Layer thickness variability for flexible pavements in North Carolina

Ground-penetrating radar (GPR) was used to identify the variability in layer thickness for 13 general pavement study sites in North Carolina. Two data sets were collected for each of 11 sites and one data set was available for 2 additional sites. Asphalt thicknesses varied over a wide range, but the standard deviation for asphalt layers less than 6 in. (15.24 cm) thick was 1 or less. GPR results were consistent with North Carolina typical base course thicknesses, whether that base be asphalt, dense-graded aggregate, or cement-treated aggregate. Variability in base course thickness was greater than variability in the surface layer. The effects of changing layer thicknesses from the mean thicknesses to the mean 1 standard deviation were examined by backcalculation and by calculating overlay thickness requirements with the North Carolina Department of Transportation overlay design procedure. Overlay thickness requirements typically changed by 3.8 mm (0.15 in.) over the range of layer thicknesses. Backcalculated moduli for asphalt and aggregate base were found to be most affected when the asphalt layer is thinner and the base course standard deviation is large. It is concluded that it is important to have an estimate of layer thicknesses for overlay design but that extensive coring is not required to obtain an adequate estimate.