Investigation of restrained shrinkage cracking in partially fixed shotcrete linings

•The problem with shrinkage cracking of shotcreted plastic drains is studied.•Steel and glass fibre additions to the shotcrete are tested for restrained shrinkage.•A newly developed test set-up for shrinking, end-restrained shotcrete slabs is presented.•The performed tests show that addition of fine glass fibres can be a solution to the cracking problem.

This study investigates shrinkage of accelerated shotcrete (sprayed concrete), especially in the case of shotcrete sprayed on drains, a part of a tunnel lining not continuously bonded to the rock. One of the goals is to find methods of avoiding shotcrete shrinkage cracks in such drain structures. If cracks yet develop the crack distribution is of great importance, i.e. several fine cracks instead of one wide. By using both steel and glass fibres this may be achieved. A newly developed test set-up for shrinking, end-restrained shotcrete slabs is also presented and evaluated. The performed tests show that the addition of very fine glass fibres could be a solution to the cracking problem. The newly developed test equipment using concrete interacting with an instrumented granite slab represents a realistic way of testing restrained shrinkage. The on-going research focuses on the optimization of the glass fibre addition and the understanding of the interaction between shrinkage and creep of shotcrete.