GFRP bars are considered an alternative to steel for concrete reinforcement. This project investigated the fatigue behavior of GFRP bars embedded in concrete, studying bond behavior at material and structural scales. GFRP bars (12 mm [0.47 in.] nominal diameter) were embedded in concrete cylinders leaving a 50 mm [2 in.] protrusion at the free end and featuring different bonded lengths. Two types of GFRP bars with different surface treatment (lacquered and unlacquered) were used. Static tests were used to determine the bonded length required for cyclic pull-out tests, Cyclic tests at 1.5 Hz showed GFRP bar failure was possible at just 20% of their reduced tensile strength (0.8ffu) as prescribed in ACI 440.1R-15. Two full-scale slabs internally reinforced with unlacquered GFRP bars were tested using a four-point bending configuration. A quasi-static test was used as a control to determine the fatigue amplitude, considering the fatigue loading provided by the ACI 440.1R-15 document and the pull-out test results with cyclic loading presented in this work. Cyclic load between 10 kN [2.25 kips] and 40 kN [9 kips] at a 1.5 Hz frequency was applied up to 5 million cycles before a subsequent quasi-static test was conducted. The load range was determined using cross-section analysis to cycle the bars between 5% and 20% of their reduced tensile strength (0.8ffu). Both slabs ultimately failed due to shear failure, with cyclic loading having little impact on the slab compliance. Displacements of the load points and supports were measured using linear variable displacement transformers (LVDTs), while digital image correlation (DIC) was utilized to obtain the full-field displacement and strain in the central region of the slab. The strain and displacement fields from DIC were used to determine the opening of flexural cracks and relate it to the stress level in the GFRP bars. A comparison between the static pull-out tests and the four-point bending tests of slabs indicated that the pull-out test could be used to describe the flexural behavior of the slab at low stress level. However, in terms of fatigue behavior, the comparison between the small- and large-scale tests indicated that the fatigue phenomenon in the slab was quite complex and could not be directly described by the results of pull-out tests.

The design, analysis, and performance of structural concrete slabs under punching shear loading conditions are topics that have been studied extensively over many decades and are well documented in the literature. However, the majority of the work reported in these areas is generally related to conventional concrete slabs subjected to highly idealized loading conditions. Structural engineers need to find new, innovative ways and methods to design new structures but also to strengthen existing infrastructure to ensure safety, resilience, and sustainability. These challenges can be addressed through the use of integrated systems and high-performance technologically advanced materials. We live in a new era of improved computational capabilities, advances in high-performance computing, numerical and experimental methods, and data-driven techniques, which give us broader access to larger and better data sets and analysis tools. These new advancements are essential to develop deeper insights into the structural behavior of concrete slabs under punching shear and to implement and analyze new materials and loading conditions. This Special Publication presents recent punching shear research and insights relating to topics that have historically received less attention in the literature and/or are absent from existing codified design procedures. Topics addressed include: the usage and impacts of alternative/modern construction materials (new concrete and concrete-like materials, nonmetallic reinforcement systems, and combinations thereof) on slab punching shear resistance, novel shear reinforcement or strengthening systems, the influence of highly irregular/nonuniform loading and support conditions on slab punching shear, impact loading, new design and analysis techniques, and the study of the punching shear behavior of footings. This Special Publication will be of interest to designers who are often faced with punching-related design requirements that fall outside of traditional research areas and existing code provisions, as well as for researchers who are performing research in related areas. Perspectives from a broad and international group of authors are included in this Special Publication, relating to a variety of punching-related problems that occur in research and practice. In particular, researchers from the United States, Canada, Ecuador, the Netherlands, Italy, Brazil, Israel, Portugal, Spain, the United Arab Emirates, and Germany contributed to the articles in this Special Publications. To exchange views on the new materials, tests, and analysis methods related to punching, Joint ASCE-ACI Committee 421, “Design of Reinforced Concrete Slabs;” Joint ASCE-ACI Committee 445, “Shear and Torsion;” and subcommittee ACI 445-C, “Punching Shear,” organized two sessions titled “Punching shear of concrete slabs: insights from new materials, tests, and analysis methods” at the ACI Spring Convention 2023 in San Francisco, CA. This Special Publication contains several technical papers from experts who presented their work at these sessions, in addition to papers submitted for publication only. Co-editors Dr. Katerina Genikomsou, Dr. Trevor Hrynyk, and Dr. Eva Lantsoght are grateful for the contributions of the authors and sincerely value the time and effort of the authors in preparing the papers in this volume, as well as of the reviewers of the manuscripts. Aikaterini Genikomsou, Trevor Hrynyk, and Eva Lantsoght Co-editors

While the punching shear behavior of centrically loaded footings has been investigated in the past, the influence of unbalanced moments has remained almost uninvestigated for footings. Nevertheless, unbalanced moments are also transferred into the column by shear stresses requiring consideration in punching shear design. Here, design approaches often use coefficients to increase the load on action side or to decrease the resistance. To fill the gap in test data necessary for validation of design approaches, tests of four centrically and fourteen eccentrically loaded footings without shear reinforcement were conducted. Here, innovative measurement techniques were used to determine the development of the compression ring at the column-footing connection. While the constriction of the concrete compression zone due to the multiaxial load transfer leads to the formation of a circumferential compression ring with multiaxial concrete strains for centrally loaded slabs, which enhances the punching shear resistance compared to one-way shear, this compression ring only develops to a reduced extent with increasing load eccentricity. Based on the test results, a new proposal for consideration of unbalanced moments is proposed and compared to existing design approaches according to ACI 318-19, Eurocode 2 and the stable version of new Eurocode 2.

This paper presents the application of a low-cost thick-shell nonlinear finite element analysis (NLFEA) procedure to estimate the punching shear resisting performance of reinforced concrete slab-column connections under variable connection shear stress conditions. Variation of connection stress conditions stems from columns with different cross section aspect ratios, different distributions of gravity loading conditions, and slabs constructed with significantly different planar reinforcement conditions in the orthogonal directions. In this regard, thirty-five isolated slab-column connection specimens presented in the literature were analyzed using a shell finite element-based analysis procedure and the results from these analyses were used to assess NLFEA model performance. All results were developed using a predefined set of material models and analysis parameters, defined on the basis of prior and unrelated validation studies, and were shown to provide good agreement with experimental findings without the need for calibration studies or the adoption of case-specific failure criteria. From the findings obtained, it was determined that the thick-shell NLFEA employed is suitable for estimating the punching shear response for slabs subjected to varied and highly non-uniform shear stresses within the connection regions and provided similar levels of precisions as that previously obtained for isolated slab-column connections constructed with idealized geometries and reinforcing conditions, subjected to idealized loading conditions.

Columns supporting reinforced concrete two-way slabs often have non-circular or non-square cross-sections. The punching shear design of alternative column geometries is addressed in ACI 318-19, although the basis for these provisions is unclear as experimental tests of irregular column geometries are limited. In particular, the punching shear behaviour of special-shaped slab-column connections, such as L-shaped connections, has received limited interest. In this paper, nonlinear finite element analysis (FEA) is used to study the influence of column geometry, column location with respect to the slab centroid and the presence of slab openings on the punching shear behaviour of interior L-shaped slab-column connections subjected to gravity loading. The FEA suggests that the diagonal portion of the critical perimeter between the column flanges assumed in ACI 318-19 is ineffective in transferring load between the slab and the column. The FEA also suggests that ideally slab openings around interior L-shaped slab-column connections should be located between the two column flanges of each connection. Locating the openings in this area minimizes their negative impact on punching capacity and is beneficial from an architectural perspective, as the openings and services can be hidden from view. The punching capacities predicted by the FEA, the ACI 318-19 concentric punching shear provisions and the eccentric shear stress model outlined in ACI 421.1R-20 are also compared.