Toughness is at least in part inï¬‚uenced by the compressive strength of the shotcrete matrix, so concrete strength is doubly important for ground control. In time-critical environments such as heading development in mines and most tunnels, the cost of excavation is often strongly inï¬‚uenced by the overall rate of progress; this, in turn, is inï¬‚uenced by the time to safe reentry. The minimum period of time that must elapse before it is safe to reenter under freshly shotcreted ground is directly related to the rate of strength gain in the shotcrete matrix.