Limited data are available on the relationship between the strengths of test cylinders and cores taken from in-place high-strength concrete. Because high-strength concrete structures may be heavily reinforced, it can be difficult to obtain cores with the same aspect ratio (core length divided by core diameter) as cylinders. The relationship between aspect ratio and compressive strength for normal-strength concrete is well-established. But does this relationship apply to high-strength concrete? The results from five-year tests of high-strength concrete help to answer this question.

Test Results

Five commercially available high-strength concretes were continuously moist cured for five years. At the end of this time, both 4- and 2-inch-diameter cores were taken and cut into various lengths to provide information about the effect of aspect ratio on strength. The following conclusions were made:

  • When the aspect ratio is between 1.0 and 2.0, core strength of high-strength concrete is unaffected by aspect ratio. In-place concrete strengths shouldn't be estimated on the basis of data from cores having aspect ratios less than 1.0.
  • Measured strengths of small cores are much more variable than those of larger cores. Cores should be at least 4 inches in diameter or eight times the maximum aggregate size.
  • Because errors in sampling can bias the data downward, it may be desirable to take more cores when sampling high-strength concrete to obtain a better estimate of in-place strength.