UNIT-1
TRADITIONAL BUILDING MATERIALS
CONCRETE
INTRODUCTION
• Composite man made material.
• Most widely used.
• Consists of rationally chosen mixture of binding material such as lime or cement,
well graded fine & coarse aggregates, water.
• Mix of sand water & cement called as matrix in concrete.
• Freshly mixed concrete is called as green concrete.
• After setting is called as set or hardened concrete.
• Major factors responsible for using cement concrete are mouldability, early
hardening, high early compressive strength, pumpability & durability.
• Versatile in nature.
• Homogenous mixture.
• The coarse aggregate acts as filler.
• The fine aggregate fills up the voids between the paste & coarse aggregate.
• The cement in conjunction with water acts as a binder.
• The mobility of mixture is aided by the cement paste, fines & now a days by
use of admixtures.
• The aim of quality control is to ensure the production of concrete of uniform
strength from batch to batch.
CLASSIFICATION ON THE BASIS OF: Cementing material
• Lime concrete
• Gypsum concrete
• Cement concrete
 Perspective specifications
• Mix proportions
 Performance oriented specifications
• Design mix concrete
 Grade of cement concrete
• Compressive strength of concrete cubes (150 mm )at 28 days
• Also classified as low strength (<20 N/mm2), medium strength (20-40 N/mm2), high
strength concrete (>40 N/mm2)
 Bulk density
• Heavy
• Dense
• Lightweight
• Extra lightweight
 Place of casting
• In situ
• precast
PRODUCTION
• The stages of concrete production are: Batching or measurement of materials.
 Mixing
 Transporting
 Placing
 Compaction
 Curing
 Finishing
BATCHING
•
For good quality concrete, proper & accurate quantity of all ingredients should be used.
•
Two methods of batching:-
 Volume batching
 Small jobs
 Guage box
 Weigh batching
 Important works
 Manual weighing or weigh batchers
 Automatic weigh batchers
MIXING
•
Objective of mixing is to make concrete mass homogenous & uniform in colour & conistency.
•
Either by hand or mixer.
 Hand mixing
 Small jobs
 On an impervious floor
 10 % of cement is added more to the mix
 Machine mixing
 Important & quality works
 Batch mixers & continuous mixers
 Batch mixers produce batch by batch with time intervals & used for small 7 medium
sized works.
 Continuous mixers produce concrete continuously & are used for large works like dams.
MIXERS
TRANSPORTING
 Should be transported at the earliest without the loss of homogeneity obtained at the time of
mixing.
 Segregation should not take place during transportation & placement.
 Methods of transportation:• Mortar pan
• Wheel barrow
• Chutes
• Dumper
• Bucket & ropeway
• Belt conveyor
• Skip & hoist
• Pumping
PLACING
• Concrete is placed on form works.
• The form works should be cleaned.
• If concrete is to be placed for foundation, the soil bed should be compacted
well & is made free from loose soil.
• Concrete should be dropped on its final position as closely as possible.
• If dropped from a height, the coarse aggregates fall early & then mortar
matrix.
• This results as segregation into weaker concrete.
COMPACTION
 In the process of placing concrete, air is entrapped.
 The entrapped air reduces the strength of concrete by 30 %.
 Hence it is necessary to remove the air which is achieved by compaction.
 Either by hand or by vibrators.
 In hand compaction method, concrete is compacted by ramming, tamping or spading.
 Concrete can be compacted by using high frequency vibrators. Vibration reduces the
friction between the particles& set the motion of particles.
 As a result entrapped air is removed & concrete is compacted. W/C ratio can be reduced
with vibrators.
 Needle or immersion vibrators
 Surface vibrators
 Form or shutter vibrators
 Vibrating tables
VIBRATORS
CURING
•
Curing may be defined as the process of maintaining satisfactory moisture & temperature
conditions for freshly placed concrete for some specified time for proper hardening of
concrete.
•
Curing in the early ages is more important.
•
Curing for 14 days is very important.
•
If curing is not done properly , strength & durability of concrete reduces.
•
Cracks develop due to shrinkage.
•
Various methods of curing are: Spraying of water :- walls, columns etc.,
 Gunny bags :- vertical surfaces
 Ponding:- slab & floors by stagnating water.
 Steam curing:- prefabricated units, steam is passed in closed chambers, accelerates
curing process.
 Curing compounds like calcium chloride is also used.
CURING
FINISHING
 To give a uniform surface.
PROPERTIES OF FRESH CONCRETE
•
Concrete has completely different properties when it is in the plastic stage & when
hardened.
•
In plastic stage it is also known as green concrete.
•
Properties of green concrete include: Workability:- ease with which concrete can be fully compacted without segregation &
bleeding. Depends on quantity of water, grading, shape & percentage of aggregates
present.
 Segregation:- separation of coarse particles in green concrete is called segregation.
Happens due to deficient quantity of fine particles or throwing concrete from grater
heights. Cohesiveness losts & honey combing results. Ultimately, loss in strength.
 Bleeding:- appearance of water along with cement particles on the surface. Happens
due to excessive quantity of water or due to excessive compaction. Results in pores
& weak concrete.
 Harshness:- resistance offered by concrete to its surface finish. Difficult to get a
smooth surface finish & concrete becomes porous. Happens due to poorly graded
aggregates or less fine aggregate or less cement mortar.
PROPERTIES OF HARDENDED CONCRETE
•
Strength:- compressive strength of 150 mm cubes at 28 days.M20 is minimum grade to be
used.
•
Resistance to wear:-
•
Dimensional changes:- concrete shrinks with age. Approximately 0.0003 of its original.
Permanent dimension change due to loading over a long period is termed as creep.
•
Durability:- resistance to weathering, chemical attacks, heat, freezing, thawing.
•
Impermeability:- resistance of concrete to the flow of water through its pores. Excess
water results into pores.
WATER CEMENT RATIO (W/C RATIO)
•
The water-cement ratio (w/c) is one of the major factors influencing the strength of
concrete.
•
It is responsible mainly for the porosity of the hardened cement paste.
•
Thus theoretically lower the w/c ratio means higher compressive strength as less voids
are created.
Definition:
•
Water-cement ratio is the water used to the quantum of cement in the mixture by weight.
•
For proper workability the w/c ratio varies from 0.4 – 0.6
• However, theoretical maximum strength is derived at
w/c = 0.4 at which minimum capillary cavities are expected to
form.
• It may be noted that for complete hydration of cement
under controlled conditions the water requirement is
about 38 per cent. (i.e. w/c = 0.38)
• When it is decreased to less than 0.4 there is improper
consistency and workability of cement and honeycombed
structure may result.
• Also, at w/c ratio more than 0.6, porosity increases
and strength decreases.
Are there any Exceptions:
• However, concrete compacted by vibrator displays
higher strength even up to w/c = 0.3.
ABHRAM LAW
•
Duff Abrahm gave the following equation to estimate the strength of concrete for a given w/c
ratio.
where,
S = Strength of cement at 28 days and A, B are constants
x = Water to cement ratio (w/c)
•
According to Abrahm’s law it is evident that strength of concrete depends only upon w/c
ratio provided the mix is workable.
TESTS ON CONCRETE
•
The tests on concrete can be divided on the following lines:
• Tests on Fresh concrete (wet concrete)
• WORKABILITY TEST
• Tests on Hardened concrete
For hardened concrete the most important tests are the assessment of strength of concrete,
which can be assessed by the following tests.
• COMPRESSION TEST
• FLEXURE TEST
• SPLIT TENSILE STRENGTH TEST
• NON DESTRUCTIVE TEST
WORKABILITY TEST
•
Measurement of workability is done by the following tests:
• Slump cone test
• Compaction factor test
• Vee-Bee Consistometer test
• Kelly ball test
• Flow table test
SLUMP CONE TEST
• Slump tests in one of the most extensively used test all over the world.
• Dimensions of the mould are bottom diameter = 200 mm, top diameter = 100
mm and height = 300 mm
• Mould is filled in with fresh concrete in four layers, each layer of approximately one
quarter of the height of the mould and tamped with 25 strokes of the rounded end of
the tamping rod.
• Strokes are distributed in a uniform manner over the cross-section
• After the top layer has been rodded, the concrete is struck off level with a trowel or
the tamping rod, such that the mould is exactly filled.
• Mould is remove immediately by raising it slowly and carefully in a vertical direction.
Then the concrete is allowed to subsidized and the slump is measured immediately
by determining the difference between the height of the mould and the highest point
of the specimen being tested.
• Slump measured is recorded in terms of millimetres of subsidence of the specimen.
APPARATUS
TYPES OF SLUMP
TYPES OF SLUMP FOR VARIOUS WORKS
COMPACTING FACTOR TEST
• This test is more accurate and sensitive than the slump test
especially for it is useful for concrete mixes of medium and low
workability.
• Here the workability is measured in terms of compaction factor
(0.4 , 0.8, 0.9)
• Concrete of very low workability (0.7 or below), this test is NOT
APPLICABLE
• It is primarily designed for laboratory work but can also be
used in the field.
COMPACTING FACTOR APPARATUS
PROCEDURE
• Sample of concrete to be tested is placed gently in the upper hopper,
and levelled.
• Trap-door is then opened to allow the concrete to fall into the lower
hopper.
• Concrete which has sticked in the upper hopper at sides is gently
pushed into lower one.
• The trap-door of the lower hopper is opened so that the concrete falls in
the cylinder.
• The excess of concrete remaining above the level of the top of the
cylinder should be cut and removed.
• Weight of the concrete in the cylinder is then determined, which is
known as weight of partially compacted concrete.
• The entire concrete is filled in cylinder and tamped with tamping rod, and
the weight of concrete in the cylinder is then determined, which is known
as weight of fully compacted concrete.
• Thus compacting factor is defined as the ratio of “weight of
partially compacted concrete to the weight of fully compacted
concrete”
• Compacting factor values for concrete are as follows:
• Higher the compacting factor, Higher the workability of
concrete.
VEE-BEE CONSISTOMETER TEST
• This test determines the time required for transforming, by
vibration a concrete specimen in the shape of a conical frustum
into a cylinder.
• Apparatus consists of a vibrator table resting upon elastic
supports, a metal pot, a sheet metal cone, open at both ends, and
a standard iron rod.
• Slump test is performed in the cylindrical pot of the consistometer
which is a good laboratory test [ONLY] to measure indirectly the
workability of concrete.
• The Slump cone is placed in the cylindrical pot, and slump is
noted. Then the electrical vibrator is switched on and the TIME
TAKEN for the concrete to spread out in the cylindrical pot is
noted in seconds and workability is measured in VEE-BEE
degree.
APPARATUS
COMPARISON OF VALUES OF SLUMP CONE, VEE-BEE,
COMPACTION FACTOR TEST
PICTORIAL REPRESENTATION OF
PROCEDURE FOR FLOW TABLE TEST
Equipment for the test Flow table, Slump cone
The cone filled with concrete, prior
to lifting.
The diameter of the resulting flow
is measured
COMPRESSIVE STRENGTH TEST
• Cement, fine aggregate and Coarse aggregate (upto 38mm) to be
used for making concrete are weighed in the required ratio to be
used in field and are thoroughly mixed, by adding requisite
amount of water until the concrete appears homogeneous.
• The test SPCIMENS are cast in the required sizes of cubes,
150mm x 150mm x 150mm or cylinders of 150mm diameter and
300mm height. [D/H = 1/2]
• Test specimens are stored at room temperature for 24hrs from the
time of addition of water to dry ingredients.
• After this time specimens are removed from the moulds and
placed in water and kept there until taken out just before the test.
• Usually specimens are tested for 7 days or 28 days strength, but
IS: 456 suggests only 28 days strength.
• Specimen is placed between the plates of compression testing
machine, gradually load is applied at the rate of
14 N/mm2/minute, until the specimen is crushed.
• Average of 3 specimen values is taken as the compressive
strength of concrete, provided individual variation is not more than
+/- 15% of the average.
• Generally the Cube specimen strength is approximately equal to
1.25 times the Cylindrical specimen strength. [This is due to the
influence of size of the specimen]
FLEXURAL STRENGTH TEST
• Flexural tensile strength test is done to determine the tensile load
at which concrete may crack.
• It is an indirect test for assessing the tensile strength of concrete.
• The size of concrete is 150mm x 150mm x 700mm.
• The specimen is placed in the testing machine on two 38mm
diameter rollers with a c/c distance of 600mm. The load is applied
through two similar rollers mounted at the third points, spaced at
200mm c/c.
• The maximum load at which the specimen fails is noted and from
basics of strength of material, the flexural strength is estimated.
TENSILE STRENGTH TEST
• The tensile strength may be determined by by split tensile strength
test.
• As it is practically very difficult to apply uniaxial tensile load, therefore
few indirect methods are developed to determine tensile strength of
concrete. Example is the split tensile strength test.
• In split tensile strength test a compressive force is applied to the
specimen such that specimen fails due to induced tensile stresses.
• Specimen is made of cylindrical shape with diameter not less than
150mm. Length is generally twice the diameter.
• The maximum load at which the specimen fails is recorded and from it
indirectly the tensile strength of specimen is calculated.
NON DESTRUCTIVE TESTING
• Non destructive testing (NDT) can be done on both fresh
concrete and hardened concrete.
• On fresh concrete Ultrasonic pulse wave test [PUNDIT] can
be done.
• On hardened concrete Pull Out test, Ultrasonic pulse wave
test [PUNDIT], Schmidt rebound hammer test, Radioactive
Methods.
REBOUND HAMMER TEST
• It is a surface hardness test for which an empirical correlation has
been established between strength and rebound number.
• It is based on the principle that the rebound of an elastic mass
depends on the hardness of the surface against which the mass
impinges.
Procedure:
• For this test, a rebound hammer [easily carried] also called
SCHMIDT HAMMER, which weighs about 1.8kg is required and
the test is suitable for both laboratory and field work.
• The Schmidt hammer has a spring-controlled hammer mass that
slides on plunger with a tubular casing.
• The hammer is forced against the surface of the concrete by the
spring and the distance of rebound is measured on a scale of the
instrument which gives indication of concrete strength.
• This test is suitable for the concrete having strength in the range
of 20 – 26 Mpa.
Limitations:
• Results are mostly affected by factors such as smoothness of
surface, size and shape of specimen, moisture condition of
the concrete, type of cement & coarse aggregate and extent of
carbonation of surface.
PICTORIAL REPRESENTATION OF
SCHMIDT REBOUND HAMMER
PICTORIAL REPRESENTATION OF REBOUND
HAMMER TEST
FIGURE OF SCHMIDT REBOUND
HAMMER TEST BEING CONDUCTED
TYPES OF CONCRETE
• The conventional cement concretes are commonly used for
structures in normal environmental conditions. With the
advancement of technology and pressing demands of better
mechanical properties and durability than the conventional
ones as well as improvements in selected properties of
interest has lead to the development of special cement
concretes.
• Following are some of the various types of Concrete used
today in the industrial and residential construction.
TYPES OF CONCRETE
• Reinforced cement concrete
• Prestressed concrete
• Fibre reinforced concrete [FRC]
• Light Weight concrete
• High strength concrete
• Ready Mix concrete
• Self Compacting concrete
• Shotcrete
REINFORCED CEMENT CONCRETE (RCC)
• Reinforced cement concrete is a composite material made up of
cement concrete and reinforcement in which the concrete resists
compression with reinforcement resisting the tension and shear. It
is the most versatile building material available and is extensively used
in the construction industry ranging from small structural elements such
as beams and columns to massive structures like dams and bridges.
Why is the purpose of the reinforcement provided in the RCC ?
• The steel bars are embedded in the tensile zone of concrete to
compensate the poor tensile resistance of concrete. The bond between
steel and the surrounding concrete ensures strain compatibility.
• Moreover, the reinforcing steel imparts ductility to this composite material.
• The reinforcing steel also supplements concrete in bearing compressive
forces, as in the case of columns.
PRE-STRESSED CONCRETE
• One of the serious limitation of reinforced cement concrete is the
cracking which is a natural phenomenon for concrete constructions.
• Once cracks occur they do not disappear even after removal of load. Ok,
so what are the issues with the presence of cracks ??
• Presence of cracks lowers the capacity of structure to bear reversal of
stresses, impact vibration and shocks.
• Also, the reinforcing bars may get corroded in due course of time and the
concrete deteriorates.
• Besides these disadvantages, the presence of cracks makes theory of
reinforced concrete quite cumbersome. Efforts were made to eliminate the
cracking of concrete by artificially introducing in it either before or
simultaneously with the application of external loads, a compressive force of
permanent nature. This force is so applied that it causes compressive stresses
in that zone of the member where tensile stress will be caused by external
loads. The tensile stress in concrete will thus be neutralized and it will not
crack.
• A prestressed concrete may thus be defined as a concrete in
which stresses of suitable magnitude and distribution are
introduced to counteract, to a desired degree, the stresses
resulting from external loads.
OK, the question is can we use the same grade of Steel and
Concrete as used in RCC ???
• In prestressed concrete high strength concrete and steel are
desirable. The former is required because of following:
1)
2)
3)
The use of high strength concrete results in smaller cross-section
of member and hence smaller self weight; longer spans become
technically and economically practicable.
High bearing stresses are generated in anchorage zones, thus we
need very high grade of concrete to resist these stresses.
The shrinkage cracks are reduced, with higher modulus of elasticity
and smaller creep strain resulting in smaller loss of prestress.
• The loss of prestress at the initial stages is very high and for this reason
high strength steel is required. High tensile strength wires with ultimate
tensile strength up to 2010 N/mm2 are the choice. For prestressed
concrete members, the high tensile steel used generally consists of bars
or strands.
• Prestressing is achieved by either pre-tensioning or post-tensioning.
• In the former the wires or cables are anchored, tensioned and concrete is
cast in the moulds. After the concrete has gained strength the wires are
released. This sets up compression in concrete which counteracts tension
in concrete because of bending in the member.
• In the post-tensioning prestressing force is applied to the steel bars or
cables, after the concrete has hardened sufficiently. After applying the full
prestress the cable passages are grouted. The minimum 28-day cube
compressive strength for concrete is 40 N/mm2 [M40 grade] for
pre-tensioned members and 30 N/mm2 [M30 grade] for
post-tensioned members.
ADVANTAGES OF PRESTRESSED CONCRETE
1)
The cracking of concrete is eliminated enabling the entire crosssection of the member to take part in resisting moment.
2)
As dead load moments are neutralized and the shear stresses
are reduced, the sections required are much smaller than those for
reinforced concrete. This reduces the dead weight of structure.
3)
In ordinary reinforced concrete (RCC) the economy is not as
pronounced as in prestressed concrete (PSC).
USES OF PRESTRESSED CONCRETE
• It is widely used for construction of precast units such as beams,
floors, roofing systems, bridges, folded plate roofs, marine structures,
towers and railway sleepers.
FIBRE REINFORCED CONCRETE [FRC]
• Conventional concrete is modified by random dispersal of short
discrete fine fibers of asbestos, steel, sisal, glass, carbon, polypropylene, nylon, natural fibres etc.,.
• The main role of fine fibers is to bridge the cracks that develop in
concrete and increase the ductility of concrete elements. Also imparts
more resistance to impact load.
• The improvement in structural performance depends on the strength
characteristics, volume, spacing, dispersion and orientation, shape and
their aspect ratio (ratio of length to diameter) of fibres.
• A fibre-reinforced concrete requires a considerably greater amount of
fine aggregate than that for conventional concrete for convenient
handling.
• For FRC to be fully effective, each fibre needs to be fully embedded in
the matrix, thus the cement paste requirement is more.
ADVANTAGES / DIS-ADVANTAGES / APPLICATIONS
Advantages:
1)
Strength of concrete increases.
2)
Fibres help to reduce cracking and permit the use of thin concrete sections.
3)
Mix becomes cohesive and possibilities of segregation are reduced.
4)
Ductility, impact resistance, tensile and bending strength are improved.
Disadvantages:
1)
Fibres reduce the workability of a mix and may cause the entrainment of air.
2)
Steel fibres tend to intermesh and form balls during mixing of concrete.
Applications:
Fibre reinforced concrete is useful in hydraulic structures, airfield pavements, highways, bridge
decks, heavy duty floors, and tunnel linings.
ACTION OF FRC
• The tensile cracking strain of cement matrix is about 1/50 of that
of yield of steel fibres. Consequently when FRC is loaded, the
matrix [CEMENT CONCRETE MATRIX] cracks long before the
fibres are fractured.
• Once the matrix is cracked the composites continue to carry
increasing tensile stress, provided the pullout resistance of fibres
at the first crack is greater than the load at the first cracking.
•
The bond or the pullout resistance of the fibres depends on the average bond strength between the fibres
and the matrix, the number of fibres crossing the crack, the length and diameter of fibres, and the aspect
ratio.
• The first flexural cracking load on a FRC member increases
[COMPARED TO ORDINARY RCC/PCC] due to crack arresting
mechanism of the closely spaced fibres. After the first crack fibres
continue to take load provided the bond is good. Thereafter the
fibres, reaching the breaking strain fracture.
LIGHT WEIGHT CONCRETE
• Conventional cement concrete is a heavy building material.
• For structures such as multistorey buildings it is desirable to reduce
the dead loads. Light weight concrete (LWC) is most suitable for such
construction works. Lightweight aggregate concrete is particularly
suitable for use where low density, good thermal insulation or fire
protection are required
• It can be obtained by anyone of the following methods:
• By making concrete with cement and coarse aggregate only. Sometimes
such a concrete is referred to as no-fines concrete. Suitable aggregates
are — natural aggregate, blast furnace slag, clinker, foamed slag, etc.
Since fine aggregates are not used, voids will be created and the concrete
produced will be light weight.
• By replacing coarse aggregate by porous or cellular aggregate. The
concrete thus produced is called CELLULAR CONCRETE.
• Types of cellular concrete are Foam concrete, gas concrete etc.,.
• Among the main shortcomings of cellular concrete are high tendency to
deformation, shrinkage.
FOAM / GAS CONCRETE
Foam Concrete:
• It is obtained by mixing cement paste or mortar with stabilized foam.
After hardening, the foam cells form concrete of a cellular structure.
• The foam is obtained by stirring a mixture of resin soap and animal glue.
The best foaming agents are alumino sulpho napthene compounds and
hydrolysed slaughter blood.
• This concrete is very suitable for heat insulation purposes.
Gas Concrete:
• It is manufactured by expanding the binding material paste, which may
or may not include aggregates. It is also known as aerated concrete.
• The approximate relative proportions of gas concrete ingredients are as
follows: 90% Portland cement, 9.75% powdered lime, 0.25% aluminium
power (for a water to cement ratio of 0.55– 0.65). About 2/3 of sand are
ground in a wet state.
HIGH STRENGTH CONCRETE (HSC)
• For mix made with normal weight aggregates, high strength concrete
(HSC) is considered to be the one having a compressive strength in
excess of 40 MPa.
• To produce concrete above this strength more stringent quality control
and more care in selection and proportioning of materials are needed.
• The tricalcium aluminate component of cement is kept as low as
possible (<8%) [REASON HIGH HEAT OF HYDRATION AND LATER IT
MIGHT LEAD TO SHRINKAGE CRACKS].
• Most cements used to produce HSC have fineness in the range of 300
– 400 m2/kg. [THAT MEANS WE WANT VERY FINE CEMENT]
• For HSC a smaller maximum size of coarse aggregate leads to
higher strength, however for Fine aggregate it should have a
F.M > 3.
APPLICATIONS OF HSC
•
The use of the highest possible strength concrete and minimum steel offers the most
economical solution for columns of high rise buildings.
•
This clearly demonstrates the economy of using HSC in multistorey buildings.
•
So far, for industrial application, HSCs are limited to structural members that are not
exposed to freeze thaw cycles.
•
Further, superplasticized, low w/c ratio HSC containing high cement content and a good
quality puzzolana has a great potential of use where impermeability or durability, not
strength, is the main consideration.
• Such applications include floors in the chemical and food industry, and bridge deck
overlays that are subject to severe chemical and physical processes of degradation.
READY MIX CONCRETE (RMC)
• Ready mixed concrete (RMC) is a concrete, delivered at site or into the
purchaser’s vehicle, in plastic condition and requires no further
treatment before being placed in a position in which it is to set and
harden.
• It is a high quality concrete of required grade produced under strictly
controlled conditions in a centralized automatic batching plant and
supplied to the customer in a transit mixer truck for its placement at
site. The concrete is mixed at the batching plant, loaded into agitator
truck mixers and water added during transportation and transported to
the site.
• Use of RMC to its full advantage requires more careful planning on the
site as compared to the site mixing. Due to better quality control
measures adopted, RMC can be considered to be almost a factory-made
product.
• It is advantageous not only for mass concreting but also for
small quantities of concrete to be placed at intervals. RMC is
extremely useful on congested sites or in road construction where
limited space is available for aggregate stock piling and mixing
plant.
• The major set back to the use of RMC is its cost. However,
though a little bit expensive, the increasing emphasis on quality,
with skilled labour becoming expensive, out weigh the cost issue
of RMC.
• Quality of RMC is generally specified in terms of performance
parameters, i.e., purchaser specifies the strength level and
intended use.
ADVANTAGES OF RMC
1)
Enhanced quality and durability resulting in lower maintenance costs and
increased speed of construction. Ready mix concrete is consistently of the
same quality and provides a high quality of construction material;
construction time is also reduced.
2)
It is an environmentally safer alternative.
3)
Convenience — Ready Mix Concrete is delivered at the site with minimum
logistical hassles.
4)
Different types of concretes can be made for different applications.
5)
Use of RMC obviates the need to set up the infrastructure required for
site manufactures of concrete [LIKE BATCHING PLANTS]. This also
reduces the working capital to be invested by the customers, as they will not
be required to maintain stock of aggregates, cement, plant and machinery
etc.
SELF COMPACTING CONCRETE (SCC)
• Self-Compacting concrete (SSC) is a very special type of concrete
which can flow and fill into every corner of formwork, even in the
presence of congested reinforcement, purely by means of its own
weight and without the need of vibrating compaction, tamping etc.
• Self-Compacting concrete as it sounds is nothing different from normal
concrete. It is just usage of extra admixtures (super plasticizers and
viscosity modifying admixtures) that makes SSC act different to
normal one.
• In SSC, high amount of supplementary cementitious materials, up to
70% of the total powder content, are added. Normally these
supplementary materials are fly ash, silica flume, blast furnace slag etc.
• Since SSC does not require any compaction, it saves time, labour and
energy. Also, good surface finish is produced.
• Self-Compacting concrete is characterized by high powder
content. The parameter that is important in SSC is waterpowder ratio, water-cement ratio is completely ignored. Other
important parameters are fly ash content, sand-aggregate
ratio, paste percentage, types of admixture used. etc. The
aggregate content in SSC is smaller than that for
conventional concrete requiring vibration.
• On the other hand, the viscosity of the paste in SSC is
highest among the various types of concrete due to is lowest
water-powder ratio. This characteristic is important in
inhibiting segregation.
• The method for achieving self-compactability involves not only high
deformability of paste or mortar, but also resistance to segregation
between coarse aggregate and mortar when the concrete flows through
the confined zone of reinforcing bars.
• To achieve selfcompactability, the aggregate content are limited, the
water-powder ratio is kept low, and super plasticizers are used.
Why is it used:
• The main reason for the employment of SCC are the shortened
construction period, assured compaction in the structural elements;
especially in confined zones where vibrating compaction is difficult
and, to eliminate noise due to vibration; effective especially at products
plants.
• SSC does not require any compaction, it saves time, labour and energy.
Also, good surface finish is produced.
BEAM COLUMN JOINT
CHARACTERISTICS OF SCC
• Non-Segregating: The aggregate stay in suspension in the mix as it flows into
the form. [STRENGTH IS SECONDARY IN THIS CASE, BUT IMPORTANT]
• Non-Bleeding: Water does not rise to the top of the mix or is observed on the
outer edges of a flow test.
• Vibration: No vibration is required during placement. SCC flows around rebar
and other inclusions in the form under its own weight.
• Flow spread: Flow spreads of 45 cm diameter or grater are obtainable.
• Set time: The initial set time in many SCC mixes increase upwards of 90
minutes, depending on the admixtures used and water content of the mix.
• Workability: Workability of Self-Compacting concrete is equilibrium of its
fluidity, deformability and resistance to segregation and filling ability. This
equilibrium has to be maintained for a sufficient time period to allow for its
transportation, placing and finishing.
SHOTCRETE
• Shotcrete is the concrete conveyed through a hose and pneumatically
projected at a high velocity on a surface. It is similar to gunite (mortar) but
with coarse aggregates.
• The normal specifications with respect to cement, aggregate and water
also apply to shotcrete but the coarse aggregate used should be harder to
account for attrition and of small size. The w/c ratio is kept quite low. The
admixtures such as accelerators are used to permit quick setting of
shotcrete.
• The high cost of shotcrete and the wastage due to rebound has to be
weighed with other techniques before recommending it. It is also to be
remembered that it is not eco friendly as lot of dusting problem and waste
due to rebound is there.
ADMIXTURES
• Admixtures are additives which are added to concrete mix at the
mixing stage to modify the properties of fresh and hardened
concrete.
• It is either chemical (liquid) or mineral (fine granular)
• However admixtures should never be regarded as a substitute for
good mix design, good workmanship and use of good materials.
• The uses of admixtures include:
• Increase workability without changing w/c ratio or reduce w/c ratio
without chaning workability.
• To decrease density, reduce segregation and bleeding, improve
pumpability, accelerate initial setting time, increase strength and rate of
gain of strength etc.,.
CLASSIFICATION OF ADMIXTURES
• Admixtures are normally categorised according to their
effect produced on concrete:
• Plasticizers (water – reducing agents)
• Super Plasticizers (high range water reducers)
• Air entertainers
• Accelerators
• Retarders
Many admixtures provide for the combination of the above effects.
PLASTICIZERS
• Chemicals to improve plasticity in fresh concrete; for improving
workability (for a given w/c ratio) to facilitate placement of
concrete in location that are not easily accesible or these are
mainly for achieving higher strength by reducing w/c ratio.

Examples include: Lingnosulphonic acids and their salts (eg. Ca, Na,
Ammonium salts); Hydroxylated caboxylic acids and their salts.
Uses:
• Plasticizers usually increase the slump of concrete with a given water
content.
• Plasticizers can reduce the water requirement of a concrete mix for a
given workability. (approximatley 10 %)
SUPER PLASTICIZERS
• These admixtures are chemically distinct from normal plasticizers
and although their action is basically the same, it is more marked.
• When the are used they produce flowing concrete and a rapid loss
of workability can be expected and therefore they should be
added just prior to placing.
• Finer the cement higher is the super plasticizer dose.
• Examples are sulphonated melamine formaldehyde condensates,
napthalene sulphonate fomaldehyde condensates, Modified
lignosulphonate (MLS) and mixture of saccharates and acid
amines.
• It is capable of reducing water requirement by 20 – 40 %
AIR ENTRAINERS
•
An air entraining agent introduces air in the form of minute bubbles that occupy upto 5% of
volume of concrete distributed uniformly throughout the cement paste.
•
Examples include salts of wood resins, animal or vegetable fats & oils [like stearic acid and
oleic acid] and sulphonated hydrocarbons.
•
Air entrainment however may reduce the strength of concrete and overdosing can cause
major loss of strength. [REMEMEBER THE ISSUES OF VOIDS]
Uses:
•
Where improved resistance of hardened concrete to damage from freezing and thawing is
required.
•
For improved workability, especially in harsh or lean mixes.
•
To reduce bleeding and segregation, especially when a mix lacks fines.
ACCELERATORS
• These admixtures (notably Calcium chloride) speed up the
chemical reaction between cement and water, thus accelerating
the setting time or early gain in strength of concrete.
• Accelerator calcium chloride (CaCl2) can be used upto amounts of
2% by mass of cement. If used in higher quantities there is
possibility of high shrinkage leading to cracks in concrete.
Uses:
• Where rapid setting and high early strengths are required
• Where faster removal of formwork is required.
Practical limitation:
• All chloride based accelerators cause corrosion of steel and thus
should not be used in RCC works.
RETARDERS
•
These admixtures slow or prolong the chemical reaction of the cement and water leading to
longer setting times and slower initial strength gains.
•
The most common retarders are hydroxylated carboxylic acids, lignins, sugar and some
phosphates.
Uses:
•
When placing concrete in hot weather
•
In concrete which has to be transported for a long time.
Practical limitations:
 Retarders often increase plastic shrinkage and plastic settlement cracking.
 Delayed addition of retarders can result in extended retardation.