GeneralQ Our construction management firm is relatively new in allowing shotcrete on our projects. In the most recent issue of Shotcrete magazine, there was a discussion of cores taken from shotcrete in the FAQ feature. Is there additional critical information we should be aware of when determining our coring plan?
ASTM C 1604, Standard Test Method for Obtaining and Testing Drilled Cores, covers cores that are obtained for determination of length, compressive strength, or split tensile strength. In addition to discovering the thickness of the applied shotcrete and its strength, a visual assessment can be made to evaluate the shotcrete quality, workmanship, shotcrete-to-substrate bond, and condition of the reinforcement. Shotcrete core strength is affected by core orientation relative to the direction of the shotcrete application. Therefore vertical, sub-horizontal, and overhead application of the same shotcrete may show variability. If obtaining cores for determination of compressive strength, cores containing wire mesh or reinforcing bars may not be used. Also, if a sample has been damaged in the process of removal, it cannot be used for strength determination. Cores must have a diameter of at least 3.0 in. unless otherwise permitted by the specifier. Cores with diameters less than 3.0 may demonstrate somewhat lower strengths and have greater variability. They may also be more sensitive to length-diameter ratio. Cores with length-diameter (L/D) ratios greater than 2.1 must be sawed to produce a capped or ground specimen with a L/D ratio between 1.9 and 2.1. Strength results from cores with L/D ratios less than 1.75 must be corrected as detailed in ASTM C42. A core having a length of less than 95% of its diameter before capping or a length less than its diameter after capping or grinding shall not be tested unless otherwise directed by the specifier. To avoid introducing the effects of moisture gradients of wetting and drying, extracted cores are to be stored in a sealed plastic bag at all times except during end preparation and a maximum of 2 hours prior to capping. Prior to capping, it is a good idea to determine the density of each core. Reported results should include the following: length of the core as drilled reported to the nearest ¼” (5 mm); length of the test specimen before capping or grinding reported to the nearest 0.1 in. (2 mm) and average diameter to the nearest 0.01 in. (0.2 mm); compressive strength reported to the nearest 10 psi (0.1 mpa) if the diameter is reported to the nearest 0.01 in. (0.2 mm) or nearest 50 psi (0.5 mpa) if the diameter is reported to the nearest 0.1 in. (2 mm); direction of the application of the load with respect to the horizontal plane of the shotcrete as placed; moisture conditioning history; date and time of test; nominal maximum size of the shotcrete aggregate; if determined, the estimated density; and any deviation from the stated test method and the reason for the deviation.
