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The optimal soaking time and nanosilica concentration were chosen by the physical properties of the nanosilica-modified recycled aggregate. Recycled aggregate concrete (RAC) and nanosilica recycled aggregate concrete (SRAC) were fabricated by using ordinary recycled aggregate and nanosilica-modified recycled aggregate. Based on the comparative experimental study of basic mechanical properties, the effects of nanosilica recycled aggregate(SRA) modification and recycled aggregate(RA) replacement percentage on the basic mechanical properties of recycled concrete were analyzed. Finally, the split-Hopkinson pressure bar (SHPB) was used to conduct comparative experimental research on the impact resistance of recycled aggregate concrete and nanosilica-modified recycled aggregate concrete. The effects of nanosilica recycled aggregate modification and aggregate replacement percentage on failure morphology, dynamic peak stress, dynamic increase factor (DIF), dynamic peak strain were analyzed.

The axial compression performance of concrete filled steel tubular (CFST) columns using high-strength recycled aggregate concrete (RAC) instead of normal concrete (NC) was studied in an attempt to use demolition debris for effective recycling in construction works. Five specimens (three RAC-filled steel tubular (RACFST) columns and two reference CFST columns) were tested to investigate the influence of tube shape (circular or square), concrete type (NC or RAC) and internal structure (installation of a steel reinforcement cage or not). The test results indicate that damage development and failure mode of RACFST columns are similar to those of CFST columns. For the same cross-sectional area, steel ratio and material strength, circular section specimens were found to have a higher load-bearing capacity and better deformability than square section specimens. The replacement of NC with RAC has relatively less influence on the axial compressive behaviour of square section specimens than on circular specimens. A steel reinforcement cage inside the square section specimen was found to strengthen confinement to the core concrete, which significantly improved the bearing capacity and deformability. The results obtained are compared with the ultimate strengths of RACFST columns predicted using existing design codes. Considering the size effect, a formula for calculation of the bearing capacity of square CFST columns is suggested; comparisons of the predicted results show good agreement with the experimental data.

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Introduction:Freeze-thaw resistance of recycled aggregate concrete with partial or total replacement of recycled aggregate compared with that of natural aggregate concrete was investigated in this paper. Method:Ninety specimens were fabricated to study the influence of different recycled aggregate replacement ratios on the surface scaling, mass loss, and residual compressive strength after 100 freeze-thaw cycles. Results:The experiment results indicate that the type of recycled aggregate and its replacement ratio have significant effects on the freezethaw performance. The cubic compressive strength of recycled aggregate concrete is overall slightly lower than that of normal concrete. After 100 freeze-thaw cycles, the compressive strength decreases and the reduction extent increases with increasing replacement rate of recycled aggregate. The surface scaling of reinforced recycled concrete prisms tends to be more severe with the increase of freeze-thaw cycles. Conclusion:Furthermore, a notable rise in mass loss and the bearing capacity loss is also found as the substitution ratio increases. Under the same replacement rate, recycled fine aggregate causes more negative effects on the freeze-thaw resistance than recycled coarse aggregate.