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2016SummerShotcreteEMag

Fig. 3—Shotcrete test panels were shot by an ACI Certified Nozzleman for both wet-mix shotcrete (left) and dry-mix shotcrete (right). (5076 to 9283 psi). Compressive strength for dry-mix shotcrete varied from 35 to 63 MPa (5076 to 9138 psi). Compressive strengths of shot wet-mix shotcrete versus cast wet-mix shotcrete and cast concrete—If one compares like mixtures (for example, cement-only mixtures with cast concrete, cast wet-mix shotcrete, and shot wet-mix shotcrete) and the same for fly ash and silica fume mixtures, then it is evident that sprayed shotcrete mixtures without accelerators consistently produce higher 7- and 28-day compressive strengths compared to cast shotcrete mixtures, or cast concrete mixtures. The differences are not large, but they are (with one exception) consistent. This supports the statement that “shotcrete, when properly applied, provides superior compaction to the cast-in-place concrete process.”9 Table 7 shows these comparisons at the age of 28 days. Compressive strength of dry-mix shotcrete—Most of the dry-mix shotcrete mixtures displayed higher 28-day strengths than like wet-mix shotcretes. This is possibly attributable to a slightly lower w/cm in some of the dry-mix shotcrete mixtures. Effect of accelerator on wet-mix and dry-mix shotcrete compressive strength—The addition of an accelerator is a common practice in the shotcrete industry—in particular, in underground shotcrete application. Accelerators help shotcrete to stick overhead and increase early-age (up to the first 24 hours) compressive strength development. However, compared to mixtures without accelerator, they tend to reduce the 7- and 28-day compressive strength, depending on the accelerator addition rate. With 5% by mass of cement of non-alkali accelerator added to the wet-mix shotcrete, the compressive strength for fly ash and silica fume mixtures decreased relative to the mixtures with no accelerator. The same effect also occurred with dry-mix shotcretes with 3% accelerator added. This is consistent with typical findings in the shotcrete industry.9 Boiled absorption (BA) and volume of permeable voids (VPV) BA and VPV tests were conducted to evaluate the porosity of the concrete and shotcrete. Test results from cores tested at 28 days are plotted in Fig. 4. ACI 506 recommends that values for BA and VPV not exceed 8% for BA and 17% for VPV. All of the 18 mixtures tested produced BA values less than 6.5% and VPV values less than 14.5%. Test results for both BA and VPV consistently decreased from cement to fly ash to silica fume irrespective of the placement method. This is consistent with the results of previous research conducted in North America, Australia, South Africa, and Europe.9 Table 8 summarizes the BA and VPV results for shot wet-mix shotcrete versus cast wet-mix shotcrete versus cast concrete. Cast concrete mixtures have almost the same BA and VPV values as cast wet-mix shotcrete. This is because the w/cm for both groups of mixtures is the same. BA and VPV test results for shot wet-mix shotcretes without accelerator are slightly lower than for cast wet-mix shotcretes for the cement and fly ash mixtures, and equal to or slightly higher for cast wet-mix shotcrete with silica fume. This indicates that, overall, the shooting process tends to produce lower permeability than the casting process. However, BA and VPV test results for the shot wet-mix shotcretes with 5% accelerator are higher than for the shot wet-mix shotcrete without accelerator, or the cast wet-mix shotcrete. This shows that when accelerator is added, the permeability of the shotcrete increases. This is consistent with findings on numerous underground support projects.9 Table 9 presents the BA and VPV results for shot dry-mix shotcrete compared to cast concrete (note: one cannot cast dry-mix shotcrete). These results show that non-accelerated dry-mix shotcrete has lower values of BA and VPV for the fly ash and silica fume mixtures and a similar value for the cement-only mixture. By contrast, the dry-mix shotcrete mixtures with 3% non-alkali accelerator had consistently higher BA and VPV values than the cast concrete. This shows that when accelerator is added into the shotcrete, the permeability increases. It should, however, be noted that all the dry-mix shotcrete BA and VPV values are consistently well below the maximum acceptable values of 8% for BA and 17% for VPV provided in ACI 506R. Rapid chloride penetration resistance (RCP) The ASTM C1202 rapid chloride penetration (RCP) test is one of the most widely used test methods to evaluate the chloride penetration resistance of concrete. Although the RCP test provides a measure of current flow, rather than the chloride ion diffusion rate or actual chloride penetration resistance, it does provide information on the electrical resistivity of concrete. Table 10 shows the results of all RCP tests. SAhCotIc Mreatet e•r Siaulms mJoeur r2n0a1l6/M ay-June 2016 37 379


2016SummerShotcreteEMag
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