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Ultra‐high performance concrete (UHPC) offers superior tensile properties in comparison to normal and high‐strength concrete. This is known to largely depend on the fibers included in the UHPC mixtures. Among the available types of fiber, steel fibers have been commonly used in the current practice. However, given the price and availability issues associated with steel fibers, there has been growing interest in benefiting from synthetic fibers in UHPC. This was the motivation for the current study to investigate the potential of five synthetic fibers by evaluating how the tensile properties of the UHPC mixtures made with a hybrid of steel and synthetic fibers are influenced under direct tension. The synthetic fibers of choice consisted of nylon, polypropylene, polyvinyl alcohol, alkali‐resistant glass, and carbon fibers. By using a dog bone test setup, the main tensile properties of the developed mixtures (i.e., direct tensile strength, elastic modulus, and stress–strain relationship) were systematically determined. The results extracted from direct tension tests were then further processed in conjunction with digital image correlation analyses to investigate the formation and propagation of cracks, as well as the post‐cracking response. The original findings reported from this study are expected to pave the way to broaden the fiber types that can be considered for the structural applications of UHPC, especially where high tensile stresses/strains are anticipated. [ABSTRACT FROM AUTHOR]

Ultra‐high performance concrete (UHPC) is an emerging class of cementitious materials with the strength and durability characteristics that far exceed those of normal‐strength concrete (NSC). While the use of UHPC in bridge structures has significantly advanced, the same cannot be stated for building structures. Among possible building applications, this study focuses on the use of UHPC in shear walls, where a significant loading demand is anticipated from the lateral loads due to wind and/or earthquake events. Upon validating a set of finite‐element models with the experimental test results, a holistic investigation has been performed in the current study to evaluate a variety of UHPC shear walls. In particular, the main structural response measures, including initial stiffness, force‐displacement relationship, and strength loss under cyclic lateral loads, have been extracted and compared. The study has been then extended to systematically investigate the effects of key design variables, such as wall thickness, aspect ratio, axial force, steel reinforcement, presence of boundary elements, and strength of embedded steel bars. The outcome of this study provides the building sector with firsthand information regarding the possibility of replacing NSC with UHPC in shear walls. This will be an important step forward to improve the safety and performance of building structures, especially where they are exposed to a high risk of wind and/or earthquake hazards. [ABSTRACT FROM AUTHOR]

Ultra‐high performance concrete (UHPC) is increasingly used for various engineering applications, owing to its superior strength properties. Such properties, however, can be accompanied by workability issues, especially if the dosage of steel fibers is significantly increased. As a potential solution, this study explored the use of a combination of steel microfibers and macrofibers through a holistic experimental testing program. This was to understand the effects of steel fiber combinations on the workability and flexural strength of various UHPC mixtures. The microfibers were investigated for five different dosages from 1.0% to 3.0% (with an increment of 0.5%). In parallel, two mixtures were prepared with a 2.0% dosage of macrofibers in the form of twisted wire and hooked fibers. At the next stage, six additional mixtures were prepared with a combination of microfibers and macrofibers of various dosages. Further to the measurements extracted from the workability and flexural strength tests, digital image correlation (DIC) analyses were employed to obtain firsthand information about the widths and depths of cracks observed in the flexural specimens tested under an increasing load. The outcome of this study showed how a combination of microfibers and macrofibers can contribute to improving the flexural response characteristics of UHPC, especially without having to increase the dosage of steel fibers to the ranges that can adversely affect the mixture's workability. [ABSTRACT FROM AUTHOR]

• Natural zeolite was studied for both internal curing and pozzolanic reactivity. • Synergistic effects of natural zeolite and silica fume were systematically identified. • Replacement of silica fume with natural zeolite improved cement hydration kinetics. • Optimal replacement dosages were determined to achieve maximum cement hydration. • Phase analyses explained in detail how the binder components change over time. Ultra-high performance concrete (UHPC) is often designed with a low water-to-binder ratio, which can potentially limit the hydration of cement particles. To address the hydration-related issues, the current study explored the use of natural zeolite, as it offers a superior porous structure for internal curing, further to pozzolanic advantages. For this purpose, the silica fume commonly used in UHPC was replaced with natural zeolite at various levels, while the cement content was maintained unchanged. The water desorption capability of zeolite in UHPC was investigated by measuring the free water content of the developed mixtures over time, supported by complementary autogenous shrinkage and SEM image analyses. Furthermore, the heat of hydration was evaluated through monitoring the temperature profile in the developed UHPC mixtures. This was paired with TGA and XRD analyses to obtain an in-depth understanding of how the hydration process of UHPC is influenced. The outcome of this holistic investigation indicated that the inclusion of natural zeolite is effective in enhancing the degree of hydration of cement particles. Additionally, the obtained results showed that if silica fume and zeolite are incorporated into the UHPC mixtures together (with optimum dosages), they further improve the entire hydration process, owing to the silica fume's high pozzolanic reactivity and zeolite's internal curing capability. [ABSTRACT FROM AUTHOR]

• Six high-performance cementitious materials were studied for repair applications. • A holistic set of compressive, splitting tensile, and slant shear tests were performed. • The bond characteristics were assessed individually and in comparison to each other. • The adequacy of bond was evaluated with respect to code-specified requirements. • Adhesion and friction coefficients were determined for future design applications. Repair of damaged concrete structures with patching materials is required for a wide variety of bridge and building applications. Despite the capabilities of conventional patching materials in replacing the spalled concrete, they are often unable to restore the original structural capacity. This has motivated research investigations to develop high-performance patching materials that can help damaged concrete structures regain their strength and durability characteristics. Such characteristics, however, are not properly achieved without a satisfactory bond between the concrete substrate and the patching material applied. To investigate this critical aspect, six high-performance patching materials, i.e., proprietary and nonproprietary ultra-high performance concrete, polypropylene and alkali-resistant glass fiber-reinforced concrete, high-early strength concrete, and shrinkage-compensating cement concrete, were systematically investigated and compared in the current study. The conducted experiments included a holistic set of splitting tensile and slant shear tests performed on the samples prepared with various surface roughnesses. The experimental test results provided firsthand information about the bond characteristics between normal-strength concrete and six high-performance patching materials. With the determination of corresponding adhesion and friction coefficient factors, the outcome of this study facilitates the selection and use of high-performance patching materials, depending on the availability of materials and repair needs. [ABSTRACT FROM AUTHOR]

• Material models for ultra-high performance fiber-reinforced concrete were studied. • The models were first examined under various stress paths at the material level. • Post-peak softening, shear dilation, and confinement effects were investigated. • Calibrated models were evaluated for simulating full-scale structures under impact. • A metamodel-based sensitivity analysis was conducted on material model parameters. Among various alternatives developed for conventional reinforced concrete, ultra-high performance fiber-reinforced concrete (UHPFRC) is proven to provide superior mechanical properties, making it appropriate for impact-resistant structures. While several experimental tests have been conducted to determine the static and dynamic response of UHPFRC, the simulation of this important category of concrete materials, especially under impact loads, is still in need of extensive effort. To address this issue, the current study investigates how two widely-used concrete constitutive models, i.e., Continuous Surface Cap Model (CSCM) and Karagozian and Case Concrete (KCC), can be reliably employed for modeling UHPFRC subjected to low-velocity impact. Utilizing the available experimental test data, a rigorous calibration process has been developed in the current study for the two constitutive models, capturing the main strength parameters, damage evolution parameters, and strain rate effects. This process begins with single-element simulations performed under various stress paths to generate the information necessary for post-peak softening and confinement factors. The investigations with single-element simulations consist of four element sizes to study the mesh size sensitivity of both constitutive models. The study is then extended to examine the capabilities of the calibrated models in simulating the response of full-scale structural elements made with UHPFRC. For this purpose, the drop hammer tests are replicated on both plain and reinforced UHPFRC, and the performance of the two constitutive models is evaluated in comparison to the experimental tests. Specifically, this evaluation examines peak impact forces and displacements, considering various hourglass coefficients and drop hammer heights. Furthermore, a metamodel-based sensitivity analysis is conducted to quantify the effects of uncertainty inherent in the input parameters on the predicted impact response measures, in terms of force, displacement, and duration. [ABSTRACT FROM AUTHOR]

• Non-proprietary UHPC mixtures were investigated under exposure to chloride ions, moisture loss, and freeze-thaw cycles. • Various performance measures were obtained and compared for non-proprietary and proprietary UHPC mixtures. • Effects of water-to-cement ratio, sand-to-cement ratio, and silica fume content were explored for an optimized mixture. • Feasibility of development of cost-effective, non-proprietary UHPC mixtures with desired properties was outlined. This study focuses on the assessment of non-proprietary ultra-high performance concrete (UHPC) mixtures exposed to the penetration of chloride ions, shrinkage due to moisture loss, and freeze and thaw cycles. For achieving a proper assessment, several non-proprietary UHPC mixtures were designed, cast, and tested for flow, compressive strength, tensile strength, surface resistivity, rapid chloride penetration, rapid chloride migration, autogenous shrinkage, drying shrinkage, and loss of mass and dynamic modulus due to freeze and thaw cycles. The performance measures of the developed mixtures were then compared with those of two proprietary UHPC mixtures. The results obtained from this rigorous set of experimental tests showed how non-proprietary UHPC mixtures can be designed to experience chloride penetration as low as their proprietary counterparts, maintain high volume stability under both autogenous and drying shrinkage, and benefit from negligible degradation under freeze and thaw cycles. This study also quantified how the water-to-cement ratio, sand-to-cement ratio, and inclusion of silica fume can have substantial effects on the properties of non-proprietary UHPC mixtures. Considering the significant cost advantages of non-proprietary UHPC mixtures, the outcome of this study shed light on the feasibility of developing non-proprietary UHPC mixtures with desired properties, taking into account short- and long-term performance considerations. [ABSTRACT FROM AUTHOR]