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2014FallShotcreteEMag

Low Compressive Strength When accelerator was used, the variation of When shotcrete cores show low compressive the core strength tended to increase. strength results, the owner will question the With a proper QC program, a minimum of 96% quality of the shotcrete being applied, and the of the shotcrete cores tested met the specified contractor will then be required to pay attention minimum compressive strength of 2900 psi to shotcrete production, transportation, and (20 MPa) at 7 days and a minimum of 96% of the application, as well as the handling and curing of shotcrete cores met the specified compressive the shotcrete test panels. One example is a test strength of 5000 psi (35 MPa) at 28 days. result from January and February. Shotcrete core Tunnel shotcrete cores show an SD of 870 to strength results (Sets 500 to 600) showed that the 1160 psi (6 to 8 MPa) for compressive strength, strengths at 7 days were lower than 2900 psi and COV of 16 to 21%. The variation can be (20 MPa), and then increased greatly to about minimized by proper mixture design, rigorous 5800 psi (40 MPa) at 28 days; later on, March quality control, proper curing and handling of test shotcrete core strength test results (Sets 600 to panels and shotcrete core samples, proper 630) showed that the strength increased by about shooting skills, and most importantly, implemen- 4350 psi (30 MPa) from 7 to 28 days. tation of a rigorous QC testing program to test The low test results from Sets 500 to 600 compressive strength for daily shotcrete production. (January and February) was attributed to the test Shotcrete cores from test panels with strengths panels being kept in the tunnel for a few days due lower than the specified compressive strength to logistic challenges, and then were moved to were rejected as nonconforming. In such cases, above ground and left uncured. Considering the in-place cores can be extracted from the tunnel cold weather conditions in the tunnel site, which lining and tested for compressive strength, with were normally 32 to 50°F (0 to 10°C) during that these results overriding test panel results. time, the strength gain at early age under low temperature conditions was not sufficient. Most References of these sets were delivered to the QC lab at about 1. Heere, R., and Morgan, D.R., “Determination of Early- 6 or 7 days, and then put into the curing tank for Age Compressive Strength of Shotcrete,” Shotcrete, V. 4, standard moist curing. The 7-day compressive No. 2, Spring 2002, pp. 28-31.2. Zhang, L., “Accelerator Dosing Pump Calibration and strength was thus tested when cores had not been Verification,”Shotcrete, V. 14, No. 3, Summer 2012, pp. 42-44. properly cured. Consequently, the 28-day strength 3. Morgan, D.R., and Heere, R., “Shotcrete Design, increased dramatically, and even doubled or Construction, and Quality Assurance for Underground Support tripled compared to 7-day compressive strength, Engineering Foundation,” Shotcrete for Underground Support as shown in Fig. 5 and 6. IV, New York, 1987.4. CSA A23.1/23.2-09, “Concrete Materials and Methods Once core compressive strength was found to of Concrete Construction/Test Methods and Standard Practices be lower than the specified strength, the shotcrete for Concrete,” Canadian Standards Association, Toronto, ON, was ruled to be nonconforming. Contractors Canada, 2014, 674 pp. were then required to extract in-place cores from the tunnel and test these in-place cores for com- pressive strength. Test results for in-place cores override the strength of cores from the test panels Lihe (John) Zhang is an Engi- and these cores were referred to as quality neer at LZhang Consulting and assurance (QA) cores. Normally, three shotcrete Testing Ltd. Zhang has over cores were extracted from one area and tested 10 years of experience in con- for compressive strength. Compressive strengths crete technology and the eval- from QA cores ranged from 5440 to 8380 psi uation and rehabilitation of (37.5 to 57.8 MPa). Although the testing age infrastructure. He received his exceeded 28 days, the strength met the 5000 psi PhD in civil engineering from (35 MPa) requirements and therefore, the the University of British Columbia, where he project owner accepted the shotcrete represented conducted research on fiber-reinforced concrete. by them. Zhang is a LEED Accredited Professional and is a member of the American Concrete Institute Summary (ACI). He is Chair of ACI Subcommittee 506.5, Underground shotcrete QC core strength is Underground Shotcrete; a member of ACI dependent on the mixture design, nozzleman Committees 130, Sustainability of Concrete; 506, skills, equipment, curing conditions, and test Shotcreting; and 544, Fiber-Reinforced Concrete; panel handling. and a member of ASTM Committee C09, Concrete Variation in compressive strength of shotcrete and Concrete Aggregates. He is also Chair of the cores at 28 days was found to be less than that ASA Education Subcommittee: Graduate Scholar- found at 7 days. ships, and an ASA Board member. Shotcrete • Fall 2014 27


2014FallShotcreteEMag
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