The motorway and trunk road system in England has a total length of over 12,000 km and an asset value of £60bn. Extrapolating this to the whole of the UK road network of some 400,000 km and allowing for the much lower value per km of non-motorway/trunk roads gives a total highway asset worth some £600bn. Maintaining and rehabilitating this asset, while at the same time sustaining undisturbed traffic flows, has placed increased emphasis on the need for high-performance and increasingly more durable pavement materials. The majority of roads in the UK and throughout the world are constructed using asphalt mixtures with over 340 million tonnes being produced in Europe in 2007. The most important factor influencing the durability of asphalt mixtures is the presence of water in the pavement structure and the detrimental effect that water has on the properties of the mixture. Moisture-induced damage is an extremely complicated mode of distress that leads to the loss of stiffness and structural strength of the asphalt and eventually to the costly failure of the road structure. An improved understanding of moisture-induced damage in asphalt and more moisture resistant materials could have a significant impact on road maintenance expenditure, particularly where rainfall is predicted to increase due to global warming. In this project, for the first time, the micro-mechanical processes that result in moisture induced damage at meso- and macro-scale in asphaltic pavements, will be analysed in a comprehensive manner in which both cohesive and adhesive types of damage will be addressed and evaluated as a function of the physio-chemical characteristics of the components of the asphalt mix. This project will involve the use of X-Ray CT to characterise the internal microstructure of the asphalt, the development of tools for the processing and conversion of these images into accurate 3D finite element meshes which will then be ised in a Finite Element simultion to investigate moisture damage in asphalt. A significant experimental programme will be required to determine the mechanical properties of the asphalt mixture components (and interfaces between the components) required by the FE analysis. From the combined experimental and computational analyses it will become possible to reach unprecedented insight into the dominant parameters controlling moisture induced damage in asphaltic mixes. On the basis of the conclusions of the combined numerical-experimental studies, recommendations for practise shall be drafted focused on the improvement of the moisture resistance of typical asphalt mixtures and contributing thus to the sustainability of the UK road network.

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The overall aim of this research is to use a combination of thermodynamic surface free energy and adhesion fracture energy measurements to understand, predict and enhance the resistance to moisture-damage of asphalt mixture pavement materials. Moisture-damage of asphalt mixtures is directly associated with the adhesive and cohesive properties of the material and how the presence of water affects these mechanisms. Although mechanical test procedures exist to quantify the moisture-damage of asphalt mixtures, they do not measure the fundamental material properties related to adhesion and cohesion. This study will use a combination of adhesive fracture energy measurements on bitumen-aggregate and bitumen-filler mastic-aggregate systems using monotonically-loaded tests together with intrinsic adhesion calculations based on thermodynamic surface free energy concepts to produce a step change in the moisture-damage performance and material screening of asphalt mixtures. The introduction and development of these new methods and novel approaches will provide the tools needed for the better selection and moisture-damage prediction of appropriate pavement materials. The study will involve collaboration between researchers working in the areas of pavement engineering materials and the mechanical engineering aspects of adhesion, adhesives and composites. This combined approach will allow the exceptionally high expertise in asphalt technology, moisture-damage characterisation, surface energy and adhesive bond testing and modelling to contribute effectively to improving the understanding and prediction of moisture-damage in asphalt mixtures and thereby provide a tool to achieve the project goal of enhancing moisture-damage performance.