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Compaction is the process by which the volume of air in an HMA mixture is reduced by using external forces to reorient the constituent aggregate particles into a more closely spaced arrangement.

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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 percent increase in air voids (above 6-7 percent), about 10 percent of the pavement life may be lost (Linden et al., 1989[1]). Keep in mind that this rule-of-thumb … Read more »

Figure 1: HMA temperature vs. compaction aspects.

Factors Affecting Compaction

HMA compaction is influenced by a myriad of factors; some related to the environment, some determined by mix and structural design and some under contractor and agency control during construction (Table 1).   Table 1: Factors Affecting Compaction Environmental Factors Mix Property Factors Construction Factors Temperature Ground temperature Air temperature Wind speed Solar flux Aggregate … Read more »


Compaction Measurement and Reporting

Compaction reduces the volume of air in HMA. Therefore, the characteristic of concern is the volume of air within the compacted pavement. This volume is typically quantified as a percentage of air voids by volume and expressed as “percent air voids”. Percent air voids is calculated by comparing a test specimen’s bulk density with its … Read more »

Figure 1: Thin lift nuclear density gauge.

Nuclear Density Gauge

  A nuclear density gauge measures in-place HMA density using gamma radiation.  Gauges usually contain a small gamma source (about 10 mCi) such as Cesium-137 on the end of a retractable rod. Gamma rays are emitted from the source and interact with electrons in the pavement through absorption, Compton scattering, and the photoelectric effect. A … Read more »

Figure 2. Large vibratory steel wheel roller (17 tonnes (18.7 tons), 213 cm (84-inch) wide drum).

Compaction Equipment

There are three basic pieces of equipment available for HMA compaction: (1) the paver screed, (2) the steel wheel roller and (3) the pneumatic tire roller.  Each piece of equipment compacts the HMA by two principal means: By applying its weight to the HMA surface and compressing the material underneath the ground contact area.  Since … Read more »

Figure 2. Probable temperature differential damage on a state route in the Cascade Mountains.

Temperature Differentials

Construction-related HMA temperature differentials are large mat temperature differences resulting from placement of a significantly cooler portion of HMA mass into the mat. This cooler mass comes from the surface layer (or crust) typically developed during HMA transport from the mixing plant to the job site. These cooler areas will reach cessation temperature more quickly … Read more »

Figure 1: Field permeability test.

Permeability vs. Aggregate Size and Lift Thickness

It is generally believed that HMA densities between 3 and 8 percent air voids will result in a pavement that would not be susceptible to water damage, oxidation, raveling, and cracking. There is some indication from recent studies that coarse-graded mixes can be excessively permeable to water at in-place air voids less than 8 percent … Read more »

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HMA Compaction

It has been said that the top three factors in real estate are “location, location, location”. It can also be said that the top three factors in HMA pavement construction are “compaction, compaction, compaction”. Compaction is the process by which the volume of air in an HMA mixture is reduced by using external forces to … Read more »

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Tender Mixes

A tender mix is an internally unstable mix that tends to displace laterally and shove rather than compact under roller loads.  Internal mix stability is a function of asphalt binder viscosity and aggregate gradation and shape.  Mix tenderness is usually one of two varieties (TRB, 2000[1]): High temperature tenderness.  At high temperatures some mixes may … Read more »

Figure 1. PQI™ from TransTech.

Electrical Density Gauges

Pavement Quality Indicator (PQI)   An electrical density gauge, like TransTech’s Pavement Quality Indicator (PQI™), measures pavement density indirectly by measuring its dielectric constant. The PQI™ passes a small current through the pavement, which creates an electrical sensing field. Density is measured by the response of this electrical sensing field to changes in the pavement’s … Read more »

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