A total of seven steel-concrete composite beams were tested to study the effectiveness of the HM CFRP strengthening system. In the first phase of the research three beams were tested to study the behavior of the strengthening system under overloading conditions. The second phase consisted of
three beams which were tested under fatigue loading conditions to investigate the durability of the strengthening system. The effect of the bonding procedure on the fatigue behavior of the beams was also examined. In the third phase, the seventh beam was tested to study the possible presence of shear-lag between the steel and the CFRP. The strengthened beams that were tested in the first and second phase of the research were also used to investigate the possible presence of a shear-lag phenomenon. The beams tested in all three phases of the experimental program consisted of scaled steel-concrete composite beams which are typical of most highway bridge construction. The typical cross-section of the tested beams is shown in Figure 1.
Figure 1: Cross-section of a typical test beam
The beams were strengthened with different levels of HM CFRP materials and tested in a four point bending configuration as shown in Figure 2.
The test matrix for the three phases of the experimental program is presented in Table 1. In the first phase, the behavior of the beams under overloading conditions was investigated. Two of the test beams were strengthened with two different levels of CFRP while the third remained unstrengthened to serve as a control beam for the overloading study. All three of the beams were unloaded and reloaded at various load levels to simulate the effect of severe overloading conditions.
The second phase was designed to study the fatigue durability of the strengthening system. Two different beams were strengthened with the same amount of CFRP materials, however, using different bonding techniques. The modified bonding technique involved increasing the final thickness of the cured adhesive and additionally including the use of a silane adhesion promoter. The third beam remained unstrengthened as a control beam for the fatigue study. All three beams were subjected to three million fatigue loading cycles with a frequency of 3 Hz. The minimum load in the loading cycle was selected as 30 percent of the calculated yield load of the unstrengthened beams to simulate the effect of the sustained dead-load for a typical bridge structure. The maximum load for the unstrengthened beam was selected as 60 percent of the calculated yield load to simulate the combined effect of dead-load and live-load. The maximum load for the two strengthened beams was selected as 60 percent of the calculated increased yield load of the strengthened beams to simulate the effect of a 20 percent increase of the allowable live-load level for a strengthened bridge.
The final beam was tested under monotonic loading conditions to investigate the possible presence of a shear-lag effect in the absence of more harsh loading conditions. The four strengthened beams which were tested in the first and second phases of the experimental program were also used to study the potential shear-lag effects.
Table 1: Test matrix for the three phases of the experimental program
*defined as the ratio of the cross-sectional area of the CFRP strengthening, accounting for the fiber volume fraction to the cross-sectional area of the steel beam **included the use of a silane adhesion promoter
The tensile yield strength and modulus of elasticity of the steel beams were determined by coupon tests according to ASTM A370-02 as 380 MPa and 200,000 MPa respectively. The compressive strength of the concrete used for the concrete deck slabs for the seven test beams was determined from cylinder tests after 28 days in accordance with ASTM C39-03. The measured concrete cylinder strengths are presented in Table 1.
To accurately represent the actual behavior of a typical strengthened highway bridge, the two strengthened beams that were tested in the fatigue study were subjected to a sustained simulated deadload prior to installation of the HM CFRP strengthening system. The simulated dead-load was applied using an independent loading apparatus as shown in Figure 3. Prior to installation of the CFRP, the simulated dead-load of 50 kN was applied by tightening nuts on a series of threaded rods as shown in the figure. The simulated dead-load was sustained on the beams while the CFRP strips were installed and during the curing process of the adhesive. After the adhesive cured, the load was transferred from the independent dead-load apparatus to the hydraulic actuator and the fatigue loading program was commenced.
Figure 3: Independent dead-load apparatus
All of the test beams were instrumented to measure deflections at midspan and at the supports as well as to measure strains at various locations on the midspan cross-section of the beam. The measured strains were used to construct the strain profiles for the strengthened beams to investigate the possible presence of a shear-lag effect between the steel and the CFRP strengthening materials.