The formworks are removed after the concrete has reached a strength at least twice its removal stress. But the members should be supported with the help of props. The reason is because of creep due to the Interfacial transition zone (ITZ).
The interfacial transition zone is formed in the concrete between the aggregates and the mortar. This zone comprises of high Water cement ratio and hence is the weakest part of the concrete. Due to the high WC ratio, the hydration reaction in the ITZ produces a porous framework in the early stages. This porous network will be filled in subsequently by stronger gels such as CSH gel as the hydration reaction proceeds. But for this to take place, sufficient time is needed. As time increases, the ITZ becomes stronger.
This is why the creep coefficient reduces as we delay the age of loading. When the props are moved, the self-weight of the structure acts on it, which is equivalent to loading the structure.
We know that the creep coefficient when loaded at
Seven days – 2.2
28 days – 1.6
One year – 1.1
Due to the high creep coefficient, the structure may become susceptible to sagging when the props are removed soon.