Porous asphalt pavement uses open-graded asphalt mixes and permeable bases to allow stormwater to infiltrate vertically into the subgrade. It provides stormwater management benefits like reduced runoff and groundwater recharge while requiring less drainage infrastructure. Proper design considers the soil, slope, and hydrology to determine appropriate reservoir sizing. Materials include specialized open-graded asphalt and aggregates with high void ratios. Construction requires protecting subgrades and compacting thin lifts, while maintenance involves periodic vacuuming and limited repair capabilities.

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Porous Asphalt Pavements for
Stormwater Management
Amir Ghavibazoo, Ph.D.
Senior Pavement Engineer
Twining, Inc.
April 2017

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Porous Asphalt Pavement
• Porous asphalt pavements use open-graded mixes on top of stone reservoir to let the storm water
flow vertically down and infiltrate into the subgrade
Porous Asphalt = Pervious Asphalt = Uncompact Asphalt
 An efficient LID technique
to manage storm water

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Porous Asphalt Applications
• Open-graded friction course (OGFC): thin, open-graded asphalt mix placed on impermeable
pavement
– The storm water will flow into the OGFC layer and then flow laterally to drain out the side of the
pavement
• Porous pavement: OGFC on permeable aggregate reservoir on uncompacted subgrade soil
– The storm water flow vertically down through the pavement layers and the infiltrate into the
subgrade

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Porous Asphalt Pavement Benefits
Porous friction course vs. dens graded HMA
Porous Pavement vs. dens conventional Pavement
• Reduces the stormwater runoff
• Reduces contamination in water runoff
• Recharges the ground water
• Reduces the need for drainage structure and
right of way
• Reduces the noise
• Improves the wet-weather visibility

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Porous Pavement Composition
• Open-graded HMA
 Va% >18%
• Choker layer (usually Asphalt Treated Permeable Base)
 Works as a transition layer and creates stabilized surface
• Aggregate Reservoir
 40% voids to store water and create structural support
• Perforated Pipe
 To remove overflow of water from the site
• Non-woven Geotextile fabric
 To support the subgrade and prevent migration of fine material to the
reservoir aggregate
• Uncompacted soil subgrade

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Design of Porous Asphalt Pavement
• Site Considerations
– Soil types
• Infiltration rates of 0.1 to 10 inches/hr (tested every ¼ acre)
– Depth of bedrock or impermeable layer (i.e. clay soil)
• More than 2 ft deep from the bottom of the reservoir
– Depth of groundwater
• More than 10 ft deep from the bottom of the reservoir
– Pavement slope
• Less than 5%
(Check the Low Impact Manual of the governing agency)
Double ring infiltrometer

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Design of Porous Asphalt Pavement
• Hydrology Design
– Measures the expected inflow
– Determines the desired size (thickness) of
reservoir
– Recommendations:
• Include an overflow in the design
• Include alternate path for stormwater to
enter the aggregate reservoir

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Design of Porous Asphalt Pavement
• Caltrans follows typical highway design manual:
– Uses R-value of the soil
– Gf of the Aggregate Reservoir = 1.0 (minimum 0.7 ft for areas with truck traffic)
– Porous asphalt layer includes:
• Open-Graded Friction Course (is not considered as part of structure) – minimum 0.1 ft
• Asphalt Treated Permeable Base (ATPB) (Following the HDM, Gf = 1.4) – min 0.25 ft
Note: Gf ≈ 1.9
• To follow AASHTO 93 design procedures, use the following coefficients

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Porous Asphalt Pavement Materials
• Open-graded asphalt mix (SSPWC,2015)
– Va >18%
– AC content (TWM) > 6% (film thickness twice as conventional HMA)
– Draindown < 0.3%
– 0.5 percent liquid antistrip
– One grade higher PG
• Other considerations
– Fibers can be added to reduce draindown
– Polymer modified binder would be recommended for street applications
– TSR test (if needed) should be conducted on dens-graded mix with same source of aggregate

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Porous Pavement Materials
• Chocker Layer
– Usually Asphalt Treated Permeable Base (ATPB)
• 3/4” open-graded aggregate following Caltrans standard specification
• 2.5% Asphalt content (DWA)
• Aggregate reservoir
– 1.5 to 3-inch maximum aggregate size
– Single size graded crushed aggregate

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Construction of Porous Asphalt Pavement
• Protect the subgrade from heavy equipment
• Cover the final grade with fabric geotextile (16 inch overlap)
• Install the overflow drainage pipe (if required)
• Place the reservoir aggregate in one lift (8 to 12 inches)
• Compact with one single pass of light roller
– Protect the reservoir aggregate from dirt and sedimentation

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Construction of Porous Asphalt Pavement
• Laydown the asphalt in 1 to 4-inch lifts
– Track pavers are recommended
• Should be compacted with two to four passes of 10-ton static roller
– No density test required
– Compaction process should be observed
• Restrict traffic for at least 24 hours after final roller
• Recommended testing and inspection during construction:
– Asphalt content
– Gradation
– Surface Infiltration test

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Maintenance of Porous Asphalt Pavement
• Vacuum at least two to four times a year
– Vacuum the surface in case of excessive dirt and sedimentation
– The surrounding improvements should be carefully selected to minimize tracking of dirt on
surface
• Damaged pavement can be repaired up to 10% of area
• Remove and replace porous pavement layer and leave the reservoir intact
• In general it has limited maintenance options

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Thank You!

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• Definition
• Benefits and Limitations
• Applications
• Design of Porous Asphalt Pavements
• Site Consideration
• Structural Design
• Hydrology Design
• Material Characteristics
• Construction
• Maintenance