Effective Bond Length of Carbon-Fiber-Reinforced Polymer Strips Bonded to Fatigued Steel Bridge I-Girders Katsuyoshi Nozaka 1 ; Carol K. Shield 2 ; and Jerome F. Hajjar 3 Abstract: After years in service, many steel girders have deteriorated to the point where fatigue cracks have initiated in the girders. In girders having cover plates that do not terminate in a compression region, a common type of crack initiates at the weld toe at the ends of the cover plate after being subjected to cyclic tensile loads due to traffic. The use of precured carbon-fiber-reinforced polymer CFRP laminates, adhered to the inside face of the girder tension flange, is one proposed method for repairing these cracked bridge girders. The main advantages of using CFRP laminates are their light weight and their durability, which result in ease of handling and maintenance. For the application of this rehabilitation method, it is important to determine the effective bond length for CFRP laminates adhered to the inside face of a cracked steel girder flange. Experimental tests using a new type of effective bond length test specimen were conducted in this research on several types of adhesives and precured CFRP laminates, in addition to several different bonding configurations. The minimum bond length required to achieve the maximum strength of the rehabilitation scheme for the materials investigated in this research was determined. The experimental results also indicated that an adhesive with relatively large ductility is required to redistribute the stresses successfully within the adhesive layer during increased loading. A simple analytical solution for the shear strain distribution in the adhesive layer was proposed for estimating the effective bond length, and the results were verified with computational analyses. Good agreement was found among the computational, analytical, and experimental results. DOI: 10.1061/ASCE1084-0702200510:2195 CE Database subject headings: Composite materials; Adhesive bonding; Bonding strength; Bridges, girder; Rehabilitation; Fiber reinforced polymers; Fatigue. Introduction In the construction of older steel bridges, it was common to weld a cover plate onto the outer face of the top flange over the interior pier of a continuous steel I-girder, where the top flange was in tension due to flexure. Typically, these plates did not extend into the positive moment region, where the top flange would be in compression due to flexure. However, many girders with these cover-plate details have developed fatigue cracks at the weld toe at the cover-plate ends due to cyclic tensile loads, e.g., from traf- fic flow. These cracks propagate through the tension flange and may eventually enter the web and cause a catastrophic failure. Since crack initiation from the weld toe of the cover-plate ends was recognized as a potential cause for catastrophic failure, ret- rofitting the fatigued bridge girders has become important. Sev- eral rehabilitation methods have been developed; they are based on two general approaches. In the first approach, peening, arc melting, grinding, and/or hole drilling are directly applied to the weld of the cover-plate ends, thus modifying the stress state around the weld or eliminating the crack Fisher et al. 1979; Fisher et al. 1980. These methods are not always successful; for example, several girders that have been rehabilitated by hole drill- ing alone had cracks reinitiate from the holes Andrea et al. 2001. In the second approach, steel plates are bolted to the outer side of the tension flange over the crack, restoring the flexural capacity and the strength of the section Sahli et al. 1984; Hassan and Bowman 1996. The rehabilitation methods listed above, except for hole drill- ing, require direct access to the top of the tension flange on which the cover plate is welded. This in turn requires removal of the concrete deck on top of the tension flange in the negative moment region and partial closing of the traffic on the bridge being reha- bilitated, resulting in expensive and time-consuming work that is inconvenient to the public. Another retrofit method in which a steel angle is bolted to the girder web near the top flange has been investigated in a recent work McKeefry and Shield 1999. This method does not require closing the bridge to traffic. In this method, the angle member to be bolted to the girder tends to be long in order to develop the full yield strength of the angle, which may result in problems associated with handling a heavy steel angle and drilling many holes in the web. There is currently a need to develop a new rehabilitation method that does not require removal of the concrete deck. The use of carbon-fiber-reinforced polymer CFRP precured laminates hereafter referred to as CFRP strips adhered to the inside face of the girder tension flange is one method for repairing cracked bridge girders Nozaka et al. 2003. The main advantages of using CFRP strips are their light weight and their durability, which result in easier handling and 1 Research Associate, Ritsumeikan Univ., Kusatsu, Shiga, 525-8577 Japan. 2 Professor, Dept. of Civil Engineering, Univ. of Minnesota, Minneapolis, MN 55455. 3 Associate Professor, Dept. of Civil Engineering, Univ. of Minnesota, Minneapolis, MN 55455. Note. Discussion open until August 1, 2005. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on January 3, 2003; approved on February 12, 2004. This paper is part of the Journal of Bridge Engineering, Vol. 10, No. 2, March 1, 2005. ©ASCE, ISSN 1084-0702/2005/2-195–205/$25.00. JOURNAL OF BRIDGE ENGINEERING © ASCE / MARCH/APRIL 2005 / 195