International Concrete Abstracts Portal

Brittle punching failures in flat plates are precluded by ensuring adequate shear strength. Typically, this is achieved by adding shear reinforcement in the design. This paper presents an experimental and analytical study of flat plates to investigate load-resisting mechanisms associated with stirrup addition. The experimental program includes four isolated flat plates with parametric variations tested under monotonic punching loads. In terms of normalized shear strength, improvements of 22% and 29% were observed in flat plates with different layouts of stirrups, respectively, when compared with the reference specimen without stirrups. The role of longitudinal and shear reinforcements in punching resistance of flat plates was assessed through strain observations. Based on test findings, a reasonable physics-based analytical procedure for punching capacity estimation is proposed and verified using a database of 72 isolated flat plate specimens. The proposed method provided reasonably accurate capacity estimates with an overall mean test-to-estimated capacity ratio of 1.06 and a low coefficient of variation (COV) of 13%. These estimates are also compared with capacity predictions using ACI 318-19 guidelines, which resulted in an overall mean capacity ratio of 1.58 with a COV of 22%. Based on experimental and analytical results, modifications to ACI 318-19 two-way shear provisions are suggested by incorporating the key parameters in shear strength estimations, which improved the prediction accuracy to a mean of 1.25 with a COV of 13%.

This paper presents research focused on the development of a test method that can be used to gauge the susceptibility of a post-tensioning (PT) grout to form soft grout. Depending on the grout formulation, soft grout may have a lower pH, retain excessive moisture, and be corrosive to the tendon. While relatively rare, it has been documented in bridge construction in the U.S. and abroad and in some cases has prompted the replacement of PT tendons.

One of the causes of the soft grout is thought to be the result of the use of low reactivity fillers such as ground limestone. When tendons are deviated significantly, these fillers can segregate and then accumulate into a mass of material that does not harden. The modified inclined tube test (MITT) was developed based on the Euronorm inclined tube test. None of the commercially available PT grouts produced soft grout when the grout was mixed and injected in accordance with the manufacturer’s recommendations and tested well before their expiration date. Additional mix water or residual water in the tube, however, produced soft grout consistently in one of the PT grouts.

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.

Compression development and lap splice length provisions in ACI 318-19 §25.4.9 and §25.5.5 are reexamined after an example is used to show that existing provisions can produce unexpected results in some design conditions, such as compression lap splices longer than tension lap splices. A historical review of ACI Building Codes shows existing compression bond length provisions are largely based on provisions adopted before test data were available. The provisions in ACI 318-19 are compared with a database of 89 test results, and are shown to poorly fit the data. Several compression and tension bond equations are also examined that fit the data better. It is shown that compression development and lap splice lengths can be based on a number of expressions available in the literature for tension development length, with minor modification, including the ACI 318-19 equation for tension development length. Using this approach would simplify design by eliminating the use of different expressions to calculate tension and compression development lengths, prevent calculated lengths from being longer in compression than in tension, and provide a better fit to available data.

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.