Cylinder/cube compressive-strength ratio in high-strength concrete

Abstract

Spanish and European Codes on Structural Concrete (Spanish Code on Structural Concrete, EHE, and Eurocode 2, EC2, respectively) allow measuring the concrete compressive strength with cubic specimens, normally 150 mm-edged cubes, instead of the standard cylinders, which are 150 mm in diameter and 300 mm in height. As both types of specimen do not usually give the same strength, codes prescribe the use of a conversion factor between them. This paper presents an experimental study, performed with eight types of high-strength concrete, showing that the prescribed conversion factor is not valid for many cases, which leads to unsafe compressive strengths (in case the real ratio is below the one prescribed by the code) or having to use a more expensive concrete that necessary (if the real ratio is bigger than the one given by the codes). Our results indicate that the conversion ratio depends on the concrete internal structure, through the maximum aggregate size, and on its fracture properties. In fact, we derive an expression that adjusts correctly the real ratios for our experimental results. Finally, we propose that codes allow the experimental calibration of the conversion ratio ad hoc whenever the control of the concrete strength is performed using cubic specimens.