To enhance the microstructure and mechanical properties and to optimize the formulations of high-strength and high-flow ultra-high-performance concrete (UHPC), the effects of different quartz powder contents, fineness, and curing temperatures of UHPC were systematically studied. It was hypothesized that using the appropriate content and fineness of quartz powder can improve the microstructure of UHPC and, thus, improve its mechanical properties, especially at higher curing temperatures. To test this hypothesis, the flowability, compressive strength, flexural strength, and tensile strength of UHPC mixtures with different quartz powder dosages (0%, 15%, 30%) and fineness (4 µm, 8 µm), cured at 20 °C, 45 °C, and 90 °C, were investigated. The results indicate that as the dosage of quartz powder increases and the particle size decreases, the flowability of UHPC decreases. The compressive strength of UHPC first increases and then decreases with an increase in quartz powder dosage. Finer quartz powder usage considerably enhances packing density and pore structure. When the content of quartz powder is 15%, UHPC achieves optimal mechanical properties and pore structure, showing an improvement of 3.6% to 14.4% compared to UHPC with a coarse particle size. Additionally, an increase in curing temperature leads to the consistent growth of the compressive strength of UHPC. Under 90 °C steam curing, UHPC incorporating 15% fine quartz powder (4 μm) achieved a peak compressive strength of 182.1 MPa, increased by 19.8% compared to that under 20 °C, which is attributed to the enhanced pozzolanic activity of fine quartz powder. These findings provide valuable guidance for the mix design of UHPC in precast concrete components, long-span bridges, and marine structures, where high early strength and durability are critical.

A three-dimensional time-dependent finite element model is developed to investigate the short-term and long-term mechanical behavior of prestressed concrete beams subjected to combined freeze–thaw deterioration, tendon corrosion, and sustained loading. The proposed framework integrates smeared cracking and tension stiffening of concrete with concrete creep and shrinkage, prestressing tendon relaxation, freeze–thaw-induced degradation of concrete mechanical properties, and corrosion-induced deterioration of reinforcing steel and prestressing tendons. These mechanisms are incorporated into a unified incremental time-stepping analysis to capture the progressive deformation and prestress loss of the beams. The model is validated against experimental results for corroded prestressed concrete beams, freeze–thaw-damaged concrete beams, and long-term sustained-loading tests. Parametric analyses are subsequently performed to investigate the effects of concrete strength, freeze–thaw cycles, tendon corrosion rate, and prestress level. The results show that increasing the concrete strength from C35 to C50 reduces the 1000-day long-term deflection from 24.1 mm to 18.0 mm. Freeze–thaw deterioration has the most pronounced effect: the 1000-day deflection increases from 13.0 mm at 0 cycles to 42.9 mm at 150 cycles. In contrast, increasing the tendon corrosion rate from 0% to 12% increases the 1000-day deflection only from 21.6 mm to 21.8 mm, although the corresponding long-term prestress loss increases from 188.2 MPa to 198.0 MPa. Increasing the prestress level from 0.45 f_ptk to 0.75 f_ptk reduces the long-term additional deflection from 11.4 mm to 8.8 mm. The results demonstrate that freeze–thaw deterioration and concrete time-dependent effects dominate the long-term deformation response, whereas the influence of tendon corrosion on global deflection is comparatively limited within the investigated corrosion range. The proposed framework provides a numerical approach for assessing the time-dependent mechanical response of prestressed concrete beams under combined environmental and mechanical deterioration.