Chapter 10: Ultra-Thin, Hot Mixed Asphalt, Bonded Wearing Course Projects
4.0 Construction of Bonded Wearing Courses
The main components of the construction process include:
• Safety and Traffic Control
• Equipment Requirements
• Mix Production and Handling
• Surface Preparation
• Application Conditions Required
• Application of BWC
• Opening to Traffic
Checklists at the end of this chapter provide guidance for project personnel about the important aspects of constructing a BWC.
4.1 Safety & Traffic Control
Traffic control is required both for the safety of the traveling public and the personnel performing the work. It is also used to ensure the new surface is compacted and allowed to cool to below 70°C (160°F) prior to reopening the surface to traffic.
Traffic control should be in place before work forces and equipment enter the roadway or work zone. Traffic control includes placing construction signs, construction cones and/or barricades, flag personnel, and pilot cars required to direct traffic clear of the maintenance operation. For detailed traffic control requirements, refer to the Manual of Uniform Traffic Control Devices.
4.2 Equipment Requirements
Equipment requirements for constructing a BWC are found in the Agency specifications. The most significant requirement is that the binder application and hot mix spreading function are combined into a single unit. The following section describes this specialized unit while the subsequent sections discuss other equipment requirements.
4.2.1 Paving Unit
The paving unit used for the construction of a BWC is a specially constructed machine. A diagram of a BWC paving unit is shown in Figure 3. Figure 4 shows a close up of the spray and spreading functions of a BWC paving unit, and Figure 5 shows a freshly laid BWC.
- The paving unit pushes the truck carrying the hot mix asphalt.
- The mix drops into a hopper at the front of the paving unit.
- The mix is transported via an auger to a screed.
- The emulsion membrane is sprayed just in front of the screed and the mix is laid on top.
Figure 3: Paving Unit
(1) |
Figure 4: Emulsion Membrane and Mix Spreading (9) |
Figure 5: Freshly Laid BWC (9) |
4.2.2 Rollers
Compaction of a BWC is required. Static and steel drum type rollers, only, should be used. The rollers must be at least 12 tons and must conform to the Caltrans Standard Specifications Section 39-5.02, “Compacting Equipment." Compaction must conform to the Agency Standard Specifications and shall consist of two complete passes before the temperature of the mat falls below 90°C (195°F). If necessary, more than two coverages may be ordered by the engineer when rolling bonded wearing course patches or joints. Rolling the gap-graded mixture is intended to seat the aggregates and to provide a smooth surface. There are no in place density requirements when rolling bonded wearing course mixes.
4.2.3 Other Equipment
Other required equipment include sweepers for cleaning the pavement before application and hand tools such as rakes, shovels etc.
4.3 Mix Production & Handling
Standard hot mix facilities and storage bins may be used for BWC mix production. The only special requirements are that the mixing temperatures for a BWC shall not exceed 177°C (350°F) and storage time shall not exceed 12 hours. A drain down test should be performed to ensure binder does not drain out of the mixture. All mixing plants must be calibrated to the Agency Test Methods. BWC mixes may be treated with an anti-stripping agent or lime if necessary.
4.4 Surface Preparation
Cracks greater than 6 mm wide (1/4 in) should be filled or sealed prior to application see Chapter 3 (Crack Sealing and Filling) of this document. The use of over-banding methods of crack sealing is not recommended for this treatment as that method can leave strips that reflect through the finished pavement. All repairs necessary to bring the pavement to the minimum requirements listed in Table 7 must also be performed prior to the application of the BWC.
Manhole covers, drains, grates, catch basins, and other utility services must be covered prior protected from the application of the BWC. Covering the services with construction paper or roofing felt can do this. Any surface irregularities deeper than 25 mm (1 in) should be filled with dense graded hot mix before applying the BWC. Prior to application, the pavement should be swept with a rotary broom equipped with metal or nylon broom stock.
4.5 Application
Application of a BWC requires the use of a specialized paving unit described above in Section 4.2.1. Additional details specific to placing bonded wearing courses are discussed in the following paragraphs.
4.5.1 Conditions Required
A BWC may be applied on damp, but not wet, surfaces. The minimum air and pavement temperature requirements are 7°C (45°F) and rising, although it is recommended that the surface temperature be above 15°C (60°F). No freezing conditions are allowed in the first 24 hours, the emulsion-based tack coat requires about one day to fully cure. Additionally, if the water in the emulsion freezes, it may rupture the bond between the pavement and the new mix.
4.5.2 Polymer Modified Asphalt Emulsion Membrane
The emulsion applicator is part of the paving equipment and applies the polymer modified emulsion membrane at a temperature between 40° and 85°C (105° and 185°F) at a rate of 0.6 to 1.2 liter/m2 (0.13 to 0.30 gal/yd2). An emulsion application shot rate of 0.9 liter/m2 (0.20 gal/yd2) can be considered typical when using a 12.5mm, gap-graded mix gradation. An emulsion application shot rate of 0.77 liter/m2 (0.17 gal/yd2) can be considered typical when using 9.5 mm gap-graded mix gradation. The application rate should be adjusted according to the surface being covered. For more absorbent or textured pavement surfaces, a higher application rate by as much as 0.40 liter/m2 (0.17 gal/yd2) extra emulsion can be used when dealing with badly pocked pavement surfaces. For very smooth pavements, a reduced application rate by as much as 0.27 liter/m2 less emulsion can be used when dealing with flushed asphalt concrete pavements or smooth Portland cement concrete pavements. Typically PCC pavements require less emulsion membrane than AC pavements.
If the screed extension is outside the spray bar width, the tack coat will need to be applied manually to coat the pavement between the end of the spray bar and the end of the screed. Care should be taken to ensure the correct application rate in such circumstances. The spray bar should be calibrated and able to be adjusted to within ± 10% of the design application rate. Coverage of the pavement must be even and uniform and, as such, it is important that there are no plugged nozzles on the spray bar.
4.5.3 Paving
Good paving practice should always be followed when constructing a BWC. Windrowing and pick up machines are not allowed for constructing bonded wearing courses. The trucks servicing the paving unit should operate in a smooth manner, causing no bumps and allow paving to proceed continuously to create a smooth ride.
BWC may be placed in lifts from 12 to 40 mm (1/2 to 11/2 in) thick when using a 9.5 mm (3/8 in) mix. Thicker lifts are allowed when using a 12.5 mm (1/2 in) mix. The spread rate associated with using a 9.5 mm (3/8 in) mix is from 36 to 47 kg/m2 (7 to 9.5 lb/ft2) and 44 to 55 kg/m2 (9 to 11 lb/ft2) when applying a 12.5 mm (1/2 in) mix. The minimum delivery temperature for a BWC is 135°C (275°F), with an upper temperature limit of 177°C (350°F).
Longitudinal joints should be straight or correctly aligned to the curvature of the roadway, and should occur only at the edge or center of a traffic lane and never in the wheel paths.
At the start and finish of the work, the existing flexible pavement should be cut out to a depth of 30 mm (1.2 in) and tapered back a distance of 3m (10 ft) to provide a key for the new surfacing. For PCC, the mix should be rolled over at the start and finish of the work. The end of each run should be squared off at the point where feathering commences, and the feathered material should be cut out before the next run is started. Figure 6 illustrates the method for making transverse butt joints between runs. Handwork should always be minimized.
Figure 6: Making Transverse Butt Joints (8) |
4.5.4 Rolling
A minimum of one steel drum tandem roller is required for compacting a BWC. Rollers must be operated in static mode only. Usually two passes using a 12 to 15 ton roller are sufficient to properly seat the aggregates. Rolling must be carried out before the temperature, at mid layer of the mix, falls below 90°C (195°F). Figure 7 shows roller positions relative to the paver.
Figure 7: Roller position During Application (9) |
4.6 Opening to Traffic
Traffic can be allowed onto the new surface once rolling is completed and the mix temperature has fallen below 70°C (160°F). Typically, no post sweeping is required unless the mix begins to ravel.
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