I’ve been testing shotcrete cores for compression strength according to ACI 506, ASTM C1604, and ASTM C1385. The only thing that we have been doing out of specifications is the panel. Our panels are 18 x 18 in. (457 x 457 mm). We have been coring at the center of the panel 2 days after it has been cast. We test these cores at 7 and 28 days, and the strength of cores reflects passing at 7 days but failing at 28 days. Can you please tell me what could be the cause of this?

ASTM C1140/C1140M-11, “Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels,” is the appropriate ASTM standard for producing and coring test panels. ASTM C1140 specifies panel size as a minimum of 24 x 24 in. (610 x 610 mm) with a minimum 3.5 in. (89 mm) depth. Without more information on the materials used in the shotcrete and the type of shotcrete, it is impossible to identify what may be causing the lower compressive strength tests.
The compressive strength should increase between 7 and 28 days on a curve equivalent to cast concrete. Strength degradation between 7 and 28 days may be a result of poor shotcrete application or problems with coring or curing of the samples.