8.3 Compaction Importance

The volume of air in an HMA pavement is important because it has a profound effect on long-term pavement performance. An approximate rule-of-thumb is for every 1% increase in air voids (above 6 to 7%), about 10% of the pavement life may be lost (Linden et al., 1989). Keep in mind that this rule-of-thumb was developed using limited project data. This rule-of-thumb applies to air voids above 6 to 7% and should be used with extreme caution. According to Roberts et al. (1996), there is considerable evidence that dense-graded mixes should not exceed 8% nor fall below 3% air voids during their service life. This is because high air void content (above 8%) or low air void content (below 3%) can cause the following pavement distresses (this list applies to dense-graded HMA and not open-graded HMA or SMA):
  1. Decreased stiffness and strength
    . Kennedy et al. (1984), concluded that tensile strength, static and resilient moduli, and stability are reduced at high air void content.
  2. Reduced Fatigue Life
    . Several researchers have reported the relationship between increased air voids and reduced fatigue life (Pell and Taylor, 1969; Epps and Monismith, 1969; Linden et al., 1989). Finn et al. (1973), concluded “...fatigue properties can be reduced by 30 to 40% for each 1% increase in air void content.” Another study concluded that a reduction in air voids from 8% to 3% could more than double pavement fatigue life (Scherocman, 1984a).
  3. Accelerated Aging/Decreased Durability
    . In his Highway Research Board paper, McLeod (1967) concluded that “compacting a well-designed paving mixture to low air voids retards the rate of hardening of the asphalt binder, and results in longer pavement life, lower pavement maintenance, and better all-around pavement performance.”
  4. Raveling
    . Kandhal and Koehler (1984) found that raveling becomes a significant problem above 8% air voids and becomes a severe problem above 15% air voids.
  5. Rutting
    . The amount of rutting which occurs in an asphalt pavement is inversely proportional to the air void content (Scherocman, 1984a). Rutting can be caused by two different mechanisms: vertical consolidation and lateral distortion. Vertical consolidation results from continued pavement compaction (reduction of air voids) by traffic after construction. Lateral distortion – shoving of the pavement material sideways and a humping-up of the asphalt concrete mixture outside the wheelpaths – is usually due to a mix design problem. Both types of rutting can occur more quickly if the HMA air void content is too low (Scherocman, 1984a).
  6. Moisture Damage
    . Air voids in insufficiently compacted HMA are high and tend to be interconnected with each other. Numerous and interconnected air voids allow for easy water entry (Kandhal and Koehler, 1984; Cooley et al., 2002) which increases the likelihood of significant moisture damage. The relationship between permeability, nominal maximum aggregate size, and lift thickness is quite important and can change significantly as these parameters change.
Air voids that are either too great or too low can cause a significant reduction in pavement life. For dense-graded HMA, air voids between 3 and 8% generally produce the best compromise of pavement strength, fatigue life, durability, raveling, rutting, and moisture damage susceptibility.