Improvement of nondestructive assessment of on-site concrete strength: Influence of the selection process of cores location on the assessment quality for single and combined NDT techniques Khoudja Ali-Benyahia a,b , Zoubir-Mehdi Sbartaï c,⇑ , Denys Breysse c , Mohamed Ghrici b , Said Kenai d a Department of Technology, University Djillali Bounaama, 44225 Khemis-Miliana, Algeria b Geomaterials Laboratory, Hassiba Benbouali University of Chlef, 02000 Chlef, Algeria c I2M, UMR 5295, University of Bordeaux, 33405 Talence, France d Department of Civil Engineering, University Saad Dahleb, 09000 Blida, Algeria highlights  A new methodology has been proposed in order to improve the concrete strength assessment in existing structures by using NDT.  Conditional coring is, a very efficient way to improve the concrete strength evaluation by NDT.  The prediction error ‘‘RMSE” is mandatory as it is the relevant indicator of the quality of the concrete strength assessment. article info Article history: Received 1 August 2018 Received in revised form 28 September 2018 Accepted 4 October 2018 Keywords: Structure Concrete strength Non-destructive evaluation Rebound Hammer Ultrasonic Pulse Velocity Calibration quality Random coring Conditional coring abstract For existing constructions, there is a need to estimate the concrete mechanical strength for a more accu- rate evaluation of their structural capacity. The destructive evaluation of strength by coring is expensive, sometimes technically difficult and it may be impossible in some situations and the evaluation is limited to a restricted surface of the structure. Non-destructive testing (NDT) in conjunction with destructive testing by coring (DT) offers an interesting alternative for the strength assessment of concrete in existing buildings. For a reliable assessment of concrete strength, it is necessary to fully master the calibration quality of the conversion models between DT and NDT used for strength estimation. Because of technical and economic constraints, the professional practice is frequently based on a very limited number of cores. If this optimal number is not reached, the risk is high that the resulting conversion model is not reliable. This practical context, in which the common number of cores is small, forces to review the calibration procedure in order to take care of the relevance of the identified model. The procedure using conditional coring, which is based on the selection of critical locations of cores after an analysis of NDT test results, is the subject of methodological developments presented in this paper. Our research works aims at analyz- ing this issue on a broad investigation program by NDT (Rebound Hammer and Ultrasonic Pulse Velocity) and coring realized on the structural elements (columns and beams) of an existing building. 205 data tri- plets (core strengths and NDT test results) are used to carry out this study. The thorough statistical data analysis allows quantifying the potential interest of the conditional coring compared to the random cor- ing with two measurement strategies of NDT (by using single or combined methods). Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction The assessment of concrete strength in existing constructions is necessary during their service life for reasons of restoration, mod- ification, change of use or after a fire or seismic damage [1–3]. The resources allocated to the assessment of such buildings by the tra- ditional methods (coring) to estimate strength has reached a sig- nificant level [4]. The assessment by non-destructive testing (NDT) in conjunction with destructive testing (DT) constitutes one of the promising ways in order to reduce these budgets in terms of cost and time [5]. NDT also allows a wider coverage of the structure and gives access to new information as spatial vari- ability of concrete properties and the identification of homoge- neous areas [6–8]. However, with a relatively low number of DT, the risk is high, for simply statistical reasons, to obtain relations https://doi.org/10.1016/j.conbuildmat.2018.10.032 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: zoubir-mehdi.sbartai@u-bordeaux.fr (Z.-M. Sbartaï). Construction and Building Materials 195 (2019) 613–622 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat