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2013FallShotcreteEMag

Shotcrete Corner It is also possible that sand lenses could have robust product that is suitable for regular been present in end-beam samples due to the mining and tunneling operations. poor condition of the shotcrete equipment used 3. Early-age strength development seems to be underground. Future testing will help provide sensitive to ambient temperatures. values that can be expected for early-age com- 4. Absorption results are higher than usual port- pressive strength in underground conditions. land cement-based, silica fume-enhanced Temperature has a big impact on early-age shotcrete mixtures. compressive strength (same as with port- land cement), but since the goal of the testing program was to show it is possible to reopen the heading when compressive strengths reach at least 7 MPa (1000 psi) (approximately 1 or 2 hours), the slight reduction in early-age compressive strength in underground conditions was not considered an issue. Therefore, ambient temperature and temperatures of the dry material and water must be con- trolled and monitored to ensure safety. The set-time results were found to meet the requirements of the testing program and were, therefore, satisfactory. The absorption values are higher than usual, but the commonly used guidelines that are proposed in literature and gener- ally accepted in the industry3 were all obtained using portland cement-based shotcrete, so the values available in this test program must be taken as data to be collected for further development. These higher values could possibly be related to shooting with too high of a w/c or poor Contractor Qualifications Nationally compaction/consolidation which could also explain the lower compressive strength results. ACI Shotcrete Nozzleman Certification The acceptable or lower rebound level Locally through California-based ACI Chapters can be explained by the combination of the different pozzolans and the fineness of the calcium sulfo-aluminate cement (CSA). CSA cements are usually finer than normal portland cement, causing increased adhesion and compaction. All nozzleman pointed out the fact that the water was easier to adjust and seemed to fluctuate less than conventional portland cement-based dry-mix shotcrete. conclusions ExaminEr 1. It is possible to obtain 20 MPa (2900 psi) or even higher compressive strengths at 2 hours in the right conditions using Shotcrete Concrete Contractors Association dry-mix shotcrete with CSA for mining applications. www.shotcrete.us 2. The rapid-strength-gain dry-mix shot- crete should be considered a very Shotcrete • Fall 2013 29


2013FallShotcreteEMag
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