Construction Management

1. 1. A. What is concrete and how is it made?
Concrete is a man-made material consisting of a hard, chemically inert particulate
substances known as fine and coarse aggregates that are bonded together by cement or lime and
water. Concrete is made by first measuring the materials or ingredients to be used. The aggregates,
cement and water should be measured with an accuracy of ±3% of batch quantity and the
admixtures by 5% of the batch quantity. Then, mix the materials to make the concrete mass
homogeneous and uniform in color and consistency. Next, the concrete is transported to the place
of deposition at the earliest without the loss of homogeneity obtained at the time of mixing. A
maximum of 2 hours from the time of mixing is permitted if trucks with agitator and 1 hour if
trucks without agitators are used for transporting concrete. After which, it is place with utmost
care securing the homogeneity achieved during mixing and the avoidance of segregation in
transporting. Curing is the next process to kept the moisture of the concrete. Finally, to achieve
pleasant finish product there are many ways to do to concrete surfaces.
B. What is curing? What is its significance?
Curing is the process of controlling moisture loss from concrete that has already been
placed. Curing ensures hydration of the cement, which in turn enhances it, s strength and durability.
Curing takes place immediately after placing of the concrete and deals with maintenance of the
desired temperature and moisture for extended periods of time.
C. Define water-cement ratio. Hos does it influence concrete strength?
The water–cement ratio is the ratio of the weight of water to the weight of cement used in
a concrete mix. A lower ratio leads to higher strength and durability, but may make the mix
difficult to work with and form. On the other hand, a mix with too much water will experience
more shrinkage as excess water leaves, resulting in internal cracks and visible fractures
(particularly around inside corners), which again will reduce the final strength. or higher-strength
concrete, lower ratios are used, along with a plasticizer to increase flowability.
2. A. What are the various types of concrete used in the construction industry?
The different types of concrete used in construction include plain concrete, high-density
concrete, lightweight concrete, precast concrete, reinforced concrete, prestressed concrete,
shotcrete, air-entrained concrete, self-consolidating concrete, and glass-reinforced concrete.
B. Describe the procedure of preparing good quality concrete.
Concrete is made by first measuring the materials or ingredients to be used. The aggregates,
cement and water should be measured with an accuracy of ±3% of batch quantity and the
admixtures by 5% of the batch quantity. Then, mix the materials to make the concrete mass
homogeneous and uniform in color and consistency. Next, the concrete is transported to the place
of deposition at the earliest without the loss of homogeneity obtained at the time of mixing. A
maximum of 2 hours from the time of mixing is permitted if trucks with agitator and 1 hour if
trucks without agitators are used for transporting concrete. After which, it is place with utmost
care securing the homogeneity achieved during mixing and the avoidance of segregation in
transporting. Curing is the next process to kept the moisture of the concrete. Finally, to achieve
pleasant finish product there are many ways to do to concrete surfaces.

2. 3. A. What are the various methods used for curing? How 28-day concrete strength cam be
predicted in one day?
There are to keep the moisture of the concrete. These are water curing, steam curing, curing
by infra-red radiation, electrical curing and chemical curing. Water curing is done by covering the
concrete surface with gunny bags and then sprinkling water over them regularly or with water
proof paper. Steam curing is done by artificial heat while the concrete is maintained in moist
condition. Electrical curing is done by passing the high ampere alternating current having low
voltage (generally 30V-60V potential difference adopted) through electrodes. Curing compounds
are chemical products usually sprayed directly over the concrete surface and allowing it to dry.
The compound forms an impermeable membrane that retards the loss of moisture from the
concrete.
B. What is gel-space ratio?
Gel space ratio is the ratio of the volume of cement gel to sum of the volume of cement gel
and that of the capillary pores.
C. Describe the factors affecting strength of concrete.
Factors affecting the strength of concrete can be from two groups including depending
upon the testing methods and independent of the testing methods. Factors depending on testing
methods are size of test specimen, rate of specimen relative to maximum size of aggregate,
moisture condition of specimen, rate of loading adopted and type of testing machine used; and
those independent testing methods are type of cement and age of cement, type and size of
aggregates, degree of compaction, water-cement ratio, aggregate-cement ratio, air-voids, curing
method and curing temperature and type of stress situation that may exist.
4. What is maturity concept of concrete?
The maturity of concrete is the summation of product of time and temperature. the
relationship between concrete temperature, time, and strength gain. It is represented by an index
value that can be measured in real time in the field.
5. A. What is concrete? What are functions played by its ingredients?
Concrete is a composite material made by mixing different material like cement, sand,
aggregate & water with or without admixture. The ingredient of concrete are cement, aggregate,
sand, water and admixtures. Cement is the main binder material used to bind other building
concrete materials together. It is used for making mortar and concrete during the construction
process. Coarse Aggregate forms the major body of concrete. The aggregates contribute to the
overall strength of the concrete by increasing density. fine aggregate such as sand used to fill in
the spaces left between the large coarse aggregate and to lock the larger pieces together. Sand helps
in reducing the quantity of cement paste required and decreases the amount of shrinkage that could
occur. Water contributes to the hardening of concrete through a process called hydration. Its role
is major in concrete because the strength of concrete extensively depends on water to cement ratio

3. and it is the critical factor in the production of perfect concrete. Admixtures are added to enhance
or to modify the properties of fresh & hardened concrete.
B. Discuss the various properties of concrete.
The different properties of concrete are grades of concrete, compressive strength,
characteristic Strength, tensile strength, workability, durability, creep, shrinkage and unit weight.
Concrete is known by its grade which is designated as M15, M20 etc. in which letter M
refers to concrete mix and number 15, 20 denotes the specified compressive strength (fck) of
150mm cube at 28 days, expressed in N/mm2.
Characteristic strength of concrete is defined as the value of the strength below which not
more than 5% of the test results are expected to fall.
The estimate of flexural tensile strength or the modulus of rupture or the cracking strength
of concrete from cube compressive strength is obtained by the relations fcr = 0.7 fck N/mm2. The
tensile strength of concrete in direct tension is obtained experimentally by split cylinder. It varies
between 1/8 to 1/12 of cube compressive strength.
Workability of Concrete describes how easily freshly mixed concrete can be mixed, placed,
consolidated, and finished with minimal loss of homogeneity.
Durability of concrete is its ability to resist its disintegration and decay. One of the chief
characteristics influencing durability of concrete is its permeability to increase of water and other
potentially deleterious materials. The desired low permeability in concrete is achieved by having
adequate cement, sufficient low water/cement ratio, by ensuring full compaction of concrete and
by adequate curing.
Creep is defined as the plastic deformation under sustained load. Creep strain depends
primarily on the duration of sustained loading. According to the code, the value of the ultimate
creep coefficient is taken as 1.6 at 28 days of loading.
Shrinkage is the property of diminishing in volume during the process of drying and
hardening. It depends mainly on the duration of exposure. If this strain is prevented, it produces
tensile stress in the concrete and hence concrete.
The unit weight of concrete depends on percentage of reinforcement, type of aggregate,
amount of voids and varies from 23 to 26 kN/m2.
6. A. What is meant by workability of concrete? How is it tested in field and in laboratory?
Workability of concrete means the ease and homogeneity with which a freshly mixed
concrete or mortar can be mixed, placed, compacted and finished. It is the amount of useful internal
work necessary to produce 100% compaction. It is tested using three methods; slump test,
compacting factor test and vee-bee consistometer method.
B. What are the requirements of good concrete?
The first requirement for good concrete is to use a cement type suitable for the work at
hand and have a satisfactory supply of sand, coarse aggregate, and water. Everything else must be
equal, the mix with the best graded, strongest, best shaped, and cleanest aggregate makes the
strongest and most durable concrete. Second, the amount of cement, sand, coarse aggregate, and
water required for each batch must be carefully weighed or measured according to project

4. specifications. Third, it should be workable enough to fill the form spaces thoroughly. Lastly, the
concrete should be properly cured, that is, properly protected against loss of moisture during the
earlier stages of setting.
C. How nominal mix concrete is different from design mix concrete?
Nominal mix concrete is used for relatively unimportant and simpler concrete works. In
this type of mix, all the ingredients are prescribed and their proportions are specified. While design
mix concrete a performance-based mix where choice of ingredients and proportioning are left to
the designer to be decided.
7. A. What are the factors affecting workability of concrete?
The factors helping concrete to achieve workability are water cement, mix proportions,
aggregate size, shape of aggregates, surface texture, grading of aggregates and admixtures. The
amount of water content available in a volume of concrete will have a significant effect on the
workability of concrete. The higher the water content per cubic meter of concrete the higher would
be the fluidity of the concrete.
Aggregate-cement ratio is also an important factor which influences the workability. If the
aggregate-cement ratio is higher, the lean concrete is formed. In this case less quantity of paste is
available for providing lubrication, hence workability is reduced. Whereas if the aggregate-cement
ratio is lower the rich mix is formed. In this case more paste is available to provide lubrication,
hence better workability of concrete is observed.
The bigger the size of the aggregates the lesser is the surface area hence less amount of
water is required for wetting the surface. Hence less paste is required for lubricating the concrete
surface to reduce internal friction, thus we can say that the bigger size of aggregates will give
higher workability. Reduction in size of aggregates will reduce the workability.
The shape of aggregates also has considerable influence on the workability of concrete.
Compared to rounded aggregates or cubical shaped aggregates the angular, elongated or flaky
aggregates makes the concrete very harsh. Thus, rounded shaped aggregates have better
workability. This is due to rounded aggregate will have less surface area and less voids than an
angular or flaky aggregate.
The surface texture of the aggregate also has a significant effect on the workability of
concrete. The total surface area of roughly textured aggregate is more than the surface area of a
smoothly rounded aggregate of the same volume. Thus, it can be inferred that roughly textured
aggregate will show poor workability and smooth or glassy texture aggregate will give better
workability of concrete.
The grading of aggregate is an important factor which has maximum influence on the
workability of concrete. A well graded aggregate has the least amount of voids in a given volume,
thus when the total voids are less an excess paste is available to give better lubricating effect.
The use of admixtures also has a greater influence on the workability of concrete. In
general, the plasticizers and super plasticizers greatly improve the workability of the concrete. Use
of air-entraining agent being surface active reduces the internal friction between the particles the
air bubbles act as a sort of ball bearing between the particles and give easy mobility to the particles.

5. B. Describe the various defects in concrete. What precautions should be exercised to
prevent them?
Different types of defects in concrete structures can be cracking, crazing, sulphate
deterioration, efflorescence, segregation, bleeding and laitance.
Cracks are formed in concrete due to many reasons but when these cracks are very deep, it
is unsafe to use that concrete structure. Various reasons for cracking are excess water, early loss
of water, Alkali aggregate reaction, steel and freeze and thaw. To prevent cracking, use low water
– cement ratio and maximize the coarse aggregate in concrete mix, admixtures containing calcium
chloride must be avoided. Surface should be prevented against rapid evaporation of moisture
content. Loads must be applied on the concrete surface only after gaining its maximum strength.
Crazing also called as pattern cracking or map cracking, is the formation of closely spaced
shallow cracks in an uneven manner. Crazing occurs due to rapid hardening of top surface of
concrete due to high temperatures or if the mix contains excess water content or due to insufficient
curing. Pattern cracking can be avoided by proper curing, by dampening the sub-grade to resist
absorption of water from concrete, by providing protection to the surface from rapid temperature
changes.
Sulfate Deterioration is one of the most damaging causes of concrete deterioration, causing
either softening and decay of the concrete matrix or expansive cracking and other disruption
associated with the formation of ettringite and other reaction products. Preventive measures
include the following: use of sulphate-resisting cement, low water-cement ratio, minimum cement
content, air-entrainment, waterproof coatings, drainage features, and special attention to
reinforcing cover.
Efflorescence is the formation of deposits of salts on the concrete surface. Formed salts
generally white in color. It is due to the presence of soluble salts in the water which is used in
making concrete mix. When concrete is hardening, these soluble salts get lifted to the top surface
by hydro static pressure and after complete drying salt deposits are formed on the surface. It can
be prevented by using clean and pure water for mixing, using chemically ineffective aggregates
etc. And make sure that cement should not contain alkalis more than 1% of its weight.
Segregation implies separation of coarse aggregate from fine aggregate, paste from coarse
aggregate and water from the mix and the ingredients of the fresh concrete no longer remain
uniformly distributed. It can be reduced by increasing small size coarse aggregate, air
entertainment, using dispersing agents and pozzolana.
Bleeding in concrete is a phenomenon in which free water in the mix rises up to the surface
and forms a paste of cement on the surface. Bleeding occurs in concrete when coarse aggregates
tend to settle down and free water rises up to the surface. It can be prevented if concrete mixtures
use uniformly graded aggregates, pozzolana, break the continuous water channel, use entertaining
agents, finer cement, alkali cement and a rich mix.
Laitance is defied as cement and water slurry coming on top and setting on the surface. If
laitance is formed in a lift, it should be removed before next lift is placed.
C. Describe the methods of testing fresh concrete.

6. Concrete is tested to ensure that the material that was specified and bought is the same
material delivered to the job site. There are a dozen different test methods for freshly mixed
concrete. Some of these are slump, air content, unit weight and compressive strength test.
Slump is a measure of consistency, or relative ability of the concrete to flow. If the concrete
can’t flow because the consistency or slump is too low, there are potential problems with proper
consolidation. If the concrete won’t stop flowing because the slump is too high, there are potential
problems with mortar loss through the formwork, excessive formwork pressure, finishing delays
and segregation.
Air content measure the total air content in a sample of fresh concrete, but does not indicate
what the final in-place air content will be, because a certain amount of air is lost in transportation
Consolidating, placement and finishing.
Compressive strength is tested by pouring cubes of fresh concrete and measuring the force
needed to break the concrete cubes at prescribed interval as they harden.
8. Write short notes on following:
A. Carbonation of concrete – It is the chemical reaction between carbon dioxide in the air and
calcium hydroxide and hydrated calcium silicate in the concrete to give mainly carbonates.
B. Split tensile strength test - A method of determining the tensile strength of concrete using a
cylinder which splits across the vertical diameter. It is an indirect method of testing tensile strength
of concrete.
C. Revibration – It is the application or process of vibration to concrete after completion of
placing and initial compaction, but preceding initial setting of the concrete.
D. Maturity of concrete - It indicates how far curing has progressed. Maturity is the relationship
between concrete temperature, time, and strength gain. It is represented by an index value that can
be measured in real time in the field.