Abstract
This paper investigates the axial behavior of slender reinforced concrete (RC) columns strengthened with concrete filled steel tube (CFST) jacketing technique. It is realized by pouring self-compacting concrete (SCC) into the gap between inner original slender RC columns and outer steel tubes. Nine specimens were prepared and tested to failure under axial compression: a control specimen without strengthening and eight specimens with heights ranging between 1240 and 2140 mm strengthened with CFST jacketing. Experimental variables included four different length-to-diameter (L/D) ratios, three different diameter-to-thickness (D/t) ratios, and three different SCC strengths. The experimental results showed that the outer steel tube provided confinement to the SCC and original slender RC columns and thus effectively improved the behavior of slender RC columns. The failure mode of slender RC columns was changed from brittle failure (concrete peel-off) into ductile failure (global bending) after strengthening. And, the load-bearing capacity, material utilization, and ductility of slender RC columns were significantly enhanced. The strengthening effect of CFST jacketing decreased with the increase of L/D ratio and D/t ratio but showed little variation with higher SCC strength. An existing expression of load-bearing capacity for traditional CFST columns was extended to propose a formula for the load-bearing capacity of CFST jacketed columns, and the predictions showed good agreement with the experimental results.
Highlights
Strengthening of reinforced concrete (RC) structures is critically important for several reasons [1,2,3,4,5,6,7]
A third reason is to improve the load-bearing capacity for deficient members as a result of design or construction errors. Common strengthening methods such as section enlargement [8], externally bonded steel plates [9, 10], and externally bonded fiber-reinforced polymer (FRP) [11, 12] have been used for many years to improve structural service performance and ultimate capacity of concrete structures. Their disadvantages may limit the further application. e concrete jacketing method obviously enlarges the cross section of concrete members, and the construction of steel cage and formwork costs a lot of labor and time. e steel plate strengthening hardly changes the appearance of concrete structures, but it requires a large amount of steel and antirust work [13,14,15]
(i) e original column is sandblasted by a hand grinder to remove the irregularities and debris (ii) e outer steel tube is carefully placed on the designed region leaving a uniform gap (iii) e self-compacting concrete (SCC) is poured into the gap between original columns and steel tube at three intervals (iv) e strengthened column is cured for 28 days in the laboratory e reinforcement ratios of concrete filled steel tube (CFST) jacketed columns range from 5.2% to 9.5%. e ratios are in the range of 3.0–20.0% which are commonly used by others’ researches [32]
Summary
Strengthening of RC structures is critically important for several reasons [1,2,3,4,5,6,7]. A third reason is to improve the load-bearing capacity for deficient members as a result of design or construction errors Common strengthening methods such as section enlargement (concrete jacketing) [8], externally bonded steel plates [9, 10], and externally bonded fiber-reinforced polymer (FRP) [11, 12] have been used for many years to improve structural service performance and ultimate capacity of concrete structures. E results showed that the preloading level had little effect on the load-bearing capacity of specimens He et al [27] studied the axial compressive behavior of CFST jacketed columns with recycled aggregate concrete. A modified model is applied to predict the load-bearing capacity of strengthened slender RC columns
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