Analysis of the single and combined non-destructive test approaches for on-site concrete strength assessment: General statements based on a real case-study

The evaluation of the compressive strength of concrete in existing structures by coring is expensive, technically difficult in certain cases, and even impossible in others. The use of non-destructive testing (NDT) is an interesting alternative method (i.e. affordable cost, portable, fast, etc.). However, the NDT estimation of strength requires a procedure of calibration of the model between NDT and compressive strength. The robustness of this calibration is a crucial point allowing better choice of the optimal number of cores. Studies which treat the calibration of proposed models are often based on laboratory experiments or synthetic data. The present study aims at identifying and optimizing the methodology of the calibration model on site. This paper is based on a broad campaign of auscultation using NDT (Rebound and Ultrasound) and coring on an existing construction with 205 triplets of data (strengths and NDT results). Statistical data analysis enables to quantify the role of: the number of cores (NC) used for the calibration, the use of only one or two-combined NDT techniques and the calibration method. The conclusions are focused on the improvement of the relevance and the effectiveness of NDT techniques in such operational situations.