Using Twist-Off Method for Measuring Surface Strength of Concretes Cured under Different Environments

Abstract

This paper presents the results of a research project whose goal was to measure the surface strength of concretes cured under different environments by using the newly developed twist-off test. A circular steel disk was glued to the surface of the concrete with an epoxy resin. The force required to twist this disk off the surface, together with an attached layer of concrete, was measured by using an ordinary torque wrench. The different curing regimes had specimens immersed in drinking water, covered with a wet hessian and polythene sheet, kept in a dry laboratory, left in open air, steam cured, and coated with a curing agent. Eight different concrete mixtures with a 28-day design compressive strength of 10–75 MPa were used. The effects of different water-cement and aggregate-cement ratios were studied for all mixes cured under the six different environments. The coefficients of variation associated with surface-strength measurement were also studied for different curing regimes.