The need to improve the conditions and sustainability of existing buildings has necessitated the development and advancement of strengthening technologies. Carbon fiber reinforced polymer (CFRP) strengthening technology, commonly used in structural reinforcement, has attracted significant attention. Its excellent low weight, high tensile strength, and corrosion resistance properties have particularly made them appropriate for strengthening both plain and reinforced concrete members. However, carbon fiber reinforced polymer-tubed concrete members experience several limitations, including high cost due to complexity and brittle failures, and outward buckling failures, common in concrete-filled steel tubular columns, are limiting their applications. Carbon fiber reinforced polymer-jacketed concrete-filled steel tubular (CFT) columns have been identified as a promising solution for the challenges above. Unlike fully jacketed carbon fiber reinforced polymer CFT columns and beams, little research has been conducted on partially jacketed carbon fiber reinforced polymer wrapping technology despite its rising use in reinforced concrete structures.
In a recent paper published in the Journal of Construction Steel Research, scientists at Hefei University of Technology: Dr. Qihan Shen, Professor Jingfeng Wang, Qiuyu Xu (Ph.D. Student), and Yubo Cui (Ph.D. Student) investigated the eccentric behavior of partially jacketed carbon fiber reinforced polymer circular concrete-filled steel tubular columns. Specifically, they evaluated the mechanical properties and the influence of various parameters: steel yield stress, column slenderness, load eccentricity, and the number of layers and spacing of the carbon fiber reinforced polymers on the eccentric compressive response of the analyzed specimens.
The experimental work entailed testing and analyzing a total of 10 circular CFT short columns and 11 circular CFT slender columns. First, the authors developed a modified finite element model of the partially jacketed carbon fiber reinforced polymer circular CFT columns based on the evaluation of the critical mechanical properties, including eccentric pressure, displacement curves, strength, strain response, failure modes, among others. This model was then used to explore the contact stress, and suitable wrapping approach for the partially CFRP jacketed circular CFT columns.
Results showed that failure patterns in eccentrically loaded CFRP jacketed CFT short columns in a partial wrapping scheme comprised of concrete crumbling, concrete cracking, rupture of CFRP, and outward bulges of the circular hollow steel sections. However, the local buckling was only observed in the unwrapped region, an indication that this approach could be used to eliminate the buckling effects. For partially jacketed CFT slender columns, failure modes included slight CFRP damages, global buckling, local tensile cracking on the concrete, and poor strain response, suggesting that CFRP jackets in partial and lateral wrapping arrangements are not favorable for strengthening concrete-filled steel tubular slender columns. Both short and slender columns exhibited abrupt decline beyond peak load attributed to CFRP rapture and global buckling, respectively.
Based on the contact stress and wrapping scheme analysis, CFRP jackets for circular concrete-filled steel tubular short columns were highly recommended, over slender columns, for enhanced strength improvements. Eventually, confining stress transferring mechanism analysis was conducted. A design formula describing the relationship between the axial and eccentric load-bearing capacity of a partially jacketed circular concrete-filled steel tubular column was proposed. The finite element model and the proposed formula were verified using existing experimental results, which compared well with the predicted results. According to the authors, their study recommendations will pave the way for future design and applications of CFRP strengthening concrete-filled steel tubular structures.
Shen, Q., Wang, J., Xu, Q., & Cui, Y. (2020). Performance and design of partially CFRP-jacketed circular CFT column under eccentric compression. Journal of Constructional Steel Research, 166, 105925.