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Focusing on sunlight that has severe effects on the degradation of asphalt pavement during service, this study examined the relationships between the film thickness of asphalt and the states of photodegradation. The results confirmed that thinner films had a higher modulus of elasticity, lower viscosity, and increased production of carbonyl groups, which is an indicator of oxidative degradation. The degree of degradation rapidly increased especially at thicknesses below 200 µm. The wavelength dependence of spectral photodegradation was also examined. Visual observations and the carbonyl index confirmed that ultraviolet irradiation in the range of 300 to 400 nm caused severe photodegradation of asphalt. Carbon black blocked light near the surface of the asphalt layer, where the damage caused by light is most severe. Addition of carbon black helps to prevent photodegradation by ultraviolet irradiation in the wavelength range of 300 to 400 nm.

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