The cracking phenomena usually start at ITZ- level. This level is considered to be a very thin region having access water and less cement. When water is consumed due to hydration of any other kind of purpose than it leaves behind pores and therefore, ITZ-level become further weak.
It will be therefore natural to expect that when a reinforced concrete member is subjected to unit strain. The stress levels in different phases (paste, aggregate) will be different. This is so because of the differences in the value of elastic modulus. This results into a kind of difficulty for the smooth stress transmission across various phases.