How to evaluate concrete core strengths

compressive strength testing of concrete cores
Concrete core strengths can help the Engineer of Record assess structural safety when the cylinder strengths don’t conform to ACI 318. Shutterstock image.

There are many reasons to take cores of in-place concrete. You might want to know the capacities of the footings and columns in an old building. Or maybe you’re conducting a forensic investigation of a structure to determine the cause(s) of its failure. In pavements, cores verify the thickness. In buildings under construction, however, concrete core strengths may be necessary when tests of concrete cylinders fail to meet the criteria of ACI 318, Building Code Requirements for Structural Concrete.

ACI 301, Specifications for Concrete Construction, has two requirements for compressive strength:

  • Every average of three consecutive tests equals or exceeds the specified strength, fc.
  • No test falls below fc by more than 500 psi if fc is 5000 psi or less, or by more than 0.10fc if fc exceeds 5000 psi.

Before taking cores, the Engineer of Record should consider less destructive alternatives. It may be that lower strengths are adequate for the part of the structure they represent. Even if that’s the case, though, the engineer should investigate whether the procedures require adjustment. For example, if the on-site curing conditions don’t conform to ASTM C31, the test results will be artificially low.

If the cylinder strengths are unacceptably low, the engineer should investigate the in-place strength of the concrete. Various nondestructive and minimally destructive tests are available. Some, such as the rebound hammer (ASTM C805), can only give a relative indication of the strength unless you calibrate them. However, if you’re trying to determine where in the structure the concrete is weaker, that’s useful information. You may want to do an overall survey of the in-place strength and then do further testing where it appears weaker.

If you need to take cores

Consider what you want the cores to tell you. ACI 318 section 26.12.6 is for verifying the in-place strength of the concrete to evaluate structural safety. But that may not be all you need to know. You may need to know why the strength is low to correct the problem or to establish responsibility for it. If so, you’ll want additional cores for petrography or other testing.

We’ve discussed previously how to take good concrete cores. For concrete core strengths, ACI 318 requires three cores conforming to ASTM C42. It’s essential that the person who takes the cores have the right qualifications. Ideally, someone familiar with ASTM C42 will witness the coring to ensure everything is done correctly. The core diameters should be nominally 4 in. or twice the maximum size of the coarse aggregate, whichever is larger, and the cores should have an aspect ratio of at least 1.0 after trimming. They should also be free of reinforcing bars and other inclusions. It may not be possible to satisfy all of these criteria. For example, a slab may be too thin or the reinforcement too closely spaced. In such cases it’s acceptable to use a smaller core diameter.

The engineer should determine where in the structure and where in the member to drill each core. Use a nondestructive test method to locate reinforcement and prestressing tendons so you don’t cut into them.

Label each core and seal it in a moisture-proof container such as a plastic bag. Drilling and sawing concrete almost always involve water for cooling. Because moisture gradients induce stresses in the concrete, you need to allow at least 5 days after any cutting of the core before you test. Shortly after you get the core into the lab, cut off the ends to provide flat surfaces perpendicular to the axis. Return the core to its moisture-proof container until it’s ready to test.

Evaluating concrete core strengths

Testing cores is similar to testing cylinders, but there’s a correction factor for cores with aspect ratios significantly less than 2.0. ACI 318 has two criteria for determining the acceptability of the concrete:

  • The average strength of three cores is at least 0.85fc.
  • No single core strength is less than 0.75fc.

The factors 0.85 and 0.75 take into account that coring, transport, and handling may induce damage in the concrete. Also, the concrete in the structure has undergone different conditions of placement, consolidation, and curing than the cylinders. The purpose of testing cores is to ensure the safety of the structure, not to evaluate the quality of the concrete delivered to the site.

If the test results fail either of these criteria, the concrete may not be structurally adequate. The Engineer of Record has some discretion here—for example, to verify that there were no errors in coring, handling, and testing. It helps if the testing lab retains the broken cores for examination, as that may provide some clues.

The engineer may decide to take additional cores for another strength test, particularly if one core tested low but the average of the three was acceptable. If the results of the second test meet the criteria of ACI 318, the structure is adequate. If not, the engineer will have to decide whether to perform other testing such as a load test.

When the concrete strength is inadequate, the Engineer of Record determines what remedial measures to take. These could include additional curing, wrapping with carbon fiber, or removal and replacement of the concrete.

Understanding why

When concrete core strengths are inadequate, it’s important to find out why. The better you understand the situation, the likelier you are to come up with a good solution. And ultimately you’ll need to determine who’s responsible. If you have additional cores or cylinders, petrography can be highly informative. Keep in mind that the cylinders represent the concrete as supplied, while cores represent the concrete as placed. If you don’t have extra specimens, you may be able to get some information from the offcuts of the cores. Similarly, if you have some cylinders you’re saving to test at a later age, you may be able to trim one of the ends to use for petrography. Use the appropriate adjustment factor to account for the change in aspect ratio on the compressive strength.

Some years ago a concrete supplier mixed up the proportions of cement and fly ash in some footings. Everyone wanted to know whether the inadvertently high fly ash concrete would ever gain sufficient strength. I recommended that the lab boil the cylinders to accelerate the strength gain. That way they’d have a conservative estimate of the later-age strength. Once we had that, we could either just wait or apply moderate heat to the footings. In this case the footings weren’t on the critical path for the project, so time was on our side. If the Engineer of Record was really concerned, he could order additional cores once the curing was complete.