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An investigation was conducted to study the behavior of plain concrete and steel fiber reinforced concrete under direct tension fatigue loading. Tests were conducted on dogbone specimens with varying amounts of steel fiber volume content (0, 0.75, and 1.5%). A new concept was introduced in deriving material damage parameters for plain and steel fiber concrete. The parameters developed were implemented into a damage evolution function to enable the prediction of concrete strength and fatigue secant modulus deterioration of steel fiber reinforced concrete. As such, the damage evolution models developed for steel fiber concrete can be implemented into steel fiber reinforced concrete constitutive models for the analysis of fatigue-damaged concrete elements. From the experimental results, the deformation profiles for plain and steel fiber concrete were similar, and the well-known relationship between the fatigue life and secondary strain rate of concrete in compression also exist for plain concrete and steel fiber concrete in tension. In addition, under the same loading parameters, the fatigue life of steel fiber concrete was found to increase as steel fiber content increased from 0 to 1.5%. individual papers. This paper is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561. © ASCE 04017130-1 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2017, 29(9): -1--1 Downloaded from ascelibrary.org by University of Toronto on 05/16/17. Copyright ASCE. For personal use only; all rights reserved. © ASCE 04017130-7 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2017, 29(9): -1--1 Downloaded from ascelibrary.org by University of Toronto on 05/16/17.

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In the literature, three basic assumptions are used to modify monotonic constitutive models in order to simplify fatigue analysis of concrete. First, the fatigue hysteresis loop at failure is assumed to intersect the monotonic stress-strain envelope. Second, it is assumed that the peak stress of a fatigue-damaged concrete element intersects the monotonic stress-strain envelope. Third, the centerlines of fatigue hysteresis loops are assumed to converge at a common point. Although the modifications supposedly lead to improved predictions, experimental verifications of these assumptions are currently insufficient to justify their implementation in the fatigue analysis of complex and large concrete structures where considerations of safety and cost-effectiveness are expedient. From experimental verifications conducted to ascertain the conservative level of these assumptions, it was found that the first and second assumptions seem reasonable, while the third assumption was inaccurate and thus in need of improvement. As such, a new convergence point is proposed. The constitutive models for high-strength and normal-strength concrete in compression were also modified as functions of the irreversible strain and residual strength. Further, a model was proposed for the irreversible strain accumulation, and its corroboration with experimental results showed good agreement.