FRP Composites for Seismic Retrofitting of Steel-Concrete Shear Walls with Steel Encased Profiles

The paper presents a part of an experimental program performed on composite steel-concrete slender shear walls, retrofitted using CFRP composites. The reinforced concrete (RC) elements were tested initially cyclic under lateral loads. The experimental program was performed on two shear walls with steel encased profiles tested prior to failure, thereafter retrofitted and retested. The retrofitting solution consists in rehabilitation of tested elements using Carbon Fiber Reinforced Polymer (CFRP) composites. Before retrofitting, the damaged specimens were repaired by replacing the crushed concrete with high-strength repair mortar. The experimental specimens were provided with CFRP strips and plates, to restore the bending resistance of the walls and to provide confinement effect at the ends. The aim of this study is to analyze the possibilities of using CFRP materials for strengthening the shear walls affected by seismic action. The retrofitting solution used and the failure mode is presented for each tested element. The behavior and envelope curves, ductility and energy dissipation and the stiffness degradation are presented comparatively for the strengthened and for the reference elements. 2 EXPERIMENTAL PROGRAM 2.1 Experimental specimens design and materials The experimental program consists of six 1:3 scale elements (CSRCW1 to 6), designed using the principles from the existing European codes (Eurocode 2, Eurocode 4 and Eurocode 8) applied to composite steel-concrete elements. Two specimens (CSRCW2 and CSRCW4) were selected for retrofitting using CFRP composites. These elements were tested prior to failure and after retrofitting were tested again in order to investigate the strengthening effect and efficiency related to the seismic behavior. The retrofitted elements were renamed CSRCW2_R and CSRCW4_R. The experimental specimens had 3000 mm height, 1000 mm width and 100 mm in thickness. The wall panels were fixed in reinforced concrete foundations with 1500 mm length, 400 mm height and 350 mm width. The structural steel profiles were connected with the concrete web by headed shear stud connectors with d=13 mm diameter and h=75 mm length placed every 150 mm. The reinforcements of the reinforced concrete (RC) web panel consists of O10 mm vertical bars (1.57% steel ratio) and O8/150 mm horizontal bars (0.67% steel ratio) and horizontal stirrups (0.67% steel ratio). The materials used for elements construction were tested in laboratory in order to determine their specific properties. The compression strength of concrete obtained from tests made on the day of composite walls test was 46 N/mm for CSRCW2 and 62 N/mm for CSRCW4. The average yield strength (fy) for the horizontal reinforcements was 479 N/mm and for the vertical reinforcements 548 N/mm, whilst the ultimate strength (fu) was 616 N/mm and 622 N/mm respectively. The average yield strength for the structural steel was 328 N/mm, whilst the ultimate strength was 516 N/mm. The parameters of the materials used for retrofitting are presented in the strengthening solutions section. The design details of the two selected specimens for strengthening are presented in Figure 1. Figure 1. Details of composite tested elements 2.2 Test set-up and instrumentation Figure 2. Testing equipment A general view of the testing equipment is presented in Figure 2. The specimens were tested under quasistatic reversed cyclic horizontal loads. Also a constant vertical load was applied while the cyclically horizontal (lateral) loads increased. The loading frame consists in two steel braced frames anchored in the laboratory floor. The alternate horizontal force was applied using two 400 kN hydraulic jacks, whereas the vertical force was provided by a 250 kN hydraulic cylinder. The vertical force was provided in order to obtain an axial load level between 0.015 and 0.025. A transverse brace system was used in order to avoid out of plane displacements of the specimen. The measurement equipment consists in pressure transducers for monitoring the loads, displacement transducers, strain gauges glued on the reinforcement bars, on the structural steel and on CFRP composites. Also optical laser measurements were performed on elements surface. The horizontal forces were applied under controlled cyclic displacements until the strength of the specimens decreased to 85 % of the peak horizontal load, like in reference (Liao et al., 2009). More details about the testing methodology, instrumentation of the tested elements and the loading history are presented in detail in references (Dan at al., 2011a and 2011b). 2.3 Behavior and results of the reference elements The tested composite shear walls with steel encased profiles showed a bending behavior. In the initial loading stages, horizontal cracks appeared in the tensioned zone due to the transfer of the stresses between steel and concrete. Diagonal cracks appear in the cycle when the elastic limit of the element was attained and developed in the specimens until a series of rhombic concrete blocks separated by these inclined cracks resulted. The measured strains indicated yielding of the vertical reinforcing bars located at the extremities and yielding of the steel profiles. CSRCW-2 1 1 7 8 1 1 1 0 0 7 0 50 70 760 70 5