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 storage reservoir aggregate layers. The benefit of porous asphalt in comparison to some other permeable pavements is that it doesn't require proprietary components, joint stabilizing aggregate, 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, filtration and detention 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 has prepared life cycle costs estimates for each design configuration, based on a drainage area composed of 1,000 m² of conventional asphalt and 1,000 m² of porous asphalt, runoff control target of 25 mm depth and 72 hour drainage period, for comparison which can be viewed below. To generate your own life cycle cost estimates customized to the development context, design criteria, and constraints applicable to your site, access the updated LID Life Cycle Costing Tool (LCCT) 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 stabilizing base aggregate layer, and a sub-base aggregate layer (or stone reservoir) for water storage. Optional components include an underdrain to remove excess water, flow restrictor to control the release rate of the facility, and surface drains to safely convey flows in excess of the storage capacity of the design.

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 base layer aggregate (19 mm dia. clear stone) depth of 50 millimetres.
• Default asphalt depth of 110 millimetres.
• Default sub-base layer (or stone reservoir) aggregate (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 catch basin, annual vacuum sweeping of the pavement surface and some repairing of potholes, but assume no rehabilitation of the porous asphalt surface is required to maintain acceptable drainage function 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: Click on each image to enlarge to view associated construction cost results.

Above you can find breakdowns of construction costs by expense type for a pavement consisting of 1,000 m² of porous asphalt and 1,000 m² of standard asphalt (I:P area ratio of 1:1) for each design configuration:

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

As can be seen, regardless of outlet configuration, Material & Installation expenses represent the largest portion of the total construction costs (74 to 77%). These include costly components such as the impermeable membrane/liner, underdrain, sub-base layer of 50 mm dia. clear stone, base layer of 19 mm dia. clear stone, and porous and standard asphalt surface course layers.

Life Cycle Costs

Below are capital and life cycle cost estimates for the three porous asphalt configurations over 25- and 50-year time periods. The estimates of maintenance and rehabilitation (life cycle) costs represent net present values. 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 restriping, some repairing of potholes and flushing of underground pipes (underdrain and overflow) after 25 years of operation. Estimates assume no rehabilitation of the porous asphalt surface is required to maintain acceptable drainage performance over the 50 year timeframe.

25-Year life cycle cost break down

Porous Asphalt: Full infiltration

Porous Asphalt: Non-infiltrating

Porous Asphalt: Partial infiltration

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

Cost Summary Tables

Total life cycle cost estimates over the 50 year evaluation period for the three porous asphalt configurations vary substantially with the No Infiltration design being highest ($260,789.11), compared to the Partial Infiltration design ($237,742.21), and followed closely by the Full Infiltration design ($228,316.69).

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.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

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

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

Sustainable Technologies Evaluation Program

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.

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.

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 perforated pipe used to assist the draining of soils.

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.

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.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

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.

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