Properties of shotcrete-shrinkage problems

Cracking of shotcrete in tunnels is a problem in Sweden. This is due to that shotcrete does not behave as a normal cast concrete. The main difference is due to the set accelerator and as a result the major cement reactions will take place in a stiff (but not hardened) material. The shotcrete is very sensitive to early shrinkage if not properly water cured. As the concrete cannot level the early shrinkage phenomena will give a basic different structure and mode of porosity, which in turn will increase shrinkage. Moreover, with alkali free set accelerator the shotcrete gets more and coarser porosity, which in turn will give significantly larger drying shrinkage than in cast concrete.