#The reduction of air voids to the optimum level in the mix produces a pavement that is nearly impermeable. In an under compacted mix the voids tend to be interconnected thereby permitting the intrusion of air and water into the pavement structure. The air oxidizes the asphalt binder, making it brittle and causing the mix to crack under the repeated deflections due to traffic loads. Water permeating into and through the pavement can lead to stripping of the asphalt from the aggregate, weakening of the base and subgrade soils under the pavement, or freeze-thaw damage in colder climates. Premature pavement distress can occur in any of these situations.
#Finally, if a high level of compaction (concurrently with items 1 and 2 above) is not achieved during construction, subsequent traffic will further consolidate the mix. This will occur principally in the wheel paths and the resulting ruts could be a safety hazard to traffic using the road.
==Fundamental principles==
The goal of compacting an asphalt concrete pavement is to achieve the optimum air void content and provide a smooth riding surface. Behind the paver, the mat has 15 to 20% air voids. It is the task of the [[steamroller|rollers]] to reduce that void content to 8% or less for dense-graded mixes. At that level, the voids are usually not interconnected and the detrimental effects of air and water are avoided. Caution must be exercised, however, not to compact the pavement to a very low air void level. This minimum void level is necessary to allow for thermal expansion.
Even with a properly designed mix, compaction of the mat is possible only if correct mix temperature exists.
At temperatures between 85°C (185°F) and 150°C (300°F), the asphalt binder is fluid enough to act as a lubricant in the compaction process aiding movement of the mix aggregates into a dense configuration. As it cools below this temperature range, asphalt becomes very stiff and sticky making further mix densification extremely difficult. Therefore, compaction must be completed before the mix cools to 85°C (185°F).
==References==