Problem 1 What is Poissons ratio and where can it be of sign

Problem 1 What is Poisson\'s ratio, and where can it be of significance in concrete work? Problem 2 What factors influence the creep of concrete? What steps can be taken to reduce creep? What is the effect of creep in reinforced concrete columns that are subjected to axial compression loads? Problem 3 Why is silica fume used in high-strength concrete? What does it do? Problem 4 Why do the surfaces of reinforcing bars have rolled-on deformations? Why are epoxy-coated bars somctimes uscd in the construction of reinforced concrete? Problem5 What is the diameter and cross-sectional area and weight per unit length for the commercially available reinforcing bars at Qatar?

Solution

solution-1)

Poisson\'s ratio is the ratio of lateral strain to the longitudinal strain under uniform uniaxial stress.For concrete a wide range of values of Poison\'s ratio are obtained which range from 0.1 to 0.3. It is a measure of how test specimen bulges when loaded under uniaxial compression testing machine.

2) There are following factors which affect the creep of concrete-:

i) High water - cement ratio increases creep of concrete

ii) High cement content increases the creep of concrete.

iii) Creep increases when aggregate content is low.

iv) Long term sustained loading increases creep of concrete.

v) It increase when air entrainment is high in concrete.

Creep can be reduce by increasing the thickness of members as small thickness of member show large amount of creep.There are various effect of creep on reinforced concrete-:

a) It increase the deflections of certain concrete element like beams and slabs.

b) It increase the deflection of long / slender columns.

c) It slowly transfers the load from concrete to reinforcing steel over a period of time.

d) It causes loss of prestress in prestressed concrete members.

3) Silica fumes is used in concrete because it can produces high strength and very dureble concrete. It is in finely divided form and in the presence of water reacts with calcium hydroxide, liberated in the hydration process at ordinary temperature to produce compounds possessing cementitious properties.It Produces less heat of hydration and offer great resistance to the attack of impurities in water.