An aggregate base course is the layer of soil installed between the surface pavement layer and subgrade (foundation soils) of either flexible (asphalt) or rigid (concrete) pavement. Their purpose is to help transition surface loads from the pavement to the subgrade, evenly support the pavement surface profile, and protect the overall pavement system from water intrusion and deformation. While these objectives of base course layers are applicable to both types of pavements, load transfer is a more focused design consideration for flexible pavement, while moisture control is a priority for rigid pavement designs. Concrete base courses are further discussed in Article 32.11.36.
In order to adequately serve their function, aggregate base course soils must contain enough larger soil particles (gravel, crushed stone) to provide strength and permeability, yet enough fine particles (clay, silt) or additives (chemical stabilizers) to maintain their cohesion and structure under loads. For this reason, aggregate base courses are usually a graded mixture of soil elements (meaning they contain various sized particles) that bind crushed stone or gravel in a matrix. If desired base course attributes need to be especially emphasized in a pavement design due to challenging site conditions, sometimes two courses of base materials are placed, each having different soil constituents. If two layers of aggregate base courses are installed above the subgrade, they are then referred to as the base and sub-base layers. Soils treated with asphalt, Portland cement, or other stabilizers (including filter fabrics) may also be used to enhance or maintain the characteristics of base course soils. Soil stabilization is discussed in greater detail in Article 31.31.00.
Specific strength concerns include proper base course compaction, and the load support characteristics of the selected base course materials. The base course, particularly with flexible pavements, must distribute traffic loads over a greater area below. When the soil particles are placed under loads, they will tend to bind together due to friction and interlocking. This occurs in roughly an inverted cone-shaped pattern beneath the source of a load (a car tire, for example), or during compaction. Enough fine soil particles in the base course will allow for loads to be transferred more efficiently at many points of contact between the larger soil particles (gravel, stone).
Specific moisture control concerns include the capillary action of water into the base course, any seepage flow of water through the base course, and excessive moisture filtering below to the subgrade soils. These conditions can compromise the strength of the pavement system. With capillary action, much like dipping a piece of paper into a glass of water, moisture will wick up into a base course from moist subgrade soils below. This can allow moisture in the base course to accumulate to detrimental levels. Moisture in the base course can likewise develop if the pavement surface is not waterproofed sufficiently. Either way, the base course can become saturated, and as water then seeps laterally or vertically through it, fine soil particles can be washed out. This leaves void spaces that reduce the load bearing capability of the base course soil matrix. In colder climates, where cycles of freezing and thawing occur, excessive moisture can result in trapped water that forms ice lenses, which then expand and contract the pavement courses, causing damage. Water that seeps downward through the base to the subgrade will change the foundational (subgrade) soils’ moisture content. This can locally reduce subgrade soil strength, creating settlement and potholes.