Analyzing the Hysteretic Behavior of Strengthened Concrete Beams

A recent study published in Engineering examined the behavior of reinforced concrete beams supported with Ultra-High-Performance Concrete (UHPC) and Carbon Fiber Reinforced Polymer (CFRP) under thermocyclic loading.

The study aimed to understand better the effects of multi-hazard loading on these reinforced structures, which is crucial for the maintenance and restoration of existing buildings. This research was conducted by Ju-Hyung Kim and Yail J. Kim.

Multiple hazards, such as seismic activity and high temperatures, can seriously threaten a building's ability to function.

These complex loading conditions often exceed the capabilities of conventional design methods. While CFRP and UHPC are known to improve the strength of concrete structures, their performance under thermocyclic stress remains unclear.

The researchers expanded upon a previous study in which load reversals were applied at temperatures ranging from 25 to 175 °C. They developed an analytical approach to quantify the uncertainty in the hysteretic behavior of the reinforced beams.

Their findings revealed that the uncertainty index increased as the drift ratio of the beams increased. When compared to the responses of a reference model, the CFRP-strengthened (CF) and CFRP/UHPC-strengthened (UC) beams had uncertainty indices of 0.35 and 0.37, respectively, at 175 °C. This increase in uncertainty was linked to a reduction in the beams’ energy capacity.

The study also analyzed the hysteretic behavior of the reinforced beams. Damage accumulation and resistance to deformation were observed through changes in the stiffness of the hysteresis loop.

Energy loss was significant during the formation of plastic hinges. The researchers proposed regression and mean hysteresis models, with the regression model performing better at higher temperatures and the mean model performing better at lower temperatures, particularly below the glass transition temperature of CFRP.

Regarding the pinching mechanism, the study found that the magnitude of drift ratios had a greater impact on pinching progression than the retrofit materials. Thermal damage between the concrete substrate and UHPC affected its performance at higher temperatures, though the UHPC jacket maintained stable hysteresis loop patterns at lower temperatures.

To assist with practical design, the researchers introduced a performance degradation factor. This factor helps estimate the degraded energy dissipation capacity of the beams under thermocyclic distress, with values ranging from 1.00 to 0.45, depending on temperature and the retrofit plan.

This study provides a better understanding of how CFRP/UHPC-strengthened reinforced concrete beams behave under thermocyclic stress. Engineers can use these findings to enhance building safety and durability when designing and retrofitting structures to withstand complex environmental challenges.

Journal Reference:

Kim, J.-H., et al. (2024) Hysteretic Uncertainty and Anomaly Quantification of Reinforced Concrete Beams Strengthened with Carbon Fiber Reinforced Polymer and Ultra-High-Performance Concrete in Thermocyclic Distress. Engineering. doi.org/10.1016/j.eng.2024.11.018.