The use of precast concrete permanent formwork systems in reinforced concrete (RC) construction can deliver meaningful benefits only when their design is optimized by balancing structural, economic, and environmental objectives. This study proposes a multi-objective optimization framework for RC beams with permanent formwork, simultaneously considering flexural performance, material cost, and CO₂ emissions. A case study is conducted in which key geometric and material parameters are treated as variables and the results are compared with those of reference RC beams using reusable steel formwork. Parametric and sensitivity analyses reveal trade-offs among cracking resistance, flexural capacity, cost, and emissions, with formwork thickness, concrete strength, and reinforcement ratio identified as dominant factors. The multi-objective optimized solutions achieve up to 250% higher cracking moment and over 30% increases in yield and ultimate capacities while maintaining comparable or lower material cost and CO₂ emissions than reference beams.

Precast concrete T-beams often develop early-age cracking during storage due to large daily temperature differences, threatening the long-term durability of bridges. To investigate the factors leading to cracking, this study establishes a coupled thermal-mechanical model that accounts for heat of hydration, thermal exchange, solar radiation, concrete shrinkage, and creep. Based on this model, the effects of curing method and formwork removal timing on the cracking risk of T-beams were analyzed. The results indicate that the use of steel formwork + PU insulation board and full shading measures can effectively mitigate the impact of temperature differences and radiation on thermal stresses generated in precast concrete T-beam. The simulated stress patterns closely match field observations, confirming the model’s reliability. This study provides valuable guidance for cracking prevention and optimized curing strategies for precast concrete T-beams in regions with large daily temperature differences.

Frost-induced deterioration remains a critical factor limiting the durability of reinforced concrete (RC) bridge deck slabs in cold regions. Repeated freeze-thaw (FT) cycles lead to the formation of horizontally layered cracks in RC bridge deck slabs, eventually leading to the fall-off of concrete and the cave-in of pavement. To elucidate the mechanism responsible for such damage, a two-stage experimental investigation was conducted using a thermoelectric cooler (TEC)-based system on concrete cylindrical specimens. In the first stage, specimens were subjected to accelerated FT cycles to simulate frost deterioration possible in RC bridge deck slabs. The extent of deterioration was quantified through ultrasound velocity measurements. Physical properties exhibited progressive degradation with an increase in the number of FT cycles. For example, after 25 FT cycles, the oven-dry and saturated weights decreased by 3.2% and 2.3%, respectively. Additionally, X-ray computed tomography (CT) analysis was employed to analyze the crack network and cracking volume. In the second stage, steady thermal conditions were imposed, with one face of the specimen undergoing freezing and the opposite face thawing in the presence of water. Horizontal cracking associated with ice lens growth was observed in all specimens that had been pre-damaged by FT cycling. The results imply a possibility of ice lens formation in inducing horizontal cracking in frost-damaged RC bridge deck slabs.

Existing functionally graded concrete structures are prone to weak interfacial zones due to sequential casting. To address this, this study presents a construction method integrating liquid nitrogen-frozen concrete with normal concrete. The core objective is to investigate the influencing factors and laws of interfacial bonding, and to clarify the interfacial bonding mechanism of the composite system. In this approach, fresh mortar is rapidly frozen into solid blocks via liquid nitrogen to halt cement hydration, and then co-cast with subsequent fresh mortar for molding. The splitting tensile strength, compressive strength and microstructural characteristics of composite specimens under different temperatures were tested. Techniques including scanning electron microscopy, microhardness testing and temperature monitoring were employed to reveal the interfacial hydration mechanism. The results indicate that with appropriate temperature control, the interfacial bonding strength of the pre-frozen composite mortar is comparable to that of monolithic cast mortar. These findings are expected to alleviate the problem of weak interfacial bonding in functionally graded concrete, realize the integral forming of graded structures, and can be applied to practical engineering scenarios such as anti-scouring of functionally graded bridge piers and anti-cracking of concrete graded structures in high-altitude regions in the future.

Single-layer electromagnetic wave-absorbing ceramsite concrete exhibits poor reflection loss and insufficient effective absorption bandwidth in the 8 to 18 GHz frequency range. To address this, expanded perlite (EP) and polypropylene fiber (PF) were used as wave-transparent agents in the matching layer, while carbon fiber (CF) and basalt fiber (BF) were used as wave-absorbing agents in the absorption layer, to prepare double-layer structured fiber-reinforced electromagnetic wave-absorbing ceramsite concrete. The arch method testing system was used to analyze its electromagnetic wave absorption properties in the 8 to 18 GHz frequency range. The experimental results indicate that the double-layer structured specimens exhibit enhanced microwave absorption properties compared to their single-layer counterpart, with improved low-frequency electromagnetic wave reflection loss and broadened effective absorption bandwidth. The microwave absorption properties vary with the changes in fiber types and volume fractions in the matching and absorption layers: when EP is incorporated into the matching layer, the absorption layer with CF achieves optimal microwave absorption parameters at 0.6% CF volume fraction, while with BF, the properties improve as the BF volume fraction increases; when PF is incorporated into the matching layer, higher PF volume fraction than CF or BF in the absorption layer yields lower minimum reflection loss and wider effective absorption bandwidth.

In this study, bischofite and dolomite were used as raw materials to investigate a low-cost production process for active Magnesium Oxide (MgO) through the chlorination roasting method, with a focus on optimizing the technics parameter. The performance differences of the prepared MgO were analyzed by X-ray diffraction (XRD), BET and other test methods. Subsequently, Magnesium Oxychloride Cement (MOC) was prepared, and the effects of the synthesized MgO on the performance of MOC were evaluated through tests including compressive strength, hydration heat, XRD, scanning electron microscopy (SEM), scanning electron microscopy (MIP) and hydration heat. Modifications of MOC were carried out using single-seed crystals addition, combined seed crystals addition, and various mineral admixtures (steel slag, blast furnace slag, and fly ash). The results indicate that MgO prepared by calcining bischofite and dolomite at 850 °C for 3 h achieved activity level nearly reaching 100%, making it suitable for MOC production, with a 28-day compressive strength of 125.5 MPa. Moreover, the prepared MgO exhibited good compatibility with mineral admixtures. When MOC was modified using seed crystals and fly ash, the softening coefficient after 30 days of water immersion reached 0.912.