International Concrete Abstracts Portal

This study investigates the feasibility of utilizing a hybrid combination of recycled steel fibers (RSF) obtained from scrap tires and manufactured steel fibers (MSF) in concrete developed for pavement overlay applications. A total of five concrete mixtures with different combinations of MSF and RSF, along with a reference concrete mixture, were studied to evaluate fresh and mechanical properties. The experimental findings demonstrate that the concretes incorporating a hybrid combination of RSF with hooked-end MSF exhibit comparable or higher splitting tensile strength, flexural strength, and residual flexural strength to that of concretes containing only hooked-end MSF, straight MSF, and RSF. This enhanced mechanical performance can be ascribed to the multiscale fiber reinforcement effect that controls different scales (micro to macro) of cracking, thereby providing higher resistance to crack propagation. The concretes containing only RSF show lower splitting tensile strength, flexural strength, and residual flexural strength compared to concrete solely reinforced with straight MSF or other steel fiber-reinforced concrete (SFRC) mixtures due to the presence of various impurities in the RSF, such as thick steel wires, residual rubber, and tire textiles. Interestingly, blending RSF with hooked-end MSF overcomes these limitations, enhancing tensile strength, flexural strength, and residual flexural strength, while significantly reducing costs and promoting sustainability. Lastly, the findings from the pavement overlay design suggest that utilizing a hybrid combination of RSF with hooked-end MSF can reduce the design thickness of bonded concrete overlays by 50% compared to plain concrete without fiber reinforcement, making it a practical and efficient solution.

Three large-scale reinforced concrete rectangular slender structural walls were subjected to cyclic displacement demands to establish whether, and under what conditions, mechanical splices can be used with Grade 100 (690) bars where yielding is expected. These tests were conducted because ACI 318-19 prohibits both lap splices and mechanical splices for high-strength longitudinal reinforcement (Grade 80 (550) and higher) in special structural walls where yielding is expected. Three mechanical splices were used that differed in connection type (one type per wall) and overall splice length. The mechanical splices were all placed starting 2 in. (50 mm) from the wall base. Mechanical splices satisfying the specified minimum tensile strength criterion of ACI 318-19 Type 2 mechanical splices resulted in better wall behavior than reported for lap splices, but satisfying Type 2 requirements alone did not prevent bar fractures at the mechanical splice. Thus, Type 2 mechanical splice requirements are not recommended as the sole qualification criteria where yielding is expected. Test results also showed that mechanical splices with a strength not less than the actual bar tensile strength, such that bars systematically fail in direct tension tests away from the splice and therefore develop their actual uniform elongation, perform well, and are recommended for use where yielding is expected in special structural walls.

Inner surface reinforcement is one of the most widely adopted techniques for upgrading or strengthening shield tunnels. An important failure mode in this method is the debonding of the thin plate from the segments, resulting in less reinforcement effect than expected. The shield tunnel lining is a discontinuous curved structure formed by connecting segments with bolts, and its structural form and internal force state are essentially different from reinforced concrete beams. However, there are few reports on the evolution process of debonding failure of similar structures. Therefore, to develop a thorough understanding of the debonding failure, a three-dimensional refined numerical model for the shield tunnel strengthened by a thin plate at the inner surface based on the mixed-mode cohesive method was proposed. The validity and rationality of the model were corroborated by a full-scale experiment. Then, the model was applied to other inner surface reinforcement schemes commonly used in practice to explore the debonding mechanism of the adhesive layer. Finally, anti-debonding measures were proposed, and their effectiveness was elucidated by numerical analysis. The results show that the proposed numerical model can accurately predict the failure process of the adhesive interface of the shield tunnel strengthened by a thin plate. There are obvious interfacial stress concentrations at the joints and the plate ends, which are the essential reasons for the debonding failure initiating from those places. Anchoring the thin plate only at the plate ends and joints can significantly and sufficiently increase the debonding load. Therefore, it is not necessary to add anchoring measures elsewhere.

This study introduces a new anchorage strategy using ultra-high-performance concrete (UHPC) to attach unbonded post-tensioning (PT) strands to existing foundations. This solution complements a seismic retrofit scheme investigated by the authors, which transforms non-ductile cast-in-place reinforced concrete (RC) shear walls into unbonded post-tensioned rocking shear walls, following concepts of selective weakening and self-centering. In the proposed PT anchorage scheme, mild steel reinforcements are inserted through the shear wall thickness and into the foundation. Subsequently, UHPC is cast around the wall base, forming a vertical extension connected to the foundation, which is used to anchor the unbonded PT strands. The feasibility and performance of the anchorage scheme were investigated through a combination of laboratory testing and numerical simulations. Pull-out testing on four scaled-down anchorage specimens was conducted in the laboratory. Hairline cracks were observed in the UHPC during testing. Additionally, 3D finite element (FE) models were created, validated, and used to study the performance of the proposed anchorage scheme under lateral loading. The simulation results support the effectiveness of the proposed anchorage strategy.

In this study, fabric formwork is used to cast I-shaped and non-prismatic tapered reinforced concrete (RC) beams that have up to a 40% reduction in concrete volume, resulting in lower embodied CO₂, compared to a rectangular prismatic beam. The primary aim of this research is to use distributed sensing to characterize the behavior of these shape-modified beams to an extent that was not previously possible and compare their behavior to that of a conventional rectilinear beam. Four RC beams (a rectangular control and three fabric-formed sections) were tested in three-point bending. Distributed fiber optic strain sensors were used to measure strains along the full length of the longitudinal reinforcement, and digital image correlation was used to acquire crack patterns and widths. The results indicate that fabric-formed RC beams can achieve the same load carrying capacity as conventional rectilinear prismatic beams and meet serviceability requirements in terms of crack widths and deflections. The longitudinal reinforcement strains along the full length of the specimens were captured by Canadian concrete design equations as they account for the effects of both flexure and shear on reinforcement demand.