Effect of sustained load combined with cold temperature on flexure of damaged steel beams repaired with CFRP sheets

This paper discusses an experimental program to examine the residual behavior of notch-damaged steel beams strengthened with carbon fiber reinforced polymer (CFRP) sheets subjected to typical long-term load configurations in cold regions. Test parameters include two levels of sustained load (40% and 60% of the ultimate capacity of a short-term strengthened beam: 40%Fu and 60%Fu, respectively) and cold temperature as low as −30 °C. A total of thirteen strengthened beams (W4 × 13) in addition to one unstrengthened control are monotonically tested in three-point bending after being exposed to the long-term load for 7000 h. Experimental results show that the sustained loads significantly influence the load-carrying capacity of the beams due to the presence of creep damage near the crack-tip field above the notch. Cold temperature also affects the capacity and flexural stiffness of the beams. A crack-bridging effect by the bonded CFRP is noted near the crack mouth of the strengthened beams; however, such an effect decreases when the long-term load is applied. CFRP-debonding consistently governs the failure of the strengthened beams, irrespective of the degree of temperature exposure. Local CFRP rupture (fiber disintegration) is observed near the notch location of the beams subjected to 60%Fu due to the combination of stress concentrations and creep damage of the fibers. Interfacial stresses along the CFRP–steel interface are controlled by the sustained loads. Cold temperature results in reduced interfacial slip of the CFRP, while the temperature effect appears to be insignificant on the magnitude of bond stress. Stress redistribution along the CFRP–steel interface is noticed because of the long-term load.