Are the days of the 6x12 concrete cylinder for compressive strength testing coming to an end? While things move slowly in the construction world, newer data has shown little difference between measured strength for 4x8 concrete cylinders compared to 6x12s. (see Nick Carino’s Q&A in the June 2010 issue of Concrete International). And with ASTM now recognizing 4x8 cylinders in C31, “Standard Practice for Making and Curing Concrete Test Specimens in the Field” and C39, “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens,” there’s little reason to continue with the old big cylinders.

The advantages of 4x8s are:

  • They are easier for the technician to make.
  • They weigh about 9 pounds compared to 29 pounds for a 6x12, making them easier to handle (and less likely to be damaged in handling).
  • The smaller size takes up less space in curing chambers.
  • With the smaller cross-sectional area, testing high strength concrete is easier with smaller test equipment.

In the past there was a general feeling that 4x8s had greater test variability and gave higher strength results but mostly that’s been overcome by a slight change to the technique for making them (rodding only twice). And the 4x8s can’t be used for mixes with aggregate greater than 1 inch, but that’s not an issue with most concrete—this despite the fact that I have long advocated using larger aggregate sizes in slabs when possible.

So are you still using 6x12 cylinders on your projects? Bryan Castles, an engineer in Arizona, tells me that almost all cylinders in his area are 4x8s. What’s holding up this change?