Study On Mechanical Properties And Damage Constitutive Mode Of Nano Silica And Steel Fiber Reinforced Recycled Concrete

There are many micro-cracks and old mortar on recycled coarse aggregate,which makes the mechanical properties of recycled concrete weaker than that of natural aggregate concrete,so,the engineering application of recycled concrete is limited.In order to improve the mechanical properties of recycled concrete,nano-silica and steel fibers were used as external materials to improve recycled concrete.The replacement rate of recycled coarse aggregate,the nano-silica content and steel fiber content are selected as the mix proportion control parameters,and a total of 18 working conditions of recycled concrete are designed.The basic mechanical properties such as compressive strength test,splitting tensile strength test and flexural strength test were carried out respectively,And the stress-strain curve test under uniaxial compression and fracture performance test of prism,The effects of steel fiber and nano-silica on recycled concrete were studied.The main research results are as follows:(1)Through mechanical property tests.The results show that steel fiber improves the mechanical properties of recycled concrete.However,it has limited improvement in compressive strength,noticeable improvement in splitting tensile strength and flexural strength.Nano-silica can improve mechanical properties of recycled concrete.At the same time,nano-silica can improve the ductility of steel fiber recycled concrete,and the integrity of the specimens is better when they are destroyed.Compared with the single addition of steel fiber and nano-silica,the mechanical properties of recycled concrete are significantly improved when the combination of nano-silica and steel fiber is added.At this time,the better nano-silica content and steel fiber contents are 1% and 1.5%,respectively.Nano-silica can improve the flexural toughness of steel fiber regenerated concrete.When the two additives work together,the improvement of flexural toughness is better than that of a single additive.According to the different flexural toughness standard characteristics,it is recommended to use JG/T 472-2015 to evaluate the flexural toughness of nano-silica and steel fiber reinforced recycled concrete.(2)The stress-strain curve of recycled concrete was analyzed through prismatic compression test.Based on the Weibull distribution theory and Lemaitre equivalent strain principle,a damage constitutive model was established and the damage evolution mechanism is analyzed.The results show that the stress-strain curve becomes fuller but is not obvious with the increase of nano-silica content,and the stress-strain curve of recycled concrete becomes fuller with the increase of the steel fiber content.The subsection expression can better express the full stress-strain curve equation of recycled concrete.When the steel fiber or nano-silica is single-doped,the peak stress increases,the peak strain decreases and the elastic modulus increases with the increase of the nano-silica content,and the peak stress and peak strain increase,and the elastic modulus decreases with the increase of the steel fiber content.When the steel fiber and nano-silica are added together,the peak stress and peak strain increase with the increase of the steel fiber content,The peak stress and elastic modulus increased,but the peak strain did not change significantly with the increase of the nano-silica content.The established damage constitutive model can better satisfy the stress-strain relationship of recycled concrete.The damage variable increases in S-shape with the increase of strain,which can better reflect the damage development of recycled concrete in a certain period of time.(3)Through three-point bending test,the effects of replacement rate,steel fiber content and nano-silica content on the fracture properties of recycled concrete were analyzed.The research shows that the fracture surface of the specimen is relatively flat when no steel fibers are added,and the fracture surface is gradually uneven when steel fiber is added;the nano-silica and the replacement rate have no obvious effect on the fracture failure form of recycled concrete.With the increase of steel fiber or nano-silica content,the double-K fracture toughness and fracture energy of recycled concrete show an increasing trend.The fracture performance of recycled concrete composite reinforced with steel fiber and nano-silica is better than that of reinforced with single material.When the content of nano-silica is fixed,with the increase of the steel fiber content,Fracture toughness and fracture energy increased.When the content of steel fiber is fixed,with the increase of the steel fiber content,Fracture toughness and fracture energy increased.When 2% nano-silica and 1.5% steel fibers were added at different replacement rates,the double-K fracture toughness and fracture energy reached the maximum.The damage model established in this paper can better reflect the stress-strain relationship of recycled concrete during fracture...