Improved Method for Bonding CFRP Overlays to Steel for Fatigue Repair

Abstract

Experiments and computer simulations were carried out at the University of Kansas to investigate the use of composite materials for strengthening and repair of fatigue-vulnerable steel bridges. Prefabricated CFRP overlays were attached to welded coverplate specimens using a layer of epoxy resin. The specimens were then subjected to three-point bending under cyclic loading at a constant stress range. Early tests showed that bonding CFRP overlays to steel was an effective method to reduce the stress demand in areas susceptible to fatigue damage, and that this strengthening method could lead to substantial increases in crack initiation life. Fatigue tests also showed that maintaining the bond between the CFRP overlays and the steel was the most critical factor in the effectiveness of this strengthening technique. It was observed that the presence of a layer of breather fabric embedded within the layer of epoxy resin was the most important parameter affecting bond behavior under fatigue loading. This paper describes monotonic and fatigue tests carried out to evaluate the effect of the presence of breather fabric and thickness of the resin layer on bond strength and on the performance of the resin layer under fatigue loading. It was found that the mode of failure of the resin under monotonic and fatigue loading changed significantly due to the presence of the breather cloth. Although the presence of breather cloth had a negligible effect on the bond strength of thin resin layers, its effect on strength increased with the thickness of the resin layer. It was also found that the presence of the breather fabric had a very significant effect on the post-peak behavior of monotonic tests, which partially explains its beneficial effect on behavior under fatigue loading. BACKGROUND Techniques to repair connections vulnerable to fatigue damage in steel bridges must be developed considering the specific geometry and other relevant characteristics of the particular connection detail. This is particularly the case for fatigue repairs using composite material overlays, in which the geometry and location of the overlays should be optimized for the shape of the connection detail. Nonetheless, several studies have found that regardless of geometry a critical factor in the implementation of this repair technique is maintaining the bond between the composite material and the steel for the intended fatigue life of the repair. There is a wealth of studies investigating bond between resins and metal. Two studies closely related to the type of repair discussed in this paper were carried out by Nozaka et al. (2005; 2005a), who investigated the effectiveness of carbon-fiber-reinforced polymer (CFRP) strips for increasing fatigue life of steel girders. Testing described in Nozaka et al. (2005) was