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2013FallShotcreteEMag

Shotcrete Corner equipment was in poor condition and required maintenance prior to shooting (refer to Fig. 4). Even after emergency maintenance was per- formed on the equipment, the material feeding rate was not as consistent as usually expected. Results from the in-place underground testing program with respect to set time, early- age compressive strength, later-age compressive strength, ambient temperature, volume of per- meable voids, and boiled absorption are shown in Table 4. Nozzlemen who shot the material underground provided the same comments as the nozzlemen who shot the material in lab/surface conditions, which were that the rebound levels were as low as or even lower than silica fume-enhanced dry- mix shotcrete. Fig. 4: King representative working with mining personnel to Discussion perform maintenance on a dry-mix shotcrete machine The early-age compressive strength curves presented in Graph 1 indicate that the material Table 4: In-place test results from underground testing temperature had the largest effect on the time Date Mar-13 Jun-13 taken to reach compressive strengths in excess Temperature 20°C (68°F) 23°C (73°F) of 20 MPa (2900 psi). It should be noted that, even with an initial material temperature of 5°C Set time 10 minutes N/A (41°F), it was possible to reach compressive Compressive strength at 1 hour strengths in excess of 20 MPa (2900 psi) within12.4 MPa0.7 MPa (101 psi) (1798 psi) 4 hours after shooting. The later-age compres- 9.6 MPa sive strength curves presented in Graph 2 Compressive strength at 1.5 hours N/A (1392 psi) indicate that the material temperature did not 15.6 MPa 14 MPa have a major impact on later-age compressive Compressive strength at 2 hours strengths, and all of the samples tested were(2031 psi)(2263 psi) shown to exceed the target compressive 30.1 MPa 41.3 MPa Compressive strength at 3 days strengths of 10 MPa (1450 psi) at 24 hours, 20 (4366 psi) (5990 psi) MPa (2900 psi) at 3 days, 30 MPa (4350 psi) Compressive strength at 7 days at 7 days, and 40 MPa (5800 psi) at 28 days. It42.4 MPa45.4 MPa (6584 psi) (6150 psi) should be noted that the lower compressive Compressive strength at 28 days strength results for the “Indoor Lab Test” (tested51.2 MPa47.5 MPa (6889 psi) (7426 psi) Feb. 2013) in Graph 2, can be attributed to the Volume of permeable voids, % 19.0 19.3 fact that the material was shot at the wettest possible consistency without sloughing. The Boiled absorption, % 8.7 9.0 flexural strength results presented in Table 2 are very similar to results that would be Various nozzlemen who shot the material in expected from a normal portland cement-based, the lab or on the surface commented that the silica fume-enhanced dry-mix shotcrete. When rebound levels were as low as or even lower than comparing the early-age compressive strength silica-fume-enhanced dry-mix shotcrete. They results between values obtained in lab/surface also added that the water was easier to adjust for conditions to underground conditions, it is the proper consistency and the rapid-strength-gain apparent that the same level of strength devel- dry-mix shotcrete seemed to be less sensitive to opment has not been shown to be present in water fluctuation than the control mixture. underground conditions. It is possible that this The in-place underground testing brought could have been caused by a higher water- minor daily challenges that a good product cement ratio (w/c) being used underground, as should be able to overcome. In this case, when it can be more difficult to visually attain the the test was conducted in March 2013, the proper consistency in underground conditions. 28 Shotcrete • Fall 2013


2013FallShotcreteEMag
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