Compaction of Hot Mix Asphalt Pavements (HMA)

Ed Lyon, PE, GE April 9, 2014

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

• “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 in a mixture produces a corresponding increase in HMA unit weight, or density (Roberts et al., 1996). “

Compaction – Defined

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

Quality Characteristics Affected by Compaction

– Stability / Strength – Fatigue Life – Durability – Moisture Sensitivity – Raveling

Importance of Compaction

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

• “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 in a mixture produces a corresponding increase in HMA unit weight, or density (Roberts et al., 1996). “

Compaction – Defined

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

• “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 in a mixture produces a corresponding increase in HMA unit weight, or density (Roberts et al., 1996). “

Compaction – Defined

The volume of air in an HMA pavement is important because it has a profound effect on long-term pavement performance.

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

– Fatigue Resistance “A reduction in air voids from 8% to 3% could more than double pavement fatigue life” (Scherocman, 1984a).

– Durability “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.” McLeod (1967)

– 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.

Positive Effect of Compaction

– 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 which increases the likelihood of significant moisture damage.” (Kandhal and Koehler, 1984; Cooley et al., 2002)

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

Negative Effects of Excess Compaction

Balance -For dense-graded HMA, for use on high traffic pavements in place air voids between 3 and 8 percent generally produce the best compromise of pavement strength, fatigue life, durability, raveling, rutting and moisture damage susceptibility.

– Decreased Stability and Strength. Kennedy et al. (1984) concluded that tensile strength, static and resilient moduli, and stability are reduced at low air void content.

– Rutting. The amount of rutting which occurs in an asphalt pavement is inversely proportional to the air void content (Scherocman, 1984a)

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

Aggregate Gradation

Aggregate Gradation

– Gradation has a direct affect on the effort required to properly compact a pavement

Fuller’s maximum density line:Passing (%) = (d/D)n ; n = 0,45 (FHWA)

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

Aggregate Gradtaion – VMA

VMA – Voids in Mineral Aggregate – The volume of the intergranular void space between the aggregate

particles of a compacted paving mixture that includes the air voids and the effective binder content AASHTO R-35

HIGH VMA = High Voids, harsh or stiff mix• High stability • Difficult to compact

LOW VMA = Low Voids, densely graded mix • Lower stability • Easier to compact

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

Relative Compaction

How are common practices set up to ensure that the finished pavements

have in place air voids between 3 and 8 percent ?

Relative compaction is the ratio expressed as a percentage between the field in place density and a laboratory test standard.

Relative to What Standard ?

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

Common Practice – Specifying Relative Compaction

LAB TEST MAXIMUM DENSITY (LTMD)

• Specified Air Voids in the lab between 3% and 5%

• Compacted Briquette ÷ Theoretical Max SpG

• Specified Relative Compaction 95% to 96% of LTMD

• Field Density ÷ Compacted Briquette

In Place Air is equal to the sum of the air voids in the lab and air voids from the field relative compaction tests

Example: 3.5% Air Voids in the lab, 96.4% Compaction

In Place Air = 3.5% + 100% – 96.4% = 7.1 %

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

LTMD - Marshall Method

• Mechanically Compacts Briquettes in steel molds using the Impact from the Marshall Hammer.

• Mix Design based on optimum asphalt content that yields air voids in the compacted briquettes of 4% when comparing briquette density to Rice Theoretical Density

The density of the compacted briquette is used as the specified Lab Test Maximum Density (LTMD ) for comparison to determine the relative compaction

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

LTMD California Kneading CompactorHveem Method

• Mechanically Compacts Briquettes in steel molds using the California Kneading Compactor.

• Mix Design based on optimum asphalt content that yields air voids in the compacted briquettes between of 4% when comparing briquette density to Rice Theoretical Density

The density of the compacted briquette is used as the specified Lab Test Maximum Density (LTMD ) for comparison to determine the relative compaction

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

Common Practice – Specifying Relative Compaction

Theoretical Maximum Specific Gravity:– Specify a minimum in place relative compaction equal to 92% of the Theoretical

Maximum Specific Gravity, commonly referred to as the Rice Test – Field Tests are performed by Nuclear Gauge or Cores– % Relative Compaction is the ratio expressed as a percentage between the in

place density of the compacted pavement and the Rice maximum specific gravity.

Agencies May specify a maximum in place relative compaction on the order of 96 to 97% to preserve stability in areas of heavy traffic

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

Theoretical Maximum Specific Gravity - Rice

• Measures the specific gravity of a loose mixture of the HMA after all of the air in the mix has been removed under vacuum.

• Used in all mix design methods to select the design asphalt content that yields air voids in compacted briquettes between 3 and 5%

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

Gyratory Compactor Super Pave Method

• Mechanically Compacts Briquettes in larger steel molds using the Gyratory Compactor.

• Mix Design based on optimum asphalt content that yields air voids in the compacted briquettes of 4% when comparing briquette density to Rice Theoretical Density.

• Specifies a Minimum VMA – stiffer mixes

Theoretical Maximum Specific Gravity (Rice) test used for comparison to field density to determine the relative compaction

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

YOUR QUESTIONSThank you for your attention.

Compaction of HMA Pavements | Ed Lyon, PE, GE | April 9, 2014

Hamburg Wheel Tracking DeviceSuper Pave Method