MECHANICAL PROPERTIES AND CRACK RETARDATION OF GLASS FIBRE MICROASPHALT

A project was undertaken in conjunction with Colas, a UK road design, maintenance and surfacing company with the use of pavement laboratory facilities provided by Liverpool John Moores University (LJMU). The focus of the project was to assess Colas’ conventional microasphalt surfacing product Ralumac 2000 with the inclusion of glass fibres with the aim of increasing the material’s resistance to reflective cracking. Ralumac is typically applied on top of an existing defective road surface in two consecutive courses. It was for the lower of these courses that the glass fibre was introduced to provide a reinforcing layer or SAMI (stress absorbing membrane interlayer). To achieve the project goal, it was necessary to collate previously unknown mechanical properties of conventional microasphalt (CM) and glass fibre microasphalt (GFM) in order to compare their attributes. An experimental and quantitative research approach was adopted, and a series of laboratory tests were carried out to determine stiffness, resistance to permanent deformation and fracture toughness by crack propagation. Due to the slurry nature of microasphalt an optimum curing technique was adopted to allow the material to gain adequate strength to undergo testing by Hot Mix Asphalt (HMA) standards. Optimum fibre quantity was also examined and 0.25 % provided the most favourable outcome. Further laboratory tests were carried out on CM and GFM, as a composite overlay to match site conditions, to determine wheel rutting resistance and its ability to resist reflective cracking by simulating crack extension by thermal cycling. Finite element modelling (FEM) of the composite GFM over a simulated crack was undertaken to determine the magnitude of displacement, stress and tensile strain at the crack tip whilst under wheel loading at varying temperatures (10 °C and 20 °C). Findings revealed that GFM exhibited lower stiffness but greater flexibility before failure and provided greater resistance in terms of permanent deformation, with wheel tracking rate and rut depth showing a 6 % improvement over CM. Under crack extension GFM demonstrated absorption of tensile strain with bottom-up cracks being delayed and dispersed away from the vertical crack propagation path. FEM corroborated the findings observed in the laboratory and revealed that the absorption of stress and strain in the GFM layer led to the reduction of stresses in the surface course by 21 % and 28 % at temperatures of 20 °C and 10 °C respectively