Q: The strengths of cores taken from a bearing wall are lower than we have reason to think they should be. In our experience, core strengths have often been lower than cylinder strengths, and we frequently wonder how reliable core strengths are. Is there any way to be sure the strength of a core is representative of the concrete it is taken from?

A:We presume you are taking three cores for each area in question, as required by the ACI Building Code, ACI 318 77. (The Code says that the average of three cores must be equal to at least 85 percent of the design strength, and that no single core should be less than 75 percent of the design strength.)Core strengths give an indication of the strength of the concrete in the structure, but cannot be equated to strengths measured on regular test cylinders because cores are not made and cured the same way as cylinders. Following are some good precautions to be taken in the preparation and testing of cores based on recommendations of the National Ready Mixed Concrete Association:

  • Test at least three cores for each area questioned.
  • Drill cores with a diamond bit to avoid irregular cross-section or other damage from drilling.
  • Be sure the core has a diameter of at least 2 1/2 inches, or twice the maximum aggregate size, whichever is greater. For usual structural concretes, 31/2- to 4-inch-diameter cores are preferred.
  • Avoid including reinforcing steel in the cores because it reduces the measured strength, and correction for the reduction in strength is uncertain.
  • Drill completely through the member if possible so that the core does not have to be broken out. If the core must be broken out, however, allow two extra inches of length so that potentially damaged concrete can be sawed off. To break the core out, use wooden wedges to minimize the likelihood of damage.
  • Before capping the core, saw the broken ends to make them plane.
  • Cap in a manner that makes the ends properly plane, using an acceptable capping material.
  • Unless the concrete in the structure will be wet during its normal condition of use, dry the cores before testing. (ASTM C 42 allows cores to be tested without the normal 40-hour soaking period when the agency ordering the tests directs testing in a drier condition.)
  • Make certain that core specimens are accurately centered in the testing machine to avoid eccentricity of the applied load during testing.