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Calculation of Axial Compression Capacity for Square Columns Strengthened with HPFL and BSP
School of Civil Engineering, Chang’an University, Xi’an, Shaanxi 710061, China
Received 24 September 2015; Revised 14 December 2015; Accepted 27 December 2015
Academic Editor: Seung-Jun Kwon
Copyright © 2016 Hua Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
The load carrying capacity and failure mechanism of 8 square columns strengthened with high-performance ferrocement laminate (HPFL) and bonded steel plates (BSP) were analyzed on the basis of experiments on the axial compression performance of these columns. Results show that the reinforcing layer worked together with the original columns as a whole, and the load-bearing capacity significantly increased. When failure of the strengthened column occurred, the mortar and concrete were crushed and bulged outward in the middle of the columns, the angle bars and longitudinal steel bars buckled, and some stirrups were pulled out. The chamfering of angle bar momentously affected the primary damage of steel strand. The values of the strength reduction factor and pressure effective utilization coefficient of the mortar were suggested. Based on the experiments and existing tests of 35 columns strengthened with HPFL, equations for the axial compression bearing capacity were proposed and all calculation results agreed well with testing results. Therefore, the calculation method could be used in the capacity design of axial compression strengthened columns.
1. Introduction
New technology for rehabilitating reinforced concrete (RC) structures has continued to emerge recently. Our research group introduced the reinforcement technique strengthened with HPFL and BSP; this technique adopts the advantages of HPFL reinforcement and bonded reinforcement technology [1–3]. At the beginning of this technique, holes on the part of column surface of steel plate position were repaired following the design requirements; other parts of the column were chamfered and scabbled according to the design requirement. Next, structure adhesive was used to stick the steel plate on the RC member surface. Afterwards, the RC member was cleaned, and then high-strength strand was twined around column and fixed on by bolts. Interfacial agent was sprayed and high-performance mortar was plastered after the structure adhesive was solidified. Finally, the strengthened column was cured. This new technique was adapted from reinforcement methods that use HPFL and BSP. Ferrocement is a form of thin wall RC using wire mesh (small diameter skeletal steel rods might be used sometimes) and mortar, and it was thought to be a ductile and durable repair material for RC structures [4–12]. The advantages of the new structure reinforcement technique were that the thickness of reinforced layer was only 25 mm, the influence on the shape of the structure was relatively small, and heavy construction machineries were unnecessary. Furthermore, high-strength steel strand as a reinforcing stirrup could fix steel plates. The bolts for fixing the steel strand served as shear connectors to avoid the slip between the reinforcement layer and the RC members. As the covering of the steel plate, the high-performance mortar could prevent or delay corrosion. The structure of the reinforced layer, which is composed of pasting steel plate, steel strand by bolts, and plastering mortar, could work together with the original columns as a whole to avoid early stripping damage.
Numerous experimental tests and finite element analyses have been conducted to study the axial force bearing capacity for a concrete column strengthened with FRP [13, 14], steel jacket [15, 16], and wire mesh [17] by researchers all over the world. However, studies that focus on square columns strengthened with HPFL and BSP are limited. This paper reports on the load carrying capacity for strengthened columns through a study carried out on 8 axially loaded RC columns combined with the obtained test results of 35 specimens. The relative calculation equation is also proposed.
2. Experimental Program
In our experiments, 3 concrete square columns and 5 RC square columns strengthened with HPFL and BSP were built. The dimensions and reinforcement bars are shown in Figure 1, and the strengthening design of the column is shown in Figure 2. RG-JS polymer mortar, Araldite XH130AB concrete interface bound rubber, bisphenol A-type modified epoxy resin, and amine curing agent were used in this study. The diameter of the steel strand for reinforcement was 3.2 mm, and its cross-sectional area was 5.1 mm2 and elastic modulus was 1.16 × 105 MPa. The size of the angle was 25 mm × 25 mm × 3 mm, and its cross-sectional area was 143.2 mm2 and elastic modulus was 2.10 × 105 MPa, similar to those of steel. The yield strength of steel and the ultimate strength are shown in Table 1, and the cube strength of concrete and mortar is shown in Table 2.