Question: We had some cores drilled in a pavement section where there was some question about compressive strength. We got the compressive strength report back from the lab, and the strength was rounded to the nearest 1 psi. Is this correct?

Answer:Compressive strength is nothing more than a ratio between applied load and the surface area. For cores or cast-in-place cylinders, the surface area is calculated with the following equation: Area = Pi * D / 4, where D is the diameter of the core or cylinder.

When the diameter of the cylinder is measured to the nearest 0.01 inch, the final compressive strength should be recorded to the nearest 10 psi for both cast-in-place cylinders and cores, according to ASTM C 39 Section 8.1 and ASTM C 42 Section 6.8.2. However, should the diameter of a core have a precision of only 0.1 inch, the final compressive strength should be rounded to the nearest 50 psi. There is no provision for reporting to the nearest 50 psi for cast-in-place cylinders, only for drilled cores. In no case should the compressive strength ever be rounded to the nearest 1 psi.

It's also important to note that in calculating the ratio between applied load and area, you also have to use a strength correction factor, determined by the ratio between the length of the cylinder and the diameter. You can find this ratio in ASTM C 39 Sec. 8.2 and ASTM C 42 Sec. 6.7.2. The precision and accuracy of the diameter measurement determines the final precision of the compressive strength.