It is a well-known empirical fact that an asphalt mixture with a high percentage of air voids fatigues rapidly. Under repeated loading, air voids grow into cracks. A void or a crack radius follows a Weibull distribution, from which it is possible to determine both the mean air void or crack size and its variance. The ability to measure the size distribution is important in all models of aging and moisture damage. Two mixtures with the same mean air void radius may have strikingly different rates of crack propagation, aging, and moisture damage because of the size of the variance of the air void or the crack size. This paper demonstrates how to make an accurate determination of the air void and subsequent growing crack radius distribution with X-ray computed tomography (CT) and a repeated direct tension test. The proposed Weibull distribution model is more realistic than the model developed solely on the basis of results from the X-ray CT system, which does not take into account air voids and cracks smaller than the minimum detectable air voids and cracks. The accurate measurement and determination of the initial distribution of air void sizes and the subsequent distributions of the growing cracks are necessary to the development of realistic predictive models of aging and moisture damage as well as of fatigue and thermal cracking.