Volume 35, Issue 6 (November 2007)
New Twist-Off Method for the Evaluation of In-Situ Strength of Concrete
This paper introduces an innovative partially destructive method called “Twist-off,” for the assessment of in situ concrete strength. In this method, a 40 mm diameter metal probe is bonded to a concrete surface by means of a high strength epoxy resin adhesive. To measure the concrete compressive strength, a torque is applied using an ordinary torque wrench and the maximum torque or shear stress at failure is used to estimate the cube compressive strength by means of a calibration graph. The relationship between the results of this new method and the compressive strengths of concrete cores as well as the correlation of this method with friction-transfer method are also presented in this paper. The average coefficient of variation of the results of this method was seen to be of the order of 8 % and the correlation coefficients of its comparative results with concrete cube and core compressive strengths were found to be 0.98 and 0.90, respectively. In order to assess the performance of this method on site, tests were undertaken on a number of concrete structures. Based on these results, this method is suggested for in situ determination of concrete strength.