We have a question about the shotcrete setup strength for the exclusion zone in underground shotcrete work. New York and others are requiring a shotcrete exclusion zone (an area excluded from personnel) based upon either time or strength. The UK asks for an engineered approach to this minimum strength. Do you have any information on this?

The minimum strength for safety must be established by the designer based on the specific structural and geotechnical aspects of the project. The minimum strength value may also be influenced on whether using fiber-reinforced shotcrete or plain shotcrete. The early-age strength was tested with a Meyco Needle Penetrometer after creating a time (early) strength curve with minimum tests at 10 minutes and 30 minutes with Needle Penetrometer; then at 3 hours and 6 hours with Hilti Studs; then at 1, 7, and 28 days with cores. Thus, the curve was calibrated for the specific mixture and environmental conditions. It was also useful to identify when early strength (and potentially long-term strength) was lacking.