Concrete Base Courses

A typical concrete pavement design will include the concrete slab on the surface, supported by a base course (or two) of aggregate soil materials, all of which are placed upon the prepared subgrade (foundational) soils of the roadbed.  Concrete pavement is discussed in more detail in Article 32.13.13.

Excessive moisture in the base layer can create structural problems for the concrete above the base course, such as pumping, heaving, reinforcing steel and dowel rod corrosion, cracks, and corner breaks. Most of these problems are particular to rigid (concrete) pavements. Heaving occurs when ice particles or ice lenses are formed in base courses, causing an expansion that pushes upward on the bottom surface of the concrete. Heaving can cause significant elevation shifts at the surface, resulting in uneven pavement edges that affect the smoothness of the pavement. Heaving also makes the pavement edges vulnerable to repeated tire impact stresses that may result in concrete corner breaks. Cycles of high moisture and drying throughout the pavement system facilitate corrosion of the reinforcing steel in the concrete and dowel rod connections between adjacent pavement sections. Cracks can form in the pavement if water beneath has washed away fine particles from the base soil matrix, causing voids and areas of reduced base course strength. With heavy loads, over time this can result in overstressing the concrete, and can lead to the formation of cracks. Pumping occurs when water trapped beneath the pavement discharges at the surface through pavement joints and edges at times when loads are applied. Pumping can likewise result in a loss of fine soil particles from within the base course.

To mitigate these issues, the best base courses are usually a mixture of gravel or crushed stone, sand of various sizes, and a small portion of fine soil particles to serve as a binding agent. Chemical additives can also be used to serve as a binder. Soil treatment is discussed in greater detail in Article 31.31.00.

Concrete base courses may be designed with lateral tie-ins to screened and perforated drainage pipes buried beneath the pavement edges to provide additional moisture control.