Assessment of Ultrasonic Pulse Velocity and Rebound Hammer Response in Steel–GFRP Hybrid Reinforced Concrete Beams

Non-destructive evaluation (NDE) methods are widely used to assess concrete quality, but their interpretation in reinforced concrete members remains challenging because measured responses may be influenced by reinforcement layout, casting and surface conditions, and testing configuration. This study investigates the effects of longitudinal reinforcement type, stirrup spacing, and stirrup material on Schmidt rebound hammer (SRH) and ultrasonic pulse velocity (UPV) responses of concrete beams. Nine beams were cast from the same concrete batch: one plain concrete reference beam, steel-reinforced beams, GFRP-reinforced beams, and hybrid steel–GFRP-reinforced beams. Two stirrup spacings, 100 and 200 mm, and two stirrup materials, steel and GFRP, were considered. In total, 1044 rebound readings and 162 UPV measurements were collected, with UPV testing performed using Right–Left direct, longitudinal direct, and Right–Left indirect configurations. Compared with the plain reference beam, the mean rebound number (RN) increased by up to 21.5% in steel-reinforced beams and 23.2% in hybrid beams, whereas the GFRP-reinforced beam with wider stirrup spacing showed a slight reduction of about 0.8%. UPV differences were more moderate, with a maximum increase of approximately 10% relative to the plain beam. Rebound results also showed a clear face effect, with the bottom face producing the highest values and the top face the lowest. The dense spatial mapping captured local variability within each beam; however, because one beam represented each configuration, the comparisons are interpreted as descriptive specimen-level trends rather than generalized reinforcement effects. Single-pair spacing comparisons indicated larger and more variable differences in RN than in UPV between the 100 and 200 mm specimens. These findings demonstrate the importance of documenting casting face, reinforcement proximity, measurement location, and transmission path when interpreting NDE results in hybrid-reinforced concrete members.