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Ultra-high performance concrete (UHPC) is a state-of-the-art cementitious composite. Since the concept of this novel concrete mixture emerged in the 1990s, significant advancements have been made with numerous benefits such as high strength, flowability, high post-cracking tensile resistance, improved durability, reduced maintenance, and extended longevity. Currently, UHPC is employed around the globe alongside recently published practice guidelines. Although numerous research projects were undertaken to examine the behavior of UHPC-incorporated structures, there still are many gaps to be explored. Of interest are the development of robust and reliable mixtures and their application to primary load-bearing members for bridges and buildings, including various site demonstration projects that would promote the use of this leading-edge construction material. This Special Publication (SP) contains nine papers selected from three technical sessions held in the ACI Spring Convention in March 2022. All manuscripts were reviewed by at least two experts in accordance with the ACI publication policy. The Editors wish to thank all contributing authors and anonymous reviewers for their rigorous efforts. The Editors also gratefully acknowledge Ms. Barbara Coleman at ACI for her knowledgeable guidance. Yail J. Kim, Steven Nolan, and Antonio Nanni Editors University of Colorado Denver Florida Department of Transportation University of Miami

Through a Change Proposal by Facca Incorporated, the Ontario Ministry of Transportation (MTO) approved the replacement of the as-tendered steel H-piles by newly designed prestressed/precast Ultra-High-Performance Concrete (UHPC) piles for supporting the west abutment of the Lily River Detour Bridge. The 300 mm (~12”) deep UHPC piles were designed and installed at the west abutment based on the previous successful development and testing of a tapered H-shaped pile at Iowa State University. The east abutment, as tendered, was designed to be supported by six steel H-shaped battered piles driven to bedrock. For the west abutment, six UHPC piles were produced and installed using the same batter. Since the site contained occasional boulders and the design intent to drive the piles to bedrock, the UHPC piles were fitted with steel shoes for the first time. All piles were successfully installed to reach the targeted load bearing capacities. After the replacement bridge was constructed, the detour bridge was removed and the UHPC piles were extracted to examine the conditions of the piles. This presentation will provide details of the innovative design of the piles, fabrication and driving of the piles, and lessons learned from analyzing the driving data and removal of the piles. Fellowship and Scholarship recipients. With the help of generous donors from the concrete community, the ACI Foundation awards high-potential undergraduate and graduate students in engineering, construction management, and other appropriate curricula.

While reinforced concrete is one of the most used construction materials, traditional reinforcement steel may cause undesirable side effects, as corrosion and the associated volume changes can lead to damages in the concrete matrix and can cause spalling, which may significantly reduce the load-bearing capacity and service life of structures. Alternative reinforcement methods, such as glass or basalt fiber reinforced polymer rebars, can serve as a viable alter-native to reduce or eliminate some of the disadvantages associated with steel reinforcement. In addition to an increased tensile strength and a reduction in weight, fiber reinforced polymer rebars also offer a high corrosion resistance among other beneficial properties. Because these materials are not fully regulated yet and the durability properties have not been conclusively determined, further research is needed to evaluate the material durability properties of FRP rebars. To determine the durability properties of GFRP and BFRP rebars in cold climates, the freeze-thaw resistance of these materials was evaluated throughout this study. Specifically, two types of materials (basalt and glass reinforced polymers) and two common rebar sizes (8 mm (#2) and 16 mm (#5) diameters) were tested. To quantify the freeze-thaw-durability, tensile tests according to ASTM D7205, transverse shear strength tests in line with ASTM D7617, and horizontal shear strength tests as specified in ASTM D4475 were conducted on numerous virgin fiber rebars and on fiber rebars that were subjected to 80 and 160 freeze-thaw cycles. While the results from the virgin materials served as benchmark values, the measurements and analysis from the aged (by freeze-thaw cycles) materials were used to quantify and determine the strength retention capacity of these bars. The results showed that a higher number of freeze-thaw cycles lead to lower strength retention for some rebar types. In addition, it was seen that rebar products respond differently to the aging process; while some material properties notably deteriorated, other material properties were insignificantly affected.

The equivalent rectangular stress block is the basis for determining the flexural strength of reinforced concrete members. Instructors commonly present the concept in two dimensions, contrary to its three-dimensional nature. Unfortunately, this can be particularly difficult for students to understand and visualize, especially for students with visual learning style preferences. This paper presents an overview of the equivalent rectangular stress block, select active learning methods, and four specific examples used by the authors to introduce the equivalent rectangular stress block in an undergraduate reinforced concrete design course. The first method focuses on understanding the terms associated with the equivalent rectangular stress block using a visual, hands-on learning activity; the second focuses on visualizing the internal forces and couple moment within a beam; the third uses a virtual three-dimensional model to derive the depth of the equivalent rectangular stress block; and the fourth illustrates the various stress blocks used in reinforced concrete flexure theory via physical three-dimensional models. Each description includes detailed instructions to create the resources and how to facilitate the related activities within a course. The overall goal is to provide educators with several examples that will help students better visualize the three-dimensional concept.

With the aging and deterioration of infrastructure, the need for repair, strengthening, and rehabilitation of existing structures continues to increase. Climate change makes extending the service life of our infrastructure critical since any demolition and new construction will trigger substantial amounts of carbon emissions. Research related to repairing and strengthening existing infrastructure is seeing major developments as new green materials and technologies become available. Improved assessment and retrofit of deficient structures, and performance-based design of new structures are also in high demand. Despite the progress, there are many challenges yet to be addressed. The main objective of this Special Publication is to present results from recent research studies (experimental/numerical/analytical) on the retrofit and repair of structural elements along with the assessment, analysis, and design of structures. Several of these papers were presented at the ACI Fall Convention “Seismic Repair/Retrofit/Strengthening of Bridges at the Element or System Level: Parts 1 and 2.” The presented studies cover various aspects of structural retrofitting and strengthening techniques including the use of rubberized engineered cementitious composite for enhancing the properties of lightweight concrete elements, high-performance concrete jacketing to strengthen reinforced concrete piers/columns, and the behavior of fiber-reinforced-polymer-wrapped concrete cylinders under different environmental conditions. Additionally, the research explores the behavior of concrete-filled FRP tubes under axial compression, innovative bridge retrofit technologies, and retrofit techniques for deficient reinforced concrete columns. There is also a focus on evaluating the seismic response of retrofitted structures, designing guidelines for seismic retrofitting using tension-hardening fiber-reinforced concrete, strengthening unreinforced masonry walls with ferrocement overlays, and developing seismically resilient concrete piers reinforced with titanium alloy bars. The seismic response of a retrofitted curved bridge was also presented where elastomeric bearings of the as-built bridge were replaced by high damping rubber bearings as a part of the seismic retrofit. Recommendations for nonlinear finite element analysis of reinforced concrete columns under seismic loading are also presented to simulate their behavior up to collapse. Overall, the presented studies in this Special Publication demonstrate the potential of new materials, methods, and technologies to improve the performance of various structural elements under different loading conditions, including seismic and environmental loads. These studies are expected to help our practitioners and researchers not only develop more effective and sustainable methods for repairing and strengthening of structures but also improve their analysis and design skills.