Steel Free Hybrid Reinforcement System for Concrete Bridge Decks ...

tensile strength and modulus of elasticity. Normal strength concrete was used for each
specimen.
Three GFRP bars and two steel bars were used in the pullout tests. Extensive
instrumentation recorded strains at six locations along the bars, as well as other strains and
reactions. In both types of bars, the tensile stress distributions in the bar varied
exponentially, decreasing as the distance from the loaded end increased. As the load
increased, the bond stress distributed itself further from the loaded end, with the maximum
bond stress moving away from the loaded end due to progressive loss of bond starting at that
end. It was found that chemical adhesion and mechanical interlock give bond strength before
slip; friction provides bond strength after slip begins.
In the beam test, the results showed bond strengths of GFRP to be in the range of
sixty to ninety percent of the strengths given by steel bars. As with steel bars, the bond stress
decreases as bar diameter increases. Additionally, the relative slip of the two ends of the bar
was shown to be roughly the same under similar stresses as the slip experienced by the steel
bars.
The bond of concrete and steel reinforcement comes largely from concrete bearing
against the deformations in the steel bar. Thus, the loss of bond comes from concrete shear
failure and crushing in this bearing zone, and not from the steel bar being deformed. GFRP
bars have a lower bond strength, due mainly to the fact that the deformations on their surface,
unlike those on steel bars, do not have the strength to cause concrete shear and crushing
failure. In this study, the concrete surfaces adjacent to the GFRP bars were observed to be
undamaged following the tests, pointing to low induced bearing stresses in the concrete. The
authors conclude that adhesion and friction are the two main bond stress components of
GFRP rebar to concrete; concrete bearing is an insignificant factor.
Results from the beam tests and the pullout tests were compared, showing the pullout
tests giving much higher values for bond strength. In the pullout test, the concrete is placed
in compression, disallowing tensile cracking, which in many cases precipitates bond failure.
Thus the bond strengths given by this test ranged from 5 to 82 percent higher than those from
the beam tests. The beam tests replicate actual conditions and strain gradients much closer,
giving better estimates for the bond strength.
Tighiouart, Benmokrane, and Gao (1998) studied the bond of GFRP reinforcement to
concrete. A total of 64 beam tests were performed, using four different bar diameters for
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