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2016SpringShotcreteEMag

Figure 4 shows the impact of three different binder systems on the compressive strength of cored samples at 1, 7, and 28 days. Similar to the trends shown in Fig. 3, at a very early age, such as 1 day, the mixture with 40% slag cement had lower strength than the OPC mixture and binary system with 25% fly ash. However, due to the improved pozzolanic reactivity at later ages, slag cement outperformed the other two mixtures starting from day 7. Figure 5 shows the impact of three different binder systems on compressive strength of cast samples at 7 and 28 days. Results correlate well with the cored samples, as similar trends were observed where slag cement exhibited the highest strength. Overall, it can be concluded that, with the aid of pozzolanic-based rheology control agent, both early-age and ultimate strength on beams, cored, and cast samples met the specification as they reached 145 psi (1 MPa) in 3 hours while exhibiting strengths higher than 6000 psi (40 MPa) and lower than 10,000 psi (70 MPa) at 28 days. Figure 6 shows the impact of OPC mixture, fly ash, and slag cement on workability. Fly ash showed the highest slump and required the lowest dosage of water reducer, which is most likely due to its particle shape and size distribution, whereas slag cement and OPC mixture had similar workability slump and water reducer dosage. This effect with slag cement is likely due to its influence on paste characteristics and absorption. Overall, all three mixtures met the requirement of having a minimum of 6.25  in. (160 mm) slump. The Effect of Alkali-Free Accelerators on Early- and Later-Age Strength There is a misconception in the industry that increasing the accelerator dosage reduces the later-age shotcrete strength. While this may seem to be the case for some applications, it should be clarified that such strength reduction is observed most likely when the shotcrete mixture has an inadequate rheology, resulting in poor compaction, which becomes more pronounced at higher accelerator dosage rates due to reduced set times. Therefore, for a shotcrete mixture with desired rheology allowing sufficient consolidation, accelerator amount used within the manufacturer’s recommended dosage rate (for example, 4 to 10% of the total cementitious materials content) does not necessarily decrease the later-age strength. Figure 7 shows the impact of increased dosage rate of TYTRO SA 530 accelerator on early-age strength of a binary binder system containing 25% fly ash (Mix 1). A significant strength improvement was observed, especially when the dosage was Fig. 4: Impact of cementitious materials on strength development of cored samples Fig. 5: Impact of cementitious materials on strength development of cast samples Fig. 6: Impact of cementitious materials on slump (Note: 1 mm = 0.0394 in.) Shotcrete • Spring 2016 25


2016SpringShotcreteEMag
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