Why shrinkage is more in Singly Reinforced section as compare to doubly Reinforced Section in RCC ?

Analyzing the problem from a theoretical point of view, “Shrinkage” essentially means that the concrete is trying to shrink which is on account of the chemical interaction between various constituents of concrete like cement, water and aggregates. Now providing reinforcement steel in a way holds the concrete together by way of restraining it and hence reduces the shrinkage.

Also, Shrinkage and temperature cracks have reasonably comparable behaviour.

Let us take a formula based approach to understand the relationship between shrinkage or temperature or shrinkage cracks and the reinforcement provided.

As per Cl. A-1.4, IS 3370 Part-II,

Smax = (Fct/Fb) x D / (2 x SR)

And

Wmax = Smax x Alpha x (T1+T2) x 0.5

Wmax is the crack width, which is quantity of interest here

From the above, we can deduce that all other variables being made constant valued,

Wmax is directly proportional to Smax.

Smax is inversely proportional to SR

It implies Wmax is inversely proportional to SR

So, if we want lesser shrinkage/temperature cracks, we have to increase this variable SR.

Now, this SR = Steel ratio = Percentage of steel provided / 100

Therefore, it is clear that, if we provide greater reinforcement, SR increases, Smax increases and Wmax decreases.

Since singly reinforced section has less reinforcement steel than a doubly reinforced section, shrinkage/temperature cracking is more in a singly reinforced section.

When we start from the starting point then we engineers design a structure not only for safety purposes but it has a good aesthetic view and hence we try to construct the dream of less cross-sectional area. We cannot compromise the width of pain hence let's modify the depth-first. 

All we know that structural members should be sound for compression as well as tension. Concrete can more sustain the compressive type of loading and steel can resist the tensile load. In the case of a singly reinforced beam the concrete on the upper portion that is a top fiber which is subjected to compression and the whole reinforcements are provided to resist tension on the lower part. In this case, however, the depth of the beam is Greater. 

When we talk about a doubly reinforced beam that the reinforcements are also used in resisting compression. Tensional reinforcements are provided as and enforcement are used as abrasion resistance element the cross-sectional area of the concrete is less subjected to the adaptation of less depth.

Thank You.

Shrinkage is defined as the change in length or volume of a material resulting due to any chemical changes, moisture content and applied loads.

In doubly reinforced structure longitudinal reinforcement provided for both tension and compression zone. Thus all the applied loads taken by the reinforcement which are compression and tension value are much higher than concrete. Thus the reduction in length or volume is negligible. i.e no shrinkage occur.

In Singly reinforced beam, reinforcement provides longitudinally only in the tension zone. Thus all the compressive loads are taken by a concrete structure, Which is weak in compression as compared to steel reinforcement. Thus the reduction in volume takes place. i.e shrinkage occur.

Hope you get the answer.

Yes, your statement is true, but I was looking for a detailed explanation of why and what is the mechanism behind this high/low Shrinkage.

Singly reinforced beam: The beam that is longitudinally reinforced only in tension zone is called a singly reinforced beam. In this beams, bending moment and tension is carried by reinforced bar and compression by concrete. It is not possible to provide a singly reinforced beam. Because we need to provide stirrups in beams. Doubly reinforced beam:

The beam that is longitudinally reinforced in both tension and compression zone is called a doubly reinforced beam. Doubly reinforced can take reverse load. i.e., tension can be taken on both sides of the beam, which is not possible in case of singly reinforced, so shrinkage will be higher in Singly Reinforced Beams.