HMA compaction is influenced by a myriad of factors including: 1) environment, 2) mix and structural design, and 3) construction (see “Table 6-4: Factors Affecting Compaction”).

Environmental factors are determined by when and where paving occurs. Paving operations may have some float time, which allows a limited choice of “when,” but paving location is determined by road location, so there is essentially no choice of “where.” Mix and structural design factors are determined before construction and, although they should account for construction practices and the anticipated environment, they often must compromise ease of construction and compaction to achieve design objectives. Obviously, construction factors are the most controllable and adaptable of all the factors affecting compaction. Although some factors like haul distance/time, HMA production temperature, lift thickness, and type/number of rollers may be somewhat predetermined, other factors associated with roller timing, speed, pattern, and number of passes can be manipulated as necessary to produce an adequately compacted mat. This subsection discusses:

HMA temperature has a direct effect on the viscosity of the asphalt cement binder and thus compaction. As HMA temperature decreases, its asphalt cement binder becomes more viscous and resistant to deformation, which results in a smaller reduction in air voids for a given compactive effort. As the mix cools, the asphalt binder eventually becomes stiff enough to effectively prevent any further reduction in air voids, regardless of the applied compactive effort. The temperature at which this occurs, commonly referred to as

cessation temperature

, is a function of the mix property factors in the “Table 6-4: Factors Affecting Compaction”. In some literature, it is reported to be about 79°C (175°F) for dense-graded HMA (Scherocman, 1984b; Hughes, 1989). Below cessation temperature, rollers can still be operated on the mat to improve smoothness and surface texture, but further compaction will generally not occur. Conversely, if the binder is too fluid and the aggregate structure is weak (e.g., at high temperatures), roller loads will simply displace, or “shove,” the mat rather than compact it. In general, the combination of asphalt cement binder and aggregate needs to be viscous enough to allow compaction but stiff enough to prevent excessive shoving.

Mat temperature, then, is crucial to both the actual amount of air void reduction for a given compactive effort and the overall time available for compaction. If the initial temperature and cool-down rate are known, the temperature of the mat at any time after laydown can be calculated. Based on this calculation, rolling equipment and patterns can be employed to:

The major factors affecting time available for compaction are (Roberts et al., 1996):

Jordan and Thomas (1976) pointed out additional factors affecting mat cool-down rate that include mat density, pavement layer thermal conductivity, specific heat, convection coefficient, incident solar radiation, and coefficients of emission and absorption of solar radiation for the pavement surface.

HMA temperature affects its binder viscosity, which affects compaction in two ways:

Many factors influence HMA temperature and cool-down rate, including initial mat temperature, mat thickness, temperature of the surface on which the mat is placed, ambient temperature, and wind speed. Warm mix additives have been used to extend compaction time resulting from lower temperatures or thinner mix lifts that cool more rapidly.

Mix aggregate and binder properties can also affect compaction. They do so by affecting the:

Gradation affects the way aggregate interlocks and thus the ease with which aggregate can be rearranged under roller loads. In general, aggregate effects on compaction can be broken down by aggregate size (TRB, 2000):

The asphalt binder grade affects compaction through its viscosity. A binder that is higher in viscosity will generally result in a mix that is more resistant to compaction. Additionally, the more a binder hardens (or ages) during production, the more resistant the mix is to compaction.

Asphalt binder content also affects compaction. Asphalt binder lubricates the aggregate during compaction. Mixes with low asphalt contents are generally more difficult to compact because of inadequate lubrication, whereas mixes with high asphalt contents will compact more easily but may shove under roller loads (TRB, 2000).

Sometimes, a combination of mix design factors produces what is known as a tender mix. Tender mixes are internally unstable within a limited mixture dependent temperature range known as the tender zone. Mixtures in this zone tend to displace laterally and shove rather than compact under roller loads. It is advisable to cease compaction of these mixtures when the mat temperature is within this zone. Compaction may resume when the mat temperature falls below the tender zone.