Experimental Study On Mechanical Properties And Freeze Thaw Deterioration Of Cement Cold Recycled Base Mixture

At present,the development of highway has entered the stage of paying equal attention to construction and maintenance.Recycling materials from asphalt pavement(RMAP)produced in the maintenance process can be used in the grass-roots renovation and reconstruction,which can ameliorate the problems of low utilization rate of recycled materials,waste of resources and environmental pollution.The cracking of base course is mostly caused by its material characteristics.So,it is an urgent problem to improve the service life of cold recyc led base course by studying the cold recycled base course technology on the basis of secondary utilization of RMAP.Therefore,this article investigates the influence of RMAP on the mechanical properties and frost resistance of cement cold recycling base,providing technical support for the promotion of cold recycling technology of cement base.Firstly,the basic physical and mechanical properties of cement,RMAP and new aggregate are tested,and the gradation and variability of RMAP in different pavement layers are analyzed in this paper;Secondly,the mechanical performance indexes of unconfined compressive strength,splitting strength,compressive resilient modulus and flexural tensile strength of cold recycled mixture under different cement content,curing age and mix proportion are tested,and the strength,stiffness and stability of cold recycled mixture under different pavement layers are compared and analyzed;Thirdly,the dry shrinkage water loss rate and dry shrinkage strain of cold recycled base are tested to study the dry shrinkage characteristics of cold recycled base;Finally,through the standard freeze-thaw cycle test,the mechanical properties,erosion resistance and anti-permeability of the cold recycled mixture after freeze-thaw are compared and analyzed,and the freeze-thaw deterioration of the cold recycled mixture with the increase of freeze-thaw cycles is analyzed by ultrasonic nondestructive testing technology.The conclusions are as follows:(1)The basic indexes of raw materials meet the requirements of cold recycling.Compared with the new aggregate,the gradation of RMAP after crushing has certain variability,and the particle size of recycled materials is small when the milling depth is shallow.When the gradation is poor,adding 20% coarse aggregate with the particle size of 10mm-30 mm can ensure that the unconfined compressive strength of cold recycling mixture meets the requirements.(2)The unconfined compressive strength,splitting strength,compressive resilient modulus and flexural tensile strength of cold recycled mixture increase with the raise of cement content and curing age,but decrease with the raise of recycled asphalt pavement(RAP)content.From the formation of unconfined compressive strength and splitting strength,the strength value increased rapidly before 28 days of curing,but tends to flat after 28 days;After replacing the recycled aggregate or recycled inorganic binder stabilized aggregate(RAI)with new aggregate,the unconfined compressive strength,splitting strength,compressive modulus of resilience and flexural strength of the cold recycled mixture are significantly enhanced.Adding RAP increases the"flexibility"of recycled base structure,and the stiffness of the cold recycled mixture is between the flexible material and the semi-rigid material.(3)The dry shrinkage water loss rate and dry shrinkage time of cold recycled mixture increase rapidly in the initial stage(within 7 days),slowly in the middle stage(from 7 days to 21 days),and tend to be stable in the later stage(after 21 days).The drying shrinkage strain increases with the increase of water loss rate,but the water loss rate and strain growth rate gradually slow down after the water loss rate is 3%;The dry shrinkage strain and dry shrinkage water loss rate increase with the increase of cement content and dry shrinkage age.Compared with 4% and 6% cement content,5% cement content keeps relatively low dry shrinkage water loss rate and dry shrinkage strain in the whole dry shrinkage age;When RAP content is 20%,the dry shrinkage water loss rate and dry shrinkage strain of cold recycled mixture are integrally at the lowest value,while when RAP content exceeds 25%,the dry shrinkage water loss rate and dry shrinkage strain upsurge.Therefore,for the sake of reduce the risk of dry shrinkage cracking,it is suggested that the cement content should not exceed 5% and RAP content is inappropriate to exceed 25%.(4)Increasing the age and cement content makes the frost resistance of cold recycled mixture improve.However,with the increase of freeze-thaw times,the damage degree of freeze-thaw-scour and freeze-thaw-seepage is intensifying;At the beginning of freeze-thaw,the wave velocity is greatly affected by the cement content,while at the later stage,it is greatly affected by the RAP content.Moreover,the freezethaw damage of cold recycled mixture occurs at 3 and 21 freeze-thaw cycles,which makes the wave velocity decrease significantly.As the RAP content is 25%,the freezethaw deterioration of cold recycled mixture is lesser,indicating that 25% RAP can significantly improve the frost resistance of cold recycled mixture.