Managing Bleeding and Flushing in Asphalt Pavements
Bleeding and flushing are common challenges in asphalt pavements, both stemming from excess asphalt binder filling gaps between aggregate particles. The main methods used to address bleeding involve either (a) modifying the texture of the current flush pavement surface or (b) applying a fresh textured surface on top of the flush pavement. According to a technical report by the Texas Tech University, there is promising potential in using a seal coat or surface treatment as future maintenance solutions. According to the research findings, the use of polymer-modified and alternative binders could enhance the effectiveness of seal coats and surface treatments, minimising the occurrence of cases of bleeding and flushing. Three potential avenues for further bleeding/flushing research and application are as follows: (a) the application of lime water, (b) the implementation of ultra-high-pressure water cutting, and (c) the use of the racked-in seal at intersections.
5 Key Takeaways for Asset Managers
The main points from the technical report has been summarised below. Please see References for access to the full paper.
1. Prevention is Key
The best approach is to prevent bleeding and flushing during construction by addressing factors like aggregate quality, binder quality, traffic loads, environmental conditions, and construction practices.
2. Bleeding Demands Immediate Action
Bleeding is an active issue that requires immediate maintenance. Solutions include bridging excess asphalt with various aggregates, cooling the pavement surface with water, or removing and rebuilding the pavement seal.
3. Flushed Pavements Can Wait
Unlike bleeding, flushed pavements typically don't demand immediate attention. Options for treating them involve re-texturing the surface or adding a new textured layer.
4. Binder Improvements Help
The use of polymer-modified and other binders has improved seal coat and surface treatment performance, reducing the occurrence of bleeding and flushing.
5. Future Solutions
Ongoing research points to promising solutions for bleeding and flushing issues, including lime water, ultra-high-pressure water cutting, and racked-in seals at intersections.
Lime water: The use of hydrated lime mixed with water is a widely adopted maintenance approach for addressing bleeding asphalt pavements in Texas. The usual procedure involves preparing lime water in a portable 3,785L water tank unit, which is placed at the rear of a dump truck. This tank unit includes an agitator pump to maintain the lime in suspension, and the lime water is then sprayed onto the bleeding pavement surface using a spray bar. Its effectiveness is described as follows:
Lime water is considered a short-term solution for addressing bleeding asphalt pavements, providing temporary relief.
The effectiveness of lime water treatment varies, lasting from 2 to 4 hours up to 3 to 5 days, often requiring multiple applications.
Lime water is believed to oxidize and age the hot liquid asphalt, reducing stickiness, forming a crust on the surface, and sometimes changing the asphalt's color to green as an indicator of its action.
Lime powder chemically reacts with and adheres to hot asphalt particles, essentially deactivating the asphalt.
Lime water treatment makes the road surface white, increasing reflectivity and reducing pavement temperature temporarily.
Lime water provides a temporary cooling effect on the pavement, which can last for 2 to 3 days, but higher air temperatures can cause pavement temperatures to rise again.
To maintain lower pavement temperatures, additional treatments are necessary as the cooling effects eventually dissipate.
Residual lime may temporarily separate tires from the asphalt surface.
Ultra-high-pressure water cutting: The emerging ultra-high pressure (UHP) watercutter technology, currently utilised in Australia/New Zealand and under consideration for implementation in Texas by TxDOT, shows potential for addressing flushed pavements. This technology, which combines watercutting and road cleaning capabilities, aims to simultaneously remove excess binder and contaminants from pavement surfaces while enhancing road surface macrotexture and aggregate microtexture.
This technology integrates a truck-mounted UHP pump, water supply, and vacuum recovery system with an independently operated umbilical deckblaster. A rotating spraybar employs specialised nozzles to direct extremely fine jets of ultra-high pressure water (36,000 psi) at ultrasonic velocity (Mach 1.5) onto the road surface, achieving the desired pavement treatment. Its effectiveness is described as follows:
The treatment's lifespan depends on factors such as the underlying cause of asphalt flushing and the likelihood of additional aggregate embedment or binder flushing.
Promotional information suggests that when the UHP watercutter is used to retexture a flushed pavement, several years may pass before further retexturing or resurfacing becomes necessary, based on data from New Zealand. However, its effectiveness under U.S. (Texas) roadway conditions is unconfirmed.
Retexturing involves removing excess asphalt binder buildup, which would otherwise hinder the application of sprayed seal treatments and increase the risk of recurring flushing or bleeding in the new seal.
Racked-in seals: The racked-in seal presents an alternative to the standard single-course seal coat. It involves the application of a single, thick layer of binder followed by two layers of cover aggregate. The second layer, known as the scatter coat, comprises smaller aggregate pieces, typically one-third to one-half the size of the first layer. This smaller aggregate serves to fill gaps and pushes the binder upward on the larger stones, creating a mechanical lock that stabilises the larger aggregate, resulting in a durable matrix. The racked-in seal has been employed in various regions, including Canada, Europe, Asia, Australia, and New Zealand, especially on roads with high traffic volume or fast-moving vehicles and is expected to last for a long time.
In summary, by understanding the causes and appropriate treatments for bleeding and flushing, pavement managers can ensure the longevity and safety of their roadways.
Reference
Lawson, W.D., Leaverton, M. and Senadheera, S. (2007). Maintenance Solutions for Bleeding and Flushed Pavements Surfaced with a Seal Coat or Surface Treatment. [online] ROSA P. Available at: https://rosap.ntl.bts.gov/view/dot/24774 [Accessed 5 Sep. 2023].