NDT measurements for the prediction of 28-day compressive strength

A procedure is proposed for the prediction of the compressive strength of concrete based on the use of NDT measurements. Prior to formwork removal, electrical resistivity can be assessed and based on an electrical resistivity versus compressive strength correlation curve, the strength of concrete can be estimated. This procedure ensures, for example, that the formwork is only removed when the concrete reaches a predefined compressive strength, dispensing with the calculation of concrete maturity. On the other hand, the whole surface of the concrete structure can be systematically measured so as to estimate 28-day compressive strengths and detect areas that could potentially not conform to the specified requirements. In this manner, a realistic overview of the in situ compressive strength of the whole structure can be obtained.The procedure proposed predicts the 28-day strength of concrete based on two models. The first model is based on an empirical equation used for the maturity assessment of concrete. The second model is based on a theoretical equation for the nucleation process of cement hydration. The effect of temperature and different curing conditions (concrete maturity) can be effectively taken into account by the conversion of all electrical resistivity measurement to the same referential. The models are used to estimate 28-day compressive strength based on the 7-day electrical resistivity measurements. The results show that the errors in estimation of the 28-day compressive strength are less than 22% and 10% for these models, respectively. If 28-day electrical resistivity measurements are used then the error in estimation is lower, circa 9% and 5% for these models, respectively.