ABSTRACT
The increasing use of shotcrete as a first pass ground support 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 ground. 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 set of problems and interpreting the collected data needs to be done with care. An understanding of what is actually being measured needs to be developed to enable the user to make sense of the results. A study done in two mines by the author has shown that penetrometer readings in particular have the potential to significantly overstate actual in-situ strengths.
