Reinforced Concrete Partition Walls Retrofitted with Carbon Fiber Reinforced Polymer

For seismically insufficient buildings, retrofitting of the reinforced concrete partition walls using the carbon fiber reinforced polymer is of particular interest at the present juncture in Taiwan after Chi-Chi Earthquake. This paper describes theoretical and experimental studies related to the seismic retrofits of the reinforced concrete frames containing partition walls using carbon fiber reinforced polymer materials. One “as built” partition wall, one structural wall, and four “retrofitted” partition walls had been tested under simulated seismic actions. The test results of the retrofitted partition walls indicated that the use of the carbon fiber reinforced polymer with sufficient end anchorages is an effective retrofitting measure. Experimental observations and theoretical analyses indicated that the shear resisting mechanism of the reinforced concrete squat walls can be modeled as the strut-and-tie actions, and that the shear strength of low-rise wall can be reasonably predicted by the softened strut-and-tie model. 1 Professor, Dept. of Construction Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan 10672, ROC Division Head of Building Structures, National Center for Research on Earthquake Engineering, Taipei, Taiwan 106, ROC 2 Lecturer, Department of Civil Engineering, De Lin Institute of Technology, Taipei, Taiwan 23654, ROC PhD Candidate, Dept. of Construction Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan 10672, ROC 3 Research Assistant Fellow, National Center for Research on Earthquake Engineering, Taipei, Taiwan 106, ROC INTRODUCTION The poor performances of the low-rise reinforced concrete (RC) buildings with nonductile reinforcing details had been widely observed during the Chi-Chi earthquake. This stimulates an urgent need of seismic assessment and retrofitting of these structures. However, seismic retrofitting of each component of the nonductile RC frames is extremely difficult and expensive. Fortunately, the existing low-rise buildings in Taiwan contain a lot of partition walls which are lightly reinforced RC walls. By retrofitting these walls as the lateral-force-resisting elements, the existing frames can be treated as members that are not proportioned to resist forces induced by earthquake motions. This retrofitting strategy may alleviate the strength and deformation requirements of a nonductile RC frame. Due to the greatly reduced cost, the retrofitting of the existing buildings with nonductile frames is much feasible. The low-rise buildings in Taiwan were usually designed as structural frames, but nevertheless the interaction of frames and partition walls due to earthquakes was not considered in the analysis. Therefore, the RC frames were provided with sufficient strengths, but the partition walls were detailed with temperature and shrinkage reinforcement only. Consequently, the frames containing RC partition walls might result in sufficient flexural strength but unfortunately insufficient shear strength. The common practice in Taiwan now is to tear down the partition walls then to recast with the RC structural walls. This retrofit scheme is time consuming and causes tremendous inconvenience to the residents. Seeking for the other alternative is of ever-increasing expectance. The repair of understrengthed or damaged reinforced concrete members by the external bonding of carbon fiber reinforced polymer (CFRP) laminates is becoming increasingly popular in the construction industry (Meier 1992). The use of CFRP laminates for this application offers several desirable attributes, such as high strength, resistance to corrosion, light weight, and ease of handling. Retrofitting the RC partition walls by the bonding of CFRP might be a feasible solution of the aforementioned problem. This paper describes theoretical and experimental studies related to the seismic retrofits of the RC frames containing partition walls using CFRP materials. Experimental tests (Yeh 2001, Chiou 2002) were conducted on wall specimens that were subjected to reversed cyclic inelastic deformations representative of earthquake loadings. Theoretical analyses, based on the softened strut-and-tie model (Hwang and Lee 2002) and the ACI 318 building code (2002), were performed to evaluate the shear strength of the walls. RESEARCH SIGNIFICANCE The techniques on assessment and retrofitting of the seismically insufficient buildings are urgently needed in Taiwan. This paper presents the experimental and analytical studies related to the seismic retrofits of the RC frames containing partition walls using CFRP materials. The test results indicate that the use of the CFRP laminates with sufficient end anchorages is quite effective in enhancing the shear strength of partition walls. Moreover, the enhanced shear strength of the retrofitted walls can be reasonably assessed by the softened strut-and-tie model. EXPERIMENTAL PROGRAM Test Specimens In all, six large-scale isolated specimens, two as-built walls and four retrofitted walls, were tested. The test specimens are identified as WF-12, WF-12-FV, WF-12-FHV, WF-12-FV-A, WF-12-FHV-A, and WF-15. The dimensions and reinforcing details are given in Fig. 1. In order to focus attention on the shear behavior of walls, the adequate strength and the ductile detailing requirements per ACI 318 building code (2002) were adopted for the design of frames for all specimens. The test wall of Specimen WF-12 (Yeh 2001) represented approximately 0.6 scale model of a prototype partition wall in a building. The test wall was 12 cm thick with 50 30× cm boundary elements. The overall length of the wall was 350 cm and the height was 155 cm. Specimen WF-12 had reinforcement #3 @ 30cm for each direction placed in one layer parallel with faces of wall (Fig. 1), which resulted in the minimum ratio of reinforcement area to gross concrete area of 0.002.