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Sponsors: Sponsored by ACI 370 Committee Editors: Eric Jacques and Mi G. Chorzepa This Symposium Volume reports on the latest developments in the field of high strain rate mechanics and behavior of concrete subject to impact loads. This effort supports the mission of ACI Committee 370 “Blast and Impact Load Effects” to develop and disseminate information on the design of concrete structures subjected to impact, as well as blast and other short-duration dynamic loads. Concrete structures can potentially be exposed to accidental and malicious impact loads during their lifetimes, including those caused by ballistic projectiles, vehicular collision, impact of debris set in motion after an explosion, falling objects during construction and floating objects during tsunamis and storm surges. Assessing the performance of concrete structures to implement cost-effective and structurally-efficient protective measures against these extreme impacting loads necessitates a fundamental understanding of the high strain rate behavior of the constituent materials and of the characteristics of the local response modes activated during the event. This volume presents fourteen papers which provide the reader with deep insight into the state-of-the-art experimental research and cutting-edge computational approaches for concrete materials and structures subject to impact loading. Invited contributions were received from international experts from Australia, Canada, China, Czech Republic, Germany, South Korea, Switzerland, and the United States. The technical papers cover a range of cementitious materials, including high strength and ultra-high strength materials, reactive powder concrete, fiber-reinforced concrete, and externally bonded cementitious layers and other coatings. The papers were to be presented during two technical sessions scheduled for the ACI Spring 2020 Convention in Rosemont, Illinois, but the worldwide COVID-19 pandemic disrupted those plans. The editors thank the authors for their outstanding efforts to showcase their most current research work with the concrete community, and for their assistance, cooperation, and valuable contributions throughout the entire publication process. The editors also thank the members of ACI Committee 370, the reviewers, and the ACI staff for their generous support and encouragement throughout the preparation of this volume.

Current computational modeling approaches used to evaluate the impact-resisting performance of reinforced concrete infrastructure generally consist of high-fidelity modeling techniques which are expensive in terms of both model preparation and computation cost; thus, their application to real-word structural engineering problems remains limited. Further, modeling shear, erosion, and perforation effects presents as a significant challenge, even when using expensive high-fidelity computational techniques. To address these challenges, a simplified nonlinear modeling methodology has been developed. This paper focuses on this simplified methodology which employs a smeared-crack continuum material model based on the constitutive formulations of the Disturbed Stress Field Model. The smeared-crack model has the benefit of simplifying the modeling process and reducing the computational cost. The total-load, secant-stiffness formulation provides well-converging and numerically stable solutions even in the heavily damaged stages of the responses. The methodology uses an explicit time-step integration method and incorporates the effects of high strain rates in the behavioral modeling of the constituent materials. Structural damping is primarily incorporated by way of nonlinear concrete and reinforcement hysteresis models and significant secondorder mechanisms are considered. The objective of this paper is to present a consistent reinforced concrete modeling methodology within the context of four structural modeling procedures employing different element types (e.g., 2D frames, 3D thick-shells, 3D solids, and 2D axisymmetric elements). The theoretical approach common to all procedures and unique aspects and capabilities of each procedure are discussed. The application and verification of each procedure for modeling different types of large-scale specimens, subjected to multiple impacts with contact velocities ranging from 8 m/s (26.2 ft/s) to 144 m/s (472 ft/s), and impacting masses ranging from 35 kg (77.2 lb) to 600 kg (1323 lb), are presented to examine their accuracy, reliability, and practicality.

This study experimentally and numerically investigated the impact responses of reinforced concrete (RC) beams with a rectangular hollow section (HCB) in comparison with a rectangular solid section (SCB). Experimental tests of the two types of RC beams were firstly conducted under the drop-weight impact of a 203.5-kg-solid-steel projectile. Numerical models of the beams under impact loads were then developed in the commercial software namely LS-DYNA and carefully verified against the experimental results. The numerical models were then used to investigate the stress wave propagation in the two beams. The effect of the top flange depth, contact area, and impact velocity on the impact responses of the beams was also investigated. The experimental and numerical results in this study showed that although the two beams were designed with similar reinforcement ratio, their impact responses were considerably different, especially when the shear failure dominated the structural response. The HCB exhibited a smaller peak impact force but higher lateral displacement than the SCB when these beams were subjected to the same impact condition. Besides, more shear cracks were observed on the HCB while that of SCB has more flexural cracks. Furthermore, the decrease of the top flange depth of the hollow section and the increase of the impact velocity changed the failure modes of the two beams from flexural failure to shear failure with concrete scabbing. The change of the contact area also shifted the failure mode of the beam from global response to direct shear, inclined shear, punching shear and concrete scabbing at the top flange of the section close to the impact location.

The impact resistance of concrete is becoming an increasingly important component of insuring the durability and resilience of critical civil engineering infrastructure. Design engineers are not currently able to use impact resistance as a performance-based specification in concrete due to a lack of a reliable standardized impact test for concrete. An improved method of the ACI standard, ACI 544.2R-89 Measurement of Properties of Fiber Reinforced Concrete, is developed that provides a resistance curve as a function of impact energy and number of blows (N) to failure. The curve provides information about the life cycle (N) under repeated sub-critical impact events and an estimate of the critical impact energy (where N=1), whereas the previous method provided only a relative value. The generated impact-fatigue curve provides useful information about damage accumulation under repeated impact events and the effectiveness of the fiber-reinforcement. In this paper, the improved method is demonstrated for three fiber types: steel, copolymer polypropylene, and a monofilament polypropylene. Additionally, the analytical solution for the specimen geometry is given as well as the theoretical considerations behind the development of the impact-life curve. The use of a specimen geometry provides a path to generalize the test results to full-scale structures.

Explosively formed projectiles (EFP) are one of the most severe explosive and impact loading threats for civil infrastructure and military vehicles. EFP warheads are commonly found in conventional anti-tank weapons. They are also regularly used by insurgent forces against armoured vehicles in conflict-affected countries. The energy of EFPs is significantly greater than that of large calibre ammunition, such that a threat is posed to the occupants of armoured vehicles both by perforation and spalling of the armour. This paper aims to present new experimental results of the hypervelocity impact of EFPs on reinforced concrete (RC) columns to demonstrate the vulnerability of infrastructure to EFP improvised explosive devices (EFP-IEDs). As a possible mitigation measure of threat against EFPs, an RC column was retrofitted with a steel-jacket. The ability of a steel-jacket to minimise RC column damage was evaluated where it was found to minimise damage to the RC column and contain concrete fragments. Threedimensional numerical simulations were performed to elucidate the different stages of EFP interaction with the RC columns. No previously published results on the EFP terminal ballistic performance of RC columns have been found in the open literature.