Multi-linear tensile stress-crack width relationships for hybrid fibre reinforced concrete using inverse analysis and digital image correlation

This study aimed to analytically develop tensile stress-crack width constitutive relationships for hybrid fibre reinforced concrete (HFRC). An inverse analysis was performed based on the fictitious crack hinge model to derive the stress-crack width relationship through careful calibrations of model parameters with the digital image correlation (DIC) measurements. The studied parameters included fibre types, volume fraction (Vf) and their combinations. Multilinear stress-crack width relationships for steel, polyolefin and hybrid fibre reinforced concrete with three different fibre volume fractions of 0.50%, 0.75% and 1.00% were derived. The hinge parameters assumed in the inverse analysis were validated with the help of full-field strain measurements using the DIC technique. The relationship between the crack tip opening displacement (CTOD) and the crack mouth opening displacement (CMOD) was verified with the help of the developed equations through the inverse analysis. Afterwards, the simplified equations for predicting the stress-crack width relationships for various types of hybrid FRC were proposed. © 2020 Elsevier Lt

Investigation of the effect of steel fibers on the shear crack-opening and crack-slip behavior of prestressed concrete beams using digital image correlation

The effect of steel fiber reinforcement on the shear behavior of prestressed concrete beam is studied using digital image correlation (DIC) technique. Steel fiber reinforced prestressed concrete (SFRPC) beams with fiber dosages of 0.5% and 1.0% by volume were cast and tested under shear. SFRPC beams were tested under a three-point bending configuration at a shear span (a) to depth (d) ratio of 2.4 to simulate shear dominant behavior. Other parameters such as compressive strength of concrete, prestressing reinforcement ratio and level of prestressing were kept constant. The kinematics of diagonal shear cracks including crack opening and crack slip were monitored using DIC based full-field strain measurement technique. The improvement in aggregate interlock and residual tensile strength at the crack tip due to the addition of steel fiber was reflected in the post-peak response of SFRPC beams. DIC analysis revealed that the full depth crack was formed in all the SFRPC beams before reaching their peak loads. The dilatant behavior was found to be consistent for all the test specimens (control and fiber reinforced concrete beams) up to peak load. The test results portray that the addition of steel fibers stiffens the post-peak response and reduces the crack opening and crack slip. Moreover, the failure mode changed from shear (brittle) to flexure-shear (less brittle) mode

Effect of steel fibers on the shear behavior of Self-Compacting reinforced concrete deep Beams: An experimental investigation and analytical model

The shear behavior of reinforced concrete deep beams made of self-compacting concrete with different volume fractions of discrete macro steel fibers (0.5%,1.0%, and 1.5%) is studied. All the beams are tested at shear span to depth ratio of 1.0, and the effect of loading plate width on shear carrying capacity is also studied. Similarly, the effectiveness of steel fibers in improving the shear behavior is compared with the beams made of plain concrete and the beams with 0.3% web reinforcement in both horizontal and vertical directions. Test results revealed that the addition of macro steel fibers significantly improved the performance of deep beams, particularly first cracking load, ultimate shear capacity, ultimate deformation, and ductility. However, the addition of steel fiber more than 1.0% volume fraction did not increase the ultimate shear capacity. A simplified analytical model is proposed to predict the ultimate shear capacity of steel fiber reinforced concrete deep beams. The existing cracking strut and tie method (CSTM) is extended to include the effect of fibers, and a simplified fiber-reinforced cracking strut and tie method (FR-CSTM) is proposed. Predictions of FR-CSTM are compared with CSTM and two-parameter kinematic theory (2PKT). The comparisons of predictions with test results and existing models show that the proposed method can capture the ultimate shear capacity of deep beams with reasonable accuracy

Effectiveness of Hybrid Fibers on the Fracture and Shear Behavior of Prestressed Concrete Beams

This study investigates the effectiveness of hybrid fibers (steel and macro‐synthetic) on the shear behavior of prestressed concrete beams. The hybrid fiber combination was selected to avoid workability issues at high volume dosages and ensure effective crack arresting over the crack opening range. Fracture studies included testing notched concrete prisms to identify the role of hybrid fibers in the crack bridging mechanism. Seven hybrid fiber reinforced prestressed concrete (HFRPC) beams were tested at a low shear span (a) to depth (d) ratio of 2.4. The effects of hybrid fibers on load–deflection behavior and strain in the strand are reported. Similarly, the crack opening, crack slip and crack angle variation regarding applied shear were investigated using the digital image correlation (DIC) technique. Test results of HFRPC beams showed considerable improvements in peak load and the post‐peak response with a higher hybrid fiber dosage. The crack opening and crack slip measurement across the major shear crack revealed continuous dilatant behavior. The kinematic response of critical shear crack reflects the sustained dilation response up to the ultimate load, which depends on the critical shear crack angle of the tested beams. As the fiber dosage increases, the shear crack slip and width are reduced, indicating the roles of hybrid fibers in improving ductility and the change in failure mode from brittle shear tension to relatively ductile shear tension. © 2022 by the authors. Licensee MDPI, Basel, Switzerland

Experimental Study on Evaluation of Replacing Minimum Web Reinforcement with Discrete Fibers in RC Deep Beams

This study investigates the possibility of replacing the minimum web reinforcement in deep beams with discrete fibers. Additionally, the equivalent dosage of fibers required to obtain similar performance of the deep beam with minimum web reinforcement is investigated. Deep beams made of plain concrete with no fibers, beams with minimum web reinforcement as per AASHTO LFRD recommendations (0.3% in both horizontal and vertical), and with a 0.5% volume fraction of steel, macro-synthetic and hybrid fibers are tested at a shear span to height ratio (a/h) of one. Test results show that the presence of 0.3% web reinforcement in horizontal and vertical directions increased the peak load by 25% compared to the plain concrete beams. However, it did not significantly change the first diagonal crack load. With the addition of 0.5% of steel, macro-synthetic and hybrid fibers, the peak load increased by 49%, 42%, and 63%, respectively, compared to the plain concrete specimen. The addition of steel fibers significantly improved the first cracking load. In contrast, macro-synthetic fibers did not affect the first cracking load but improved the ductility with higher deflections at peak. Hybridization of steel and macro synthetic fibers showed improved performance compared to the individual fibers of the same volume in peak load and ductility. Test results showed that a 0.5% volume fraction of discrete macro steel or synthetic or hybrid fibers can be used to completely replace the minimum web reinforcement (0.3% in both directions). © 2021 by the authors. Licensee MDPI, Basel, Switzerland