Evaluation of Fly ash Treated Reclaimed Asphalt Pavement for Base/Subbase Applications

Reclaimed asphalt pavement (RAP) is a term given to removed and/or milled pavement materials containing asphalt and aggregates. This material is obtained from recycling the existing flexible pavements which have reached the end of their design life or shown excessive cracking. The use of RAP has grown widely reducing the use of virgin material thus helps to conserve the natural resources. It also reduces the amount of construction debris reaching the landfills. RAP can be used in construction of base/subbase of a new pavement which results in huge savings. A 100 % RAP cannot be used directly in base/subbase layers because of its inferior properties such as unconfined compressive strength (UCS), California bearing ratio (CBR) and resilient modulus (Mr). This is because RAP material has a very low or limited bonding as the particles are coated with aged bitumen. These properties can be improved by using RAP in combination with virgin aggregates (VA) and/or calcium based additives such as lime, cement or fly ash. Calcium rich fly ash and a fraction of VA have been used in the present study to evaluate the strength in terms of UCS and stiffness in terms of Mr of RAP to promote as a pavement base/subbase material. The RAP material was collected from an ongoing national highway (NH) expansion project in India. Class “C” fly ash was collected from Neyveli Lignite Corporation (NLC), Tamil Nadu for stabilization studies. Specimens were prepared with 100 % RAP, 80 % RAP + 20 % VA and stabilized with 10, 20, 30 and 40 % dosages of fly ash by weight. In this study, emphasis is given to the mixing methodologies adopted to amend base materials and stabilizer. Traditionally, addition method is preferred for stabilization studies and replacement is preferred for mixture of two materials by weight of the principal material. Two methodologies were compared in terms of results obtained from the Mr and UCS studies and a balanced method was suggested. To know the level of interaction of fly ash with the bitumen coated aged aggregate, X-ray powder diffraction (XRPD) and Scanning Electron Microscope (SEM) studies were also performed. It was observed that the balanced addition method of mixing the stabilizer by weight of principal material would be more beneficial in terms of the strength and stiffness of the mixes. The new design mix consisting of 80:20 proportion of RAP and VA stabilized with 40 % fly ash by weight of the total mix, has met the design specifications laid down by Indian roads congress (IRC) for a base/subbase material of low volume roads.