Bridge Analysis, Design and Repair
341578 - Investigation of Eliminating Prestress in Bridge Girders with the Use of Non-Prestressed Ultra-High-Performance Fiber-Reinforced Concrete Girders
Saturday, April 21
10:00 AM - 11:30 AM
Precast, prestressed concrete has long been used in bridge construction. The production of prestressed concrete members involves the use of special prestressing equipment, requiring a prestress bed and skilled labor, which eventually increases unit cost and production time. Thus, prestressing is uneconomical for short span members unless the same unit is used repeatedly. Often, camber-related issues pose challenges to designers, fabricators and contractors. Additionally, prestressed concrete members are prone to long-term loss of prestressing forces due to creep, shrinkage, and relaxation. The prediction of long-term prestress losses is in general cumbersome and by no means accurate. This study explores a new concept of replacing prestressed concrete structures with non-prestressed ultra-high-performance fiber-reinforced concrete (UHP-FRC) structural members. UHP-FRC is an innovative concrete construction material with excellent mechanical properties. UHP-FRC has very high compressive strength (generally greater than 22 ksi (150 MPa)), flexural strength (up to 6 ksi (42 MPa) or higher), great durability, low permeability, and exceptional compressive ductility with a maximum useable compressive strain of 0.015–0.03. Maximum useable compressive strain, εcu, specified in the ACI 318 Building Code and AASHTO LRFD Bridge Design Specifications are limited to 0.003 for conventional plain concrete with little ductility and a maximum compressive strength of about 15 ksi (103 MPa). Since the maximum useable compressive strain of UHP-FRC is 5 to 10 times that of plain concrete, it allows designer to provide more reinforcement and hence increase the flexural capacity of members. The higher amount of reinforcement also increases the cracking resistance of UHP-FRC due to the enhanced moment of inertia. The very high modulus of rupture in UHP-FRC allows no cracks under service load, which was the purpose of prestressed concrete when invented several decades ago by Freyssient and others. This research investigates the flexural and shear behavior of UHP-FRC beams, reinforced with Grade 60 steel (ASTM A615/A615M) and Grade 100 steel (ASTM A1035/A1035M) bars. Experimental outcomes have shown promising results by retaining high stiffness up to very large loads, which can effectively control the deflection. UHP-FRC beams also have high initial cracking strengths with a very high modulus of rupture of approximately 3 ksi (21 MPa). The authors extended these experimental results to propose a new modified decked bulb tee (DBT) girder, which has nearly the same cracking resistance as typical prestressed DBT but without camber. This would allow a fast construction of such bridges. This pioneering solution of replacing prestressed members with non-prestressed UHP-FRC members offers efficient design and construction practices by eliminating the issues with prestressing.