Study On The Design Index And Method Of Long Life Asphalt Pavement

The semi-rigid base course asphalt pavement structure has been mostly adopted in China. But the damages of asphalt pavement in expressway are serious and shorter than design period. In view of this condition, we can use long-life asphalt pavement to solve this problem. On the basis of study on the design index, allowable standard and structure-design method, the paper hopes to constitute the design method system for long-life asphalt pavement in China. The main contents and conclusions of study are as follows:The first chapter discusses the background and signification of the long-life asphalt pavement. Through reviewing the development of the design index and design method for asphalt pavement and the study of long-life asphalt pavement at home and abroad, it concludes that the study on the long-life asphalt pavement is meaningful and important. Finally, the contents of study in the following chapter are determined.The second chapter researches on the structure of the long-life asphalt pavement. Through studying on foreign design ideas of long-life asphalt pavement, the long-life asphalt pavement for our country is defined. The semi-rigid sub-base asphalt pavement and granular sub-base asphalt pavement are suitable for long-life asphalt pavement. Based on the layer elastic theory, orthogonal design and range analysis, the sensitivity of the different layer parameters to pavement response are analyzed. The results indicate that the thickness of the asphalt layer has much influence on the tensile strain at the bottom of the asphalt layer and the compression strain at the surface of the sub grade. So 24 cm is suggested as the minimum thickness for the long-life asphalt pavement.Structural analysis indicates that the high modulus binder layer is not only attributed for rutting resistance, but also helpful for decreasing the tensile strain at the bottom of the asphalt layer and the thickness of the asphalt layer. According to the function and the mechanical character, the design principle and the requirement of the material for different layers of the long-life asphalt pavement are ascertained. And the representative structural design plan is recommended.Due to the significance of permanent deformation for long-life asphalt pavement, the third chapter studies on it. The permanent deformation of the long-life asphalt pavement should be controlled according to the type of the structure. For the semi-rigid sub-base asphalt pavement the permanent deformation of the asphalt base and the grading macadam course should be restricted; and not only the asphalt base and the grading macadam layer, but also the sub-grade should be controlled for granular sub-base asphalt pavement. Based on modifying to AASHTO model and numerical simulation, the permanent deformation forecasting models for the asphalt base, grading macadam layer, the granular layer and the sub-grade are constituted. The method and process of forecasting permanent deformation are put forward.The forth chapter studies the character of the fatigue limit of different asphalt mixtures. On the basis of testing on the compacted hot-mix asphalt subjected to repeated flexural bending, the relationship between the flexural stiffness, the dissipation energy, the accumulative dissipation energy and the number of repetition are established.According to AASHTO T321 and the simplified dissipation energy ratio, the fatigue behaviors of the different asphalt mixture are analyzed. The results indicate that fatigue limit exists in the asphalt mixture. Given the same test condition and only changing the binder content, fatigue limits for the mixtures with basic asphalt are more or less the same. The limits value is about 100μεfor the optimum asphalt content and rich asphalt content (optimum+0.5%). But the value is clearly higher, and about 100με-200μεfor the mixture with modified asphalt.At the same time, the influence of overloading to fatigue limit is studied through the low strain and high strain alternate fatigue test. The outcomes make clear that limited overloading should not destroy the fatigue limit characteristic, if certain over loading are considered during the structural analysis and design period.The fifth chapter introduced the method for structural design of the long-life asphalt pavement. At first the aim and principle for structural design is discussed. It concluded that the structural design should meet the requirement of the heavy traffic in the future. Based on the analysis of the deflection in current asphalt pavement design criterion, it concludes that this index can not evaluate pavement performance correctly even though it is convenience to measure and easy to master. So it is not suitable for the design index of long-life asphalt pavement, but can be used for construction control. On the basis of analysis of many structure combinations, a simple equation of deflection at the surface of different layers is created by mathematical statistics.It concludes that the design indexes for long-life asphalt pavement should include the tensile strain at the bottom of the asphalt layer, the compression strain at the surface of the sub grade and the permanent deformation of structure. It is suggested that tensile strain limit can use 100μεand 110μεfor optimum asphalt content and rich asphalt content (optimum asphalt content+0.5%) of the AC25 mixture with basic asphalt respectively, but 145μεfor modified asphalt. The compression strain criterion at the surface of the sub grade is 170μεPermanent deformation of structure is 12mm and 20mm for the semi-rigid sub base asphalt pavement and the granular sub base asphalt pavement respectively. At last, the design process and calculation example for the long-life asphalt pavement are given.Finally, the test road of long-life asphalt pavement is introduced in the sixth chapter. Based on laboratory study and theoretical analysis, four different structures are chosen. The materials for different layers are analyzed and determined in response to the project and the design requirement of long-life asphalt pavement. And the main points of quality control of materials are also discussed. The modulus of resilience and compressive stress at the surface of the sand base course are also tested. Through analyzing the difference of cost between test road and normal road, it concludes that the life cycle cost of the long-life asphalt pavement is lower.