Quantification of the effects of thin spray-on liner application on shotcrete tensile strength

Masethe, Richard
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In underground and surface excavations, rock mass instability often occurs, and one method of remedying this is to spray support liners onto the unstable rock surfaces. Thin Spray-on Liners (TSLs) have been used in both the mining and civil engineering industries as sealants, and as surface support to maintain rock mass stability. As a replacement of shotcrete, the use of TSLs for rock support is gaining increasing acceptance in the mining industry due to experience with its exposure. The literature describing success stories of the application of the products have been emerging lately. A significant amount of testing has been carried out to determine the properties of TSLs and to investigate how the products provide support to the excavations. The research described in this research report describes a laboratory investigation into the extent to which various spray-on liners, coated on unreinforced and reinforced shotcrete specimens, will enhance the tensile strength of the shotcrete. Physical properties and the mechanisms of behaviour of these sprayed liners on shotcrete under laboratory conditions are compared as well. Brazilian indirect tensile strength tests were performed on shotcrete specimens, both uncoated and hand-coated with TSL material for this research. Tests were carried out for a specified range of curing times, and thus the performance of the TSLs was established as a function of curing time. This is seen as a significant parameter to be checked for sensitivity. The laboratory test results of a total of 96 unreinforced and reinforced shotcrete specimens, using three different TSL products as coatings, showed that the spray-on liners significantly enhanced the tensile strength of the shotcrete. Strength gains 2 hours after application were in the order of 20%, and this increased to approximately 40% after 28 days. Slightly higher strength gains were recorded for the fibre-reinforced shotcrete specimens. In summary, the tests showed that the performance of the TSLs coated on the shotcrete depends on the type and quality of the liner, the curing time and the added material in the shotcrete, eg fibre product. The Brazilian test provided a satisfactory method of evaluating and comparing the enhancement of shotcrete tensile strength provided by different TSL products.