Study On Strength Characteristics Of Foamed Asphalt Cold Recycled Mixture

Foamed asphalt cold recycling mixture is a certain proportion of asphalt pavement milling material,new aggregate,cement and mineral powder mixed with water mixing,and then use the asphalt foam to glue it into a whole and compacted molding material.Because this kind of pavement material has the advantages of saving resources,reducing pollution and reducing transportation cost,it is more and more widely used in asphalt pavement maintenance engineering.Foamed asphalt cold recycling mixture is a semi flexible material.Its structure and mechanical response are very complex.At present,it is mainly applied to the underlying layer or base.The mechanical properties of foamed asphalt cold recycled mixture have always been one of the main bottlenecks limiting the application scope and application level of cold recycling technology.It is very important to explore the strength performance characteristics of cold recycled asphalt mixture.In this paper,the foaming mechanism,foaming evaluation index and foaming test method of foamed asphalt were discussed.Through indoor foaming test,two indexes of expansion rate(ER)and half life(HL)were used to evaluate the foaming effect of asphalt,and the best conditions of asphalt foaming were determined by combining foaming characteristic curve method and foaming index(FI)method.The results show that the influence of asphalt temperature on foaming effect is less than that of foaming water consumption;Compared with the foaming characteristic curve method,the foaming index(FI)is greatly affected by the expansion rate and less affected by the half-life.It is suggested to use it as an auxiliary means to evaluate the foaming effect;The optimum foaming condition of asphalt is that the asphalt temperature is 165℃,and the foaming water consumption is 2.5%.Secondly,by using industrial CT scanning technology,the air gap characteristics of Marshall test pieces of foamed asphalt cold recycled mixture under different gradation types and mixing water consumption were studied.The concept of "Gap gradation" was introduced,and the dual parameter Weibull distribution model was used to further study the characteristics of void distribution of recycled asphalt.The results show that the measured void fraction of foamed asphalt cold recycled mixture is larger than that of CT image processing under different gradation types and mixing water consumption,the difference is about 1%.The proportion of fine aggregate and mixing water consumption of foamed asphalt cold recycling mixture have significant influence on the void fraction and void size,but have little influence on the number of voids.With the increase of the proportion of fine aggregate in recycled material,the "Scale parameter" λ in Weibull distribution model The smaller the,the finer the void gradation,and the larger the "Shape parameter" k,the more uniform the void distribution;Compared with 60%and 100% mixing water consumption,the "Scale parameter" λ under 80% mixing water consumption Minimum,"Shape parameter" k is the largest,that is,the void gradation is the smallest and the void distribution is the most uniform under 80% mixing water consumption.Then,based on atomic force microscopy(AFM),the asphalt phase of foamed asphalt cold recycling mixture was tested under different cement content and mixing water consumption,and the surface morphology characteristics of the asphalt mixture were studied,and the mechanical properties of the asphalt mixture were quantitatively characterized.The results show that after adding cement,the surface roughness and inclination angle of recycled asphalt phase increase significantly,the density distribution of asphalt phase bee structure increases significantly,the height increases and the length decreases,and the adhesion,DMT modulus and dissipation energy of asphalt phase increase by 28.3%～55.9%;Compared with 60% and100% mixing water consumption,the surface roughness and inclination of recycled asphalt phase are the largest under 80% mixing water consumption,the density distribution of asphalt phase bee shaped structure is the largest,the height is the largest and the length is the smallest.The adhesion and dissipation energy of asphalt phase are increased by 6.3%～14.6% and4.4%～25.8% respectively,while the DMT modulus is increased and decreased by26.5%～37.4% and 10.1%～14.5% respectively;Under different cement content and mixing water consumption,the roughness,inclination,adhesion,DMT modulus and dissipation energy of asphalt phase in the interface area of recycled material are greater than those in the non interface area.Finally,60℃ Marshall stability test,15℃ splitting strength test and-10℃ semi-circular bending test were used to test the foamed asphalt cold recycled mixture Marshall specimens with different gradation types,cement content and mixing water consumption.Correlation analysis was also made between the macroscopic mechanical strength properties of recycled aggregate and its microscopic pore distribution characteristics as well as the microscopic interface adhesion characteristics.The results show that the strength gradation of foamed asphalt cold recycled mixture can be significantly improved by adding new fine aggregate and mineral powder to different grades of temperature and humidity."Scale parameters" λ in Weibull distribution model parameters under different gradation And "Shape parameter" k have the best correlation with Marshall stability and low-temperature flexural tensile strength of recycled materials,and have the best correlation with dry and wet splitting strength of recycled materials under different mixing water consumption;The adhesive force and dissipated energy obtained from AFM micro test have a good correlation with the strength properties of recycled materials at different temperatures.Therefore,the macro strength properties can be evaluated by micro mechanical analysis and test.