Evaluation of cracking tendency due to early-age concrete shrinkage

Cracks can reduce the service life of a concrete structure by allowing aggressive agents to penetrate through it in easy ways. Free shrinkage evaluation alone is not enough to determine if cracking can be expected in a structure since concrete creep behaviour, stiffness and toughness also influence the potential for cracking. Consequently, it is rather interesting to perform restrained shrinkage tests, such as the ring test according to a provisional standard developed by AASHTO. The testing procedure involves concrete ring specimens restrained by an inner steel ring on which strain gauges are placed to determine the age of cracking, since abrupt changes in the steel strain occur when concrete is cracked. Both the ring test and free shrinkage test should be carried out in the same exposure conditions, usually 20°C and 50% relative humidity. Moreover, compressive and tensile strengths of concrete should be evaluated on cubic specimens at the time of its cracking. By means of analytical and numerical models of the ring specimen, some useful information on the stress induced in the material and on the tensile creep behaviour of concrete can be extrapolated thus allowing to better interpret the experimental results. This experimental procedure enables to study the influence of concrete mixture composition on the potential for early-age cracking of concrete. Several concretes can be prepared having the same mechanical performance (such as 35 MPa compressive strength class), and same workability (preferably fluid consistency). The addition of superplasticisers can be tested in order to verify their ability to reduce concrete shrinkage by lowering the cement dosage and hence by increasing the aggregate to cement ratio. Moreover, steel fiber addition can be experimented to determine its effectiveness in reducing concrete vulnerability to cracking.