Carbonation depth model for loaded reinforced concrete (RC) beams under time-dependent relative humidity conditions

Steel bar corrosion caused by concrete carbonation is one of the important factors of the durability failure of reinforced concrete (RC) structures. The annual relative humidity (RH) in the upper reaches of the Yangtze River shows obvious time-dependent characteristics. The important wharf components represented by RC beams bear flexural loads during their long-term service. The carbonation of RC structures under the coupling of the service environment and flexural load is very complicated. However, corresponding studies on the carbonation of loaded RC structures are mostly carried out in a constant RH environment, which is quite different from the timedependent RH (T-RH) conditions in the upper reaches of the Yangtze River. For this study, the RC beam of a wharf in the upper reaches of the Yangtze River is treated as the research object. According to the long-term observation data of the 24-year daily mean RH at this wharf site, the periodic (time-dependent) characteristics of RH in a typical year are obtained by statistical analysis. On this basis, a physical experiment for RC beam specimens subjected to flexural loads under an accelerated carbonation environment with T-RH is carried out to explore the influence of the coupling effect of the T-RH and flexural load on the concrete carbonation behaviour. The results showed that the range of RH is basically distributed from 45 to 95%, and the monthly mean RH over a typical year shows a 'W'-shaped periodic characteristic. The compressive strength and concrete carbonation depth under the T-RH environment are larger than those under the C-RH carbonation environment; hence, the carbonation degree of concrete under a T-RH environment is more serious. When the flexural load level is less than 1.0, the flexural tensile stress can accelerate the carbonation degree of RC specimens, and the carbonation depth of concrete is positively correlated with the flexural load level. When the flexural load does not reach the concrete compressive strength, the flexural compressive stress can inhibit concrete carbonation. Based on the experimental measurements, the carbonation depth model for loaded RC beams under the T-RH is established, and the accuracy of the model is verified by the original test data collected during 27 years of the natural carbonation of RC beams. Through the studies and achievements presented in this paper, the carbonation trends of loaded RC beams under T-RH are revealed, and the corresponding mathematical model is established, which can be used to evaluate the carbonation depth of loaded RC beams considering T-RH in general atmospheric environments. The research results can provide a basis for RC structure durability evaluation in the upper reaches of the Yangtze River and have important guiding significance for practical engineering.