Is there a good way to correlate core compressive strength and flexural strength? I have a job in which there was only one beam broken, at 28 days, and there are not enough sets to perform any statistical analysis. We are planning to core the pavement and are looking for a comparison between the two.

A correlation can be made between cast compression samples (cubes or cylinders) and cored samples, which will be the first thing you will have to establish. Correlation of flexural and compressive test results can also be determined, but it is only an approximation.

Just as background, concrete is usually assumed to be about 10% as strong in tension as it is in compression. That tensile strength is the basis for its ability to resist bending, or its flexural strength. ACI 207R, Effect of Restraint, Volume Change, and Reinforcement on Cracking of Mass Concrete, states in Chapter 3 that concrete’s tensile strength is often taken as 6.7 times the square root of its compressive strength. It also notes that where a conservative estimate is in order, you can use a minimum tensile strength of 4 times the square root of the compressive strength.

For pavement-specific applications, you can also look at Section 2.5 of ACI 330-R, Guide for Design and Construction of Concrete Parking Lots. The relationship is given in terms of the modulus of rupture, a more direct measure of flexural strength, which can be found by raising the compressive strength to the 2/3 power and multiplying that by 2.3.

For concrete with a compressive strength of 3000 psi, the corresponding calculated tensile strengths using these four approximations are 300, 367, 219 and 478 psi, respectively.

The type of coarse aggregate in the concrete also significantly affects the compressive/tensile strength relationship. All other things being equal, concrete made with rounded aggregate will have lower tensile strength than concrete made with crushed aggregate.

Back to your particular situation: You likely will have to say something like the strength of the core samples is 80% of the strength of the compression samples, and the flexural strength is 10% of that. Both ratios would have to be established, and combining approximations provides plenty of room for error.

If you are using cylinders for samples, it may be easier to determine the correlation between indirect tensile strength and flexural strength. Alternatively, if you go through the trouble of coring, you might consider cutting beams from the pavement and testing them to get the in situ flexural strength and proceed from there.

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