Porous Asphalt: Life Cycle Costs

Porous Asphalt in parking stalls (Source: EOR).

Overview

Porous Asphalt is an alternative to traditional impervious pavements that allow stormwater to drain through them and into a storage reservoir below. Porous asphalt's performance and integrity is similar to that of other standard asphalt pavements. Porous asphalt contains air pockets, which are created during the development process of the paver due to the inclusion of less fines and sand content in comparison to traditional asphalt. The "air pockets" or greater void spaces are what allow stormwater to infiltrate through the surface level to the underlying stone aggregate layer bed. The benefit of porous asphalt in comparison to some other permeable pavements is that it doesn't require third party, proprietary components, nor specialized paving equipment for installation.

Depending on the native soil properties and site constraints, the system may be designed for full infiltration, partial infiltration, or as a non-infiltrating detention and filtration only practice. They can be used for low traffic roads, parking, driveways, and walk ways, and are ideal where space for other surface BMPs is limited.

The information found here relates to Porous Asphalt, for costs and information associated with Permeable Interlocking Concrete Pavers click here. STEP conducted life cycle costs estimates for each of permeable pavements 3 design configurations which can be viewed below. To design your own life cycle cost estimates that can be adapted to fit your project budget and unique development needs access the updated LCCT Tool here.

Design Assumptions

Porous asphalt is ideal for sites with limited space and projects such as low traffic roads, parking lots, driveways and walkways. Components include: a porous asphalt surface, a stabilizer course, and a base layer for storage. Additional components include an underdrain to remove excess water and surface drains to deal with runoff flows in excess of design capacity.

Design and operation and maintenance program assumptions used to generate cost estimates are based on tool default values and the following STEP recommendations:

• Native soil infiltration rates for Full, Partial and No Infiltration Design scenarios were assumed to be 20 mm/h, 10 mm/h and 2 mm/h, respectively, and a safety factor of 2.5 was applied to calculate the design infiltration rate.
• Operation and maintenance (O&M) cost estimates assume annual inspections, removal of trash and debris twice a year, and vacuum sweeping annually. Verification inspections are included every 5 years to confirm adequate maintenance, and every 15 years to confirm adequate drainage performance through in-situ surface infiltration rate testing.
• Impervious drainage area to permeable surface area (I:P area) ratio of 1:1
• Default Stabilizing course/ Choker layer (19 mm dia. clear stone) depth of 50 millimetres.
• Default Asphalt depth of 110 millimetres.
• Default Stone resevoir (50 mm dia. clear stone) depth of 230 millimetres.
• A 150 mm diameter perforated underdrain pipe is included in Partial Infiltration and No Infiltration design configurations only.
  

Notes

• Operation and maintenance cost estimates include removal of sediment from the catchbasin, by annual surface vacuum sweeping, repairing of potholes and assume no rehabilitation of the porous asphalt surface is required over the 50 year timeframe.
• The tool calculates costs for new (greenfield) development contexts and includes costs for contractor overhead and profit, material, delivery, labour, equipment (rental, operating and operator costs), hauling and disposal.
  • Land value and equipment mobilization and demobilization costs are not included, assuming BMP construction is part of overall development site construction.
  • Design and Engineering cost estimates are not calculated by the tool and must be supplied by the user.
  • The tool adds 10% contingency and additional overhead as default.
• All cost estimates are in Canadian dollars and represent the net present value (NPV) as the tool takes into account average annual interest and discount rates over the 25 and 50 year operating life cycle periods.
• Unit costs are based on 2018 RSMeans standard union pricing.
• Additional costs associated with retrofit or redevelopment contents is assumed to be 16% higher than the cost for new (greenfield) development contexts.
  • Retrofit construction cost estimates are included in the 'Costs Summary' section for comparison.
    

Construction Costs

Construction Costs Per Unit Drainage Area (CAD$/m²) - No Infiltration Design, 25 mm Retention

Construction Costs Per Unit Drainage Area (CAD$/m²) - Full Infiltration Design, 25 mm Treatment

Construction Costs Per Unit Drainage Area (CAD$/m²) - Partial Infiltration Design, 25 mm Retention

Note: Please click on each image to enlarge to view associated construction cost results.

Above you can find a cost breakdown of a 1000m² in three different configurations:

1. Non-infiltrating/filtration only Porous Asphalt,
2. Partial infiltration Porous Asphalt
3. Full infiltration Porous Asphalt

As can be seen the greatest cost for this practice (regardless of configuration type) will be Material & Installation, which include costly components such as the, impermeable membrane/liner, the underdrain, the porous asphalt sub-base of 50mm clear stone, the conventional asphalt base course (stone reservoir) of 230mm Granular A, the 19mm Stabilizing course and the asphalt (both conventional and permeable).

Life Cycle Costs

Below are both the capital and life cycle costs of the three porous asphalt configurations over a 25- and 50-year time horizon based on a detailed assessment of local input costs, maintenance requirements, rehabilitation costs and design scenarios relevant to Canadian climates. The costs of maintenance and rehabilitation (Life cycle costs) are set at "Present Value" of these activities in 2022. The estimates of maintenance and rehabilitation (life cycle) costs represent net present values (NPV). Operation and maintenance costs are predicted to represent between 26 to 29% of total life cycle costs over the 25-year evaluation period, and increase to between 37 to 40% of total life cycle costs over the 50-year period, due to costs associated with increased surface vacuuming, restriping, repairing of potholes and cleaning out underground pipes (underdrain and overflow) to prevent clogging assumed to be continually required after 25 years of operation.

25-Year life cycle cost break down

Porous Asphalt: Full infiltration

Porous Asphalt: Non-infiltrating

Porous Asphalt: Partial infiltration

Note: Please click on each image to enlarge to view associated life cycle cost results.

50-Year life cycle cost break down

Porous Asphalt: Full infiltration

Porous Asphalt: Non-infiltrating

Porous Asphalt: Partial infiltration

Note: Please click on each image to enlarge to view associated life cycle cost results.

Total Cost & Design Summary

Total life cycle cost estimates for the three porous asphalt configurations vary substantially with the No Infiltration design being highest ($160,519.90), compared to the Partial Infiltration design ($138,144.27), and followed closely by the Full Infiltration design ($131,972.30).

It is notable that a sensitivity analysis was conducted in 2019 to compare construction cost estimates generated by the tool to actual costs of implemented projects. The analysis found that tool estimates were typically within ±14% of actual construction costs.

Full Infiltration

An example of installed porous asphalt in a designated biking lane, with curb cut inlets leading to adjacent bioswale features, located in the in the City of Gresham, Oregon (Source: Sightline Institute, 2012).

Partial Infiltration

Non-Infiltrating/filtration only

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

Soil particles with a diameter less than 0.050 mm.

The natural ground material characteristic of or existing by virtue of geographic origin.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

The technique of removing pollutants from runoff as it infiltrates through the soil.

Sustainable Technologies Evaluation Program

A perforated pipe used to assist the draining of soils.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The natural ground material characteristic of or existing by virtue of geographic origin.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

Credit Valley Conservation