My firm is a general contracting entity that frequently uses shotcrete subcontractors. When project specifications are not clear on testing, I have been relying on the advice of my shotcrete subcontractors on the frequency of taking tests for compliance with strength requirements. We always shoot a test panel prior to starting construction. How much testing should we be doing during construction?

ACI 506.2, “Specification for Shotcrete,” recommends that a test panel be produced for every 50 yd3 (38 m3) of shotcrete placed or one per day, whichever is less. A minimum of three cores are to be cut from the test panel for compressive strength testing in accordance with ASTM C 42, “Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete.” Testing must be performed in accordance with ASTM C 1140, “Standard Practice for Preparing and Testing Specimens from Shotcrete Panels.” The average of the strength results from the cores must be at least 85% of the specified strength with no individual core less that 75% of the specified strength.