Concrete Technology: Concrete Production, Properties and Testing Unit 4 Part 5

Que 4.23. What are the affecting factors of modulus of elasticity of

concrete ?


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

Concrete :

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

concrete strength,

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

concrete ?


Creep :

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

concrete :

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

weight aggregate.

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

applied loads.

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

with time.

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

relative humidity.

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.

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