A rapid method for highlighting shotcrete performance issues in underground mines

摘要

An underground mine was experiencing slower than acceptable re-entry times, i.e. in excess of four hours, to their priority development heading. This was due to slow setting of the accelerated shotcrete being sprayed in-cycle as part of the ground support system. A QA/QC investigation of the shotcrete used semi-adiabatic calorimetric testing to determine the hydration efficiency and set time effects of the shotcrete with different admixtures and cements to identify causal factors for the slow setting. Calorimetric testing, typically used in the civil and construction industry as a QA/QC measure for conventional high performance concrete, produces heat of hydration curves and monitors temperature rate rises over time to provide indications of early age behaviour and long term performance. Initial testing was completed onsite using a number of small scale mix designs, tested using an F-Cal Field Calorimeter, with heat of hydration results available within 12 hours. Calorimetry was successful in determining the most likely factors leading to slow setting, which were concluded to be cement type and super plasticiser performance. The calorimetric testing accelerated the QA/QC investigation, when compared to in situ batch testing, and allowed the mine operation to rapidly resolve the slow re-entry times and significantly increase development advance rates through implementing changes to the shotcrete mix design.