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)
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.