Incorporating Size Effects into Multiscale Adhesion Modelling of Bitumen-Mineral Interfaces

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.