Equipment for the analysis of the behaviour of concrete under restrained shrinkage at early ages

Abstract

This paper describes an experimental set-up which was built to evaluate the visco-elastic deformations that can occur when a specimen of concrete is placed in a fully restrained condition from the time it is cast. It consists of two distinct devices. The first one allows the measurement of the free autogenous shrinkage from time t = 0. The second one, which includes a movable head, is used to determine the increase in load due to autogenous shrinkage, also from time t = 0. In both cases, the specimen is cast directly into the mould after mixing, and is then sealed. In the equipment with a movable head, the specimen is allowed to shrink (or swell) freely until it reaches a predetermined strain, at which time a sufficient force is applied to pull (or push) it back to its original position. It is then left again to shrink (or swell) while the force that is applied is maintained constant until the predetermined strain is once more reached, at which point the force is increased in order to pull (or push) it back to its original position. Such a procedure allows the determination of the stress build-up in the specimen as a function of time. It also allows the determination of the visco-elastic deformation (and thus of the stress relaxation) in the restrained shrinkage specimen by subtracting (at any given point in time) the cumulative sum of the strains in the restrained shrinkage specimen from the free shrinkage in the companion specimen. The results of a first series of tests on a mortar mixture are presented and discussed.