
paste is capable of holding large amount of water. Like the solid and void phases discussed
above, water can exist in hydrated cement paste in many forms. The classification is based
upon the degree of difficulty or ease with which water can be removed from the hydraulic
cement paste.
A. Capillary water
This is the water presents in the voids larger than about 50 A which is free from the
influence of attractive forces exerted by the solid surface. From the behavior of capillary
water in hydrated cement paste, it is desirable to divide the capillary water into two
categories. Water in larger voids (greater than 50 nm) which may be called free water
(because its removal doesn’t cause any volume change) and water held by capillary tension
in capillaries (5 to 50 nm), the removal of which may cause shrinkage of the system.
B. Adsorbed water
This is the water which is very close to the solid surface and is under the influence of the
attractive forces of solids. Water molecules are physically adsorbed unto the surface of
solids in hydraulic cement paste. Up to six molecular layers of water (15 A ) can be held by
hydrogen bonding because bond energies at individual water molecules decreases with
distance from solid surface a major portion of adsorbed water can be lost when hydraulic
cement paste is dried to 30 percent of its relative humidity. The loss of adsorbed water is
responsible for shrinkage of hydraulic cement paste.
C. Interlayer water
Water associated with C-S-H structure i.e monomolecular water layer between the layers of
C-S-H is strongly held by hydrogen bonding. The interlayer water is lost only on the strong
drying.
D. Chemically combined water
It Is the water which is an integral part of the microstructure of various cement hydration
products. This water is not lost on drying; it is evolved when hydrates decompose on
heating.
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2.4 Transition zones in concrete
At macro level, aggregates particles are seen to be dispersed in the matrix of the cement
paste and thus the concrete looks to consists of only two phases i.e. aggregate and cement
paste phase. But at the microscopic level, complexities arise in vicinity of large aggregate
particles. This area in the vicinity of the large aggregate particles represents the interfacial
region between the particles of the coarse aggregates and hydrated cement paste is actually
a third phase of the concrete also known as transition zone.
2.4.1 Structure of Transition zone
The transition zone is defined as the region between large aggregate particles and the hcp
(or mortar). It exists as a thin shell, typically 10-50 micron thick. Formation of transition zone
can be attributed to poor packing and the formation of water films around large particles
during mixing. Owing to higher w/c ratio, the transition zone is more porous than the bulk
cement paste or mortar matrix.
Scanning electron microscopy indicates the following:
HCP and transition zone have the same constituent phases but at different fractions. The
transition zone is more porous and large CH crystals are present, providing smooth planes
for cracks to go through. This observation cast light on the ‘engineering’ of microstructure to
improve concrete strength. By adding silica fume, a very small particle that can react with
CH to form additional C-S-H, the CH crystals are removed and the interface becomes much
denser. Indeed, silica fume is a very common constituent in high strength concrete used in
practice.
2.4.2 Effects of transition zone
Transition zone is the weakest region among all the three phase and has greater influence
on the mechanical behavior of the concrete. If a concrete falls under load or due to any
reason, it is the transition zone which fails first. It is for this reason; transition zone is the
weakest link of the chain and is the strength limiting zone of the concrete.
Although the transition zone is made up of same bulk cement paste, quality of paste in
transition zone is of poor quality. Due to internal bleeding, water accumulates below
elongated, flaky and large pieces of aggregate which reduces bond between paste and
aggregate in general.
If we further go into detail, size and concentration of crystalline compounds such as
Ca(OH)2 and ettringite are also large in transition zone. Such situation also account for the
lower strength of the transition zone. Numerous experimental studies have shown that in
transition zone, there is increase in w/c ratio of 0.1 and reduction in cement content by 100
kg/m3 which may be due to internal bleeding.
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Due to drying shrinkage and temperature variations, transition zone develops micro-cracks
even before loading. When structure is loaded and at a high stress level, these microcracks
propagate and bigger cracks are formed resulting in failure of bond between constituents of
concrete.
The existence of transition zone can be used to explain why:
 Cement paste or mortar will always be stronger than concrete provided that they
have the same w/c ratio and be tested at same age.
 The permeability of concrete is much higher than cement paste.
 Under the same loading, components of concrete (aggregate and hcp) can show
linear behavior while concrete itself shows a nonlinear behavior
The first two questions can be easily explained by the high porosity and existing of microcracks in transition zone. For the third question, we should note that it does not take a lot of
energy for the propagation of pre-existing micro-cracks in the transition zone. Even at 40
percent of the ultimate strength of concrete, nonlinear behavior can be observed.
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Chapter - 3 Chapter Three: Mix design of concrete and
property of green concrete
3.1
Workability and its test
Many research workers tried to define workability. But it signifies much wider properties
and qualities of the concrete and does not project any one particular meaning. One general
definition is that workability is the ease with which a freshly mixed concrete can be placed,
compacted and finished. According to Road research laboratory of U.K., Workability is the
amount of useful internal work necessary to produce full compaction. The useful internal
work is a physical property of concrete alone and is the work or energy required to
overcome the internal friction between the individual particles in the concrete. Also
additional energy is required to overcome the surface friction between the formwork and
the reinforcement.
Sometimes consistency is also used to define workability but Consistency is a general term
to indicate the degree of fluidity or degree of mobility. In case of concrete, consistency is
sometimes taken as degree of wetness. A concrete which has high consistency and which is
more mobile need not be of right workability for a particular job because every job requires
particular workability. For example,
A concrete which is considered workable for mass concrete foundation may not be
workable for roof construction and even in roof, concrete considered workable when
vibrator is used is not workable when hand compaction is done.
3.1.1 Factors affecting the workability
A. Water content
Higher the water content, higher will be the fluidity of the concrete which is one of the
important factors improving workability. Increasing water content is the easiest measure to
improve workability of a concrete mix but it should be taken only as a last resort to improve
the workability if there are no any other options. Increasing water content significantly
affects the strength of the concrete and thus sometime cement is also added in addition to
water to maintain the constant water cement ratio so that the strength of concrete remains
the same.
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B. Mix proportioning (aggregate cement ratio)
In a concrete mix, if aggregate cement ratio is higher i.e. in a leaner mix, less quantity of
paste is available for lubrication per unit surface area of aggregate and hence the
workability is lower. In case of richer mix, more cement paste is available for lubrication and
hence the workability is higher in such mix.
C. Size of aggregate
Bigger the size of the aggregates less will be the surface area and hence less amount of
matrix or paste is sufficient for lubrication. As such, for a given quantity of water and paste,
bigger size of the aggregate will improve the workability of the mix.
D. Shape of aggregates
Shape of the aggregates also significantly affects the workability. Angular, flaky and
elongated aggregates will make the concrete very harsh or unworkable in comparisons to
rounded or cubical aggregates. It is because of the fact that for a given volume or weight,
rounded aggregates will have less surface area and less voids than angular aggregates. Also
the frictional resistance in case of rounded aggregates is much lower and hence less amount
of paste is required for lubrication.
E. Surface texture of aggregate
Smooth textured aggregates have less surface area as well the internal friction between
them is less in comparison to rough textured aggregates and hence the workability will be
more in case of smooth textured aggregates.
F. Grading of aggregate
Grading of aggregates also affects the workability to a great extent. Well graded aggregate
will have the least amount of voids in a given volume of concrete mix. When the voids is
minimum, excess amount of paste is available for lubrication and hence the workability
becomes higher.
G. Use of admixture
Admixtures such as plasticizers and super plasticizers are capable of greatly influencing the
workability up to many folds. Likewise, some air entraining agents and pozzolanic materials
also increases the workability by increasing lubrication between aggregate particles.
3.1.2 Measurement of workability
Workability is determined in the laboratory by
Slump test
Compaction factor test
Vee Bee Consistometer Test
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A. Slump test
Slump test is a most commonly used method of measuring the consistency of the concrete
which can be employed either in laboratory or at site of work. This method is not suitable
for very dry and very wet concrete and is generally used for concrete of medium
workability.
The apparatus for conducting the slump test essentially consist of metallic mould in the
form of a frustum of a cone have the internal dimensions as below:
Bottom diameter: 20 cm, Top diameter: 10 cm, Height =30 cm
A steel tamping rod 0.6 m long and 16 mm diameter with bullet end is used for tamping the
concrete.
Procedure
 Inner surface of the mould is thoroughly cleaned and is placed on a smooth,
horizontal, rigid and non absorbent surface.
 The mould is filled with freshly mixed concrete in 3 equal layers.
 Each layer is tamped 25 times by tamping rod. After the top layer has been rodded,
the concrete is struck off level with trowel.
 The mould is then removed immediately by raising it slowly and carefully in a vertical
direction to allow the concrete to subside.
This subsidence is termed as the slump of the concrete i.e. the difference in level between
height of the mould and the highest point of the subsided concrete.
Figure 3-1 Slump Apparatus and Types
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If the concrete slumps evenly, it is called true slump. If one half of the cone slides down, it is
called shear slump. Shear slump indicates that the concrete mix is non cohesive and is a
sign of segregation of concrete. If the concrete slumps significantly and collapse as shown
in figure, it is called collapse slump. It is a sign of greater water or cement content in a mix.
B. Compaction factor test
It is more precise and sensitive than slump test and is particularly used for the concrete of
very low workability. This test works on the principal of determining the degree of
compaction achieved by a standard amount of work done by allowing the concrete to fall
through a standard height. The degree of compaction, called the compaction factor is
measured by the density ratio i.e. the ratio of the density actually achieve in the test to
density of same concrete fully compacted.
Compaction factor= wt of partially compacted concrete / wt of fully compacted concrete
Procedure
 Take a full concrete into the upper hopper up to the brim.
 Open the trap door of upper hopper so that the concrete falls into the lower hopper.
 Then the trap door of the lower hopper is opened and the concrete is allowed to fall
into the cylinder.
 The excess concrete remaining above the top level of the cylinder is then cut off with
the help of plane blades supplied with the apparatus.
 The outside of the cylinder is then cleaned and weighted to the nearest of 10 g
which is the weight of the partially compacted concrete.
 Now the cylinder is emptied, refilled with the same sample in layers of 5 cm each
with heavy vibration or ramming to obtain full compaction. The cylinder is weighed
which is the weight of fully compacted concrete.
The distance between the upper hopper, lower hopper and cylinder is 20.3 cm each.
The dimensions of the apparatus are as below:
Dimension
Upper hopper (cm)
Lower hopper (cm)
Cylinder (cm)
Top internal diameter
25.4
22.9
15.2
Bottom internal diameter
12.7
12.7
Height
27.9
22.9
30.5
Table 3-1 Dimension of the Compaction factor test apparatus
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Figure 3-2 Compacting factor apparatus
C. Vee-bee Consistometer test
 This test consists of a vibrating table, a metal pot, a sheet metal, cone, a standard
iron rod. The apparatus is shown in figure below:
 Concrete is placed at top pot and vibrator is then switched on and simultaneously a
stop watch is started.
 The vibrator is continued till such a time that the conical shape of the concrete
disappears and the concrete assumes a cylindrical shape
 The time required for the shape of concrete to change from
slump cone to cylindrical shape in seconds is known as Vee Bee degree.
Figure 3-3 Vee-Bee Consistometer Apparatus
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3.2 Water cement ratio:
Water cement ratio is defined as the ratio of weight of water to the weight of cement in a
concrete mix. It is the amount of water used per unit weight of cement in a mix. For
example, if 210 kg of water is used in mix with cement content of 420 kg, water cement
ratio of the mix is said to be 0.5. In terms of volume, it is defined as the amount of water in
liters required per bag of cement. Water cement ratio is very important parameters in
concrete mix which mostly affects the workability and strength of the concrete.
The more is the water cement ratio, the more will be the fluidity of the cement paste which
means the lubricating power of the paste is higher. As such, the concrete mix is more
workable. On the other hand, water cement ratio has inverse relation with the strength of
the concrete mix i.e. strength of the concrete decreases with the increase in water cement
ratio and increases with increase in cement water ratio. When the water content per unit
weight of the cement in a mix increases, the cement paste will lose its shear strength as well
as the bond strength between cement paste and aggregates. This accounts for the decrease
in strength.
In 1918 Abrams presented
his classical law relating the strength of concrete with water cement ratio in the form:
𝐴
S=𝐵𝑥 where, x= water cement ratio by volume, s= strength of concrete
A and B are the constants and for 28 days constants A and B are 14000lbs/sq.inch and 7
respectively
Abrams water/ cement ratio law stated that the strength of concrete is only dependent
upon water/cement ratio provided the mix is workable.
Figure 3-4 Relation between compressive strength and Water cement ratio of concrete
The graphs above shows the variation of compressive strength of the concrete with water
cement ratio and cement water ratio respectively. Water cement ratio can be controlled in a
mix by the use of plasticizers and super plasticizers.
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3.3 Introduction to nominal mix
The mix in which all the ingredients are already prescribed and their proportions are
specified is called nominal mix of the concrete. Therefore, there is no any deviation that can
be done by the designer in the mix. Such types of mix are generally designed to have a fixed
strength depending upon the predefined ratio of cement, sand, aggregate and water.
Nominal mix is generally used for less important and simpler concrete works which do not
required in depth analysis and also when there is not much time available for detail mix
design. Nominal mix is designed for the compressive strength of M20 and lower value.
Some of the commonly used nominal mix , their respective proportions of cement, sand,
aggregate and their use in structural works are listed as below :
S.N
Grade
Concrete Mix
Types of Construction
1.
M10
1:3:6
Mass concrete in piers, abutments, massive reinforced concrete
members
2.
M15
1:2:4
Normal RCC works i.e. beam, slabs, columns, walls and small
span arches
3.
M20
1:1.5:3
Water retaining structures i.e. in reservoirs, columns and piles
Table 3-2 Nominal mix, Grade and Use of Concrete
3.4 Probabilistic concept in mix design approach
The strength of concrete varies from batch to batch and within the same batch so it is
difficult to assess the strength of the concrete. It is not possible to have a large number of
destructive tests and so we have to depend on sample test. Further, it is not practical to
have very rigid criteria and reject a structure on the basis of single or few samples.
Probabilistic concept in mix design is a method of quality control which provides a scientific
approach to a concrete designer to understand the realistic variability of the material so as
to lay down design specification with proper tolerance. By devising a proper sampling plan,
it is possible to ensure a certain quality at specified risk.
Compressive strength test of cubes from random sampling of a mix exhibit variations due to
various operations involved in making and testing of concrete. Also it is observed that the
compressive strength of a sample increase with the decrease in cube size. If the numbers of
cube test are plotted on a histogram against their compressive strength, the results are
found to follow a bell shaped curve called normal distribution curve.
The results are said to follow a normal distribution curve if they are equally spaced about
the mean value and if larger number of cubes have strength closer to the mean value and
only very few results have value much greater or less than mean value.
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Arithmetic mean of the test results does not give indication of variation of strength but this
variation can be ascertained by relating individual strength with mean strength and
determining the variations with the help of properties of normal distribution curve.
The characteristics of the normal distribution curve are fixed by the average values and the
standard deviation. The spread of the curve along the horizontal scale is governed by
standard deviation and position of curve along the vertical scale is fixed by mean value.
Figure 3-5 A histogram of Strength Values
Figure 3-6 Normal Distribution Curve showing percentage of sample value in one standard deviation
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General terms used in Normal Distribution
A. Mean strength
It is the average strength of the samples considered under discussion.
B. Standard Deviation
Standard deviation is an index number used to represent the variation of the results or data
with the mean value. It also shows how dispersed are the results from the mean value. If the
standard deviation is high, we can say that the results are more deviated from the mean and
the sample in non uniform.
This value of standard deviation is used to determine the target value of the mix or the
average design strength of the mix and the formula is:
f (t) = fck + 1.64 s
Where f (t) = target strength of the mix
fck = characteristic strength
s = standard deviation
C. Coefficient of variation
It is an alternative method of expressing the variation of results. It is a non dimensional
measure of variation obtained by dividing the standard deviation by arithmetic mean and is
expressed as:
C.V = (Standard Deviation/mean) * 100 %
D. Characteristic Strength
It is that strength of the sample below which not more than 5 percent of the results are
expected to fail. For example, if 100 samples are tested for the design compressive strength
of the 20 MPa, then more than 95 samples show the strength of 20 MPa.
E. Grade of Concrete
Grade of concrete refers to the characteristics compressive strength of concrete defined in
terms of strength of 15x15x15 cm3 cubes at 28 days of casting in N/mm2.
3.5 Concrete mix design
Concrete mix design is a method of selecting the suitable ingredients of concrete and it’s
proportioning with the objective of producing concrete of certain minimum strength and
durability as economically as possible. From the above definitions, the purpose of concrete
mix design can be divided into two parts. The first is to achieve the stipulated minimum
strength and durability and the second is to make the concrete in the most economical
manner.
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Cost wise all concrete depend upon two factors; namely cost of material and cost of labor.
Labor cost is nearly same for good and bad concrete so main attention is directed to the
cost of material. Since the cost of cement is many more times than the cost of other
ingredients, attention is directed mainly to the use of as little cement as possible consistent
with strength and durability.
General steps of mix design
 Required grade or characteristics strength of concrete is found according to
structure and strength (Grade of concrete depends upon the type of structure and
its functional requirements.
 Mean target strength is fixed using the relation as below
fmean = fck + Kσck
fck + 1.65σck
Where,
Grade
Assumed σck
fck=characteristics strength of concrete
M10, M15
3.5
∑(𝑥−𝑥𝑚𝑒𝑎𝑛)2
M20, M25
4.0
M30-M50
5.0
fmean= Target strength of concrete
σck= standard deviation = √
𝑛−1
x= strength of cube test
n= number of test
k= Him worth constant =1.65 (for 95%confidence level or 5% level of significance)
Note: 5% level of significance means strength below which not more than 5% of the test
results are expected to fall
 Select a type of cement and its amount considering both the w/c ratio and durability
criteria.
 Fixed nominal size, shape and type of aggregate and determine its amount
 Mixed ingredient in desired proportions
 The strength of concrete proportion is found out which must be equal to target
strength of concrete. If not, re-proportioning is required.
Before designing any mix, following information are necessary
 Characteristics strength of concrete (fck)
 Workability of concrete
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Workability of concrete measures the ability and easiness to work. It can be measured by
slump test, compaction factor test and Vee-Bee Consistometer test
Workability of concrete as per placing conditions as per IS 456 -1978 is shown below:
Degree of
Workability
Placing Conditions
Compacting
factor
Vee-Bee
(sec)
Slump
(mm)
Very Low
Shallow section with vibration
0.75-0.8
20-10
-
Low
Highly reinforced section with vibration
0.80-0.85
10-5
-
Medium
Highly reinforced section without
vibration or heavily reinforced section
with vibration
0.85-0.92
5-2
25-75
High
Heavily reinforced section without
>0.92
vibration
Table 3-3 Workability of concrete as per placing conditions
75-125
The values of slump are for 20 mm aggregate only and will be lower for smaller aggregate.
 Zoning of sand
Zone
I
II
III
IV
Passing through 600 micron sieve
15-40 %
40-60%
60-80%
80-100% -not suitable for reinforced concrete
 Water cement ratio
 Exposure condition of concrete (as per IS 456 of 2000 )
Environment
Mild
Moderate
Severe
Very Severe
Extreme
Exposure Conditions
Concrete surface protected against weather or aggressive conditions except those
situated in coastal areas
Concrete surface sheltered from severe rain of freezing whilst wet
Concrete continuously under water
Concrete in contact or buried under non aggressive soil or ground water
Concrete surface exposed to severe rain, alternate wetting and drying or occasional
freezing whilst wet or severe condensation
Concrete completely immersed in sea water
Concrete surface exposed to sea water spray, corrosive fumes or severe freezing
conditions whilst wet
Concrete in contact or buried under aggressive subsoil / ground water
Concrete exposed to coastal environment
Surface of members in tidal zone, members in direct contact with the liquid / solid
aggressive chemicals
Table 3-4 Exposure conditions of concrete (IS 456 of 2000)
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 Durability of concrete
Durability of the concrete is defined as ability of concrete to resist weathering action,
chemical attack, abrasion or any other process of deterioration. Therefore durable concrete
will remains its original forms, quality and serviceability when exposed to its environmental
factor
Exposure
Plain concrete
Max w/c by
Wt.
Reinforced concrete
Min Cement Min Grade of Max w/c by Min Cement
content
concrete
Wt.
content
(kg/m3)
(kg/m3)
Mild
0.6
220
-
0.55
300
M20
Moderate
0.6
240
M15
0.50
300
M25
Severe
0.5
250
M20
0.45
320
M30
Very Severe
0.45
260
M20
0.45
340
M35
Extreme
0.4
280
M25
0.4
360
M40
Table 3-5 Water cements ratios, cement content and concrete grade for durability
(Based on IS 456-2000)
3.5.1 Methods of Mix Design
A. Department of Environment (DOE) method
Steps involved in DOE methods are as follows
 Find the target mean strength from the specified characteristics strength by using
probabilistic approach
fmean = fck + Kσck
Where k = risk factor = 1.64
6 = standard deviation = 4 for M20 or M25
= 5 for > M30
 Determine water cement ratio based on target strength using compressive strength
vs. water cement ratio curve and check for maximum water cement ratio based on
durability criteria. Refer Figure 3-7 Compressive Strength Vs Water cement ratio.
 Decide water content for the required workability, expressed in terms of slump or
Vee-bee time, taking consideration the size of aggregate and its type from Table 3-6.
 Find the cement content knowing the water/cement ratio and water content. The
cement content is calculated simply dividing the water content by W/C ratio and
should be compared with the minimum cement content specified from the durability
consideration and higher of these two values should be adopted. Refer to actual
guidelines by British Method or general idea can be taken from Table 3-5 .
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 Next step is to find out the total aggregate content. This requires an estimate of the
wet density of the fully compacted concrete. This can be found out from Figure 3-8.
If the specific gravity of aggregate is unknown, the value of 2.6 for uncrushed
aggregate and 2.7 for crushed aggregate can be assumed. The aggregate content is
obtained by subtracting the weight of cement and water content from weight of
fresh concrete.
 Then, proportion of fine aggregate is determined in the total aggregate depending
upon water cement ratio, maximum size of aggregate, grading zone of fine aggregate
and workability level from Figure 3-9 shown below.
 Final proportion is obtained in the form of 1:X:Y // W/c ratio (Cement: sand: gravel //
W/c ratio)
Figure 3-7 Compressive Strength Vs Water cement ratio
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Table 3-6 Approximate free water content required to give various levels of workability according to 1988 British
method
Water Content ( kg/ m3)
Aggregate
Max Size
Type
Slump 0-10
10-30
30-60
60-180
Vebe >12
6-12
3-6
0-3
uncrushed
150
180
205
225
crushed
180
205
230
250
uncrushed
135
160
180
195
crushed
170
190
210
225
uncrushed
115
140
160
175
crushed
155
175
190
205
(mm)
10
20
40
Table 3-7 Requirement of Cement to ensure durability under specified conditions of exposure of plain concrete
Figure 3-8 Estimated wet density for fully compacted concrete
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Figure 3-9 Recommended percentage of fine aggregate in total aggregate as a function of free water/cement ratio for
various values of workability and maximum size of aggregate: a) 10
mm, b) 20 mm, c) 40 mm. Numbers on each
graph are the percentage of fines passing a
600 microns sieve
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B. ACI method of concrete mix design
This method suggested by the ACI committee 211 is widely used in USA. One method is
based on the estimated weight of the concrete per unit volume while the other is based on
calculation of The ACI method takes into consideration the requirements for workability
consistency, strength and durability etc.
Steps or procedures of design by ACI Method
 Find the target mean strength from the specified characteristics strength by using
probabilistic approach
fmean = fck + Kσck
 Determine the workability in terms of slump according to the work requirements.
General guidance can be taken from Table 3-3 though the actual values
recommended by ACI differ slightly.
 Determine the maximum size of coarse aggregate that is economically available and
consistent with dimensions of the structures. Generally, for RCC works 20 mm and
for prestressed concrete, 10 mm size is used.
 Determine the water cement ratio based upon the required compressive strength as
per the recommendation made in table below. Also check the maximum water
cement ratio from the durability criteria and adopt the lower value.
Average Compressive Strength
at 28 days (MPa)
Effective water cement ratio by mass
Non-air entrained concrete
Air entrained concrete
45
0.38
-
40
0.43
-
35
0.48
0.40
30
0.55
0.46
25
0.62
0.53
20
0.70
0.61
15
0.80
0.71
Table 3-8 Water cement ratio as per the required compressive strength at 28 days
 Determine the approximate amount of mixing water and air content for the given
slump and maximum size of aggregate from table below.
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Water content Kg/m3 for indicated maximum aggregate size
(mm)
12.5
20
25
40
50
70
150
Workability air content
10
Slump (mm)
Non air - entrained concrete
30-50
205
200
185
180
160
155
145
125
80-100
225
215
200
195
175
170
160
140
150-180
240
230
210
205
185
180
170
-
Entrapped air content
(Approx)
Slump (mm)
3
2.5
2
1.5
1
0.5
0.3
0.2
30-50
180
175
165
160
145
140
135
120
80-100
200
190
180
175
160
155
150
135
150-180
215
205
190
185
170
165
160
-
Air entrained concrete
Table 3-9 Water content as per ACI Method for maximum size aggregate and given workability
 Cement content of the mix is computed by dividing total water content by water
cement ratio. Check the cement content for minimum value for durability criteria.
 Durability criteria for water cement ratio and cement content given by ACI slightly
differs from one another but general guidance can be taken from Table 3-5 .
Determine the amount of coarse aggregate required for a unit volume of concrete from
 Table 3-10 given below. The value obtained is multiplied by the dry rodded unit
weight of the aggregate to get the required dry weight
Max Size of Aggregate
(mm)
Bulk volume of dry rodded coarse aggregate per unit volume of
concrete for given fineness modulus of sand
Fineness Modulus
2.4
2.6
2.8
3.0
10
0.5
0.48
0.46
0.44
12.5
0.59
0.57
0.55
0.53
20
0.66
0.64
0.62
0.60
25
0.71
0.69
0.67
0.65
40
0.75
0.73
0.71
0.69
50
0.78
0.76
0.74
0.72
70
0.82
0.80
0.78
0.76
150
0.87
0.85
0.83
0.81
Table 3-10 Bulk volume of dry rodded coarse aggregate per unit volume for given maximum
aggregate size
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 Determine the amount of fine aggregate. At completion of step7, all ingredients
except fine aggregate have been estimated. If the weight of concrete per unit
volume is assumed, the weight of fine aggregate is simply the difference between
the weight of fresh concrete and the total weight of all other ingredients. An
estimate of weight of fresh concrete can be made either by using table below :
Maximum size of aggregate
(mm)
First estimate of unit weight of fresh concrete
Non air entrained (Kg/m3)
Air entrained (Kg/m3)
10
2285
2190
12.5
2315
2235
20
2355
2280
25
2375
2315
40
2420
2355
50
2445
2375
70
2465
2400
150
2505
2435
Table 3-11 Unit weight of fresh concrete for maximum aggregate size as given by ACI Method
An estimate of unit weight of concrete can be made using the equation as well which is
given below:
Wm= 10 Ga (100-A)+γc(1-Ga/Gc)-Yw(Ga-1)
Where
Wm= Weight of fresh concrete, kg/m3
Ga= Weighted average specific gravity of combined fine and coarse aggregate
Gc= Specific gravity of cement =3.15
Yc= Cement requirement, kg/m3
Yw= Mixing water requirement, kg/m3
A= Air content in percentage
 Adjust the mixing water quantity based on the moisture content in the aggregate
and obtained the final result
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C. Indian mix design method
IS method of mix design is applicable to design of normal concrete mixes (non-air entrained)
for different grades of concrete based on their 28 day strength. The procedure of the mix
design is as follows:
 Determine the target or design laboratory strength based on 28 day characteristic
strength and standard deviation specified in code (IS:456)
 Determine the minimum W/C ratio for the design strength from the graph
(Figure 3-7) of 28 days compressive strength of concrete Vs water cement ratio.
Check the water cement ratio for the durability criteria and adopt the lower value.
 Determine the amount of entrapped air from the table below
Nominal maximum size of aggregate,
Entrapped air, as percentage of volume of
concrete
mm
10
3.0
20
2.0
40
1.0
 The water content and percentage of sand in total aggregate are selected from table
below
Table: Approximate sand and water contents per m3 of concrete
W/C= 0.60, Workability = 0.50CF (compaction factor)
(Applicable for concrete up to grade M35)
Water content per m3 of
concrete
Sand as % by total aggregate by
absolute volume
10
208
40
20
186
35
40
165
30
Maximum size of aggregate
(mm)
Table: Approximate sand and water contents per m3 of concrete
W/C= 0.350, Workability = 0.80 CF (compaction factor)
(Applicable for concrete above grade M35)
Water content per m3 of
concrete
Sand as % by total aggregate by
absolute volume
10
200
28
20
190
25
Maximum size of aggregate
(mm)
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Kantipur Engineering College
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 Make adjustment in water content and percentage of fine aggregate for other
conditions of workability, water/cement ratio, grading of fine aggregate and
uncrushed aggregate from table below
Change in condition
Adjustment required in
Water content
Sand% in total aggregate
0
+1.5 % for zone I
0
-1.5 % for zone III
0
-3.0% for zone IV
Increase or decrease in the value of compaction
factor by
± 3%
0
Each 0.05 increase or decrease in water cement
ratio
0
±1 %
For rounded aggregate
-1.5 kg/m3
-7 %
For sand conforming to grading zone I, zone III or
Zone IV (IS: 383-1970)
 Determine the cement content from W/C ratio and the final water content is
obtained after adjustment. Compare the resulting value with the minimum or
maximum cement content specified based on durability or any other consideration
and modify if necessary.
 Determine the quantities of coarse and fine aggregate from the following equations:
𝑊𝑐
𝑊𝑠
V= [ Ww+ 𝐺𝑐 +𝑝.𝐺𝑠 ]
𝑊𝑐
WCA
V= [ Ww+ 𝐺𝑐 +(1−𝑝).𝐺𝑐𝑎 ]
Where,
V= Net volume of fresh concrete = 1 m3-volume of entrapped air
Gc= specific gravity of cement
Gs= specific gravity of sand (SSD)
GCA= specific gravity of coarse aggregate (SSD)
Ww= Mass of water per m3of concrete
Wc= Mass of cement per m3of concrete
WCA= Mass of coarse aggregate per m3of concrete
P= ratio of fine aggregate to total aggregate by absolute volume
 Obtain the actual amount of water to be added after making correction for water
absorption by aggregate and free moisture present in the aggregate. Similarly, get
actual quantities for fine aggregate and coarse aggregate required.
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Kantipur Engineering College
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3.6 Segregation and bleeding
3.6.1 Segregation:
Segregation can be defined as the separation of the constituent materials of concrete. A
good concrete is the one in which all the ingredients are properly distributed to produce the
homogenous mixture. In case of concrete, it is the differences in the size and specific
gravity of the constituents which makes concrete vulnerable to segregation.. we can
generally categorize segregation into following three types :
 The coarse aggregate separating out or setting down from the rest of the matrix
 The paste or matrix separating away from coarse aggregate
 Water separating out from rest of the material being a material of lowest specific
gravity
Such separation of ingredients in concrete not only makes the concrete weak but also
induces all the undesirable properties in a concrete mix such as shrinkage, cracks, improper
finishing and many others.
Now, the conditions favorable for segregation can be summarized as:
 Badly proportioned mix where sufficient matrix is not there to bind and contain the
aggregates
 Insufficiently mixed concrete with excess water also shows the sign of segregation
 Dropping of concrete from heights as in the case of placing concrete in column
concreting will result in segregation
 When a concrete is discharged from a badly designed mixer or a mixer with worn out
blades, concrete shows a sign of segregation
 Conveyance of concrete by conveyer belts, wheel barrow, long distance haul by
dumper, long lift by skip and hoist are also some other situations promoting the
segregations of concrete.
 If a too wet mix is excessively vibrated or if the vibration is continued for time longer
than optimum time, segregation of concrete occur due to settlement of coarse
aggregate in the matrix.
 While finishing concrete floors or pavement with a view to achieve smooth surface,
masons are likely to work too much with the trowel, float or tamping rule
immediately after placing concrete. This immediate working on concrete without any
time interval is likely to press the coarse aggregate down resulting in segregation.
A well made concrete taking into consideration of various parameters such as grading size,
shape and surface texture of the aggregate with optimum quantity of water makes a
cohesive mix. Such mix concrete will not show any tendency of segregation. The cohesive
and fatty characteristics of the mix do not allow the aggregates to fall apart and at the same
time the matrix itself is sufficiently contained by the aggregate.
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Kantipur Engineering College
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Similarly, others factors apart from mix proportioning which do contribute to control
segregation are proper handling, mixing, transporting, placing, compacting and finishing of
concrete. At any stage, if segregation is observed, remixing for a short time would make the
concrete again homogenous. Also the use of certain workability agents, pozzolanic materials
and air entraining agents helps to reduce the segregation.
3.6.2 Bleeding:
Bleeding is sometimes referred as water gain. It is a particular form of segregation, in which
some of the water from the concrete comes out to the surface of the concrete, being of the
lowest specific gravity among all the ingredient of concrete. This is caused by the inability of
the solid constituents of the mix to hold all of the mixing water when they settle
downwards. Bleeding can be expressed quantitatively as the total settlement per unit height
of the concrete. Bleeding capacity as well as the rate of bleeding can be determined
experimentally.
As a result of bleeding, top surface of every lift of concrete may become too wet and if the
water is trapped by superimposed concrete, porous, weak and non durable aggregate will
be formed. If the bleeding water is remixed during finishing of the top surface, a weak
wearing surface will be formed. This can be avoided by delaying the finishing operations
until the bleeding water has evaporated. On the other hand, if evaporation of water from
the surface of the concrete is faster than the bleeding rate, plastic shrinkage cracking may
result.
According to ASTM, bleeding water is expressed in terms of the amount of accumulated
water as the percentage of net mixing water in the sample.
3.7 Quality control in site (mixing, handling, placing,
compacting and curing)
Concrete is usually produced in batches at the site with the locally available materials of
varied characteristics. Therefore, it is likely that strength, cost and other characteristics of
concrete may vary from batch to batch. Such variations depend upon various factors such
as:




Variations in quality of constituents
Variations in mix proportions
Variation in mixing
Quality of overall workmanship and supervision
The aim of quality control is to reduce all these variations and produce uniform batches of
concrete having fixed strength with the fixed amount of cost and resources. On such
account, quality of concrete is defined in terms of desired performance characteristics,
economics, safety and others.
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Advantages of quality control
 Quality control ensures the optimum quantity in the mix, proper workmanship with
supervision which results in optimum strength and durability of the structures.
 Quality control is the rational use of available resources after testing their strength
and characteristics and this leads to reduction in material cost.
 Quality control reduces the maintenance cost.
 Quality control checks any attempts of overdesign made by the designer which
reduces the overall cost.
3.7.1 Mixing
The primary purpose of mixing is to coat the surface of all the aggregate particles and to
blend all the ingredients of concrete mix into a homogenous and uniform mass. As such,
thorough mixing of the materials is necessary for the production of uniform concrete and a
good mixing should ensure that mass become homogenous, uniform in color and
consistency. Generally, there are two methods adopted for mixing. They are
A. Hand mixing
Hand mixing is practiced for small scale and unimportant works. As such type of mixing is
not thorough and efficient; it is desirable to add 10 percent more cement to cater for the
inferior concrete produced by this method. It is of particular significance that water is
sprinkled over the mix and not poured during hand mixing. At the end, water in small
quantity should be added to get just the required consistency.
B. Machine mixing
Mixing of concrete is generally done by machine for reinforced concrete works and for
medium or large scale mass concrete works. Machine mixing is not only efficient but also
economical when the quantity of concrete to be produced is large. Many types of mixers are
available for mixing concrete which can be classified as batch mixers and continuous mixers.
Batch mixers produce concrete batch by batch with time interval and are mostly used in
normal concreting works. Continuous mixers produce concrete continuously without
stoppage till the machine is working and are mostly used in large works such as dams and
abutments.
3.7.2 Handling
Concrete can be transported from the place of mixing or mixer to the site by various
methods. The choice of the methods may depends upon the quantity of concrete to be
transported and economic considerations but the major factor to be considered during this
whole operation is that the homogeneity obtained at the time of mixing should not be
disturbed while being transported to the final place of deposition of concrete.
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Another thing to consider is that the fresh concrete should not be exposed directly to
various environmental factors such as sunlight, humidity and others. Also the vessels used
to carry the concrete should be kept wet and clean during the entire operations.
As such depending upon the requirements of the situations and economic factors, there are
several methods of transportation of concrete and some of them are listed as below:










Mortar pan
Wheel barrow and hand cart
Crane, bucket and rope way
Truck mixers and dumpers
Belt conveyors
Chute
Skip and hoist
Transit mixer
Pump and pipeline
Helicopter
3.7.3 Placing
Not only designing, mixing, batching and transporting, it is of utmost importance that the
concrete must be placed in a systematic manner to yield optimum results. Some of the
precautions that need to be taken while placing the concrete are listed as below:
The most important factor to be considered during placing of concrete is to deposit the
concrete as close as to its final positions so that the segregations of the constituents of the
mix are avoided.
The above situation often takes place in heavily reinforced concrete columns, at the junction
of beam and column and in deep beam where the concrete is to be poured from a
considerable height. To avoid this, concrete is directed by tremie, drop chute or by any
other means so that concrete gets deposited as close as to its final position.
Formwork shall be designed and adjusted so as to remain sufficiently rigid at the time of
placing of concrete. The joints are plugged to prevent the loss of slurry from the concrete.
Before placing the concrete in foundations or subgrade, all the loose earth, pool of water
and organic matters like grass, roots, leaves etc must be removed from the bed. The earth
must be properly compacted and made sufficiently damp to prevent the absorption of the
water from the concrete.
Before placing the concrete, the surface of the previous lift is thoroughly cleaned with water
jet and scrubbing by wire brush. The old surface is sometimes hacked and made rough by
removing all the laitance and loose material.
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3.7.4 Compacting
Compaction of concrete is the process of expelling air from the mix which gets trapped
during mixing, transporting and placing of concrete. If this entrapped air is not removed, the
concrete loses its strength considerably. It is to be noted that 100 percent compaction is not
only important from the view point of workability but also from the point of durability.
Insufficient compaction increases the permeability of the concrete resulting in easy entry of
aggressive chemicals in the mix which attacks concrete and reinforcement to reduce the
durability of the concrete.
Dry mix has high percentage of entrapped air and so the maximum compacting efforts is
required in such case. Too wet mix is easy for compaction but has less strength. In order to
achieve full compaction and maximum density with reasonable compacting efforts available
at the site, it is necessary to use a mix with adequate workability. For maximum strength,
driest possible mix should be compacted 100 percent. Following two methods are adopted
for compacting the concrete:
A. Compaction by hand
Hand compaction of concrete in case of unimportant concrete works of small magnitude.
Sometimes, this method is also applied in such situation where a large quantity of
reinforcement is used which can't be compacted by mechanical means. In case of hand
compaction, workability required is higher and so the strength of such concrete is lower as
compared to dry mixed compacted by mechanical means. Hand compaction consists of
rodding, ramming or tamping.
B. Compaction by vibration
it is noted that compaction by hand if properly carried out on concrete with sufficient
workability gives satisfactory results but the strength of hand compacted concrete is low
because of higher water cement ratio required for full compaction. Where high strength is
required, stiff mix with low water cement ratio is used and to compact such concrete,
mechanically operated vibratory equipments must be used. The different types of vibratory
equipments used are as follows;
 Internal vibrator (Needle vibrator )
 Formwork vibrator ( external vibrator )
 Table vibrator
 Platform vibrator
 Surface vibrator (Screed vibrator)
 Vibratory roller
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3.7.5 Curing
It is known that cement requires water cement ratio of 0.38 for complete hydration and for
filling the voids in gel pores if a concrete is made and contained in a sealed container.
However, in the field and actual work, it is a different story. Since the concrete is open to
atmosphere, water used in the concrete evaporates and water available in the concrete will
not be sufficient for effective hydration so if the hydration is to continue unabated, extra
water must be added to replenish the loss of water on account of absorption and
evaporation. This operation of adding extra water is called curing. Thus curing can be
described as keeping the concrete moist and warm enough so that the hydration of cement
can continue. More elaborately, it can be described as the process of maintaining
satisfactory moisture content and a favorable temperature in concrete during the period
immediately following placement so that hydration of cement can continue until the desired
properties are developed to a sufficient degree to meet the requirement of service.
Curing can be broadly divided into two categories:
A. Water curing
This is by far the most satisfactory method of curing as it satisfies all the requirements of
curing namely promotion of hydration, elimination of shrinkage and absorption of the heat
of hydration. Water curing can be done in following ways
(a) Immersion
(b) Ponding
(c) Spraying or Fogging
(d) Wet covering
The precast concrete items are normally immersed in curing tanks for certain durations.
Pavement slabs, roof slabs etc covered under water by making small ponds. Vertical
retaining walls or plastered surfaces or concrete columns etc are cured by spraying water. In
some cases, wet covering such as wet gunny bags, hessian cloth, jute matting, straw etc are
wrapped to vertical surface for keepign the concrete wet.
B. Membrane curing
In this method concrete is covered with membrane which will effectively seal off the
evaporation of the water from the concrete. This method is useful especially where there is
acute shortage of water or where the curing is entirely up to the workmen who do not quite
understand the importance of uninterrupted curing. In such case, water loss from the
concrete is prevented by using membrane or sealing compounds.
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3.8 Concreting in extreme weather temperature
Concreting in hot weather
Concreting in cold weather
3.8.1 Concreting in hot weather
It is difficult to define what hot weather condition is. However just for convenience, it is
regarded that the concrete placed at an atmospheric temperature above 40 degree
centigrade is considered as hot weather concreting
Problems encountered during concreting in hot weather:
A. Rapid rate of hydration of cement, quick setting and early stiffening.
Due to increase in temperature, hydration of cement is very fast as well as the development
of early strength but the long term strength or ultimate strength is considerably reduced.
Also setting time is decreased resulting in early stiffing and thus reduces workability. It also
causes partial setting of the concrete.
B. Rapid evaporation of mixing water
Water mixed with concrete to give required workability is lost on account of high
temperature and more compacting efforts are required. If the compaction is not proper, it
will create large voids in the concrete and this introduces all sorts of problems in the
concrete mix.
C. Greater plastic shrinkage
The rate of evaporation of the water will be more than the rate of movement of water from
the interior to the surface. As a result, a moisture gradient will be set up which results in
surface cracks known as plastic shrinkage cracks.
D. Absorption of water from the concrete by the sub grade and formwork
The subgrade or formwork is usually dry in hot weather region and if they are not properly
wetted before placing concrete, water from the mix is absorbed by surface in contact.
E. Curing
In hot weather region, concrete surface dries up fast and hydration get affected in such
cases. As such, in hot weather region, continuous and early curing becomes necessary.
F. Difficult in incorporation of air entrainment
If air entraining agents are used, they should be used more than required to compensate for
the loss of air entrainment due to high temperature.
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G. Ready mixed concrete
In hot weather region, if ready mixed concrete is transported to a long distance, it may
creates a problem regarding faster loss of slump or workability.
Precautions
(i) Kept temperature as low as possible by shading the aggregate piles and the
mixture
(ii) Reduce the temperature of aggregate by sprinkling water on it. The evaporation of
sprinkled water will cool the aggregate
(iii) Keep water supply cool by insulating or shading pipes and tanks
(iv) Use crushed ice with mixing water however ice should change to liquid form when
the mixing operation is over.
(v) Effect of hot weathering can also be reduced by working at night time
(vi) For curing, covering with wet burlap (jut bag) or by sprinkling or by other moisture
retaining materials has been found better as it has a definite cooling value.
3.8.2 Concreting in cold weather
In general, any concreting works done below freezing temperature is called cold weather
concreting. The normal procedures adopted for concreting in fair weather can't be adopted
in such cases and special measures have to be taken. Concreting in cold weather introduces
problems such as delay in setting and hardening, damage to concrete in plastic condition
etc. some of the major problems faced in cold weather concreting are listed as below.
Effects of cold weather on concreting
A. Delay in setting and hardening
Rate of hydration of cement depends upon temperature. If temperature is low, concrete
takes a long time to set and longer time to harden i.e. for development of strength. Delay in
setting time makes concrete vulnerable to frost attack and other disturbances while delay in
hardening delays the early removal of formwork.
B. Freezing of concrete at early age
When the temperature goes below freezing point, free water contained in the plastic
concrete freezes. Freezing of water not only prevents the hydration of cement but also
makes the concrete expand. This expansion causes disruption of concrete due to which
irreparable loss of strength and quality takes place.
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C. Freezing and thawing:
It is likely that in cold weather regions, fresh or hardened concrete is subjected to freezing
and thawing cycles and these alternate cycles of freezing and thawing seriously affects the
durability of the concrete and also introduces fatigue in it.
Precaution
(i)
To increase the temperature of fresh concrete, mixing water can be heated
but not above the temperature of 60 to 80 degree centigrade as more hot
water will cause flash set of the cement.
(ii)
If heating of water does not raise the temperature of concrete, aggregates
may be heated but not above 52 degree centigrade. Aggregates should not be
heated by fire as it leads to non uniform heating but should be heated by
passing steam over them.
(iii) Use of rich mixes with low water cement ratio also increases the rate of
hydration and heat development.
(iv) Use of cement of high rate of heat generation (i.e. cement having high C3S
and C3A produced high heat during reaction)
(v)
Rate of hydration can also be increased by using various accelerating
admixtures in the mix
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Chapter - 4 Properties of hardened concrete
In the theory of reinforced concrete, it is assumed that concrete is elastic, isotropic,
homogenous and that it conforms the Hook's law. Actually, none of these assumptions are
strictly true and concrete is not a perfectly elastic material. Concrete deforms when load is
applied but this deformation does not follow any simple set rule. The deformation depends
upon the magnitude of the load, the rate at which the load is applied and the elapsed time
after which the observation is made. In other words, the rheological behavior of concrete
i.e. the response of concrete to applied load is quite complex.
The knowledge of rheological properties of concrete is necessary to calculate deflection of
the structures and design of the concrete structures with respect to their section, quantity
of steel and stress analysis. When reinforced concrete is designed by elastic theory, it is
assumed that a perfect bond exists between the concrete and steel. The stress in steel is m
times the stress in concrete where m is the ratio of modulus of elasticity of steel and
concrete known as modular ratio. The accuracy of design will generally be dependent upon
the value of modulus of elasticity of concrete because modulus of elasticity of steel is more
or less a definite quantity.
It is to be further noted that concrete exhibits very peculiar rheological behavior because of
its being a heterogeneous, multiphase material whose behavior is influenced by elastic
properties and morphology of gel structures.
4.1 Modulus of elasticity
Modulus of elasticity is defined as the ratio of stress and strain or the slope of stress strain
curve.
Types of elastic moduli
4.1.1 Static modulus of elasticity
The modulus of elasticity found out from the actual loading of the structure or specimen is
called the static modulus of elasticity. It can be determined by subjecting a cube or cylinder
specimen to uniaxial compression and measuring the deformations by means of a dial
gauge.
From the stress strain curve of the concrete, it is seen that concrete does not behave as an
elastic material even under the short term loading. However, up to 10-15 % of the ultimate
strength of the concrete, the stress strain graph is not very much curve and hence can give
more accurate values. For higher stresses, the stress strain relationship will be greatly
curved and as such it will be inaccurate.
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A. Initial tangent modulus
Young's Modulus of elasticity can strictly be applied only to the straight part of stress strain
curve. In concrete, since no part of the graph is concrete, modulus of elasticity is found out
with reference to the tangent drawn to the curve at origin. The modulus found out from this
tangent is then called initial tangent modulus. This gives satisfactory results only at low
stress value. For higher stress value, it gives misleading picture.
B. Tangent modulus
Tangent can also be drawn at any other point on the stress strain curve. The modulus of
elasticity calculated with reference to this tangent is then called the tangent modulus. The
tangent modulus also does not give the realistic value of modulus of elasticity for the stress
level much below or above the point at which tangent is drawn.
C. Secant modulus
A line can be drawn connecting a specified point on the stress strain curve to the origin of
the curve. If the modulus of elasticity is calculated with reference to the slope of this line,
the modulus of elasticity is referred as secant modulus. If the modulus of elasticity is found
out with reference to the chord drawn between two specified points on the stress strain
curve, then such value of modulus of elasticity is known as chord modulus.
The modulus of elasticity most commonly used in practice is secant modulus. Since the
value of secant modulus decreases with increase in stress, the stress at which the secant
modulus has been found out should always be stated.
Figure 4-1 Different modulus of elasticity
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4.1.2 Dynamic modulus of elasticity
The static modulus of elasticity does not truly represent the elastic behavior of the concrete
due to the phenomenon of the creep. Attempts have been made to find out the modulus of
elasticity from the data obtained by non destructive testing of the concrete. The modulus of
elasticity can be determined by subjecting the concrete member to longitudinal vibration at
their natural frequency. This method involves the determination of either resonant
frequency through a specimen of concrete or pulse velocity travelling through the concrete.
The modulus obtained by this method is called dynamic modulus of elasticity. In this
method, modulus of elasticity can be determined from the following relationship
Where
is the dynamic modulus of elasticity
is the constant
is the resonant frequency
is the length of the specimen
is the density of the concrete
The value of dynamic modulus of elasticity computed from ultrasonic pulse velocity method
is somewhat higher than those determined by static method because dynamic modulus is
unaffected by creep.
4.1.3 Flexural modulus of elasticity
The modulus of elasticity can also be determined by subjecting a concrete beam to bending
and then using the formula for deflection and substituting other parameters. Modulus of
elasticity thus obtained is called flexural modulus of elasticity. For a beam simply supported
at the ends and loaded at mid span, ignoring the shear deflection, the approximate value of
the modulus is calculated from:
Where Δ is the mid span deflection due to load P
L the span length,
And I is the moment of inertia.
The flexural modulus is commonly used for design and analysis of pavements.
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4.1.4 Non-uniformity of the stress strain curve
It is interesting to note that the stress strain relationship of the aggregate alone shows fairly
a good straight line. Similarly, stress strain relationship of the cement paste also shows a
fairly good straight line. But the stress strain relationship of concrete which is combination
of aggregate and cement paste shows a curved relationship. Perhaps, this is due to the
development of micro cracks at the interface of the aggregate and paste. Because of the
failure of the bond at the interface increases at a faster rate than that of the applied stress,
the stress strain curve continue to bend faster than increase of stress.
Figure 4-2 Stress - Strain curve for aggregate, cement and concrete
This cause of non-linearity of the stress strain curve of concrete can be described better by
dividing the behavior of concrete into four distinct stages. Under normal atmospheric
exposure conditions (when a concrete element is subjected to drying or thermal shrinkage
effects) due to the differences in their elastic moduli differential strains are set up between
the matrix and the coarse aggregate, causing cracks in the interfacial transition zone.
Therefore, even before the application an external load, microcracks already exist in the
interfacial transition zone between the matrix mortar and coarse aggregate. The number
and width of these cracks in a concrete specimen depend, among other factors, on the
bleeding characteristics, and the curing history of concrete.
Below about 30 percent of the ultimate load, the interfacial transition zone cracks remain
stable; therefore, the σ −ε curve remains linear .This is Stage 1 in Fig. Above 30 percent of
the ultimate load, with increasing stress, the interfacial transition zone microcracks begin to
increase in length, width, and numbers. Thus, theε/σ ratio increases and the curve begins
to deviate appreciably from a straight line. However, until about 50 percent of the ultimate
stress, a stable system of microcracks appears to exist in the interfacial transition zone. This
is Stage 2 and at this stage the matrix cracking is negligible.
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At 50 to 60 percent of the ultimate load, cracks begin to form in the matrix. With further
increase in stress level up to about 75 percent of the ultimate load, not only does the crack
system in the interfacial transition zone becomes unstable but also the proliferation and
propagation of cracks in the matrix increases, causing the σ −ε curve to bend considerably
toward the horizontal. This is Stage 3. At 75 to 80 percent of the ultimate load, the rate of
strain energy release seems to reach the critical level necessary for spontaneous crack
growth under sustained stress, and the material strains to failure. In short, above 75 percent
of the ultimate load, with increasing stress very high strains are developed, indicating that
the crack system is becoming continuous due to the rapid propagation of cracks in both the
matrix and the interfacial transition zone. This is the final stage (Stage 4).
Figure 4-3 Different stages of cracks in transition zone
4.2 Shrinkage and creep
4.2.1 Creep
Creep can be defined as the time dependent part of the strain resulting from stress. The
degree of curvature of the stress-strain relationship depends upon many factors among
which the intensity of stress and time for which the load is acting is of significant interest. It
clearly shows that relation between stress and strain for concrete is a function of time. The
gradual increase in strain without increase in stress with the time is due to the creep. From
this explanation, creep can also be defined as the increase in strain under sustained stress.
All materials undergo creep under some conditions of loading to a greater or smaller extent.
But concrete creep significantly at all stresses and for a long time. Further, creep of
concrete is approximately linear function of stress up to 30 to 40 percent of its strength.
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Creep of concrete is said to be an isolated rheological phenomenon associated with the gel
structure of the concrete. One of the explanations given to the mechanics of creep to the
concrete is based on the theory that the colloidal particles slide against each other to
readjust their position displacing the water held in gel pores and capillary cavities. This flow
of gel and consequent displacement of water is responsible for complex deformation
behavior and creep of concrete.
4.2.2 Shrinkage
One of the most objectionable defects in concrete is the presence of cracks, particularly in
floors and pavements. One of the most important factors that contribute to the cracks in
floors and pavements is that due to the shrinkage. The term shrinkage is loosely used to
describe the various aspects of volume changes in concrete due to loss of moisture at
different stages due to different reasons.
It is not difficult to make a concrete which does not shrink and crack. It is only a question of
magnitude. As shrinkage is an inherent property of the concrete, it demands greater
understanding of the various properties of concrete which influences its shrinkage
characteristics. It is only when the mechanism of all kinds of shrinkage and the factors
affecting the shrinkage are understood , an engineer will be in a better position to control
and limit the shrinkage in the body of concrete.
To understand shrinkage more closely, it is better to classify shrinkage in the following ways
A. Plastic shrinkage
Loss of water by evaporation from the surface of the concrete or by absorption by aggregate
or subgrade is believed to be the reason for plastic shrinkage. The loss of water results in
decrease in volume. The aggregate particles or the reinforcement comes in the way of
subsidence due to which cracks may appear at the surface or internally around the
aggregate or reinforcement.
Sometimes, even if the concrete is not subjected to severe drying but poorly made with high
water cement ratio, large quantity of water bleeds and accumulates at the surface. When
this water at the surface dries out, the surface concrete collapses causing cracks.
Plastic concrete is sometimes subjected to unintended vibration or yielding of formwork
support which again causes plastic shrinkage cracks as the concrete at this stage has not
developed enough strength.
From above, it can be inferred that high water cement ratio, badly proportioned concrete,
rapid drying, greater bleeding, unintended vibrations etc are the reasons for plastic
shrinkage.
Plastic shrinkage can be reduced mainly by preventing the rapid loss of water from the
surface or by revibrating the concrete in a controlled manner.
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B. Drying shrinkage
Just as the hydration of cement is an everlasting process, the drying shrinkage is an
everlasting process when concrete is subjected to drying conditions. The drying shrinkage of
concrete is analogous to the mechanism of drying of timber specimen. The loss of free
water contained in the hardened concrete does not result in any appreciable dimension
change. It is the loss of water held in the gel pores that causes the change in volume and
shrinkage. Under drying conditions, the gel water is lost progressively over a long time, as
long as the concrete is kept in drying conditions.
For a given humidity and temperature conditions, the total shrinkage of concrete is most
influenced by the total amount of water present in the concrete at the time of mixing and to
a lesser extent, by the cement content.
Cement paste shrinks more than mortar and mortar shrinks more than the concrete.
Concrete made with smaller size shrinks more than concrete made with bigger size
aggregate. The magnitude of drying shrinkage is also a function of the fineness of the gel.
The finer the gel, the more is the shrinkage. It has been pointed out earlier that the high
pressure steam cured concrete with low specific surface gel shrinks much less than that of
normally cured cement gel.
C. Autogeneous shrinkage
In a conservative system i.e. where no moisture movement to or from the paste is
permitted, when temperature is constant, some shrinkage may occur. The shrinkage of such
conservative system is called Autogeneous shrinkage. Autogeneous shrinkage is of minor
importance and is not applicable in practice to many situations except that of mass concrete
in the interior of the concrete dam.
D. Carbonation shrinkage
Carbonation shrinkage is a phenomenon very recently recognized. Carbon dioxide present in
the atmosphere reacts in present of water with the hydrated cement and converts calcium
hydroxide and also some other compounds are decomposed. Carbonation shrinkage is
probably caused by the dissolutions of crystals of calcium hydroxide and deposition of
calcium carbonate in its place. As the new product is less in volume than the product
replaced, shrinkage takes place.
E. Thermal shrinkage
It is the shrinkage associated with the heat of hydration of cement. A large quantity of heat,
up to about 500 j/g is released during the hydration of the cement. Since, the thermal
conductivity of the concrete is low; a very high temperature gradient could be generated in
the interior of the mass of the concrete. At the same time, the exterior of the concrete mass
loses heat. As a result, a steep temperature gradient may get established. During
subsequent cooling of the interior, a serious cracking may take place.
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The rate of evolution of heat as well as the total heat generated depends upon the chemical
composition of the cement and also on the fineness of it. The faster rate of heat
development is more harmful than total heat of hydration which develops slowly.
Therefore, for mass concrete and hydraulic structures, it is wise to use cement with low C3A
and C2S content. Blended cement with high pozzolanic material content gives out heat
rather slowly because of slow pozzolanic reaction, during which time certain quantity of
heat gets dissipated, virtually reducing the temperature difference between interior and
exterior and also the resultant shrinkage.
4.3 Fatigue, impact and dynamic loading
Two types of failure in fatigue can take place in concrete. In the first, failure occurs under a
sustained load (or a slowly increasing load), near but below, the strength under an
increasing load, as in a standard test; this is known as static failure or fatigue failure.
The second type of failure occurs under a cyclic or repeated loading and is simply known as
fatigue. We are more concerned with this type of fatigue here. In many of the structures like
road, pavements, airport pavements, concrete is subjected to repeated or cyclic loads.
When a material falls under such load below the static strength, failure in fatigue is said to
take place.
For a constant range of alternating stress, fatigue strength decreases as the number of cycle
increases. This is illustrated in figure by S-N curve where S is the ratio of the maximum stress
to the short term static strength and N is the number of cycles at failure. The maximum
value of S below which no failure occurs is called endurance limit. Whereas mild steel has an
endurance limit of about 0.5, which means that when S<0.5, N is infinity, concrete does not
appear to have a corresponding limit. Therefore, it is necessary to define the fatigue
strength of concrete by considering very large numbers of cycles, say 1 million. In reality, the
S-N curve for concrete has very large scatter due to uncertainty of short-term strength of
the actual fatigue specimen and due to the stochastic nature of the fatigue. This means, for
a give cycle of stress, it is difficult precisely to determine the number of cycles to failure.
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Figure 4-4 Relation between number of cycles and fatigue strength
If the breaking of the structure or specimen takes place under a very short time of loading
(i.e. fraction of number of cycles), strength under dynamic loading is observed. This is
basically the interaction of the
versus logN curve with the
axis. This
can be termed as strength under impact load.
The CFB-FIP model code (1990) recommends that the increase in compressive strength due
to impact with rates of loading less than 100 MPa/sec can be computed using the
relationship.
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The ultimate strength is also affected by rate of loading. The figure below suggest as the
rate of loading decreases (or as the duration of test increases), the application of steadily
increasing load leads to a lower recorded strength than in the case of standard test (i.e.
short duration of about 2 to 4 minutes).on the other hand, the load if applied extremely
rapidly or instantaneously (impact loading), a higher strength is recorded and the strain at
failure is smaller.
Under low rates of loading, static fatigue occurs when the stress exceeds about 70-80
percent of the short term strength; this threshold value represents the onset of rapid
development of cracks, which eventually links and cause failure. Thus, when the stress
exceeds the threshold value, concrete will fail after a period which is indicated by the failure
envelope.
Figure 4-5 Effect of rate of loading on strength and strain capacity in compression
4.4 Effect of porosity, water cement ratio and aggregate size
Porosity in concrete increases the permeability. We have already discussed about how the
permeability, volume change and cracks are interrelated and how the permeability leads to
finally disintegration of concrete.
Water cement ratio has its effect on strength and durability in hardened concrete.
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4.4.1 Aggregate size
At one time, it was thought that use of larger size of aggregate leads to higher strength. That
was due to the fact that larger aggregate size results in lower surface area which will
decrease the quantity of water in a mix and as the low water cement ratio would be
possible, strength would be higher.
However later, it was found that use of larger size of aggregate do not contribute to higher
strength as expected from theoretical considerations due to the various reasons as stated
under:
The larger the maximum size of the aggregate, the specific surface area for bonding
between aggregate and cement paste would be smaller which will decreases the bond
strength of concrete. Also, bigger aggregate size causes more heterogeneity in the concrete
which will prevent the uniform distribution of load when stressed. When large aggregate
size is used, due to internal bleeding, the transition zone will become much weaker due to
the development of the cracks which will result in lower compressive strength.
Figure 4-6 Influence of aggregate size on compressive strength of concrete
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4.4.2 Effect of gel space ratio
Many research workers commented on the validity of water/cement ratio law as
propounded by Duff Abrams and argued that it can only be consider a rule. Some of the
limitations are that strength at any water cement ratio depends upon degree of hydration of
cement and its physical and chemical properties, the temperature at which hydration takes
place, change in effective water cement ratio, shrinkage and bleeding etc.
Instead of relating strength to the water cement ratio, it can be related more accurately to
the solid products of hydration of cement to the space available for formation of this
product known as gel space ratio. The ratio developed by Powers and Brownyard is defined
as the ratio of the volume of hydrated cement paste to the sum of the volumes of hydrated
cement and capillary pores.
Power's experiment showed that the strength of the concrete bears a specific relationship
with gel space ratio. He found the relationship to be 240x3 where x is the gel space ratio and
240 represents the intrinsic strength of gel in MPa for the type of cement and specimen
used. The expression holds good for ideal case. The figure below shows the relationship
between strength and gel space ratio.
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It is pointed out that the relationship between the strength and water cement ratio will hold
good primarily for a period of 28 days strength for fully compacted concrete whereas the
relationship between the strength and gel space ratio is independent of the age. Gel space
ratio can be calculated at any age for any fraction of hydration of cement. The following
example illustrates how to calculate gel/space ratio.
Calculation of gel space ratio for complete hydration
Let
C = weight of cement in gm
VC = specific volume of cement = 0.319 ml/gm
WO = volume of mixing water in ml
Assuming that 1 ml of cement on hydration will produce 2.06 ml of gel
Volume of gel = C x 0.319 x 2.06 = 0.657 C
Space available = C x 0.319 + WO
Therefore, Gel/Space ratio = X =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑔𝑒𝑙
𝑆𝑝𝑎𝑐𝑒 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑒
0.657 𝐶
= 0.319 𝐶+𝑊
Calculation of gel/space ratio for partial hydration
Let α = fraction of cement that has hydrated
Volume of the gel = C x 0.319 x 2.06 x α
Total space available = C x VC x α + WO
Therefore, Gel/Space ratio = X =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑔𝑒𝑙
𝑆𝑝𝑎𝑐𝑒 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑒
=
0.657 𝐶
0.319 𝐶+𝑊
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Chapter - 5 Testing of concrete and quality control
5.1 Various strength of concrete: compressive, tensile, shear
and bond
In concrete design and quality control, strength is the property which is generally specified.
It is because, compared to most other properties, testing of strength is relatively easy.
Furthermore, many properties of concrete such as elastic modulus, water tightness and
impermeability and resistance to weathering agents including aggressive water are directly
related to strength and can therefore be deduced from the strength data.
5.1.1 Compressive strength of concrete
Compressive strength is the maximum compressive stress that, under a gradually applied
load, a concrete can sustain without fracture. Compressive strength of concrete is many
times greater than other types of strength and majority of concrete elements are designed
to take advantage of higher compressive strength of material. Although, in practice, most of
the concrete is subjected to simultaneously to a combination of compressive, tensile, shear
stresses in two or more directions, the uniaxial tests are easiest to perform in the laboratory
and the 28 days of compressive strength of concrete determined by standard uniaxial
compression test is accepted universally as a general index of concrete strength.
5.1.2 Tensile strength of concrete
Concrete being a brittle material is very weak in tension and is not expected to resist tensile
force from external loading condition. But due to restrained conditions of shrinkage and
temperature effects, tensile stress is developed in concrete. The design strength of concrete
should be enough to resist these tensile stresses.
Concrete is normally ten times weaker in tension than in compression .The reason behind is
in the structure. Concrete is made of a collection of materials (several aggregate types,
cement, pozzolans, water, air...), which are glued together with a cement paste. The
"interface" zone is the weakest link in the structure. When compressing, that interface only
serves to transfer compressive stresses from one aggregate to the next. That does not
require exceptional strength.
Under tension, the aggregates are trying to pull away from each other, and the glue is what
holds the whole system together. Since it is significantly weaker than the aggregates, it is
where the failure starts at much lower stresses.
Steel, on the other hand, is uniform and therefore has similar strength in tension and
compression.
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5.1.3 Bond strength
It is defined as the resistance to slipping of the steel reinforcing bars, which are embedded
in concrete. This resistance is developed by adhesion between the concrete and steel,
friction and bearing of the lugs of deformed bars. It is also affected by shrinkage of concrete
relative to steel. Bond involves not only the properties of concrete but also the mechanical
properties of steel and its position in the concrete member. In general, bond strength is
approximately proportional to the compressive strength of concrete up to about 20 MPa.
For higher strengths of concrete, the increase in bond strength becomes progressively
smaller.
Bond strength varies considerably with the type of cement, admixtures and water cement
ratio i.e. on quality of the paste. It is generally not affected by air entrainment. It has been
observed that bond strength of concrete increases with delayed vibration. It is greater for
dry concrete than wet concrete. Bond strength reduces at high temperature. At 200 to 300
degree centigrade, bond strength has been found 50 percent of the bond strength at room
temperature. It is also reduced by alternations of wetting and drying, freezing and thawing.
Bond strength is determined by pull out test. The bond strength of deformed bar may be
taken 40 percent more than ordinary bar of the same diameter.
5.1.4 Shear strength
Shear is the action of two equal and opposite parallel forces applied in planes, a short
distance apart. In general, shear stress exists with tensile and compressive stresses. Pure
shear can be applied through torsion of a cylindrical specimen in which case the stresses are
equal in primary shear. As the concrete is weaker in tension than in shear, failure in tension
invariably occurs in diagonal tension. Direct determination of shear is very difficult. Hence,
shear strength of concrete is taken about 12 percent of the compressive strength.
5.2 Compressive strength test
In order to determine the compressive strength of concrete, three types of specimen are
used.
5.2.1 Cube test
Specimens are cast into steel or iron moulds of 150 mm diameter of cubical shape.
The cubes are filled in three layers and well compacted.
After compaction, the top surface is smoothened by means of a trowel.
The finished surface is left undisturbed for 24 hours at a temperature of 66 to 70 degree
Fahrenheit and relative humidity not less than 90 percent.
After 24 hours, the mould is stripped and specimen is stored in water for further curing. The
curing temperature is maintained at 66 to 70 degree Fahrenheit. The curing period is usually
28 days.
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Then, the sample is tested in compressive testing machine.
5.2.2 Cylinder test
The standard cylinder mould is 150 mm in diameter and 300 mm in height made of cast iron
or steel. Cylinder specimens are prepared and tested same as cube specimen. As the
finishing surface of cylinder may not be smooth, capping is done before testing.
Cylinder specimen are used for the determination of compressive strength of concrete
generally in USA, France, Australia, Canada while cubes are used in UK, Germany, India,
Nepal etc.
According to BS 1881 part-4 1970, the strength of cylinder is equal to four-fifth of the
strength of the cube but the experiments have shown that there is no simple relation
between the strength of specimen of two shapes. It is difficult to say which type of
specimen is better but there seems to be tendency at least for research purpose to use
cylinders rather than cubes. Cylinders are believed to give a greater uniformity of results for
nominally similar specimens as their failure is less affected by the properties of the coarse
aggregate used in the mix, and the stress distribution on horizontal planes in a cylinder is
more uniform than on the specimen of square cross section.
5.3 Tensile strength test
Tensile strength of concrete is tested in various ways and the values as such also differ to
each other.
5.3.1 Direct tension
This type of test is carried out under axial loading. The strength obtained under this type of
loading is small compared to other types of tensile tests. The schematic diagram of pure
tension test is as shown below:
Figure 5-1 Schematic diagram of direct tensile strength of concrete specimen
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Based on various standards, the direct tensile strength of concrete is calculated using the
following relationship
Modulus of rupture
This is the tensile strength under bending. Central point loading and third point loading are
the two loading a scheme in which modulus of rupture is determined. Third point loading is
the most appropriate method and gives lesser value compared to central point loading. The
testing scheme for modulus of rupture is as shown below:
Figure 5-2 Schematic diagram of modulus of rupture for central and third point loading
The modulus of rupture is also calculated from the following bending equation
Based on various standards, the modulus of rupture of concrete is calculated using the
following relationship.
5.3.2 Splitting test
Splitting test is also used to determine the tensile strength of the concrete. This test is also
known as Brazilian method as it was used for the first time in Brazil. This is an indirect
method to obtain the tensile strength of concrete. In this test, a cylinder of the type used in
compression test is placed with its axis horizontal between the plates of testing machine
and the load is increased until the failure by splitting along the vertical diameter takes place.
The load is applied through narrow strips of packing material such as plywood. Usually,
these strips are 3 mm thick and 12.5 to 25 mm wide.
The horizontal tensile strength is then obtained by the formula
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Splitting test is simple to perform and gives more accurate results than other tension tests.
It gives strength very close to the true tensile strength of the concrete then the modulus of
rupture. However, this method gives 5 to 12 percent higher strength than direct tensile test.
5.4 Variability of concrete strength and acceptance criteria
The strength of concrete varies from batch to batch and within the same batch so it is
difficult to assess the strength of the concrete. Strength test of concrete from random
sampling of a mix exhibit variations due to various operations involved in making and testing
of concrete. Also it is observed that the strength of a sample varies with the variation in
shape and size of the sample as well as the type of testing method used. Since the variation
in concrete is inevitable, it is not wise to reject a structure on the basis of few samples. As
such, IS 456-1978 has developed a certain terms and condition which if fulfilled by the
concrete will be considered acceptable.
The supply concrete may be considered to be satisfactory (i.e. acceptable) in case
Every sample has a test strength not less than the characteristic strength of the concrete
The strength of one or more samples though less than characteristic value in each case ,
must not be less than the greater of the following
The characteristics strength minus 1.35 times the standard deviation
0.8 times the characteristics strength
Whichever is greater in above equations a and b and the average strength of all the samples
should fulfill the following conditions
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5.5 Non destructive testing of the concrete
In this method, samples or concrete are not destroyed and remain intact after the test.
These methods have been in use for about four decades and now they are considered a
powerful method for evaluating existing concrete structures with regard to their strength
and durability apart from assessment and quality control of the hardened concrete. In
certain cases, the investigation of crack depth, micro cracks and progressive deterioration
are also studied by this method.
Though such methods are relatively simple to perform, the analysis and the interpretation
of the test results are not so easy. Therefore, special knowledge is required to analyze the
hardened properties of concrete in the non destructive method of testing. These method
attempts to measure some other properties of the concrete from which an estimate of its
strength, durability and elastic parameters are obtained. Some such properties of concrete
are hardness, resistance to penetration of projectiles, rebound number, resonant frequency
and ability to allow ultrasonic pulse velocity to pass through it.
Two of the above methods are briefly described here
5.5.1 Schmidt's Rebound Hammer
It consists of a spring controlled hammer that slides on a plunger within a tubular
housing. When the plunger is pressed against the surface of the concrete, the mass rebound
from the plunger. It retracts against the force of the spring. The hammer impacts against the
concrete and the spring control the mass rebound which is converted into rebound number
by rider.
Although, rebound hammer provides a quick inexpensive means of checking uniformity of
concrete, it has serious limitations and these must be recognized. Investigation have shown
that there is general correlation between compressive strength of concrete and rebound
number; however; there is wide degree of disagreement among various research workers
regarding the accuracy of estimation of strength from rebound reading. The variation of
strength of a properly calibrated hammer may lie between
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5.5.2 Pulse velocity method
This can be sub-divided into two parts:
Mechanical sonic pulse velocity method which involves measurement of the time of travel
of longitudinal or compression waves generated by a single impact hammer blow or
repeated blows.
Ultrasonic pulse velocity method which involves measurement of the time of travel of
electronically generated mechanical pulses through the concrete.
Out of these two, the ultrasonic pulse method has gained considerable popularity all over
the world. Ultrasonic pulse velocity method consists of measuring the time of travel of an
ultrasonic pulse, passing through the concrete to be tested. The time travel between the
initial onset and the reception of the pulse is measured electronically. The path length
between transducer divided by the time of travel gives the average velocity of the wave
propagation.
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Chapter - 6 Durability of Concrete
The durability of cement concrete is defined as its ability to resist weathering action,
chemical attack, abrasions or any other process of deterioration. Durable concrete will
retain its original form, quality and serviceability when exposed to its environment.
For a long time, concrete was considered to be a durable material requiring little or no
maintenance. The assumption is largely true except when it is subjected to highly aggressive
environment. Nowadays, we build concrete structures in highly polluted industrial and
urban areas, aggressive marine environment, harmful sub-soil water in coastal areas and
many other hostile conditions where other materials of construction are found to be non
durable. Since the use of concrete has spread to highly harsh and hostile conditions, the
earlier impression that the concrete is very durable material is being challenged particularly
on account of premature failure of number of recent structures in the recent past.
In the past, only the strength of the concrete was considered in mix design assuming a
strong concrete is also a durable concrete. Although, compressive strength is a good
measure of durability to a great extent, it is not always true that a strong concrete is also a
durable one. As such, in recent years, in addition to strength, environmental conditions or
the exposure conditions has also become important factor for durability.
6.1 Effect of water and permeability on concrete durability
We know that the fundamental factor responsible for deterioration of concrete are cracks
which induced by volume changes in the concrete. Also, permeability is the contributory
factor for the volume change and higher water content or higher water cement ratio is the
fundamental cause of higher permeability. Therefore, use of high water cement ratiopermeability-volume change-cracks-disintegration-failure of concrete is a cyclic process.
Therefore, for a concrete to be durable, it should have lowest water content and lowest
permeability.
With low water cement ratio, the permeability decreases to such a level that these concrete
are impervious to water. This does not mean that they do not contain interconnected
network of capillaries but these capillaries are so fine that water cannot flow any more
through them. Further it has also been proved that low water cement ratio concrete are less
sensitive to carbonation, external chemical attack and other detrimental effects that cause
lack of durability of the concrete. Practically, it is very difficult to reduce the water cement
ratio below 0.4 but with the advent of plasticizers and super plasticizers, it is now possible
to make flowing concrete with water cement ratio as low as 0.2. In such low water cement
ratio concrete, the water available can only hydrate the surface of cement particles and
there exist plenty of unhydrated particles which can play an important role in durability.
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If for any reasons, concrete gets cracked, the unhydrated cement particles begin hydration
as soon as the moisture starts penetrating through cracks. This is to say that unhydrated
cement particles on hydration offers self healing potential to seal up the cracks and improve
the durability of the concrete.
It can be viewed that permeability leads to volume change, volume change results in cracks
and cracks promotes more permeability and thus it becomes a cyclic action till such time
that concrete undergoes deterioration, degradation, disruption and eventual failure.
Permeability can be viewed as permeability of the cement paste and permeability of the
concrete.
In cement paste, gel pores are so small that hardly any water can pass through them under
normal conditions so permeability in cement paste is contributed by water filled or empty
capillary cavities. The size and extent of the capillary cavities depend upon the water
cement ratio .The capillary cavities resulting at low water cement ratio will get filled up
within a few days by the hydration products of the cement. Only unduly large cavities
resulting from higher water cement ratio (greater than 0.7) will not get filled up by the
products of hydration and will remain as unsegemented cavities which is responsible for the
permeability of the paste.
Theoretically, the introduction of aggregates of low permeability into cement paste is
expected to reduce the permeability of the system because the aggregate particles intercept
the channels of flows and make it take a circuitous course but the test data in practice
shows it is not the case. The introduction of the aggregate, particularly larger size
aggregates increase the permeability considerably. The explanation lies in the development
of the micro cracks that are produced in the transition zone and it may be that drying
shrinkage, thermal shrinkage and externally applied load causes cracks in weak transition
zone at young age.
6.2 Physical and chemical causes of concrete deterioration
6.2.1 Physical causes
Physical causes of concrete deterioration can be broadly divided into deterioration of
concrete by surface wear and deterioration by cracking.
A. Surfaces wear (Deterioration by abrasion, erosion and cracking)
Concrete used in certain situations is required to exhibit good abrasion and erosion
properties. Abrasion refers to the wearing away of surface by friction. Erosion refers to the
same action by fluids. The cavitations refer to the damage due to non-linear flow of water at
velocities more than 12 meters per second. The concrete used in the roads, floors,
pavements and hydraulic structures should exhibit resistance against abrasion, erosion and
capitation.
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The resistance against abrasion is closely connected with the compressive strength of
concrete. The higher is the compressive strength. The shape and surface texture of the
aggregate also plays an important role in abrasion resistance of concrete. A smooth rounded
aggregate when subjected to lateral load may be dislodged due to lack of bond and
interlocking effect of the aggregate. Once the aggregate is dislodged and removed, paste
cannot withstand abrasive action. Rough and angular aggregates with better bond and
interlocking effect stands up well against abrasion.
In closed conduits or in a sheet of water flowing over a weir, vapor bubbles are formed in
running water whenever the pressure at a point in the water is reduced to its vapor
pressure at the existing temperature. Vapor bubbles flow downstream and when they enter
a region of high pressure, they collapse with great impact which is called capitation. The
energy released during the collapse of bubbles can cause serious wearing of the concrete
surface called the capitation damage. Apart from careful designing of structures, making
smooth surface free from irregularities is another effective step of controlling capitation.
B. Deterioration by cracking
Cracking is one of the major ways through which deterioration of concrete starts leading to
ultimate failure. Here, we shall discuss various types of cracks and factors responsible for
causing them:
(a) Plastic Shrinkage Cracks
When the loss of water from the surface of concrete is faster than the migration of water
from the interior to the surface, the surface dries up. This creates moisture gradient which
results in surface cracking while concrete is still in plastic condition known as plastic
shrinkage cracks. The magnitude of the plastic shrinkage and plastic shrinkage cracks are
dependent upon ambient temperature, relative humidity, wind velocity etc. in other words,
it depends upon rate of evaporation of water from the surface.
(b) Settlement cracks
If the concrete is free to settle uniformly, then there are no cracks. If there is any
obstruction to uniform settlement by way of reinforcement or larger piece of aggregate, it
creates some voids or cracks. This is called settlement cracks and it generally happens in
deep beam.
(c) Bleeding
The internal bleeding water trapped below flat pieces of aggregate and reinforcement
affects the bond between hardened cement pastes, aggregate and reinforcement on
account of higher local water cement ratio. This interface is easily prone to micro cracking
due to shrinkage stress induced on dissipation of heat of hydration and drying shrinkage. On
loading, micro cracks propagate further making the concrete susceptible to degradation by
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Kantipur Engineering College
A course manual on Concrete Technology
environmental agencies. Also when the bleeding water emerged on the top of surface, top
layer will consist of very fine materials again susceptible to cracking.
(d) Delayed curing
If curing is delayed, moisture from surface will be lost rapidly due to exposure to
environment which will leads to plastic shrinkage cracks as described earlier.
(e) Constructional effects
If formworks fails to maintain their rigidity and firmness when wet concrete is placed and
vibrated: sinking, bending or settlement of formwork may occur which causes cracks or
deformation in plastic concrete.
If concrete is vibrated beyond the limit, segregation occurs and segregated concrete shrinks
more than homogenous concrete and exhibit high shrinkage cracks.
While finishing, we should be careful such that the cement paste is contained by fine
aggregates and matrix is contained by coarse aggregate. Such a uniform mix devoid of
excess paste on the surface will have almost no shrinkage.
(f) Early frost damage
Freshly mixed concrete should not be exposed to freezing conditions to protect it from the
disruptive action of the ice lens. Ice lens will assume 9 percent more volume than the
equivalent water volume. The cumulative effect of increased volume disrupts the integrity
of the fresh concrete. Once frozen, it is difficult to bring back the integrity of the concrete
subsequently. It is reported that significant ultimate strength reductions up to 50 percent
can occur if concrete is frozen within a few hours after placement or before it attains a
compressive strength of 3.5 MPa.
(g) Unsound materials
Cement or aggregate is considered unsound when they cause unacceptable extent of
volume change in hardened concrete or mortar which causes cracks and affects durability.
In cement, if cement contains more lime that can combine with other acidic oxides or if the
raw materials are not properly burnt to the required temperature for the lime to get fully
combined with other oxides, cement becomes unsound. Similarly, the presence of
magnesium oxide which reacts with water in similar manner as calcium oxide also causes
unsoundness.
(h) Shrinkage
Shrinkage is another important factor which contributes to the cracks in concrete especially
in floors and pavements. Shrinkage is an inherent property of concrete and is difficult to
make a concrete which does not shrink and crack. It is only question of magnitude.
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Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
The term shrinkage is loosely used to describe the various aspects of volume changes in
concrete due to loss of moisture at different stages due to different reason. One of the most
important factors that affect the shrinkage is the drying conditions i.e. the relative humidity
at which the concrete specimen is kept. If the concrete is kept at 100 percent relative
humidity for any length of time, there will not be any shrinkage; instead there will be slight
swelling. Other important factors which affect the shrinkage of the concrete are water
cement ratio and richness of the mix. Both have direct relationship with the shrinkage.
6.2.2 Chemical causes of concrete deterioration
Chemical attack or chemical causes are another factors leading to concrete deterioration.
Under chemical causes, we will discuss about sulphate attack, alkali aggregate reaction,
carbonation, acid attack and effect of sea water.
A. Sulphate attack
Solid sulphates do not attack the concrete severely but when the chemicals are in solutions,
they find entry into porous concrete and reacts with hydrated cement product. Due to the
chemical action between the products of hydration of cement and solution containing
sulphates, an increase in volume of the cement paste or concrete results which is called
sulphate attack. A characteristics whitish appearance in the concrete is an indication of
sulphate attack.
In the hardened concrete, calcium acuminate hydrate reacts with sulphate salts from
outside. The product formed is the calcium sulphoaluminate and since the volume of
calcium sulphoaluminte is 227 percent greater than original aluminates, it causes gradual
disintegration of concrete.
Of all the sulphates, magnesium sulphates causes most damage to the concrete because it
reacts not only with calcium hydroxide and hydrated calcium aluminates like other
sulphates but also decomposes hydrated calcium silicates completely and makes it a friable
mass.
B. Alkali aggregate reaction
For a long time, aggregate were thought to be inert material but later after 1940, it was
clear that they were not fully inert. Some aggregate contains reactive silica which reacts
with alkalis present in the cement. This reaction starts with attack on reactive siliceous
minerals on aggregates by alkaline hydroxide derived from cement. As a result, alkali silicate
gel of unlimited swelling type is formed. The progressive swelling of this gel results in
disruption of the concrete with spreading of pattern cracks and eventually failure of
structures.
There are certain factors which promotes this reaction. They are
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
 High alkali content
 Reactive type of aggregates
 Availability of moisture
 Optimum temperature and conditions
C. Acid attack
Concrete is not fully resistant to acids. Most acid solutions will slowly or rapidly disintegrate
the Portland cement concrete depending upon the types and concentration of the concrete.
The most vulnerable part of the cement hydrate is calcium hydroxide but C-S-H gel can also
be attacked. Siliceous aggregates are more resistant than calcareous aggregates. Concrete
can be attacked by liquids with pH value below 6.5. But the attack is severe only at a pH
value below 5.5. At a pH value 4.5, the attack is very severe. As the attack proceeds, all the
cement compounds are eventually broken down and leached away. If acids or salt solutions
are able to reach the reinforcement of concrete through cracks or pores, corrosion of
reinforcement occur which causes cracking.
D. Effect of sea water
The offshore and coastal structures are exposed to simultaneous action of number of
physical and chemical deterioration process. Sodium and Magnesium sulphates present in
sea water induce sulphate attack but their expansive effect is retarded by the presence of
chlorides in seas water. Further, the concrete in sea water is subjected to chloride induced
corrosion of steel, freezing and thawing, salt weathering, abrasion by sand held in water and
other floating bodies.
E. Carbonation
This is discussed elaborately on next heading.
6.3 Carbonation
Carbonation is a process by which carbon dioxide from the air penetrates into the concrete
and reacts with calcium hydroxide to form calcium carbonate. Formation of calcium
carbonate from calcium hydroxide due to action of carbon dioxide results in small shrinkage
in concrete.
Let us see another aspect of carbonation:
In presence of moisture, carbon dioxide changes into dilute carbonic acid which attacks the
reinforcement and reduces the alkalinity of the concrete. The pH value of pore water in the
hardened concrete is generally from 12.5 to 13.5. The high alkalinity forms a thin passivating
layer around steel reinforcement and protects it from the action of oxygen and water.
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
In actual practice, carbon dioxide present in atmosphere in smaller or greater proportion
permeates into concrete and carbonates the concrete to reduce its alkalinity. The pH value
of pore water in the hardened cement paste which was around 13 will be reduced to around
9. When all the calcium hydroxide has become carbonated, the pH value will reduce up to
8.3. In such low pH value, the protective layer gets destroyed and steel is exposed to
corrosion.
Rate of carbonation depends upon following factors:
(a) The level of pore water i.e. relative humidity
If pore water is filled with water (100 % relative humidity), the diffusion of carbon dioxide
into the concrete is very slow and so is the carbonation. Also, if the pores are rather dry,
carbon dioxide remains in the gaseous form and does not react. Therefore, highest rate of
carbonation occurs at a relative humidity between 50 and 70 percent.
(b) Grade of Concrete
The depth of carbonation will be slower in stronger concrete because the stronger concrete
is much denser with low water cement ratio and also the permeability is also low in low
water cement ratio concrete.
(c) Permeability of concrete
If the concrete is more permeable, diffusion of carbon dioxide is more easy and also the
penetration of moisture from the environment which both increased the rate of
carbonation in concrete.
(d) Whether the concrete is protected or not
If the concrete is protected by various coating, obviously, carbonation will be slower.
(e) Depth of cover
Also when depth of cover is high, carbonation is slower.
(f) Time
Depth of carbonation increases with time but rate will decrease.
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Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
6.4 Corrosion of steel in concrete
Corrosion is defined as "destruction or deterioration" of materials in environments to which
they are exposed.
Iron is not naturally stable metal and it achieves stability in the form of different oxide
because these oxides have lower energy level.
To extract iron from these oxides, extra energy is supplied in the manufacturing process.
These metals so extracted tend to slip back to lower energy level which is the fundamental
reason behind the corrosion.
Corrosion of steel in concrete is an electrochemical process. When there is difference in
electric potential along the steel reinforcement in the concrete, an electrochemical cell is
set up in the steel. One part becomes anode and other part becomes cathode connected by
electrolyte in the form of pore water. The positively charged ferrous ion ( ) at the anode
pass into the solution while the negatively charged free electrons ( ) pass through the steel
into the cathode where they are absorbed by constituents of the electrolyte and combine
with water and oxygen to form hydroxyl ions ( ). These travel through the electrolyte and
combine with the ferrous ions to form ferric hydroxide which is converted by further
oxidation to rust.
Anodic reactions
Cathodic reactions
It can be noted that no corrosion takes place if the concrete is dry or probably below the
relative humidity of 60 percent because enough water is not there to promote corrosion. It
is also true that corrosion does not take place if concrete is fully immersed in water because
diffusion of oxygen does not take place in such situation.
The products of corrosion occupy a volume as much as six times the original volume of the
steel. The increased volume of rusts exerts thrust on cover concrete resulting in cracks or
spalling of concrete.
6.4.1 Corrosion control
If we make good concrete with low permeability and improved microstructure, it will be
durable by itself and also can protect the reinforcement from corrosion. However, it is not
always possible to make such ideal concrete so following are the some methods in which we
can control the corrosion:
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Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
A. Metallurgical methods
Steel can be made more resistant to corrosion by altering its structure through metallurgical
process by different methods such as rapid quenching of hot bars by series of water jets or
by keeping the hot steel bars for a short time in water bath etc.
B. Corrosion inhibitors
Corrosion can be delayed or prevented by chemical methods by using certain corrosion
inhibiting chemicals like nitrites, phosphates, benzoates etc
Coating to reinforcement
The reinforcement itself can be coated by various corrosion resisting materials to provide
durable barriers to the aggressive materials such as chlorides.
C. Cathodic protection
In this method, artificial electrode is inserted into concrete which acts as anode and the
steel acts as a cathode. Obviously, anode is subjected to corrosion and cathodic steel is
protected from it.
D. Coating to concrete
Here, surface coating is applied to concrete member as a whole to protect it from the
environmental effects.
E. Design and detailing
The structural engineer should take all the precautions in designing and detailing with
respect to the spacing between the bars for the concrete to flow between reinforcements,
to facilitate vibration of the concrete, to give proper cover to the reinforcements, to restrict
the crack width etc.
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A course manual on Concrete Technolgoy
Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
Contents
Chapter - 1 Introduction to concrete and concrete materials ........................................................... 1
1.1
Use of concrete in structure and types of concrete ............................................................... 1
1.1.1
Structural use of concrete ............................................................................................... 1
1.1.2
Types of concrete ............................................................................................................. 3
1.2
Concrete materials (Role of different materials, aggregate cement, water and admixture)
5
1.2.1
Aggregate (properties of aggregate and their gradation) ............................................... 5
1.2.2 Cement (Manufacturing of cement, compound composition of Portland cement,
structure and reactivity of compounds) ...................................................................................... 13
1.2.3
Use of water in concrete:............................................................................................... 19
1.2.4 Admixtures (classification of admixture, introduction to commonly used admixture
(super-plasticizer, water proofing agent, retarders), use of mineral admixtures in concrete) .. 19
Chapter - 2 Structure of concrete ..................................................................................................... 23
2.1
Concrete as a three phase system ........................................................................................ 23
2.2
Structures of the aggregate phase ........................................................................................ 24
2.3
Structure of the hydrated cement phase.............................................................................. 24
2.3.1
Solids in hydrated cement phase. .................................................................................. 25
2.3.2
Voids in hydrated cement paste .................................................................................... 26
2.3.3
Water in the hydrated cement paste ............................................................................ 27
2.4
Transition zones in concrete ................................................................................................. 28
2.4.1
Structure of Transition zone .......................................................................................... 28
2.4.2
Effects of transition zone ............................................................................................... 28
Chapter - 3 Chapter Three: Mix design of concrete and property of green concrete ..................... 30
3.1
Workability and its test ......................................................................................................... 30
3.1.1
Factors affecting the workability ................................................................................... 30
3.1.2
Measurement of workability ......................................................................................... 31
3.2
Water cement ratio: .............................................................................................................. 35
3.3
Introduction to nominal mix ................................................................................................. 36
3.4
Probabilistic concept in mix design approach....................................................................... 36
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Er. Simpson Lamichhane
Kantipur Engineering College
3.5
Concrete mix design .............................................................................................................. 38
3.5.1
3.6
A course manual on Concrete Technology
Methods of Mix Design .................................................................................................. 41
Segregation and bleeding...................................................................................................... 50
3.6.1
Segregation: ................................................................................................................... 50
3.6.2
Bleeding: ........................................................................................................................ 51
3.7
Quality control in site (mixing, handling, placing, compacting and curing) .......................... 51
3.7.1
Mixing............................................................................................................................. 52
3.7.2
Handling ......................................................................................................................... 52
3.7.3
Placing ............................................................................................................................ 53
3.7.4
Compacting .................................................................................................................... 54
3.7.5
Curing ............................................................................................................................. 55
3.8
Concreting in extreme weather temperature....................................................................... 56
3.8.1
Concreting in hot weather ............................................................................................. 56
3.8.2
Concreting in cold weather ............................................................................................ 57
Chapter - 4 Properties of hardened concrete................................................................................... 59
4.1
Modulus of elasticity ............................................................................................................. 59
4.1.1
Static modulus of elasticity ............................................................................................ 59
4.1.2
Dynamic modulus of elasticity ....................................................................................... 61
4.1.3
Flexural modulus of elasticity ........................................................................................ 61
4.1.4
Non-uniformity of the stress strain curve ..................................................................... 62
4.2
Shrinkage and creep .............................................................................................................. 63
4.2.1
Creep .............................................................................................................................. 63
4.2.2
Shrinkage........................................................................................................................ 64
4.3
Fatigue, impact and dynamic loading ................................................................................... 66
4.4
Effect of porosity, water cement ratio and aggregate size ................................................... 68
4.4.1
Aggregate size ................................................................................................................ 69
4.4.2
Effect of gel space ratio ................................................................................................. 70
Chapter - 5 Testing of concrete and quality control ......................................................................... 72
5.1
Various strength of concrete: compressive, tensile, shear and bond .................................. 72
5.1.1
Compressive strength of concrete ................................................................................. 72
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Er. Simpson Lamichhane
Kantipur Engineering College
A course manual on Concrete Technology
5.1.2
Tensile strength of concrete .......................................................................................... 72
5.1.3
Bond strength ................................................................................................................ 73
5.1.4
Shear strength ................................................................................................................ 73
5.2
Compressive strength test .................................................................................................... 73
5.2.1
Cube test ........................................................................................................................ 73
5.2.2
Cylinder test ................................................................................................................... 74
5.3
Tensile strength test .............................................................................................................. 74
5.3.1
Direct tension ................................................................................................................. 74
5.3.2
Splitting test ................................................................................................................... 75
5.4
Variability of concrete strength and acceptance criteria...................................................... 76
5.5
Non destructive testing of the concrete ............................................................................... 77
5.5.1
Schmidt's Rebound Hammer ......................................................................................... 77
5.5.2
Pulse velocity method .................................................................................................... 78
Chapter - 6 Durability of Concrete .................................................................................................... 79
6.1
Effect of water and permeability on concrete durability ..................................................... 79
6.2
Physical and chemical causes of concrete deterioration ...................................................... 80
6.2.1
Physical causes ............................................................................................................... 80
6.2.2
Chemical causes of concrete deterioration ................................................................... 83
6.3
Carbonation ........................................................................................................................... 84
6.4
Corrosion of steel in concrete ............................................................................................... 86
6.4.1
Corrosion control ........................................................................................................... 86
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Er. Simpson Lamichhane