The aim of this investigation is to raise awareness of the shear stresses and strains that develop during wheel loading of a pavement structure. The design of fully flexible pavements requires the strain at the base of the asphalt layer and at the top of the subgrade to be limited to a predetermined level, depending on traffic loading. However, research carried out at TRL and elsewhere, over the last decade, has demonstrated that the deterioration of thick, well-constructed, fully flexible pavements is not structural. Generally, the deterioration starts at the road surface in the form of rutting and cracking. The evidence suggests that roadbase fatigue and structural deformation, originating deep within the pavement structure, are not the prevalent forms of deterioration. Data collected from a range of pavement structures show that cracks start at the top and propagate towards the bottom of the asphalt layers. This contradicts the conventional design concept. This research work suggests the main contributory factors to surface cracking that result from wheel loading. The presence of shear in the vertical plane of the pavement is shown to be particularly important in most cases. The shear strain was found to exceed the tensile strain at both the base and the surface of the asphalt layer of the structures analysed. A 3-D finite element model was used to analyse the pavement structures. To realistically reflect the behaviour of pavements, infinite elements were also adopted at the structure boundaries. A three-dimensional 'wheel-pavement' loading interaction was adopted. Different parameters were studied, including the effect of wheel type. material stiffness and asphalt layer thickness on the stress and strain distribution over each of the pavement structures. The paper also describes the implication for maintenance and condition assessment procedures of pavement structures.

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