# Experimental Study on Slender CFRP-Confined Circular RC Columns under Axial Compression
## 1 Introduction  
Many slender reinforced concrete (RC) columns exist in actual concrete structures. These columns must be strengthened because of function changes and increasing loads. One solution method is possible using carbon fiber-reinforced polymer (CFRP) jackets to confine them, but the columns are sensitive to buckling instability as the slenderness ratio increases [1]. Column slenderness has a substantial influence on the confinement action of slender columns, and this effect must be considered in the design.  
Contrary to the numerous existing tests on axially loaded short RC columns constrained by CFRP, the research on CFRP-confined circular RC columns with larger slenderness remains quite limited. S lenderness ratio is one of the important parameters that affect the axial behavior of slender CFRP-confined circular RC columns and control the lateral constraint effect provided by CFRP tubes. S lenderness ratio is (\(\lambda=kL/r\)), where \(k\) and \(L\) are the column effective length factor and the unsupported length of the column, respectively; and r is the radius of gyration of the column section. [PERSON] et al. (2001) conducted the study with 10 concrete-filled fiber-reinforced polymer tubes (CFFT) columns (\(4\leq\lambda\leq 36\)), which showed that the stable load-carrying capacity of columns rapidly drops as the slenderness increases [2]. The existing studies conducted by several research scholars showed that the working mechanism of CFRP-confined circular RC columns is different from that of CFPT columns [3,4]. [PERSON] et al. (2012) presented a study on the axial compression of 48 circular RC columns with CFRP wrapped and their height-to-diameter ratios (H/D) were 2, 5.08, and 6.45. The results indicated that the load-carrying capacity of columns slightly decreases as the slenderness increases [5]. [PERSON] and [PERSON] (2013) established an analysis model and proposed an upper slenderness limit for slender FRP-confined circular RC columns. The theoretical results showed that the slenderness ratio has a more significant influence on the ultimate axial load of slender specimens compared with shortspecimens [6]. [PERSON] et al. (2018) studied the axial stability behavior of slender CFFT columns. The results showed that the bearing capacities of the specimens decreases by 22% when the slenderness ratio increases from 8 to 20 [7]. [PERSON] et al. (2020) studied the eccentric compressive behavior of 10 FRP-confined circular RC columns with a diameter of 300 mm and H/D varying from 3 to 11, which indicated that the ultimate bearing capacity of specimens decreases rapidly as the eccentricity or slenderness ratio increased [8]. Several studies have indicated that a size effect for columns with FRP wrapped may exist [9; 10; 11; 12]. Some experiments have been conducted on the effect of eccentric loading for circular RC columns with FRP wrapped [13; 14; 15].  
Against this background, the slenderness ratio of RC columns with CFRP wrapped adopted by most of the current studies was relatively small (\(2\leq\lambda\leq 36\)). Existing test results also indicated that the slenderness ratio has a more significant influence on the ultimate axial load of FRP-confined circular RC columns compared with unwrapped columns. To provide references for engineering designers and encourage the extensive application of reinforcement technology for slender columns, the working mechanical behavior of CFRP-confined RC columns with a greater slenderness ratio (\(\lambda>36\)) must be investigated. The purpose of this experiment is to study the influence of a larger slenderness on the axial bearing capacity of CFRP-confined circular RC columns.