Stabilized aggregate sounds great. Is it?
We can appreciate that stabilized aggregate is often considered for its affordability, to provide visual diversity of pavements, and incorporate green qualities. However, our experience over many years with many different products/binders, installation techniques, and applications, supports our conclusion that this product has very limited application.
We have used several different products on many projects here at CRJA. Generally, we find these stabilizing amendments have a limited lifespan, require ongoing maintenance and require periodic replacement over time; on their own, these are not necessarily deal breakers but combined with the additional concerns that follow, have resulted in our recommending against its use.
In particular we are concerned with:
1. Erosion: Erosion can be expected where there is overland or channelized flow across the stabilized aggregate. Where overland flow crosses flush rigid pavements, it is likely that runnels will develop on the up-grade side of the flush pavement and washouts of the stabilized aggregate will follow. This in turn will create toe catchers and, even worse, a lip that can catch plow blades. Control of water flow is essential when using these stabilized aggregates. After all, we long ago determined that bitumen and cement were the best means of stabilizing aggregate.
2. Infiltration: Considering the desire to infiltrate as much storm water as possible into the ground, the stabilized aggregate may be contemplated as being part of the stormwater infiltration system. We have found that the products that bind aggregates together into a stable surface also make the aggregates impervious. This is why there is so much concern on our part about controlling the direction and concentration of overland flow of stormwater.
3. Snow: The stabilized aggregate will need to be swept of snow. Plow blades can run away with stabilized aggregates. Property owners may have to utilize rotating brushes on snow removal equipment. Also, if the edges of hard pavements need to be marked with snow stakes then the stabilized aggregates will significantly impede snow staking unless sleeves are installed.
4. Pollution: The aggregates will be carried into buildings on pedestrians’ feet. Owners/facility managers should consider whether or not there are walk-off grates or grills at building entries to capture this aggregate. The LEED process recognizes that the pollution carried into buildings by people can degrade indoor air quality.
We have two examples of installations we consider successful. The first is a trail at Battle Road Trail, Minute Man National Historic Park, Concord, MA where we consider it a success because the material is so well suited to the context:
The trail’s location and use make this a suitable paving solution; however, although it has been in place for a number of years now, it has required regular maintenance and replacement in some areas. Additionally, erosion issues have had to be addressed though in this instance, the source of the erosion problem (off the trail itself) was dealt with. The second success is at New England Executive Park, Burlington, MA where stabilized aggregate is the only paving surface under a grove of trees:
It has done well because the area is perfectly flat so erosion has not been an issue. Also, snow is never removed from it.
Landscape architects remain interested in exploring new products. Stabilized aggregates looked to be a great alternative to asphalt and concrete pavement. But after a decade or more of using these products on many projects in many climates CRJA has concluded that this innovation did not live up to our expectations. We continue to try new products, systems and paradigms cautiously, always with the interests of clients and public health and safety in mind. Stabilized aggregates remain a tool but one with application far more narrow than we once thought.
by Horace Aikman, ASLA