Experimental investigation and comparison of prestressed single-layer and repaired two-layer reinforced concrete beams

This study continues a series of theoretical and experimental investigation of two-layer beams (TLBs), performed by the authors. Such beams consist of normal strength concrete (NSC) layer in the tensile zone and steel fibered high strength concrete (SFHSC) in the compressed one. The present research is focused on comparison of prestressed single layer beam and three repaired prestressed two-layer beams. The research includes experimental investigation of full-scale simple supported beams under four-point bending. A TLB represents possibility of strengthening by addition of high strength steel fibered concrete layer. Therefore, nonrepaired beam section height is equal to that of the NSC layer in repaired ones. All prestressed specimens with equal section height were produced at real industrial conditions. After that, a SFHSC layer was added in laboratory conditions to obtain three identical TLBs (one single-layer beam was used as a control specimen). The main aim of the research is to study the effect of repairing, using a SFHSC layer on load-bearing capacity. All specimens were tested from initial loading up to failure. The study was focused on cracking and deflections in TLBs, compared to single-layer one. It was found that initial cracks' development in the investigated beams starts at same mid span deflection, but the corresponding load in the repaired beams is about 50% higher, compared to the control specimen. Additionally, proper interaction between the layers without de-bonding was verified. The load bearing capacity in the control specimen is about 77% lower, compared to the repaired ones. Results, obtained in the present study, enable to recommend using SFHSC layer for strengthening prestressed beams. This method is a promising way for repairing existing elements and creating attractive structural systems.