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2013FallShotcreteEMag

Sustainability The sustainable mixture had 32%, 36%, and w/cm ratio was different than that measured 19% lower compressive strength at 3, 7, and 28 experimentally, due to absorption and adsorption days, respectively, compared to the control of water on the dried aggregates. Furthermore, mixture. The blast-furnace slag aggregates are the values of boiled absorption, volume of perme- more porous, with a specific gravity of 2.3 for the able voids, and chloride ion permeability are all fine aggregates and 1.7 for the coarse aggregates. higher in the sustainable mixture, indicating it is The natural concrete sand has a specific gravity less dense or more porous. of 2.7, showing the slag aggregates are less Even the control mixture values for these dense.12 Furthermore, the sustainable shotcrete criteria are above those proposed by ACI 506.5R, probably suffered from less compaction and more illustrating that the porous coarse slag aggregates voids than the control mixture due to lower air-flow alone greatly influence the total porosity and rate used to project it, also increasing its overall permeability of the final hardened shotcrete. The porosity. According to strength theory, as voids 42.7% reduction in rebound measured for the and porosity increase in a material, strength sustainable mixture can be partly attributed to the decreases.1 The sustainable mixture had a 7.6% lower air-flow rate used to project it on the steel lower hardened density than the control mixture. receiving plate. Future experiments at optimized The experimental densities were close to those air-flow rates will provide a better comparison. determined mathematically, with 3560 lb/yd3 Both the sustainable and the control mixtures did (2110 kg/m3) versus 3600 lb/yd3 (2140 kg/m3) for not meet the ACI 506.5R criteria for compressive the control mixture and 3280 lb/yd3 (1950 kg/m3) strength, boiled absorption, and volume of perme- versus 3330 lb/yd3 (1970 kg/m3) for the sustain- able void limits.16 According to ASTM C1202, able mixture, perhaps indicating that the actual both mixtures are still classified as having low chloride ion permeability.17 All these factors illustrate the chal- lenges and future opportunities of studying shotcrete mixtures based on lightweight slag aggregates. The water content and the air flow could be optimized in future experiments to increase success and achieve higher compressive strengths, while reducing permeability and porosity, especially when using lightweight blast- furnace slag aggregates. conclusions Although the experimental phase did not meet performance and durability cri- teria for a suitable underground ground control shotcrete application as defined by ACI 506.5R, “Guide to Specifying Under- ground Shotcrete,” it did meet the objective of sustainability, achieving an economi- cally and technically feasible mixture composed of 77.7% recycled materials— including slag cement, slag aggregates, and silica fume—with an average compressive strength of 4570 psi (31.5 MPa) at 28 days and an average density 7.6% lighter than the control mixture. The comparatively low rebound of 14.8% on a vertical steel plate can be an additional benefit, increasing the yield of fresh shotcrete per unit of dry shotcrete material, along with the lower density, for the sustainable shotcrete mixture. In general, this article has illustrated the 36 Shotcrete • Fall 2013


2013FallShotcreteEMag
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