Slaking of clay-bearing rocks (shales, claystones, mudstones, siltstones), upon interaction with water, causes numerous problems in engineering construction. The slaking behavior of clay-bearing rocks is generally evaluated by three tests: the jar slake test, the slake index test, and the slake durability test. Among these, the latter is the most commonly used test. When the slake durability test is performed on a sample of clay-bearing rock, the material retained in the 2-mm mesh drum consists of disintegrated fragments of various sizes. The American Society for Testing and Materials method for the slake durability test requires that the retained fragments be categorized as type I material (primarily large fragments), type II material (mixture of large and small fragments), or type III material (primarily small fragments). This paper presents a new quantitative method of describing the nature of material. The method quantifies the fragment size distribution of the slaked material in terms of “disintegration ratio,” defined as the ratio of the area under the fragment size distribution curve to the total area encompassing the curve. A durability classification system based on disintegration ratio is recommended for assessing the durability of clay-bearing rocks.