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2014FallShotcreteEMag

than 28%. This shows that the 28-day compressive However, it was found that the accelerator strength for Stage II shotcrete cores is higher than dosing pump was not always operated properly. that in Stage I shotcrete. Excessive accelerator addition occurred at times and was found to be detrimental to the Compressive strength at 28 days 7- and 28-day compressive strength.4 This influenced the variability of compressive versus 7 days strength test results. Generally, concrete develops about 70 to 75% 4. Nozzlemen shooting technique: Shotcrete is of its 28-day compressive strength at 7 days. For a method of placing concrete, and the quality shotcrete cores, similar compressive strength of the shotcrete is dependent on the nozzleman’s development is expected, provided the cores are skills. Hand nozzling requires proficient cured properly. Figures 5 and 6 show the increases operation of the nozzle, prompt adjustment of of the compressive strength from 7 days to 28 days. The compressive strength ratio is defined as the ratio of compressive strength at 28 days versus the compressive strength at 7 days. The ratio ranges from 1.5 to 2.5 for both Stage I and Stage II shotcrete. This variability could be attributed to the following reasons: 1. Curing of the testing panels: The shotcrete QC test panels were typically shot in the tunnel, and left there for 2 days before being transported to the surface and then to the testing laboratory. The moisture content in the tunnel was generally above 80%, and was considered to provide natural curing as defined in ACI 506.5R-09, “Guide for Specifying Underground Shotcrete.” But the temperature in the tunnel was generally in the 40 to 50°F (5 to 10°C) range. In addition, Sets 541 and 542 had greater increases in compressive strength from 7 to 28 days due to the fact that the cores were left on site without proper curing (the weather was cold during February when cores were extracted), and were only delivered Fig. 5: Compressive strength of concrete at 28 days versus 7 days for to the lab at 7 days of age. Therefore, these Stage I shotcrete cooler temperatures caused a slower rate of strength development in the 7-day test results, compared to cores with standard laboratory curing at 73°F (23°C) from 2 days of age. 2. Handling the QC test panels: Due to the challenges in shotcrete QC panel handling, some panels, when moved to the surface, were found to have been left outside for a few days without proper curing before cores were extracted for testing. In a few cases, the shotcrete cores were extracted at 7 days and then sent to the laboratory for testing without even being stored in the laboratory curing tank. 3. Addition of accelerator: An alkali-free accelerator was added to the shotcrete. The dosage of the accelerator was found to not be consistent during shotcrete production. This was largely due to the challenges of properly operating the accelerator dosing pump in the underground environment. The author worked with the contractor to accurately calibrate the accelerator dosing pump, establish operational Fig. 6: Compressive strength of concrete at 28 days versus 7 days for procedures, and train the shotcrete crew.3 Stage II shotcrete Shotcrete • Fall 2014 25


2014FallShotcreteEMag
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