Study On Frost Resistance Of Recycled Aggregate Concrete Based On Flexural Fatigue Damage

Faced with the increasing shortage of natural sand and gravel materials in China,the construction solid waste is crushed and processed to prepare recycled aggregate,and Recycled Aggregate Concrete（RAC）can be obtained by partially or completely replacing natural aggregate,which can not only solve the resource shortage caused by over-exploitation,but also meet the requirements of green and low carbon,and has remarkable social,economic and environmental benefits.In the northern cold region of China,the concrete pavement is often subjected to bending fatigue load caused by vehicle driving,which greatly affects the frost resistance of RAC,therefore,the study of the frost resistance of RAC after fatigue damage has significant engineering significance for the engineering application of RAC in northern cold regions.Through the bending and tensile strength test,bending fatigue test and freeze-thaw cycle test of RAC,the effect law of recycled coarse aggregate replacement rate and basalt fiber doping on the bending and tensile strength,degree of bending fatigue damage and frost resistance of RAC,and the effect law of bending fatigue damage on the frost resistance of RAC are studied in this paper,and combined with nuclear magnetic resonance test,the influence mechanism of basalt fiber content and fatigue damage on the frost resistance of RAC is analyzed from the microscopic level.The main conclusions are as follows:（1）The crushing index,voiding ratio,water absorption rate and the content of particles below 75μm of recycled crude aggregate are higher than that of natural coarse aggregate,while the apparent density and accumulation density are less than that of natural aggregate.（2）Increasing the replacement rate of recycled coarse aggregate will reduce the bending strength of RAC,accelerate the decay rate of relative dynamic elastic modulus of RAC after fatigue,and affect the fatigue performance of RAC.The incorporation of basalt fiber within0.3%can effectively improve the bending strength of RAC,reduce the decay rate of RAC after fatigue,and improve the fatigue performance of RAC.However,when the basalt fiber mixing exceeds 0.2%,the improvement effect of basalt fiber on the bending and tensile strength and fatigue performance of RAC begins to weaken.（3）Improving the replacement rate of regenerated coarse aggregate will directly accelerate the deterioration rate of RAC test surface,the growth rate of mass loss rate and the relative dynamic elastic modulus in the freezing-thaw environment,reduce the freezing resistance of RAC,and reduce the freeze-thaw life of RAC test.However,when the amount of basalt fiber does not exceed 0.2%,basalt fiber can fill the harmful and harmful holes of RAC and improve the freezing resistance of RAC.When the basalt fiber exceeds 0.2%,the proportion of harmful and harmful holes will be increased due to the fiber"agglomeration",which affects the improvement of fiber on RAC.（4）0.6 Stress level and appropriate bending fatigue load can improve the proportion of RAC harmless holes and less harmful holes through improving pores,inhibit the deterioration of RAC test surface,the growth of mass loss rate and relative dynamic elastic modulus decay under the action of freezing and thawing cycle,and improve the freezing resistance of RAC.However,excessive bending fatigue load will increase the proportion of RAC and harmful holes and affect the frost resistance of RAC.In this paper,the fatigue of RAC specimens and non-regenerated RAC specimens mixed with fiber exceeds 20,000times,the improvement effect of bending fatigue damage begins to weaken,and the fatigue of fully regenerated concrete exceeds 10,000 times,the bending fatigue damage will greatly reduce the freezing resistance of fully regenerated concrete.（5）The EGM model of RAC related to the substitution rate and fatigue number of regenerated coarse aggregate was established to predict the prediction value of the remaining relative elastic modulus of RAC after 30 years of freeze-thaw cycle f_yc,and found that although the prediction accuracy of RAC was secondary and above,and most of them are first level,with relatively good accuracy.（6）The EGM model of RAC related to fiber incorporation and fatigue number was established,and the modified model was used to predict the predicted relative dynamic elastic modulus f_yxc of RAC after 30 years of freeze-thaw cycle,and found that the prediction accuracy of RAC after fiber incorporation was first level and had excellent fitting accuracy.