Porous Asphalt Pavements Kent Hansen Director of Engineering National Asphalt Pavement Association

Resources Tom Cahill Cahill Associates Environment Consultants Newt Jackson Nichols Consulting Engineers Larry Scofield Arizona DOT

What are Porous Pavements? Open-Graded HMA ~ 2 ½” ½” Agg. (#57) ~ 1 – 2” Thick Clean Uniformly Graded 2”-3” Crushed Agg. (#2) – 40% Voids Non-Woven Geotextile Uncompacted Subgrade

-20

Infiltration Wetlands Wet Ponds Filtering Swales

TSS

Zink

Nitrate

Total Nitrogen

Soluable Phosphorus

Total Phosphorus

Median Polutant Removal Efficiency 120

100

80

60

40

20

0

Dry Ponds

Comparison of Detention vs. Infiltration Design Systems

Porous bituminous pavement ƒ Developed by the Franklin Institute – 1972 ƒ Tested in pilot projects during 1970’s ƒ Development of geotextiles in 1979 ƒ Current design since 1980 ƒ CA has built over 150 projects since 1980 ƒ Outstanding engineering project - 2000

Keys to Success – Site Conditions ƒ Soil permeability/infiltration rate ƒ EPA recommends 0.5”/hour ƒ 0.1”/hour still OK

ƒ Depth to bedrock > 2’ ƒ Depth to high water > 3’ ƒ Fill – not recommended ƒ Frost ƒ Pavement section should exceed frost depth

Soils Investigation ƒ Borings and/or test pits ƒ Test permeability ƒ Determine depth to high water table ƒ Determine depth to bedrock

Keys to Success - Design ƒ Slope – limit surface slope to 5% ƒ Terrace when necessary ƒ Use conventional HMA for steeper slopes

ƒ Avoid piping water long distances ƒ Spread infiltration over largest area possible ƒ 5:1 Impervious: Infiltration

Avoid piping long distances

Bottom Must Be Flat Recharge Bed

Recharge Bed

Design Regulations ƒ Rainfall ƒ Typical designs for 6 month/24 hr storm ƒ Conservative design for 20 year/24 hr storm range from 1.4 to 15 in./24 hr.

ƒ Meet Local & State wastewater mitigation requirements.

Keys to Success – Design Usage / Vehicle Loading Lightly loaded areas ƒ ƒ ƒ

Parking lots Low volume roads (limited truck use) Recreational Areas

ƒ Meet structural requirements ƒ Roads?

What about roads?

It does rain in Arizona

18 Years Later

Roads ƒ Challenges ƒ ƒ ƒ ƒ

Cuts and fills Slope Variable soil conditions Utilities

ƒ Limited use

Keys to Success – Construction ƒ Build porous pavement last ƒ Protect from construction debris ƒ Protect from soil laden runoff ƒ Protect site from heavy equipment ƒ Don’t compact subgrade ƒ Excavate to subgrade (soft footprint) ƒ Place filter fabric

Keys to Success – Construction ƒ Place reservoir course 1.5 to 3 in. stone (if gravel source then 95% double fracture) ƒ Place 1-2 in layer of ½ in stone to stabilize the surface of the reservoir course ƒ Place porous asphalt course (2 to 4 in.) usually compacted / seated with 2-3 passes with 10 ton roller.

Porous HMA Surface 100 90

NAPA Ohio Oregon

80

Percent Passing

70

FHWA TA

60 50 40 30 20 10 0

#200

#8

#4 Sive Size

3/8 in

1/2 in

3/4 in

1 in

Open-Graded HMA ƒ Binder Content 6.0-6.5% ƒ Should consider using stiffer asphalt ƒ Consider modified asphalt ƒ Consider fibers ƒ Thick OG HMA – 2 layers?

Construction Guidelines Construction ƒ Restrict traffic for 24 hrs. ƒ Protect porous pavement from contamination. ƒ Runoff sediment ƒ Construction debris

Construction Guidelines ƒ Post Construction ƒ Inspect for design compliance during storm event. ƒ Confirm vegetation is established before removing temporary storm water measures ƒ Do not sand or ash for snow or ice, liquid deicing compounds may be used. ƒ Sign for maintenance.

Maintenance ƒ Inspect several time first few months during storm events. ƒ Inspect annually thereafter. ƒ Pavement surface may be flushed or jet washed. ƒ Damage pavement can be repaired using dense hot mix provided