Study On Failure Mechanism And Mechanical Properties Of Total Recycled Aggregate Concrete Improved By Nano-SiO₂

With the continuous development of our economy and the acceleration of urbanization,the demand for natural gravel resources grows day by day.At the same time,the disposal of abandoned buildings is becoming more and more prominent.Using construction waste to produce recycled aggregate instead of natural sand to produce concrete can well solve the dependence on natural resources.At present,the researches on recycled concrete mainly focus on recycled coarse aggregate and its performance strengthening,while there are few researches on fully recycled aggregate concrete prepared by using recycled coarse and fine aggregate instead of natural sand and stone.Therefore,it is necessary to study the performance of fully recycled aggregate concrete.And according to the reason of the inferior performance of the total recycled aggregate concrete,effective ways are adopted to strengthen and improve the performance of the total recycled aggregate concrete.In this paper,recycled coarse aggregate and recycled fine aggregate are used to produce fully recycled aggregate concrete.Through experiments,the influence of water addition method and water-cement ratio on mechanical properties of fully recycled aggregate concrete is explored,and the main failure characteristics and weak areas of fully recycled aggregate concrete in mechanical tests are analyzed.Nano-SiO₂ was used to modify the total reclaimed aggregate concrete,and the improvement effect of nano-sio2 solution on the macro-physical properties of reclaimed coarse and fine aggregate under different concentrations and pre-soaking time was explored,and the improvement effect of Nano-SiO₂ on the basic physical properties of total reclaimed aggregate concrete was studied.Finally,the strengthening mechanism of Nano-SiO₂ on the performance of fully recycled aggregate concrete was analyzed through microscopic technology.The main research contents and results are as follows:（1）The fully recycled aggregate concrete prepared by saturated surface dry method shows better working performance and long-term mechanical properties;With the increase of water-cement ratio,the mechanical properties of FRAC decreased under different preparation methods.Compared with FRAC prepared by saturated surface dry method,with the increase of water-cement ratio,the strength reduction range of FRAC prepared by no additional water method gradually decreased,while the strength reduction range of FRAC prepared by additional water method gradually increased.Based on the experimental data fitting,the conversion relationship between compressive strength and tensile strength of total recycled aggregate was establelished,and showed a high correlation.（2）Interfacial fracture and non-interfacial new mortar fracture are the main failure forms of total recycled aggregate concrete,and the probability of aggregate fracture is small.The performance of interfacial transition zone and the strength of new mortar are the key factors affecting the performance of total recycled aggregate concrete,and the interfacial bonding property is the most important factor.（3）In the reclaimed aggregate premodified by Nano-SiO₂,soaking the coarse aggregate for 48h with 2%solution concentration and soaking the fine aggregate for24h with 2%solution concentration can significantly improve the apparent density,water absorption and crushing index of aggregate.However,further increase of solution concentration and soaking time has little effect on macroscopic physical properties of reclaimed coarse and fine aggregates.（4）The modified Nano-SiO₂ will reduce the fluidity of fully recycled aggregate concrete,but the modified compressive strength of fully recycled aggregate concrete increases by about 14%and tensile strength by about 7%.The experimental results show that increasing solution concentration and soaking time has no significant effect on improving mechanical properties of total reclaimed aggregate concrete.（5）The microhardness test shows that the modification of Nano-SiO₂ can effectively reduce the width of interfacial transition zone and improve the microhardness of new and old mortar.However,for old mortar,the improvement effect of microhardness is not as obvious as that of new mortar.XRD analysis shows that Nano-SiO₂ can penetrate into reclaimed aggregate effectively and mix with new mortar to promote hydration reaction in concrete.TG analysis confirmed that the content of Ca（OH）₂ decreased significantly in the transition zone of the interface of the modified total recycled aggregate concrete.Scanning electron microscopy（SEM）results show that the interfacial transition zone can be densified by Nano-SiO₂ modification...