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2016SummerShotcreteEMag

Goin’ Underground (31°C). Afterwards, a consistent decrease was observed, stabilising at around 73°F (23°C) at the age of 40 hours. The panel temperature histories in conjunction with Equation (1) were used to estimate the degree of hydration and are correlated to the strength histories. The input parameters for the rate of hydration equation were determined by isothermal calorimetric testing using an I-Cal 4000, manufactured by Calmetrix. The detailed results will be published in later publications. In the analysis, it was assumed that the shotcrete had achieved initial degree of hydration of 0.05 by the end of spray and started gaining strength immediately. Figure 5 shows the fc-ξ relationship deduced from the panel strength and temperature histories. Using the fc-ξ relationship shown in Fig. 5, the panel strength development was obtained from the calculated degree of hydration. Figure 6 provides a plot comparing strengths measured by in-situ tests and the strengths calculated using SMUTI. The average error between the in-situ and calculated strengths was approximately 7% while the maximum error was less than 17%. This may be due to the variability of the in-situ strength tests rather than inaccuracy of the SMUTI calculation. The maximum error occurred for the panel achieving the strength of 2900 psi (20 MPa). Since this strength was measured beyond the limit of the stud-driving range, it may not be reliable. Next, the lining hydration was calculated using its temperature history. Figure 7 and Fig. 8 are comparative plots showing temperature histories and calculated degree of hydration, respectively, for the panels and lining section. It can be observed that as the temperature histories of the panels and the lining section were very different, so were the hydration developments. Further, the lining strength development was estimated using the fc-ξ relationship and calculated degree of hydration shown in Fig. 5 and Fig. 8, respectively. These estimates are shown in Fig. 9, which is a comparative plot for strength histories of the five panels and three key locations of the lining (calculated using SMUTI). Discussion Figure 7 shows the lining surfaces were warmer than the panel surfaces, which means the lining experienced higher rate of hydration in its early age than the panels. Thus, the degree of hydration of the lining is always greater than that of the panels, in this case. This higher degree of Fig. 4: Shotcrete panel strength and temperature histories for EBRT-W section Fig. 5: fc-ξ relationship from shotcrete panels corresponding to EBRT-W lining section Fig. 6: Panel strengths—in-situ measurements and SMUTI estimation 52 Shotcrete • Summer 2016


2016SummerShotcreteEMag
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