United States Department of Transportation
i
Steel fiber replacement of mild steel in prestressed concrete beams.
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2011-01-01
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[PDF-8.96 MB]
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Abstract:In traditional prestressed concrete beams, longitudinal prestressed tendons serve to resist bending moment and transverse mild
steel bars (or stirrups) are used to carry shear forces. However, traditional prestressed concrete I-beams exhibit early-age cracking
and brittle shear failure at the end zones despite the use of a high percentage of stirrups (4.2%). Moreover, producing and placing
stirrups require costly labor and time. To overcome these difficulties, it is proposed to replace the stirrups in prestressed concrete
beams with steel fibers. This replacement concept was shown to be feasible in a TxDOT project (TxDOT project 0-4819) recently
completed at the University of Houston.
The replacement of stirrups by steel fibers in highway beams requires a set of shear design provisions and guidelines for
prestressed Steel Fiber Concrete (PSFC) beams. The development of rational shear provisions with wide applications must be
guided by a mechanics-based shear theory and must be validated by experimental tests on I- and box-beams. A rational shear
theory, called the Softened Membrane Model (SMM), has been developed at the University of Houston for reinforced concrete
beams. This theory satisfies Navier’s three principles of mechanics of materials, namely, stress equilibrium, strain compatibility
and the constitutive relationship between stress and strain for the materials.
The first phase of the research consisted of testing 10 full-size prestressed PSFC panels. This was done to establish the effect of
fiber factor and the level of prestress on the constitutive models of steel fiber concrete and prestressing tendons. From this data a
set of constitutive models was developed to predict the behavior of prestressed PSFC. Notable findings include the fact that
increasing steel fiber content has a beneficial effect on the softening properties of prestressed PSFC. Additionally, the findings
show that increasing steel fiber content increases tension stiffening in prestressed PSFC under tensile loading.
The second phase of this research project generalizes the SMM shear theory for application to prestressed PSFC beams. This was
achieved by feeding the new constitutive models of fiber concrete and prestressing tendons into a finite element program
(OpenSees). The accuracy of the new shear theory was evaluated by testing full-size prestressed PSFC I- and box-beams that fail
in shear modes. The developed finite element program was used to simulate the shear behavior of the beams with acceptable
accuracy. Finally, a design equation and recommendations were provided for use when designing PSFC beams. Using the design
e quations, a series of four design examples, was also provided.
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