See discussions, stats, and author profiles for this publication at: https://ww

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/314163212 Shear Strength Enhancement Mechanisms of Steel Fiber-Reinforced Concrete Slender Beams Article in Aci Structural Journal · May 2017 DOI: 10.14359/51689449 CITATIONS 0 READS 48 2 authors, including: Some of the authors of this publication are also working on these related projects: Bond Stress-Slip of Reinforcing Bars and Prestressing Strands in HPFRC Composites View project Full-Scale RC and HPFRC Frame Subassemblages Subjected to Collapse-Consistent Loading Protocols for Enhanced Collapse Simulation and Internal Damage Characterization View project Shih-Ho Chao University of Texas at Arlington 65 PUBLICATIONS 708 CITATIONS SEE PROFILE All content following this page was uploaded by Shih-Ho Chao on 03 September 2017. The user has requested enhancement of the downloaded file. 729 ACI Structural Journal/May-June 2017 ACI STRUCTURAL JOURNAL TECHNICAL PAPER An experimental study was conducted to identify the shear-enhance­ ment and failure mechanisms behind the ultimate shear strength of steel fiber-reinforced concrete (SFRC) slender beams by using the full field-deformation-measuring capability of digital image correla­ tion (DIC) technology. A total of 12 large-scale simply supported SFRC and RC beams with an overall height from 12 to 48 in. (305 to 1220 mm) were tested under monotonic point load up to failure. The greater shear strength in SFRC beams originates from the ability of the fiber bridging effect that delays the propagation of the cracks into the compression zone, whose shear strength is enhanced by the compressive stresses induced by the higher load. The slow progres­ sion of the cracks keeps the compression zone depth large, thereby enabling it to contribute to a higher shear resistance. In contrast with the traditional assumption for either plain concrete or SFRC beams, where the shear contribution resulting from dowel action is completely neglected, this research clearly shows that the dowel action has an appreciable effect on the ultimate shear strength. Its contribution varies from 10 to 30% when the beam depth increases from 12 to 48 in. (305 to 1220 mm). On the other hand, the compres­ sion zone’s contribution decreases from 69 to 36% with the increase in beam depth. In addition, the shear contribution from the fiber bridging effect along the critical shear crack stays approximately unchanged at 20%, irrespective of the beam depth. In this study, the minimum shear strength obtained was in the range of 5√fc′ psi (0.42√fc′ MPa) for the beams with the greatest depth. This indicates that the maximum allowed shear stress limit of 1.5√fc′ psi (0.125√fc′ MPa) specified in ACI 318-14 is on the very conservative side. Keywords: dowel action; hooked-end steel fiber; shear strength; steel fiber-reinforced concrete. INTRODUCTION While there is a rather universal acknowledgement of diagonal shear failure in plain concrete (PC) beams (without transverse reinforcement), different models with distinct perspectives have been proposed to explain the shear resis­ tance mechanism of PC beams. One explanation is found in the Modified Compression Field Theory (Vecchio and Collins 1986), where the aggregate interlock (based on aggregate sizes), crack width/spacing, and straining effect due to longitudinal reinforcement are used to explain the shear behavior. Another popular approach is the Compres­ sion Force Path Theory (Kotsovos 1988) in which the resis­ tance against shear is assumed to be provided by a compres­ sion path through which the external force is transmitted to the supports. Finally, the Critical Shear Crack Theory (Muttoni and Fernández Ruiz 2008) introduces arch action as the possible shear-carrying mechanism of a PC beam upon formation of a critical shear crack. Generally, the mechanical behavior of fiber-reinforced concrete (FRC) is significantly different from PC, depending on the fiber volume fraction, fiber geometry, fiber orienta­ tions, and fiber-to-matrix bond characteristics. Numerous studies have shown that steel fiber-reinforced concrete (SFRC) can considerably improve the post-cracking tensile behavior and toughness of concrete (Mobasher 2012). When considering SFRC behavior at the structural element scale, the addition of steel fibers in concrete beams can also result in a substantial increase in the ultimate shear capacity in comparison with identical plain concrete beams. The enhanced shear strength is usually attributed to the fiber bridging stress across shear cracks (Choi et al. 2007). For design purposes, ACI Committee 318 (2014) allows the use of steel fibers as minimum shear reinforcement when ϕ0.5vc ≤ vu ≤ ϕvc, where compressive strength of concrete does not exceed 6 ksi (41 MPa), depth is no more than 24 in. (610 mm), and shear stress vu is no more than ϕ2√fc′ (that is, 1.5√fc′ where fc′ is in psi). ACI Committee 318 (2014) also requires a minimum volume fraction of steel fibers of 0.75% (100 lb steel fibers per cubic yard of concrete). To date, a limited number of models have been proposed for the shear resistance mechanisms of SFRC beams. Choi et al. (2007) proposed a theoretical strain-based model to account for the effect of flexural deformation on shear capacity of an SFRC beam. The shear resistance from aggregate inter­ lock and dowel action has been ignored because the intact compression zone was assumed to prevent slip of the crack interface (Choi et al. 2007; Kotsovos and Pavlović 1998). They assumed that the shear resistance of an SFRC beam is provided by the intact compression zone and the bridging tensile strength of steel fibers crossing the critical shear crack. The location of the critical shear crack was first deter­ mined by their strain-based formulations through an iterative process, and then the shear contribution of the compression zone was determined by Rankin’s failure criteria. Contrary to the procedure used by Choi et al. (2007), in this study, the full field strains (thus, stresses) were measured by digital image correlation (DIC) directly along the critical crack right before failure, thereby eliminating any assumption and itera­ tive procedure to establish shear strength. Dinh et al. (2011) used a similar approach where the shear contribution of Title No. 114-S59 Shear Strength Enhancement Mechanisms of Steel Fiber- Reinforced Concrete Slender Beams by Mohammad Reza Zarrinpour and Shih-Ho Chao ACI Structural Journal, V. 114, No. 3, May-June 2017. MS No. S-2016-005.R2, doi: 10.14359/51689449, received July 7, 2016, and reviewed under Institute publication policies. Copyright © 2017, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published ten months from this journal’s date if the discussion is received within four months of the paper’s print publication. 730 ACI Structural Journal/May-June 2017 aggregate interlock and dowel action were ignored. Dinh et al. (2010) reported that 18 of the 23 SFRC specimens failing in shear were caused by tension failure. Nevertheless, in their model, they assumed that crushing in the compression zone was the cause of failure, and employed the compression failure criterion established for plain concrete by Bresler and Pister (1958) with the average ultimate compressive stress of 0.85fc′ in Whitney’s stress block. In general, the current models used to predict the ulti­ mate shear strength of SFRC beams were developed on the basis of simplified assumptions concerning shear failure mechanism. Even though some shear-resisting components have proven negligible for RC beams, their contribution to SFRC beams has not yet been assessed. Furthermore, the strength-enhancement mechanism of SFRC beams is still not well defined. Establishment of the strength-enhance­ ment mechanism serves as a preliminary step to investigate size effect in ultimate shear strength of SFRC beams and to determine the design shear strengths of SFRC beams, which has a conservative low value in current ACI 318 provisions (ACI Committee 318 2014). This research aims at deter­ mining the shear-enhancement and failure mechanisms of SFRC beams by means of a deformation-monitoring digital image correlation (DIC) technique. DIC captures full field deformations and their progression with load increase. RESEARCH SIGNIFICANCE Shear strength-enhancement and the resistance mecha­ nism of SFRC beams were assessed. The primary compo­ nents were identified through experiments on large-scale SFRC beams with a range of depths varying from 12 to 48 in. (305 to 1220 mm). The results obtained from this experi­ mental study were particularly valuable because of the full field deformation (that is, state of strain on each arbitrary plane and displacement in all directions) obtained from DIC. Identification of the key factors contributing to the shear strength was performed based on the visualization of strains and deformations provided by the DIC as well as a mechan­ ical-based computation. EXPERIMENTAL PROGRAM Specimens A total of 12 simply supported SFRC and RC beams were monotonically loaded up to failure. Each pair out of the first four pairs of SFRC beams consisted of two duplicated beams, whereas the last pair had two identical SFRC specimens differing in width (6 and 24 in. [152 and 610 mm]), which allowed the investigation of the effect of width on the shear behavior of SFRC beams (Fig. 1(a) and 1(b)). Two identical RC beams with no stirrup and an overall height of 18 in. (457 mm) were used as control specimens. For all specimens, Fig. 1—Geometry and reinforcement details of large-scale RC and SFRC beams: (a) uploads/Voyage/ zarrimpour-pdf.pdf

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