the accelerator dosage to meet the ground (50.0 MPa). Second, core strength variations condition requirements, and instant were found between different nozzlemen. communication with other crew members, During QC testing, it was found that the including the pump operator, to operate safely. shotcrete core strength from one particular During shotcrete construction, two items nozzleman was typically 725 to 1450 psi (5 to which influenced compressive strength were 10 MPa) lower than the other nozzlemen. observed and found to be related to the Shotcrete cores were examined prior to and nozzleman’s shooting technique. First, a after testing and it was found that there was variation in strength between two cores typically a layer of overspray in the middle of extracted from the same test panel was shot by the cores (Fig. 7). This nozzleman was then one nozzleman. One set (Set 282) had a core trained by the author to properly operate the with 28-day compressive strength of 7140 psi nozzle and improve nozzling skills. (49.2 MPa), and the strength of the other core was only 3550 psi (24.5 MPa). Another set Variability of Shotcrete Core (Set 323) had one core with a compressive strength of 4680 psi (32.3 MPa) and the other Compressive Strength core had a compressive strength of 7250 psi 1. Table 2 summarizes the SD and COV for 7- and 28-day compressive strength for both Stage I and Stage II shotcrete. It shows that the COV for 7-day strength is 20 to 21%, and 15 to 17% for 28-day strength. This shows that the 7-day strength tends to have higher variation than the 28-day strength. The reasons have been discussed in the previous sections. Those include curing, panel handling, and core extraction. 2. The Canadian Standard Association (CSA) A23.1/23.2-20094 requirements for cores drilled from a concrete structure are: “… for standard-cured concrete specimens, the strength can be accepted if concrete has an average strength of (1) 1.4 times the SD above the specified strength when the SD is not more than 3.5 MPa (500 psi); and (2) 2.4 times the SD minus 3.5 MPa (2.4 x SD – 3.5 MPa) above the specified strength when the SD is more than 3.5 MPa (500 psi). The SD should be based on at least 30 consecutive strength tests, representing concrete made from a Fig. 7: Low-strength shotcrete cores from one nozzleman, single mix design.” The CSA-required with overspray in the middle strength is calculated and listed in Table 2. It appears that the actual average strength for 7 days and 28 days for Stage I and Stage II are slightly lower than the CSA-required average strength. This is due to the fact that several compressive strength tests did not meet the specified compressive strength of 2900 psi (20 MPa) at 7 days and 5000 psi (35 MPa) at 28 days. Based on the testing data from this project, it appears that the COV of shotcrete core strength should be less than 20%. The least variation can be achieved by proper mixture design, rigorous quality control, proper curing and handling of test panels and shotcrete core samples, proper shooting skills, and most impor- tantly, rigorous implementation of a QC testing program to test compressive strength for daily shotcrete production. 26 Shotcrete • Fall 2014

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