۱- Introduction

۲- Experimental study

۳- Test results

۴- Conclusion

References

Abstract

Fiber-reinforced polymers (FRPs) are widely used in the strengthening of concrete structures due to their light weight, high strength, and good durability. For precast concrete structures, bundled FRP/steel bars can substantially ease the construction process. In this paper, experimental studies were conducted on five concrete beams with different types of bundled reinforcements. The test results showed that all the beams exhibited concrete crushing failure modes after the steel bar yielded, and the plastic development of the steel bar was restrained by the elastic FRP bar. As the reinforcement concentration increased, the bond behavior between the longitudinal reinforcement and the surrounding concrete decreased; the postcracking stiffness and crack quantity of the corresponding concrete beam decreased, whereas the crack width increased. Both the initial stiffness and postyield stiffness of the concrete beam with 3-bar bundles were approximately 50% of that of the beam with double-bar bundles. The displacement ductilities of all the concrete beams were greater than six. Due to the differences in the bond behavior, the ultimate displacements of the beams with 3-bar bundles and 6-bar bundles were approximately 1.6 and 1.9 times the ultimate displacement of the beam with single-bar reinforcement, respectively.

Introduction

A large number of laborers and a large quantity of formwork are needed in the construction of cast-in-place concrete structures, in which the use of tremendous formwork inevitably affects the environment, and the labor cost continually increases with economic development [1]. Prefabrication of components/members in the factory can greatly reduce the on-site labor requirements, and the construction speed and the quality of the structure can be ensured [2,3]. When the conjunction of a precast concrete member is not well treated, steel bar corrosion will develop very fast, and the corresponding structural performance will be degraded [4]. Fiber-reinforced polymer (FRP) is a kind of composite with high strength, low density, and high durability that can be placed near the corner of the concrete cover to delay the deterioration of the structure from corrosion [5]. In terms of hybrid reinforcement, experimental studies on concrete beams reinforced by FRPs and steel bars have been conducted [6,7]. The longitudinal reinforcements were made of steel bars and glass FRP (GFRP) bars, which were arranged in different layers on the concrete beam sections. The results showed that the yielding of the steel bar can ensure ductility, and the high strength of the FRP improves the bearing capacity of the beam. Tests of twenty-four concrete beams reinforced by steel bars, GFRP bars or hybrid steel/GFRP bars (GFRP bars were placed on the outer tension side) have been conducted [8,9]. These studies found that the failure modes of the hybrid reinforced beams were concrete crushing after the steel yielded, the hybrid reinforced beams had better ductility than the beams reinforced by pure FRP bars, and according to ACI 440.1R-06 [10], the minimum reinforcement ratio for an FRP-reinforced beam could be reduced by 25%. Regarding the seismic performance of a concrete structure, due to the elastoplastic characteristics of ordinary steel bars, the deformation of a reinforced concrete (RC) structure after yielding cannot be effectively controlled [11], and the failure probability of the structure will subsequently increase [12]. Under a stable load that is greater than the yield load, the damage in a concrete column is mainly concentrated on the plastic hinge near the column foot, and the residual deformation would be excessively large after an earthquake [13]; thus, the column collapses more easily during the aftershocks [14,15].