Study On The Durability Improvement Of Nano-reinforced Recycled Aggregate Concrete Beams In Chloride Environment

Recycled Aggregate Concrete?RAC?is new concrete prepared by recycling,crushing and grading of building waste concrete.Making full use of recycled aggregate can solve the environmental pollution and economic pressure caused by construction waste,and is also one of the main methods to slove the sustainable development of the construction industry.However,the mechanical properties and durability of recycled concrete have a certain degree of weakening compared with conventional concrete,which limits its use in special environments.Therefore,it is necessary to study the performance strengthening method of recycled concrete.With the support of the National Natural Science Foundation"Research on the Durability Improvement Technology and Design Theory of Marine Prestressed Concrete Structures"?51778272?,this paper proposes a method for wrapping recycled aggregates with nano-strength slurry for the weak links of recycled aggregates.Modification was carried out to study the mechanical properties and durability of the modified recycled concrete materials and components.The main contents are as follows:1.Research on the basic properties of nano-reinforced recycled coarse aggregate concrete.From the perspective of macro cracks,microstructures,mechanical properties and durability,the experiments compared the modification effects of four nano-strengthened slurries?nano-SiO₂,nano-CaCO₃,nano-Al₂O₃ and nano-GP?on recycled concrete.The research results show that the nano-strengthened slurry after ultrasonic dispersion can effectively strengthen the interface transition zone of recycled aggregates and improve the overall performance of recycled concrete.Among the four nanomaterials tested,nano-SiO₂ reinforced slurry has the most significant effect on improving the interface properties of recycled aggregate.2.Research on durability of nano-reinforced recycled concrete load-bearing beams under chloride environment.Nine nano-strength recycled aggregate concrete corbels were designed and poured,and a total of 50 chlorine salt dry-wet cycle tests were carried out for300 days with continuous loading.At the same time,considering the replacement rate of recycled aggregates,nano SiO₂ content and crack width,the corrosion potential of the steel bars,the change of crack shape and the distribution of chloride ion concentration were tested,and the equivalent chloride ion diffusion coefficient under different cracking conditions was analyzed.The research results show that:1)The corrosion data of the steel bars measured by the double-electrode potential difference method can more easily and quickly determine the location of the steel bar corrosion defects in the components without damaging the concrete components;2)A certain degree of self-healing occurred in the cracks of the concrete during the dry-wet cycle of chloride salt,and the self-healing rate of the crack was the largest when the nano-SiO₂ content was 0.4%;3)The chloride ion content at the crack shows wave declining trend along the depth direction;4)Controlling w?0.12mm and adopting about 0.2%of nano-SiO₂ modified recycled aggregate can effectively reduce the relative chloride ion content in recycled concrete,which is 44.8%lower than that of unreinforced recycled concrete cracks;5)According to the calculated results of the modified equivalent chloride ion diffusion coefficient model are in good agreement with the experimental results.3.Research on bearing capacity of nano-strengthened recycled concrete corroded beam.Nine nano-strength recycled aggregate concrete beams were designed and poured,and the internally stressed steel bars were energized and non-uniformly accelerated to rusted by the method of built-in cathode.At the same time,factors such as the replacement rate of the recycled aggregate,the amount of nano-SiO₂,and the corrosion rate of the steel bar are considered to affect the shape of the rust expansion cracks and loading cracks,failure mode and bearing capacity.The research results show that the improved energized non-uniform accelerated corrosion method can achieve the effect of longitudinal bars bidirectional non-uniform corrosion.Compared with natural aggregate concrete beams,the flexural bearing capacity of recycled aggregates has been reduced under the same degree of corrosion.Among them,the bearing capacity degradation is the most serious at the replacement rate of 25%recycled aggregate,reaching 19.3%.According to the bearing capacity test results,the bearing capacity calculation formula of the recycled concrete beam after rust expansion cracking was revised?the erosion degree and aggregate replacement rate were successively corrected on the basis of the existing specifications?,which provides a reference for the calculation of bearing capacity of regenerated concrete beams with non-uniform rust expansion and cracking.4.Mesoscopic numerical simulation of durability prediction of recycled concrete beams.Considering the multy heterogeneous composition of recycled aggregates,the random distribution of aggregates,and the time decay characteristics of chloride ion diffusion coefficients,etc.,a two-dimensional mesoscopic co-simulation simulation combining MATLAB and COMSOL was carried out to realize the durability life prediction of the recycled concrete component in the marine environment.The simulation results show that the two-dimensional concentric circle meso model reveals that the ultrasonic dispersion nano-enhanced slurry coating technology can reduce the“accelerating field”effect of the chloride ion transmission near the coarse aggregate,thereby reducing the speed of the rapid chloride ion transmission into the surrounding matrix and increasing the durability of recycled concrete.At the same time,the functions of the thickness of the protective layer and the diffusion coefficient of the nano-cladding layer on the durability limit state of the recycled concrete are obtained.