Flexural behavior of long-term loaded RC beams strengthened by ultra-high performance concrete

This study aims to investigate the flexural behavior of reinforced concrete (RC) beams strengthened with ultrahigh-performance concrete (UHPC) after being loaded with long-term sustained loads for more than 20 years. Seven RC beams, originally fabricated in 1998, are categorized as the one-time pouring unnotched beam (Overall beam), one-time pouring notched beam (Notched beam), and two-time pouring beam (Repaired beam). These RC beams were firstly tested under the effect of long-term sustained loads which lasted 23.6 years. Subsequently, notched beams were strengthened using UHPC (hereafter named as Strengthened beam), which are tested under four-point bending test to investigate the significance of strengthening using the UHPC. Long-term load effects on the flexural performance of RC beams, including creep, cracking, concrete carbonization, and steel rebar corrosion were tested and analyzed. Test results showed that the maximum carbonization depth was less than the concrete cover depth and the steel rebars corrosion was among 1.51-2.47%, which indicates that the steel rebars inside the beam are well protected and are almost free of rust. The bearing capacity of the strengthened beams were increased by 24.6-48.6% compared with that of the Notched beam, which emphasizes the significance of strengthening using the UHPC. The displacement ductility of the strengthened beams was decreased by 82-94% compared with that of the Overall beam, however no brittle failure was observed during the test. It is worth mentioning that the internal changes, caused by the long-term sustained loading, could not be completely changed by adopting UHPC strengthening, while the stiffness reduction caused by long-term loading considerably influenced the flexural behavior of the UHPC strengthened beams. In addition, more attention shall be paid to the UHPC-NC vertical interface which remarkably affected the flexural performance of the Strengthened beams.