page 1/1 Q. 1 why grading is important for normal aggregate? Importance of aggregate grading. Ideally coarse and fine aggregate should be graded in such a way as to minimize the void age. After compaction the volume of the cement paste must be more than the void age between particles. Under filling will result in entrapped air and an unworkable mix. An extreme example of this is a no-fines concrete where the sand fraction protect minimized, the course aggregate interlocks, but nothing fills the voids. This will not reinforcement from corrosion or provide a weather tight structure. Similarly achieving a paste volume to ‘just fill the voids will result in a mix where the coarse aggregate will interlock but not necessarily in an optimum compacted state making placing difficult and leaving voids. Some overfilling of the void space between the coarse particles by the sand fraction and between the sand particles by a cement paste is necessary for workability place ability and durability of the concrete. Q. 2 Write a note on some qualities of water used for mixing and curing of concrete? Qualities of water used for mixing concrete. Concrete is a chemically combined mass which is manufactured from binding materials and inert materials with water. Function of Water in Concrete: (1) Three water serves the following purpose: To wet the surface of aggregates to develop adhesion because the cement pastes
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page 1/1
Q. 1 why grading is important for normal aggregate?
Importance of aggregate grading. Ideally coarse and fine aggregate should be graded in such a way as to minimize the
void age. After compaction the volume of the cement paste must be more than the
void age between particles. Under filling will result in entrapped air and an unworkable
mix. An extreme example of this is a no-fines concrete where the sand fraction protect
minimized, the course aggregate interlocks, but nothing fills the voids. This will not
reinforcement from corrosion or provide a weather tight structure. Similarly achieving a
paste volume to ‘just fill the voids will result in a mix where the coarse aggregate will
interlock but not necessarily in an optimum compacted state making placing difficult and
leaving voids. Some overfilling of the void space between the coarse particles by the
sand fraction and between the sand particles by a cement paste is necessary for
workability place ability and durability of the concrete.
Q. 2 Write a note on some qualities of water used for mixing and curing of concrete?
Qualities of water used for mixing concrete.
Concrete is a chemically combined mass which is manufactured from binding materials
and inert materials with water.
Function of Water in Concrete:
(1) Three water serves the following purpose: To wet the surface of aggregates to develop adhesion because the cement pastes
adheres quickly and satisfactory to the wet surface of the aggregates than to a dry
Surface.
(2) To prepare a plastic mixture of the various ingredients and to impart workability to con-
crete to facilitate placing in the desired position.
(3) Water is also needed for the hydration of the cementing materials to set and harden
during the period of curing.
(4) The quantity of water in the mix plays a vital role on the strength of the concrete. Some
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water which have adverse effect on hardened concrete. Sometimes may not be harm- less or even beneficial during mixing. So clear distinction should be made between the
effect on hardened concrete and the quality of mixing water.
Potable water as mixing water:
The common specifications regarding quality of mixing water is water should be fit for
drinking. Such water should have inorganic solid less than 1000 ppm. This content
lead to a solid quantity 0.05% of mass of cement when w/c ratio is provided 0.5 resu-
lting small effect on strength.
But some water which are not potable may be used in making concrete with any sign-
ificant effect. Dark color or bad smell water may be used if they do not posses delet-
erious substances. PH of water to even 9 is allowed if it not tastes brackish. In coastal
areas where local water is saline and have no alternate sources the chloride concent-
ration up to 1000 ppm is even allowed for drinking. But this excessive amount of alkali
carbonates and bicarbonates, in some natural mineral water, may cause alkali-silica
reaction.
Determination of Suitability of Mixing Water:
A simple way of determining the suitability of such water is to compare the setting time
of cement and the strength of mortar cubes using the water in question with the corres-
ponding results obtained using known suitable or distilled water. About 10% tolerance
is generally allowed. Such tests are recommended when water for which no service
record is available containing dissolved solids in excess of 2000 ppm or, in excess of
1000 ppm. When unusual solids are present a test is also advisable.
Quality parameters Maximum limit (ppm)
Chlorides 500
SO3 1000
Alkali Carbonates and Bicarbonates 1000
Turbidity 2000
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The effect on concreting for different types of contamination or impurities are described Suspended Solids:
Mixing water which high content of suspended solids should be allowed to stand in a
setting basing before use as it is undesirable to introduce large quantities of clay and slit
into the concrete.
Acidity and Alkalinity:
Natural water that are slightly acidic are harmless, but presence of humid or other orig-
anic acids may result adverse affect over the hardening of concrete. Water which are
highly alkaline should also be tested.
Algae:
The presence of algae in mixing water causes air entrainments with a consequent loss
of strength. The green or brown slime forming algae should be regarded with suspicion
and such water should be tested carefully.
Sea Water:
Sea water contains a total salinity of about 3.5%(78% of the dissolved solids being
NaCl and 15% MgCl2 and MgSO4), which produces a slightly higher early strength
but a lower long-term strength. The loss of strength is usually limited to 15% and can
therefore be tolerated. Sea water reduces the initial setting time of cement but do not
effect final setting time.
Chloride:
Water containing large amount of chlorides tends to cause persistent dampness and
surface efflorescence. The presence of chlorides in concrete containing embedded
steel can lead to its corrosion.
Moisture Content of Aggregate:
Aggregate usually contains some surface moisture. Coarse aggregate rarely contains
more than 1% of surface moisture but fine aggregate can contain in excess of 10%.
This water can represent a substantial proportion of the total mixing water indicating a
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significant importance in the quality of the water that contributes surface moisture in
aggregate.
Qualities of water used for Curing of concrete.
Curing is the process in which the concrete is protected from loss of moisture and 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 con-
crete, which severely impacts durability. Cracks allow open access for harmful material
to bypass the low permeability concrete near the surface. Good curing can help mitigate
the appearance of unplanned cracking.
When smart, suitable, and practical curing is used, the amount of cement required to
a given strength and durability can be reduced by either omission or replacement with
supplementary cementations materials. Since the cement is the most expensive and
energy intensive portion of a concrete mixture, this leads to a reduction in the cost as
well as the absolute carbon footprint of the concrete mixture. Additionally, practical
curing methods can enhance sustainability by reducing the need for resource intensive
conditioning treatments, should the curing method be incompatible with the intended
service environment.
Q. 3 What is slump? Describe three types of slump with the help of rough diagram.
Slump is a relative measurement in concrete consistency. It is not an indicator of quality
of the material. The slump of a mix with the same aggregate, cement and water can
vary significantly by adding an admixture. The admixture does not reduce the quality
of the material.
Simply defined, slump is a measure of the consistency of fresh concrete.
The slump test is a very simple test. The slump cone is a right circular cone that is 12
inches high. The base of the cone is 8 inches in diameter and the top of the cone is 4
inches in diameter. The cone is filled with fresh concrete in three layers of equal volume
volume. Each layer is stroked 25 times with a rod that is ¾ inch in diameter. The end of
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the rod is bullet shaped. After the cone has been filled with concrete and the concrete
has been cut off level with the top of the cone, the cone is raised vertically allowing the
concrete to fall or slump. The distance that the concrete falls or slumps from the orignal
height is the slump of the concrete. Slump is measured in inches and is reported to the
nearest ¼ inch.
In the early days of concreting when concrete was composed of cement, aggregate
and water, the coarse aggregates determined the water content and the water determ-
ined the slump. During this time a lower slump value meant a lower water content, whi-
ch also meant a higher quality of concrete.
Today, concrete is not only a blend of the three primary ingredients. Today's typical
concrete may also contain admixtures, fibers and polymers. Therefore, the coarse
aggregate may not be the
lone factor determining the water content of today's concrete. That means a high or low
slump may not be a clear indication of the quality of the concrete. In other words, the
slump cannot be used to directly determine the water content of a concrete mix.
Using modern technologies, a traditional concrete mix design with a natural 2-inch
slump may actually have a higher water content than that of a modern concrete mix
design with a 9-inch slump that contains a chemical superplasticizer. (A Superplastici-
zer is a chemical that is added to a concrete in order to increase the slump without
adding additional water.)
A mixture of stone, sand and water without cement will not flow like concrete, no matter
how much water you add. The reason for this is that it is the cement that gives concrete
its flowing properties. By chemically dispersing the cement grains a super plasticizer
enables cement to become a more effective lubricant, thereby increasing the slump
without adding additional water.
With the advent of chemical admixtures, slump can now be seen as either a water slu-
mp or as a plasticized slump. If the slump is determined only by the water content, it is
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said that the slump is a water slump. If the slump is determined by the water content
and the effects of a chemical admixture, it is said to be a plasticized slump.
Almost everything that we put into concrete can affect the slump. As the air content of
concrete is increased, the slump will also increase. Placing fibers into concrete will
decrease the slump. All factors have to be taken into account when designing a concr-
ete mix in order to produce concrete with a workable slump. The best slump from the
standpoint of workability and consolidation is 4 to 5 inches. Slump values in the 1 to 2-
inch range are not only hard to place they are also hard to consolidate. On the other
hand, slump values above 6 inches may be prone to segregate and produce excessive
bleed water.
It should be pointed out that the slump test can only be used to determine the quality of
concrete from batch to batch within a given mix design. It cannot be used to determine
the quality of concrete from mix design to mix design. The slump does not determine
the strength of the concrete. That determination is made by the water-to-cement ratio.
With the use of modern admixtures in the mix designs of today, we can set the slump
of the concrete at any practical range we choose and still have good quality concrete.
Three type of slump. The slumped concrete takes various shapes, and according to the profile of slumped
concrete, the slump is termed as;
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(1) COLLAPSE SLUMP.(2) SHEAR SLUMP.
(3) TRUE SLUMP.
(1) COLLAPSE SLUMP.
In a collapse slump the concrete collapses completely. A collapse slump will generally
me that the mix is too wet or that it is a high work a mix, for which slump test is not app-
ropriate.
(2) SHEAR SLUMP.
In a shear slump the top portion of the concrete shears off and slips sideways. OR
If one-half of the cone slides down an inclined plane, the slump is said to be a shear
slump.
(i) If a shear or collapse slump is achieved, a fresh sample should be taken and the test is
repeated.
(ii) If the shear slump persists, as may the case with harsh mixes, this is an indication of
lack of cohesion of mix.
(3) TRUE SLUMP.
In a true slump the concrete simply subsides, keeping more or less to shape
(i) This is the only slump which is used in various tests.
(ii) Mixes of stiff consistence have a zero slump, so that in the rather dry range no
variation can be detected between mixes of different workability.
However, in a lean mix with a tendency to harshness, a true slump can easily change
to the shear slump type even to collapse, and widely different values of slump can be
obtained in different samples from the same mix the slump test is unreliable for lean
mixes.
Q. 4 Write briefly the procedure of compacting factor test?
Compacting Factor Test is an experiment that determines the consistency of freshly
mixed concrete.
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APPARATUS. Compaction factor apparatus’ trowels, hand scoop (15.2 cm long), a rod of steel or
other suitable material (1.6 cm diameter, 61 cm long rounded at one end) and a balanc
SAMPLING.
Concrete mix (M15) is prepared as per mix design in the laboratory.
PROCEDURE.
(i) Place the concrete sample gently in the upper hopper to its brim using the hand scoop
and level it.
(ii) Cover the cylinder.
(iii) Open the trap door at the bottom of the upper hopper so that concrete fall in to the low
hopper .Push the concrete sticking on its sides gently with the road.
(iv) Open the trap door of the lower hopper and allow the concrete to fall in to the cylinder
below.
(v) Cut of the excess of concrete above the top level of cylinder using trowels and level it.
(vi) Clean the outside of the cylinder.
(vii) Weight the cylinder with concrete to the nearest 10 g. This weight is known as the
weight of partially compacted concrete (wi).
(viii) Empty the cylinder and then refill it with the same concrete mix in layers approximately
5cm deep, each layer being heavily rammed to obtain full compaction.
(ix) Level the top surface.
(x) Weigh the cylinder with fully compacted. This weight is known as the weight of fully
compacted concrete (w2).
(xi) Find the weight of empty cylinder (W).
The test is sufficiently sensitive to enable difference in work ability arising from the
initial process in the hydration of cement to be measured. Each test, there for should
be carried out at a constant time interval after the mixing is completed, if strictly
comparable results are to be obtained. Convenient time for releasing the concrete
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from the upper hopper has been found to be two minutes after the completion of mixing CALCULATION
The compaction factor is defined as the ratio of the weight of partially compacted concrete to the weight of fully compacted concrete. It shall normally to be stated to the
nearest second decimal place.
compaction factor = (w1-w2/w2-w) RESULT
Compactio0n factor of the concrete =
Q. 5 Write a brief note a types of cement?
there are following types of cement uses in construction work.
(1) Rapid Hardening cement.
(2) Quick setting cement
(3) Low Heat Cement
(4) Sulphates resisting cement
(5) Blast Furnace Slag Cement
(6) High Alumina Cement
(7) White Cement
(8) Coloured cement
(9) Pozzolanic Cement
(10) Air Entraining Cement
(11) Hydrographic cement
Rapid Hardening cement.
In this type of cement the lime content increased. and used for obtained high strength
in early days it is used in concrete where form work are removed at an early stage.
Quick setting cement
Small percentage of aluminum sulphate as an accelerator and reducing percentage of
Gypsum with fine grinding.
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Used in works is to be completed in very short period and concreting in static and running water.
Low Heat Cement
Manufactured by reducing tri-calcium aluminate. It is used in massive concrete constru-
ction like gravity dams.
Sulphates resisting cement
It is prepared by maintaining the percentage of tricalcium aluminate below 6% which
increases power against sulphates. It is used in construction exposed to severe
sulphate action by water and soil in places like canals linings, culverts, retaining walls,
siphons etc.,
Blast Furnace Slag Cement
It is obtained by grinding the clinkers with about 60% slag and resembles more or less
in properties of Portland cement. It can used for works economic considerations is
predominant.
High Alumina Cement
It is obtained by melting mixture of bauxite and lime and grinding with the clinker it is
rapid hardening cement with initial and final setting time of about 3.5 and 5 hours
respectively. It is used in works where concrete is subjected to high temperatures,
frost, and acidic action.
White Cement
It is prepared from raw materials free from Iron oxide. It is more costly and is used for
architectural purposes such as pre-cast curtain wall and facing panels, terrazzo surfa-
ce etc.,
Coloured cement
It is produced by mixing mineral pigments with ordinary cement. They are widely used
for decorative works in floors.
Pozzolanic Cement
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It is prepared by grindin pozzolanic clinker with Portland cement. It is used in marine
structures, sewage works, sewage works and for laying concrete under water such as
bridges, piers, dams etc.,
Air Entraining Cement
It is produced by adding indigenous air entraining agents such as resins, glues, sodium
salts of Sulphates etc during the grinding of clinker. This type of cement is specially
suited to improve the workability with smaller water cement ratio and to improve frost
resistance of concrete.
Hydrographic cement
It is prepared by mixing water repelling chemicals. This cement has high workability
and strength.
Q. 6 Write a short notes on any of the following types of special concretes.
(a) Ferro cement.
(b) Roller Compacted concrete.
(c) High performance concrete.
(a) Roller Compacted concrete.
Roller Compacted Concrete pavement is best described as a zero slump concrete that
is placed with standard or high-density paving equipment and consolidated/compacted
using steel-drum or rubber-tired rollers to achieve a durable, wear resistant surface.
Roller-Compacted Concrete pavements were first used in the 1970’s for stabilization
of logging roads in the US and Canada. It has proven to be a very effective heavy duty
pavement that can stand up to harsh climates, heavy wheel loads and difficult
operating conditions. Typically, Roller-Compacted Concrete has been used for heavy
duty pavements such as log handling yards, intermodal terminals, freight depots, and
other industrial applications. However, more recently there has been an increase in
the use of Roller-Compacted Concrete to create cost-effective pavements for many
conventional highway and street applications.
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(c) High performance concrete.
High performance concrete is a concrete mixture, which possess high durability and
high strength when compared to conventional concrete. This concrete contains one or
more of cement materials such as fly ash, Silica fume or ground granulated blast
furnace slag and usually a super plasticizer. The term ‘high performance’ is somewhat
pretentious because the essential feature of this concrete is that it’s ingredients and
proportions are specifically chosen so as to have particularly appropriate properties for
the expected use of the structure such as high strength and low permeability. Hence
High performance concrete is not a special type of concrete. It comprises of the same
materials as that of the conventional cement concrete. The use of some mineral and
chemical admixtures like Silica fume and Super plasticizer enhance the strength,
durability and workability qualities to a very high extent.
High Performance concrete works out to be economical, even though it’s initial cost is
higher than that of conventional concrete because the use of High Performance
concrete in construction enhances the service life of the structure and the structure
suffers less damage which would reduce overall costs.
Q. 7 What is curing of concrete? write a note on steam curing.
Curing is the process in which the concrete is protected from loss of moisture and kept
within a reasonable temperature range. This process results in concrete
with increased strength and decreased permeability. Curing is also a key player in
mitigating cracks, which can severely affect durability.
Most construction projects all for concrete in some form, whether as footings,
walls, or flatwork. As a frequently used material, concrete is also a common
source of problems. foul-ups can occur at any stage, from batching, mixing, and
transporting to placement, finishing, or even curing, the final step. concrete that
has been handled correctly all along can still be ruined if it's not properly cured.
curing means taking steps to keep the concrete under the right temperature
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and moisture conditions during the first few days of hardening after placement.
proper curing is vital because the concrete will eventually be much harder and
stronger if it is cured correctly.
Steam curing.
Steam curing is a process for hardening concrete, cement, and mortar that involves
exposure to warm steam. Materials subjected to this hardening technique tend to cure
more uniformly and also much more quickly than those hardened via other processes.
There are some disadvantages to this process that must be considered before
deciding to use it for curing, and there may be certain applications where this method.
is not appropriate.
In steam curing, objects to be cured are placed inside a chamber or room. Using a
control panel, an operator can set the temperature and humidity level. Variations in
pressure may also be possible, depending on the device. The heat and moisture
penetrate the materials quickly to fully hydrate and harden them. Steam curing
requires a fraction of the time involved with traditional curing and quickly strengthens
the products so they can be used immediately.
The primary reason for using steam in the curing of concrete is to produce a high early
strength This high early strength is very desirable to the manufacturers of precast and
prestressed concrete units~ which often require expensive forms or stress beds. They
want to remove the forms and move the units to storage yards as soon as possible
The minimum time between casting and moving the units.is usually governed by the
strength of the concrete. Steam curing accelerates the gain in strength at early ages$
but the uncontrolled use of steam may seriously affect the growth in strength at later
ages.
The research described in this report was prompted by the need to establish realistic
controls and specifications for the steam curing of pretension prestressed concrete
bridge beams and concrete culvert pipe manufactured in central plants. The complete
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project encompasses a series of laboratory and field investigations conducted over a
period of approximately three years.
Q. 8 Write short notes on any two topic.
(a) Relation between compressive and tensile strength
(b) Factor effecting workability.
(c) Slump and types of slump.
(a) Relation between compressive and tensile strength
Compressive Strength
The main measure of the structural quality of concrete is its compressive strength. This
property of concrete is commonly considered in structural design. Depending on the
mix (especially the water-cement ratio) and time and quality of the curing, compressive
strength of concrete can be obtained up to 14,000 psi or more. Commercial production
of concrete with ordinary aggregate is usually in the 3,000 to 12,000 psi range with the
most common ranges for cast-in-place buildings from 3,000 to 6,000 psi. On the other
hand, precast and prestressed applications often expect strengths of 4,000-8,000 psi.
tensile strength
The tensile strength of concrete can be measured by radically different tests, namely
flexure, direct tension and splitting, and the resulting values of strength are not the
same. A direct application of a pure tension force, free from eccentricity, is very difficult
A direct tension test, using bond end plates, is prescribed by the U.S Bureau of reclam-
ation. In flexural strength tests, a plain (unreinforced) concrete beam is subjected to
flexure using symmetrical two-point loading until failure occurs.
British Standard BS 1881: part 118:1993 and ASTM C78-94 prescribed third point
loading on 150 by 150 by 750 mm beams supported over a span of 450 mm. The
modulus of rupture is determined from this test. In splitting tension test, a concrete
cylinder, of the type used for compression tests, is placed with its axis horizontal
between the platens of a testing machine, and the load is increased until failure by