This experimental project will address the problem of wheel track rutting that develops in asphalt road pavements under repeated traffic loading. A new torsional Hollow Cylinder Apparatus will be developed to reproduce, more accurately than hitherto, the field loading regime in the laboratory, so that high quality measurements can be made of the permanent strain that accumates under cyclic loading. Collaboration with the University of California at Berkeley and at Davis will allow use to be made of their established but less accurate asphalt shear testing equipment using identical material. Pilot scale wheel tracking tests will be conducted in the Nottingham Pavement Test Facility to generate rutting performance data and use will be made of full scale test data from the Californian team. The outcome of the project is aimed at improving prediction methods for rut development in asphalt pavements and to assess the reliabilty of a simple practical test for use by industry to estimate the rut resistance of asphalts.

The UK's road network totals over 250,000 miles of paved roads providing a means for efficient distribution of goods and services, economic security and social prosperity. The entire road network has an asset value of £750 billion and as the UK's main transport infrastructure provides a vital service to road users, commerce and industry. However, the network requires constant upgrading, maintenance and rehabilitation with a predicted spend of £181bn required over the next 20 years. Over 95% of these paved roads are constructed from asphalt mixtures which comprise three principal components, namely, mineral aggregates (microns to centimetres), natural or added filler (< 63 microns) and bitumen (film thickness 10-20 microns). However, in spite of their importance, the deterioration of asphalt mixtures has never been fully understood or accurately predicted. The key reason is that the current means of assessing and predicting adhesive behaviour between the bitumen and the mineral aggregates does not account for size effects at the different material dimensional scales. These size effects result mainly from the variations of the bitumen film thickness, mineral surface roughness, air void radius, bitumen polarity distribution (molecular sizing) and mineral compositional distribution. Neglect of these size effects makes it impossible to accurately predict the asphalt mixture's distresses such as material fracture (traffic load induced fatigue cracking, non-load associated thermal cracking and age related cracking), moisture damage susceptibility (material disintegration and softening, stripping and fretting), potholes and other forms of severe surface deterioration, all of which are directly affected by the bitumen-mineral interfacial adhesive properties. The project aims to develop an overall 'adhesion analysis framework (AAF)' focusing on the measurement and prediction of interfacial adhesive properties between bitumen (binder) and mineral aggregates in asphalt mixtures using a size-affected multiscale experimental and modelling approach. Using a combination of experimental techniques, adhesion theories and material modelling approaches, size-dependent and size-independent material properties will be determined and scaled up from nano to micro to macroscale to predict the bitumen-mineral interface adhesive debonding properties of a range of asphalt mixture types. The research will use a combination of microscopy and spectroscopy imaging and molecular dynamics (MD) modelling at the nanoscale to predict bitumen-mineral interface adhesion and a range of size-independent material properties. The viscoelastic Griffith energy equilibrium principle will then be used at the microscale to produce a mechanics-based debonding initiation criterion incorporating the critical material size effects and the size-independent material properties obtained from the nanoscale MD simulations. The theoretical bitumen-mineral debonding criterion will then be verified by means of pull-off adhesion and cohesion testing incorporating different materials and size effects as well as loading and environmental conditions. The final scaling up effect will deal with crack (debonding) propagation developed through a Paris' law propagation model incorporating both size-dependent and size-independent materials parameters determined at the nano and microscales. These theoretical predictions will then be experimentally verified by a novel 'sandwich-cracking' test with prefabricated initial cracking dimensions together with material and conditioning variables. Finally, all these different multiscale effects will be incorporated into a multiscale modelling hierarchy for predicting adhesive failure and overall material response and delivered as a web-based opensource software and database. This user-friendly software will be used to design and produce better and long-lasting asphalt materials to ensure long-term sustainability of this key national asset.

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.

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.

Abstracts are not currently available in GtR for all funded research. This is normally because the abstract was not required at the time of proposal submission, but may be because it included sensitive information such as personal details.