Regarding sample size for compressive strength, the core length-to-diameter ratio should be in the range of 1:1 to 2:1, with length-to-diameter core strength correction factors applied as per the requirements in ASTM C 42, Clause 7.9.1. Shotcrete test panels are typically between 3.5 to 5 in. (89 to 127 mm) deep. Thus, either 3 or 4 in. (76 to 102 mm) diameter cores should be drilled for compressive strength testing, depending on test panel thickness. We would also suggest referring to ASTM C 1604/C 1604M for securing and testing cores of shotcrete. This new test method allows smaller core diameters for shotcrete in an effort to provide for increased length-to-diameter ratios. Care should be taken when interpreting the compressive strengths using smaller-diameter cores because of the possible presence of voids, which may result in compressive strengths that are not representative of the actual in-place shotcrete.