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A nondestructive evaluation (NDE) method is developed to detect the dielectric constant and surface roughness of concrete highway pavement. The system is based on Leader's slope-expansion scattering theory. Different from most other scattering theories, the slope-expansion method does not pose stringent requirements about the nature of the rough surface. Yet, it can calculate the in-plane scattered field (or power), including the nonzero slope terms up to the second order. The explicit expression of the scattered power is given, provided that the incident wave is plane wave, and the surface is Gaussian distributed. A microwave scatterometer operating in X-band is implemented. The measurement system consists of a pair of horn antennas. The measured scattered power is used as the input to the software application. The program then computes the dielectric constant and surface roughness inversely by the least square inversion technique. Several samples have been measured in the laboratory.To verify the test results, a laser profiler and a radar system were used to provide a direct measurement result.Index Terms-Dielectric constant, Gaussian distribution, plane wave, roughness, rough-surface scattering, slope expansion. Ce Richard Liu (M'90-SM'97) received the B.S., M.S., and Ph.D. degrees in radio engineering from