Cheng-Tzu Thomas Hsu, 1 Wonsiri Punurai, 2 Yongxin Jia, 3 and Haitao Bian 3 Direct Shear Behavior of Carbon Fiber- Reinforced Polymer Laminates and Concrete ABSTRACT: This paper explores the direct shear behavior between carbon fiber-reinforced polymer CFRP laminates and concrete. To study the push-off strength and slip relationship of externally bonded strengthening system, a new test setup is proposed herein. In this research, a total of 27 specimens are tested. The test variables include the maximum compressive strength of concrete, from 4000 to 12000 psi. With this test setup, it has been found to be able to investigate the direct shear condition between the CFRP laminates and concrete. Based on the present test results, empirical formulas to account for the push-off strength and slip relationship of various concrete compressive strengths are developed. This relationship enables further understanding of the transfer mechanism between the CFRP laminates and concrete. Also, the effect of salt water on the ultimate direct shear strength of the CFRP strengthening system is discussed. KEYWORDS: direct shear, push-off strength, slip, CFRP laminates, concrete, salt water, tests Conversion factors 1 in. =25.4 mm 1 lb. mass =0.436 kg 1 lb. force =4.448 N 1 psi =6.895 kPa 1 kip =4.448 N 1 ksi =6.895 MPa 1 in. 2 =6.451 cm 2 Introduction Rehabilitation of structurally deteriorated or functionally obsolete reinforced concrete structures is one of the major tasks for the con- struction industries in the United States and around the world. Uses of externally bonded carbon fiber-reinforced polymer CFRP to strengthen reinforced concrete structures have been proved to be one of the solutions to this tough task. It has been found by several studies Alexander and Cheng 1996; Arduini et al. 1997; Bian, Hsu, andWang 1997; Chajes et al. 1997; Fanning and Kelly 2001; Grace 2001; Meier 1987; Meier et al. 1992; Nanni 1995, 1997; Norris et al. 1997. that externally epoxy bonding a CFRP strip to the tension face of the beam is an effective technique for repair and retrofit of reinforced concrete beam under various loads. It has also been demonstrated analytically and experimentally Bian, Hsu, and Wang 1997; Grace 2001; Fanning and Kelly 2001; Quan-trill et al. 1996; Bizindavyi et al. 1999. that in some cases, the CFRP strengthen reinforced concrete beams cannot attain their ultimate flexural capacities due to either debonding of the CFRP strips from the concrete or development of horizontal cracking and subsequent separation of the concrete cover delamination at the reinforcing steel level. These types of failure mechanisms are possibly insti- gated by high local interfacial bond stresses and high peeling forces developed at the ends of CFRP strips. To prevent such premature failures, several methods have been developed over the years. These include the use of end anchorage steel plates Bian, Hsu, and Wang 1997.in particular for a shorter beam, the development of anchorage length or bond development length Bizindavyi et al. 1999; Hsu, Bian, and Jia 1997.for a longer beam, etc. It is well known that the bonding strength of externally bonded CFRP laminates and concrete depends not only on the quality of epoxy resin, and the CFRP strips, but also the quality of surface preparation, the perfect and compact bonding, and the concrete it- self. Thus the following objectives of this research have been estab- lished: • To determine the bonding strength of the interface between adhesives and concrete in CFRP strengthened reinforced concrete beams. • To study the push-off strength and slip relationship of the interface between adhesives and concrete in CFRP strength- ened beams. • To develop an empirical push-off strength and slip equation of the interface between adhesives and concrete for possible use in analytical modeling or finite element analysis of CFRP strengthened beams. • To develop empirical formulas for direct shear strength for use in design which may be functions of concrete compres- sive strength , etc. • To investigate the interface characteristics between adhe- sives and concrete when the direct shear specimens are in a sea water environment. Research Significance A new experimental setup is proposed herein to study the bond strength versus slippage relationship between the externally Manuscript received July 16, 2004; accepted for publication August 23, 2005; published December 2005. 1 Professor and Director of structures Laboratory, Department of Civil and Environmental Engineering, New Jersey Institute of Technology, Newark, N.J. 07102. 2 Lecturer, Department of Civil Engineering, Mahidol University, Nakornpa- tom, Thailand. 3 Former graduate students, Department of Civil and Environmental Engi- neering, New Jersey Institute ofTechnology, Newark, N.J. 07102. Journal of Testing and Evaluation, May 2006,Vol. 34, No. 3 Paper ID JTE12694 Available online at: www.astm.org Copyright © 2006 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. 1 Copyright by ASTM Int'l (all rights reserved); Fri Jun 13 04:01:16 EDT 2008 Downloaded/printed by wonsiri punurai () pursuant to License Agreement. No further reproductions authorized.