3.2 Early strength development and ground support performance Field tests confirmed that the in-cycle shotcrete mix gained strength rapidly underground, consistent with lab results. Penetrometer measurements and a limited number of early-age core breaks indicated compressive strength on the order of 1-1.5 MPa at 1 hour after spraying, and 3-4 MPa at 3 hours. In contrast, the mine’s previous standard shotcrete (a slower-setting mix with lower accelerator dosage and no fibers) reached only about 0.5 MPa at 1 hour and ~2 MPa at 3 hours. By 24 hours, the ICS mix exceeded 20 MPa, and ultimately attained >40 MPa at 28 days, comparable to the long-term strength of the standard mix. The improved early strength allowed safe re-entry in under 3 hours, as intended, without observed failures. During the trial, no instances of sloughing or collapse were reported in areas that had been shotcreted in-cycle, whereas historically some headings experienced minor falls of ground in the interval between blasting and the next shift’s shotcreting. The fiber reinforcement proved effective in controlling cracking and enhancing toughness of the liner. Observations showed that even when blasting the subsequent round, the fresh (only a few hours old) shotcrete remained well-adhered and suffered only hairline cracks, but did not dislodge. This indicates it had developed sufficient integrity to withstand blast vibrations and stress relaxation. This performance is significant: it demonstrates that with proper design, an expedited shotcrete can serve as a competent temporary liner, even when subjected to early-age disturbances (Bernard, 2010). Table 4 shows a summary of Early-Age Compressive Strength Results for ICS Shotcrete Panels (MPa). Figure 2 presents an example of an early strength gain curve for the ICS mix versus the baseline mix, highlighting the much steeper initial strength ramp-up of the ICS approach. Figure 2 – (ex., panels 1 & 2) Early compressive strength gain of the ICS shotcrete mix compared to the baseline traditional mix (strength measured via penetrometer and core tests). The ICS mix achieved approximately 1 MPa in 1 hour (dotted line indicates re-entry threshold), we show results from several panels development with ICS shotcrete mix.
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