, Professor, Civil, Environmental and Geo-Engineering
In order to design a successful hot mix asphalt (HMA) mixture, several factors must be considered, including rutting, thermal cracking, fatigue cracking, and durability. Low durability often manifests itself as fatigue or thermal cracking, but is generally associated with asphalt aging and/or film thickness. The Minnesota Department of Transportation (Mn/DOT), like many other owner agencies, currently uses voids in the mineral aggregate (VMA) as a volumetric criterion to ensure durable HMA in the design and construction process. VMA is simply the air and the non-absorbed asphalt by volume of an HMA mixure. A limited number of owner agencies specify film thickness; one of the major benefits of film thickness over VMA is that the calculation recognizes varying gradations (e.g. fine versus coarse) and the corresponding surface areas. The VMA concept and corresponding calculation were first proposed in 1956 and this is in essence what is still used today. Issues that surround both film thickness and VMA calculations are that the calculations as they are currently used have never been measured and thus proven to exist as calculated. More recently, alternative methods for calculating film thickness that consider particle shape have been proposed; the current method only considers particle size.