The use of construction waste for concrete is an effective way to reduce the environmental burden while improving the sustainability of construction materials. Nano-SiO₂ (NS) has excellent volcanic ash activity, which can effectively improve the strength of concrete. In this study, the synergistic effects of NS and 30% recycled concrete aggregate (RCA) on the mass loss, compressive strength, ultrasonic sound velocity values and microstructure of RAC after 25, 50, 75 and 100 repetitions of a dry–wet cycle (DWC) of sulfuric acid were investigated. The results show that NS has a significant role in improving RAC performance. The RAC specimens showed the lowest mass loss rate, the highest compressive strength, the lowest ultrasonic velocity value and the best resistance to sulfate erosion when the NS doping was 4%. Meanwhile, under CT and SEM, RAC with NS has lower porosity and higher hydration, which can effectively inhibit the crack generation and has stable volume growth within 0–50 repetitions of DWC. In addition, the addition of NS improves not only the microstructure of the substrate but also the interfacial transition zone (ITZ). When 4% NS was added, the porosity was the lowest and the durability improvement was the best performance. This study not only improves the production performance of RAC but also provides a strong reference for the integrated application of nanomaterials in concrete.

The progressive collapse of buildings induces a variety of catastrophic consequences, such as casualties and property loss over the past few decades. The corner column is more prone to abnormal load events compared to the inner column and outer column; thus, it is easier to trigger progressive collapse. By considering the effects of floor slabs and adjacent bays on progressive collapse behavior, the pseudo-static loading method was used to study the progressive collapse test of a 1/3 scaled, one story, 2 × 2-bay cast-in-place reinforced concrete frame substructure under the removal condition of a corner column. The test results show that the flexural deformation principally concentrates upon the components of a directly affected part (DAP), and compressive arch actions are observed in members of the indirectly affected part (IAP). Moreover, the slab adjacent to the removed column and periphery elements contributes great resistance to a progressive collapse.

Under the complex working conditions in cold areas, in order to achieve health monitoring of engineering structures, carbon fiber and iron tailings sand were added to ordinary cement-based materials to prepare cement-based piezoelectric composites, and the deterioration of their pressure-sensitive properties and mechanical properties under the action of the sulfate-freeze-thaw cycle was studied. Six groups of specimens and a set of benchmark specimens were prepared according to different contents of carbon fiber and iron tailings sand, and the specimens of each group were analyzed qualitatively and quantitatively after 50, 100, and 150 freeze-thaw cycles. Based on the external damage analysis, it was concluded that with the increase in the number of freeze-thaw cycles, the apparent morphology of the specimens in each group continued to deteriorate. After 150 freeze-thaw cycles, the addition of a certain proportion of carbon fiber and iron tailings can improve the compactness of cement-based composites, effectively inhibit the development of cracks, maintain the integrity of the apparent morphology of the specimen, and the quality loss rate of the specimen does not exceed 5%. Based on the internal damage analysis, it is concluded that the specimen mixed with carbon fiber and iron tailings has undergone the freeze-thaw cycles, and its relative dynamic elastic modulus generally shows a trend of first rising and then falling, and after 150 freeze-thaw cycles, the relative dynamic elastic modulus of C04T30 specimen is 85.5%, and its compressive strength loss rate is 20.2%, indicating that its freeze resistance is optimal. The compressive stress and resistivity change rate of each group of cement-based piezoelectric composite specimens that have not undergone freeze-thaw cycles are approximately consistent with the linear attenuation relationship. Those that have undergone 150 freeze-thaw cycles approximately conform to the polynomial attenuation relationship. The correlation coefficient between the compressive stress and the resistivity rate of the change fitting curve are all above 0.9, and the correlation is high; therefore, the deterioration of the structural mechanical properties after freeze-thaw cycles can be reflected by the resistivity change rate. After 150 freeze-thaw cycles, the pressure sensitivity coefficient of the C04T30 specimen is 0.007294, which has good pressure sensitivity. So, cement-based piezoelectric composite material can be embedded as an impedance sensor to monitor the health of engineering structures.

This study improved the thermal damping of concrete with rice husk ash (RHA)–fly ash (FA) matrix and three phase-change material (PCM) aggregates with phase change temperatures between −15 and 5 °C, which are expected to reduce winter energy consumption in cold regions when used as building envelope structures. Firstly, the strength of concrete was studied. Secondly, the dynamic and transient thermal response of concrete was evaluated through thermal conductivity and thermal diffusivity. Based on nuclear magnetic resonance experiments, the changes in the pore volume and fractal dimension of RHA–FA matrix and PCM aggregate added to concrete were studied. Through correlation analysis, a macroscopic performance prediction model based on pore characteristics was obtained. The results indicated that the incorporation of PCM aggregate reduced concrete strength, while an appropriate RHA–FA matrix contributed to enhancing concrete strength. Both the PCM aggregate and RHA–FA matrix were beneficial for improving the thermal damping properties of concrete. For 15% RHA–30% FA 100% PCM concrete, the thermal conductivity can be reduced by 53%, the thermal diffusivity can be reduced by 64%, the limiting temperature decreased by 5.5 °C, and the thermal damping coefficient increased by 48%. The nuclear magnetic resonance test results showed that PCM aggregate increased the pore volume and decreased the fractal dimension, while an appropriate RHA–FA matrix helped to reduce the pore volume. The macroscopic properties of RHA–FA–PCM aggregate concrete were highly correlated with the capillary pore volume and fractal dimension. A two-parameter prediction model based on pore characteristics can effectively predict the macroscopic properties of concrete.