DEF Grilli Andrea PhD Thesis 2007.pdf

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Advanced Testing And Theoretical Evaluation Of Bituminous Mixtures For Flexible Pavements • Confinement of materials and enhancement of the load distribution capacity; • Enhancement of ductility at low temperatures; • Reduction of tensile stress. To note that the reinforcement behaviour in bound layers differs from the reinforcement behaviour in unbound layers. This thesis focuses on the use of geosynthetics in bound layers, hence, the following paragraphs deal with this specific issue. The reinforcement function in asphalt layers can be further summarized in: • Extension of fatigue life or reduction of thickness layers (Brown et al., 1985, Brown et al., 2001); • Elimination or limitation of reflective cracking (Cantabiano and Bruton 1991, Brown et al. 2001, Gillespie and Roffe, 2002, Elseifi & Al Qadi 2003, Mostafa and Al-Qadi, 2003, Montestruque et al., 2004, Sobhan et al., 2004); • Reduction of rutting (Brown et al., 1985, Komatsu et al., 1998). Fatigue failure occurs when a body is subjected to repeated loading cycles. The tyre weight causes a local bending of the asphalt pavement and tensile stresses are generated at the bottom of bound layers. Even though the induced-tensile stresses are lower than the failure strength of materials, the repeated action of these stresses reduces the material resistance and causes microcracking and then their propagation up to fracture. In this case, stiff geosynthetics contribute to absorb tensile stresses enabling longer service life. The reflective cracking failure typically occurs in asphalt overlay by propagation of existing cracks or discontinuities in old asphalt pavement. Repeated loading and thermal cycles generate tensile stresses on the tip of predefined cracks leading to premature failure. Once reflection cracking propagates the overlay becomes more susceptible to adverse environmental factors such as water infiltration and premature oxidation. In this case, geosynthetics give a contribution to resist against these tensile stresses at the interface and delay the rising of cracks. It is worth note that the correct action of a reinforcing system at the interface should satisfy both mechanical 26
Chapter 1. Literature review – Part I needs of multilayered road pavements in terms of bending and their shear performance (Brown et al., 2001, Caltabiano and Brunton, 1991). Rutting is due to the accumulation of permanent deformations or densification under loading cycles and high temperatures and appears as longitudinal depressions in the wheel paths. Rutting reduces the comfort of the pavement and, by affecting vehicle handling characteristics, creates serious hazards for highway users. In this case, the geosynthetic have to avoid plastic deformation and distribute shear loading. The most important characteristic of the geosynthetic is the geometry and the size of meshes and ribs which have to allow mechanical interlocking between the geosynthetic and the surrounding material. Currently, two different approaches are used to study the mechanical properties of asphalt pavements: empirical approach and theoretical approach. The empirical approach is based on physical principles, experiences and relative standard testing methods. This approach generally provides easy response related to the material behaviour but it can be rather approximate when applied to complex cases. The theoretical approach, such as finite element method or finite difference method, is based on numerical modelling which can deal with complex situations and offer different solutions. This approach can take into account the unhomogeneity and the non linear behaviour of materials. However, the reliability of this approach depends on the schematization of materials and on the constitutive law used. Both approaches have been used to understand the behaviour and the effects of geosynthetics in asphalt layers. The attempt of providing tensile reinforcement for asphalt pavements goes back to 1950’s. At the beginning all attempts experienced installation difficulties in laying the mesh flat but afterwards, with new technology, there were evidences of possible benefits related to the use of geosynthetics in road construction. However, the use of geosynthetics in asphalt layers is still considered a high-risk solution due to a lack of detailed design procedure. 1.1 Location of geosynthetics in asphalt layer Reinforcement can be placed in different alternative locations in a road structure. Investigations carried out at the Nottingham University established that the use of geosynthetics in asphalt layers could 27
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Part II Influence of water and temp
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Conclusions Conclusions This thesis
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Conclusions high tensile stress is
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Conclusions mixtures, higher air vo
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References References Al-Khateeb G.
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References the symposium on polymer
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References EN 12697-31, 2004. “Bi
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References Kennepohl G., Kamel N.,
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References pavement materials using
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