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In this study, ultrahigh-performance concrete (UHPC) was utilized in precast segmental beams to reduce the self-weight, shorten the construction time, and improve the performance and durability of bridges. Owing to the discontinuity in the joints, shear behavior plays a critical role in the overall structural performance of precast UHPC segmental beams (PUSBs). Therefore, four dry-jointed segmental specimens along with one monolithic specimen were designed and tested under a two-point concentrated load with various joint types, shear span-to-depth ratios (λ), and numbers of shear keys. Two types of shear failure modes were observed in the tests: shear compression failure of the web (λ = 1.44 and 2.56) and local shear failure of the flanges at the joint (λ = 3.67). The shear capacity, stiffness, and cracking load of the dry-jointed segmental specimens were lower than those of the monolithic specimen, and the single-keyed specimen exhibited better shear behavior than the three-keyed specimen. Increasing λ decreased the shear strength and stiffness of the segmental beams and considerably affected their failure modes and crack distributions. Additionally, four UHPC design codes were evaluated for their accuracy in estimating the shear strength of the specimens, and a simplified strut-and-tie model was developed to predict the shear strength of externally pre-stressed PUSBs. Finally, several design recommendations were proposed. This study is expected to facilitate the research and application of PUSBs. [ABSTRACT FROM AUTHOR]

Precast Ultra-High-Performance Concrete (UHPC) segmental bridges (PUSBs) are among the most promising structural systems owing to their potential for application in advanced philosophies, such as green and intelligent construction technology, industrialization, prefabrication, and automation. The joints, where precast segments are connected, play a significant role in the construction and structural performance of PUSBs. This study reviews the state-of-the-art knowledge on the shear behavior of UHPC joints, which includes the following aspects: (1) the introduction of joints in PUSBs, including their role, types, experimental methods, and typical shear responses; (2) the development of the experimental database of the push-off tests conducted on UHPC joints; (3) a discussion of the influence of critical factors (e.g., UHPC strength, joint type, confining stress, fiber characteristics, and shear key design) on the shear behavior of UHPC joints; (4) a summary and evaluation of the current analytical models and formulae for predicting the shear resistance of UHPC joints; and (5) the identification of current challenges and recommendations of future research directions for UHPC joints. This review is expected to comprehensively understand the shear performance of UHPC joints and facilitate the research and application of UHPC structures. [Display omitted] • A state-of-the-art review is conducted on the shear performance of joints in precast UHPC members. • The critical factors affecting the shear behavior of UHPC joints are discussed. • Analytical models for predicting the shear strength of UHPC joints are summarized and compared. • Existing formulae for the shear strength of UHPC joints are evaluated based on the experimental database. [ABSTRACT FROM AUTHOR]

• An updated database consisting of 532 UHPC beams that failed by shear is established. • Ten ML models with different algorithms are developed to predict the shear strength of UHPC beams. • The performance of the ML models is evaluated and compared to empirical models. • The ML models are interpreted using the SHAP methods. • The impact of critical features on the shear strength of UHPC beams is identified. To provide more accurate and reliable predictions of the shear strength of ultrahigh-performance concrete (UHPC) beams, in this study, the machine learning (ML) approaches were employed to develop the data-driven models, and the ML models were interpreted using the Shapley additive explanations (SHAP) method. It was found that the ensemble models, particularly CatBoost, outperform individual ML models and traditional empirical models. The geometric dimensions and shear span-to-depth ratio were the most influential features for predicting the shear strength of UHPC beams, followed by the parameters of reinforcement and material properties of the UHPC. [ABSTRACT FROM AUTHOR]

• The shear behavior of externally prestressed UHPC beams without stirrups is experimentally investigated. • Effects of the a / d ratio, stirrups, and longitudinal reinforcement ratio are discussed. • An equation for predicting the shear-cracking strength of prestressed UHPC beams is proposed. • Effects of key parameters on the shear strength of prestressed UHPC beams without stirrups are analyzed. • Prediction equations of shear strength of prestressed UHPC beams without stirrups are proposed. Externally prestressed ultra-high-performance concrete (UHPC) beams without stirrups (EPUBs-WS) that simplify the reinforcement design and construction procedure are becoming a competitive option in bridge engineering. To investigate the shear behavior of EPUBs-WS, five specimens were designed and tested considering several critical parameters, such as the shear span-to-depth ratio (λ), shear stirrups, and longitudinal reinforcement ratio. The EPUBs-WS exhibits diagonal tension failure as the fibers pull out and the specimen shears off into two parts along the critical diagonal crack. However, the beam with λ = 3.67 exhibited flexural failure due to the large λ. The stiffness, shear-cracking strength, and shear strength of EPUBs-WS decrease as λ increases. Stirrups enhance the shear resistance of EPUBs but result in a more brittle shear failure mode. A higher reinforcement ratio augments the dowel action in EPUBs-WS, which contributes to a ductile shear failure mechanism. The web-shear cracking force accounts for 46% to 87% of the shear capacity, implying that the shear-cracking strength is a significant factor in the design of EPUBs-WS. An equation for predicting the shear-cracking strength of prestressed UHPC beams is proposed and validated using a database of experimental results reported in the literature. A parametric analysis is performed using 40 available specimens to investigate the effects of compressive strength, λ , reinforcement ratio, fiber reinforcing index, and prestressing level on the shear strength of prestressed UHPC beams without stirrups. By considering the key parameters, equations for estimating the shear strength are proposed and exhibit good accuracy. [ABSTRACT FROM AUTHOR]

• The mechanism of precast segmental UHPC beams (PUSBs) with epoxy joints subject to combined bending and shear was investigated. • The PUSBs with epoxy joints exhibit comparable structural performance with the monolithic beam. • The effect of the shear span-to-depth ratio, stirrups, and joints on the mechanical behavior of PUSBs were discussed. • Analytical models for predicting the cracking strength and ultimate capacity of PUSBs were proposed. Precast ultra-high-performance concrete (UHPC) segmental bridges (PUSBs) have become a competitive bridge type in modern civil engineering. To establish a thorough mechanistic understanding of PUSBs, seven specimens were fabricated and experimentally investigated. These specimens were designed to have different longitudinal reinforcements, construction methods, joint types, shear span-to-depth ratios (λ), stirrup ratios, and numbers of shear keys. Two failure modes were observed in the tests: flexural failure with crushing of the UHPC at the joints, and shear failure with shear tension sliding and concrete spalling of the web. Additionally, it was found that although the ultimate capacity, stiffness, and cracking load of the segmental beams were slightly lower than those of the monolithic beams, the segmental beams tended to exhibit better ductility and deformation capacity. As λ increased, the ultimate capacity and stiffness of the segmental beams decreased, whereas the growth rates of the deflection and tendon stress increased. Moreover, the stirrups showed a positive effect on the cracking control ability and shear strength of the beams. The three-keyed specimen exhibited better structural behavior than the single-keyed specimen. Finally, calculation methods were proposed to predict the cracking strength and ultimate capacity of PUSBs. [ABSTRACT FROM AUTHOR]

• Shear behavior of UHPC joints was experimentally examined. • The failure modes, crack patterns, and load–displacement responses were obtained. • The effect of test parameters on the shear behavior of UHPC joints was discussed. • The calculation method for the shear strength of UHPC joints was developed. • Four calculation models for the shear strength of UHPC joints were proposed. The joints in precast ultra-high-performance concrete (UHPC) segmental bridges (PUSBs) play significant roles in the structural performance of PUSBs, which require systematic investigation to establish a thorough understanding of their shear behavior. To this end, the push-off tests for 11 UHPC joints, including 9 epoxy joints and 2 dry joints, were carried out to obtain the failure modes, crack distribution, and the load-relative vertical displacement relationships. The confining stress, number of shear keys, and joint types were the critical test parameters considered in this study. Experimental results show that the failure mode of flat epoxy joints was the spalling of the epoxy, and no visible damage was observed on the surface of joints. For the keyed epoxy joints, the brittle shear-off of shear keys led to the failure of joints. However, dry joint specimens showed severe damage and spalling of UHPC. The shear capacity of UHPC joints effectively improved as the confining stress and number of keys increased, and the epoxy joints exhibited a higher shear capacity than the dry joints. The shear strength formulas of UHPC joints were developed and the calculated results from the proposed formulas agreed well with the test results with an average ratio of 1.05 and a coefficient of variation of 0.079. Moreover, four calculation models for the UHPC joint shear strength were proposed based on the forms of existing shear strength formulas of UHPC joints and 61 push-off test results available in the previous studies were used to evaluate the calculation models. [ABSTRACT FROM AUTHOR]