The performance of pavement is measured by how well a pavement meets functional objectives such as providing a safe, convenient and smooth traveling surface for vehicles; as well as structural objectives such as durability under all sorts of environmental and traffic conditions. Most pavement research is aimed at finding ways to design, construct and maintain pavements in order to maximize their performance and service life at the lowest possible cost.

    Traditional pavements are constructed in layers of material that must work together to achieve the expected performance. A pavement system includes natural subgrade soil, with selected or modified soil and several layers of processed material built on top of the subgrade. To achieve a successful pavement performance, all of these layers must complement one another and interact harmoniously.

    The AASHTO pavement design guide assigns a structural coefficient to each pavement layer material, and allows the substitution of one material with another one of equivalent thickness or “equal strength” for economic or other practical reasons. However, recent studies have shown that the interaction between layers is more complex than indicated by simple layer coefficients. While the exchange of materials may not appear to cause any harm to the pavement structure at first, it may eventually lead to pavement performance problems. Therefore, a pavement needs to be designed as an integral system, including the subgrade, and any deviation from the original design must be carefully evaluated for its appropriateness.

    Today, we will discuss the latest findings from the Ohio Research Institute for Transportation and the Environment on ways to select the best base material to achieve the highest possible performance of individual layers as well as the entire pavement system. We theorized that pavement structure design should consider not only layer strength, but also the interaction between layers. Base types considered included gravel (GB), lean concrete (LCB), asphalt treated (ATB), cement treated (CTB), and permeable asphalt treated (PATB) bases.

    Based on the findings of our research, it is recommended that engineers possess enough data to understand the soil properties for a given project and be able to design a suitable pavement structure for these highly variable conditions. The selection of base type for flexible pavements is sensitive to subgrade strength. So, engineers must have complete subgrade strength data in order to select the appropriate base for a given project.

    The construction of rigid pavement is not sensitive to subgrade strength. LTPP Data support the hypothesis that relative stiffness affects slab fatigue life. Therefore, it is most important to consider base-slab interaction when selecting an appropriate base for a rigid pavement project.