Toward A Non-Prestressed Precast Long-Span Bridge Girder Using UHP-FRC

Abstract

The exceptional compression strength and ductility of ultra-high-performance fiberreinforced concrete (UHP-FRC) can revolutionize the design of reinforced concrete structural members. While the maximum useable compressive strain, εcu, for conventional plain concrete is assumed to be 0.003 in current design codes (ACI 318 Building Code and AASHTO LRFD Bridge Design Specifications), UHP-FRC’s εcu is 5 to 10 times higher. Underestimating the compressive ductility of UHP-FRC limits the allowable maximum amount of longitudinal reinforcement, which in turn leads to limited flexural capacity of the members. Conventional reinforced concrete members are designed with a smaller amount of reinforcement to meet tension-controlled behavior. This design approach in turn leads to 1) a small ultimate flexural capacity, 2) a large amount of cracking and wider crack widths under service loads, which lead to a reduced member stiffness, 3) cracks that are less likely to close after overloading, 4) a small compression zone depth that allows cracks to propagate deeply, which further reduces the stiffness, 5) large strains in rebars, which reduce aggregate interlock and shear strength, and 6) considerable yielding of rebars, which causes bond deterioration. Contrary to the conventional design concept, a new ductileconcrete strong-reinforcement (DCSR) design concept is investigated in this study. A maximum useable compressive strain of 0.015 is considered for UHP-FRC, which allows a concrete member to maintain tension-controlled behavior while using a high amount of steel rebars. Accordingly, the flexural capacity of the section increases. This approach allows the UHP-FRC’s high compressive strength to be effectively utilized in the compression zone. The synergistic interaction of strong steel and tensile strength of UHP-FRC considerably increases the cracking resistance of the member. In addition, the number and size of initial microcracks are limited due to the strong bridging effect of a high amount of steel. Therefore, the member maintains its stiffness and small deflection under service loads. This feature permits eliminating prestressing in bridge girders, where an uncracked section is desired under service loads. Besides experimental evidence, a prototype single-span 250-ft long non-prestressed UHP-FRC decked bulb-tee (DBT) girder was designed using the DCSR concept. Finite element analysis with AASHTO loading confirms that the new UHP-FRC girder satisfies code requirements. The experimental and analytical results show that conventional precast prestressed concrete girders can be replaced by the new nonprestressed decked UHP-FRC girders.