The Research On The Durable Asphalt Pavement Structure Based On The Composite Subgrade

By the end of 2008, China's total expressway mileage has reached 60,300 kilometers. More than 90% of these expressways were built with semi-rigid asphalt pavement structure. Such kind of pavement structure has obvious advantage in structure with high bearing capacity, better water stability and lasting quality, but its fatal drawback is that semi-rigid base is universally easy to crack and easy to make some road reflective cracking, leading to structural damage (early cracking and reflection cracks caused by temperature and drying shrinkage of semi-rigid base material are apt to lead to cracks up through all the structure). Through reviewing nearly 20 years of asphalt pavement structural design and its application in our country, it can be seen that the inconsistent control index of design with the road damages and the single pattern of pavement structure have seriously affected road performance and life expectancy. In view of this, following the principle of the most economical life-cycle cost, the design concept of durable asphalt pavement is introduced in this paper in order that no structural damages occur in the entire design life or just a regular fuctional maintenance are needed. On this basis, control index and structural design methods of durable asphalt pavement suitable for China's climate, transportation, materials, economy and other conditions are proposed to improve the quality of design and service of our asphalt pavements.In this paper, regarding asphalt pavement structural damage, application of high-grade asphalt pavement at home and abroad was analyzed and adaptability of durable asphalt pavement especially with composite base was systematically studies. So it is considered that composite base can effectively prevent semi-rigid base from cracking and reflection cracks. Comparatively speaking, it is more suitable to be used as the base of durable asphalt pavement.A finite element model was set up and impact of the interlayer bonding state on the internal mechanical response of asphalt pavement with composite base was analyzed. Particularly different contact states between layers and impact of pavement structure index on shear stress were deeply investigated. With "multifunction shear apparatus for pavement interlayer materials", shear resistence of different types of pavement layer materials under different temperature conditions was tested and analyzed and suitable materials and optimum dosage for prime layer, sealing layer and adhesive layer were recommended. According to the damage mechanism and its changing pattern in the shear failure process of interlayer material, shear failure was defined into three stages and it is believed that adhesive force between materials, machinery bite force and friction gradually act in the whole process(of shear failure).In the research, a road design index system of durable asphalt pavement with composite base corresponding to asphalt pavement structural damage has been proposed. The tension strain is taken (as the design index) to control the strained condition of the asphalt layer while the tension stress in the bottom layer (as the design index) to control the strained condition of semi-rigid base and the compressive strain on the top of the subgrade is to control the subgrade plastic deformation and the design value of compressive strain on the top of the subgrade is recommended as 200με. The road deflection is to be taken as the index (target) of design and completion in order to make the design corresponding to the quality control. It is recommended that maximum shear stress in the asphalt mixture and between surface layer and base layer should be recomputed in order to control local damage in the surface layer and shear damage between layers, especially analysis should be focused on the shear stress of the structure layer in 10cm depth and its shear strength should be checked.It is the base for the design of durable asphalt pavement to determine a reasonable fatigue limit of the asphalt mixture, but there is no uniform standard for the determination of the value either at home or abroad. Based on the analysis of fatigue damage of asphalt mixture under normal strain condition, computing methods for fatigue limit recommended by ASSHOT T321 was modified. According to the variation trend of mixture stiffness modulus, the fatigue failure process is divided into three stages. Test data of the second stage are proposed to be used to calculate the fatigue life of the specimen under low strain condition. Trabecular repeated bending fatigue tests were conducted with UTM fatigue testing machine to test fatigue performance of asphalt mixture at different strain conditions and ascertain that fatigue limit of durable pavement ATM-25 and AC-25 is 70μεto 80μεin Bin Zhou, Shandong province. The fatigue tests also show that the fatigue life and strain level of asphalt mixture can still keep consistent well with power function fatigue equation even under low strain condition. So, the fatigue life of asphalt mixture under low strain condition can be inferred by its fatigue life under normal strain condition.Based on computation and analysis, structural design principles for durable asphalt pavement were proposed here. According to the destruction model and load-bearing characteristics of semi-rigid base, it is divided into upper anti-cracking function layer and lower anti-fatigue function layer with reasonable thicknesses for both layers. For anti-cracking function layer, it is recommended to choose skeleton void structure and skeleton dense structure as well as to reduce the amount of binder in the mixture to effectively control the occurrence of cracking. For anti-fatigue function layer, skeleton dense structure and higher amount of binder are to be selected to improve the fatigue life of structure layer.Through enormous amount of theoretical analysis and experimental studies, structure of durable asphalt pavement with composite base was confirmed and a trial road section was built. By in-situ tests on the trial road section, modulus values of all structure layers were inverted and the pavement structrue was calculated. It was found that stress and strain values of the durable asphalt pavement with composite base can meet the design requirements and it is safer than other structures. As analysis of life-cycle cost show, in 40 to 50 years of design period, great economic benifit of durable asphalt pavement with composite base can be expected.