The traffic of vehicles on the surface of roads yields deformations in a pavement structure that are a function of both travelling loads and the mechanical characteristics of the pavement. With the cyclic application of traffic loads, these deformations degrade the pavement and irregularities, ruts, and longitudinal asphalt cracks appear on the surface. The structural strength of flexible pavement is related to its constitutive elements: the asphalt layer, the granular base, the in-situ subgrade soil and also the geosynthetic reinforcement. Geosynthetics are commonly used in flexible road base reinforcement at the interface between the aggregate base course and the subgrade. This paper advises the design steps for asphalt concrete flexible pavements, utilizing the current AASHTO design manual, modified to account for the structural contribution of integral extruded geogrids. The modification of the AASHTO method due the use of biaxial geogrids for reinforcement of flexible pavements is based upon extensive laboratory testing and has been verified by means of several full scale tests by different authors. The empirical evaluation of the structural contribution of geosynthetics reinfofcement to pavement systems has allowed the development of a sound design algorithm. The data collected have been conservatively analysed and a full design methodology has been generated, applicable only to high strength stiff integral geogrids having high tensile modulus, junction strength and characterized by great interlock capacity. Existing design methods for flexible pavements include empirical methods, limiting shear failure methods, limiting deflection methods, regression methods and mechanistic-empirical methods. The AASHTO method is a regression method based on empirical results from AASHTO road test conducted in the 1950s.
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