It’s important when testing for compressive strength of concrete and mortar products the right methods are used. Mortar products should be tested in accordance with ASTM C109 using 2x2x2 cube molds as seen in the brass 3 gang mold in this picture. Concrete products should be tested in accordance with ASTM C39 using cylinders like the ones shown on the right side of the picture. The difference between a mortar and concrete is that concrete products have number 8 sized rock or larger in them while mortars have aggregates smaller than number 8 rock.