| In recent years, in order to make better use of resources of existed roads and reduce the impact to the environment, cold-recycled techniques for asphalt pavement bases have got more and more extensive application in highway maintenance and reconstruction in the world. However, the application in roads which used the cold-recycled techniques in the asphalt pavement base shows that its service life is obviously lower than that of the conventional semi-rigid pavement, and the differences will be more and more obvious with the increase of the traffic volume and vehicle loads.The fatigue life of a new asphalt pavement mainly depends on the fatigue property of both asphalt concrete materials on the surface and the base, under the action of comprehensive effect of traffic load and environmental factors, In the current bituminous road design standard of China (JTG D50-2006), it gives clear rules on the service life of the road, but no corresponding rules about the calculation formula can be found to calculate the fatigue property of asphalt pavements using cold recycled materials on road bases. Although the strength of base materials constituted by aggregates or binder meets the lowest limit according to the standard, its fatigue property under traffic loads needs to be further investigated to clearly understand whether it meets requirements because of absolute differences from cement or fly ash stabilized base materials in the new road. Therefore, it has important theoretical significances and application values since the deep reaearch on the fatigue property of cold recycled materials is the foundation of scientific use of the cold-recycled technology, and it is also a problem needed to be clear and solved in design and management as well as management of asphalt pavement.This dissertation focuses on the fatigue property of stable cold-recycled base materials with the inorganic binder (cement), and the experimental researchs about the fatigue property and the related road performance are performed. The two types of cold-recycled road bases, which are RAP-BS type (which is made of the surface and road base materials of the existed road) and RAP-S type (which is made of surface materials of the existed road) respectively, the cement and fly ash are used as binder in the test. The fatigue mechanics model of asphalt pavements with cold recycled materials is set up based on the fatigue test results. Based on the elastic multi-layer system theory and the fracture mechanic theory, the fatigue property was analyzed and the fatigue design parameters of asphalt pavements with cold-recycled materials was put forward and the main research contents are as follows:(1) The experimental study on the road performance about cold-recycled base materials was performed. The test contents included researches on the unconfined compressive strength, the cleavage strength and the resilient modulus. The experimental results showed that the unconfined compressive strength, the cleavage strength and the resilient modulus of both the two types of cold-recycled materials met the corresponding rules in standards about the recycled technology of asphalt pavements for highway (JTG F41-2008). When the dosage of cement reaches five percent, the unconfined compressive strength of cold-recycled mixture in seven days used in the test met the technical requirements of various highway bases and the road bases lowered than level two, but cannot meet that of the highway of level one, according to corresponding rules.(2) Fatigue property of cold-recycled base materials was developed. The fatigue tests were performed by using specimens with non-recycled traditional semi-rigid base materials, RAP-S type and RAP-BS type materials added little new aggregate and RAP-S type and RAP-BS type materials added new aggregate by the proportion of ten percent to forty percent, respectively. The test results showed that the fatigue property of the cement-stabilized base with pure cold recycled materials was not better than that contained twenty percent gravel, the fatigue property of cement-stabilized base with cold-recycled materials contained twenty percent gravel was lower than contained forty percent gravel. Within the range of the test stress ratio, the fatigue property of the cement-stabilized base with cold-recycled materials contained gravel is better than that with pure cold-recycled materials, and the higher the stress level was, the smaller the difference of the fatigue life between the two type materials was. Conversely, the larger the difference of the fatigue life between the two type materials was.(3) The structural model with elastic multi-layer system of the asphalt pavement was set up with cold-recycled base materials, based on levels of the typical asphalt pavement structure and the basic mechanical parameters of cold-recycled base materials. The mechanical response law of the pavement structure with cold-recycled base materials under the vehicle loads was analyzed, by using finite element method. According to vehicle load calculation results, the fatigue damage of the cold-recycled asphalt pavement occurred at the bottom of the base at the initial stage. The tensile stress at the bottoms of different types of base materials was arranged from big to small, in order of the cement-stabilized gravel base materials, cold-recycled base materials with forty percent gravel, cold-recycled base materials with twenty percent gravel and pure cold-recycled base materials. The temperature stress of different pavement structures was analyzed considering the influence of temperature changes during a year, based on the temperature shrinkage coefficient of the materials. Although the stress affected by temperature in the structure of cold-recycled pavement cannot be ignored, the main factor leading to the changes of stress was still vehicle load.(4) The fatigue model with elastic multi-layer system of asphalt pavement was also set up, based on the results of cold recycled base material fatigue test. Using the results of finite element analysis and combining with the current standards, the fatigue equation was established in terms of allowable tensile stress and load times. Considering vehicle and the coupling action of temperature, the modified coefficients of temperature were introduced which indicating the effects of temperature stress. The analysis results showed that the allowable stress of all of the base materials reduced along with the increase of load times. The reduced rate of cement-stabilized base with cold-recycled materials was higher than that of conventional semi-rigid base materials, which meant that conventional semi-rigid base materials was better than that of cement-stabilized base with cold-recycled materials in fatigue property. Through the comparison with experimental road, it can be found that the fatigue life of conventional semi-rigid base materials was as twice as the cement-stabilized base with pure cold-recycled materials, and the fatigue equation was closer to the real situations when considering the temperature stress.(5) Based on fracture mechanics and Forman formula, and combined with the tests of cold-recycled base materials, the fatigue equation was established for the application of cold-recycled base materials. Through the numerical analysis, it can be concluded that the initial cracks and the change of stress amplitude had a major influence on the fatigue life. The fatigue life significantly reduced with the increase of initial cracks or stress amplitude. The fatigue reliability function was established by using Weibull distribution, and the established model was used to calculate the reliability of fatigue life of cold-recycled materials. The results showed that the pure cold-recycled base materials can get good fatigue reliability only when it suffers light traffic load and the fatigue reliability can be improved by adding a percentage of gravel.(6) The fatigue design index of cold-recycled base asphalt pavements was analyed. The fatigue design index of asphalt pavements with cold-recycled base was put forward based on the results of experimental and theoretical researches. It was reasonable to make the bottom stress as an index to control the fatigue life of cold-recycled base. The fatigue properties of cold recycled materials can meet design requirements, when prameters of material, the loading, and the environmental influence were all in a reasonable range. |
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