Flexural strengthening of timber-concrete composite beams using mechanically fastened and externally bonded combining mechanically fastened strengthening techniques

Timber-concrete composite (TCC) beams fail due to the tensile failure of the bottom timber lamellas, with the upper concrete remaining elastic, indicating a low utilization ratio of materials. Consequently, two types of strengthening techniques, namely mechanically fastened (MF) and externally bonded combining mechanically fastened (EB + MF), were used in this study to strengthen the TCC beams. Four-point bending tests were conducted to evaluate the strengthening efficiency of the MF and the EB + MF strengthening techniques for the TCC beams. Experimental variables include different strengthening techniques (MF, and EB + MF), types of reinforcement (steel and CFRP), and thickness of reinforcement. The test results indicate that the ultimate load capacity of the TCC beams was significantly improved, with a maximum improvement of up to 85.4% for the EB + MF and 30.6% for the MF strengthening techniques, respectively. The MF and the EB + MF strengthening techniques improved the bending stiffness of the TCC beams ranging from 10.8% to 41.6%. Generally, the MF strengthening technique shows a slightly lower strengthening efficiency in ultimate load capacity and bending stiffness compared with the EB + MF strengthening technique, respectively, indicating that the MF strengthening technique is also an effective way for strengthening the TCC beams. The comparison between the experimental and calculated bending stiffness indicates that the γ method reported in Eurocode 5 underestimates the bending stiffness by 40% on average for all the tested TCC beams.