FUNDAMENTAL CHARACTERISTICS OF NEW HIGH MODULUS CFRP MATERIALS. FOR STRENGTHENING STEEL BRIDGES AND STRUCTURES
Mina Dawood, Emmett Sumner and Sami Rizkalla
Department of Civil, Construction, and Environmental Engineering. North Carolina State University,
USA, E-mail: sami_rizkalla@ncsu. edu
Due to corrosion and the continuous demand to increase traffic loads, there is a great need for an effective, cost-efficient system which can be used for the repair and strengthening of steel highway bridge girders. Recently, research has been conducted to investigate the use of carbon fiber reinforced polymer (CFRP) materials to address this need. This paper describes the details of an experimental program which was conducted to investigate the fundamental behavior of steel-concrete composite bridge girders strengthened with new high modulus CFRP (HM CFRP) materials. The behavior of the beams under overloading conditions and fatigue loading conditions was studied as well as the possible presence of a shear-lag effect between the steel beam and the CFRP strengthening. A series of proposed flexural design guidelines are presented which can be used to establish the allowable live – load increase for a strengthened beam and to design the required HM CFRP strengthening.
The use of fiber reinforced polymer (FRP) materials for the repair and strengthening of concrete structures has gained widespread acceptance. Due to the success of this technique, several researchers have investigated the use of externally bonded CFRP materials for the repair and retrofit of steel bridges and structures. A number of different approaches have been investigated to assess the effectiveness of using CFRP materials for the retrofit of steel bridge members including repair of overloaded girders (Sen et al., 2001), repair of naturally deteriorated girders (Mertz and Gillespie, 1996), strengthening of undamaged girders (Tavakkolizadeh and Saadatmanesh, 2003c) and repair of girders with simulated corrosion damage (Al-Saidy et al., 2004). Other research has been conducted to study the fatigue durability of CFRP strengthening systems (Miller et al., 2001).
Early research focused on the use of conventional CFRP materials to repair steel-concrete composite bridge girders which were damaged due to severe overloading conditions (Sen et al., 2001). The CFRP strengthening system helped to increase the yield load and post-elastic stiffness of the beams.
Researchers have also investigated the use of CFRP materials to repair naturally corroded bridge girders (Mertz and Gillespie, 1996). Installation of the CFRP materials restored the elastic stiffness and moment capacity of the girders to levels comparable to those of the undamaged girders.
In another study, three undamaged steel-concrete composite beams were strengthened with one, three and five layers of CFRP strips respectively (Tavakkolizadeh and Saadatmanesh, 2003c). The CFRP materials also increased the ultimate capacity of the strengthened beams by up to 76 percent, however, the increase of the elastic stiffness was minimal. In a companion study, the tension flange of three other steel-concrete composite beams were notched with a 1.3 mm wide notch at midspan to simulate 25, 50 and 100 percent loss of the tension flange due to corrosion (Tavakkolizadeh and Saadatmanesh, 2003b). The repair restored the elastic stiffness and ultimate capacity of the girders to levels comparable to the undamaged state and helped to reduce the measured residual deflections due to overloading.
Other researchers have simulated corrosion damage by removing a uniform portion of the tension flange along the entire length of the girders (Al-Saidy et al., 2004). The repair technique was capable of restoring the lost strength of the damaged beams to levels higher than those of the unstrengthened girders. However, only 50 percent of the lost stiffness of the beams was recovered.
M. Pandey et al. (eds), Advances in Engineering Structures, Mechanics & Construction, 215-226.
© 2006 Springer. Printed in the Netherlands.
Investigations on the fatigue durability of steel beams strengthened with CFRP materials have been limited. Tavakkolizadeh and Saadatmanesh (2003a) demonstrated that externally bonded CFRP patches can be used to reduce crack propagation rates and increase the fatigue life of cracked steel members. Prestressed CFRP patches can also be installed to help promote crack-closure effects and further extend the fatigue life of cracked steel members (Bassetti et al., 2000). The fatigue durability of naturally corroded steel bridge girders which were repaired with CFRP materials has also been used investigated (Miller et al., 2001).
The majority of the previous research has focused on the use of conventional modulus CFRP materials for the repair of steel bridge members. While substantial strength increases have been achieved, typically large amounts of strengthening were required to achieve substantial increases of the elastic stiffness of the beams. This is due to the relatively low modulus of elasticity of the CFRP as compared to steel and also possibly due to the presence of shear-lag effects between the steel beam and the CFRP strengthening.
Recently, a strengthening system has been developed at North Carolina State University which uses new high modulus CFRP (HM CFRP) materials. These materials have a modulus of elasticity approximately two times greater than that of steel. The effectiveness of using HM CFRP materials to repair steel bridge girders was demonstrated by testing three large-scale steel-concrete composite beams strengthened with different configurations of HM CFRP materials (Schnerch, 2005). The elastic stiffness and ultimate moment capacity of the beams were increased by up to 36 percent and 45 percent respectively. The testing demonstrated that prestressing the HM CFRP strips prior to installation on the steel beam increased the efficiency of utilization of the CFRP and only half the amount of strengthening was required to achieve a comparable increase of stiffness as a beam strengthened with unstressed CFRP laminates.
This paper presents the details and relevant findings of an experimental investigation which was conducted to study the behavior of the strengthened beams under overloading conditions and fatigue loading conditions. Also, the possible presence of a shear-lag effect between the steel and the CFRP, which can limit the effectiveness of the strengthening system, was investigated in detail for a number of different loading conditions. Additional details about the research program are available in Dawood (2005). A series of guidelines are also proposed which can be used by practitioners to design the required HM CFRP strengthening for a given steel-concrete composite beam.