Study On Compressive Strength And Frost Resistance Of Composite Reinforced Recycled Coarse Aggregate Concrete

With the development of economy,people’s demand for concrete is increasing.Many old buildings have been dismantled and a large amount of construction waste has been accumulated during the renovation project in our city.The use of recycled coarse aggregate instead of natural coarse aggregate is an efficient means to solve the large amount of construction waste hoarding.However,it is found that the physical properties of recycled coarse aggregates have major defects,which limit the utilization rate of recycled coarse aggregates.Therefore,this thesis investigates the physical properties of recycled coarse aggregate and the compressive strength and frost resistance of recycled coarse aggregate concrete after composite reinforcement to investigate the reinforcing effect of composite reinforcement on the properties of recycled coarse aggregate and recycled coarse aggregate concrete.The main research contents and conclusions are as follows.（1）Study of physical properties of single reinforced recycled coarse aggregate.According to the results of the single recycled coarse aggregate physical properties test,two groups of optimal reinforcement in a single reinforcement were determined.The results show that the best reinforcement method of cement-based slurry is RA3 group,i.e.,the recycled coarse aggregate is soaked with a cement-based slurry made of partial kaolin and silica fume with a mass ratio of 1:3for 12 hours and maintained for 7 days.The best reinforcement method of hydrochloric acid solution is RAH2-2 group,i.e.,the recycled coarse aggregate is soaked with hydrochloric acid solution with a mass fraction of 2% for 2 hours.The recycled coarse aggregate is reinforced by the above two reinforcement methods,and the aggregate grade is improved from Class III to Class II.（2）Compressive strength study of recycled coarse aggregate concrete with composite reinforcement.The two optimal single reinforcement methods obtained through the physical properties of the aggregates are combined to form two composite reinforcement groups.Through the physical properties test of the composite reinforced recycled coarse aggregate and the compressive strength test of recycled coarse aggregate concrete,the difference between the compressive strength values of the reinforced recycled coarse aggregate concrete and the natural concrete was compared to determine the reinforcement effect of the composite reinforcement method,so as to obtain the best composite reinforcement group.The results show that the best strengthening method is the composite strengthening HR group（RAH2-2+RA3）,i.e.,the recycled coarse aggregate is soaked with hydrochloric acid solution with mass fraction of 2% for 2 hours,then soaked with cement-based slurry made of partial kaolin and silica fume with mass ratio of 1:3for 12 hours and cured for 7 days.The 28 d compressive strength value of the specimens in this group is 62.3 MPa,which is 59.7% higher than that of the specimens in the unreinforced group.The 28 d compressive strength value of this group is 62.3MPa,which is 59.7% higher than that of the unreinforced group and 11.8% higher than that of the single reinforced optimal group.The grade of reclaimed coarse aggregate after composite strengthening is class I.（3）Study on frost resistance of recycled coarse aggregate concrete with composite reinforcement.Natural coarse aggregate,single reinforced recycled coarse aggregate and composite reinforced recycled coarse aggregate were used to make the frost resistance test specimens.Under the same conditions,the "walk-in environmental simulation chamber" was carry out the freeze-thaw cycle test of concrete to study the strengthening effect of the composite strengthening method on the frost resistance of recycled coarse aggregate concrete,so as to verify the optimal strengthening method.The results show that in the single-reinforced group,none of the test blocks meet the frost resistance requirement of D125.In the composite reinforcement group,the best composite reinforcement HR group can meet the frost resistance requirement of D150,and the reinforcement effect of this composite reinforcement on the frost resistance of recycled coarse aggregate concrete is higher than that of other reinforcement methods.In summary,it can be seen that the composite strengthening method of HR（RAH2-2+RA3）has the best strengthening effect on the physical properties of recycled coarse aggregate and the compressive strength and frost resistance of recycled coarse aggregate concrete.