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2014FallShotcreteEMag

Table 1: Wet-mix shotcrete mixture proportions Material Mass per 1.3 yd3 (1 m3), SSD aggregate Cement type GU 900 lb (410 kg) Silica fume 88 lb (40 kg) Coarse aggregate (0.2 to 0.4 in. 5 to 10 mm, SSD) 1180 lb (535 kg) Fine aggregate (SSD) 2480 lb (1125 kg) Water 49 gal. (185 L) Water-reducing admixture 0.142 gal. (0.538 L) Hydration-controlling admixture 0.227 gal. (0.860 L) Air content: at pump 3 to 6% As shot, ±1.5 3.5% Total 5072 lb (2301 kg) every test panel. Two cores were tested for compressive strength at 7 days and two cores were tested at 28 days. Shotcrete construction included Stage I, when shotcrete was applied primarily on the upper walls and overhead parts of the tunnel, and Stage II, when shotcrete was applied primarily in the lower half of the tunnel. A total of over 1000 cores were extracted from QC test panels and tested during the whole construction stage of the project. 7-day Compressive Strength A total of 566 cores were tested for 7-day compressive strength in Stage I of the project and 482 cores in Stage II. Test results are graphically shown in Fig. 1 and 2 for Stage I and Stage II, respectively. Figures 1 and 2 show that the 7-day compressive strength for shotcrete cores extracted from the test panels ranged from 1450 to 7300 psi (10 to 50 MPa) with the majority of results meeting the Fig. 1: 7-day compressive strength for shotcrete cores from test panels, specified minimum (7-day requirement of 2900 psi March to December (Note: 1 MPa = 145 psi) 20 MPa). For Stage I shotcrete, the average compressive strength was 4400 psi (30.5 MPa) and there were 22 cores out of 556—that is, 22/566 = 3.9%—where the 7-day strength did not meet the specified 2900 psi (20 MPa) com- pressive strength requirement. For Stage II shotcrete, the average 7-day compressive strength was 4200 psi (28.7 MPa) and there were 30 cores out of 482—that is, 30/482 = 6.2%—where the 7-day strength did not meet the specified compressive strength requirement. The standard deviation (SD) for shotcrete core 7-day compressive strength from both Stage I and Stage II shotcrete was less than 1300 psi (9 MPa), with 95% of them being less than 730 psi (5 MPa). The coefficient of variation (COV) for 7-day compressive strength was 21% for Stage I shot- crete and 20% for Stage II shotcrete. It should be noted that 95% (540/566) of the core strengths in Stage I had a COV of less than 15%, while 95% (460/482) of the core strengths in Stage II had a Fig. 2: 7-day compressive strength for shotcrete cores from test panels, COV of less than 20%. This shows that the July to August (Note: 1 MPa = 145 psi) Shotcrete • Fall 2014 23


2014FallShotcreteEMag
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