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

A possible way to reduce CO₂ emissions linked to cementitious materials is to use alternative resources, particularly co-products from other industries. Oyster shell co-products are a calcareous resource produced by aquaculture currently available in coastal areas and must be valorized. The present study investigates the impact of crushed oyster shells used as aggregates in concrete on its mechanical behavior. Thus, concrete samples with 50% aggregates replaced by crushed oyster shells were formulated. Two different types of cement were used: CEMI for reference and low-carbon cement CEMIII-C. Mechanical strength and Young’s modulus were assessed at 28 days, and cracking under compression was followed by acoustic emission technique. Results show that oyster shell aggregates slightly reduce concrete's mechanical resistance but significantly decrease its Young’s modulus. However, cracking behavior under compression remains similar during compression loading.

Serviceability checks in Reinforced Concrete (RC) elements involves the verification of crack width mainly aimed to limit the exposure of the steel reinforcement to corrosion and chemical attack and, thus, improve durability. Classical approaches for assessing the crack width in RC elements provide the calculation of two terms: 1) the average crack spacing, and 2) the average difference between the strain in the steel reinforcement and in the concrete in tension referred to the average crack spacing. A similar approach can be assumed valid also for RC elements strengthened with externally bonded Fiber Reinforced Polymer (FRP) materials, taking into account the additional tension stiffening effect provided by the external reinforcement.

This paper presents the comparisons of some existing code formulations for predicting crack spacing and crack width in RC elements with the experimental results of a database collected by the Authors and concerning tests on RC beams and ties externally bonded with different types and configurations of FRP materials. The paper is mainly aimed to check the reliability of the existing equations provided by codes in order to address the future assessment of reliable design provisions for cracking verifications in RC elements strengthened with FRP materials. The comparisons have evidenced, indeed, some useful issues for the design provisions: 1) larger scatter in the predictions of crack width than in crack spacing and, in particular, for ties, 2) limited effect of shrinkage on crack width, 3) necessity of taking into account the external reinforcement in crack spacing formulations, 4) good reliability of mechanical models for calculating cracks width.

The use of FRP tendons has become an attractive alternative to steel tendons in prestressed concrete structures to avoid strength and serviceability problems related to corrosion of steel. There is however a lack of knowledge in serviceability behavior related to deflection after cracking for beams prestressed with FRP tendons. Conventional approaches used to compute deflection of cracked members prestressed with steel is problematic at best, and the situation is exacerbated further with the use of FRP tendons having a lower modulus of elasticity than steel. Deflection of FRP reinforced (nonprestressed) concrete flexural members computed with Branson’s effective moment of inertia 𝐼􀀁 requires a correction factor (called a softening factor) that reduces the member stiffness sufficiently to provide reasonable estimates of post-cracking deflection. For FRP prestressed concrete however, this approach does not always work as expected and deflection can be either underestimated or overestimated significantly.

This study investigates the accuracy of different models proposed for estimating deflection of cracked FRP prestressed members using a database of 38 beams collected from the literature. All beams are fully prestressed. Results indicate that using Branson’s effective moment of inertia 𝐼􀀁 with a generic softening factor can produce reasonable estimates of deflection provided the 𝐼􀀁 response is shifted up to the decompression moment or adjusted with an effective prestress moment defined by an effective eccentricity of the prestress force. The former approach overpredicts deflection by 20% on average while the latter overpredicts deflection by not more than 5% based on the beams available for comparison. Assuming a bilinear moment deflection response overpredicts deflection by 12%, while an approach proposed by Bischoff (which also shifts the 𝐼􀀁 response upwards) overpredicts deflection by 23%. These last two approaches work reasonably well without the need for a correction factor.

This paper presents the crack monitoring of reinforced concrete beams strengthened with fiber reinforced polymer (FRP) sheets. Emphasis is placed on the development of a smart FRP bonded material that can measure the crack opening of a reinforced concrete beam strengthened by FRP. The reliability measured by a conventional digital image correlation (DIC) and by the proposed smart FRP is employed to assess the contribution of the FRP to control the crack. The monitoring process is based on a large set of experimental database consisting of 19 test beams. The effect of FRP to control the crack opening is studied depending on the steel ratio, FRP ratio and the level of damaged of RC beams when FRP is applied. The results were compared with the theoretical values of crack width and spacing predicted using the Eurocode 2 (EC2) formula, calibrated for non-strengthened RC elements. The corresponding results were compared in order to clarify the effect of external bonded FRP on the cracking behaviour of RC beams.