Ultra-high-performance concrete (UHPC) can be vulnerable to variations in materials properties and environmental conditions. In this paper, the sensitivity of UHPC to changes in mixing, casting, curing, and testing temperatures ranging between 10 and 30 ± 2°C (50 and 86 ± 3.5°F) was investigated. The investigated rheological properties, mechanical properties, and shrinkage of UHPC are shown to be significantly affected by temperature changes. UHPC made with either binary or ternary binder containing fly ash (FA) or slag cement exhibited greater robustness than mixtures prepared with 25% silica fume. UHPC made with 60% FA necessitated the lowest high-range water-reducing admixture demand. With temperature increase, the yield stress of UHPC mixtures increased by up to 55%, and plastic viscosity decreased by up to 45%. This resulted in accelerating initial and final setting times by up to 4.5 and 5 hours, respectively. The increase of temperature from 10 to 30 ± 2°C (50 ± to 86 ± 3.5°F) led to a 10 to 75% increase in compressive, splitting tensile, and flexural strengths and modulus of elasticity and 15 to 60% increase in autogenous shrinkage.

Jointed plain concrete pavement (JPCP) repair slabs experience high incidences of early-age cracking due to high temperature rise and increased autogenous shrinkage of high-early-strength (HES) concrete mixtures. This paper presents an investigation to evaluate early-age cracking mitigation strategies of JPCP repair slabs. Finite element analyses were performed to understand the effects of physical phenomena leading to early-age cracking in JPCP repair slabs. While the analyses indicate the importance of concrete hydration kinetics and viscoelastic behavior on the early-age stress development in slabs, concrete moisture loss to the base was found to be the most significant phenomenon. Numerical modeling of concrete slabs was found to be useful in predicting the stress development in advance of costly field trials. Therefore, the proposed modeling approach can be applied to evaluate the performance of concrete mixtures prior to slab placement and thus improve and economize the current rigid pavement maintenance practices.

This study presents the application of an analytical model to describe the rheological behavior of cement pastes containing silica fume at replacement rates of up to 30% by mass. The analytical model hypothesizes how water interacts with particles in a cementitious system. The coating thickness of water surrounding each particle in the system is estimated. This coating thickness is shown to correlate strongly with measured rheological properties when fit to the Herschel-Bulkley model. To calculate coating thickness, it is necessary to account for the water absorbed by nonhydraulic components in the system, whether aggregate, supplementary cementitious materials, or mineral. The results suggest that silica fume particles may be absorptive, and this absorption capacity, although small, must be considered when designing water-starved cementitious materials. The experimental investigation involved the rheological testing of three water-binder ratios (0.20, 0.30, 0.45), three silica fume replacement levels (10%, 20%, 30%), and eight different silica fume products.

This study investigated the effects of different synthetic and mineral fibers and limestone powder on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC). For the purpose of this study, 16 mixture designs and 204 prism specimens were prepared and cured under either of wet or autoclave conditions. Measurements revealed that mixtures containing synthetic fibers recorded considerable compressive and flexural strengths close to the steel fiber-reinforced mixtures. Specimens reinforced with nylon fibers as the best fibers in this study exhibited a much better flexural performance in terms of flexural strength, deflection capacity, and post-peak ductility than did those containing ceramic and polyester fibers. Finally, specimens containing limestone powder recorded acceptable flexural strength, which was close to those only containing silica fume. The X-ray diffraction (XRD) test showed that limestone powder increased ettringite content due to the dilution effect at 180 days as the main reason for decreasing of compressive strength of mixtures.

In this paper, a numerical model for radial-direction conduction through ultra-high-performance concrete (UHPC) cylinders was developed and experimentally validated. The focus of this research is ultimately to improve understanding of UHPC’s thermal behavior, with this portion focused on determining a convection heat-transfer coefficient for appropriate boundary condition modeling. Although constant heat-transfer coefficients are commonly assumed in literature, this paper shows that many commonly assumed constant coefficients overestimate convection performance for natural convection boundary condition environments. For well-studied geometries, numerical solutions including a temperature and time-dependent convection coefficient produced errors under 1%, whereas constant heat-transfer coefficient assumptions produced errors up to 10%. However, for less-common geometries, numerical model errors ranged from 2 to 5%.