Huraian
The concrete which is generally cast immediately after mixing at the time t=0 i.e. 0-hr concrete is called as fresh concrete and possess highest strength. If the concrete is kept exposed to the open air for certain time lag t and becomes partially set called old concrete. This concrete is unserviceable and does not achieve the target strength, should be discarded. Now if the partially set concrete i.e. t-hr. concrete is mixed with the freshly prepared concrete i.e. t=0.