Hot Mix Asphalt Compaction. Hot Mix Asphalt Compaction
8/16/2013
Hot Mix Asphalt Compaction
Hot Mix Asphalt Compaction “Compaction is the final step in the construction of a durable, smooth riding asphal...
Hot Mix Asphalt Compaction “Compaction is the final step in the construction of a durable, smooth riding asphalt pavement.” Close attention to detail must be provided during this critical process.
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Hot Mix Asphalt Compaction Reasons for Compaction – To prevent further consolidation – To provide shear strength or resistance to rutting – To ensure the mixture is waterproof (impermeable) – To prevent excessive oxidation of the asphalt binder
Purpose of Compaction Increases Stability Reduces Air Voids Increases Density Smoothes Surface
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Compaction is the process by which the asphalt and the aggregate are compressed into reduced volume
Density is the unit weight of the asphalt concrete achieved through the compaction process.
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Hot Mix Asphalt Compaction Asphalt binder holds particles together – Provides lubrication at high temperatures – provides cohesion at in-service temperatures
Prevents air and water intrusion into mat.
Hot Mix Asphalt Compaction Air voids are the spaces in the pavement not filled by aggregate or asphalt after placement and compaction - no more than 8% in-place air voids air voids will be reduced under traffic amount of reduction depends on air voids of produced mix best performance at 3-5% in-place air voids
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Effect of Air Voids on Life
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Typical Permeability vs. Voids
Factors Affecting Compaction Mix Properties –Aggregate –Asphalt –Mix Temperature
Layer Thickness Environmental Factors Rollers
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Compaction Equipment Compaction equipment exerts force into the pavement through weight and/or dynamic vibration Two types of equipment – vibratory screed on paver (10-20% air voids) – rollers (no more than 8% air voids)
Self Propelled Paving Machine
HMA Flow
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Hot Mix Asphalt Compaction
10 - 20 % air voids behind paver
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Hot Mix Asphalt Compaction
Temperature control is critical
Hot Mix Asphalt Compaction Typical Compaction Temp. Range
79°C (175°F)
150°C (300°F)
Intermediate rolling should be completed by 175 degrees F. Finish rolling can be performed at lower temperatures
Hot Mix Asphalt Compaction Compaction Equipment – Static steel wheeled rollers Compact by dead weight – Ballast can be added to some Two axle - tandem drums Three steel wheels Scrapers and wetting system
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Steel-Wheeled Rollers Section 401.3.11.2 8-12 ton tandem drum rollers required Min. 250 lbs. of pressure per inch of roller width Drums must be smooth and clean Drums must have adjustable scrapers and wetting system to prevent sticking
Hot Mix Asphalt Compaction
Static Three-Wheeled Roller
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Hot Mix Asphalt Compaction Compaction Equipment – Pneumatic - tired rollers Compact by tire contact pressure – Weight of machine - ballast – Tire pressure Typically have different number of front and rear wheels – Offset front to back (min. ¼ in. overlap) – Oscillate - yes; Wobble - no Maintain consistent tire pressure
Pneumatic-Tired Rollers Section 401.3.11.3 Effective rolling width of not less than 60 inches Average contact pressures: – 60-80 psi recommended by adjusting ballast and tire inflation – Tire pressure should not vary more than 5 psi
Tires must be hot to avoid pickup Kneading action reorients aggregate particles
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Hot Mix Asphalt Compaction
Hot Mix Asphalt Compaction Compaction Equipment – Vibratory rollers Compact through weight and dynamic force – Rotating eccentric weights – Can adjust vibration frequency and/or amplitude Vibration helps to re-orient aggregate particles Can be used as static rollers
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Hot Mix Asphalt Compaction Compression force of vibratory rollers is a combination of: – Weight of equipment – Dynamic force from vibration
Tandem Drum Vibratory Rollers Section 401.3.11.4 8 tons minimum weight Either one or two vibrating drums Amplitude: min. high and low Frequency: 2000-4000 impacts/min. Speed adjusted to obtain minimum of 10 impacts/ft
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Hot Mix Asphalt Compaction
Vibratory Steel Wheeled Roller
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Three Rolling Stages Breakdown Intermediate Finish
Three Rolling Stages Breakdown rolling – Provides nearly all needed density
Intermediate rolling – Provides final density level – Seals surface (pneumatic)
Finish rolling – Removes roller marks
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Rolling Pattern Speed & lap pattern for each roller No. of passes for each roller Min. temperature by which each roller must complete pattern
IMPORTANT: Paver speed must not exceed that of the compaction operation!!!
Rolling Procedures Breakdown rolling – Best with steel wheeled rollers – Static or vibratory mode – Be consistent Speed Number of passes Vibration
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“Bumping the Screed” with the vibratory roller
2 Breakdown Rollers Close to Paver
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Rolling Procedures Intermediate rolling – Roll while mat is still hot – Steel or pneumatic rollers – Pneumatic advantages More uniform degree of compaction Better surface seal Kneading action re-orients particles
– Be consistent
Hot Mix Asphalt Compaction Compaction Train (note three rollers)
2 Breakdown rollers split mat to cover entire mat quickly Intermediate roller obtains final required density
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Rolling Procedures Finish rolling – Provides surface aesthetics Removes roller marks
– Steel wheels - static mode – Mix needs to be warm
Finland
Hot Mix Asphalt Compaction Static Steel Wheel Roller
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Rolling Procedures General guidelines to follow – Keep drums moist to retard pick up (static and vibratory rollers) – Keep tires hot to retard pick up (pneumatic rollers) – Maintain slow, constant speed – Drive wheel toward paver – Reverse direction slowly – Don’t stand before mat cools
Longitudinal Joint Compaction Section 401.4.23 Longitudinal joints shall be rolled directly behind the paver – This means first pass is always on the hot joint
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Rolling Procedures
How Many Passes Are Required?
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Hot Mix Asphalt Compaction Mixture Confinement – Mix is held in place from every direction – Consists of three forces Top - compressive forces from rollers Bottom - supporting forces from base/subgrade under mat Sides - internal, resistive forces within mix or adjacent in-place mat
Hot Mix Asphalt Compaction
Unconfined Mats are more difficult to compact Unconfined edge will have low density and high air voids
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Hot Mix Asphalt Compaction Measuring Density
Using cores…... Or a nuclear density gauge
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Hot Mix Asphalt Compaction Reference Density Chart 100 % of Maximum 99 Theoretical Density 98 For 4% Voids Mix Design
97
3
100
96
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99
95
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98
94
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97
93
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100
96
92
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99
95
91
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% of Laboratory Density
% of Control Strip Density
0 In-Place Air 1 Voids 2
Hot Mix Asphalt Compaction
Goal of compaction operation
Smooth, impermeable asphalt pavement (4 to 8% air voids)
