Determination of In Situ Concrete Compressive Strength Using Core, Rebound Hammer, and Ultrasonic Pulse Velocity Tests: A Regional Study in Erzincan

Reliable estimation of the in situ compressive strength of concrete is an essential component of the assessment of existing reinforced concrete structures, particularly in earthquake-prone regions. This study investigates the regional calibration and applicability of relationships based on the Schmidt rebound hammer, ultrasonic pulse velocity, and core tests for concrete used in the Erzincan region of Türkiye. Field investigations were carried out in reinforced concrete buildings constructed before and after the regulatory requirement to use ready-mixed concrete; the former were generally built with traditionally produced site-mixed concrete and the latter with ready-mixed concrete. Laboratory specimens produced from locally used ready-mixed concrete and core samples obtained from existing buildings were investigated. Linear regression models relating the rebound number and the ultrasonic pulse velocity to compressive strength were developed, and a combined rebound–ultrasonic (SonReb) model was also established. Model performance was evaluated using goodness-of-fit and error metrics, predictive performance was assessed by repeated k-fold cross-validation, and the relationships were independently validated against core samples from additional buildings. The rebound-based models showed strong relationships with compressive strength for laboratory specimens and for cores from both ready-mixed and traditionally produced concrete, and the combined SonReb model predicted strength more accurately than either single-parameter model. In the independent validation, the rebound-based relationships produced deviations from core strength that are acceptable in view of the material safety factor for ready-mixed concrete, but considerably larger deviations for traditionally produced concrete, while the ultrasonic-only relationship was too imprecise to be used on its own. These results indicate that locally calibrated non-destructive relationships can provide useful estimates of in situ concrete strength when applied to concrete with characteristics comparable to those represented in the calibration dataset. However, the proposed relationships should not be transferred directly to other regions or to substantially different materials and construction practices without additional calibration and validation. The principal contribution of this study is the regional calibration and validation of non-destructive concrete strength relationships and the quantitative evaluation of the combined rebound–ultrasonic approach for ready-mixed concrete in the Erzincan region.