ABSTRACT
Abstract Modern tunnel construction methods have gained increased importance on a world wide basis. Demand for both, the wet and the dry shotcreting methods, is steady on the increase. Currently, strong caustic alkaline accelerators are being used with a resultant significant reduction in concrete strength at later ages. Aside from the demand for higher quality and improved economy, today, there must also consideration be given toward ecological viewpoints as well as toward a safer working environment. The described new alkalifree powder accelerator, ap plicable in all wet and dry shotcrete processes, can be viewed to meet these de mands and ensures an improved ecology in tunnel construction. The new accelerator is not caustic, not hazardeous and not toxic (according to Eu ropean regulations). The transport declaration is MNo dangerous goods”. Tests and experiences gathered in a research tunnel and on several construction sites have shown positiv results and will be reported. Compared to conventional accelerators, no ultimate strength reductions can be observed. On the contrary, increased ultima te strength values can be possible. Leaching tests with de-ionized water have indicated significantly less alkalies and calcium hydroxide in the water that was in contact with shotcrete samples accelera ted by the powder. That means reduced clogging of tunnel drains and conduits, less alkali pollution of ground waters and less contamination of shotcrete waistes du ring long term depositing. The simultaneous development of a practical dosage equipment ensures a simple handling, also the continuous precise addition of powder accelerator is guaranteed. Keywords: alkalifree, dry spraying, ecology, non-toxic, powder accelerator, shotcrete, strength increase, wet spraying.
