Study Of Nano-SiO₂ Modification On Fracture Performance Of Recycled Aggregate Concrete By Using Three-point Bending Beam Test

A large amount of waste concrete are being produced in the process of demolition in China every year,waste concrete that has not been treated effectively not only occupies the land resource,but also causes problems such as environmental pollution,excessive energy consumption,etc.Waste concrete can be made into recycled aggregates through a series of processing technology,which makes it available for recycling and turns it into a kind of renewable resource.This process can reduced the pollution caused by waste concrete and also reduce the use of natural aggregates.Therefore,it meets the requirements of sustainable development.However,Compared with natural aggregate,recycled aggregate has lower bulk density and apparent density,higher porosity,higher water absorption and larger crushing index.Therefore,there are some defects in the performance of recycled aggregate concrete.In this paper,the fracture properties of recycled concrete were modified by using nano-SiO₂,and the influence of the replacement rates of recycled aggregate and the content of nano-SiO₂ on the fracture properties of the recycled concrete was studied.The results of the study are listed as follows:1.Taking the replacement rates of recycled aggregates and nano-SiO₂ content as variables,13 groups of mix proportion were designed.The performance of recycled concrete fracture was investigated by using three-point bending beam method.Specimen size was L×h×t=750mm×150mm×75mm,precast crack length was a0=45mm,five specimens were made for each group.The fracture performance test was performed by using MTS fatigue testing machine?250 kN?.2.Initial cracking load of recycled concrete beam was determined by electrical measuring method,the effects of the replacement rates of recycled aggregates and nano-SiO₂ content on initial cracking load of specimens were analyzed.The results showed that,the initial cracking load of recycled aggregate concrete decreased with the increase of recycled aggregate replacement rates?0%,30%,50%,70%,100%?.At the replacement rates of 50%and 100%?1.0%nano-SiO₂?,the initial cracking load improved most significantly.The 1.0%content of nano-SiO₂ might be the optimal amount of recycled concrete specimen to delay cracking and improve specimen bearing capacity.3.Double-K fracture parameters of recycled concrete were calculated by using the curves of load-fracture opening displacement?P-CMOD?data obtained from the experiment,and the effects of the replacement rates of recycled aggregates and nano-SiO₂ content on Double-K fracture parameters of recycled concrete were studied.The results showed that,Double-K fracture parameters of recycled concrete decreased with the increase of recycled concrete replacement rates.Appropriate amount of nano-SiO₂ improved Double-K fracture parameters of recycled concrete.When the replacement rates of recycled aggregate were 50%and 100%respectively?1.0%nano-SiO₂?,the enhancement for Double-K fracture parameters of recycled concrete was the highest.4.The power curve function is fitted to the descending section of the curves of load-displacement?P-8?obtained from the test to calculate the fracture energy of recycled concrete,and the effects of the replacement rates of recycled aggregates and nano-SiO₂ content on the fracture energy of recycled concrete were studied.The results showed that,The fracture energy of recycled concrete decreased with the increase of recycled aggregate replacement rate;The fracture energy of recycled concrete can be improved by adding a proper amount of nano-SiO₂.With recycled aggregate replacement rate of 50%,1.0%nano-SiO₂ content had the best effect on the fracture energy of recycled concrete;With recycled aggregate replacement rates of 100%,0.8%nano-SiO₂ was the optimal amount to improve the fracture energy of recycled concrete.