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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. This reduction of air volume produces a corresponding increase in HMA density (Roberts et al., 1996).

Figure 1: A Steel wheel and a pneumatic tire roller working side-by-side.

Compaction is the greatest determining factor in dense graded pavement performance (Scherocman and Martenson, 1984; Scherocman, 1984; Geller, 1984; Brown, 1984; Bell et. al., 1984; Hughes, 1984; Hughes, 1989). Inadequate compaction results in a pavement with decreased stiffness, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and moisture damage (Hughes, 1984; Hughes, 1989).

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, which is typically quantified as a percentage of air voids in relation to total volume and expressed as “percent air voids”. Percent air voids is calculated by comparing a test specimen’s density with the density it would theoretically have if all the air voids were removed, known as "theoretical maximum density" (TMD) or "Rice density" after the test procedure inventor.

Although percent air voids is the HMA characteristic of interest, measurements are usually reported as a measured density in relation to a reference density. This is done by reporting density as:

HDOT and most other agencies and owners express density as a percentage or TMD or "Rice" density.

HDOT uses cores for density acceptance testing, while the Counties will accept nuclear gauge readings (although County inspectors can request core samples be taken).

Pavement air voids are measured in the field by one of two principal methods:

Each contracting agency or owner usually specifies the compaction measurement methods and equipment to be used on contracts under their jurisdiction.

picture of a core or core extraction
picture of nuclear density gauge
Figure 2: Core Extraction Figure 3: Pavement Core


Figure 4: Thin Lift Nuclear Density Gauge Figure 5: Taking a Nuclear Density Reading


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 (see Table 1).

Table 1: Factors Affecting Compaction

Environmental Factors   Mix Property Factors   Construction Factors
Temperature Aggregate Rollers
  Ground temperature
Air temperature
Wind speed
Solar flux
Fractured faces
Speed and timing
Number of passes
Lift thickness
      Asphalt Binder   Other
        Chemical properties
Physical properties
    HMA production temperature
Haul distance
Haul time
Foundation support

A Note on the Time Available for Compaction
HMA temperature directly affects asphalt binder viscosity and thus compaction. As HMA temperature decreases, the constituent asphalt binder becomes more viscous and resistant to deformation resulting 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 often reported to be about 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.

Mat temperature is crucial to both the actual amount of air void reduction for a given compactive effort, and the overall time available for compaction. If a mat's 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:

MultiCool, developed by Professor Vaughn Voeller and Dr. David Timm, is a Windows based program that predicts HMA mat cooling.  MultiCool can be used to predict the time available for compaction and is available on the National Asphalt Pavement Association's A Guide for Hot Mix Asphalt Pavement CD-ROM or for download at several locations:

Compaction Equipment

There are three basic pieces of equipment available for HMA compaction: (1) the paver screed, (2) the steel wheeled roller and (3) the pneumatic tire roller. Each piece of equipment compacts the HMA by two principal means: 

  1. By applying its weight to the HMA surface and compressing the material underneath the ground contact area. Since this compression will be greater for longer periods of contact, lower equipment speeds will produce more compression. Obviously, higher equipment weight will also increase compression.
  2. By creating a shear stress between the compressed material underneath the ground contact area and the adjacent uncompressed material. When combined with equipment speed, this produces a shear rate. Lowering equipment speed can decrease the shear rate, which increases the shearing stress. Higher shearing stresses are more capable of rearranging aggregate into more dense configurations.

These two means are of compacting HMA are often referred to collectively as “compactive effort”.

Steel Wheel Rollers

Steel wheel rollers (see Figures 6 and 7) are self-propelled compaction devices that use steel drums to compress the underlying HMA. They can have one, two or even three drums, although tandem (2 drum) rollers are most often used. The drums can be either static or vibratory and usually range from 35 to 85 inches in width and 20 to 60 inches in diameter. Roller weight is typically between 1 and 20 tons (see Figures 5 and 6). 

Some steel wheel rollers are equipped with vibratory drums. Drum vibration adds a dynamic load to the static roller weight to create a greater total compactive effort. Drum vibration also reduces friction and aggregate interlock during compaction, which allows aggregate particles to move into final positions that produce greater friction and interlock than could be achieved without vibration. As a general rule-of-thumb, a combination of speed and frequency that results in 10 - 12 impacts per foot is good. At 3000 vibrations/minute this results in a speed of 2.8 - 3.4 mph.

picture of steel wheel roller

another picture of a steel wheel roller
Figure 6 and 7: Steel Wheel Rollers

Pneumatic Tire Rollers

Pneumatic tire rollers are self-propelled compaction devices that uses pneumatic tires to compact the underlying HMA. Pneumatic tire rollers employ a set of smooth tires (no tread) on each axle; typically four or five on one axle and five or six on the other. The tires on the front axle are aligned with the gaps between tires on the rear axle to give complete and uniform compaction coverage over the width of the roller. Compactive effort is controlled by varying tire pressure, which is typically set between 60 and 120 psi (TRB, 2000). In addition to a static compressive force, pneumatic tire rollers also develop a kneading action between the tires that tends to realign aggregate within the HMA. Because asphalt binder tends to stick more to cold tires than hot tires, the tire area is sometimes insulated with rubber matting or plywood to maintain the tires near mat temperature while rolling (see Figures 8 and 9).

Pneumatic tire roller
Close-up of pneumatic tires
Figure 8: Pneumatic Tire Roller Figure 9: Pneumatic Tires


Video 1: Rollers in Action

Compaction Sequence

HMA compaction is typically accomplished by a sequence of compaction equipment. This allows each piece of equipment to be used only in its most advantageous situation resulting in a higher quality mat (both in density and in smoothness) than could be produced with just a single method of compaction. A typical compaction sequence consists of some or all of the following (in order of use):


Figure 10: Paving operation showing a steel wheel breakdown roller
and a pneumatic tire itermediate roller.

Finish roller

Figure 11: Finish roller.

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