Chapter 10: Ultra-Thin, Hot Mixed Asphalt, Bonded Wearing Course Projects
3.0 Project Selection
3.1 Distress & Application Considerations
While a bonded wearing course is a flexible pavement surface, it is not considered a structural layer. A BWC is a viable application for treating structurally sound, worn pavements and has shown some ability to retard cracking due to its membrane and gap-graded aggregate structure. BWC’s are used on both flexible and PCC pavements to correct non-structural surface defects such as skid resistance, noise dampening and splash-and-spray control. They are typically selected for use when speed of construction and user delay are issues. Table 7 outlines the appropriate surface distresses on which a BWC can be placed. Note that the definitions of pavement condition in Table 7 are taken from SHRP Manual P-338 (1).
Table 7: Distress Severity or Extent That Can Be Treated With a BWC (2)
| Pavement
Type |
Cracking |
Patching/
Potholes |
Surface
Deformation |
Surface Defects |
Joint Deficiencies |
| AC |
- Longitudinal & Transverse (Medium)
- Block (Moderate)
- Edge (Moderate)
|
Patches: Moderate
Potholes:
Moderate |
Rutting: <12.5 mm
Shoving: No |
Bleeding: Moderate
Polished Agg:
OK
Raveling:
Severe |
N/A |
| PCC |
- Corner Breaks (Moderate)
- Material Related Distress (Low)
- Longitudinal (Moderate)
- Transverse (Moderate)
|
N/A |
N/A |
Map cracking and scaling:
<10 m2 to
100 m2 |
Spalling:
Moderate |
| Note: For PCC, a BWC will not treat blowups, pumping, faulting of joints, or crack widths > 9.5 mm |
3.2 Performance
3.2.1 Performance
Although never measured, bonded wearing courses have been estimated to last 7 to 12 years (3, 4, 5). The main method of failure is wear; that is, the surface oxidizes and is abraded over time. Premature failure occurs from placement on highly deflecting and cracked surfaces; base failures and delamination occur when placed on dirty or poorly prepared surfaces.
The main performance benefits associated with using a BWC are improved skid resistance, reduced traffic noise, increased ride quality and spray reduction. Figures 1 and 2 shows how the characteristics of a BWC compare with those of other mixture types (3, 4, 7). The figures indicate that a BWC retains good skid resistance characteristics over time and that it is comparable to other wearing courses that provide good skid resistance characteristics. The skid resistance of a BWC varies with increasing speed in a manner similar to stone mastic asphalt (SMA) as shown in Figure 2.
It can be seen that BWCs rate well in comparison to other surface treatments. The data listed in Table 8 have been collected from several sources (3, 7). Splash and spray are important surface characteristics and may be measured in various ways. One method is by hydraulic conductivity. This is done by pressing a special cylinder against the road surface and measuring conductivity. A high number indicates faster drainage. Table 8 shows the results of hydraulic conductivity tests performed on three surface treatments. As the results indicate, BWCs had the highest drainage characteristics of the three surface treatments types tested.
Figure 1: Change in Skid Resistance Over Time (4) |
Figure 2: Change in Skid Resistance with Speed (3)
(UL-M is Ultra Thin Polymer Modified HMA - 25mm) |
Table 8: Hydraulic Conductivity as an Indication of Spray
Reduction Characteristics (3)
| Material |
Hydraulic Conductivity (s-1) |
| 14 mm SMA |
0.03 |
| 12.5 mm BWC |
0.06 |
| 10 mm UL-M |
0.01 |
| 12.5 mm OGFC |
0.12 |
3.2.2 Relative Cost Effectiveness
BWC costs could be expected to range between $21M per LM and $50M per LM. Costs for BWCs vary depending on the materials used (night work vs. day work, quantities, work windows) and the locations in which they are placed. BWCs are new and costs could reasonably be expected to decrease as more experience is gained with their use.
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