Influence of Test Direction on Rebound Hammer Measurements: Quantifying Anisotropy for Reliable In-Situ Concrete Strength Estimation

The Rebound Hammer (RH) test is a widely used non-destructive method for assessing the surface hardness and estimating the compressive strength of concrete. However, its accuracy is influenced by various factors, including the direction of testing relative to the casting orientation. This study systematically investigates the directional dependence of RH readings on concrete specimens. M25 grade concrete specimens were tested at 7, 14, and 28 days using a Schmidt hammer in three perpendicular orientations: Downward (D), Rightward (R), and Upward (U). Results demonstrated a statistically significant (p < 0.05) directional bias. The Upward direction consistently yielded the highest average rebound numbers (e.g., 23.1, 21.4, 28.7 at 7, 14, 28 days, respectively), followed by the Rightward and Downward directions, a trend that persisted in the derived compressive strength values. One-way Analysis of Variance (ANOVA) confirmed that orientation is a significant factor affecting RH measurements at all curing ages. This anisotropy is attributed to the microstructural gradient formed during casting, including particle segregation, bleeding water migration, and surface finishing, which create a surface layer with variable hardness. The study concludes that ignoring test direction can introduce a systematic error of up to 18% in estimated strength. A practical multi-directional testing protocol is proposed, wherein the mean rebound number from tests in three orthogonal directions is recommended as a more reliable, orientation-independent parameter for empirical strength models. This approach enhances the reliability of in-situ assessments, contributing to more accurate structural evaluation and quality control.