Design Considerations for a Permanent Granular Subbase Under Concrete Pavements: Stability, Drainability and Durability Requirements

Subbase layers under concrete pavements play several functional roles in providing a uniform support, minimizing pumping potential, facilitating drainage, and minimizing adverse effects of erodible and problematic subgrade soils. This paper presents a design alternative for a daylighted granular subbase that provides uniformity, drainability, stability, and durability for jointed plain concrete pavements in Illinois. The performance trends of concrete pavements with unbound granular layers in Illinois were first evaluated. Several case studies of well-performing concrete pavements with granular subbases, high traffic levels, and low distress levels and severity were observed. Next, the practices of surrounding states were evaluated, and several Midwestern states were found to regularly use unbound granular layers under concrete pavements with no negative performance implications. Based on the case study reviews and evaluations, a stable, drainable, and durable daylighted granular subbase design is recommended for traffic levels up to 10 million equivalent single axle loads. Stability of the functional and structural layer is ensured by limiting the ratio of gravel-to-sand fractions in the aggregate mix ranging from 1.3 to 1.9. Drainability requirements are met by limiting the percentage of fines passing the No. 200 sieve (0.075 mm) to 4% and maintaining the quality of drainage at least in fair condition based on the time required to drain 50% of the water. Lastly, a geotextile fabric is recommended for use below the granular subbase for separation to ensure drainability and maintain durability, or structural integrity, of the granular subbase throughout its design life.