Portland cement concrete paving is also referred to as rigid paving. Concrete pavement is supported by a base layer of compacted aggregates or treated soils, which is in turn placed upon the foundational subgrade soils of the prepared roadbed. If two base layers are utilized, they are referred to as the base and sub-base layers (or courses). Rigid (concrete) pavement differs from flexible (asphalt) pavement in that rigid pavement is designed to carry traffic loads within the pavement layer itself.
One of the most popular concrete pavement design approaches in the United States is the American Association of State Highway and Transportation Officials (AASHTO) method, although other methods will all involve the same basic design considerations: magnitude and repetition of loads experienced during the pavement’s service, desired pavement condition at the end of its service, the type and characteristics of base and subgrade materials beneath the concrete, and the concrete mix’s designed strength. Evaluating these parameters allows for a determination of required concrete pavement thickness and features, including doweled connections between adjacent pavement sections and reinforcing steel in the pavement slab.
Heavy vehicles such as trucks provide much greater potential damage to concrete paving than automobiles. For this reason, concrete pavement design often begins with an assessment of the amount of truck equivalent single axle loads (ESALs) that a pavement will experience during its lifetime. For average accuracy and consistency, a truck axle using the AASHTO method is considered to be an 18 kip load. (A “kip” is a thousand pound multiplier applied to static forces, so an 18-kip load is an 18,000 pound load.) Each passage of a truck axle load across the concrete pavement’s surface causes incremental structural damage to the pavement system. Estimating the extent of these load applications (often projected as several million during a pavement’s service life) based on traffic predictions is important.
The base course (and possibly a sub-base course) beneath a concrete pavement is designed for moisture control and stability against the creation of pockets or voids. For this reason, concrete base courses often contain a mixture of large soil particles for strength and permeability, and smaller soil particles for stability and cohesiveness. Base course soils are sometimes treated with small percentages of cement or lime additives to better achieve these desired qualities. Article 32.11.23 Aggregate Base Courses and Article 32.11.36 Concrete Base Courses provide greater details regarding pavement base courses. The AASHTO design method assigns values to base courses for their ability to resist deflection (called an AASHTO k-value) and a drainage coefficient to rate its ability to remove moisture from under the pavement. Buried drainage systems linked to the base layer(s) are often designed along the edges of a concrete pavement system to assist with water removal. The foundational sub-grade soils beneath the base layer(s) are similarly evaluated and scored by the AASHTO design method.
The AASHTO design method considers other pavement features, such as joint treatments and the interfaces between adjacent pavement sections, through their established load transfer coefficients. Many modern pavement designs call for adjacent slabs to be "doweled" into each other with reinforcing steel sections of about 18 inches in length. Many designs also include reinforcing steel (mesh or bars) within the pavement cross section to provide strength in tension when loads are applied.
Assessing these factors using the AASHTO method, or a similarly effective approach, results in an optimal concrete design thickness for a planned pavement construction project. Concrete pavement thicknesses ranging from 8-14 inches are common, reflecting particular loads and site conditions. Concrete pavements are generally classified by the type of reinforcing used and the type of connections that exist between adjacent pavement slabs. Concrete shrinkage and cracking can be controlled through the placement of isolation (full depth) joints and control (partial depth cut) joints throughout the pavement area.