Question: We recently supplied controlled low-strength material (CLSM) for backfill. Specifications called for a maximum 28-day compressive strength of 200 psi to allow excavation by backhoe at a later date. We designed for a strength between 50 and 150 psi, but cylinder breaks indicated compressive strengths exceeding 200 psi. Why would these breaks be so much higher than our design strengths? And is there a chance that the in-place strength is lower than the cylinder breaks indicate?

Answer: We can think of a few materials-related reasons for the high test breaks. If you normally use a Class F fly ash in the CLSM but switched to a Class C ash, 28-day strength would increase. If your design mix had a high entrained-air content, but air was lost in transit or when test cylinders were made, the decreased air content would increase strength. Finally, if there was leftover concrete in the trucks when the CLSM material was batched, a higher-than-normal cement content and amount of coarse aggregate could account for some strength gain.At least some of the higher-than-expected cylinder strength might also result from using a testing machine that isn't sensitive enough. Standard concrete testing machines shouldn't be used to test CLSM since the dials on many are readable only to the nearest 250 pounds. Samples of CLSM are often fabricated and tested in accordance with ASTM D 4832, Strength Test Method for Preparation and Testing of Soil-Cement Slurry Test Cylinders.

This test method states, "Since the compressive strength of soil-cement slurry cylinders will typically be 100 to 300 psi, the testing machine must have a loading range such that valid values of compressive strength can be obtained... in accordance with ASTM C 39." ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, states, "In no case shall the loading range of a dial be considered to include loads below the value that is 100 times the smallest change of load that can be read on the scale." If you tested 6x12-inch cylinders, it would take only a 2,830-pound load to generate a 100-psi stress. That means the dial readability must be 2,830/100 or about 25 pounds. Thus if you're reading to the nearest 250 pounds, it would be easy to have a dial reading error of about 10% for a 100-psi material. It's likely that the in-place strength will be higher than the cylinder strength. That's because the cylinders are made in impervious molds, and little water is lost up to the time of testing. At the jobsite, some water migrates out of the CLSM and into the surrounding soil. Because of this, the water-cementitious materials ratio is likely to be lower and the strength higher-for in-place CLSM. There's also some long-term strength gain for the in-place material. If excavation is done a year after placement, there's a good chance that the CLSM in-place strength will be higher than the 28-day cylinder strength.