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Section 3: Pavement Surface Preparation

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Surface Condition

The performance of a flexible pavement under traffic is directly related to the condition of the surface on which the pavement layers are placed. For a full-depth asphalt pavement, if the condition of the subgrade soil is poor, the ultimate durability of the roadway may be reduced. For hot mix asphalt (HMA) layers placed on top of a new, untreated granular base course, the base material should not be distorted by the trucks carrying the mix to the paver. For HMA placed as an overlay on top of an existing HMA layer, the surface should be free of major distresses, smooth and clean.

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Prime Coat - Flexible Pavements

For flexible pavements, the graded subgrade or the top granular base layer may be prepared with a prime coat. A prime coat is a sprayed application of a cutback (MC-30 or MC-70) or emulsion asphalt applied to the surface of untreated subgrade or base layers. The prime coat serves several purposes:

  • fills the surface voids and protect the base from weather
  • stabilizes the fines and preserve the base material
  • promotes bonding to the subsequent pavement layers.

The project plans specify the rate at which the prime coat is applied, and rates will vary from one project to another. Generally, however, medium curing cutback asphalt is applied at the range between 0.2 to 0.5 gal/yd2, and emulsified asphalts are applied between 0.1 to 0.3 gal/yd2. For emulsions it is recommended to mix the emulsion with the top 1 in. of the base.

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Underseals

Existing Surface Preparation for Overlays includes an underseal which is a sprayed application of asphalt binder (emulsion or hot applied asphalt binder) immediately covered by a layer of one-sized aggregate. The underseal provides several benefits, such as waterproofing the surface, sealing small cracks and protecting the underneath surface from solar radiations. The application of an underseal should be strongly considered when placing a thin ACP layer on top of a flexible base. The Seal Coat and Surface Treatment Manual covers in detail the design and construction of seal coats.

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Existing Surface Preparation for Overlays

Overlays make up a large portion of the roadway paving done today. The degree of surface preparation for an overlay is dependent on the condition and type of the existing pavement. Generally, the existing pavement should be structurally sound, level, clean and capable of bonding to the overlay. To meet these prerequisites, the existing pavement is usually repaired, leveled (by milling, leveling or both), cleaned and then coated with a binding agent. This subsection covers:

  • repair
  • tack coats.

A tack coat material can be a PG binder or an emulsion layer applied between HMA pavement lifts to promote bonding. Adequate bonding between constructed lifts (especially between the existing road surface and an overlay) is critical for the constructed pavement structure to behave as a single unit and provide adequate strength. If adjacent layers do not bond to one another they essentially behave as multiple independent thin layers - none of which are designed to accommodate the anticipated traffic-imposed bending stresses. Inadequate bonding between layers can result in delamination (debonding) followed by longitudinal wheel path cracking, alligator cracking, potholes, and other distresses such as rutting that greatly reduce pavement life.

Application

Tack coats should be applied uniformly across the entire pavement surface and result in more than about 90% surface coverage. In order for this uniformity to be consistently achieved, all aspects of the application must be considered and carefully controlled. Specific aspects are:

  • the condition of the pavement surface receiving the tack coat
  • the application rate
  • type of tack coat according to standard TxDOT specifications on Limestone Rock Asphalt Pavement, Item 330, and hot-mix items 334, 340, 341, 342, 344, and 346.

Condition of the Pavement Surface Receiving the Tack Coat

The pavement surface receiving the tack coat should be clean and dry to promote maximum bonding. Emulsified tack coat materials may be applied to cool and/or damp pavement; however, the length of time needed for the set to occur may increase (Flexible Pavements of Ohio, 2001). Since existing and milled pavements can be quite dirty and dusty, their surfaces should be cleaned off by sweeping or washing before any tack coat is placed, otherwise the tack coat material may bond to the dirt and dust rather than the adjacent pavement layers. This can result in excessive tracking of the tack coat material. Construction vehicles and equipment pick up the tack-dirt mixture on their tires and leave the existing roadway with little or no tack coat in the wheel paths (see Figure 6-1). Slippage cracking and delamination are distresses typically seen when cleanliness is lacking (Flexible Pavements of Ohio, 2001).

Tack coat tracking resulting in no tack
coat in the wheel path.
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Figure 6-1. Tack coat tracking resulting in no tack coat in the wheel path. © Copyright 2006 University of Washington

Application Rate

Tack coat application should result in a thin, uniform coating of tack coat material covering approximately 90% of the pavement surface (Flexible Pavements of Ohio, 2001). To achieve this result, application rate will vary based on the condition of the pavement receiving the tack coat. Too little tack coat can result in inadequate bonding between layers. Too much tack coat can create a lubricated slippage plane between layers, or can cause the tack coat material to be drawn into an overlay, negatively affecting mix properties and even creating a potential for bleeding in thin overlays (Flexible Pavements of Ohio, 2001). Table 6-1 shows recommended application rates from Flexible Pavements of Ohio (2001.)

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Basic application rate considerations are:

  • Roughness of the pavement surface receiving the tack coat. Rough surfaces require more tack coat than smooth surfaces. For instance, milling produces a rough, grooved surface, which will increase the existing pavement’s surface area when compared to an ungrooved surface. The surface area increase is dependent on the type, number, condition and spacing of cutting drum teeth but is typically in the range of 20 to 30%, which requires a corresponding increase in tack coat (20 to 30% more) when compared to an unmilled surface (TRB, 2000).
  • Distributor vehicle. Several vehicle-related adjustments and settings are critical to achieving uniform tack coat placement. Essentially the nozzle patterns, spray bar height and distribution pressure must work together to produce uniform tack coat application. Generally, the best applications results from a "double lap" or "triple lap" coverage. "Double/triple lap" means that the nozzle spray patterns overlap one another such that every portion of the pavement receives spray from exactly two/three nozzles (see Figure 6-1; Figure 6-2; Figure 6-3; Figure 6-4; Figure 6-5; Figure 6-6; Figure 6-7; and Figure 6-8). Specific guidance follows:
    • Nozzle spray patterns should be identical to one another along a distributor spray bar. Differing coverages will result in streaks and gaps in the tack coat.
    • Spray bar height should remain constant. As tack coat is applied, the vehicle will become lighter causing the spray bar to rise. The tack coat application vehicle should be able to compensate for this. Excessively low spray bars result in streaks, while excessively high spray bars cause excessive nozzle overlap resulting in an excessive application rate.
    • Pressure within the distributor must be capable of forcing the tack coat material out the spray nozzles at a constant rate. Inconsistent pressure will result in non-uniform application rates.
    • Temperature within the distributor should be maintained between 24°C (75°F) and 54°C (130°F). Excessive heating may cause the emulsion to break while still in the distributor.

Tack coat application picture.
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Figure 6-2. Tack coat application picture. © Copyright 2006 University of Washington

Tack Coat Distributor Truck.
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Figure 6-3. Tack Coat Distributor Truck. © Copyright 2006 University of Washington

Distributor Truck Spray Bar.
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Figure 6-4. Distributor Truck Spray Bar. © Copyright 2006 University of Washington

Tack Coat Distributor Control Panel.
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Figure 6-5. Tack Coat Distributor Control Panel. © Copyright 2006 University of Washington

Jet dryer used ahead of the tack coat distributor.
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Figure 6-6. Jet dryer used ahead of the tack coat distributor. © Copyright 2006 University of Washington

 Tack coat application.
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Figure 6-7. Tack coat application. © Copyright 2006 University of Washington

Tack coat application showing nozzle coverage
area.
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Figure 6-8. Tack coat application showing nozzle coverage area. © Copyright 2006 University of Washington

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Other Tack Coat Aspects

Timing

Generally, a tack coat should be allowed enough time to break and set (emulsion) before applying the next layer of hot mix asphalt (HMA).

Tracking

Tracking is the pick-up of tack coat material by vehicle tires. Tracking deposits tack coat material on adjacent pavement surfaces. Although this material is unsightly, it generally has little effect and wears away quickly. In extreme cases, tracking may deposit enough tack coat material to distort pavement surfaces or hinder a driver's ability to navigate (Flexible Pavements of Ohio, 2001). Rubberized tack coats have an especially high propensity to stick to vehicle tires. Allowing tack coats to set (emulsions) before driving on them can substantially reduce tracking.

Traffic on Tack Coats

Generally, traffic should not be allowed on tack coats. When a tacked road surface is exposed to traffic, the potential exists for reduced skid resistance, especially during wet weather (Flexible Pavements of Ohio, 2001). When tack coat surfaces must be opened to traffic, they should be covered with sand to provide friction and prevent pick-up. A typical rate for applying a sand cover is 4 to 8 lb./yd.2 (Flexible Pavements of Ohio, 2001).


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