CHAPTER 3 DEFINITION AND THEORY3.1 INTRODUCTION The chapter
gives the description about theory part of the project. The main
factors related to the project are described here, such as HSC,
workability, curing and its importance, gain in strength,
Superplasticizers along its working mechanisms, PPC and its
function. Concrete is classified as a Normal Strength Concrete
(NSC), High Strength Concrete (HSC) and Ultra High Strength
Concrete (UHSC). There is no clear cut boundary for the above
classification. Indian Standard Recommended Methods of mix design
denotes the boundary at 35 MPa between NSC and HSC.3.2 HIGH
STRENGTH CONCRETEThe methods and technology for producing HSC are
not basically different from those required for concrete of normal
grade except that the emphasis on quality control is greater with
HSC. HSC can be produced with all of the cements including Portland
cement, sulfate-resisting Portland cement and combinations with
pulverized fuel ash and ground granulated blast furnace, silica
fume slag. There are special methods of making high strength
concrete. They are given below a) Revibrationb) High speed slurry
mixingc) Use of admixtured) Inhabition of crackse) Sulphur
impregnationf) Use of cementitious aggregate.g) Seeding.High early
strength cements should preferably be avoided as a rapid rise in
hydration temperature may cause problems of (internal) cracks or
micro-cracks due to the higher cementitious material content. HSC
can be produced with a wide range of aggregates, but smooth and/or
rounded aggregates may tend to exhibit aggregate bond failure at a
relatively low strength. Crushed rock aggregates, of 10 to 20 mm
size, which are not too angular and elongated, should preferably be
used. However, it has been found that bond strength between smaller
size aggregates is greater than between larger size aggregates and
for that reason, smaller size aggregates (say 10 to 20 mm) tend to
give better results. Fine sands should be avoided, particularly
those with high absorption.Superplasticizer should be used to
achieve maximum water reduction, although plasticizers may be
adequate for lower strength HSC. The basic proportioning of an HSC
mix follows the same method as for normal strength concrete, with
the objective of producing a cohesive mix with minimum voids. This
can be done by theoretical calculations or subjective laboratory
trials. It is essential to ensure full compaction at these levels.
A higher ultimate strength can be obtained by designing a mix with
a low initial strength gain and cementitious additions. This is
partially due to avoidance of micro-cracking associated with high
thermal gradients. Increasing the cement content may not always
produce higher strength. Above certain levels it may have little
effect. An optimum amount of total cementitious material usually
appears to be between 450 and 550 kg/m3. HSC mixes tend to be very
cohesive and a concrete with a measured slump of 50 mm may be
difficult to place. As HSC is likely to be used in heavily
reinforced sections, a higher workability should be specified if
honeycombing is to be avoided. When superplasticizer is used,
concrete tends to lose workability rapidly. HSC containing such
materials must therefore be transported, placed and finished before
they lose their effect. Many modern superplasticizer can retain
reasonable workability for a period of about 100 minutes, but care
is still needed, particularly on projects where ready-mixed
concrete delivery trucks have long journey times. Often, in order
to avoid drastic decreases in slump and resultant difficulty in
placing, superplasticizer are only partly mixed on batching, the
balance being added on site prior to pouring. The same production
and quality control techniques for normal strength concrete should
also be applied to HSC. In general, production should include not
only correct batching and mixing of ingredients, but also regular
inspection and checking of the production equipment, e.g. the
weighing and gauging equipment, mixers and control apparatus. With
ready mix concrete supply, this control should extend to transport
and delivery conditions as well. The main activities for
controlling quality on site are placing, compaction, curing and
surface finishing. Site experience indicates that more compaction
is normally needed for high strength concrete with high workability
than normal strength for similar slump. As the loss in workability
is more rapid, prompt finishing also becomes essential. Particular
attention needs to be given to vibration at boundaries. To avoid
plastic shrinkage, the finishing concrete surface needs to be
covered rapidly with water-retaining curing agents. As the quality
of the structure with HSC is the main objective, it is essential
that, in addition to the above, the accuracy of the formwork and
the fixing details of the reinforcement and/or pre stressing steel
should also be part of the control activities. It is also desirable
to assess the in-situ strength of the concrete in the actual
structure by some non-destructive method (such as hammer test or
ultrasonic pulse velocity measurements) for comparison with
compliance cube test results, to establish that no significant
differences exist between the two sets of results. It should be
kept in mind that factors which have only a second-order effect at
lower strength levels may become of major importance at higher
levels.3.3 ADMIXTUREChemical admixtures are the ingredients in
concrete other than Portland cement, water, and aggregate that is
added to the mix immediately before or during mixing. Producers use
admixtures primarily to reduce the cost of concrete construction;
to modify the properties of hardened concrete; to ensure the
quality of concrete during mixing, transporting, placing, and
curing and to overcome certain emergencies during concrete
operations.Successful use of admixtures depends on the use of
appropriate methods of batching and concreting. Most admixtures are
supplied in ready-to-use liquid form and are added to the concrete
at the plant or at the job site. Certain admixtures, such as
pigments, expansive agents, and pumping aids are used only in
extremely small amounts and are usually batched by hand from
premeasured containers.The effectiveness of an admixture depends on
several factors including: type and amount of cement, water
content, mixing time, slump, and temperatures of the concrete and
air. Sometimes, effects similar to those achieved through the
addition of admixtures can be achieved by altering the concrete
mixture-reducing the water-cement ratio, adding additional cement,
using a different type of cement, or changing the aggregate and
aggregate gradation. Admixtures being considered for use in
concrete should meet specifications. Trial mixtures should be made
with the admixture and the job material at temperatures and
humidifies anticipated on the job. In this way the compatibility of
the admixture with other admixtures and job materials, as well as
the effects of the other admixture on the properties of the fresh
and hardened concrete, can be observed. The amount of admixture
recommended by the manufacturer or the optimum amount determined by
laboratory test should be used. The major reasons used for using
admixtures are.1. To reduce the cost of concrete construction.2. To
achieve certain properties in concrete more effectively than by
other means.3. To maintain the quality of concrete during the
stages of mixing, transporting, placing, and curing in adverse
weather conditions.4. To overcome certain emergencies during
concreting operations. 3.3.1 SUPERPLASTICIZER Superplasticizers
arewellknown chemical admixtures for concrete used in the reduction
of water to cement ratio without affecting workability, and to
avoid particle segregation in the concrete mixture. These are also
known as high range water reducers (HRWR) and dispersants as these
are capable of reducing water to cement ratio up to 40%. These
chemical admixtures are added in the concrete just before the
concrete is placed. These admixtures help to improve strength and
flow characteristics of the concrete. Superplasticizer are
essential sulfonic compounds attached to the polymer backbone at
regular intervals. These can be added at a range of 0.15% to 3.0%
of the weight of cement that is higher as compared to plasticizers.
Flow characteristics and slump of concrete varies with type,
dosage, and time of addition of concrete superplasticizer.
Superplasticizer can be classified into four types such as,
Sulfonated melamine-formaldehyde condensates (SMF), Sulfonated
naphthalene-formaldehyde condensates (SNF), Modified
lignosulfonates (MLS), and Polycarboxylate derivatives (PC). The
selection of concrete superplasticizer is based on the type of
concrete used, namely ready mix, precast, high strength, high
performance, self-compacting, shotcrete, etc. The SMF, SNF, and MLS
are very old but are still the highly consumed in todays concrete
applications. The only drawback with these concrete
superplasticizer is high slump loss and not preferable in cold
weather conditions that can be overcome with polycarboxylates.
Polycarboxylates, with superior performance characteristics, are
the new generation superplasticizer, capable of water reduction up
to 40%, and are also preferable in hot weather conditions.
3.3.2 EFFECT OF SUPERPLASTICIZER ON CONCRETEWhen cement mixes
with water, cement particles always flocculate and agglomerate then
electrostatic attractive forces are generated by the electric
charge on particle surface as a results large amount of free water
being trapped in flocs, leads to reduce the homogeneity of
concrete. The water reducing agents or workability agents such as
plasticizer and superplasticizer among which superplasticizer is
more consistence and viscous even at low w/c ratio. Further, to
achieve high filling ability, it is necessary to reduce
inter-particle friction among solid particles in concrete by using
superplasticizer and reducing coarse aggregate contents. The
incorporation of a superplasticizer not only reduces the
inter-particle friction but also maintain the deformation capacity
and viscosity.3.4 WORKABILITYThe behavior of green or fresh
concrete from mixing up to compaction depends mainly on the
property called workability of concrete. Workability of concrete is
a term which consists of the following four partial properties of
concrete namely, Mix ability, Transportability, Mouldability and
Compatibility. In general terms, workability represents the amount
of work which is to be done to compact the compact the concrete in
a given mould. The desired workability for a particular mix depends
upon the type of compaction adopted. A workable mix should not
segregate. 3.4.1 PARTIAL PROPERTIES OF WORKABILITY (i) Mixability:
It is the ability of the mix to produce a homogeneous green
concrete from the constituent materials of the batch, under the
action of the mixing forces. A less mixable concrete mix requires
more time of mixing to produce a homogeneous and uniform mix.(ii)
Transportability: Transportability is the capacity of the concrete
mix to avoid the homogeneous concrete mix from segregating during a
limited time period of transportation of concrete, when forces due
to handling operations of limited nature act.(iii) Mouldability: It
is the ability of the fresh concrete mix to fill completely the
forms or moulds without losing continuity or homogeneity under the
available techniques of placing the concrete at a particular job/
this property is complex, since the behavior of concrete is to be
considered under dynamic conditions.(iv) Compactibility:
Compactibility is the ability of concrete mix to be compacted into
a dense, compact concrete, with minimum voids, under the existing
means of compaction at the site. The best mix from the point of
view of compactibility should close the void an extent of 99% of
the original voids present, when the concrete is placed in the
mould.3.4.2 FACTORS AFFECTING WORKABILITYThe factors helping
concrete to have more lubricating effect to reduce internal
friction for helping easy compaction are:(i) Water Content: Water
content in a given volume of concrete, will have significant
influences on the workability. The higher the water content per
cubic meter of concrete, the higher will be the fluidity of
concrete, which is one of the important factors affecting
workability. At the work site, supervisors who are not well versed
with the practice of making good concrete resort to adding more
water for increasing workability. This practice is often resorted
because this is one of the easiest corrective measures that can be
taken at the site. It should be noted that from the desirability
point of view, increase of water content is the last recourse to be
taken for improving the workability even in the case of
uncontrolled concrete. For controlled concrete one cannot
arbitrarily increase the water content. In case all other steps to
improve workability fail, only as last recourse the addition of
more water can be considered. More water can be added, provided a
correspondingly higher quantity of cement is also added to keep the
water/cement ratio constant, so that the strength remains the
same.(ii) Mix Proportions: Aggregate/cement ratio is an important
factor influencing workability. The higher the aggregate/cement
ratio, the leaner is the concrete. In lean concrete, less quantity
of paste is available for providing lubrication, per unit surface
area of aggregate and hence the mobility of aggregate is
restrained. On the other hand, in case of rich concrete with lower
aggregate/cement ratio, more paste is available to make the mix
cohesive and fatty to give better workability.(iii) Size of
Aggregate: The bigger the size of the aggregate, the less the
surface area and hence less amount of water is required for wetting
the surface and less matrix or paste is required for lubricating
the surface to reduce internal friction. For a given quantity of
water and paste, bigger size of aggregates will give higher
workability. The above of course will be true within certain
limits.(iv) Shape of Aggregates: The shape of the aggregate
influences the workability in good measure. Angular, elongated or
flaky aggregate makes the concrete very harsh when compared to
rounded aggregates or cubical shaped aggregates. Contribution to
better workability to rounded aggregate will come from the fact
that for the given volume or weight it will have less surface area
and less voids than angular or flaky aggregate. Not only that,
being round in shape, the frictional resistance is also greatly
reduced. This explains the reason why river sand and gravel provide
greater workability to concrete than crushed sand and aggregate.
The importance of shape of the aggregate will be of great
significance in the case of present day high strength and high
performance concrete when we use very low w/c about 0.25.In years
to come, natural sand will be exhausted or costly. One has to go
for manufactured sand. Shape of crushed sand as available today is
unsuitable but the modern crushers are designed to yield well
shaped and well graded aggregates.(v) Surface Texture: The
influence of surface texture on workability is again due to the
fact that the total surface area of rough textured aggregate is
more than the surface area of smooth rounded aggregate of same
volume. From the earlier discussions it can be inferred that rough
textured aggregate will show poor workability and smooth or glassy
textured aggregate will give better workability. A reduction of
inter particle frictional resistance offered by smooth aggregates
also contributes to higher workability.(vi) Grading of Aggregates:
This is one of the factors which will have maximum influence on
workability. A well graded aggregate is the one which has least
amount of voids in a given volume. Other factors being constant,
when the total voids are less, excess paste is available to give
better lubricating effect. With excess amount of paste, the mixture
becomes cohesive and fatty which prevents segregation of particles.
Aggregate particles will slide past each other with the least
amount of compacting efforts. The better the grading, the less is
the void content and higher the workability. (vii) Use of
Admixtures: Of all the factors mentioned above, the most important
factor which affects the workability is the use of admixtures. It
is to be noted that initial slump of concrete mix or what is called
slump of reference mix should be about 2 3 cm to enhance the slump
many fold at a minimum doze. Without initial slump of 2-3 cm, the
workability can be increased to higher level but it requires higher
dosage hence uneconomical.3.5 GRADATION OF COARSE AGGREGATECoarse
aggregate used in concrete contain various sizes. This particle
size distribution of the coarse aggregates is termed as Gradation.
The sieve analysis is conducted to determine this particle size
distribution. Grading pattern is assessed by sieving a sample
successively through all the sieves mounted one over the other in
order of size, with larger sieve on top. The material retained on
each sieve after shaking represents the fraction of aggregate are
coarser than the sieve in lower and finer than the sieve
above.Proper gradation of coarse aggregate is one of the most
important factors in producing workable concrete. Proper gradation
ensures that a sample of aggregates contains all standard fraction
of aggregate in required proportion such that the sample contains
minimum voids will require minimum paste to fill up the voids in
the aggregates. Minimum paste means less quantity of cement and
less quantity of water, leading to increased economy, higher
strength, lower shrinkage and great durability. The workability is
improved when there is an excess of paste above that required to
fill the voids in the sand, and also an excess of mortar (sand plus
cement) above that required to fill the voids in coarse aggregate
because the material lubricate the larger particles.(i) Well
Graded: Well-graded aggregate has a gradation of particle size that
fairly evenly spans the size from the finest to the coarsest. (ii)
Poor Graded: Poor-graded aggregate is characterized by small
variation in size. It contains aggregate particles that are almost
of same size. This means that the particles pack together, leaving
relatively large voids in the concrete. It is also called
uniform-graded.(iii) Gap Graded: Gap-graded aggregate consist of
aggregate particle in which some intermediate size particle are
missing. Poorly graded concrete generally require excessive cement
paste to fill the voids making them uneconomical. Gap-graded
concrete fall in between well graded and poorly graded in terms of
performance and economy. Well graded aggregates are tricky in
proportion. The goal of aggregate proportioning and sizing is to
maximize the volume of aggregate in the concrete while preserving
the strength, workability and finishing. Aggregate graded to
maximum density gives a harsh concrete that is very difficult in
ordinary concreting. So the proportioning should be based on the
surface area to be wetted. Other things remaining same, it can be
said that the concrete made from aggregate grading having least
surface area will require least water which will consequently be
the strongest. 3.6 PORTLAND POZZALANA CEMENT (PPC)3.6.1 COMPOSITION
AND ACTION OF PPCCement is a material that can bind solid particles
e.g. gravel, sand and aggregate etc. within a compact structure. A
variety of materials may exhibit cementitious properties. In the
concrete industry, hydraulic cements such as Portland cement have
the ability to set and harden in the presence of water. They are
usually manufactured from calcareous raw materials containing
silicates, aluminates and iron oxides. Raw materials such as
limestone and clay are heated in a kiln at 1400-1450C to form
predominantly clinker, which is then finely ground together with
additives such as gypsum to obtain Portland cement. Portland cement
is the most common type of cement used in construction
applications, but it is an expensive binder due to the high cost of
production associated with the high energy requirements of the
manufacturing process itself. Other cheap inorganic materials with
cementitious properties such as natural pozzolana e.g. volcanic
tuff and clay, and waste products from industrial plants e.g. slag,
fly ash and silica fume can be used as a partial replacement for
Portland cement i.e. blended cements. In addition, to reduce the
cost of binder, there are potential technological benefits from the
use of pozzolanic materials as those blended with Portland cement
in concrete applications. These include increased workability,
decreased permeability and increased resistance to sulphate attack,
improved resistance to thermal cracking and increased ultimate
strength and durability of concrete. Pozzolanic cement is a ground
product of a mixture containing 20-40% natural pozzolana and 60-80%
Portland cement clinker with the addition of a small amount of
gypsum. Increase in the natural pozzolana content of cement would
reduce the permeability of the paste with the implication of a high
resistance to chemical attack, i.e. increase in durability. The
addition of natural pozzolana (up to 20-30%) could also improve the
compressive, splitting and flexural strengths of the concrete in
the long term, for example, over a 365 day period. Mortar or
concrete samples prepared from blended cement must produce 7- and
28-day compressive strengths of higher than 16 and 32.5 N/mm2,
respectively. Pozzolana cannot develop hydraulic properties in the
absence of hydrated lime. Hydrated lime or material that can
release it during its hydration (e.g. Portland cement) is then
required to activate the natural pozzolanas as a binding material.
The activity of a natural pozzolana, which is essentially
determined by the reactive silica content, is also closely
controlled by its specific surface area, chemical and mineralogical
composition. Reactive silica is readily dissolved in the matrix as
Ca(OH)2 becomes available during the hydration process. These
pozzolanic reactions lead to the formation of additional C-S-H with
binding properties. Silicate minerals including feldspar, mica,
hornblende, pyroxene and quartz or olivine present in volcanic
rocks can easily undergo alteration to form secondary mineral
phases such as clays, zeolites, calcite and various amphiboles.
However, every natural pozzolana with a strong acidic character
does not show pozzolanic activity, and hence the assessment of
pozzolanic activity of a given natural pozzolana is a prerequisite
for its use in the cement industry. Table 3.1 Chemical composition
of natural raw material in cementCompositionContent (%)
Lime(CaO)62-67
Silica(SiO2)17-25
Alumina(Al2O3)3-8
Calcium sulphate(CaSO4)3-4
Iron oxide(Fe2O3)3-4
Magnesia(MgO)0.1-3
Sulphur(S)1-3
Alkalies0.2-1
3.6.2 SETTING ACTION OF CEMENT Following are important compounds
formed during the setting action of cement:(1) Tricalcium aluminate
(3Cao, Al2O3): This component is formed within about the 24 hours
after addition of water to the cement. The main of this compound is
to give the early strength to cement. It hydrates and hardens very
quickly. It liberates a large amount of heat almost immediately and
contributes somewhat to early strength. Therefore gypsum is added
to cement to slow down the action of C3A(2) Tetra-calcium
alumino-ferrite (4CaO, Al2O3, Fe2O3): This compound is also formed
within about 24 hours after addition of water to the cement it
provides the prolong strength to cement which plays an important
role for gain in strength. The C4AF compound hydrates rapidly but
contributes very little to strength. Its use allows lower kiln
temperatures in Portland cement manufacturing. This compound is so
acts as flux in clinker manufacture and imparts grey colour. (3)
Tricalcium silicate (3CaO, SiO2): This component is formed within a
week after addition of water to the cement and it is mainly
responsible for imparting strength to the cement in early period of
setting. (4) Dicalcium silicate (2CaO, SiO2): This component is
formed very slowly and hence it is possible for giving progressive
strength to the cement. 3.6.3 BENEFITS OF USING PPCPPC is produced
when pozzolanas are used in the mixture. A pozzolanas is a cement
extender improving the strength and durability of the cement or
even reducing the costs of producing concrete. The term came from
the root word pozzolana which is a form of volcanic ash. The
introduction of pozzolana into a hydraulic cement like OPC, or any
similar material, leads to a pozzolanic reaction. This, in turn,
leads to a cementitious material that uses less cement but has the
same or even greater material durability than without this
addition. A pozzolanic material by itself has few, if any,
cementitious properties by itself, but adding it into a cement
mixture will result in the above-mentioned results (provided the
cement has a greater volume in relation to the pozzolanic material
added). PPC may take a longer time to settle than OPC, but it will
eventually produce similar results given time. Though volcanic ash
is the first form of pozzolana used, this now includes natural and
artificial siliceous or siliceous, aluminous materials such as
clay, slag, silica fume, fly ash, and shale. Note that some of
these are effectively waste materials from other processes but are
ideal to produce PPC.3.7 CURING3.7.1 DEFINITON OF CURINGCuring can
be described as keeping the concrete moist and warm enough so that
the hydration of cement can continue. More elaborately, it can be
described as the process of maintaining a satisfactorymoisture
content and a favorable temperature in concrete during the period
immediately following placement, so that hydration of cement may
continue until the desired properties are developed to a sufficient
degree to meet the requirement of service. If curing is neglectedin
the early period of hydration, the quality of concrete will
experience a sort of irreparable loss. An efficient curing in the
early period of hydration can be compared to a good and wholesome
feeding given to a new born baby.A concrete element is expected to
last a certain number of years. In order to meet this expected
service life, it must be able to withstand structural loading,
fatigue, weathering, abrasion, and chemical attack. The duration
and type of curing plays a big role in determining the required
materials necessary to achieve the high level of quality. Curing is
the process in which the concrete is protected from loss
ofmoistureand kept within a reasonable temperature range. The
result of this process is increased strength and decreased
permeability. Curing is also a key player in mitigating cracks in
the concrete, which severely impacts durability. Cracks allow open
access for harmful materials to bypass the low permeability
concrete near the surface. Good curing can help mitigate the
appearance of unplanned cracking.3.7.2 REASONS FOR CURING(i)
Predictable strength gainLaboratory tests show that concrete in a
dry environment can lose as much as 50 percent of its potential
strength compared to similar concrete that is moist cured. Concrete
placed under high temperature conditions will gain early strength
quickly but later strength may be reduced. Concrete placed in cold
weather will take longer to gain strength, delaying from removal
and subsequent construction.(ii) Improved durabilityWell-cured
concrete has better surface hardness and will better withstand
surface wear and abrasion. Curing also makes concrete more water
tight, with pavement moisture and water-borne chemicals from
entering into concrete, thereby increasing durability and service
life. (iii) Better requirements for curingA concrete slab that has
been allowed to dry out too early will have a soft surface with
poor resistance to wear and abrasion.3.7.3 WATER CURING This is by
far the best method of curing as it satisfies all the requirements
of curing, namely, promotion of hydration, elimination of shrinkage
and absorption of the heat of hydration. It is pointed out that
even if the membrane method is adopted, it is desirable that a
certain extent of water curing is done before the concrete is
covered with membranes. Water curing can be done in the following
ways:1. Immersion.2. Ponding.3. Spraying or Fogging.4. Wet
Covering.The precast concrete items are normally immersed in curing
tanks for a certain duration. Pavement slabs, roof slab etc. are
covered under water by making small ponds. Vertical retaining wall
or plastered surfaces or concrete columns etc. are cured by
spraying water. In some cases, wet coverings such as wet gunny
bags, hessian cloth, jute matting, straw etc., are wrapped to
vertical surface for keeping the concrete wet. For horizontal
surfaces saw dust, earth or sand are used as wet covering to keep
the concrete in wet condition for a longer time so that the
concrete is not unduly dried to prevent hydration.
3.7.4 MEMBRANE CURINGSometimes, concrete works are carried out
in places where there is acute shortage of water. The lavish
application of water for water curing is not possible for reasons
of economy. Curing does not mean only application of water; it
means also creation of conditions for promotion of uninterrupted
and progressive hydration. It is also pointed out that the quantity
of water, normally mixed for making concrete is more than
sufficient to hydrate the cement, provided this water is not
allowed to go out from the body of concrete. For this reason,
concrete could be covered with membrane which will effectively seal
off the evaporation of water from concrete. Large numbers of
sealing compounds have been developed in recent years. The idea is
to obtain a continuous seal over the concrete surface by means of a
firm impervious film to preventmoisturein concrete from escaping by
evaporation. Some of the materials, which can be used for this
purpose, are bituminous compounds, polyethylene or polyester film,
waterproof paper, rubber compounds etc. When waterproofing paper or
polyethylene film are used as membrane, care must be taken to see
that these are not punctured anywhere and also see whether adequate
lapping is given at the junction and this lap is effectively
sealed.3.7.5 APPLICATION OF HEATThe development of strength of
concrete is a function of not only time but also that of
temperature. When concrete is subjected to higher temperature it
accelerates the hydration process resulting in faster development
of strength. Concrete cannot be subjected to dry heat to accelerate
the hydration process as the presence ofmoistureis also an
essential requisite. Therefore, subjecting the concrete to higher
temperature and maintaining the required wetness can be achieved by
subjecting the concrete to steam curing.A faster attainment of
strength will contribute to many other advantages mentioned below.
The exposure of concrete to higher temperature is done in the
following manner:1. Steam curing at ordinary pressure2. Steam
curing at high pressure3. Curing by Infra-redMany a time an
engineer at site wonders, how early he should start curing by way
of application of water. This problem arises, particularly, in case
of hot weather concreting. In an arid region, concrete placed as a
road slab or roof slab gets dried up in a very short time, say
within 2 hours.Concrete should not be allowed to dry fast in any
situation. Concrete that are liable to quick drying is required to
be covered with wet gunny bag or wet hessian cloth properly
squeezed, so that the water does not drip and at the same time,
does not allow the concrete to dry. This condition should be
maintained for 24 hoursor at least till the final setting time of
cement at which duration the concrete will have assumed the final
volume. Even if water is poured, after this time, it is not going
to interfere with the water/cement ratio. However, the best
practice is to keep the concrete under the wet gunny bag for 24
hours and then commence water curing by way of ponding or spraying.
Of course, when curing compound is used immediately after bleeding
water, if any, dries up, the question of when to start water curing
does not arise at all.There is a wrong concept with common builders
that commencement of curing should be done only on the following
day after concreting. Even on the next day they make arrangements
and build bunds with mud or lean mortar to retain water. This
further delays the curing. Such practice is followed for concrete
road construction by municipal corporations also. It is a bad
practice. It is difficult to set time frame how early water curing
can be started.It depends on, prevailing temperature, humidity,
wind velocity, type of cement, fineness of cement, w/c used and
size of member etc. The point to observe is that, the top surface
of concrete should not be allowed to dry. Enough moisture must be
present to promote hydration.Regarding how long to cure, it is
again difficult to set a limit. Since all the desirable properties
of concrete are improved by curing, the curing period should be as
long as practical. For general guidance, concrete must be cured
till it attains about 70% of specified strength. At lower
temperature curing period must be increased. Since the rate of
hydration is influenced by cement composition and fineness, the
curing period should be prolonged for concretes made with cements
of slow strength gain characteristics.Pozzolanic cement or concrete
mixed with pozzolanic material is required to be cured for longer
duration. Mass concrete, heavy footings, large piers, abutments,
should be cured for at least 2 weeks.3.8 STRENGTH GAINStrength can
be defined as ability to resist change. One of the most valuable
properties of the concrete is its strength. Strength is most
important parameter that gives the picture of overall quality of
concrete. Strength of concrete usually directly related to cement
paste. Many factors influence the rate at which the strength of
concrete increases after mixing. Hardening is the process of growth
of strength. This is often confused with 'setting' but setting and
hardening are not the same. Setting is the stiffening of the
concrete after it has been placed. Hardening may continue for weeks
or months after the concrete has been mixed and placed.Voids in
concrete can be filled with air or with water. Broadly speaking,
the more porous the concrete, the weaker it will be. Probably the
most important source of porosity in concrete is the ratio of water
to cement in the mix, known as the 'water to cement' ratio.3.8.1
MEASUREMENT OF CONCRETE STRENGTHTraditionally, this is done by
preparing concrete cubes or cylinders, then curing them for
specified times. Common curing times are 3, 7, 28 and 90 days. The
curing temperature is typically 20 degrees Centigrade. After
reaching the required age for testing, the cubes/cylinders are
crushed in a large press. The SI unit for concrete strength
measurement is the Mega Pascal, although 'Newton per square
millimeter' is still widely used as the numbers are more
convenient. The table below shows the compressive strength gained
by concrete after 1, 3, 7, 14 and 28 days with respect to the grade
of concrete we use.
Table 3.2 Age-Strength relationAgeStrength %
1 Day16%
3 Days40%
7 Days65%
14 Days90%
28 Days99%
From above table, it is clear that concrete gains its strength
rapidly in the initial days after casting, i.e. 90% in only 14
days. When, its strength have reached 99% in 28 days, still
concrete continues to gain strength after that period, but that
rate of gain in compressive strength is very less compared to that
in 28 days. After 14 days of casting concrete, concrete gains only
9% in next 14 days. So, rate of gain of strength decreases. We have
no clear idea up to when the concrete gains the strength, 1 year or
2 year, but it is assumed that concrete may gain its final strength
after 1 year. So, since the concrete strength is 99% at 28 days,
its almost close to its final strength, thus we rely upon the
results of compressive strength test after 28 days and use this
strength as the base for our design and evaluation.3.9 IS:
10262-2009 CONCRETE MIX PROPORTIONINGThis code provides the
guidelines for proportioning concrete mixes as per the requirements
using the concrete making materials including others supplementary
material identified for this purpose. The proportioning is carried
out to achieve the specified characteristics at specified age,
workability of fresh concrete and durability required.3.9.1 DATA
REQUIRED FOR MIX PROPORTIONThe following data are required for mix
proportioning of a particular grade concrete a) Grade designationb)
Type of cementc) Maximum nominal size of aggregated) Minimum Cement
Contente) Maximum water-cement ratiof) Workabilityg) Exposure
condition as per Table 4 and Table 5 of IS 456h) Maximum
temperature of concrete at the time of placingi) Method of
transport and placingj) Early age strength requirement, if
requiredk) Type of aggregatel) Maximum cement contentm) Whether
admixture shall or shall not be use and the type of admixture and
the condition of use3.9.2 TARGET STRENGTH FOR MIX PROPORTIONING:As
sufficient test results for a particular grade of concrete are not
available, the value of standard deviation is taken from table 1
IS: 10262-2009In order that the specified proportion of test
results are likely to fall below the characteristic strength, the
concrete mix has to be proportioned for higher target mean
compressive strength fck. The margin over characteristic strength
is given by the following relation:fck = fck+ 1.65STable 3.3
Assumed Standard Deviation (IS: 10262 -2009)Sl No.Grade of
concreteAssumed standard Deviation
a)M103.5
b)M15
c)M204.0
d)M25
e)M305
f)M35
g)M40
h)M45
i)M50
j)M55
3.9.3 SELECTION OF MIX PROPORTIONTable 3.4 Maximum W: C ratio
for different exposure condition (IS: 456-2000)Exposure
ConditionReinforced Concrete
Minimum Cement Content(Kg/m3)W:C ratio
Extreme3600.40
3.9.4 SELECTION OF WATER CONTENTWater content of concrete is
influence by a number of factors, such as aggregate size, aggregate
shape, aggregate texture, workability, water-cement ratio, cement
and other supplementary cementious material type and content,
chemical admixture and environmental condition. An increase in
aggregate size, a reduction in water-cement ratio and slump, and
use of rounded aggregate and water reducing admixture will reduce
the water demand. On the other hand increase temperature, cement
content slump, water-cement ratio, aggregate angularity and
decrease in proportion of coarse aggregate to fine aggregate will
increase water demand.The quantity of maximum mixing water per unit
volume of concrete may be determined from Table 2.3 .The Table 2.3
is for 25 to 50 mm slump. For desired workability other than 25 to
50 mm slump range, the required water content may be increased by
about 3 percent for every 25mm slump or by use of chemical
admixtures conforming to IS 9103. Water reducing admixture or
superplasticizing admixture usually decrease water content by 5 to
10 percent and 20 percent and above respectively at appropriate
dose.Table 3.5 Maximum water content per cubic meter of concrete
for nominal maximum size aggregate for 25 to 50mm slump (IS: 10262
- 2009)Sl No.Nominal Maximum size aggregateMaximum water
content
a)10 mm208 kg/m3
b)20mm186 kg/m3
c)40mm165 kg/m3
3.9.5 ESTIMATION OF COARSE AGGREGATE PROPORTIONAggregate of
essentially the same nominal maximum size, type and grading will
produce concrete of satisfactory workability when a given volume of
coarse aggregate per unit volume of total aggregate is used.
Approximate value for this aggregate volume is given in Table 2.4
for water cement ratio of 0.5 which may be suitably adjusted for
the other water-cement ratios. It can be seen that for equal
workability, the volume of coarse aggregate in a unit volume of
concrete is dependent only on is nominal maximum size grading zone
of fine aggregate. Differences in the amount of mortar required for
workability with different aggregate, due to difference in particle
shape and grading, are compensated for automatically by difference
in rodded void content.For more workable concrete mixes which is
sometimes required when placed is by pump or when the concrete is
required to be worked around congested reinforcing steel, it may be
desirable to reduce the estimated coarse aggregate content
determined using Table 2.4 up to 10 percent. However, caution shall
be exercised to assure that the resulting slump, water-cement ratio
and strength properties of the concrete are consistent with
recommendation of IS 456 and met the project specification
requirements as applicable.
Table 3.6 Volume of coarse aggregate per unit volume of total
aggregate for different zones of fine aggregate (IS: 10262-2009)Sl
No.Nominal maximum Size of Aggregate(mm)Proportion of CA with
respect to the zone
Zone 1Zone 2Zone 3Zone 4
1100.500.480.460.44
2200.660.640.62 0.60
3400.750.730.71 0.69
3.9.6 COMBINATION OF DIFFERENT COARSE AGGREGATEThe coarse
aggregate used shall conform to IS 383. Coarse aggregate of
different size may be combined in suitable proportions so as to
result in an overall grading conforming to Table 2 of IS 383 for
particular nominal maximum size of aggregate.3.9.7 ESTIMATION OF
FINE AGGREGATE PROPORTIONThese quantities are determined by finding
out the absolute volume of the cementious material, water and
chemical admixture; by dividing their mass by specific gravity,
multiplying by 1/1000 and subtracting the result of their summation
from unit volume. The values so obtained are divided into Coarse
and Fine Aggregate fraction by volume in accordance with the coarse
aggregate proportion. The coarse and fine aggregate content are
then determined by multiplying with their respective specific
gravities and multiplying by 1000.3.9.8 TRIAL MIXESThe calculated
mix proportion shall be check by means of trial batches.Workability
of the Trial Mix NO.1 shall be measured. The mix shall be carefully
observed for freedom from segregation and bleeding and its
finishing properties. If the measured workability of Trial Mix No.1
is different from the stipulated value, the water or admixture
content shall be adjusted suitably. With this adjustment, the mix
proportion shall be recalculated keeping the free water-cement
ratio at the pre-selected value, which will comprise Trial Mix No.2
and varying the free water-cement ratio by 10 percent of the
pre-selected value. More Mix No. will provide sufficient
information, including the relationship between compressive
strength and water-cement ratio, from which the mix proportions for
field trial may be arrived at. The concrete for field trial is
produced by methods of actual concrete production.28