Fractal Law Of Crushing Process Of Mixed Recycled Aggregates And Reasonable Gradation For Pavement Base Or Sub-base Applications

China's rapid urbanization process has produced a large number of construction and demolition waste.According to relevant statistics,China produces about 1 billion tons of construction waste each year.The usual practice of disposing of construction waste is to fill it or stack in the open air.It often causes the construction waste to encircle the city or secondary pollution,which seriously threaten the urban environment and human health.How to make full use of construction waste is a problem to be solved.It is one of the effective ways to replace natural aggregates with recycled aggregates for pavement base or sub-base applications.The way not only can make up for the huge demand gap of materials for urban road development,but also improve the cost performance of recycled products.China's old building structure is mainly brick-concrete structure.The most common component of the construction waste is concrete and clay bricks.Studying the recycling of waste concrete and clay bricks,especially the clay bricks,is of great significance for the recycling of construction waste.At present,there are few studies on the crushing characteristics of recycled aggregates.The physical and mechanical properties of the recycled concretes aggregates and clay bricks are very different.The interaction between the two kinds of materials in the process crushing is still difficult to determine.In addition,the use of mixed recycled aggregates instead of natural aggregates in the pavement base or sub-base must consider the grading to meet the requirements of compactness.This article uses the demolition construction waste from a university building in Taiyuan.After pretreatment,the coarse aggregates of same particle size recycled clay bricks and recycled concrete are mixed according to the ratio of components.The crushing test is carried out by changing the condition variables such as material strength,particle size,component ratio and vertical pressure.After crushing,weighing the quality of aggregates of bricks and concrete.By introducing the fractal geometry to analyze the test results,a method for determining the fractal dimension by the crushing test and then determining the gradation by the fractal dimension is put forward.The specific research results can be summarized as follows:(1)The effects of material strength,pre-crushing particle size,component ratio and vertical pressure on the crushing characteristics of mixed aggregates were discussed.The crushing process of mixed aggregates can be expressed by fractal geometry.The fractal dimension is an objective index reflecting the crushed aggregates.This paper determines the minimum energy level required to make fractal dimension meet physics meaning.(2)The effects of material strength,pre-crushing particle size and component ratio on fractal dimension were discussed.There is a negative correlation between the fractal dimension and the strength of the materials(or the ratio of recycled concrete).There is a positive correlation between the fractal dimension and pre-crushing particle size.(3)The relationship between the ratio of recycled concrete and the crushing index,the vertical deformation is discussed.In the process of mixed aggregates crushing,there is a crushing mode of “discard weak aggregates to protect strong aggregates”.The recycled clay brick aggregates are preferentially broken,and the recycled concrete aggregates is protected.(4)In this paper,a method for determining the fractal dimension by the crushing test and then determining the gradation by the fractal dimension is put forward.This method is of great significance for the application of mixed recycled aggregates in pavement base or sub-base.It provides theoretical basis and method guidance for this engineering practice.(5)Considering the gradation results and the restrictive requirements of clay bricks,the limit of the ratio of recycled clay bricks for pavement base or subbase applications is given.The ratio of recycled clay bricks in primary roads should not exceed 20%,and the ratio of recycled clay bricks in secondary roads should not exceed 25%.