Toughness is at least in part influenced 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 influenced by the overall rate of progress; this, in turn, is influenced 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.