Urban neighborhoods are seeing more intense rain and tighter budgets. Streets and parking lots that used to take water away are now part of the problem, directing runoff into sewers and streams. Permeable asphalt (see here for more), or porous asphalt, provides a simple idea with great potential: let rain go through the surface and store it below. For urban planners, a useful notion that handles stormwater and allows them to develop complete streets.
How Porous Mixes Work
Permeable asphalt has the same general look as standard pavement, but it allows water to pass through the pavement. The surface mix uses less fine particle material and more uniform stone material, which generates the interconnecting voids that act like mini-channels. Rainwater passes through the mat into a stone reservoir below the mat and is either infiltrated into native soil or routed away via an underdrain.
The system consists of many layers. On top is the open-graded asphalt. Underneath the asphalt is a thick clean angular stone drainage layer that stores water–often several inches or better depending on the design storms. Under optimal site conditions with well-draining soils, under-drains would be unnecessary. Where soils are dense, the under-drain can be connected to a controlled outlet, typically located at a low spot, allows for the RT to leave the site slowly. Teams of people providing commercial paving services know that the success of the system is based on the gradation, compaction, and preventing the sediment from entering the system while being installed.
“Will it Freeze in Winter?”
A common concern is would the water frozen in the pavement crack the surface? The reality is, permeable asphalt will drain more quickly than it can freeze. The large air voids also afford the ice room to expand without stressing the mix. Because it dries rapidly, black ice can be less a hazard than dense, impervious asphalt.
Winter performance is still determined by good design principles and maintenance practices. For snow removal, a rubber-tipped blade should be used to reduce any raveling. The use of de-icing salt may be less because the water drains through and does not re-fracture freeze across the top. In places with severe winter conditions, municipalities will often use permeable sections, and traditional hot-mix for heavier loading areas.
Check this link https://www.sciencedirect.com/topics/engineering/asphalt-mixture for more details.
Environmental Advantages
Permeable asphalt does more than provide a place for puddles. It is a way for cities to comply with their stormwater permits, allows planting of street trees, and provides cooling to hot blocks. With the water moving into the stone reservoir, and then into the soil, peak flows to the sanitary and combined systems are reduced. This reduces flows in the systems and reduces the chance of overflows during intense storms.
Planners weighing the merits of an investment with multiple benefits will note:
- Runoff control and flood resilience. By temporarily storing water into the base layer, streets and parking lots are reducing peak discharge to drains and streams.
- Cleaner water at the outfall. As the stormwater passes through the permeable pavement and stone, the sediments, which hold attached pollutants, are shed.
- Healthier urban landscapes. The base layer provides some infiltration to provide moisture support to the adjacent street trees and planting strips, and helps to reduce heat island impacts.
Installation Considerations
The first decision for use of permeable asphalt is site selection. Permeable sections perform best when installed on gentle slopes, away from heavy sediment areas, and within native soil that will accept some infiltration. Utility conflicts, curb lines, and driveway crossings should be located early. Materials matters: an open-graded surface with a clean and angular stone reservoir provides performance and space. The next prompt is traffic loading. For bus stops or loading docks and turning areas, designers may replace permeable pavement with dense-graded asphalt, and/or dense-graded asphalt in any park bays.
Construction sequencing is an important step: construction operators must keep subbase clean; no driving or stockpiling on the stone. Ensure inlets are protected from sediment clogging due to work being conducted involving larger work. Make vacuum sweeping a schedule item; vacuum sweeping is a major part of maintenance for core asphalt to keep the voids open. Avoid sealcoating on permeable pavements. If some sections do clog many years later, it is possible to allow for targeted milling and overlay, which may renew the paving surface but allow the reservoir to reside. Local area experts or depot consult with a crew who understands compaction timeframes, edge resistance, and winter care. For instance, a paving contractor Vacaville advises for regional storm weather patterns and soils.
Plan approach in advance: the organization of any sweeping schedule planned, snow plow adjustable; then keep sediment off the surface. Getting firms providing various commercial paving services to coordinate during the design and warranty timeframes provides clear signaling to the maintenance phase of the pavement structure.
Case Studies in Cities
Cities already use permeable asphalt to enact some very urban solutions. The comments below reflect results from pilot block, alley, and parking field installations.
- Philadelphia Alleys. In older rowhouse neighborhoods, alleys were reconstructed with porous asphalt over deep stone. Alley residents indicated their alleys had less standing water to navigate and noise from tire impacts was mitigated. The city combined the work with tree trenches to route the runoff away from the combined sewer system.
- Seattle Curbside Parking. On traffic calmed roads the parking lane and bicycle buffer were the areas installed with permeable asphalt, and the travel lanes remained dense-graded. This design reduced splash-over in the parking lane and fed adjacent bioretention cell.
- School Districts, Midwest. School districts installed permeable asphalt as a replacement for cracked parking lots. The permeable asphalt addresses runoff created by larger roofs and bus circulation. The stone reservoir also acts as a structural base for the play courts added during maintenance. As a remediation step, they include vacuum sweeping quarterly and inspection of the pavement.
Looking Ahead
Permeable asphalt is not a panacea, but manageable resilience in a streetscape context. Permeable asphalt occupies a space at the edges and margins of curbs, utilities, and grades of traffic in the right-of-way, and also works well with trees and rain gardens. If the design is intentional, successful installation is methodical, and operations is regular, permeable asphalt can help convert more hardscape back into hydrology—one block, one parking bay, and one campus lot at a time.