Effect Of Fly Ash On Mechanical Properties And Damage Mechanism Of Recycled Concrete

With the vigorous development of China's construction industry,people are mining large quantities of rocks and firing cement,resulting in the gradual depletion of natural aggregates and cement.With the urbanization and economic development,old city reconstruction,old road concrete demolition and firepower Coal power generation,leaving a lot of waste concrete and fly ash,if not treated in time will seriously affect the surrounding environment,and properly handled can play the role of "turning waste into treasure",while also alleviating natural aggregates and cement The shortage of resources avoids the environmental damage caused by rock mining.Solve the problem of non-renewable energy.This paper uses the concrete obtained from the demolition of old pavement as the source of recycled concrete,and studies the stressstrain curve and constitutive relationship of recycled concrete from different fly ash content,curing age,and freeze-thaw cycles,and aims to promote its application.Fly ash recycled concrete provides theoretical support.The main research work is as follows:(1)Considering different fly ash contents as design parameters,7 kinds of fly ash were mixed into ordinary concrete and recycled concrete,and uniaxial compression test was carried out on test blocks with different fly ash contents.Deformation and failure characteristics of recycled concrete and ordinary concrete specimens;considering different curing ages as design parameters,six different curing age tests were carried out on fly ash recycled concrete and recycled concrete,and test blocks of different curing ages were carried out In the uniaxial compression test,the uniaxial compression stress-strain curves under different maintenance ages were obtained,and the relationship between peak stress,peak strain,elastic modulus and maintenance age was analyzed.The results show that the compressive strength of recycled concrete and ordinary concrete is the highest when the fly ash content is 10%;as the curing age increases,the shape of the stress-strain curve of recycled concrete and fly ash recycled concrete changes greatly in the early stage After the curing age reaches 28 d,the shape of the curve is basically similar.The compressive strength of recycled concrete gradually increases with age,and the compressive strength of fly ash recycled concrete is higher in the later period.(2)Using different freeze-thaw cycles as design parameters,seven different freeze-thaw cycles were tested on fly ash recycled concrete and recycled concrete,and the mass loss rate and relative dynamic elastic modulus were measured.A uniaxial compression test was carried out on the concrete block after freezing and thawing.Observation of the change and failure morphology of the surface of the test block after two kinds of concrete undergoing different freezing and thawing cycles.,The relationship between peak stress,peak strain,elastic modulus and the number of freeze-thaw cycles was analyzed.The results show that as the number of freeze-thaw cycles increases,the mass loss rate and relative dynamic elastic modulus gradually decrease,the shape of the stress-strain curve has a large difference,and the compressive strength gradually decreases.(3)Based on statistical damage theory and macro test phenomena,the uniaxial compression damage constitutive model of fly ash recycled concrete at different curing ages and freeze-thaw times is established,assuming that there are fractures and yields on the mesoscale The two damage evolution processes change with the age of curing and the number of freeze-thaw cycles.The composition and mechanical properties of the internal microstructure of the concrete have changed,which respectively show the strengthening and deterioration of the concrete,and also changed the microcrack initiation and expansion.The morphology and law of the above;the above effects can be characterized by the five characteristic parameters in the damage model.Through theoretical prediction and result analysis,the internal relations between the chemical physical mechanism,meso-damage evolution mechanism and macroscopic nonlinear mechanical behavior of recycled concrete and fly ash recycled concrete at different curing ages and freeze-thaw cycles are discussed respectively.Using the polynomials given in the Zhen hai model to fit the freeze-thaw cycle test curve,the stress-strain full curve equation and corresponding parameters of the freeze-thaw cycle of fly ash recycled concrete were established,and the parameters of the equation were analyzed to obtain the parameters The relationship between freezing and thawing times is in good agreement with the test curve.