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2014FallShotcreteEMag

strength variation of Stage II shotcrete cores is higher than in the Stage I shotcrete cores. 28-day Compressive Strength Figure 3 shows the compressive strength development at 28 days for Stage I shotcrete. It ranged from 4400 to 10,300 psi (30 to 71 MPa) with an average compressive strength of 7140 psi (49.2 MPa). For Stage I shotcrete, there were 15 cores that have strengths lower than the specified 5000 psi (35 MPa), which means that 97% (551/566) of the shotcrete cores met the specified compressive strength of 5000 psi (35 MPa) at 28 days. For Stage II shotcrete, except for one core which had a compressive strength of 2000 psi (14 MPa), the 28-day compressive strengths ranged from 3900 to 9700 psi (27 to 67 MPa) with Fig. 3: 28-day compressive strength for shotcrete cores from test panels, an average compressive strength of 6600 psi March to December (Note: 1 MPa = 145 psi) (45.4 MPa). Figure 4 shows there were 16 cores with strengths lower than the specified minimum 5000 psi (35 MPa). This means that 97% (464/482) of the shotcrete cores met the specified compressive strength of 5000 psi (35 MPa) at 28 days. The SD and COV of the 28-day compressive strength for Stage I and Stage II were calculated and are summarized in Table 2. The SD of 28-day compressive strength for Stage I shotcrete is 1100 psi (7.8 MPa) and the COV is 16%. Except for three cores that show higher SD and COV, the SD for Stage I shotcrete 28 days core strength is less than 870 psi (6 MPa). This shows that 99.5% (553/556) of the Stage I shotcrete cores had a SD of less than 870 psi (6 MPa) and a COV of less than 15% at 28 days. The SD of 28-day compressive strength for Stage II shotcrete is 1100 psi (7.5 MPa) and the COV is 17%. Except for four cores, the SD for Stage II shotcrete core strength is lower than 1450 psi (10 MPa). This means that 99.4% Fig. 4: 28-day compressive strength for shotcrete cores from test panels, (479/482) of the Stage II shotcrete cores had a SD July to August (Note: 1 MPa = 145 psi) of less than 1450 psi (10 MPa) and a COV of less Table 2: Summary of average strengths, standard deviation, and coefficient of variation at 7 and 28 days Stage I shotcrete compressive Stage II shotcrete compressive strength strength Variation 7 days 28 days 7 days 28 days Specified, psi (MPa) 2900 (20) 5000 (35) 2900 (20) 5000 (35) Average, psi (MPa) 4420 (30.5) 7140 (49.2) 4160 (28.7) 6580 (45.4) SD, psi (MPa) 910 (6.3) 1130 (7.8) 840 (5.8) 1090 (7.5) COV, % 21 16 20 17 CSA required average strength, psi (MPa) 4580 (31.6) 7280 (50.2) 4410 (30.4) 7180(49.5) 24 Shotcrete • Fall 2014


2014FallShotcreteEMag
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