Porous Asphalt: Life Cycle Costs

==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 layersCity of Toronto. 2017. Toronto Green Streets Technical Guidelines. Version 1.0. August, 2017. https://www.toronto.ca/legdocs/mmis/2017/pw/bgrd/backgroundfile-107515.pdf. 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 installationSpeight, J.G., 2016. Asphalt materials science and technology (pp. 437-474). Butterworth-Heinemann is. https://link.springer.com/article/10.1557/mrs.2016.267#article-info.
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==
thumb|right|400px|Construction Costs Per Unit Drainage Area (CAD$/m2) - No Infiltration Design, 25 mm Retention
thumb|left|400px|Construction Costs Per Unit Drainage Area (CAD$/m2) - Full Infiltration Design, 25 mm Treatment
thumb|center|400px|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:
#Non-infiltrating/filtration only Porous Asphalt, 
#Partial infiltration Porous Asphalt
#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===
thumb|left|400px|Porous Asphalt: Full infiltration
thumb|right|400px|Porous Asphalt: Non-infiltrating
thumb|center|400px|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===
thumb|left|400px|Porous Asphalt: Full infiltration
thumb|right|400px|Porous Asphalt: Non-infiltrating
thumb|center|400px|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 costsCredit Vally Conservation (CVC). 2019. Life-cycle costing tool 2019 update: sensitivity analysis. Credit Valley Conservation, Mississauga, Ontario. https://sustainabletechnologies.ca/app/uploads/2020/04/LCCT-Sensitivity-Analysis_March2020.pdf.
===Full Infiltration===
700px
===Partial Infiltration===
700px
===Non-Infiltrating/filtration only===
700px
==References==