UNIVERSITY OF NAIROBI
DEPARTMENT OF CIVIL AND CONSTRUCTION ENGINEERING
FCE 246 – Civil
Engineering Materials
Class Notes
MWERO
2016/2017
FCE 246 – Civil Engineering Materials
2016/2017
Contents
List of Figures .......................................................................................................................................... 6
List of Tables ........................................................................................................................................... 7
Course Outline ........................................................................................................................................ 8
1
MANUFACTURE AND PROPERTIES OF INORGANIC CEMENTS – PORTLAND CEMENT......................... 9
1.1
Introduction ............................................................................................................................. 9
1.2
History of Portland Cement ...................................................................................................... 9
1.3
Cement Overview..................................................................................................................... 9
1.4
Manufacture of Portland cement ........................................................................................... 11
1.5
The Role of the Laboratory ..................................................................................................... 15
1.6
Environmental Implications .................................................................................................... 16
1.6.1
Dust Emissions ............................................................................................................... 16
1.6.2
CO2 ................................................................................................................................. 17
1.6.3
Quarry and Plant Water Runoff ...................................................................................... 17
1.6.4
Chrome Bricks ................................................................................................................ 17
1.7
2
HYDRATION OF PORTLAND CEMENT .............................................................................................. 18
2.1
Portland Cement – Chemical Composition.............................................................................. 18
2.2
Hydration of Portland Cement................................................................................................ 20
2.2.1
Tri & Di Calcium silicates ................................................................................................. 22
2.2.2
Calcium Aluminate and Aluminoferrite ........................................................................... 23
2.3
3
Cement Industry in Kenya ...................................................................................................... 17
Portland Cement Hydration Products ..................................................................................... 23
2.3.1
Ettringite ........................................................................................................................ 23
2.3.2
CSH gel (Calcium Silicagte Hydrate)................................................................................. 23
2.3.3
Heat of Hydration ........................................................................................................... 24
FRESH CONCRETE .......................................................................................................................... 28
3.1
Properties of Aggregates ........................................................................................................ 28
3.1.1
Classification of Natural Aggregates................................................................................ 29
3.1.2
Sampling ........................................................................................................................ 29
3.1.3
Particle Shape and Texture ............................................................................................. 30
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3.1.4
Bond of Aggregate .......................................................................................................... 32
3.1.5
Strength of Aggregate..................................................................................................... 32
3.1.6
Other Mechanical Properties of Aggregates.................................................................... 32
3.1.7
Specific Gravity ............................................................................................................... 34
3.1.8
Bulk Density ................................................................................................................... 34
3.1.9
Porosity and Absorption of Aggregate ............................................................................ 35
3.1.10
Moisture Content of Aggregate ...................................................................................... 37
3.1.11
Bulking of Fine Aggregate ............................................................................................... 37
3.1.12
Deleterious Substances in Aggregates............................................................................. 38
3.1.13
Organic Impurities .......................................................................................................... 38
3.1.14
Clay and Other Fine Material .......................................................................................... 39
3.1.15
Salt Contamination ......................................................................................................... 39
3.1.16
Unsound Particles........................................................................................................... 40
3.1.17
Soundness of Aggregate ................................................................................................. 41
3.1.18
Alkali-Silica Reaction ....................................................................................................... 41
3.2
Quality of Mixing Water ......................................................................................................... 42
3.3
Workability ............................................................................................................................ 42
3.3.1
Definition ....................................................................................................................... 42
3.3.2
The Need for Sufficient Workability ................................................................................ 43
3.3.3
Factors Affecting Workability.......................................................................................... 43
3.3.4
Measurement of Workability .......................................................................................... 44
3.3.5
Effect of Time and Temperature on Workability ............................................................. 48
3.4
Segregation ............................................................................................................................ 48
3.5
Bleeding ................................................................................................................................. 49
MANUFACTURE OF CONCRETE ...................................................................................................... 51
4.1
The Mixing of Concrete .......................................................................................................... 51
4.2
Concrete Mixers ..................................................................................................................... 51
4.3
Uniformity of Mixing .............................................................................................................. 53
4.4
Mixing Time ........................................................................................................................... 54
4.5
Hand Mixing ........................................................................................................................... 54
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4.6
Ready-Mixed Concrete ........................................................................................................... 55
4.7
Concrete Admixtures.............................................................................................................. 55
4.7.1
Benefits of Admixtures ................................................................................................... 55
4.7.2
Types of Admixtures ....................................................................................................... 56
PROPERTIES OF HARDENED CONCRETE .......................................................................................... 57
5.1
Strength ................................................................................................................................. 57
5.1.1
5.2
Characteristic Material Strengths.................................................................................... 58
Creep ..................................................................................................................................... 59
5.2.1
Influence of Aggregate ................................................................................................... 59
5.2.2
Influence of Mix Proportions .......................................................................................... 59
5.2.3
Influence of Age ............................................................................................................. 59
5.3
Shrinkage ............................................................................................................................... 60
5.3.1
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Types of Shrinkage ......................................................................................................... 60
NON DESTRUCTIVE TESTS .............................................................................................................. 63
6.1
Introduction ........................................................................................................................... 63
6.2
Crack monitor ........................................................................................................................ 63
6.3
Pocket tachometer ................................................................................................................. 63
6.4
Concrete Test Hammer (Schmidt Hammer) ............................................................................ 64
6.5
Rebar Locator ......................................................................................................................... 64
6.6
Ultrasonic Concrete Testing System ....................................................................................... 65
6.7
Impact Echo System ............................................................................................................... 65
6.8
Maturity Meter System .......................................................................................................... 66
6.9
Portable Moisture Meter with Probe ...................................................................................... 66
7
DURABILITY QUALITY CONTROL OF CONCRETE .............................................................................. 68
8
CONCRETE MIX DESIGN ................................................................................................................. 72
9
8.1
Introduction ........................................................................................................................... 72
8.2
British Method of Selection of Mix Proportions ...................................................................... 72
8.3
Example ................................................................................................................................. 79
CORROSION OF STEEL IN CONCRETE .............................................................................................. 81
9.1
Introduction ........................................................................................................................... 81
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9.2
Concerns of Corrosion of Steel ............................................................................................... 81
9.3
Process of Corrosion of Steel in Concrete ............................................................................... 81
9.3.1
Introduction ................................................................................................................... 81
9.3.2
The corrosion mechanism............................................................................................... 82
9.3.3
Corrosion Prevention...................................................................................................... 84
10
EFFECTS OF FIRE ON CONCRETE ................................................................................................. 85
10.1
Introduction ........................................................................................................................... 85
10.2
Effects of High Temperature on Hardened Cement Paste ....................................................... 85
10.3
Effects of High Temperatures on Aggregates .......................................................................... 85
10.4
Performance of Concrete in Fires ........................................................................................... 85
10.5
Factors Infliencing Fire Resistance of Concrete ....................................................................... 85
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List of Figures
Figure 1.1: Cement Manufacturing Process ............................................................................................ 16
Figure 2.1: Cement model reactions (numbers below the equation indicate volume stoichiometries) ... 21
Figure 2.2: Variation of hydration of C3S with time: (a) the pre-induction period, (b) the induction period,
(c) acceleratory period, and (d) the deceleratory period ......................................................................... 22
Figure 2.3: Effect of member thickness on temperature of concrete (numbers on curves denote wall
thicknesses in mm, and inches in brackets) ............................................................................................ 25
Figure 2.4: Average heat of hydration of a Type I cement at various ages ............................................... 26
Figure 2.5: Typical calorimetry heat curve ............................................................................................. 27
Figure 3.1: Typical Riffler (Also available in the highways laboratory – UoN) .......................................... 30
Figure 3.2: Typical slump test results: collapse, shear, and true slump ................................................... 44
Figure 3.3: Typical Compaction Factor Equipment ................................................................................. 46
Figure 4.1: Photo of a typical tilting mixer (A) and non tilting mixer (B) .................................................. 52
Figure 4.2: Pan type mixer ..................................................................................................................... 52
Figure 5.1: Variation in material properties ........................................................................................... 58
Figure 6.1: Crack monitor ...................................................................................................................... 63
Figure 6.2: Typical pocket tachometer ................................................................................................... 63
Figure 6.3: Schmidt Hammer ................................................................................................................. 64
Figure 6.4: Typical Rebar Locator ........................................................................................................... 64
Figure 6.5: Typical Ultrasonic Concrete Test System .............................................................................. 65
Figure 6.6: Typical Impact Echo System ................................................................................................. 66
Figure 6.7: Typical Maturity Meter System ............................................................................................ 66
Figure 6.8: Typical Portable Moisture Meter .......................................................................................... 67
Figure 8.1: Relation between compressive strength and free water/cement ratio for use in the British
mix selection method ............................................................................................................................ 74
Figure 8.2: Estimated wet density for full compacted concrete, (specific gravity is given for saturated and
surface dry condition)............................................................................................................................ 76
Figure 8.3: Recommended proportion of fine aggregate (expressed as a percentage of the total
aggregate) as a function of the free water/cement ratio for various workabilities and maximum sizes
(numbers refer to percentage of fine aggregate passing 600 μm sieve) ................................................. 77
Figure 9.1: The expansion of corroding steel creates tensile stresses in the concrete, which can cause
cracking, delamination, and spalling ...................................................................................................... 82
Figure 9.2: When reinforcing steel corrodes, electrons flow through the bar and ions flow through the
concret .................................................................................................................................................. 82
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List of Tables
Table 1.1 Major Mineral Constituents of Portland Cement ..................................................................... 11
Table 2.1: Typical composition limits of Portland cement ...................................................................... 19
Table 2.2: Cement Chemists’ Notation and Other Properties .................................................................. 20
Table 3.1 Minimum Mass of Samples for Testing (BS 812: Part 102: 1989) ............................................. 29
Table 3.2: Particle Shape Classification .................................................................................................. 30
Table 3.3: Surface Texture of Aggregates (BS 812: Part 1: 1975) with examples ..................................... 31
Table 3.4: Porosity of Some Common Rocks .......................................................................................... 36
Table 3.5: Description of Workability and Magnitude ............................................................................ 45
Table 3.6: Classification of Workability and Magnitude of Slump according to European Standard ENV
206: 1992 .............................................................................................................................................. 45
Table 3.7: Description of Workability and Compaction Factor ................................................................ 47
Table 7.1: Classification of Exposure Conditions (BS 8110-1:1997: Table 3.2) ......................................... 69
Table 8.1: Approximate Compressive Strengths of Concretes Made with a Free Water/Cement Ratio of
0.5 According to the 1998 British Method ............................................................................................. 73
Table 8.2: Approximate free water contents required to give various levels of workability according to
the British Method ................................................................................................................................ 75
Table 8.3: Proportion of Coarse Aggregate Fractions According to the 1988 British Method .................. 79
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Course Outline
1) Manufacture and properties of inorganic cements – Portland cement
2) Hydration of Portland cement
3) Rheology of fresh concrete – workability, segregation
4) Properties of hardened concrete
5) Manufacture of concrete
6) Ready mixed concrete
7) Mix design
8) Non destructive tests
9) Creep shrinkage
10) Durability quality control of concrete
11) Corrosion of steel in concrete
12) Thermal properties
13) Effects of fire on concrete
14) Materials laboratory
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1 MANUFACTURE AND PROPERTIES OF INORGANIC CEMENTS –
PORTLAND CEMENT
1.1 Introduction
Concrete is an extremely versatile material, being used in the production of anything from
nuclear radiation shields to playground structures, bridges, yachts, etc. It can be used in such a
wide variety of applications because it can be poured into any shape, reinforce with steel or glass
fibers, precast, coloured, has a variety of finishes and can even set under water. Modern concrete
is made by mixing aggregate (sand, stone and shingle) with Portland cement and allowing it to
set. Of these ingredients, the most important is Portland cement.
Cement is a fine grey powder which when reacted with water hardens to form a rigid chemical
mineral structure which gives concrete its high strength. Cement is in effect the glue that holds
concrete together. The credit for its discovery is given to Romans, who mixed lime (CaCO3) with
volcanic ash, producing a cement mortar which was used during construction of such impressive
structures as the Colosseum. When the Roman empire fell, the information on how to make
cement was lost and was not rediscovered until the 16th century.
1.2 History of Portland Cement
Cement has been made since Roman times but over time the recipes for making cement have
been refined. The earliest cements were made from lime and pozzolana (a volcanic ash
containing significant quantities of SiO2 and Al2O3) mixed with ground brick and water. The
cement was not improved upon until 1758, when Smeaton noticed that using a limestone that
was 20 – 25% clay and heating the mixture resulted in a cement that could harden under water.
He called this new cement ‘hydraulic lime’. When the mixture was heated, a small quantity of it
was sintered (resulting from atomic diffusion forming a solid piece). Normally this was
discarded as waste, but in 1800s Aspdin and Jonson discovered that when the entire batch was
sintered and then ground a superior cement was formed. This substance became designated
Portland cement (after the region in which they were working) and is the most common cement
in use today.
1.3 Cement Overview
Cement can be defined as a material with cohesive and adhesive properties which when mixed
with water, is capable of bonding material fragments into a compact whole. Portland cement is
the most common type of cement in general usage in many parts of the world, as it is a basic
ingredient of concrete mortar, stucco and most non specialty grout.
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Portland cement is currently defined as a mixture of argillaceous (ie clay like) and calcaneous (ie
containing CaCO3 or other insoluble calcium salts) materials mixed with Gypsum (CaSO4.2H2O)
sintered and then pulverized into a fine powder. The precise definition of Portland cement varies
between different coutries
Portland cement can also be defined as a fine powder produced by grinding clinker (more than
90%), a limited amount of calcium sulphate which controls the setting time, and up to 5% minor
constituents (as allowed by various standards). The fineness and chemical composition of a
portland cement are the key factors in determining cement strength characteristics.
Portland cement consists of at least two thirds by mass of calcium silicates (3CaO.SiO2 and
2CaO.SiO2), the remainder consisting of aluminium and iron containing clinker phases and other
compounds. Generally, the ratio of CaO to SiO2 must be less than 2.0 while the magnesium
content (MgO) must not exceed 5% by mass.
The grinding process is controlled to obtain a powder with a broad particle size range, in which
typically 15% by mass consists of particles with diameter below 5μm, and 5% of particles above
45μm. The measure of fineness commonly used is the “specific surface”, which is the total
particle surface area of a unit mass of cement. The rate of initial reaction (up to 24 hours) of the
cement on addition of water is directly proportional to the specific surface. Typical values are
320 – 380 m2.kg-1 for general purpose cements, and 450 – 650m2.kg-1 for “rapid hardening”
cements.
The finer the cement is, however, the faster it deteriorates on exposure to the atmosphere and
also the more expensive it is to produce due to the extra grinding. The particle size distribution
also, together with the parking density and rate of hydration, has an influence on the porosity of
the cement, and its bleeding characteristics.
The original size, spatial distribution, and composition of Portland cement particles have a large
influence on hydration kinetics, microstructure development, and the ultimate properties of
cement based materials. The particle size distribution is sometimes described using the two
parameter Weibull distribution given by equation 1.
( )=
Where:
(2.1)
f(T) is a probability density function
T is a random variable
β is the shape parameter, and
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η is the scale parameter
The Weibull distribution is a continuous probability distribution function that can be used to
describe the size distribution of particles. The shape parameter β determines the shape and hence
the slope of the distribution whereas the scake parameter η, describes the sharpness or the
gentleness of the bell of the distribution curve. A change in the value of the scale parameter η has
the same effect on the distribution as a change of the abscissa scale.
In principle, the strength of a cement mortar continues to rise slowly as long as water is available
for continued hydration, but concrete is usually allowed to dry out after a few weeks, and this
slows down the strength increase. Cement based materials, however, are unique in that water that
is necessary for hydration reactions and microstructural development, is on the other hand,
largely responsible for the degradation of the material in service.
Setting and hardening of the Portland cement is caused by the formation of hydrated compounds,
forming as a result of reactions between cement components and water. Usually, cement reacts in
a plastic mixture at water cement (w/c) ratios between 0.25 and 0.75. For w/c below 0.25, the
water supplied is insufficient to fully hydrate the cement, whereas above w/c ratio of 0.75,
complete hydration occurs but the water forms too many voids within the cement resulting in a
very weak matrix. The reaction and the products of reaction are reffered to as hydration and
hydrates (or hydrate phases) respectively.
1.4 Manufacture of Portland cement
Portland cement is made by heating raw materials rich in oxides of silicon, calcium, aluminium,
and iron to temperatures of around1200 – 1400 oC. The chemical reactions that occur within the
partially molten mass result from the formation of the four main cement materials
Table 1.1 Major Mineral Constituents of Portland Cement
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Of these compounds, C3S and C3A are mainly responsible for the strength of the cement. High
percentages of C3S (low C2S) result in high early strength but also high heat generation as the
concrete sets. The reverse combination of low C3S and high C2S develops strengths more slowly
(over 52 rather than 28 days) and generates less heat.
C3A causes undesirable heat and rapid reacting properties, which can be prevented by adding
CaSO4 to the final product. C3A can be converted to the more desirable C4 AF by the addition of
Fe2O3 before heating, but this also inhibits the formation of C3S. C4AF makes the cement more
resistant to sea water and results in somewhat slower reactions which evolve less heat.
The balance of the formed compounds versus the performance characteristics required from the
cement is a chemically controlled parameter. For this reason, considerable efforts are made
during the manufacturing process to ensure the correct chemical compounds in the correct ratios
are present in the raw materials before introduction of the materials to the kiln.
The cement manufacturing process involves four distinct stages as outlined below:
Step 1 - Quarrying
The raw material for cement manufacture is a rock mixture which is about 80% limestone (which
is rich in CaCO3) and 20% clay or shale (a source of silica, alumina and Fe2O3).
These are quarried and stored separately. The lime and the silica provide the main strength of the
cement, while the iron reduces the reaction temperature and gives the cement its characteristic
grey colour.
Step 2 – Raw Material Preparation
The steps involved here depend on the process used. There are two main cement manufacturing
processes: the dry process, and the wet process. The dry process uses more energy in grinding
but less in the kiln, and the wet process has lower overheads than the dry process. The two
processes are discussed separately below
The Dry Process
The quarried clay and limestone are crushed separately until nothing bigger than a tennis ball
remains. Samples of both rocks are then sent to the laboratory for mineral analysis. If necessary,
minerals are then added to either the clay or the limestone to ensure that the correct amounts of
aluminium, iron, etc are present.
The clay and limestone are then fed together into a mill where the rock is ground until more than
85% of the material is less than 95μm in diameter.
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The Wet Process
The clay is mixed to a paste in a washmill – a tank in which the clay is pulverized in the presence
of water. Crushed lime is then added and the whole mixture further ground. Any material which
is too coarse is extracted and reground. The slurry is then tested to ensure that it contains the
correct balance of minerals, and any extra ingredients blended in as necessary.
Step 3 - Clinkering
This is the step which is characteristic of Portland cement. The finely ground material is dried,
heated (to enable the sintering reactions to take place) and then cooled down again. While it is
being heated various chemical reactions take place to form the major mineral constituents of
cement.
The powder from the dry process doesn’t contain much moisture, so can be dried in a pre-heater
tower. As it falls through the tower (which takes 30 seconds) it is heated from 70oC to 800oC.
The moisture evaporates, up to 20% of the decarbonation (loss of CO2) occurs and some
immediate phases such as CaO.Al2O3 begin to appear. The mixture is then fed into the kiln.
The slurry from the wet process contains too much moisture to be successfully dried in a
preheater tower. Instead the slurry is fed directly intot he kiln where it is formed into dry balls by
the heat and rotation of the kiln. Because of this extra role of the kiln, wet process kilns are
generally longer (say ~ 100m) than dry process kilns (say ~ 60m). The kilns used in both
processes are inclined on a shallow angle and lined with heat-resistant bricks.
The Kiln
The kiln shell is steel, 60m long and inclined at an angle of 1 in 30. The shel is supported on
rollers and weighs in at over 1100T. The kiln is heated by injecting pulverized coal dust into the
discharge end where it spontaneously ignites due to the very high temperatures. Coal is injected
with air into the kiln at a rate of 9 – 12 T.hr-1.
The reaction processes occurring within the kiln are not easily understood due to the wide
variations in raw-mix chemistry, raw-mix physical properties and kiln operating conditions, and
the physical difficulties of extracting hot materials from the process for investigations before
they cool.
Breaking the reaction processes into a number of simple zones means we can make some
approximations about the cement formation process.
Zone 1: 0 – 35 min, 800 – 1100oC
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Decarbonation. Formation of 3CaO.Al2 O3 above 900oC. Melting of fluxing compounds Al2O3
and Fe2O3.
Zone 2: 35 – 40 min, 1100 – 1300oC
Exothermic reactions and the formation of secondary silicate phases as follows:
Zone 3: 40 – 50 min, 1300 – 1450 – 1300oC
Sintering and reaction within the melt to form ternary silicates and tetracalcium alumino-ferrates:
Zone 4: 50 – 60 min, 1300 – 1000oC
Cooling and crystallization of the various mineral phases formed in the kiln.
The Cooler
Immediately following the kiln is a large cooler designed to drop the temperature of the clinker
(as the fused material is now called) from 1000 oC to 150oC. This is achieved by forcing air
through a bed of clinker via perforated plates in the base of the cooler. The plates within the
cooler slide back and forth, shuffling the clinker down the cooler to the discharge point and
transport to a storage area.
At this point in the process the materials have been formed into all the required minerals to make
cement. Like cement, the clinker will react with water and harden, but because it is composed of
1 – 3 cm diameter fragments it is too coarse to be used.
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Step 4 – Cement Milling
To produce the final product the clinker is made with gypsum (CaSO4.2H2O), which is added as
a retarder, and ground for approximately 30 minutes in large tube mills. The cement flow from
the inlet to the outlet of the mill (a rotating chamber), being first ground with 60mm then 30mm
diameter steel balls. The first grinding breaks up the material, and the second grinds it to a fine
powder.
The amount of grinding is governed by the volume of cement fed into the mill: the greater the
volume the coarser the grind. This has practical limits, with too much cement clogging up the
mill and not enough actually increasing the particle size.
The particle size is measured by laser diffraction analysis and the quantity of material entering
the mill adjusted accordingly {this is a continuous process in modern cement manufacturing
plants}. Over time the charge (steel grinding balls) wear out so when they reach a certain size
they fall through a sieve and then are replaced.
The cement grinding process is highly energy intensive. The rotating mill generates significant
quantities of heat energy and water is used to cool the product and the mill itself.
1.5 The Role of the Laboratory
The laboratory forms an integral part of the control systems on site with testing from raw
materials to finished product. The laboratory operates a 24 hour facility in line with a continuous
manufacturing facility responsible for the following aspects:
 Testing raw materials prior to blasting in the quarry and assisting with the development
of quarrying strategies forms the first step in the process
 Analyzing rock samples from the raw mill at regular intervals during the day and night
and fine tuning the process to ensure chemical control is estimated
 Analysing clinker at the end of the cooler (before grinding) to ensure that the
manufactured process meets specification.
 Checking that cement mills are undertaking grinding correctly and that customers receive
the right product
 Checking dispatched materials for quality and compliance with the requirements of the
relevant standards. Certificates of conformance are issued to customers based on these
analyses
 Product development
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Testing work within the laboratory ranges from simple air permeability measurements to high
technology x-ray fluorescence analysis
Figure 1.1 gives a summary of the cement manufacturing process.
Figure 1.1: Cement Manufacturing Process
1.6 Environmental Implications
Many of the aspects of the cement making process are potentially environmentally damaging,
although these risks can be minimized. The areas of potential concern are listed below:
1.6.1
Dust Emissions
The manufacture of cement generates large quantities of dust. These must be prevented (both on
environmental and economic grounds) from escaping to the atmosphere. The two areas where
dust has the potential to escape are via air streams that have been used to carry cement (e.g. the
mills or kiln) and directly from equipment used to transport cement (e.g. the various conveyor
belts). Thus to prevent dust emissions all transport equipment is enclosed and the air both from
these enclosures and from the kiln and mills is treated in an electrostatic precipitator to remove
its load of dust. Here dust-laden air passes between an electrode carrying 50,000 volts and an
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earthed connection plate. The electrostatic discharge between the electrode and the plate forces
the dust onto the plates, from which it is removed.
The current European emission limits are about 50mg/m3. This poses a significant challenge to
the manufacturing operation both in capital cost to reduce emissions and monitoring of emissions
to ensure compliance of the existing limits.
1.6.2
CO2
Cement manufacture is an energy intensive process. One of the most significant challenges
facing the industry into the 21st century is a requirement to reduce CO2 emissions. CO2 is
produced during the calcinations phase of the manufacturing process and also as a result of
burning fossil fuels. Opportunity to reduce emissions through increased energy efficiency is only
possible on the latter of the CO2 emissions.
1.6.3
Quarry and Plant Water Runoff
Runoff of storm water and treatment of waste water from quarries is a problem for almost all
quarry operations. Usually this is trapped in wetland areas where the water is treated in a
controlled manner. Within the factory runoff can be contaminated by oil ad lubricants, but the
runoff is monitored and training programs are regularly undertaken to ensure this does not
happen.
1.6.4
Chrome Bricks
Kiln bricks used to be made of hexavalent chrome, which is a carcinogen and causes dermititus
in some people. Since the problems associated with its use were identified, its use has gradually
been moved.
1.7 Cement Industry in Kenya
The main cement manufacturers in Kenya include; Bamburi Cement, Athi River Mining and East
African Portland Cement among others. In 2009, per capita consumption of cement in Kenya
stood at 52kg. This consumption has since risen with the large infracture development projects
that the country has engaged in since then.
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2 HYDRATION OF PORTLAND CEMENT
2.1 Portland Cement – Chemical Composition
As mentioned earlier, the raw materials used in the manufacture of Portland cement consist
mainly of lime, silica, alumina and iron oxide, these compounds interact with one another at high
temperature to form a series of more complex products and, apart from a small residue of
uncombined lime remaining due to insufficient reaction time, a state of chemical equilibrium is
reached. However, equilibrium is not maintained during cooling, thus cement is considered as
being in frozen equilibrium, i.e. the cooled products are assumed to reproduce the equilibrium
existing at clinker temperature. This assumption is made of the compound composition of
commercial cements: the “potential composition is calculated from the measured quantities of
oxides present in the clinker as if full crystallization of equilibrium products has taken place.
Four compounds are usually considered as the major constituents of cement as given in Table 1.
In reality, the silicates in cements are not pure compounds, but contain minor oxides e.g. TiO 2,
P2O5 and MnO, in solid solution.
Table 2 gives the oxide limits for Portland cement. This oxide analysis is what is obtained when
laboratory analysis is carried out on either the raw material or the finished product. To convert
this oxide analysis into the cement phases given in Table 1, several methods can be used. The
commonest of these methods is referred to as the Bogue calculation carried out using a set of
equations as given below:
= 4.0710
− 7.6024
= 8.6024
+ 1.0785
= 2.6504
− 1.4297
+ 5.0683
− 1.6920
= 3.0432
− 6.7187
− 3.0710
(1)
(2)
(3)
(4)
An adjustment is necessary in order to use of these equations for Portland cement. This is
achieved by deducting 0.7xSO3 from the total CaO, to catter for gypsum [118]. The molar mass
of CaO (56.08 g/mol) is about 0.7 that of SO3 (80.07 g/mol). Since gypsum (CaO.SO3.2H2O)
comprises of one mole of both CaO and SO3, if it is assumed that all the SO3 in the cement is
used in creating gypsum, a mass of CaO of 0.7 that of the SO3 will be utilized, thus the reduction
of 0.7xSO3 mass of CaO to get the CaO available for creation of the other cement phases.
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Table 2.1: Typical composition limits of Portland cement
Oxide
Content, percent
CaO
60-67
SiO2
17-25
Al2O3
3-8
Fe2O3
0.5-0.6
MgO
1.4 -1.5
Alkalis (as Na2O)
0.3-1.2
SO3
2.0-3.5
The amount of CaO tends to play a significant role in the production of calcium hydroxide which
is susceptible to acid attack finally leading to deterioration of hardened concrete material. This is
because acid medium attacks mainly calcium hydroxide followed by hydration products in the
cement matrix leading to hydrolytic decomposition of hydration cement based materials.
For ease of presentation, cement chemists have come up with a notation removed from the
standard chemistry notation. The cement chemists notation is summarized in the Table 2.2.
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Table 2.2: Cement Chemists’ Notation and Other Properties
Molar
Volume
(cm3/mo
le)
Heat of
Formation
(Kj/mole)
Dens
ity
(Mg/
m3)
228.33
71.00
-2,927.82
3.21
2CaO. SiO2
172.25
52.00
-2,311.60
3.28
C3A
3CaO.Al2O3
270.20
89.10
-3,587.80
3.03
Tetracalcium Alumino Ferrite
C4AF
4CaO.Al2O3. Fe2O3
485.96
128.00
-5,090.30
3.73
Gypsum
CSH2
CaO. SO3.2H2O
152.20
74.20
-2,022.60
2.32
Calcium Silicate Hydrate
C1.7SH4
1.7 CaO. SiO2.4 H2O
227.49
108.00
-3,283.00
2.12
Calcium Hydroxide
CH
CaO. H2O
74.10
33.10
-986.10
2.24
Ettringite
C6AS3H32
6 CaO. Al2O3.3 SiO2.32H2O
1,195.22
735.00
-17,539.00
1.70
Monosulfate
C4ASH12
4 CaO. Al2O3. SiO2.12H2O
602.56
313.00
-8,778.00
1.99
Hydrogarnet
C3AH6
3 CaO. Al2O3. 6H2O
378.30
150.00
-5,548.00
2.52
Iron Hydroxide
FH3
Fe2O3.3H2O
213.73
69.80
-823.90
3.00
Water
H
H2O
18.02
18.00
-285.83
1.00
Name of Element
Chemists
Notation
Chemical Formular
Tricalcium Silicate
C3S
3CaO. SiO2
Dicalcium Silicate
C2S
Tricalcium Aluminate
Molar
Mass
(g/mole)
2.2 Hydration of Portland Cement
The addition of water to dry cement powder results in a thin cement slurry that can be easily
manipulated and cast into different shapes. In time the slurry sets and develops strength through
a series of hydration reactions.
The hydration reactions are summarized in Figure 2.1. The figure also gives the volume
stoichiometries for each of the reactions.
A stoichiometric amount or ratio of a reagent is the optimum amount or ratio where, assuming
the reaction proceeds to completion:
i.
ii.
iii.
all reagent is consumed
there is no shortfall of reagent
no residues remain.
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Silicate Reactions
+ 5.3
1
→
1.34
1.521
+ 4.3
1
+ 1.3
.
→
0.61
+ 0.3
.
1.49
2.077
0.191
Aluminate and Ferrite Reactions
+ 6
1
1.21
̅
+ 3
0.4
+4
0.2424
0.575
1
̅
+
2.426
1
+ 10
1
1.41
1.278
+
3.3
→
+ 2
1.278
+
1.17
+
0.15
̅
→3
0.294
̅
→3
→
+ 12
0.348
3.3
0.098
+ 30
1
→
2.1
+
+3
1.69
+ 26
1
2
2
→
0.26
0.09
0.31
+2
0.19
+
0.545
Figure 2.1: Cement model reactions (numbers below the equation indicate volume
stoichiometries)
The products of hydration reactions are denser than the reactants, resulting in shrinkage of
hydrating cement. This volume change is called chemical shrinkage and occurs in two phases:
Dissociation of cement into pore solution leading to volume swelling, followed by the
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crystallization process which produces volume shrinkage. Crystallization is therefore necessary
for chemical shrinkage to occur in hydrating cement. This volume change continues to occur as
long as cement hydrates.
After the initial set, the paste is able to resist deformation, causing the formation of voids in the
microstructure. Autogenous shrinkage is a volume change of cement paste, mortar, or concrete
caused by chemical shrinkage resulting when there is no moisture transfer to the surrounding
environment. It is most prominent in high performance concretes when or where the water
cement ratio is under 0.42.
2.2.1
Tri & Di Calcium silicates
Hydration of C3S Consumed
Heat Evolved
C2S and C2S comprise over 80% by weight of most cement. C3S is the most important phase in
cement for strength development in the first month, while C2S reacts much more slowly and
contributes to the long term strength of the cement.
The hydration of C3S proceeds via four distinct phases as shown in Figure 2.2.
1.0
0.8
0.6
0.4
0.2
0
3
6
9
Time (Hours)
60
Time (Days)
120
Figure 2.2: Variation of hydration of C3S with time: (a) the pre-induction period, (b) the
induction period, (c) acceleratory period, and (d) the deceleratory period
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The first 15 – 20 minutes, termed the pre-induction period, are marked by rapid heat evolution
and swelling. The early hydration reactions are exothermic thus the rapid heat escxvolution.
During this period, calcium and hydroxyl ions are released into solution. The induction period is
characterized by low reactivity. The reactions that cause the induction period are not precisely
known but it is clear that some form of activation barrier must be overcome before hydration can
continue. It has been suggested that this is the period required for calcium ions to become super
saturated in solution, or the induction period may be caused by the development of a small
amount of impermeable calcium-silicate-hydrate (C-S-H) gel at the surface of the particles.
The rate of C-S-H increases with the amount of C-S-H formed, approximately three hours into
the reaction, in the acceleratory period. Setting occurs towards the end of this period, it continues
to about 24 hours and it is characterized by shrinkage. At the end of this period, about 30% of the
cement is reacted.
2.2.2
Calcium Aluminate and Aluminoferrite
Aluminates and aluminoferrites comprise less than 20% of the bulk of the cement. C3A reacts
very fast, compared to the reaction of C3S. If the very rapid and exothermic hydration of C3A is
allowed to proceed unhindered in cement, then the setting occurs too quickly and the cement
does not develop strength.
Therefore, gypsum (calcium sulfate hydrate, CaSO4.2(H2O)) is added to slow down the reaction.
The hydration of aluminoferrite (C4AF) is much slower than that of C3 A. C3S and C3 A
chemically generate more heat, and at a faster rate than the other cement compounds.
2.3 Portland Cement Hydration Products
The products of hydration of cement have a very low solubility in water. This is evidenced by the
stability of hydrated cement pastes in contact with water.
2.3.1
Ettringite
Ettringite is a Hexacalcium aluminate trisufate, (CaO)6(Al2O3)(SO3)3.32 H2O. It is found in
hydrated Portland cement system as a result of the reaction of calcium aluminate with calcium
suphate, both present in Portland cement. The formular used by cement scientists is C6AS3H32.
2.3.2
CSH gel (Calcium Silicagte Hydrate)
This is the main product of hydration of Portland cement and is primarily responsible for the
strength in cement based materials. Calcium silicate hydrate (also known as C-S-H) is a result of
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the reaction between the silicate phases of Portland cement and water. This reaction is typically
expressed as:
2
+ 7
→3
.2
.4
+3
(
) + 173.6
(5)
The stoichiometry of C-S-H in cement paste is variable and the state of chemically and
physically bound water in its structure is not clear. This is why it is illustrated as “C-S-H”. The
crystal structure of C-S-H has not been fully resolved yet and there is still ongoing debate over
its nano structure. The poorly crystalline calcium silicate hydrate (C-S-H) phases that form near
room temperature, which includes the technically important C-S-H gel phase formed during the
hydration of Portland cement, have a broad similarity to the crystalline minerals tobermorite and
jennite.
2.3.3
Heat of Hydration
Cement hydration is an exothermic reaction. The rate of evolution of heat is an indication of the
rate of hydration. The rate of evolution of heat shows that there are three peaks in the first three
days or so. The higher the concrete temperature, the faster the rate of hydration and the more
rapidly heat is generated in the concrete member. The higher the cement content the greater the
potential temperature rise in the concrete. The finer the cement, the more surface area there is for
reaction and the more rapid the heat development. The total heat of hydration, Hcem, can be
quantified in the equation 6 below:
= 500
+ 260
+ 866
+ 420
+ 64
+ 1186
+ 850
(6)
Where:
Hcem = total heat of hydration of the cement (J/g),
pi
= weight ratio of i-th compound in terms of the total cement content
The heat of hydration coefficients (equation 6) represent the contribution of 1% of the
corresponding compound to the heat of hydration, assuming 100% hydration of the compound.
This is such that if 100% of a phase was to hydrate, the percentage multiplied with the
coefficients would give the total heat of hydration evolved. These values are not varying since
they are characteristic of the complounds, ie they are unique to each particular cement phase.
The weight ratio represents the percentage of the corresponding compound to the whole paste.
This percentage changes with the cement composition.
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For many applications, the avoidance of a rapid initial heat release is beneficial in limiting the
development of thermal stresses and minimizing early age cracking problems. In this aspect, the
use of a coarser cement may offer a performance benefit relative to a finer one.
The heat of hydration causes temperature rise in concrete and this temperature rise is dependednt
on size of the member. The thicker the concrete the higher the temperature as illustrated in Figure
2.3.
Figure 2.3: Effect of member thickness on temperature of concrete (numbers on curves denote
wall thicknesses in mm, and inches in brackets)
The heat of hydration depends on the type of cement. Generally, the types of cement in common
use can be classified into the following groups:




Type I (Normal Use) – General purpose cement suitable for all uses where the special
properties of other types are not required.
Type II (Moderate Sulfate Resistance) – Used where precaution against moderate supfate
attack is important. C3A content is limited to not more than 8%.
Type III (High Early Strength) – Provides strength at an early period usually a week or
less. Chemically similar to Type I, but the particles are more finely ground, to bring the high
early strength development.
Type IV (Low Heat of Hydration) – Used where the rate and amount of heat generated
from hydration must be minimized, this is achieved by allowing slightly larger particle sizes
and limiting C3S < 50%, thus slowing down hydration reactions. Strength development is at
a lower rate than other cement types.
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Type V (High Sulfate Resistance) – Used for concrete exposed to severe sulfate action. C3 A
content is limited to not more than 5%. Strength gain is also slower than Type I cement.
Portland cement is known to evolve heat for a long time. Figure 2.4 shows the heat of hydration
data up to13 years. The rate of heat generation is greatest at early ages.
Figure 2.4: Average heat of hydration of a Type I cement at various ages
The heat of hydration of cement is measured experimentally by calorimetry. A graphical
representation of temperature over time is referred to as a heat curve. Heat curves are mainly
used to give an initial indication of performance of a cement mix. Figure 2.5 shows a typical
heat curve derived from an calorimetric measurements of cement pastes.
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Figure 2.5: Typical calorimetry heat curve
A – Empty calorimeter temperature, if logging was started before loading sample into calorimeter.
B – Initial adjustment caused by concrete being warmer or cooler than the calorimeter sensor
C – Concrete mix temperature after initial adjustment
D – Dormant period,
E – Approximate point of initial set
F – Approximate point of final set
G – Most of the broad heat rise comes from strength giving alite hydration.
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3 FRESH CONCRETE
Although fresh concrete is only of transient interest, it is important to note that the strength of a
given concrete mix is very seriously affected by the degree of its compaction. It is vital therefore,
that the consistency of the mix be such that the concrete can be transported, placed, compacted,
and finished sufficiently easily and without segregation. Before discussing properties of
concrete, it is prudent to look at the composition and properties of the other constituents of
concrete namely, aggregates, and water.
3.1 Properties of Aggregates
About three quarters of the volume of concrete is occupied by aggregates. Aggregates quality is
therefore a very important contributor to the final quality of concrete. The quality of aggregates
has an effect not only on the strength of the resulting concrete but also on its durability and
structural performance characteristics.
Aggregate was initially regarded as an inert material dispersed throughout the concrete mainly
for economic reasons. Aggregate can however be viewed differently as a building material
connected into a cohesive whole by means of the cement paste, in a manner similar to masonry
contruction. Aggregate is known not to be truly inert and its physical, thermal and sometimes
also chemical properties influence the performance of concrete.
Aggregate is cheaper than cement and it is therefore economical to put into the mix as much of
the former and as little of the latter as possible. In addition to economy, inclusion of aggregates
provides better volume stability and durability that if the cement paste was acting alone.
General classification of aggregates
The size of aggregate used in concrete ranges from tens of millimeters to particles less than one
tenth of a millimeter in cross section. The maximum size actually used varies, but in any mix,
particles of different sizes are incorporated, with the particle size distribution being referred to as
grading.
In making low grade concrete, aggregates from deposits containing a whole range of sizes, from
the largest to the smallest, is sometimes used. This is referred to as all-in or pit-run aggregate.
The alternative, always used in manufacture of good quality concrete, is to obtain the aggregate
in at least two size groups, the main division being between fine aggregate, often called sand (eg
BS 882:1992 not larger than 5mm), and coarse aggregate, which comprises material at least
5mm in size.
Natural sand is generally considered to have a lower size limit of 70 or 60 μm. Material between
60 μm and 2 μm is classified as silt, and particles smaller still are termed clay. All natural
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aggregate originally formed a larger parent mass. This may have been fragmented by natural
processes of weathering or abrasion or artificially by crushing. Thus many properties of the
aggregate depend on the properties of the parent rock eg chemical and mineral composition,
petrological character, specific gravity, hardness, strength, physical and chemical stability, pore
structure, and color. On the other hand properties like particle shape and size, surface texture and
absorption are independent of the parent rock properties. All these properties have an influence
on the quality of concrete, fresh or hardened.
3.1.1
Classification of Natural Aggregates
So far, we have discussed only aggregate formed from naturally occurring materials. This
discussion will concentrate on naturally occurring aggregates although there are also artificially
prepared aggregates in existence.
(See Attached photocopy – Table 3.1 – Extract from Properties of Concrete, A. M. Neville)
The classification of BS 812: Part 1: 1975 is the most convenient
3.1.2
Sampling
Tests of various properties of aggregate are performed on samples of the material and, therefore,
the results of the tests apply, strictly speaking, only to the aggregate in the sample. Since
however we are interested in the bulk of the aggregate as supplied, or as available for supply, we
should ensure that the sample is typical of the average properties of the aggregate. Such a sample
is said to be representative and, to obtain it certain precautions in procuring the sample have to
be observed.
The main sample is made up of portions drawn from different parts of the whole. The minimum
number of these portions, called increments, is ten, and they should add up to a mass not less
than that given in Table 3.1 for particles of different sizes as prescribed in BS 812: Part 102:
1989. If however, the source from which the sample is obtained is variable or segregated, a
larger number of increments should be taken and a larger sample size be tested.
Table 3.1 Minimum Mass of Samples for Testing (BS 812: Part 102: 1989)
Maximum particle size
substantial proportion (mm)
present
in Minimum mass of sample dispatched for
testing (Kg)
28 or larger
50
Between 5 and 28
25
5 or smaller
13
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Once the right size of sample has been taken, it can be reduced using a riffler (Figure 3.1), to
ease the effort in testing.
Figure 3.1: Typical Riffler (Also available in the highways laboratory – UoN)
3.1.3
Particle Shape and Texture
The shape of three dimensional bodies is rather difficult to describe. It is therefore convenient to
define certain general geometrical characteristics of such bodies.
3.1.3.1 Roundness
This measures the relative sharpness or angularity of the edges and corners of a particle.
Roundness is controlled largely by the strength and abrasion resistance of the parent rock and by
the amount of wear to which the particle has been subjected. A convenient broad classification of
roundness is as given in Table 3.2.
Table 3.2: Particle Shape Classification
Classification
Description
Well Rounded
No original faces left
Rounded
Faces almost gone
Sub-rounded
Considerable wear, faces reduced in area
Sub-angular
Some wear but faces untouched
Angular
Little evidence of wear
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3.1.3.2 Surface Texture
This affects an aggregates bond to the cement paste and also influences the water demand of the
mix, especially in the case of fine aggregate.
The classification of surface texture is based on the degree to which the particle surfaces are
polished or dull, smooth or rough, the type of roughness has to be described. Surface texture
depends on the hardness, grain size and pore characteristics of the parent material (hard, dense
and fine grained rocks generally having smooth fracture surfaces) as well as the degree to which
forces acting on the particle surface has smoothed it or roughed it. Table 3.3 gives an appropriate
classification for surface roughness.
Table 3.3: Surface Texture of Aggregates (BS 812: Part 1: 1975) with examples
Group
Surface Texture
Characteristics
Examples
1
Glassy
Conchoidal fracture
Black flint, vitreous
slag
2
Smooth
Water-worn,
or Gravels, chert, slate,
smooth due to fracture marble,
some
of laminated or fine rhyolites
grained rock
3
Granular
Fracture
showing Sandstone, oolite
more or less uniform
rounded grains
4
Rough
Rough fracture of Basalt,
felsites,
fine- or medium- porphyry, limestone
grained
rock
containing no easily
visible
crystalline
constituents
5
Crystalline
Containing
easily Granite,
visible
crystalline gneiss
constituents
6
Honeycombed
With visible pores and Brick,
pumice,
cavities
foamed slag, clinker,
expanded clay
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3.1.4
2016/2017
Bond of Aggregate
Bond between aggregate and cement paste is an important factor in the strength of concrete,
especially the flexural strength, but the nature of bond is not fully understood. Bond is due, in
part, to the interlocking of aggregate and the hydrated cement paste due to the roughness of the
surface of the former. A rougher surface, such as that of crushed particles, results in better bond
due to mechanical interlocking. Better bond is usually obtained with softer porous, and
minerologically heterogeneous particles.
Generally, texture characteristics which permit no penetration of the surface of the particles are
not conducive to good bond (eg ceramic recycled aggregates have been found to return less
strength in concrete due to the reduced bonding between aggregates and cement paste).
The determination of the quality of bond of aggregate is rather difficult and no accepted tests
exist. Generally, when the bond is good, a crushed specimen of normal strength concrete should
contain some aggregate particles broken right through, in addition to the more numerous ones
pulled out from their sockets. An excess of fractured aggregate however may suggest that the
aggregate is too weak.
3.1.5
Strength of Aggregate
Clearly, the compressive strength of concrete cannot significantly exceed that of the major part
of the aggregate contained therein, although it is not easy to state what id the strength of an
individual particle.
Since it is difficult to test the strength of aggregate particles, indirect tests such as crushing value
of bulk aggregate, force required to compact bulk aggregate and performance of aggregate in
concrete are used. A good average value of crushing strength of aggregate is about 200MPa but
many excellent aggregates range in strength down to 80 MPa. One of the highest values recorded
is 530 MPa for a certain quartzite
3.1.6
Other Mechanical Properties of Aggregates
Several mechanical properties of aggregates are of interest, especially when the aggregate is to
be used in pavement construction or is to be subjected to high wear.
The first of these is toughness, which can be defined as the resistance of a sample rock to failure
by impact. Although this test would disclose adverse effects of weathering of rock, it is not used.
It is also possible eto determine the impact value of bulk aggregate, and toughness determined in
this manner is related to the crushing value, and can in fact be used as an alternative test. The
size of the particles tested is the same as in the crushing value test, and the permissible values of
the crushed fraction smaller than 2.36mm test sieve are also the same. The impact is applied by a
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standard hammer falling 15 times under its own weight upon the aggregate in a cylindrical
container. This results in fragmentation in a manner similar to that produced by the pressure of
the plunger in the aggregate crushing value test. Details of the test prescribed in BS 812:112:
1990, and BS 882:1992 give the following maximum values: 25 percent when the aggregate is to
be used in heavy duty floors; 30 percent when the aggregate is to be used in concrete for wearing
surfaces; and 45 percent when it is to be used in other concretes.
One of the advantages of this test is that it can be performed in the field with some
modifications, such as the measurement of quantities by volume rather than by mass, but the test
may not be adequate for compliance purposes.
It may be worth noting that some limestone rocks are subject to wear, and their use in concrete
pavement should be conditional to abrasion testing. In other respects, many limestone
aggregates, even when porous, can produce satisfactory concrete.
Abrasion of rock specimens is no longer determined, and in keeping with the tendency to test
aggregate in bulk, and abrasion value test on aggregate particles is prescribed in BS 812: Part
113: 1990. Aggregate particles between 14.0 and 10.2mm, with flaky particles removed, are
embedded in resin in a single layer. The sample is subjected to abrasion in a standard machine,
the grinding lap being turned 500 revolutions with sand being fed continuously at a prescribed
rate. The aggregate abrasion value is defined in terms of the percentage loss in mass on abrasion,
so that a high value denotes a low resistance to abrasion.
The attrition (Deval) test uses aggregates in bulk but is no longer in use because it gives only
small numerical differences between widely differing aggregates.
An American test combining attrition and abrasion is the Los Angeles Test which is also
frequently used in other countries as well. This test shows good correlation not only in the actual
wear of aggregate when used in concrete but also with the compressive and flexural strengths of
concrete made with the given aggregate. In this test, aggregate of specified grading is placed in a
cylindrical drum mounted horizontally, with a shelf inside. A charge of steel balls is added, and
the drum is rotated a specified number of revolutions. The tumbling and dropping of the
aggregate and the balls result in abrasion and attrition of the aggregate, and this is measured in
the same way as in the attrition test.
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Specific Gravity
Because aggregates generally contain pores, both permeable and impermeable, the meaning of
the term specific gravity has to be carefully defined, and there is indeed several types of specific
gravity
The absolute specific gravity refers to the volume of the solid material excluding all pores, and
can, therefore, be defined as the ratio of the mass of the solid, refered to vacuum, to the mass of
an equal volume of gas free distilled water, both taken at a stated temperature. Thus, in order to
eliminate the effect of totally enclosed impermeable pores the material has to be pulverized, and
the test is both laborious and sensitive. Fortunately, it is not normally required in concrete
technology work
If the volume of the solid is deemed to include the impermeable pores, but not the capillary ones,
the resulting specific gravity is prefixed by the word apparent. The apparent specific gravity is
then the ratio of the mass of the aggregate dried in an oven at 100 to 110 oC for 24 hours to the
mass of water occupying a volume equal to that of the solid including the impermeable pores.
The later mass is determined using a vessel which can be accurately filled with water to a
specified volume. Thus, if the mass of the oven dried sample is D, the mass of the vessel full of
water is B, and the mass of the vessel with the sample and topped up with water is A, then the
mass of the water occupying the same volume as the solid is B-(A-D). The apparent specific
gravity is then
−
+
Calculations with reference to concrete are generally based on the saturated and surface dry
condition of the aggregate because the water contained in all the pores in the aggregate does not
take part in the chemical reactions of the cement and can, therefore, be considered as part of the
aggregate.
3.1.8
Bulk Density
It is well known that in the SI unit system the density of a material is numerically equal to its
specific gravity although, of course, the latter is a ratio while density is expressed in kilograms
per liter. However, in concrete practice, expressing the density of kilograms per cubic meter is
more common.
This absolute density, it must be remembered, referrers to the volume of the individual particles
only, and of course it is not physically possible to pack these particles so that there are no voids
between them. When aggregate is to be actually batched by volume (as opposed to batching by
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weight as in the laboratory work for this unit) it is necessary to know the mass of aggregate that
would fill a container of unit volume. This is known as the bulk density of the aggregate, and this
density is used to convert quantities by mass to quantities by volume.
The bulk density clearly depends on how densely the aggregate is packed, and it follows that, for
a material of a given specific gravity, the bulk density depends on the size distribution and shape
of the particles: particles all of one size can be packed to a limited extent, but smaller particles
can be added in the voids between the larger ones, thus increasing the bulk density of the packed
material. The shape of the particles greatly affects the closeness of the parking that can be
achieved.
For a coarse aggregate of a given specific gravity, a higher bulk density means that there are
fewer voids to be filled by fine aggregate and cement, and the bulk density test was at one time
used as basis of proportioning of mixes.
The actual bulk density of aggregates depends not only on the various characteristics of the
material which determine the potential degree of packing, but also on the actual compaction
achieved in a given case. For instance, using spherical particles all of the same size the densest
parking is achieved when their centers lie on the apexes of imaginary tetrahedra. The bulk
density is then 0.74 of the absolute density (specific mass) of the material.
Knowing the apparent specific gravity for the saturated and surface-dry condition, s, the voids
ratio can be calculated from the expression:
=
If an aggregate contains surface water, it will pack less densely owing to the bulking effect
3.1.9
Porosity and Absorption of Aggregate
The presence of internal pores in the aggregate particles was mentioned in connection with the
specific gravity of aggregate, and indeed the characteristics of these pores are very important in
the study of its properties. The porosity of aggregate, its permeability, and absorption influence
such properties of aggregates as the bond between it and the hydrated cement paste, and
resistance of concrete to freezing and thawing, as well as its chemical stability and resistance to
abrasion. As stated earlier, the apparent specific gravity of aggregate also depends on its porosity
and, as a consequence, the yield of concrete for a given mass of aggregate is affected {less
parking therefore if batching is done by volume, less aggregate will be batched}.
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The pores in aggregate vary in size over a wide range, the largest being large enough to be seen
under a microscope or even with the naked eye, but even the smallest aggregate pores are larger
than the gel pores in the cement paste. Pores smaller that 4 μm are of special interest as they are
generally believed to affect the durability of aggregates subjected to alternating freezing and
thawing. Table 3.4 gives the porosity of some common rocks.
Table 3.4: Porosity of Some Common Rocks
Rock Group
Porosity, Per Cent
Gritstone
0.0 – 48.0
Qaurtzite
1.9 – 15.1
Limestone
0.0 – 37.6
Granite
0.4 – 3.8
Some of the aggregate pores are wholly within the solid; others open onto the surface of the
particle. The cement paste, because of its viscosity, cannot penetrate to a great depth any but the
largest of the aggregate pores, so that is the gross volume of the particle that is considered solid
for the purpose of calculating the aggregate content in concrete. Since aggregates represents
some three quarters of the volume of concrete, it is clear that the porosity of aggregate material
contributes to the overall porosity of concrete.
When all the pores in an aggregate are full, it is said to be saturated and surface dry. If aggregate
in this condition is allowed to stand free in dry air, e.g. in the laboratory, some of the water
contained in the pores will evaporate and the aggregate will be less than saturated, i.e. air dry.
Prolonged drying in an oven would reduce the moisture content of the aggregate still further
until, when no moisture whatever is left, the aggregate is said to be bone dry.
The water absorption of aggregate is determined by measuring the increase in mass of an ovendried sample when immersed in water for 24 hours (the surface water being removed). The ratio
of the increase in mass to the mass of the dry sample, expressed as a percentage, is termed
absorption. Standard procedures are prescribed in BS 812: Part 1: 1975. Generally, gravel has a
higher absorption than crushed rock of the same petrological character because weathering
results in the outer layer of the gravel particles being more porous and absorbent.
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Although there is no clear cut relation between the strength of concrete and the water absorption
of aggregate used, the pores at the surface of the particle affect the bond between the aggregate
and the cement paste, and may thus exert some influence on the strength of concrete. Normally,
it is assumed that, at the time of setting of concrete, the aggregate is in a saturated and surface
dry condition. If dry aggregate is used in the mix, the water supplied is assumed to be enough to
bring it to a saturated, surface dry condition. The absorption of water by the particles can also
cause some loss of workability with time.
3.1.10 Moisture Content of Aggregate
It was mentioned in connection with the specific gravity that, in fresh concrete, the volume
occupied by the aggregate is the volume of the particle including all the pores. If no water
movement into the aggregate is to take place, the pores must be full of water. On the other hand,
any water on the surface of the aggregate will contribute to the water in the mix and will occupy
a volume in excess of that of the aggregate particles. The basic state of aggregate is thus
saturated and surface dry.
Aggregate exposed to rain collects a considerable amount of moisture on the surface of the
particles, and except at the surface of the stockpile, keeps this moisture over long periods. This is
particularly true of fine aggregates.
Coarse aggregate rarely contains more than 1 per cent of surface moisture but fine aggregate can
contain in excess of 10 per cent. The surface moisture is expressed as a percentage of the mass of
the saturated and surface-dry aggregate, and is termed moisture content.
Important
Since absorption represents the water contained in aggregate in a saturated and surface dry
condition, and the moisture content is the water in excess of that state, the total water content
of a moist aggregate is equal to the sum of the absorption and the moisture content.
Moisture content is determined by simply finding the loss in mass of an aggregate sample when
dried on a tray over a source of heat. Care is necessary to avoid over-drying the aggregate
3.1.11 Bulking of Fine Aggregate
The presence of moisture in aggregate necessitates correction of the actual mix proportions: the
mass of water added to the mix has to be decreased by the mass of the free moisture in the
aggregate, and the mass of the wet aggregate must be increased by a like amount, In the case of
sand, there is a second effect of the moisture: bulking. This is the increase in the volume of a
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given mass of sand caused by the films of water pushing the sand particles apart. While bulking
does not really affect the proportioning of materials by mass, in the case of volume batching,
bulking results in a smaller mass of sand occupying the fixed volume of the measuring box. For
this reason, the mix becomes deficient in fine aggregates and appears “stony”, and the concrete
may be prone to segregation and honeycombing. Also the yield (amount) of concrete is reduced.
The remedy of course is increasing the apparent volume of fine aggregate (sand) to allow for
bulking.
The extent of bulking depends on the percentage of moisture present in the sand and on its
fineness. The increase in volume relative to that occupied by a saturated and surface-dry sand
increases with an increase in moisture content of the sand up to a value of some 5 to 8 per cent,
when the bulking of 20 to 30 percent occurs. Upon further addition of water, the films merge and
the water moves into the voids between the particles so that the total volume of sand decreases
until, when fully saturated (flooded), its volume is approximately the same as the volume of dry
sand for the same method of filling the container. Finer sand bulks more and reaches maximum
bulking at higher water content than does coarse sand.
Coarse aggregate shows only a negligible increase in volume due to the presence of free water,
as the thickness of moisture films is very small compared with the particle size.
3.1.12 Deleterious Substances in Aggregates
There are three broad categories of deleterious substances that may be found inn aggregates:
impurities which interfere with the process of hydration of cement; coatings preventing the
development of good bond between aggregate and the hydrated cement paste; and certain
individual particles that are weak or unsound in themselves. All or part of an aggregate can also
be harmful through the development of chemical reactions between the aggregate and the cement
paste.
3.1.13 Organic Impurities
Natural aggregates may be sufficiently strong and resistant to wear and yet they may not be
satisfactory for concrete making if they contain organic impurities which interfere with the
chemical reactions of hydration. The organic matter found in aggregate consists usually of
products of decay of vegetable matter (mainly tannic acid and its derivatives) and appears in the
form of humus or organic loam. Such materials are more likely to be present in the sand than in
coarse aggregate, which is easily washed.
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Not all organic matter is harmful and it is best to check its effects by making actual compression
test specimens {like the ones you will prepare in the concrete lab under this unit}. Generally,
however, it saves time to ascertain first whether the amount of organic matter is sufficient to
warrant further tests.
3.1.14 Clay and Other Fine Material
Clay may be present in aggregate in the form of surface coatings which interfere with the bond
between aggregate and the cement paste. Because good bond is essential to ensure a satisfactory
strength and durability of concrete, the problem of clay coatings is an important one.
There are two more types of fine material which can be present in aggregate: silt and crusher
dust. Silt is material between 2 and 60 μm, reduced to this size by natural processes of
weathering; silt may thus be found in aggregate won from natural deposits. On the other hand,
crusher dust is a fine material formed during the process of breaking rock into crushed stone or,
less frequently, of gravel into crushed fine aggregate. In a properly laid out processing plant, this
dust should be removed by washing. Silt and dust should not be present in excess because they
not only coat the aggregates reducing bonding with hydrating cement, but also, due to their large
surface area, reduce the amount of water available for aggregate wetting and cement hydration.
BS 882: 1992 imposes a limit on the maximum amount of material passing the 75 μm (No 200)
sieve.



In coarse aggregate: 2 per cent, increased to 4 per cent when it consists wholly of crushed
rock;
In fine aggregate 4 per cent, increased to 16 per cent when it consists wholly of crushed rock;
and
In all aggregate: 11 per cent
For heavy duty floors, the limit is 9 percent
3.1.15 Salt Contamination
Sand won from the seashore or dredged from the sea or a river estuary, as well as desert sand,
contains salt, and has to be processed. The simplest method is to wash the aggregate in fresh
water, but special care is required with deposits just above the high water mark I which large
quantities of salt, sometimes over 6 percent of mass of sand, can be found. Generally, sand from
the sea bed, washed even in sea water, does not contain harmful quantities of salt.
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Because of the danger of chloride induced corrosion of steel reinforcement, BS 882:1992 limits
chloride ion content by mass, expressed as a percentage of the mass of the total aggregate, are as
follows:



For pre-stressed concrete
0.01
For reinforced concrete made with sulfate resisting cement
For other reinforced concrete
0.05
0.03
Apart from the danger of corrosion of steel reinforcement, if salt is not removed, it will absorb
moisture from the air and cause efflorescence – unsightly white deposits on the surface of the
concrete
Sea dredged coarse aggregate may have a large content of shell. This usually has no adverse
effect on strength but workability of concrete made with aggregate having a large shell content is
slightly reduced.
3.1.16 Unsound Particles
Tests on aggregates sometimes reveal that the majority of the component particles are
satisfactory but that a few are unsound: the quantity of such particles must clearly be limited.
There are two broad types of unsound particles: those that fail to maintain their integrity, and
those that lead to disruptive expansion on freezing or even on exposure to water. The disruptive
properties are characteristic of certain rock groups.
Shale and other particles of low density are regarded as unsound and so are soft inclusions, such
as clay lumps, wood and coal as they lead to pitting and scaling. If present in large quantities
(over 2 to 5 per cent of the mass of the aggregate) these particles may adversely affect the
strength of the concrete and should certainly not be permitted in concrete which is exposed to
abrasion.
Coal, in addition to being a soft inclusion, is undesirable for other reasons, it can swell causing
disruption of concrete and, if present in large quantities in a finely divided form, it can disturb
the process of hydration of the cement paste.
The presence of coal and other low density materials can be determined by floatation in a liquid
of suitable specific gravity.
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Mica should be avoided because, in the presence of active chemical agents produced during the
hydration of cement, alteration of mica into other forms can occur. Mica, in fine form, also
affects the water requirement and strength of concrete.
Gypsum and other sulfates must not be present; their content in aggregate must be determined by
BS 812: Part 118:1988.
3.1.17 Soundness of Aggregate
This is the term used to describe the ability of aggregate to resist excessive changes in volume as
a result of changes in physical conditions. Lack of soundness is thus distinct from expansion
caused by the chemical reactions between the aggregate and the alkalis in cement.
Physical causes of large or permanent volume changes or aggregate are freezing and thawing,
thermal changes at temperatures above freezing and alternating wetting and drying.
Aggregate is said to be unsound when volume changes, induced by the above causes result in
deterioration of the concrete. This may range from local scaling and so called pop outs to
extensive surface cracking and to disintegration over a considerable depth, and thus can vary
from no more than impaired appearance to a structurally dangerous situation.
3.1.18 Alkali-Silica Reaction
In the recent years, an increasing number of deleterious chemical reactions between and the
surrounding hydrated cement paste has been observed. The most common reaction is that
between active silica constituents of the aggregate and the alkalis in cement. The reactive forms
of silica are opal (amorphous), chalcedony (cryptocrustalline fibrous), and tridynamite
(crystalline). These reactive materials occur in: opaline or chalcedonic cherts, siliceous
limestones, rhyolites and rhyolitic tuffs, tuffs and dacite tuffs, etc.
The reaction starts with the attack on the siliceous materials in the aggregate by the alkaline
hydroxides in pore water derived from the alkalis (Na2O and K2O) in the cement. As a result,
the alkali-silicate gel is formed, either in planes of weakness or pores in the aggregate (where
reactive silica is present) or on the surface of the aggregate particles. In the latter case, a
characteristic altered surface zone is formed. This may destroy the bond between the aggregate
and the surrounding hydrated cement paste.
The gel is of the unlimited swelling type: it imbibes water with a consequent tendency to
increase in volume. Because the gel is confined by the surrounding hydrated cement paste,
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internal pressures result and may eventually lead to expansion, cracking and disruption of the
hydrated cement paste. Thus expansion appears to be due to the hydraulic pressure generated
through osmosis, but expansion can also be caused by the swelling pressure of the still solid
products of the alkali silica-reaction.
3.2 Quality of Mixing Water
The quality of the mixing water has an influence on the strength of the resulting concrete. Above
this, the quality of water also plays a significant role: impurities in water may interfere with the
setting of the cement and may adversely affect the strength of the concrete or cause staining of
its surface, and may also lead to corrosion of the reinforcement. For these reasons, the suitability
of water for mixing and curing purposes should be considered.
Mixing water should not contain undesirable organic substances or inorganic constituents in
excessive proportions. In many project specifications, the quality of water is covered by a clause
saying the water should be fit for drinking.
While the use of potable water as mixing water is generally satisfactory, there are some
exceptions; for instance, in some arid areas, local drinking water is saline and may contain an
excessive amount of chlorides. Also some natural mineral waters contain undesirable amounts of
alkali carbonates and bicarbonates which could contribute to the alkali-silica reaction.
(For further water quality discussion ref to “Properties of Concrete” by AM Neville Pp 182 –
184)
3.3 Workability
3.3.1
Definition
A concrete which can be readily compacted is said to be workable, but to say merely that
workability determines the ease of placement and the resistance to segregation is too loose a
description of this vital property of concrete. Furthermore the desired workability in any
particular case would depend on the means of compaction available; likewise, a workability
suitable for mass concrete is not necessarily sufficient for thin, inaccessible, or heavily
reinforced setions. For these reasons, workability should be defined as a physical property of
concrete alone without reference to the circumstances of a particular type of construction.
{This also helps standardize the measure of workability}.
To obtain this definition it is necessary to consider what happens when concrete is being
compacted. Whether compaction is achieved by ramming or vibration, the process consists
essentially of the elimination of entrapped air from the concrete until it has achieved as close a
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configuration {as close a packing} as is possible for a given mix. Thus the work done {in
compacting} is used to overcome friction between individual particles in the concrete and also
between the concrete and the surface of the mould or of the reinforcement. These two can be
called internal friction and surface friction, respectively. Some work is also absorbed in the
vibration of the concrete mass and the mould. This constitutes “wasted” work as opposed to
“useful” work shich constitutes the effort to overcome friction.
Because only the internal friction is an intrinsic property of the mix, workability can be best
defined as the amount of useful internal work necessary to produce full compaction. The
ASTM C 125-93 definition of workability is somewhat more qualitative: “property
determining the effort required to manipulate a freshly mixed quantity of concrete with
minimum loss of homogeneity”
Another term used to describe the state of fresh concrete is consistency. In ordinary English
usage, this word refers to the firmness of form of a substance or to the ease with which it will
flow.
3.3.2
The Need for Sufficient Workability
Workability is a vital property as far as the finished product is concerned because concrete must
have a workability such that compaction to maximum density is possible with a reasonable
amount of work or with the amount that was prepared to put it under given conditions.
The strength of concrete is directly dependent on the degree of compaction. It is convenient to
express the degree of compaction as a density ratio betweenthe actual density of the concrete to
that of the same concrete when fully compacted. The presence of voids in concrete greatly
reduces its strength. 5 percent voids can lower the strength by 30 percent.
3.3.3
Factors Affecting Workability
The main factor is the water content of the mix, expressed in kilograms (or liters) of water per
cubic meter of concrete.
If the water content and the other mix proportions are fixed, workability is governed by the
maximum size of aggregate, its grading, shape and texture. Generally, the higher the
water/cement ratio, the finer the grading required for the highest workability. The fineness of the
cement can also affect workability, but its effect is still subject of controversy.
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Measurement of Workability
Unfortunately, there is no acceptable test which will measure directly the workability as given by
any of the definitions given above. The available tests for workability therefore only serve to
provide indication of how workable the concrete is.
3.3.4.1 Slump Test
This test is used extensively in site work all over the world. This test does not measure the
workability of concrete, but its consistency {how easy the concrete flows, can be moulded}
The slump test is described in ASTM C 143-90a and BS 1881: Part 102: 1983. The mould for the
slump test is a frustum of a cone, 300mm high. It is placed on a smooth surface with the smaller
opening at the top, and filled with concrete in three layers. Each layer is tamped 25 times with a
standard 16mm diameter rod, rounded at the end and the top surface is truck off by means of
sawing and rolling motion of the tamping rod. The mould must be firmly held against its base
during the entire operation.
Immediately after filling, the cone is slowly lifted, and the unsupported concrete will now slump
– hence the name of the test. The decrease in height of the slumped concrete is called the slump,
and is measured to the nearest 5mm. the decrease is measured to the highest point according to
BA 1881: part 102: 1983.
If instead of slumping evenly all round as in a true slump (Figure 3.2), one half of the cone slides
down an inclined plane, a shear slump is said to have taken place, and the test should be
repeated. If shear slump persists as may be the case with harsh mixes, this is an indication of lack
of cohesion in the mix.
Figure 3.2: Typical slump test results: collapse, shear, and true slump
Mixes of stiff consistency have a zero slump, so that, in the rather dry range, no variation can be
detected between mixes of different workability {it is known that once slump values get below
25mm, the slump test looses its accuracy}. Rich mixes behave satisfactorily, their slump being
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sensitive to variations in workability. However, in a lean mix with a tendency to harshness {bony
or too much coarse aggregate}, a true slump can easily change to the shear type or even to
collapse, and widely different values of slump can be obtained in different samples from the
same mix
The approximate magnitude of slump for different workabilities is given in Table 3.5. Table 3.6
gives the proposed European classification of ENV 206:1992.
Table 3.5: Description of Workability and Magnitude
Description of Workability
Slump (mm)
No Slump
0
Very low
5 – 10
Low
15 – 30
Medium
35 – 75
High
80 – 155
Very High
160 – collapse
Table 3.6: Classification of Workability and Magnitude of Slump according to European
Standard ENV 206: 1992
Classification of Workability
Slump (mm)
S1
10 – 40
S2
50 – 90
S3
100 – 150
S4
≥ 160
It should also be noted that slump in itself does not measure the ease of compaction of concrete,
rather it reflects the ‘yield’ of the concrete.
Despite this, slump is very useful on site in measuring the batch-to-batch variation in the
materials being fed into the mixer. Too high or too low slump gives an immediate warning and
enables the mixer operator to remedy the situation.
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3.3.4.2 Compacting Factor Test
There is generally no accepted method of directly measuring the amount of work necessary to
achieve full compaction, which is the definition of workability. Probably the best test yet
available uses the inverse approach: the degree of compaction achieved by a standard amount of
work is determined.
The degree of compaction, called the Compaction Factor, is measured by the density ratio, i.e.
the ratio of the density actually achieved in the test to the density of the same concrete fully
compacted.
The test known as the compaction factor test is described in BS 1881: Part 103: 1993 and in ACI
211.3-75 and is appropriate for concrete with a maximum size of aggregate up to 40mm. the
apparatus consists of two hoppers each in the shape of a frustum of a cone, and one cylinder, the
three being above one another. The hoppers have hinged doors at the bottom, as shown in photo
in Figure 3.3. All inside surfaces are polished to reduce friction.
Figure 3.3: Typical Compaction Factor Equipment
The upper hopper is filled with concrete, this being placed gently so that at this stage no work is
done on the concrete to produce compaction. The bottom door of the hopper is then released and
the concrete falls into the lower hopper. This is smaller that the upper one and is, therefore, filled
to overflowing, and thus always contains approximately the same amount of concrete in a
standard state; this reduces the influence of the personal factor in filling the top hopper. The
bottom door of the lower hopper is then released and the concrete falls into the cylinder. Excess
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concrete is cut by two floats slid across the top of the mould, and the net mass of the concrete in
the known volume of the cylinder is determined.
The density of the concrete in the cylinder is now calculated, and this density divided by the
density of the fully compacted concrete is defined as the compacting factor.
Table 3.7 lists values of the compacting factor for different workabilities. Unlike the slump test,
variations in the workability of dry concrete are reflected in a large change in the compacting
factor, ie. The test is more sensitive, at the low workability end of the scale, than at high
workability.
Table 3.7: Description of Workability and Compaction Factor
Description
of Compacting Factor
Corresponding
Workability
(mm)
Very Low
0.78
0 – 25
Low
0.85
25 – 50
Medium
0.92
50 – 100
High
0.95
100 – 175
Slump
3.3.4.3 ASTM Flow Test
This laboratory test gives an indication of the consistency of concrete and its proneness to
segregation by measuring the spread of a pile of concrete on a table subjected to jolting. This test
also gives a good assessment of consistency of stiff, rich, and cohesive mixes. The test was
covered by ASTM C 124-39, which was withdrawn in 1974 because the test was little used,
rather than because it was thought to be inappropriate.
3.3.4.4 Other Tests
Other tests that may be used in the determination of consistency of concrete include: remounding
test, vebe test, flow test, ball penetration test, Nasser’s K-tester and the two point test.
Assignment:
Go research on these (other) tests and prepare a three page summary write-up of the same.
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Effect of Time and Temperature on Workability
Freshly mixed concrete stiffens with time. This should not be confused with setting of cement. It
is simply that water from the mix is absorbed by the aggregate if not saturated, some is lost by
evaporation, particularly if the concrete is exposed to the sun or wind, and some is removed by
the initial chemical reactions.
Because workability decreases with time, it is important to measure, say, slump after a
predetermined time lapse since mixing. There is value in determining slump immediately after
the discharge of the concrete from the mixer for the purpose of control of batching. There is also
value in determining slump at the time of placing the concrete in the formwork for the purpose of
ensuring that the workability is appropriate for the means of compaction to be used.
3.4 Segregation
Fresh concrete should not segregate i.e. should remain cohesive. However, strictly speaking, the
absence of a tendency to segregate is not included in the definition of a workable mix,
Nevertheless, the absence of appreciable segregation is essential as full compaction of a
segregated mix is impossible.
Segregation can be defined as the separation of the constituents of a heterogeneous mixture so
that their distribution is no longer uniform. In the case of concrete, it is the differences in the
sizes of particles and in the specific gravity of the mix constituents that are the primary cause of
segregation, but its extent can be controlled by the choice of suitable grading and by care in
handling.
It is worthy noting that a higher viscosity of the fresh cement paste component militates against
the downward movement of the heavier aggregate particles; consequently, mixes with low
water/cement ratios are less prone to segregation.
There are two forms of segregation. In the first, the coarser particles tend to separate out because
they tend to travel further along a slope or to settle more that finer particles. The second form of
segregation, occurring particularly in wet mixes, is manifested by the segregation of grout
(cement plus water – “mchuzi”) from the mix. With some gradings, when a lean mix is used, te
first type of segregation may occur if the mix is too dry; addition of water would improve the
cohesion of the mix, but when the mix becomes too wet the second type of segregation would
take place.
The actual extent of segregation is dependent on the method of handling and placing concrete. If
the concrete does not have far to travel, and is transferred directly from the skip or bucket to the
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final position in the form, the danger of segregation is small. On the other hand, dropping
concrete from a considerable height, passing along a chute, particularly with changes in
direction, and discharging against an obstacle – all these could encourage segregation so that
under such circumstances a particularly cohesive mix should be used.
It should be stressed that concrete should be placed directly in the position in which it is to
remain and must not be allowed to flow or be worked along the form. This prohibition includes
the use of a vibrator to spread a heap of concrete over a large area. Vibration itself must not be
allowed for too long, since in many mixes, this could cause separation of coarse aggregate
towards the bottom of the form and of the cement paste towards the top.
As far as proneness to segregation on over-vibration is concerned, a good test is to vibrate a
concrete cylinder or cube for 10 minutes and then strip it and observe the distribution of coarse
aggregate: any segregation will be easily seen.
3.5 Bleeding
Bleeding, also known as water gain, is a form of segregation in which some of the water in the
mix tends to rise to the surface of freshly placed concrete. This is caused by the inability of the
solid constituents to hold all of the mixing water when they settle downwards, water having the
lowest specific gravity of all the mix constituents. Bleeding can be expressed quantitatively as
the total settlement per unit height of concrete or as a percentage of the mixing water; in extreme
cases, this may reach 20 per cent.
The initial bleeding proceeds at a constant rate but subsequently the rate of bleeding decreases
steadily. Bleeding of concrete continues until the cement paste has stiffened sufficiently to put an
end to the process of sedimentation.
If 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 bleed water has
evaporated, and also by use of wood floats and avoidance of overworking the surface. On the
other hand, if evaporation of the water from the concrete surface is faster than the bleeding rate,
plastic shrinkage cracking may result.
Some of the rising water becomes trapped on the underside of coarse aggregate particles or of
reinforcement, this creates zones of poor bond. This water leaves behind air pockets or lenses,
and because all the voids are oriented in the same direction, the permeability of a concrete in a
horizontal plane may be increased. Hence, ingress of an attacking medium into the concrete is
facilitated.
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The bleed water can alter the water/cement ratio in the upper parts of the elements resulting in
weaker concrete in those regions. If bleed water finds an escape path along the surface of the
formwork, this could lead to surface streaking on the concrete.
Bleeding decreases with increase in cement fineness, possibly because finer particles hydrate
earlier and also because their rate of sedimentation is lower. There is also less bleeding when the
cement has a high alkali content, a high C3 A content, or when calcium chloride is added.
The presence of an adequate proportion of very fine aggregate particles (especially smaller than
150μm) significantly reduces bleeding. Rich mixes are less prone to bleeding than lean ones.
Reduction in bleeding is obtained by the addition of pozzolanas or other fine material or
aluminium powder.
A higher temperature, within the normal range, increases the rate of bleeding, but the total
bleeding capacity is unaffected. Very low temperature however, may increase the bleeding
capacity, probably because there is more time prior to stiffening for bleeding to occur.
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4 MANUFACTURE OF CONCRETE
4.1 The Mixing of Concrete
Mixing must be done properly so that all of the aggregate particles are coated with cement paste
and which is homogeneous on the macro scale and therefore possessing uniform properties.
4.2 Concrete Mixers
Concrete mixers must not only achieve the uniformity of the mix, but they must also discharge
the mix without disturbing the uniformity. In fac, the method of discharging is one of the bases
of classification of concrete mixers. Several types exist.
In the tilting mixer (Figure 4.1-A), the mixing chamber, known as the drum is tilted for
discharging. In the non tilting mixer (Figure 4.1-B), the axis of the mixer is always horizontal,
and the discharge is obtained either by inserting a chute into the drum or by reversing the
direction of rotation of the drum. There is also a pan type mixer (Figure 4.2 – also similar to the
one in concrete lab).
Tilting mixers usually have a conical or bowl shaped drum with vanes inside. The efficiency of
the mixing operation depends on the details of design, but the discharge action is always good as
all the concrete can be tripped out rapidly and in an unsegregated mass as soon as the drum is
tilted. For this reason, tilting mixers are preferable for mixers of low workability and for those
containing large aggregates.
On the other hand, because of a rather slow discharge rate from a non-tilting drum mixer,
concrete is sometimes susceptible to segregation. In particular, the largest size of aggregate may
tend to stay in the mixer, so that the discharge sometimes starts as mortar and ends as a
collection of coated coarse aggregate particles. This problem has made non tilting mixers to
become less popular.
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A
B
Figure 4.1: Photo of a typical tilting mixer (A) and non tilting mixer (B)
Figure 4.2: Pan type mixer
Non tilting mixers are always charged by means of a loading skip, which is also used with the
larger drum mixers. It is important that the whole charge from the skip can be transferred into the
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mixer every time, ie, no sticking must occur. Sometimes a shaker mounted on the skip assists in
emptying it.
The pan mixer is usually not mobile and is used at a central mixing plant, at a precast concrete
plant, or in a small version in a concrete laboratory. The mixer contains essentially of a circular
pan rotating about its axis, with one or two stars of paddles rotating about a vertical axis not
coincident with the axis of the pan {NB: this feature no longer works in the pan mixer in our
concrete laboratory}. Sometimes the pan is static and the axis of the star travels along a circular
path around the axis of the pan. In either case, the relative movement between the paddles and
the concrete is the same, and concrete in every part of the pan is thoroughly mixed. Scrapper
blades prevent mortar from sticking to the sides of the pan, and the height of the paddles can be
adjusted so as to prevent a permanent coat of mortar forming on the bottom of the pan.
Pan mixers offer the possibility of observing the concrete in them, and therefore of adjusting the
mix in some cases. They are particularly efficient with stiff and cohesive mixes and are,
therefore, often used in the manufacture of precast concrete. They are also suitable, because of
the scrapping arrangements, for mixing very small quantities of concrete – hence their use in the
laboratory.
In the drum type mixers, no scrapping of the sides takes place during mixing so that a certain
amount of mortar adheres to the sides of the drum and remains there until the mixer is cleaned.
Because of this, the first batch from this mixer leaves behind substantial amounts of its cement
paste and therefore should not be used. If the first batch is to be used, mortar is first introduced
into the mixer to coat the surface of the mixer drum before putting in the batch. This procedure is
referred to as “buttering” or priming the mixer.
All mixers considered so far are batch mixers, in that one batch is mixed at a time, as opposed to
continuous mixers. Continuous mixers discharge concrete continuously without interruption,
being fed by a continuous volume- or weigh-batching system. Modern continuous mixers
produce concrete of high uniformity. This type of mixers enable the mixing, placing, compaction
and finishing to be carried out within 15 minutes of the introduction of water into the mix. Other
mixers include, revolving drum truck mixers, etc.
4.3 Uniformity of Mixing
In any mixer it is essential that sufficient interchange of materials between different parts of the
chamber takes place, so that uniform concrete is produced. The efficiency of the mixer can be
measured by the variability of the mix discharged into a number of receptacles without
interrupting the flow of concrete.
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4.4 Mixing Time
On site, there is often a tendency to mix concrete as rapidly as possible, and it is, therefore,
important to know the minimum mixing time necessary to produce a concrete uniform in
composition, and as a result, of satisfactory strength. This time varies with the type of mixer and,
strictly speaking, it is not the mixing time but the number of revolutions of the mixer that is the
criterion of adequate mixing. Generally about 20 revolutions are sufficient. Because there is an
optimum speed of rotation recommended by the manufacturer of the mixer, the number of
revolutions and the time of mixing are interdependent.
Generally, within the first minute, the influence of mixing time on strength of concrete is of
considerable importance. The average strength of concrete increases with mixing time within this
first minute. Mixing beyond the first minute results in reduction of the strength of the concrete.
When mixing is done over a long period of time, evaporation of water from the mix occurs, with
a consequent decrease in workability and an increase in strength. A secondary effect is the
grinding of the aggregate, particularly if soft: the grading the aggregate thus becomes finer,
which lowers the workability further. The friction effect also produces a temperature increase in
the mix.
Intermittent mixing up to about 3 hours, and in some cases up to 6 hours, is harmless as far as
strength and durability are concerned, but the workability falls off with time unless loss of
moisture from the mix is prevented. Adding water to restore workability, known as retempering,
will lower the strength of the concrete.
Generally a small amount of water is fed into the mixer followed by all the solid materials,
preferably fed simultaneously into the mixer. If possible the greater part of the water should be
fed at the same time. If simultaneous feeding is not possible, the larger aggregate is fed first
followed with the finer components and water comes in last.
4.5 Hand Mixing
In some occasions, small quantities of concrete are mixed by hand. In such cases, uniformity is
hard to achieve calling for particular care and effort.
The aggregate should be spread in a uniform layer on a hard, clean and non-porous base; cement
is then spread over the aggregate, and the dry materials are mixed by turning over from one end
to the other and ‘cutting’ with the shovel until the mix appears uniform. Turning three times is
usually required. Water is then gradually added so that neither water by itself nor with cement
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can escape. The mix is turned over again, usually three times, until it appears uniform in color
and consistency.
4.6 Ready-Mixed Concrete
Ready mixed concrete is particularly useful on congested sites or in road construction where
space for a mixing plant and for extensive aggregate is unavailable. However, perhaps the single
greatest advantage of ready mixed concrete is that it is made under better conditions of control
than are normally possible on any but large construction sites. Control has to be enforced but,
since the central mixing plant operates under near-factory conditions, a really close control of all
operations of production of fresh concrete is possible. Proper care during transportation of the
concrete is also ensured by the use of agitator trucks, but the placing and compaction remain, of
course, the responsibility of the personnel on site.
There are two principal categories of ready-mixed concrete. In the first, the mixing is done at a
central plant and the mixed concrete is then transported, usually in an agitator truck which
revolves slowly so as to prevent segregation and undue stiffening of the mix. Such concrete is
known as central-mixed as distinct from the second category – transit-mixed or truck-mixed
concrete.
In the former, materials are batched at a central plant but are mixed in a mixer truck either in
transit to the site or immediately prior to the concrete being discharged. Transit mixing permits a
longer haul and is less vulnerable in case of delay, but the capacity of a truck used as a mixer is
only 63 per cent, or even less of the drum while for central mixed concrete it is 80 per cent.
Sometimes the concrete is partially mixed at a central plant in order to increase the capacity of
the agitator truck. The mixing is completed en route. Truck mixers usually have capacity of 6m3
or 7.5m3.
The main problem in the production of ready mixed concrete is maintaining the workability of
the mix right up to the time of placing
4.7 Concrete Admixtures
While admixtures, unlike cement, aggregate and water, are not an essential component of the
concrete mix, they are an important and increasingly widespread component.
4.7.1
Benefits of Admixtures
The reason for the large growth in the use of admixtures is that they are capable of imparting
considerable physical and economic benefits with respect to concrete. These benefits include the
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use of concrete under circumstances where previously there existed considerable difficulties.
They also make possible the use of a wider range of ingredients in the mix.
4.7.2
Types of Admixtures
An admixture can be defined as a chemical product, which, except in special cases, is added to
the concrete mix in quantities no larger than 5 per cent by mass of cement during mixing or
during an additional mixing operation prior to the placing of concrete, for the purpose of
achieving a specific modification, or modifications, to the normal properties of concrete.
Admixtures may be organic or inorganic in composition but their chemical character, as distinct
from mineral, is their essential feature. Admixtures are commonly classified by their function in
concrete but often they exhibit some additional action. The classification of ASTM C 494-92 is
as follows:
Type A
Water-reducing
Type B
Retarding
Type C
Accelerating
Type D
Water-reducing and retarding
Type E
Water-reducing and accelerating
Type F
High-range water-reducing or superplasticizing, and
Type G
High-range water-reducing and retarding, or superplasicizing and retarding
The effectiveness of any admixture may vary depending on its dosage in the concrete and also on
the constituents of the mix, especially the properties of the cement
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5 PROPERTIES OF HARDENED CONCRETE
Properties of hardened concrete include:
i.
Strength of Concrete
ii.
Concrete Creep
iii.
Shrinkage
iv.
Modulus of Elasticity
v.
Water tightness (Impermeability)
vi.
Rate of strength gain of concrete
5.1 Strength
The strength of concrete is basically referred to compressive strength and it depends on three
factors
 Paste strength – this is mainly due to the binding properties of the cement that the
ingredients are compacted together. If the paste has higher binding strength, the strength is
likely to have higher strength if the other constituents are similar in quantity and quality.
 Interfacial bonding – interfacial bonding between the paste and aggregates is important
regarding the strength of the resulting concrete. Clay hampers the bonding between paste and
aggregate. The aggregate should be washed for better bonding between paste and aggregate
 Aggregate strength – It is mainly the aggregate that provide strength to concrete especially
coarse aggregates. Rough and angular aggregates provide better bonding thus higher
strength.
Concrete strength can be affected by several factors as previously discussed. Chief among these
factors are:
 Water cement ratio – This has a direct impact on how much hydration will occur. It also has
an influence on the final porosity of the hardened cement. Generally, within the limits, the
lower the w/c ratio the higher the strength, though a lower water cement ratio brings with it
challenges of workability, etc.
 Cement type – as discussed earlier, the type of cement has a bearing on not only the strength
but also durability, among other concrete properties. Finer cement tends to produce higher
early strength, with an increase in chemical shrinkage and heat of hydration.
 Aggregate type – the aggregate strength, reactivity, shape, size and porosity, among other
properties has a direct influence on the strength of concrete.
 Admixtures – concrete admixture have the ability to alter the behavior of both plastic and
hardened concrete in terms of strength, resistance to corrosion, etc.
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Characteristic Material Strengths
All manufactured material properties vary because the molecular structure of the material is not
uniform and also because of inconsistencies in the manufacturing process. The variations in the
manufacturing process are dependent on the degree of control. These variations must be
recognized and incorporated into the design.
The strengths of materials on which design is based are those strengths below which results are
unlikely to fall. These are called “characteristic” strengths. It is assumed that for a given
material, the distribution of strength will be approximately “normal”, so that a frequency
distribution curve of a large number of sample results would be of the form shown in the figure
below.
Figure 5.1: Variation in material properties
The strength to be used as a basis for design must be selected from the variation of values shown
in the figure above. This strength, when defined, is called the characteristic strength. In
determining this strength for steel, a risk factor of 5 % is accepted to strike a compromise
between safety and economy. This strength is therefore calculated using the expression:
fk = fmean – 1.64…………………………………………. 3.1.3
Where  is the standard deviation of the total number of samples.
The strength used for design is then given by the characteristic strength divided by a partial
factor of safety.
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5.2 Creep
Concrete creep is defined as: deformation of structure under sustained load. Basically, the long
term pressure or stress on concrete can make it change shape. This deformation usually occurs in
the direction the force is being applied. Like a concrete column getting more compressed or a
beam increasing in deflection under the same load over a long period of time.
Factors affecting creep include:
i.
Aggregate
ii.
Mix proportions
iii.
Age of concrete
5.2.1
Influence of Aggregate
Aggregate undergoes very little creep, it is really the paste which is responsible for most of the
creep. However the aggregate influences the creep of the concrete through a restraining effect on
the creep. The stronger the aggregate, the more the is the restraining effect and hence the smaller
the magnitude of creep. The modulus of elasticity of aggregate is one known to have a direct
influence on creep of the final concrete. Lightweight aggregate shows substantially higher creep
than the normal weight aggregate.
5.2.2
Influence of Mix Proportions
The amount of paste content and its quality also has an impact on creep. A poorer paste structure
undergoes higher creep. Therefore, it can be said that creep increases with increase in water
cement ratio. Creep is inversely proportional to the strength of the concrete.
5.2.3
Influence of Age
The age at which the concrete member is loaded will have a predominant effect on the magnitude
of creep. This can be easily understood from the fact that the quality of gel improves with time.
Such gel creeps less, whereas a young gel under load being not so stronger creeps more.
Creep could have varied effects on concrete elements depending on the type of element and the
place/position within the element where creep occurs. Some of these effects include:



In reinforced concrete beams, creep increases the deflection with time and may be a
critical consideration in design.
In eccentrically loaded columns, creep increases the deflection and can lead to buckling.
In cases of statically indeterminate structures and column and beam junctions creep may
relieve the stress concentration induced by shrinkage, temperature changes or movement
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

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of support. The creep property of concrete is useful in all concrete structures to reduce
internal stresses due to non-uniform or restrained shrinkage.
In mass concrete structures such as dams, on account of differential temperature
conditions at the interior and surface, creep is harmful and by itself may be a cause of
cracking in the interior of dams. Therefore, all precautions must be taken to see that
increase in temperature does not take place within the concrete mass in such structures.
Loss of pre-stress due to creep of concrete in pre-stressed concrete structures.
5.3 Shrinkage
Concrete is subjected to changes in volume either autogenous or induced. Volume change is one
of the most detrimental properties of concrete, which affects the long term strength and
durability. To the practical engineer, the aspect of volume change in concrete is important from
the point of view that it causes unsightly cracks in concrete. The term shrinkage is loosely used
to describe the various aspects of volume changes in concrete due to an array of different
reasons.
Volume change in concrete can be caused by thermal properties of aggregates under temperature
variations, alkali/aggregate reaction, due to sulfate action, hydration reactions, loss of moisture
etc.
One of the most objectionable defects in concrete is the presence of cracks, particularly in floors
and pavements. Shrinkage is one of the most common causes for this cracking. The designer is
therefore faced with the question of how to reduce the shrinkage and by extension shrinkage
cracking in concrete structures.
5.3.1
Types of Shrinkage
Shrinkage can be classified into the following categories:
a) Plastic shrinkage
b) Drying shrinkage
c) Autogeneous shrinkage
d) Carbonation shrinkage
5.3.1.1 Plastic Shrinkage
This type of shrinkage manifests itself soon after the concrete is placed in the forms while the
concrete is still in the plastic state. Loss of water by evaporation from the surface of the concrete
or by the absorption by the aggregate or subgrade (in the case of roads), is the main cause of this
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type of shrinkage. The loss of water results in volume reduction. Cracks may appear on the
surface or internally around the aggregates and the reinforcement.
In the case of floors and pavements where the surface area exposed to drying is large as
compared to the depth, when this large surface is exposed to hot sun and drying wind, the surface
of concrete dries very fast which results in plastic shrinkage. Sometimes, even if the concrete is
not subjected to severe drying, but poorly made with a high water cement ratio, large quantity of
water bleeds and accumulates at the surface. When the water at the surface dries out, the surface
concrete collapses causing cracks.
Plastic concrete is sometimes subjected to unintended vibration or yirlding of formwork support.
This again causes plastic shrinkage cracks as the concrete at this stage has not developed enough
strength. From the above it is inferred that high water/cement ratio, badly proportioned concrete,
rapid drying, greater bleeding, unintended vibration, etc, are some of the reasons for plastic
shrinkage. It can also be further added that richer concrete undergoes greater plastic shrinkage.
Plastic shrinkage can be reduced mainly by preventing the rapid loss of water from the surface.
This can be done by covering the surface with polythene sheeting immediately on finishing
operation; by fog spray which keeps the surface moist, or by working at night. Use of small
quantity of aluminium powder is also suggested to offset the effect of plastic shrinkage.
5.3.1.2 Drying Shrinkage
Just as the hydration of cement is a continuous process, drying shrinkage is also a continuous
process that can proceed over a long time, when concrete is subjected to drying conditions. The
drying shrinkage of concrete is analogous to the mechanism of drying a timber specimen. The
loss of free water contained in hardened concrete, does not result in any appreciable dimension
change. It is the loss of water held in gel pores that cause the change in volume. Under drying
conditions, the gel water is lost progressively over a long time, as long as the concrete is kept in
drying conditions. Cement paste shrinks more than mortar and mortar shrinks more than
concrete. Concrete made with smaller size aggregate shrinks more than concrete made with
bigger size aggregate. The magnitude of drying shrinkage is also a function of the fineness of
gel. The finer the gel the more is the shrinkage.
5.3.1.3 Autogeneous Shrinkage
In a conservative system, ie where no moisture movement to or from the paste is permitted, when
temperature is constant some shrinkage may occur. The shrinkage of such a conservative system
is known as autogeneous shrinkage. Autogeneous shrinkage is of minor importance to concrete
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as a whole and is not applicable in practice to many situations except that of mass of concrete in
the interior of a concrete dam. Autogeneous shrinkage is directly related to the amount of
hydration reactions occurring in a cement paste and its measurement has been used to give an
indication of the amount of hydration reactions occurring in a cement paste.
5.3.1.4 Carbonation Shrinkage
Carbon dioxide present in the atmosphere reacts in the presence of water with hydrated cement.
Calcium Hydroxide [Ca(OH)2] gets converted to calcium carbonate and also some other cement
compounds are decomposed. Such a complete decomposition of calcium compound in hydrated
cement is chemically possible even at low pressure of carbon dioxide in normal atmosphere.
Carbonation penetrates beyond the exposed surface of concrete very slowly.
The rate of penetration of carbon dioxide depends also on the moisture content of concrete and
the relative humidity of the ambient medium. Carbonation is accompanied by an increase in
weight of the concrete and by shrinkage.
Carbonation shrinkage is probably caused by a dissolution 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.
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6 NON DESTRUCTIVE TESTS
6.1 Introduction
Coring is the method most often used to recover a concrete specimen for determination of the
properties of a concrete. In situations where this is not possible or appropriate, non-destructive
tests can be used to evaluate hardened concrete.
Several non destructive testing techniques for concrete are available. These include:
6.2 Crack monitor
Detect and record movement of cracks in concrete, brick or masonry structures. These are simple
to use and inexpensive devices. They consists of two overlapping acrylic of pvc plates. One plate
is marked with a millimeter grid, the other with cross hairs centered over the grid. Once installed
any movement can be seen and recorded easily
Figure 6.1: Crack monitor
6.3 Pocket tachometer
This checks vibration in RPM. It is used to check the effectiveness of the vibrators setting
concrete in forms. It comprises an aluminium body, steel wire reed and turning slide.
Figure 6.2: Typical pocket tachometer
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6.4 Concrete Test Hammer (Schmidt Hammer)
Tests in situ concrete compressive strength within a range of 10 – 55 N/mm2. The hammer is
placed on the test surface and a standard impact force released. The rebound value of the
hammer is noted and compared to a supplied table (for the analogue types) to get the
compressive strength of the concrete. Some digital models are able to convert the rebound value
to a compressive strength value within the equipment so as to give a reading of the compressive
strength directly.
Figure 6.3: Schmidt Hammer
6.5 Rebar Locator
This equipment is used to locate rebar, conduit pipe or other metallic objects in concrete or other
non metallic structures. It’s also able to give an indication of the size of bars located. Early
models of this used a low frequency magnetic field to locate ferrous objects within a structure.
The latter models use ground penetrating radar to locate steel and other objects within structures.
Figure 6.4: Typical Rebar Locator
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6.6 Ultrasonic Concrete Testing System
These systems are able to accurately identify basic characteristics of coarse grained materials.
Such are mainly used for quality control and inspection of concrete structures. They use the
measurement of speed of ultrasonic pulses through the concrete to correlate concrete strength to
standard strength. They are able to identify non-homogeneous conditions in the structure such as
honeycombs, voids, cracks, frozen concrete, etc. They can be used to survey whole structures,
new, old, fire damaged or weathered.
Figure 6.5: Typical Ultrasonic Concrete Test System
6.7 Impact Echo System
This is used to measure accurately the thickness or quality of concrete slabs without drilling
cores or using similar destructive techniques. It can be used to locate delaminating and voids in
concrete slabs and other elements. This uses impact generated stress waves that propagate
through concrete and are reflected by both the material’s external surfaces and internal flaws.
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Figure 6.6: Typical Impact Echo System
6.8 Maturity Meter System
This determines concrete strength based on the temperature history of the concrete. The maturity
index is expressed as equivalent age in hours referred to a specific temperature, and is a function
of time, temperature and the activation energy of the cement
Figure 6.7: Typical Maturity Meter System
6.9 Portable Moisture Meter with Probe
Microwave technology used to measure moisture content of sand, aggregates and other coarse
grained materials up to 25mm diameter having moisture content up to 20% by weight. This
provides instantaneous reading.
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Figure 6.8: Typical Portable Moisture Meter
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7 DURABILITY QUALITY CONTROL OF CONCRETE
Concrete lack of durability involves the expenditure of vast sums of money the world over on
inspection maintenance and repair. This is mainly attributed to inadequate attention to the
problem at the design and construction stages. Attention being applied both to extrinsic and
intrinsic causes. The former is environmentally related and can be assessed by a thorough
examination of the ambient conditions in which the concrete will pass the best part of its life
span. Durability is no different than other properties. In fact it is well documented that the
stronger the concrete the greater the durability.
The introduction of durability requirements for structural concrete since the replacement of
CP110 and BS8110, intended to ensure that concrete structures are designed and constructed so
that they maintain their required durability and performance for a sufficiently long period of time
– in excess of 50 years.
Modern durability technology has been built up from engineering models incorporating
knowledge and experience from a wide range of technical disciplines such as:
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
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Statics (structural behavior)
Materials technology
Design (codes, structural forms, design traditions)
Execution (workmanship, local traditions)
Statistics
Economy
Based on the above, to ensure durability, one should consider the following interrelated factors:
 The expected environmental conditions
 The use of the structure
 The required performance criteria
 The composition, properties and performance of the materials
 The shape of the members and structural detailing
 The quality of workmanship and level of control
 The particular protective measures
 The likely maintenance during the intended life
For most buildings, the general provisions in the code will ensure a satisfactory life with the
provision that the required level of performance and its duration be considered at an early stage
in the design.
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The code EC2, deals with four main deterioration mechanisms namely:
 Corrosion of reinforcement
 Alkali-silica reaction
 Chemical attack (such as sulfates)
 Freeze-thaw
The first primarily attacks and destroys the reinforcement resulting in cracking and spalling of
the concrete. the other three attack the concrete directly. All require the presence of water.
Water, essential for hydration, turns from usefull to destructive. It is well documented that the
rate of deterioration diminishes with the relative dryness of the concrete
BS 8110 classifies exposure conditions in its Table 3.2, an extract of which presented here as
Table 11.
Table 7.1: Classification of Exposure Conditions (BS 8110-1:1997: Table 3.2)
The resistance of concrete to degradation both from chemical and physical causes must be
obtained through the two processes of design and execution to achieve a dense impermeable
concrete. To achieve this, any designer must take account of the proper choice of materials,
proportions, desired mechanical properties, and mixing, placing, compacting and curing of the
concrete.
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It is well known that the potential for rebar corrosion is enhanced in the presence of chloride
ions. To this end EC2, as does BS 8110, bans the use of calcium chloride based admixtures in
any concrete incorporating embedded metal. The 0.4% limit of chloride ions by weight of
cement still applies.
The presence of chloride ions will manifest itself within a few years in the medium to high risk
category particularly if associated with concretes of low cement of low cement content and high
permeability in a moist environment.
The quality of an appropriate cover is an essential factor in the protection of the concrete from
rebar-corrosion. Apart from proper compaction of the appropriate concrete of the right grade and
w/c, curing is the main factor affecting the quality of skincrete. The provision of an impermeable
cover is seriously impaired should the structure or any element crack. The formation of such
cracks can be minimized by following the general requirements in the code for durability,
cracking, deformation, detailing, strength, etc. as detailed in the codes.
The role played by the ‘man on site’ cannot be overemphasized and the standard of workmanship
should ensure that the planned durability is achieved. The combination of materials and
procedures should be such as to ensure satisfactory resistance to aggressive media to both
concrete and the steel.
EC2 specifies the minimum necessary control measures for the design and construction of
concrete structures and comprise essential actions and decisions as well as checks to be made in
compliance with specifications and standards to ensure that these are met. It identifies three basic
control systems, each exercised by a different party with different objectives. These are:
 Internal control – effected by the designer, sub/contractor or supplier each within the scope
of his specific task and is carried out on his own initiative or according to ‘external’
requirements by client.
 External control – comprises all measures for the client and is carried out by an independent
organization employed by the client. This normally consists in the verification of internal
control measures required by external specifications and additional checking procedures
independent from internal control systems.
 Conformity control – normally part of the external control, is exercised to ascertain that a
particular service or function has been carried out in accordance with specifications.
The frequency and intensity of control depends on the consequences ensuing from mistakes and
errors in the various stages of construction. These stages may be distinguished as:
 Control of design
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Control of the production and construction
Control of the completed structure
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8 CONCRETE MIX DESIGN
8.1 Introduction
It can be said that the properties of concrete are studied primarily for the purpose of selection of
appropriate mix ingredients, and it is in this light that the various properties of concrete are
considered.
In the British usage, the selection of the mix ingredients and their proportions is referred to as
mix design.
8.2 British Method of Selection of Mix Proportions
The current British method is that of the Department of Environment revised in 1988. The
British method recognizes the need for durability in the mix requirements. The method is
applicable to normal weight concrete made with Portland cement only or also incorporating
ground granulated blast furnace slag or fly ash, but it does not cover flowing concrete or pumped
concrete, nor does it deal with lightweight aggregate concrete. Three maximum sizes of
aggregate are recognized: 40, 20 and 10 mm.
The British method consists of 5 steps, as follows:
Step 1
This deals with the compressive strength for the purpose of determining the water/cement ratio.
The concept of target mean strength is introduced, this being equal to the specified
characteristic strength plus a margin to allow for variability.
Certain strengths are assumed at a water/cement ratio of 0.5 for different cements and types of
aggregate (Table 8.1). The latter factor recognizes the significant effect of aggregate on strength.
The data of Table 8.1 applies to a hypothetical concrete of medium richness cured in water at
20oC, richer mixes would have a relatively higher early strength because they gain strength more
rapidly.
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Table 8.1: Approximate Compressive Strengths of Concretes Made with a Free Water/Cement
Ratio of 0.5 According to the 1998 British Method
Type of Cement
Type of
Coarse
Aggregate
Compressive Strength (MPa) at the age (days)
3
7
28
91
Ordinary Portland
(Type 1)
Uncrushed
22
30
42
49
Sulfate; Resisting
Portland (Type V)
Crushed
27
36
49
56
Uncrushed
29
37
48
54
Crushed
34
43
55
61
Rapid Hardening
Portland (Type
III)
From the Table 8.1, we find the appropriate value of strength (at a water/cement ratio of 0.5)
corresponding to the type of cement, type of aggregate and age which are to be used. Turning
to Figure 8.1, we mark a point corresponding to this strength at a water/cement ratio of 0.5.
Through this point, we now draw a curve ‘parallel’ to the neighboring curves. Using this new
curve we read off (as abscissa) the water/cement ratio corresponding to the specified target mean
strength (as the ordinate)
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Figure 8.1: Relation between compressive strength and free water/cement ratio for use in the
British mix selection method
Step 2
This deals with the determination of the water content for the required workability, expressed
either as slump or as vebe time, recognizing the influence of the maximum size of aggregate and
its type, namely crushed or uncrushed. The relevant data is given in Table 8.2. It can be noted
that the compacting factor is not used in mix selection, although it can be used for control
purposes.
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Table 8.2: Approximate free water contents required to give various levels of workability
according to the British Method
Aggregate
Water content, kg/m3 for:
Max size Type
(mm)
10
20
40
0 - 10
10 - 30
30 – 60
60 – 180
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
Slump
(mm)
Step 3
This determines the cement content, which is simply the water content divided by the
water/cement ratio. This cement content must not conflict with any minimum value specified for
reasons of durability or a maximum value specified for reasons of heat development.
Step 4
This deals with the determination of total aggregate content. This requires an estimate of the
fresh density of fully compacted concrete, which can be read off from Figure 8.2 for the
appropriate water content (Step 2) and specific gravity of aggregate. If this 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 from fresh density the value of cement content and of the
water content.
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Figure 8.2: Estimated wet density for full compacted concrete, (specific gravity is given for
saturated and surface dry condition)
Step 5
This determines the proportion of fine aggregate to the total aggregate, using the recommended
values of Figure 8.3, only data for 20mm and 40mm maximum aggregate size is shown. The
governing factors are: the maximum size of aggregate, the level of workability, the water/cement
ratio, and the percentage of fine aggregate passing the 600 μm sieve. Other aspects of the grading
of the fine aggregate are ignored and so is the grading of the coarse aggregate. Once the
proportion of the fine aggregate has been obtained multiplying it by the total aggregate content
gives the content of the fine aggregate.
The content of coarse aggregate is then the difference between the total aggregate content and
the content of fine aggregate. The coarse aggregate, in turn, should be divided into size fractions
depending on the aggregate shape. As a general guide, the percentages of Table 8.3 can be used.
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Figure 8.3: Recommended proportion of fine aggregate (expressed as a percentage of the total
aggregate) as a function of the free water/cement ratio for various workabilities and maximum
sizes (numbers refer to percentage of fine aggregate passing 600 μm sieve)
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Figure 8.3: - Continued
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Table 8.3: Proportion of Coarse Aggregate Fractions According to the 1988 British Method
10 – 20 mm
20 – 40 mm
Total
Coarse 5 – 10 mm
Aggregate
100
33
67
-
100
18
27
55
Following the above calculations, trial mixes must be made. It should also be remembered that
the British method is based on the experience of British materials so that the various values given
in the tables and figures may not be applicable in other parts of the world.
8.3 Example
{Properties of Concrete by AM Neville, Fourth Edition, Page 768}
With a brief description of what each step of the design process entails, design a concrete mix of
the following requirements using the British Method of Mix Selection:
 28 day mean compressive strength (measured on standard cubes) of 44 MPa,
 Slump of 50mm,
 Uncrushed aggregates with a maximum size of 20mm,
 Specific gravity of aggregate of 2.64
 60% of fine aggregate passes the 600 μm sieve,
 No air entrainment required, and
Ordinary Portland cement to be used
Solution
Step 1
This deals with the compressive strength for the purpose of determining the water cement ratio.
From Table 8.1, for the ordinary Portland cement and uncrushed aggregate, we find 28 day
strength to be 42 MPa. We enter this value on the ordinate corresponding to a water cement ratio
of 0.5 in Figure 8.1; this point is marked A. Through A, we draw a line ‘parallel’ to the nearest
curve until it intersects the ordinate corresponding to the specified strength of 44 MPa; this is
point B. The ordinate through this point gives the water cement ratio of 0.48.
Step 2
This step uses the aggregates type and size, and the required slump to approximate the amount of
water required in the mix.
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From the Table 8.2, for 20mm uncrushed aggregate and a slump of 50mm we find the water
requirement to be 180 kg/m3.
Step 3
This step uses the water cement ratio together with the water content from Step 2 to calculate the
amount of cement that will be required in the mix.
The cement content is 180/0.48 = 375 kg/m3.
Step 4
This step uses the water content and the specific gravity of the aggregates to approximate the
density of the resulting fresh concrete mix, which then is used along with the values of water and
cement to calculate the value of the total mass of aggregates.
From Figure 8.2, for a water content of 180 kg/m3 and aggregate with a specific gravity of 2.64,
we read off the density of the fresh concrete to be 2400 kg/m3.
The total aggregate content is thus:
2400 – 375 – 180 = 1845 kg/m3
Step 5
This step uses the percentage of fines below size 600 μm, the maximum aggregate size, the target
slump, and the water cement ratio to determine the percentage of fine aggregate in the total
aggregate content.
In Figure 8.3, we find the particular diagram for the maximum size of aggregate of 20mm and a
slump encompassing the value of 50mm. On the line representing fine aggregate with 60%
passing the 600 μm sieve, at a water cement ratio of 0.48, the proportion of fine aggregate is 32
per cent (by mass of total aggregate). Hence, the fine aggregate content is
0.32 x 1845 = 590 kg/m3
And the coarse aggregate content is
1845 – 590 = 1255 kg/m3
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9 CORROSION OF STEEL IN CONCRETE
9.1 Introduction
ASTM defines corrosion as the chemical or electrochemical reaction between a material, usually
a metal, and its environment that produces a deterioration of the material and its properties. For
steel embedded in concrete corrosion results in the formation of rust which has two to four times
the volume of the original steel and none of the good mechanical properties. Corrosion also
produces pits or holes in the surface of reinforcing steel, reducing strength capacity as a result of
the reduced cross sectional area.
9.2 Concerns of Corrosion of Steel
Reinforced concrete uses steel to provide the tensile properties that are needed in structural
concrete. It prevents the failure of concrete structures which are subjected to tensile and flexural
stresses due to traffic, winds, dead loads, and thermal cycling, etc. However, when reinforcement
corrodes, the formation of rust leads to a loss of bond between the steel and the concrete and
subsequently de-lamination and spalling. If left unchecked the integrity of the structure can be
affected. Reduction in the cross sectional area of steel reduces its strength capacity. This is
especially detrimental to the performance of tensioned strands in pre-stressed concrete.
9.3 Process of Corrosion of Steel in Concrete
9.3.1
Introduction
Steel in concrete is usually in non corroding, passive condition. However, steel reinforced
concrete is often used in severe environments where sea water or deicing salts are present. When
chloride moves into the concrete, it disrupts the passive layer protecting the steel, causing it to
rust and pit.
Carbonation of concrete is another cause of steel corrosion. When concrete carbonates to the
level of the steel rebar the normally alkaline environment, which prevents steel from corrosion is
replaced by a more neutral environment. Under these conditions the steel is not passive and rapid
corrosion begins. The rate of corrosion due to carbonated concrete cover is slower than chloride
induced corrosion.
Occasionally, a lack of oxygen surrounding the steel rebar will cause the metal to dissolve,
leaving a low pH liquid.
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9.3.2 The corrosion mechanism
For corrosion to occur, four elements must be present; there must beat least two metals (or two
locations on a single metal) at different energy levels, an electrolyte, and a metallic connection.
In reinforced concrete, the rebar may have many separate areas at different energy levels.
Concrete acts as the electrolyte, and the metallic connection is pro-vided by wire ties, chair
supports, or the rebar itself.
Figure 9.1: The expansion of corroding steel creates tensile stresses in the concrete, which can
cause cracking, delamination, and spalling
Corrosion is an electrochemical process involving the flow of charges (electrons and ions).
Figure 9.2 shows a corroding steel bar embedded in concrete. At active sites on the bar, called
anodes, iron atoms lose electrons and move into the surrounding concrete as ferrous ions. This
process is called a half-cell oxidation reaction, or the anodic reaction, and is represented as:
Figure 9.2: When reinforcing steel corrodes, electrons flow through the bar and ions flow
through the concret
The electrons remain in the bar and flow to sites called cathodes, where they combine with water
and oxygen in the concrete. The reaction at the cathode is called a reduction reaction. A common
reduction reaction is:
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To maintain electrical neutrality, the ferrous ions migrate through the concrete pore water to
these cathodic sites where they combine to form iron hydroxides, or rust
This initial precipitated hydroxide tends to react further with oxygen to form higher oxides. The
increases in volume as the reaction products react further with dissolved oxygen leads to internal
stress within the concrete that may be sufficient to cause cracking and spalling of the concrete
cover.
The corrosion of steel reinforcement inside a concrete structure is undesirable in the following
ways:
(i) The presence of rust impairs the bond strength of deformed reinforcement because corrosion
occurs at the raised ribs and fills the gap between ribs, thus evening out the original deformed
shape. In essence, the bond between concrete and deformed bars originates from the mechanical
lock between the raised ribs and concrete. The reduction of mechanical locks by corrosion results
in the decline in bond strength with concrete.
(ii) The presence of corrosion reduces the effective cross sectional area of the steel
reinforcement. Hence, the available tensile capacity of steel reinforcement is reduced by a
considerable reduction in the cross sectional area.
(iii) The corrosion products occupy about 3 times the original volume of steel from which it is
formed. Such drastic increase in volume generates significant bursting forces in the vicinity of
steel reinforcement. Consequently, cracks are formed along the steel reinforcement when the
tensile strength of concrete is exceeded.
Corrosion problems in concrete have led researchers to investigate the viability of new
corrosion-free reinforcement, such as fiber-reinforced polymer (FRP) bars. Those bars are
expensive, but if long-term costs are taken into account, the economic picture changes
dramatically (Balafas 2003; Burgoyne 2004).
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Various methods of limiting rust have been suggested, among them use of vapour phase
corrosion inhibitors and rust convertors.
9.3.3
Corrosion Prevention
The first line of defense against corrosion is use of quality concrete and concreting practices,
including provision of sufficient cover to the reinforcement. Quality concrete has a water/cement
ratio low enough to slow down the penetration of chloride salts and the development of
carbonation. The water/cement ratio should be less than 0.5 to slow the rate of carbonation and
less than 0.4 to minimize chloride penetration.
Air entrainment can also be used to produce good quality concrete. This reduces bleeding and the
corresponding increased permeability due to the bleed channels. Spalling and scalling can
accelerate corrosion damage of the embedded reinforcement bars.
Correct amount of steel will keep cracks tight. In general, the maximum allowable crack widths
are 0.175 mm in deicing salt environments and 0.15 mm in marine environments.
Provision of adequate cover is also essential. Chloride penetration and carbonation will occur in
the outer surface of even low permeability concretes. Increasing cover will delay the onset of
corrosion.
The concrete must be adequately consolidated and cured. Moist curing for a minimum of seven
days is needed for concrete with a 0.4 water/cement ratio, whereas six months is needed for a 0.6
water/cement ratio.
Additives can also be used to increase resistance to corrosion. Silica fume, fly ash, and blast
furnace slag reduce the permeability of the concrete to the penetration of chloride ions. Water
repellents may also be used to control ingress of corrosive elements.
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10 EFFECTS OF FIRE ON CONCRETE
10.1 Introduction
To understand the effects of fire on concrete, the effect of fire will be looked at independently for
the different constituents of concrete.
10.2 Effects of High Temperature on Hardened Cement Paste





Loss of evaporable water starting at around 105oC
Decomposition of C-S-H to produce dicalcium silicate (C2S) and water
Dehydration of calcium hydroxide (CH) to produce calcium oxide (CaO) and water, at 500 –
600oC
Increase in porosity
Destruction of C-S-H at 900oC
10.3 Effects of High Temperatures on Aggregates




Aggregates subjected to heat during their formation perform better than siliceous aggregates
Siliceous aggregates contain quartz which undergoes endothermic crystalline transformation
as temperature increases
Carbonate aggregates calcine from 600-900oC during fire driving off CO2.
Most aggregates expand at high temperatures
10.4 Performance of Concrete in Fires



Different expansion rates of hardened cement paste and aggregates
Thermal gradient between inner and outer layers of concrete (especially during rapid heating)
Building up of vapor pressure in pores
10.5 Factors Infliencing Fire Resistance of Concrete
With consideration of the above therefore, the fire resistance of concrete is influenced by:
 Tensile strength – the higher the strength the better the concrete will be able to resist stresses
arising from differential expansion and pore pressures
 Aggregate type – some aggregates perform better than others as discussed above
 Moisture content – the more moisture the concrete has the worse the effects of heating will
be
 Permeability of the concrete – can assist escape of building up pore pressures thereby
reducing the negative effects of heating
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Density of the concrete
Thickness of the elements – thicker elements are more prone to excessive stresses caused by
differential thermal expansion
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