THE WIDESPREAD USE of shotcrete as a "ground control" system in tunnels and mines has led to a requirement to be able to accurately assess how soon it is safe to re-commence work under the freshly sprayed shotcrete. There are a number of different methods adopted to estimate the compressive strength of shotcrete in the first few hours after application. These include: direct compressive strength readings from moulded samples that are crushed; penetrometer tests in which the resistance to pushing a probe or needle into the surface is measured; and pull out tests such as the Hilti nail system. Each of the methods presents a unique Each of problems and interpretation of the collected data needs to be done with care.
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