Authors: Dight, P; Hulls, IR


Cite As:
Dight, P & Hulls, IR 2009, 'Maturity and shotcrete strength for early re-entry', in PM Dight (ed.), Proceedings of the First International Seminar on Safe and Rapid Development Mining, Australian Centre for Geomechanics, Perth, pp. 81-100.

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Abstract:
A significant amount of effort is being placed on achieving early re-entry for the drilling jumbos in order to reduce mining costs. In this context, shotcrete and fibrecrete have been used to stabilise the back and side walls to facilitate early re-entry. Determining the early age strength, using a soil pocket penetrometer has been shown to be inappropriate (Clements, 2004). A more reliable penetrometer, provided commercially by BASF (the Meyco® penetrometer), tends to show that the skin of the shotcrete can be relatively strong while the sub-strata is still weak. The early age strength tests performed on cylindrical examples, and Rusty Morgan beam tests, also appear to underestimate the strength of the placed shotcrete. In this paper the authors will show why the shotcrete on the back behaves (in general) much better than the routine quality assurance testing would indicate. A major determinant on the strength and curing behaviour of the shotcrete (once the shotcrete mix, additives and placement techniques have been sorted out) is the inertial temperature of the rock being excavated. This temperature has a profound influence on the early strength behaviour. It is a characteristic of mining that as underground mines get deeper the rock temperature increases. Hence the observation that rock temperature influences the behaviour of the early age shotcrete is much clearer from mining operations, than civil engineering projects where in the latter the thermal gradient is less likely to change during the course of the construction. It can also be demonstrated that the temperature of the samples being collected to undertake early strength testing are affected by the thermal inertia of the steel cylinders/formers and hence the strength is typically underestimated at ambient temperatures (approximately 22°C). The strength is a function of time and temperature, also known as maturity. This is commonly used in the concrete industry where the early strength of concrete beams is needed to be understood in construction. In this paper the authors show simple techniques for measuring the temperature underground and correlating to the early strength of shotcrete.

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