Base/Subbase Material Characteristics

Unstabilized bases, also frequently referred to as granular bases, are the most commonly used base types for concrete pavements. Adequately designed and properly constructed, unstabilized bases exhibit excellent field performance at a lower cost than stabilized bases. A wide variety of materials can be used as unstabilized bases, including crushed stone, sand-gravels, sands, and a variety of waste and byproducts. The materials for unstabilized base should meet the requirements of AASHTO M 147. In general, the materials for unstabilized base should meet the following criteria:

• Less than 10 percent passing No. 200sieve.
• Plasticity index of 6 or less and liquid limit of25 or less.
• Maximum particle size not exceeding onethird of layer thickness.
• Los Angeles (L.A.) abrasion resistance(AASHTO T 96) of 50 percent or less.
• Permeability of approximately 150 ft/dayand not exceeding 350 ft/day.

Limiting the amount of fines is the most important criterion for preventing pumping, base erosion, and frost action.

The AASHTO M 147 gradations shown in Table 1 were developed for both asphalt and concrete pavements, and the standard facilitates a rather wide range of gradations. All of the gradations except gradations A and C allow more than 15 percent passing the No. 200 sieve. Therefore, to utilize gradations B, D, E, and F, the requirement for percent passing the No. 200 sieve should be adjusted to limit the amount of fines.

The material requirements for cement-stabilized bases do not need to be as strict as those for unstabilized bases. As for cement-treated bases, which typically contain 2 to 5 percent cement, the material requirements may be relaxed to allow up to 35 percent passing the No. 200 sieve and a plasticity index of 10. All gradations specified in Table 1 without further modification work well for cement-treated bases. In addition, granular soils with plasticity index of 10 or less (more specifically, AASHTO classification A-1, A-3, A-2-4, and A-2-5 soils) may be used for these base types.

Lean concrete base, also known as econocrete, contains more cement than cement-treated base but less than conventional concrete. Due to the increased cement content, the material requirements may be further relaxed, allowing for the use of locally available, lower quality aggregates that do not meet the requirements for unstabilized base or conventional concrete.

Stabilized open-grade drainage layers have very little aggregate passing the No. 200 sieve. Asphalt cement contents typically range between 1.6 and 1.8 percent by mass of aggregates. Cement-treated open-graded drainage layers are typically produced with a water-to-cement ratio of 0.37 and a cement content of 185 to 220 lbs/yd3.

The material requirements for the asphalt-treated bases should generally follow the agency’s existing requirements for asphalt surfaces. Although a lower grade asphalt binder may be used for asphalt-treated bases, it is important to use durable aggregates to obtain satisfactory pavement performance.