Que 4.23. What are the affecting factors of modulus of elasticity of
Following are the factor affecting the modulus of elasticity of concrete :
1. Coarse Aggregate Properties : Coarse aggregate properties like elastic
modulus of aggregate, type of aggregate (crushed or natural), petrology
and mineralogy, and quantity of aggregate. The higher the volume of
aggregate in the mix, the higher the elastic modulus.
2. Mix Design : Mix design includes total cementitious content and w/c
ratio. Less paste is good for higher elastic modulus.
3. Curing Conditions : Moist cured specimen showed better results
than that of dry cured, due to shrinkage and associated cracks.
4. Loading Rate : High loading rate will result in higher compressive
strength and higher elastic modulus.
5. Chemical Admixture : It does not have much influence on elastic
modulus. But some type of admixture can produce higher cement
dispersion and thus will result in higher compressive strength and elastic
6. Mineral Admixture : Mineral admixture as they affect the strength of
concrete, they affect the elastic modulus too.
Que 4.24. Discuss the relationship between modulus of elasticity
and strength concrete.
Relation between Modulus of Elasticity and Strength of
1. Modulus of elasticity of concrete is a key factor for estimating the
deformation of structural elements, as well as a fundamental factor for
determining modular ratio, n, which is used for the design of structural
members subjected to flexure.
2. The modulus of elasticity of concrete is directly proportional to the square
root of characteristic compressive strength in the range of normal
3. The IS 456 : 2000 gives the modulus of elasticity of concrete as :
Ec = 5000 ck f
where, E = Modulus of elasticity.
fck = Characteristic strength of concrete.
Que 4.25. Explain the procedure for determining the dynamic
modulus of elasticity using ultrasonic pulse velocity equipment.
Test for Determining Dynamic Modulus of Elasticity :
1. In this method pulses of compression waves are generated by an electroacoustical
transducer that is held in contact with one surface of the
prismatic or cylindrical concrete specimen.
2. After traversing through the concrete, the pulses are received and
converted into electrical energy by a second transducer located at a
distance L from the transmitting transducer.
3. The pulse velocity V = L/T is related to the physical properties of a solid
by the eq. (4.25.1)
Que 4.26. What is creep ? What are the factors influencing creep of
1. When concrete is subjected to compressive loading it deforms
instantaneously. This immediate deformation is called instantaneous
strain. Now, if the load is maintained for a considerable period of time,
concrete undergoes additional deformations even without any increase
in the load. This time-dependent strain is termed as creep.
Factor Affecting Creep : Following are the factors affecting creep of
1. Concrete Mix Proportion :
i. Creep increases with increase in water/cement ratio.
ii. A poorer paste structure undergoes higher creep.
iii. The amount of paste content and its quality is one of the most important
factors influencing creep.
iv. Creep is inversely proportional to the strength of concrete.
2. Aggregate Properties :
i. Light weight aggregate shows substantially higher creep than normal
ii. The higher the modulus of elasticity the less is the creep.
iii. Aggregates influence creep of concrete through a restraining effect on
the magnitude of creep.
3. Age at Loading :
i. Age at which a concrete member is loaded will have a predominant
effect on the magnitude of creep.
ii. The moisture content of the concrete being different at different age
also influences the magnitude of creep.
4. Curing Condition : Larger the curing smaller the creep.
5. Cement Properties :
i. The type of cement effects creep in so far as it influences the strength of
the concrete at the time of application of load.
ii. Fineness of cement affects the strength development at early ages and
thus influences creep.
iii. The finer the cement the higher its gypsum requirement so that re
grinding of cement in laboratory without the addition of gypsum produces
an improperly retarded cement, which exhibits high creep.
6 Temperature :
i. The rate of creep increases with temperature up to about 70 °C when,
for a 1:7 mix and 0.6 w/c ratio. It is approximately 3.5 times higher than
at 21 °C.
ii. Between 70 °C and 96 °C it drops off to 1.7 times than at 21 °C.
iii. As far as low temperature is concerned, freezing produces a higher
initial rate of creep but it quickly drops to zero.
iv. At temperature between 10 °C and 30 °C, creep is about one half of
creep at 21 °C.
7. Stress Level :
i. Higher the stress higher will be the creep.
ii. There is no lower limit of proportionality because concrete undergoes
creep even at very low stress.
Que 4.27. What is the effect of creep on concrete structures ?
Effects of Creep on Concrete Structures :
1. In reinforced concrete beams, creep increases the deflection with time
and may be a critical consideration in design.
2. In eccentrically loaded columns, creep increases the deflection and can
lead to buckling.
3. Loss of prestress due to creep of concrete in prestressed concrete structure.
4. Creep property of concrete will be useful in all concrete structures to
reduce the internal stresses due to non-uniform load or restrained
5. In mass concrete structures such as dams, on account of differential
temperature conditions at the interior and surface, creep is harmful and
by itself may be a cause of cracking in the interior of dams.
Que 4.29. What is shrinkage of concrete ? Explain about
classification of shrinkage.
A. Shrinkage :
1. Shrinkage of concrete is the time-dependent strain measured in an
unloaded and unrestrained specimen at constant temperature.
2. Shrinkage is shortening of concrete due to drying and is independent of
B. Types of Shrinkage : Following are the various types of shrinkage :
1. Plastic Shrinkage :
i. Plastic shrinkage occurs very soon after pouring the concrete in the
ii. The hydration of cement results in a reduction in the volume of concrete
due to evaporation from the surface of concrete, which leads to cracking.
2. Drying Shrinkage :
i. The shrinkage that appears after the setting and hardening of the
concrete mixture due to loss of capillary water is known as drying
ii. Drying shrinkage generally occurs in the first few months and decreases
3. Carbonation Shrinkage :
i. Carbonation shrinkage occurs due to the reaction of carbon dioxide
(CO2) with the hydrated cement minerals, carbonating Ca(OH)2 to
ii. The carbonation slowly penetrates the outer surface of the concrete.
iii. This type of shrinkage mainly occurs at medium humidity and results
increased strength and reduced permeability.
4. Autogenous Shrinkage :
i. Autogenous shrinkage occurs due to no moisture movement from
concrete paste under constant temperature.
ii. It is a minor problem of concrete and can be ignored.
Que 4.30. What are the different factors affecting of shrinkage.
Affecting Factors of Shrinkage :
1. Drying Conditions :
i. The most important factor is the drying condition or the humidity in the
ii. No shrinkage will occur if the concrete is placed in one hundred percent
2. Time :
i. The shrinkage rate will decrease rapidly with time.
ii. It has been documented that 14 to 34 % of the 20 year shrinkage will
occur within two weeks of it being poured.
iii. Within one year of the concrete being poured, shrinkage will be about
66 to 85 % of the 20 year shrinkage.
3. Water Cement Ratio :
i. The water to cement ratio will influence the amount of shrinkage that
ii. The concrete’s richness also affects the shrinkage.
iii. The process of swelling and then drying affects the concrete’s integrity
and the shrinkage.
Que 4.31. What are the effect of shrinkage on concrete and how is
it reduces ?
Effects of Shrinkage : Following are the effects of shrinkage on concrete :
1. Shrinkage of concrete between movement joints causes joints to open
or makes it wider. Therefore joints must be designed to accommodate
the widening caused by shrinkage.
2. Where other materials, such as ceramic tiles, are fixed on top of concrete
surface, shrinkage of the concrete causes relative movement between
the different materials. The resulting stresses can cause failure at the
3. If shrinkage is restrained, the concrete is put into tension and when
tensile stress becomes equal to tensile strength, the concrete cracks.
4. Shrinkage of the concrete causes the concrete to grip reinforcing bars
more tightly. This increases friction between concrete and steel and so
improves bond strength, especially for plain bars
5. The deflection of flexural members is increased by shrinkage. This is
because the lightly reinforced compression zone is free to shrink more
than heavily reinforced tension zone.
6. Shrinkage causes a reduction in pre stressing force.
Prevention of Shrinkage : Following are the measures to be taken to
reduced shrinkage :
1. Provide sun shades in case of slab construction to control the surface
2. Dampen the subgrade of concrete before placement it is liable to water
absorption but should not over damp.
3. Try to start the curing soon after finishing.
4. Use chemical admixtures to accelerate the setting time of concrete.