The freeze-thaw resistance of concrete is significantly lower in salt water than in fresh water. Concrete deteriorates through repeated freezing and thawing, but in salt water, freezing alone leads to destruction. This paper investigated the effect of calcium hydroxide in concrete on the failure of concrete under such low temperatures. Calcium hydroxide precipitates at the transition zone between aggregate and cement paste due to the hydration of cement. The lower the temperature and the higher the concentration of salt water, the more calcium hydroxide dissolves. From concrete, more calcium hydroxide is eluted in salt water than in fresh water. This accelerates the deterioration of mortar and concrete due to freeze-thaw action. Mortar and concrete using ground granulated blast-furnace slag produces less calcium hydroxide. In mortar and concrete using blast-furnace slag sand, calcium hydroxide precipitated around the aggregate reacts with cement paste and blast-furnace slag sand to modify the transition zone. From these results, it was clarified that concrete using blast-furnace slag exhibits high freeze-thaw resistance even in salt water.

This paper is the English translation of the authors’ previous work [Ayano, T., Fujii, T. and Okazaki, K., (2023). “Freeze-thaw resistance of concrete using ground granulated blast-furnace and blast-furnace slag sand in salt water.” Japanese Journal of JSCE, 79(12), 23-00042. (in Japanese)].

To explore the dynamic compressive mechanical properties of carbon fiber modified polymer reinforced concrete (CFMPRC), the 100 mm diameter SHPB (Split Hopkinson Pressure Bar) test system was used to carry out impact compression tests of CFMPRC with different carbon fiber volume content (0.1%, 0.2%, 0.3% and 0.4%). The dynamic stress-strain curves and fracture morphology of CFMPRC under different strain rates (37.7 s^-1 to 132.2 s^-1) were obtained, and the effects of strain rate and carbon fiber content on dynamic compressive strength, deformation and toughness of CFMPRC were analyzed. The results show that the dynamic compressive strength, deformation and toughness of CFMPRC have obvious strain rate strengthening effect and carbon fiber strengthening effect. The dynamic compressive strength, dynamic increase factor (DIF), dynamic peak strain and impact toughness of CFMPRC increase with strain rate gradually. The dynamic compressive mechanical properties of polymer reinforced concrete are improved by adding carbon fiber, and the optimal carbon fiber content is 0.2%. When carbon fiber content is 0.2%, the strain rate sensitivity of CFMPRC is the strongest, and the increase of strength is the maximum. Carbon fiber can bridge the internal cracks of concrete, and shows the co-modification effect with polymer. Polymer can enhance the carbon fiber/concrete matrix interface, which makes carbon fiber exert its effect more effectively.

Recycling construction demolition waste as recycled coarse aggregate (RCA) is a value-added process. However, the utilization rate of recycled aggregate concrete (RAC) is greatly reduced due to the poor material properties of RCA itself, so it is necessary to improve the related performance of RAC for its practical engineering application. Herein, taking natural aggregate concrete (NAC), recycled aggregate concrete (RAC), recycled aggregate concrete modified with nano-CaCO₃ (NC-RAC) and natural aggregate concrete with nano-CaCO₃ (NC-NAC) as the targets, the slump test, compressive strength test, flexural strength test, static axial compressive strength test and fatigue test under different stress levels were carried out. Combined with SEM images, the basic mechanical properties and fatigue characteristics of the test concrete are analyzed, and the effects of RCA and NC are evaluated. The results indicate that the replacement of natural coarse aggregate (NCA) by RCA has a significant negative impact on the mechanical properties and fatigue properties of concrete, and the negative impact increases with the increase of RCA replacement rate. However, the incorporation of nano-CaCO₃ can significantly improve the compressive strength, flexural strength, elastic modulus and fatigue life of RAC, and effectively improve the performance of RAC materials. The recommended amount of nano-CaCO₃ is 1%. In addition, the fatigue life of the test concrete conforms to the Weibull distribution. The S-N-P fatigue equation considering the survival probability is established by fitting analysis, which has guiding significance for the application of RAC and NC-RAC in structures that need to withstand cyclic loads.