Are there data relating w/cm to flexural strength that are as reliable as those for compressive strength?

Q. Are there data relating w/cm to flexural strength that are as reliable as those for compressive strength?

A. No. Flexural strength is more sensitive to aggregate characteristics, especially the particle shape, surface texture, and concrete moisture content. A small amount of surface drying of flexural strength test specimens will make a large reduction in apparent flexural strength but a very minor increase in the compressive strength of comparison test specimens. Therefore, where concrete is to be accepted or rejected on the basis of flexural strength tests, the curve should be selected on a conservative basis until job tests can be made.

References: SP-1(02)

Topics in Concrete: Mixture Proportioning; Testing of Concrete