New Approaches for Computing Film Thickness

The objective of this study is to develop a more accurate measure of film thickness (FT). The concept of FT was introduced in the 1950’s and assumes that all the asphalt exists as uniform films. The equation was expressed in English units and uses total binder volume, weight of aggregate, aggregate surface area, and a constant to convert to microns. The equation was later improved to account for absorbed asphalt binder. Numerous studies over the years examined FT as a durability parameter, but lack of strong correlation usually eliminated FT as a primary parameter. Most studies applied the FT equation without any question about its accuracy. The procedure for determining surface area is part of the Hveem mix design method. The surface area values are not a direct expression of particle surface area and do not account for differences in aggregate specific gravity. This is a critical limitation. Studies that examine differences in FT values in an attempt to identify trends in HMA mixture performance are not comparing equivalent FT values when the mixtures have aggregates with dissimilar specific gravities. In 2003, a new procedure introduced a three-dimensional mixture model. The approach models the random orientation of size-graded spheres and distributes the binder into the remaining available space. It takes into account that FT is not a uniform coating of each particle. This study generates two different film thickness values. The first is an extension of the past practice of a uniform coating “index” and the second is based on the “virtual” three-dimensional model. The proposed Index Model uses the fundamental principles of weight, volume, specific gravity, and particle geometry to calculate a surface area of each aggregate particle and accounts for the differences between aggregate sources. However, it still uses the simplification that the surface area coated by the asphalt is a flat surface and each particle is separately and uniformly coated. The Virtual Model examines the spatial relationship between particles and requires knowledge of the mixture volumetrics, binder, and aggregate proportions. This three dimensional model achieves a film thickness value that approaches the true film thickness. If our primary interest in film thickness is to define the durability of the mixture, then the nominal thickness of the asphalt between the aggregate surface and the void space represents the minimal depth of exposed coating where binder aging occurs. The study ran a sensitivity analysis on a family of generic mixes and examined the differences between the Standard Model, the Index Model, and the Virtual Model. Each analysis isolated a mixture parameter to check the reasonableness of the proposed models. The analysis included gradation, aggregate specific gravity, amount and gradation of the mineral filler, particle shape, level of mixture compaction, and multiple source blended gradations. The sensitivity analysis demonstrates that the Standard Model does not accurately express FT. The proposed Index Model is a better two-dimensional approach. The Virtual Model is a new concept and can reflect differences in mixture density.