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2015SpringShotcreteEMag

improvement on rebound loss is related to the 10 minutes after spraying until 3 hours after mixture design of the reference mixture. The spraying, mixtures containing the RC agents reference mixture containing 5% SF was already outperformed the mixture containing SF. This is optimized, and thus had the optimum combined likely due to the specific surface area difference aggregate gradation along with the cementitious between these two materials because the rate of materials content and w/cm. Because the rebound pozzolanic reaction is proportional to the amount loss of the baseline mixture was already of surface area available for reaction.1 In ad - considered to be very low, the impact of the RC dition, colloidal silica in the RC agents also agents in achieving similar low rebound loss at reacts with the calcium hydroxide released by much lower dosage rates was a significant the cement hydration, forming additional CSH improvement. However, case studies have shown gel, and also accelerates the primary CSH gel that when the performance of the RC agents on formation, which is responsible for strength rebound is compared with those of mixtures development. Distinct from the RC agents, SF containing portland cement only, or mixtures only reacts to form CSH gels and does not containing SF with rebound losses higher than contribute to the acceleration of CSH formation the one obtained in this study, the decrease in at typically used dosage rates. rebound with mixtures containing RC agents is more dramatic. Later-Age Strength Strength at 28 days is commonly specified in Early Strength Development project specifications that are often used for The first few hours are critical in mining/ quality control of the structure. Figure 4 shows tunneling operations for re-entry; therefore, it is the effect of SF mixture and mixtures containing desirable for shotcrete mixtures to have a high RC agent on 7- and 28-day cube strength. The strength-development rate. Figure 3 shows the two tested RC agents provided similar strength to comparison of the early-age strength between SF at 7 and 28 days. Generally, admixtures the SF mixture and mixtures containing the RC accelerating the strength development rate at early agents. From the initial testing conducted at ages may cause an ultimate strength reduction. Fig. 3: Effect of RC agents and SF on early strength development Shotcrete • Spring 2015 29


2015SpringShotcreteEMag
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