Goin’ Underground hydration is shown in Fig. 8. It can be observed that the panels have an average degree of hydration of 0.12, 0.24, 0.38 and 0.49 at the ages of 3 hours, 6 hours, 12 hours, and 24 hours, respectively. On the other hand, the lining areas achieve an average degree of hydration of 0.15, 0.29, 0.44, and 0.61 at the ages of 3 hours, 6 hours, 12 hours, and 24 hours, respectively. The lining also experienced faster strength gain as while the panels had an average compressive strength of 16.6 MPa at 12 hours, the average lining strength was 2860 psi (19.7 MPa). Additionally, with an average degree of hydration of 0.61 at 24 hours, the lining had achieved an average compressive strength of 4120 psi (28.4 MPa). It must be pointed out that from the fc-ξ relationship shown in Fig. 5, it could be asserted that the shotcrete can achieve an average long-term strength of more than 6960 psi (48 MPa). This relationship was reasonably verified as the mean 90-day strength of the lining cores was determined to be 6860 psi (47.3 MPa). Conclusions and Future Works From the results shown in the previous section, the following can be concluded: 1. The Arrhenius equation-based temperaturesensitive maturity function is a useful tool to estimate shotcrete strength through the remote and nondestructive approach adopted in SMUTI. 2. With an average variation of 7% between the measured and calculated panel strengths, SMUTI appears to provide useful estimates that are in close agreement with the in-situ tests. 3. The fc-ξ relationship deduced from the panel testing was reasonably verified by available 90-day lining core strengths averaging to 6860 psi (47.3 MPa). While a promising step has been taken, further laboratory testing and on-site application is the most logical next step. This will improve understanding of degree of hydration development of shotcrete, especially when various admixtures, such as accelerator and superplasticizer, are key participants in its application. It will also enable the reliability of the method to be assessed. SMUTI has the potential to provide the strength gain of the whole shotcrete lining (as against local tests on a panel) from a remote location. This is a step-change in safety and quality control of shotcrete tunneling. As a final remark, the authors envisage that integration of the thermal imaging capability into tunnel setting out and convergence monitoring survey systems will further simplify the workflow Fig. 7: Temperature histories for tested panels and corresponding EBRT-W lining section Fig. 8: Degree of hydration determination using temperature histories of EBRT-W section Fig. 9: Comparative plot showing in-situ panel strength and estimated strength for EBRT-W lining section Shotcrete • Summer 2016 53

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