Compaction Importance

The volume of air in an HMA pavement is important because it has a profound effect on long-term pavement performance. The exact nature of that effect is nuanced, and likely not linear.

• Air void content above a high-end threshold value results in a predictable reduction of pavement service life. That threshold is usually identified as 7-8%. Linden et al. (1989)^[1] stated, “The rule-of-thumb that emerges is that each 1 percent increase in air voids (over a base air void level of 7 percent) results in about a 10 percent loss in pavement life (or about 1 year less).”
• Air void content below a low-end threshold value is indicative of an unstable mixture prone to distortion and flushing. This threshold is often reported around 2-3%.

As to what happens between the high (7-8%) and low (2-3%) thresholds, there is still some uncertainty. While some assume a linear relationship between air void content and pavement life at, between, and beyond these thresholds, the true relationship may be different. Howell et al. (2021)^[15] suggest that between these thresholds (3-7% range approximately) it may be that air voids are just at an acceptable value that maximizes pavement life (with all other parameters held constant) for a typical 1/2-inch NMAS dense-graded mixture.

Over time, the most common recommendations for acceptable air void content have been in the 3-8% range. For example, Seeds et al. (2002)^[12] concluded a good construction target was 5-6%, Cominsky et al. (1998)^[14] recommended 2-7% as a reasonable range, and Willoughby (now Schofield) and Mahoney (in 2007)^[11] and Tran et al. (in 2016)^[13] showed that a majority of states used 8% as a minimum compaction value.  Further, according to Roberts et al. (1996^[2]), there is considerable evidence that dense graded mixes should not exceed 8 percent nor fall below 3 percent air voids during their service life. This is because high air void content (above 8 percent) or low air void content (below 3 percent) 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^[3]) 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^[4]; Epps and Monismith, 1969 ^[5]; Linden et. al., 1989^[1]). Finn et al. (1973)^[6] concluded “…fatigue properties can be reduced by 30 to 40 percent for each one percent increase in air void content.” Another study concluded that a reduction in air voids from eight percent to three percent could more than double pavement fatigue life (Scherocman, 1984^[7]).
3. Accelerated Aging/Decreased Durability. In his Highway Research Board paper, McLeod (1967)^[8] concluded “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^[9]) found that raveling becomes a significant problem above about eight percent air voids and becomes a severe problem above approximately 15 percent air voids.
5. Rutting. The amount of rutting which occurs in an asphalt pavement is inversely proportional to the air void content (Scherocman, 1984^[7]). 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, 1984^[7]).
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^[9]; Cooley et al., 2002^[10]) 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 percent generally produce the best compromise of pavement strength, fatigue life, durability, raveling, rutting and moisture damage susceptibility.

[[11]]Willoughby, K., and J. P. Mahoney. An Assessment of WSDOT’s Hot-Mix Asphalt Quality Control and Assurance Requirements. Report WA-RD 517.2, Washington State Department of Transportation, 2007.

[[12]]Seeds, S. B., G. R. Hicks, G. E. Elkins, H. Zhou, and T. V. Scholz. NCHRP Project 20-50(14): LTTP Data Analysis: Significance of “As-Constructed” AC Air Voids to Pavement Performance, Final Report. Transportation Research Board, Washington, D.C., 2002.

[[13]]Tran, N., P. Turner, and J. Shambley. Enhanced Compaction to Improve Durability and Extend Pavement Service Life: A Literature Review. NCAT Report 16-02R, National Center for Asphalt Technol, 2016.

[[14]]Cominsky, R. J., B. M. Killingsworth, R. M. Anderson, D. A. Anderson, and W. W. Crockford. NCHRP Report 409: Quality Control and Acceptance of Superpave Designed Hot-Mix Asphalt. Transportation Research Board, Washington, D.C., 1998.

[[15]] Howell, R.; Muench, S.T.; Ashtiani, M.Z.; Feracor, J.; Russell, M. and Uhlmeyer, J. (2021). Using Large Linked Field Data Sets to Investigate Density’s Impact on the Performance of Washington State Department of Transportation Asphalt Pavements. Transportation Research Record: Journal of the Transportation Research Board. https://doi.org/10.1177/03611981211006101.