Laboratory evaluation of shrinkage in shotcrete sprayed on soft drains

The soft plastic drains used to lead away un-wanted water in hard rock tunnels are often covered with shotcrete (sprayed concrete) for mechanical protection. This soft surface gives little resistance to the shotcrete shrinkage which may lead to stress concentrations and severe cracking problems. With a recently developed test method the behaviour of shrinking shotcrete slabs on a plastic surface and with end restraints only have been investigated. The laboratory test set-up consist of instrumented granite slabs used for the monitoring of strains and corresponding stresses that are the result of drying shrinkage in the shotcrete.