When a flexural member under load is also subjected to imposed deformations, such as drying shrinkage, the restraint provided by the steel reinforcement causes a tensile restraining force to be applied to the concrete at the level of the reinforcement. This shrinkage induced restraining force is in addition to the load induced tension. It depends on the bond-slip relationship at the steel concrete interface and may cause additional cracking. This additional cracking in effect reduces the bond stresses and reduces tension stiffening of the intact concrete between the cracks. Although it has been recognized that the transfer of stresses through bond in the tensile zone varies with time, largely due to the effects of shrinkage, the time-dependent change in tension stiffening is not well understood.
In this paper, the results of an experimental program are presented in which a series of identical reinforced concrete one-way slabs were tested under different loading and drying conditions in order to assess the effects of shrinkage on deflection and tension stiffening. Shrinkage prior to loading reduces the cracking moment and causes initial deflection, but does not appear to reduce tension stiffening. However, the effect of shrinkage on the deformation of a cracked slab under load was observed to be greater than that caused by the full service load. For typical reinforced concrete slabs, the time-dependent nature of tension stiffening becomes an important consideration in the design for serviceability.