Recent research on ultrahigh-performance concrete (UHPC) for connection elements in highway bridges indicates an improvement in the performance of the overall bridge superstructure. However, there is limited information on the effect of the material and bond strength of the UHPC on the load transfer mechanism between adjacent box girders. A three-dimensional finite-element (FE) model was developed to study the interface between UHPC and high-strength concrete (HSC) highway bridge connections. The UHPC-HSC interface was modeled using traction-separation, damage initiation criteria, and damage evolution while taking into account adhesion, friction, and the nonlinear material behavior. Material properties were determined directly from laboratory testing, and the interface parameters were identified through model calibration using direct tension test results and friction coefficients reported in the previous work. The model was validated by simulating the laboratory tests conducted at the Federal Highway Administration (FHWA) Turner-Fairbank Highway Research Center (TFHRC), and good agreement between numerical and experimental results was obtained. Finally, numerical simulations of two adjacent box-girder models using the proposed interface parameters and other interface models from past research were compared, and the results highlight the importance of using a high-fidelity model to accurately represent the system behavior.

For decades, synthetic fibers have been utilized to improve the performance of concrete structures by enhancing the flexural strength, modulus of elasticity, and other mechanical properties of concrete. However, there are uncertainties about how synthetic fibers affect the coefficient of thermal expansion (CTE), dynamic modulus of elasticity, and flexural strength of the concrete under freeze-thaw cycles. This study was an investigation of the effect of temperature change on the mechanical properties of fiber-reinforced concrete, including CTE, dynamic modulus of elasticity, and flexural strength. Two types of synthetic fiber, polypropylene (PP) and polyvinyl alcohol (PVA) fibers, with three dosages for each type, 6, 7, and $9\mathrm{kg}/{\mathrm{m}}^3$ , were studied. The CTE was obtained using the severe temperature range specified by AASHTO specifications for measuring thermal effects in concrete bridge superstructures. Up to 300 freeze-thaw cycles were applied to specimens, while mass loss, dynamic modulus of elasticity, flexural strength, and CTE were measured in accordance with ASTM protocols. Adding fiber to concrete enhanced the flexural strength, increased the flexibility, decreased the dynamic modulus of elasticity, and increased the CTE. Specimens reinforced with PP fiber showed more flexural strength and flexibility than those reinforced with PVA fiber. Polyvinyl alcohol fiber–reinforced specimens exhibited greater CTE values compared with those reinforced with PP fiber. The added synthetic fiber enhanced the concrete’s resistance to freeze-thaw cycles. The optimum fiber dosage appears to be $7\mathrm{kg}/{\mathrm{m}}^3$ , which generally had the best performance in terms of flexural strength, dynamic modulus of elasticity, and CTE while best preserving structural integrity under repeated freezing and thawing.

Precast adjacent box girder bridges have become a preferred solution in several states due to their simple structure, economy, and ease of construction. However, longitudinal cracks often appear along the shear keys between adjacent box girders. With its superior strength and durability, ultrahigh-performance concrete (UHPC) has been used as an alternative grout material to reduce shear key cracking and enhance load transfer. The objective of the present research was to evaluate the performance of UHPC in shear keys using direct shear, direct tension, and flexural tests. Also, this study determined the effectiveness and functionality of shear reinforcement bars by applying direct shear load on specimens with shear reinforcement. In the direct shear test, a steel frame was placed around the specimens to allow the shear key to experience pure shear force. The direct shear test established the cracking threshold and ultimate shear strength of each shear key with and without shear reinforcement bars. The test results show that the average maximum shear capacity for the UHPC shear key without shear reinforcement bars exceeded that determined from past research for any grout material and any shear key configuration. Furthermore, the average maximum shear capacity increased by a factor of 1.81 with shear reinforcement bars. The enhanced shear strength of the UHPC shear key may have an impact on the analysis and design of UHPC connections in bridges.

Ultrahigh-performance concrete (UHPC), with its enhanced strength and durability, has been proposed as the joint material in adjacent box-beam bridges to address the issue of longitudinal cracking. Recently, the Federal Highway Administration (FHWA) performed laboratory testing of two adjacent box beams connected by a UHPC shear key containing transverse shear reinforcement bars (SRBs), and the results suggest that an improvement in the overall performance of adjacent box-beam bridges may be achieved by using the new shear key design. However, there is no information about the load transfer mechanism within the new connection. The aim of this research is to perform finite-element (FE) analysis of the two adjacent box-beam models to investigate the load transfer mechanism in, and ultimate load capacity of, the new FHWA shear key design. The three-dimensional FE model used in the study was validated based on the FHWA laboratory test results. The model is used to identify the roles of the UHPC material, the transverse SRBs, and the shear key surface roughness on the load transfer mechanism between adjacent box beams at different load levels, and to investigate the suitability of widely accepted criteria for shear key condition/performance for the new shear key design. The results of the analysis show that a shear key with a smooth surface and minimum transverse SRBs is sufficient for load transfer between adjacent box beams for the load level used in the FHWA testing and that a shear key with a sandblasted surface and minimum transverse SRBs is capable of transferring load up to failure of the box-beam system. The results of the analysis provide valuable useful insight into the shear key behavior, which can be used to optimize the design of future UHPC shear keys.

Synthetic fibers have recently been used in a concrete mix in an attempt to produce a new concrete pipe system that is cheaper, lighter, and more flexible than conventional steel reinforced concrete pipes. However, no structural design codes have been introduced for synthetic fiber reinforced concrete pipes evaluation. There is little in the literature regarding synthetic fiber applications in concrete pipes. This study focuses on the evaluation of the synthetic fiber reinforced concrete pipes in terms of ASTM requirements for strength, stiffness, and ductility. The performance of the synthetic fiber reinforced concrete pipes was evaluated using a three-edge bearing test in accordance with ASTM protocols and using three pipe diameters: 600, 1,200, and 1,500 mm. Fiber dosages ranged from 4.75 to 18 kg/m³, and different areas of one steel cage layer were used to reinforce the concrete pipes. The results show that using synthetic fiber increased the cracking load (produces 0.3-mm crack width), ultimate load, stiffness, and ductility of tested pipes. Also, using synthetic fiber lowered the production cost, as the reduction in the steel cage area ranged from 51 to 100 %.