Moisture Diffusion Coefficients

Moisture diffusion measurements are still not performed very commonly for as­phaltic materials. There are currently two main test procedures being utilized. The first is an overall measurement of the increase of weight as a sample is ex­posed to a controlled moisture conditioning (Cheng et al., 2003). The second is a slightly more complicated procedure using Fourier transform infrared spectroscopy (Nguyen et al., 1992). It is quite challenging though, to utilize the available test data, since the values can vary greatly. For instance, for the AAD-1 bitumen, values have been published ranging from 4.79 mm2/h (Cheng et al., 2003) to 9.0 x 10-5 mm2/h (Nguyen et al., 1992). Comparing these values with published diffusion coefficients of, rubber, PVC and polyethylene (Abson & Burton, 1979), the lower diffusion value for the AAD-1 asphalt binder (bitumen) seems to be more plausible. Since the mastic matrix in an asphaltic mixture generally consists of asphalt binder as well as sand particles and filler material, a higher diffusion value than for the binder alone can be expected. In the simulations described in this chapter, a diffusion coefficient of 1.0 x 10 3 mm2/h has been utilized for the mastic film.