A recent study published in Engineering delves into the behavior of reinforced concrete beams strengthened with Carbon Fiber Reinforced Polymer (CFRP) and Ultra-High-Performance Concrete (UHPC) under thermocyclic loading. This research, conducted by Ju-Hyung Kim and Yail J. Kim, aims to understand the effects of multi-hazard loading on these strengthened structures, which is crucial for the maintenance and rehabilitation of existing buildings.
Multi-hazards, such as the combination of seismic events and high temperatures, pose a significant threat to the functionality of buildings. Conventional design practices often fall short in addressing these complex loading conditions. The use of CFRP and UHPC in strengthening concrete members has been proven effective, but their performance under thermocyclic distress remains unclear.
The researchers built on a previous experimental study where load reversals were carried out at temperatures ranging from 25 to 175 °C. They developed an analytical approach to quantify the uncertainty in the hysteretic behavior of the strengthened beams. By comparing the responses of a reference model with experimental measurements, they found that the uncertainty index increased with the drift ratio of the beams. At 175 °C, the uncertainty indexes of CFRP-strengthened (CF) and CFRP/UHPC-strengthened (UC) beams reached as high as 0.35 and 0.37, respectively. This increase in uncertainty correlated with a reduction in the energy capacity of the beams.
The study also explored the hysteretic system of the strengthened beams. The adjusted stiffness of the hysteresis loop was found to indicate damage accumulation and deformation resistance. When plastic hinges formed, a large amount of energy was dissipated. Two hysteresis models, mean and regression, were proposed. The mean model was more suitable when the temperature was below the glass transition temperature of CFRP, while the regression model was better for higher temperatures.
Regarding the pinching mechanism, the researchers found that the magnitude of drift ratios had a greater impact on the progression of pinching compared to the retrofit materials. Although the installation of a UHPC jacket was beneficial in maintaining the stable pattern of hysteresis loops at lower temperatures, thermal damage between the concrete substrate and UHPC at higher temperatures affected its performance.
To aid in practical design, the researchers proposed a performance degradation factor. This factor can be used to estimate the degraded energy dissipation capacity of the beams under thermocyclic distress. The values of this factor range from 1.00 to 0.45, depending on the temperature and the retrofit scheme.
This research provides valuable insights into the behavior of CFRP/UHPC-strengthened reinforced concrete beams under thermocyclic distress. The findings can help engineers make more informed decisions when designing and retrofitting structures to withstand multi-hazard conditions, contributing to the safety and durability of buildings in the face of complex environmental challenges.
The paper "Hysteretic Uncertainty and Anomaly Quantification of Reinforced Concrete Beams Strengthened with Carbon Fiber Reinforced Polymer and Ultra-High-Performance Concrete in Thermocyclic Distress," authored by Ju-Hyung Kim, Yail J. Kim. Full text of the open access paper: https://doi.org/10.1016/j.eng.2024.11.018
