Review of Variables that Influence Measured Concrete Compressive Strength

Abstract

This paper summarizes the many variables that can affect the measured compressive strength of concrete cylinders. The paper includes the whole testing process: sampling, casting, initial curing, transporting, laboratory curing, capping, and testing. Most nonstandard techniques render lower measured strengths, some by as much as 75%. Improper specimen casting techniques that can cause significant measured strength losses include insufficient consolidation effort or inappropriate consolidation technique, use of flexible wall molds, especially if mistreated, and poor cylinder end conditions (even if capped properly). Initial job site curing conditions can render low results. Problems can occur from low or high temperatures, insufficient moisture, and disturbance. Rough handling during cylinder transport can lower measured strengths. Use of improper capping materials and techniques are some of the worst mistakes, and include overly used capping material and nonplane end cap surfaces. Improper load testing techniques that are significant include inappropriate specimen moisture content at time of testing, misalignment of specimen in the load frame, and inaccurate machine calibration. Discussion of machine characteristics is included. Postfailure inspection of specimens and reporting techniques are recommended.