Test Rigs for Long-term Tension Tests

摘要

Concrete is a material which is strong in compression but weak in tension. Consequently, steel reinforcement is provided to resist the tensile forces which are developed in concrete. After a member is loaded at the level above which the tensile strength is exceeded, it cracks. At crack locations, concrete can clearly carry no tension forces and hence these are all borne by the reinforcement. However, cracks are discrete occurrences and so, between cracks, the concrete continues to carry some tension. This average tension reduces the stress that the reinforcement has to carry, thus reducing the deformation of the reinforcement. Consequently, the actual stiffness of the member is greater than it would be if the calculation was based on the assumption that the concrete carried no tension - this additional stiffness being referred to as tension stiffening. Tension stiffening has only a relatively minor effect on the deformation of heavily reinforced members but is highly significant in lightly reinforced members such as slabs. Tension stiffening is a structural problem related to the interaction of the reinforcement and the concrete. The precise mechanisms involved are not properly understood and, as far as can be ascertained, no formulae exist to predict how tension stiffening changes as a function of time. All that is known is that, in the long term, the tension-stiffening effect reduces to about half its initial value. How rapidly this reduction occurs has not been studied and this lack of information causes difficulties when attempting to calculate long-term deflections of beams and slabs.