Experimental Study On Mechanical Properties Of Recycled Coarse Aggregate Concrete Strengthened With Nano SiO₂

In recent years,the process of urban reconstruction and urbanization in China has gradually accelerated,the number of construction waste resulting from the demolition of buildings is increasing.In today's extremely scarce natural resources,the recycled aggregate is made from waste concrete by crushing,cleaning and sieving,instead of natural aggregate for concrete mixing,to realize the recycling of construction waste is the main way to develop ecological green concrete.However,the recycled coarse aggregate prepared by simple crushing has many edges,rough surface,large porosity,high water absorption and high crushing index,various unfavorable factors limit the popularization and application of recycled coarse aggregate,therefore,to improve the basic physical properties of recycled coarse aggregate is the focus and foundation of recycled concrete research.In twenty-first Century,nano technology is permeating in various fields,at the same time,it has injected new vitality into the development of concrete,and has opened up a new direction.It is well known that hydration of cement produces a large amount of Ca?OH?₂,the residual Ca?OH?₂ in waste concrete has become its potential active source,nanomaterials with extremely small particle sizes have very high chemical activity.For example,highly active nano SiO₂ can react with residual Ca?OH?₂ in recycled coarse aggregate to produce hydrated calcium silicate gel?C-S-H?,thus,the pore of waste concrete is filled,the activity of waste concrete is induced,the porosity and water absorption of recycled coarse aggregate are reduced,the compactness of aggregate is improved,and the crushing index of aggregate is reduced.Therefore,in this paper,nano new technology combined with recycled concrete technology is adopted to improve the basic physical properties of recycled coarse aggregate by nano SiO₂ material,and then the mechanical properties of recycled concrete are studied.Firstly,the basic physical properties of simple broken recycled coarse aggregate are measured,the experimental results show that the recycled coarse aggregate has the disadvantages of large porosity,high water absorption,small apparent density,small bulk density and high crushing index.The difference of water absorption and crushing index is more obvious than that of natural coarse aggregate.The water absorption rate of recycled coarse aggregate is almost 4 times of natural coarse aggregate,and the crushing index is 55.6%higher than that of natural coarse aggregate.Secondly,the nano SiO₂ solution with 1%,2%and 3%concentrations of three kinds of concentration was prepared to soak the recycled coarse aggregate,the soaking time of each concentration is divided into three cases:24h,48h and 72h,at last,2%concentration was selected as the best soaking concentration,and 48h was the best soaking time,after soaking in the optimum soaking conditions,the water absorption and crushing indexes of recycled coarse aggregate were reduced by 29.9%and 31%respectively.thirdly,the early and late cube compressive strength of natural concrete,ordinary recycled coarse aggregate concrete and nano SiO₂ recycled coarse aggregate concrete were studied,the test shows that the compressive strength of ordinary recycled coarse aggregate concrete is obviously lower than that of natural concrete,and decreases gradually with the increase of recycled coarse aggregate replacement rate;Compared with ordinary recycled coarse aggregate concrete,the strength of nano SiO₂ recycled coarse aggregate concrete has been greatly improved,especially,the early strength increases more greatly,even more than natural concrete.Finally,the stress-strain curve of recycled aggregate concrete under uniaxial compression is studied,the variation law of peak stress,peak strain,elastic modulus and Poisson's ratio of the curve with the replacement rate of recycled coarse aggregate was analyzed;The advantages and disadvantages of several constitutive equations are analyzed,and the fitting analysis of the experimental data is carried out,it is found that the piecewise function proposed by Zhenhai Guo is more suitable for the fitting of the experimental curve,and the higher fitting degree is obtained.