Once the foundation is in place, actual paving begins. Asphalt is usually placed in layers, which can range from several inches thick to a fraction of an inch thick, depending upon the mix, position in the structure, and the purpose (A, C, and D in the figure).
Asphalt mixtures are like recipes, there are a lot of options — the size of the aggregates, the Performance Grade of the binder, recycled materials, additives, etc. — and the goal is to build a strong, long-lasting pavement. Ideally, the mixes should be designed and placed in a way that distributes the strains of traffic loading throughout the pavement in a way that localizes any distresses to the surface. If an asphalt pavement cracks at the surface (A in the figure), it is a fairly quick and easy process to mill off the surface and to replace it with a new surface, allowing the structure to remain in service indefinitely. If cracks happen at the bottom (regardless of pavement type), the pavement full depth of the pavement has to be replaced/repaired — a much more time intensive and costly proposition.
Asphalt mixes are produced at an asphalt production facility according to the mix design formula (or recipe) and loaded into a haul truck for delivery to the job site. On the site, the mix is transferred to a paver that places the mix at the desired width and thickness. The paver is followed by a compactor that tamps down the asphalt to the desired level of compaction. Once compaction is achieved and the pavement has sufficiently cooled, the next layer can be placed (or the road opened to traffic). Between each layer of a pavement, a tack coat (usually an emulsion of asphalt cement) is used to glue the layers together, improving the bond between the layers and strengthening the structure (B in the figure).
If an existing road is being repaired or replaced, the work can be as simple as milling off a few inches and replacing it with a few inches of new asphalt mix. (It’s worth noting that since asphalt is 100% recyclable, when a pavement is milled the millings are almost always saved and reused, usually to replace a percentage of the virgin asphalt cement and aggregates in a new pavement.) Or it may be a more extensive project that involves an entirely new structure, such as when an old concrete pavement is rubblized to create a foundation for a new road.