We have a vertical shaft that is (right now) 70 ft (21.3 m) deep and we do blasting every 5 ft (1.5 m) after applying shotcrete to the vertical surface for protection. My concern is that if we have less than 48 hours between successive blasting, is it allowable? How does one measure if the shotcrete reaches the required percentage of strength?

The best guidance on this subject can be found in ACI 506.5, “Guide for Specifying Underground Shotcrete” (available through the ASA Bookstore), and some articles from past issues of Shotcrete magazine might be of interest:

• “Shotcrete Spraying Machines for Immediate Support in Tunnels”
• “Slope Stabilization in an Open Pit Mine”
• “Where Are We Now with Sprayed Concrete Lining in Tunnels?”
• “The Danger of Fallouts in Overhead Shooting”
• “Incline Tunnel—S&S Quarries, Inc.”
• “Reaching 20 MPa (2900 psi) in 2 Hours is Possible”
• “Rapid-Setting Cement in Shotcrete”

With properly qualified nozzlemen, a good shotcrete mixture, and high-quality accelerator added at the nozzle, the re-entry time can be minimal—normally 24 hours.