Study on Fracture Behavior and Shear Capacity of Steel Fiber Reinforced Concrete Beams

Gali, Sahith and K V L, Subramaniam (2019) Study on Fracture Behavior and Shear Capacity of Steel Fiber Reinforced Concrete Beams. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

In the design of reinforced concrete structures, strength-based procedures are conventionally adopted, which ignore the contribution of tensile stress carried by concrete after cracking. Any delay in the onset of cracking produced by improvement in the material performance is considered through an increase in the tensile strength of the material. To assess the true potential of discrete fiber reinforcement the delay in the onset of cracking due to suppression of microcracking, and the post-cracking stress transfer across the crack have to be accounted for. The post-cracking stress transfer is particularly important in shear where the shear transfer by aggregate interlock contributes significantly to the shear capacity of reinforced concrete elements. Shear displacements along rough cracks also produce dilatancy across the shear crack. The crack bridging provided by fibers can potentially provide for increased mobilization of aggregate interlock, thereby increasing the shear capacity. An understanding of the influence of fibers on the post-cracking shear stress transfer across rough cracks surface and its influence on the shear capacity of reinforced concrete beams needs to be understood to develop design provisions which consider the influence of fibers on shear capacity. This study aims to investigate the shear behavior of reinforced concrete with discrete hooked end steel fibers. The influence of hooked-end steel fibers on the shear transfer across rough cracks in concrete and its influence on the shear behavior of reinforced concrete are investigated. A two stage investigation which involves obtaining information of the material behavior and relating it to the structural response is developed. In the first stage of evaluation, the fracture behavior of steel fiber reinforced concrete (SFRC) is investigated using flexure tests on notched specimens. Crack propagation and post-cracking behavior in the flexural load response of SFRC is evaluated using full-field displacements obtained from digital image correlation technique. Surface displacements and strains during crack propagation from a notch are presented at volume fractions of steel fibers (Vf) equal to 0.5% and 0.75%. An analysis procedure for determining the crack opening width over the depth of the vii fiber-reinforced beam in a flexural test is presented. From the analysis of displacements and strain, the crack opening width is established as a function of crack tip opening displacement and the residual flexural strength for the SFRC. Analysis of displacements shows that crack propagation in the cementitious matrix produces softening in the load response. At the volume fractions of fibers considered in this study, the softening in the post-peak load response is shown to be associated with the rapid propagation of crack in the material. Fibers control the rate of load decrease produced by crack propagation in the matrix with increasing crack opening in the softening response. The load recovery in the SFRC is associated with a hinge-type behavior in the beam. Fibers provide resistance to opening of the hinge, which results in a load recovery. For the stress gradient produced by flexure, the hinge is established at a crack tip opening displacement before load recovery is initiated. At 0.75% fiber volume fraction, there is a significant decrease in the crack advance for a given crack opening. An analytical framework for implementing a multi-linear stress-crack separation (σ-w) relationship within the cracked hinge model is presented. Multilinear σ-w relations are obtained for SFRC with different Vf using an inversion procedure. The σ-w relationship for SFRC exhibits an initial softening to values lower than the tensile strength, which is followed by a stress recovery with increasing crack separation. In SFRC, the stress attains a constant value with increasing crack separation, larger than 1 mm. For Vf equal to 0.75%, application of cracked hinge model predicts a constant stress of magnitude less than the tensile strength with increasing crack separation in the part of the load response associated with multiple cracking. In the second stage of evaluation, the shear behavior of reinforced concrete beams with and without steel fibers is investigated. Reinforced concrete beams with discrete hooked-end steel fibers at 0.5% and 0.75% volume fractions were tested with a shear span-to-depth ratio equal to 2.25. Full-field surface displacements from the beam during the load response were obtained using digital image correlation (DIC). The formation and propagation of a shear crack which directly influences the load response and peak load, is monitored. The displacement measurements from across viii the shear crack indicate a continuous increase in crack opening associated with increasing slip between the two crack faces. The relation between slip and crack opening suggests that the dilatant behavior measured within the shear region is identical in control and SFRC beams. At a given load, the crack opening in SFRC specimens is smaller than the value obtained from the control beams. Failure in control beams is brittle and is produced by the opening of the dominant shear crack in the shear span. Analysis of shear response of reinforced concrete beam shows that control specimens failed when compression generated by rebar is insufficient to sustain aggregate interlock. In SFRC beams, the crack closing stresses provided by the steel fibers allow shear stress transfer across the shear crack, which contributes to increased ductility and to residual load carrying capacity after the peak load. In SFRC beams with 0.5% volume fraction there is a continuous opening of the shear crack even after the peak load which leads to a post-peak response with decreasing residual load carrying capacity. In SFRC beams with 0.75% fiber volume fraction, the increased resistance to crack opening provided by the fibers results in further increase in the peak load. Experimental tests are conducted to study the effect of shear slenderness on the shear behavior of steel fiber reinforced concrete (SFRC) beams. Shear beams ranging from non-slender to slender were tested at shear-span-to-depth (a/d) ratios equal to 1.8 and 3.0 in addition to 2.25. At an a/d of 1.8, shear failure is very sensitive to the loading and support conditions. For the intermediate and the slender beams, flexure-shear failure is produced. Shear capacity decreases with an increase in the slenderness of the beams. DIC is used to study the propagation of cracks leading to the formation of the critical shear crack. Critical shear crack is formed at the location of the highest applied moment in the shear span. The horizontal projection of the critical shear crack is equal to the effective depth of the beam (d). At the peak load, the moment (Mu) to shear (Vu) ratio given by Mu/(Vud) at critical shear crack increases with increasing slenderness. A continuous dilatant behavior identified with a continuous increase in the crack opening displacement with progressive slip across the crack faces of the critical shear crack is observed throughout the load response. The dilatancy measured across the critical shear crack depends on the slenderness of the beam and is not altered with the addition of fibers. The applied moment contributes ix to the measured dilatancy across the critical shear crack. There is a larger crack opening with increasing slip displacement across the shear crack with an increase in the applied moment at the location of the shear crack. There is an increase in the shear capacity and the energy absorption in SFRC beams, and the load is sustained for large crack openings. The efficiency of fibers on increasing the shear capacity decreases with an increase in the Mu/(Vud) ratio at the shear crack. Based on the two stage investigation of fracture and shear behavior of fiber reinforced concrete a discrete crack model is developed for predicting shear capacity of reinforced beams without stirrups. The experimental observations of cracking and mechanism of the pivoting action of the critical shear crack are included in the formulation of the discrete crack model. The internal contact forces on the crack faces and the cohesive stress from fibers, are considered in deriving the equilibrium. The fiber contribution in providing shear resistance is quantified in terms of a cohesive stress-crack separation relationship. The prediction of the model includes an increase in the contact forces with an increase in the fiber force at peak shear resistance with an increasing volume fraction of fibers. In SFRC beams, the additional contact force mobilized across the crack by the fibers maintains the shear transfer across the crack and hence the load carrying capacity is sustained for a larger crack opening. The model derived from laboratory-sized specimens accurately predicts the scaling the shear capacity with size of the beam. The main findings of this study are as follows: (a) Improvement in the tensile fracture response of concrete results in an improvement in the shear capacity of reinforced concrete; (b) The resistance to crack opening directly contributes to the contact stresses across the shear crack; (c) Increase in shear capacity of reinforced concrete is derived from an increase in the shear transfer ability of the frictional interface. This study establishes the potential for using discrete steel fibers as structural shear reinforcement.

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