POROUS ASPHALT PAVEMENT

Erik W. Edwards

Problem Report submitted to the

Benjamin M. Statler College of Engineering and Mineral Resources

At West Virginia University

in partial fulfillment of the requirements

for the degree of

Master of Science

in

Civil Engineering

John P. Zaniewski, Ph.D., Chair

John Quaranta, Ph.D., P.E.

Andrew Morgan, P.E.

Department of Civil and Environmental Engineering

Morgantown, West Virginia

2012

Keywords: Porous, Asphalt, Permeable, Pervious, Stormwater, Runoff

Abstract

Porous Asphalt Pavement

Erik W. Edwards

Pervious pavement is an increasingly popular new practice within the construction industry. One type of pervious pavement is known as porous asphalt. Also known as pervious, permeable, or open-graded asphalt, porous asphalt is a type of hot-mix asphalt, with less sand or fines then a dense graded mix. The reduced fines leave stable air pockets in the asphalt, which allow water to drain through it. Once through the asphalt, the water drains into an underground aggregate recharge basin, where it is retained and treated before entering the groundwater.

As stormwater runoff becomes an increasingly prominent issue, greater funds continue to be allocated for the construction of onsite rainwater retention ponds and runoff pools. Porous asphalt reduces the need for these ponds without requiring the additional cost. In addition to the cost savings from removing further construction, porous asphalt can minimize a projects overall footprint.

Porous asphalt also allows for the decentralizing of rainwater seepage into the groundwater system. The importance of containing stormwater onsite has lately become increasingly evident. Environmental groups prefer localized stormwater projects rather than those that contribute to large sewer systems. Porous asphalt is considered by the Environmental Protection Agency a Best Management Practice.

The United States Green Building Council’s green building rating system, Leadership in Energy and Environmental Design awards several credits to buildings or sites that can contain all runoff generated without adding to local sewer systems. Porous asphalt pavement allows a site to do just that, allowing the infiltration of all locally generated storm water.

Construction is similar to traditional asphalt projects with only a few minor changes and omissions. Less compaction and a greater focus on site selection are just a few of these considerations.

At the current time, porous asphalt pavement projects are mainly used for parking lots and other low traffic areas. One reason for this is a potential lack of centralized technical information on the subject. Several projects are being designed empirically off of the results of a previous project. With further research, it may be possible for porous asphalt to become used in future roadway construction, thus alleviating the need for all other storm water management techniques. This report serves as a step towards gathering the available information on the topic into one central location.

August 3, 2012 iii

Table of Contents

1 Introduction...................................................................................................................... 1

1.1 Background ............................................................................................................... 1

1.2 Purpose ..................................................................................................................... 1

1.3 Potential Future Uses ................................................................................................ 2

1.4 Porous Pavement Structure – ................................................................................... 2

2 Porous Asphalt Pavement Design .................................................................................... 4

2.1 Site Selection ............................................................................................................. 4

2.1.1 Hazardous Materials ............................................................................................. 4

2.1.2 Geological Conditions ............................................................................................ 5

2.1.3 Percolation Testing ................................................................................................ 5

2.1.4 Design Storm ......................................................................................................... 6

2.1.5 Frost Depth ............................................................................................................ 7

2.2 Pavement Bed Design ............................................................................................... 7

2.3 Filter Fabric Selection ................................................................................................ 8

2.4 Pavement Base Design ............................................................................................ 10

2.4.1 Base Course ......................................................................................................... 10

2.4.2 Choker Course ..................................................................................................... 12

2.5 Surface Design ......................................................................................................... 13

2.5.1 Mixture Design .................................................................................................... 13

2.5.2 Alternative Asphalt Concrete Mixes ................................................................... 15

3 Construction and Maintenance...................................................................................... 17

3.1 Pavement Bed ......................................................................................................... 17

3.2 Geotextile ................................................................................................................ 17

3.3 Pavement Base ........................................................................................................ 18

3.3.1 Base Course ......................................................................................................... 18

3.3.2 Choker Course ..................................................................................................... 18

3.4 Porous Asphalt Surface ........................................................................................... 19

3.5 Maintenance ........................................................................................................... 20

August 3, 2012 iv

3.5.1 Sediment Removal .............................................................................................. 20

3.5.2 Snow Removal ..................................................................................................... 20

3.5.3 Awareness ........................................................................................................... 21

4 Advantages and Disadvantages of Porous Asphalt Pavement....................................... 22

4.1 Advantages .............................................................................................................. 22

4.1.1 Elimination of Stormwater Runoff ...................................................................... 22

4.1.2 Footprint Size ...................................................................................................... 22

4.1.3 Runoff Quantity and Quality ............................................................................... 23

4.1.4 LEED Rating System ............................................................................................. 23

4.1.5 Natural Drainage ................................................................................................. 25

4.1.6 Reduced Costs ..................................................................................................... 25

4.2 Disadvantages ......................................................................................................... 26

4.2.1 Clogging ............................................................................................................... 26

4.2.2 Slopes .................................................................................................................. 27

4.2.3 Bed Design ........................................................................................................... 27

5 Case Study Performance Observation ........................................................................... 28

5.1 Oregon Neighborhood Streets ................................................................................ 28

5.2 University of Rhode Island Parking Lot ................................................................... 28

5.3 Arizona SR-87 .......................................................................................................... 29

6 Additional Research Needs ............................................................................................ 31

6.1 Mix Design ............................................................................................................... 31

6.2 Applications............................................................................................................. 31

7 Conclusion ...................................................................................................................... 32

8 References ...................................................................................................................... 33

List of Tables

Table 1 - Filter Fabric Requirements ............................................................................................. 10

Table 2- Gradation for Porous Asphalt Base Courses. (Roseen et. al., 2007) .............................. 11

Table 3. Standard Porous Asphalt Mixes (Wisconsion Asphalt Pavement Association, 2011) .... 14

Table 4. Porous Asphalt Mixture Requirements ........................................................................... 15

August 3, 2012 v

List of Figures

Figure 1 – Typical Porous Asphalt Pavement Section ..................................................................... 3

Figure 2 - Average US Frost Line Depths (Waterer, 2012) .............................................................. 8

Figure 3 - Geotextile for Separation of Subgrade and Aggregate .................................................. 9

Figure 4 – Recommended Geotextile Fabric Overlap ................................................................... 18

Figure 5 Construction Practices to Minimize Compaction of Pavement Base ............................. 19

Figure 6. Porous vs. Non-porous asphalt, (Rose, 2010) ................................................................ 22

August 3, 2012 1

1 Introduction

1.1 Background

Traditional Hot Mix Asphalt (HMA) is an impervious material, and extensive design

considerations are used to get it that way. A good deal of research has been done in the area

of dense graded asphalt mix designs. Dense graded asphalt eliminates the opportunity for

water to seep into the subgrade, jeopardizing its strength and support. In addition, asphalt

concrete is designed to minimize weathering related distresses stemming from permeability.

A drawback to this impervious surface is the issue of the surface water runoff. Pervious

pavements are designed to allow percolation or infiltration of stormwater through the surface

into the soil. The water is naturally filtered and pollutants are removed. Porous asphalt

pavement is one type of pervious pavement.

Porous asphalt is achieved by altering the aggregate gradation to an open graded blend.

This creates interconnected voids, allowing water to flow through the asphalt surface and into

the pavement structure. Once through the asphalt, the water enters the aggregate base, which

slows, stores, and allows the water to infiltrate into the native ground.

The concept of “Porous Asphalt Pavement” was conceived in 1968 at the Franklin Institute

Research Laboratories. It was further developed with support and funding from the U.S.

Environmental Protection Agency (EPA) during 1970 and 1971. Interest in the concept

prompted Edmund Thelen and Leslie Fielding Howe to publish a book about its development

that included a design guide, Porous Pavement (Thelen & Howe, 1978). Since then, individual

states have examined different applications of porous pavements. The main uses have focused

on parking lots, with additional uses on local roads, walking trails, and service roads. In

addition, Arizona has constructed a porous pavement for heavy use roadways (Thelen and

Howe, 1978). In addition, the National Asphalt Pavement Association, NAPA, published a

technical guide for the use of porous pavements (NAPA, 2012).

1.2 Purpose

Through the past twenty years, porous asphalt pavements have been gaining acceptance.

The movement towards sustainable design is one of the larger factors encouraging the use of

August 3, 2012 2

porous pavements. Storing stormwater within the pavement provides several sustainable

benefits, including:

1. Eliminating or reducing the need for retention basins

2. Reducing the volume of stormwater runoff

3. Reducing hydrocarbon transport in stormwater

4. Uses a natural filtration process to clean the water captured in the pavement structure.

There is a lack of centralized technical information on the design and materials used in

porous pavements. Many states do not have design specifications for porous asphalt

pavements. Projects in different areas of the country are borrowing design techniques from

other areas. There is very little documentation about what techniques work well and where.

This document gathers available information into a state of the practice. It will discuss what

information is known, and what needs determined by further research.

1.3 Potential Future Uses

Currently, the majority of applications of porous asphalt pavement have been focused on

only a few types of applications. The main use so far has been on parking lots, service roads,

and other low volume areas. Ideally porous asphalt pavement could become the material of

choice for larger scale projects, including high volume roadways. However, at this time

strength requirements have yet to be met using a porous asphalt pavement and more research

is needed to get to this point. Though it has been used successfully in public use trials, it is not

believed to be suited for heavy road use at this time.

1.4 Porous Pavement Structure –

Figure 1 presents a cross-section for a porous pavement structure (NAPA, 2012). Different

designs may not include all of the layers seen in Figure 1. The layer terminologies used in the

balance of this report are:

Porous Asphalt Surface – Open graded asphalt mix

Choker Course – provide a construction platform for the surface course and provide limited

filtering capabilities

August 3, 2012 3

Base Course – also referred to as the recharge bed, makes up the majority of the pavement

base. Serves as both storage for collected stormwater in addition to the structure for the

overlying pavement

Non-woven Geotextile – A non-woven geotextile is recommended to maximize infiltration

and increase separation

Pavement Base – Consists of both the base and choker courses

Pavement Bed – Uncompacted subgrade beneath the base course

Figure 1 – Typical Porous Asphalt Pavement Section

August 3, 2012 4

2 Porous Asphalt Pavement Design

2.1 Site Selection

Porous pavements have traditionally been limited to parking lots, low traffic roadways, and

pedestrian-bike paths. A few important guidelines should be considered during the site

selection process. The factors that should be considered include hazardous contamination of

the subgrade, the ability of the subgrade to allow infiltration, depth to bedrock and the ground

water table, and frost depth.

Traditionally, stormwater management systems, including retention and detention basins,

are designed to channel and direct runoff to the lowest point on a site. These low points are

commonly plagued with poor drainage capabilities as they have been draining the undeveloped

site for years, accumulating small particles often carried in runoff (Adams, 2003). This low spot

is also often next to the streams or wetlands. Porous asphalt pavement systems perform best

on pre-developed soils. As infiltration is vital to the success of a porous asphalt pavement,

avoiding previously developed and compacted soils may be necessary. These areas are typically

at higher elevations than these low spots (Adams, 2003).

Infiltration systems work best when the water is allowed to infiltrate over a large area. As a

rule of thumb, one should design to a ratio of 5:1 impervious area to infiltration area. That is,

the runoff from 5 acre of impervious area would require a 1 acre infiltration bed (Adams, 2003).

When the porous pavement is designed to store only the water from the pavement surface the

ratio is 1:1. Use of the porous pavement to store runoff from other impervious surfaces, such

as building roofs and sidewalks should follow the recommended 5:1 ratio guideline.

Clay, poorly draining soils, can serve as the pavement bed. The thickness of the aggregate

bed is designed to accommodate the drainage ability of the subgrade soil. The depth required

for the base course can be determined from a percolation test.

2.1.1 Hazardous Materials

Hazardous Materials Loading and Unloading Areas

Porous pavements should not be used in areas where there is loading and unloading of

hazardous products and materials. In addition, it should not be used where there is a potential

August 3, 2012 5

for spills and fuel leakage. The direct link from the surface to the groundwater beneath could

result in ground water contamination in the event of a spill. Examples include fueling stations,

airports, and truck depots. (United States Environmental Protection Agency, 2009).

Brownfields

A brownfield is defined as a former industrial or commercial site where future use is

affected by real or perceived environmental contamination (United States Environmental

Protection Agency, 2009). Traditional asphalt pavements have been used to seal the surface to

prevent water from penetrating into the contaminated zone and transporting the

contaminates. Currently there are no regulations about using porous asphalt pavement,

however it would not be advisable to construct a porous asphalt surface over a known

brownfield. The entirety of the environmental risk associated with a brownfield is not always

known, and should not be dealt with unless plans include complete remediation of the site and

soils before construction. Though every project is unique, the introduction of fresh stormwater

into a brownfield would only increase the ability of the underground pollutants to migrate to

additional areas (United States Environmental Protection Agency, 2009).

2.1.2 Geological Conditions

The Environmental Protection Agency (EPA) has determined that an area deemed fit for the

use of porous asphalt must have at least 4 feet of clearance between the bottom of the

recharge bed and the bedrock. In addition, the agency recommends a minimum of 4 feet

clearance between the bottom of the recharge bed and the seasonally high water table (United

States Environmental Protection Agency, 2009). This precaution is necessary to minimize the

chances of seepage of groundwater up through the base course, and into the pavement

surface.

2.1.3 Percolation Testing

Before any infiltration system is designed, percolation tests are used to determine the

water infiltration rate of the soil. This test is commonly used in the design of a septic drain field

or "leach field". The depth of the constructed recharge bed underneath the pavement will be a

direct reflection of the percolation limits of the supporting soil. In general, sandy soil will absorb

August 3, 2012 6

more water than soil with a high concentration of clay or where the water table is close to the

surface (Machmeier & Gustafson, 2009).

In its broadest terms, percolation testing is simply observing how quickly a known volume of

water dissipates into the subsoil of a drilled hole of known surface area. While every

jurisdiction may have its own laws regarding the exact specifications of the percolation test, the

testing procedures are usually very similar.

The most commonly used tests are the double ring infiltrometer and percolation tests. The

double ring infiltrometer test estimates the vertical movement of water through the bottom of

the test area. The outer ring helps to reduce the lateral movement of water in the soil from the

inner ring. ASTM D 3385-03, Standard Test Method for Infiltration Rate of Soils in Field Using a

Double-Ring Infiltrometer, and ASTM D 5093-90, Standard Test Method for Field Measurement

of Infiltration Rate Using a Double Ring Infiltrometer with a Sealed Inner Ring are the standard

methods for performing an infiltration test. The percolation test allows water movement

through both the bottom and sides of the test area. Because of this, the infiltration rate for

percolation tests needs to be adjusted to account for any infiltration that occurs through the

sides of the hole. These tests must be performed at multiple locations to determine the

average infiltration rate for the site.

2.1.4 Design Storm

It is good practice to design porous asphalt pavements to function properly in at least a one

hundred year storm. In order to do this, local rainfall data for the area is needed. The rainfall

data can be obtained from the Hydrometeorological Design Studies Center. NOAA has online

information for storm analysis (NOAA, 2012). The design storm information may also be

obtained from local sources, e.g., the West Virginia Flood Protection Plan uses a 100 year

frequency, 6 hour duration, for the analysis of reservoir capacity for dams (WV Flood Protection

Task Force, 2010)

Once the data are obtained, the recharge basin is designed to capture that required amount

of water released during the 100 year storm and slowly infiltrate it into the supporting soil over

August 3, 2012 7

time. The difference between the rainfall and the infiltration determines the volume of water

that must be stored in the voids of the base aggregate.

2.1.5 Frost Depth

Another important site consideration is the depth of the frost line. Historically, it was

assumed for it to be vital that the bottom of the bed be below the frost line. This is to prevent

freezing of water in the pavement base. If freezing within the base course occurs, it is possible

the aggregates will heave, causing distresses in the pavement surface.

After recent research, this may not be entirely true. A number of porous pavements have

been installed in freezing climates with total depths much shallower than the frost depth.

These include walkways at Swarthmore College in Pennsylvania, with a depth of 12 inches, and

a parking lot at the Walden Pond visitor center in Massachusetts, with a bed depth of 12 inches.

Neither of these pavements has shown damage due to frost heave (Hansen, 2008).

The only research on frost depth has occurred at the University of New Hampshire, where

the frost depth is 48 inches. While the porous pavement at the site extends to below the frost

depth, their data from 2006 shows frost penetration in the recharge bed of less than one foot

(Roseen et.al., 2007). Prior to construction, it is still important that the engineer determine the

location of the frost depth in the area. The University conservatively recommends the depth of

the bed be 65 percent of the frost depth in their design specifications (Hansen, 2008)

2.2 Pavement Bed Design

The pavement bed is the undisturbed soil under the porous pavement. Since this is the

native soil, there is no material specification. However, in designing the porous pavement, the

template of the surface of the pavement bed must be design to minimize the rate of horizontal

flow across the surface One of the most important features of the bed is that the bottom is

approximately level; slopes should never exceed a six percent (Roseen et. al. , 2007). This

allows the water to infiltrate into the subgrade at an even rate. Movement along this plane

may create voids, which could allow settling.

August 3, 2012 8

Figure 2 - Average US Frost Line Depths (Waterer, 2012)

2.3 Filter Fabric Selection

When placing stone aggregate on fine grained soils, there are two simultaneous

mechanisms that tend to occur over time. One is that the fine soils enter into the voids of the

stone aggregate, decreasing its drainage capability. The other mechanism is that the larger

aggregate penetrate into the fine soil, thereby ruining the aggregates strength. These

mechanisms are displayed in Figure 3. The two methods used to mitigate this problem are by

the addition of extra aggregate thickness or separation of the subgrade and base with a filter

fabric. Additional aggregate added into the design are referred to as sacrificial aggregate. Use of

filter fabric is the preferred alternative (Koerner, 1998).

August 3, 2012 9

Figure 2 - Geotextile for Separation of Subgrade and Aggregate (Koerner, 1998)

Placing the geotextile separation layer between dissimilar materials maintains and even

improves the integrity and function of both material layers (Koerner, 1998). The separation

fabric minimizes infiltration of anything other than water, and also to serve as a filtration for

the water. The geotextile fabric also ensures that none of the underlying soil travels upwards

into the aggregate bed (Cahill, Adams, & Marm, 2005). It is important when selecting this fabric

that the choice is appropriate for the soil conditions and the current design at the site.

For porous asphalt pavements, the largest requirement of the fabric is the separation. To

maximize infiltration and increase separation, a non-woven geotextile is recommended to

prevent fines in the subgrade from migrating into the stone recharge bed (Koerner, 1998).

Commonly recommended non-woven geotextile used for separation are the Mirafi® N-Series,

Amoco 4547, and Geotex 451 (Hansen, 2008). Typical specifications for geotextiles used in

porous pavements are given in Table 1:

August 3, 2012 10

Table 1 - Filter Fabric Requirements

Test Requirement

Grab Tensile Strength

ASTM - D4632 > 120 lbs.

Mullen Burst Strength

ASTM - D3786 > 225 psi

Flow Rate

ASTM - D4491 > 95 gal.min/ft²

UV Resistance after 500 hrs.

ASTM - D4355 > 70 %

2.4 Pavement Base Design

The pavement base is typically constructed of the base course and a choker course, as seen

in Figure 1. On top of the geotextile fabric is the base course. On top of that course and below

the asphalt is the choker course.

2.4.1 Base Course

The base course, sometimes referred to as the reservoir course, stores the water until it can

infiltrate into the underlying soil. Design of the base course requires both material and

thickness considerations.

In order to maximize storage capacity and still maintain its structural integrity, the base

course should consist of approximately 40% air voids (Adams, 2003). Similarly, The Franklin

Institute recommended that the percentage of voids in the reservoir should be equal to or

greater than 40% in order to store the precipitation (Thelen and Howe, 1978). ASTM C29 is used

to determine the voids in the aggregates.

The base course should consist of a uniformly graded 1.5- to 2.5-in. clean-washed stone

mix, such as an AASHTO No. 3 as seen in Table 2. Depending on local aggregate availability,

both larger and smaller size stones have been used (Adams, 2003). The important requirements

August 3, 2012 11

of the stone include that they be uniformly graded to maximize void space, and also that they

be clean washed as to avoid sediment buildup on the filter fabric. Stones that are dusty or dirty

will clog the infiltration bed and must be avoided.

Table 2- Gradation for Porous Asphalt Base Courses. (Roseen et. al., 2007)

US Standard Sieve Size

Inches (mm)

Percent Passing

Choker course

AASHTO No. 57

Base Course

AASHTO No. 3

6 (150) - -

2 ½ (63) - 100

2 (50) - 90-100

1 ½ (37.5) 100 35-70

1 (25) 95-100 0-15

¾ (19) - -

½ (12.5) 25-60 0-5

3/8 (9.5) - -

#4 (4.75) 0-10 -

#8 (2.36) 0-5 -

#200 (0.075) - -

The stone bed is usually between 18 and 36 in. deep, depending on stormwater storage

requirements, frost depth considerations, and site grading. This depth also provides a

significant structural base for the pavement (Adams, 2003).

The thickness of the base can be calculated from the inflow, infiltration, and void space

between the base course aggregates. This process can be modified to fit the specific project

(Machmeier & Gustafson, 2009).

An example of these calculations is shown below:

A starting point for the design of the depth is to assume that all rainfall from the storm

events will enter the base course from the pavement and an adjacent impervious surface. In

this example, a one hundred year storm is modeled for both the water that falls on the

pavement area (A1), and also the water that falls on the adjacent impervious surface area (A2).

The local 100 year storm in West Virginia is 4.5 inches of rainfall (R) in 6 hours (WV Flood

August 3, 2012 12

Protection Task Force, 2010). An extremely low soil permeability (P) value of 0.1 inches/hour is

assumed. Assuming the area of the adjacent impervious surfaces is half of that of the parking

lot, an equivalent to 6.75 inches of rainfall in 6 hours falls on the porous pavement surface.

With 40 percent voids in the recharge bed, the bed would have to be 16.875 inches to hold this

amount. Over 6 hours, the poorly draining soil will drain 0.6 inches, leaving the required bed

depth at 16.275 inches.

Calculations:

100 year storm = 4.5 inches in 6 hours

Impervious Area A2 = ½(Pavement Area A1)

4.5 x 1.5 = 6.75 inches in 6 hours

6.75 inches / 40 percent voids= 16.875 inches

Percolation P of 0.1 inches/hr.

0.1 inches/hr. x 6 hours = 0.6 inches of percolation

16.875 – 0.6 = 16.275 inches

With a permeability rate of 0.1 inches/hr. it would take approximately 16.275/0.1 = 163

hours, or about seven days for complete removal of the water in the base course. Any rain

during this period would recharge the base course. Hence, this calculation is a starting point for

determining the required base course thickness. This should be modified based on local

experience (Machmeier & Gustafson, 2009).

2.4.2 Choker Course

The purpose of the filter or choker course in the structure is to provide a construction

platform for the surface course and provide limited filtering capabilities. This is typically done

with AASHTO 57 aggregates with the gradation given in Table 1. The main function of this

course is to stabilize the larger aggregate below by locking of the aggregates on the surface. It

is not required that the base course be completely covered, simply that the surface voids be

slightly filled and the surface stabilized (Hansen, 2008).

August 3, 2012 13

2.5 Surface Design

While it is possible to design the thickness of the layers in a porous pavement using

conventional design methods, the thickness of the surface can be estimated based on

guidelines available from the National Asphalt Pavement Association, NAPA (National Asphalt

Pavement Association, 2009). The recommended minimum thicknesses are:

Parking lots with little or no trucks, 2.5” minimum

Residential streets, some trucks, 4.0” minimum

Heavy truck traffic, 6.0 “ minimum

The major differences between the asphalt concrete used in a standard asphalt pavement

versus a porous asphalt pavement is that the porous asphalt mix has a lower concentration fine

aggregate than traditional asphalt and the percent air voids in the compacted mix will be much

greater. In most other manufacturing aspects, porous asphalt is similar to conventional asphalt

and can be mixed at a standard asphalt batch plant. With fewer fines, the asphalt concrete is

porous and allows water to drain though the material through very small interconnected

openings (Adams, 2003). There are several variations of the mix, including gradations

developed by various state transportation departments used as highway overlays and friction

courses. However, for the purposes of stormwater management, a common mix with the good

performance is the mix indicated in Table 3 (Wisconsion Asphalt Pavement Association, 2011).

2.5.1 Mixture Design

Research has determined that sufficient asphalt content is essential to pavement durability.

In sites where lower asphalt content was used, some surface scuffing was observed. In different

situations, various commercial additives intended to improve strength or performance in cold

weather have been added, but in general most proprietary mixes or additives have not been

used. (Adams, 2003).

August 3, 2012 14

Table 3. Standard Porous Asphalt Mixes (Wisconsion Asphalt Pavement Association, 2011)

Mix Properties 12.5 mm Mix 9.5 mm Mix Test Standard Note

Binder Content

5.5% min 5.5% min

1

Binder Grade

PG 64 - 22 PG 64 - 22

2

% Air Voids (Va @ 50 gyrations)

18 - 20 18 - 20

Tensile Strength Ratio (TSR @ 5 cycles freeze/thaw)

80% min 80% min ASTM D4867 3

Draindown at Production Temperature

0.3% max 0.3% max

4

Aggregate Properties

LA Abrasion (% Loss)

AASHTO T 96

- 100 Revolutions

13 max 13 max

- 500 Revolutions

45 max 45 max

Soundness (% Loss) using sodium sulfate

12 max 12 max AASHTO T 104

Freeze / Thaw (% Loss)

18 max 18 max AASHTO T 103

Fractured Faces

ASTM D5821

- 2 Faces

90% min 90% min

- 1 Face

100% min 100% min

Thin or Elongated

5% max 5:1 ratio

5% max 5:1 ratio ASTM D4791

Mixture Gradation Sieve

3/4" 100 -

1/2" 85 - 100 100

3/8" 55 - 75 90 - 100

#4 10 - 25 30 - 40

#8 5 - 12 10 - 20

#16 - 5 - 15

#30 - 3 - 10

#200 1 - 4 1 - 4

VMA (%)

25 min 25 min

Footnotes

1. 5.75 – 6.0% Recommended 2. Minimum high temperature of 64 C Recommended 3. Following national guidance, the Cantabro Abrasion test was not included in the mix

design guidelines 4. Effective measures to reduce draindown include the use of washed manufactured

sand in lieu of crusher screenings and fibers. Also a slight reduction in production temperature may also be considered.

August 3, 2012 15

The mix design used shall be determined by the local state or federal specifications. The

asphalt mix used for a porous pavement has similar characteristics to an open graded friction

course (OGFC) placed on conventional pavements. The Federal Highway Administration

developed a mix design method for OGFC in 1974 (Smith et. al., 1974) which was updated in

1996 to Technical Advisory T 5040.31 (USDOT, 1990). These documents have served as the

basis for the mix designs developed by other agencies.

One mix design that was used in Michigan has been duplicated in other areas across the

country with relative success. This design was for a 12.5mm open-graded mixture. The mix

design met the following criteria (APAM, 2008).

• Course aggregates will be steel slag, limestone or crushed gravel with 100% having one or

more fractured faces, and at least 90% two fractured faces.

• Binder selection will be PG 76-22 for high volume lots or PG 70-22 for lower volume,

though this fluctuates depending on the temperature and other site specific details.

• Air void will be >=16% using ASTM D 6752, Vacuum Sealing method

• VMA should be >=26% using ASTM D 6752, Vacuum sealing method

• Draindown test will be <=0.3% (open graded mixtures may incorporate fibers)

• Gyratory compaction shall be 50 gyrations at 260±9° F.

Table 4. Porous Asphalt Mixture Requirements

Parameter Target Value

Air Voids % 16.0

AC Content % 5.0-6.5

Draindown % (max.), ASTM D6390 0.3

TSR % (min.), AASHTO T283 80

2.5.2 Alternative Asphalt Concrete Mixes

The lack of fine aggregates in porous asphalt mixes has a negative effective of potentially

allowing the asphalt to drain off the aggregate during production and construction. This

phenomenon is termed draindown. ASTM D 6390 and AASHTO T 305 test methods are used to

evaluate the draindown potential of a mix. As described in footnote 4 of the WAMA mix design

August 3, 2012 16

table, draindown may be altered by selecting different aggregates or by reducing the

production and construction temperatures of the mix. If these measures are not effective the

designer can consider three technologies for reducing the draindown problem, use of cellulose

fibers, use of polymer modified asphalt binder, or use of warm mix technologies.

Engineers have researched the use of fibers in porous asphalt (Wu, 2006). The idea is that

the fibers will prevent adverse effects stemming from draindown problems. The experimental

results indicate that fibers mainly stabilize asphalt binder and thicken the asphalt film around

aggregates. Furthermore, the use of fibers results in an improvement of the mechanical

strength of porous asphalt mixes at high temperature. In addition, in a comparison analysis,

cellulose fibers appear to perform better than polyester fibers in porous asphalt mixes (Wu,

2006). While fibers are effective in controlling draindown, they do require additional equipment

at the asphalt production plant to introduce the fibers into the asphalt concrete mix.

Polymer modified binders have become more common place since the implementation of

the Superpave mix design method and the associated Performance Grade specifications for

asphalt binders. Polymer modified binders have a higher viscosity than conventional asphalt

cement at a given temperature, so in a porous pavement application, the use of a polymer

modified binder at an appropriate temperature reduces the draindown due to the stiffness of

the binder. For conventional construction polymer modified mixes are heated to greater

temperatures than conventional binders to accommodate the compaction of the mix to a dense

mat. Since compaction of the porous pavement mix does not require the same effort as a

dense grade mix, the polymer modified mix can be produced and placed at lower temperatures

taking advantage of the low flow characteristics of the polymer modified binder.

Kuennen (Kuennen, 2011) reported on the construction of a porous pavement that used

both polymer modified binder and warm mix technology for the construction of a porous

pavement. The warm mix technology, combined with the modified binder met draindown

requirements without the need for fibers.

August 3, 2012 17

3 Construction and Maintenance

3.1 Pavement Bed

Great effort must be taken to minimize compaction of the bed surface, as this will be the

infiltration surface once construction is complete. It is recommended that the engineer design

and plan the method for removing the earth without significantly compacting the subgrade

soils. Berms are sometimes used to operate machinery from, and then removed before

construction (Cahill, Adams, & Marm, 2005).

Because construction sites are typically dirty and dusty places, it is often smart to install the

porous pavement toward the end of the construction period. Cahill Associates, one of the

leaders in porous asphalt pavement design and construction advise excavating the aggregate

bed area to within 6 in. of the final grade and use the empty bed area as a temporary sediment

basin and stormwater structure (Cahill et. al., 2005). Care must be taken to prevent heavy

equipment from compacting the subgrade, but sediment is allowed to accumulate (Adams,

2003). In the later stages of the project, this accumulated sediment is removed with light

machinery, the bed is excavated to final grade, and the porous pavement system is installed.

This also avoids the need for a separate sediment basin during construction.

Light machinery traffic is acceptable when necessary, but all efforts must be made to keep

heavy machinery off the surface as to not further compact it. If the soil becomes compacted,

infiltration values will decrease.

3.2 Geotextile

Immediately after the subgrade has been excavated to the desired grade, the filter fabric

should be installed. The fabric should overlap at all junctions at least 16 inches. NAPA

recommends the filter fabric extend at least four feet outside the bed, as seen in Figure 4 to

prevent sediment laden runoff from entering the bed (Hansen, 2008).

August 3, 2012 18

Figure 4 – Recommended Geotextile Fabric Overlap

3.3 Pavement Base

3.3.1 Base Course

The base course of aggregate is then placed on the filter fabric, taking care not to damage

the fabric. The aggregate should be dumped at the edge of the bed and placed in layers of 8 to

12 inches. The spreading of the aggregate should all be done with track vehicles. After each lift

is complete, it should be compacted with a single pass of a light roller or vibratory plate

compactor. Machinery used on the surface should always avoid the use of narrow rubber tires,

as they will increase compaction and therefore reduce infiltration (Hansen, 2008).

3.3.2 Choker Course

Again, the aggregates of the choker course should be dumped at the edge of the bed and

placed in one layer of 1-2 inches. The spreading of the aggregate should be done with track

vehicles. A few contractors have reported leaving out this layer as they have not seen a benefit

from its use. However, the consensus among the industry is that the choker course remains a

August 3, 2012 19

necessary element. If a choker course is used, it is important the aggregate be sized

appropriately to interlock with the aggregate in the base course.

3.4 Porous Asphalt Surface

The porous asphalt layer is placed on top of the choker course. The asphalt layer is placed

with track pavers. The procedure should also follow state or national guidelines for the

construction of open graded asphalt mixes (USDOT, 1990). Figure 5 shows the use of a material

transfer vehicle, MTV, used to transfer the asphalt from the delivery trucks to the paver

(Kuennen, 2011). The MTV and the delivery trucks drove on sheets of plywood to minimize

disruption and compaction of the base course.

Figure 5 Construction Practices to Minimize Compaction of Pavement Base (Kuennen, 2011)

Once the proper thickness has been placed, the asphalt layer should be compacted with

two to four passes of a ten ton static roller. Normally, only a few passes are necessary, and

over compaction should be avoided. In many cases, it has been necessary to let the mix cool

slightly before beginning compaction. Additional passes might be required to remove roller

marks. If these are needed, a lighter roller should always be used to remove roller marks at the

surface (Hansen, 2008).

August 3, 2012 20

After final compaction, traffic should be restricted for at least 24 hours, to allow the

pavement to gain stability as it cools.

3.5 Maintenance

3.5.1 Sediment Removal

Preventing sediment from entering the porous pavement is imperative throughout the life

of the pavement. It is important to take measures to protect the porous pavement from high

sediment loads.

The most prevalent concern is the potential for the pores in the asphalt pavement to

become clogged. Regardless of maintenance efforts, clogging will eventually occur as the

pavement ages. Studies of infiltration over the age of porous asphalts have shown that though

infiltration rates will initially decrease with time, they will eventually level off to an acceptable

value. Even when highly clogged, surface infiltration rates have been shown to exceed 1 inch

per hour (University of Rhode Island, 2008). This is far below the initial values, but will still

effectively manage heavy rainfall events. In an effort to maintain or even gain back some of the

lost permeability due to clogging, occasional vacuum sweeping has been used with successful

results. Vacuum cleaning is not required to maintain the functionality of the porous pavement,

however it can increase the drain through effectiveness. For this reason it is often included in

the maintenance plan of agencies charged with maintaining pavements.

3.5.2 Snow Removal

One concern about porous asphalt is its durability in the cold weather. Early experiments

conducted by The Franklin Institute in the late 1970’s suggested that when properly designed,

installed, and maintained, freeze-thaw damage was not observed. Through several hundred

laboratory freeze-thaw cycles, no damage or stresses were observed. Thelen stated that the

freeze-thaw resistance was achieved through larger voids sufficient for expansion of the water

(Thelen & Howe, 1978).

Another significant concern for porous asphalt is winter treatment for snow and ice.

Snowplowing can be performed as with any other pavement, however plowed snow piles

August 3, 2012 21

should not be left on the porous pavements as they regularly contain high sediment loads. It is

also imperative that cinders or sand not be applied onto porous asphalt or onto adjacent

pavements. Any application of cinders and/or sand could fully clog the interconnected pores of

the pavement. This would in turn eliminate the ability of the pavement to drain stormwater.

Salt is acceptable if necessary, but not recommended due to the proximity to the groundwater.

Studies have shown that in moderate climates experienced by most of the country, little to

no snow removal maintenance is routinely required on areas of porous pavement (Roseen et.

al., 2007). Because the water can transmit through the surface quickly, it does not have a

chance to sit on the surface and freeze.

3.5.3 Awareness

Due to the significant changes in design and construction, it is recommended that those

traveling over this new pavement be notified and aware that this surface they are on top of is

different from standard asphalt. The major concern is that users must be made aware that

there is a direct link between the new asphalt surface and the ground underneath them. Any

liquids released on the surface will enter the ground beneath them. This includes automotive

chemicals commonly found in parking lots, but also any other waste.

Another issue to be mindful of is the threat of clogging. Any small particles dropped onto

the surface can potentially become lodged in the asphalt surface and contribute to pavement

clogging over time. For this reason, some porous asphalt lots use rumble strips prior to the

entrance of the lot in an attempt to dislodge any particles, such as dirt and rocks.

August 3, 2012 22

4 Advantages and Disadvantages of Porous Asphalt Pavement

4.1 Advantages

4.1.1 Elimination of Stormwater Runoff

While there are several benefits of porous asphalt pavement, none can be as advantageous

as the removal of stormwater runoff. Because the water infiltrates through the pavement, it

does not enter the surrounding areas. This eliminates the need for runoff ditches, trenches,

culverts, additional tie-ins to the storm line, and any other conventional stormwater systems.

The water infiltrates into the soil as it would if the porous pavement surface was not there.

This also fulfills the requirement that no stormwater runoff leave the site, as the water

infiltrates exactly where it falls. In addition, the pavement can be designed to handle additional

stormwater from other impervious surfaces. Figure 6 compares a porous asphalt surface to a

standard HMAC surface; porous asphalt pavement eliminates the standing water on the

pavement surface.

Figure 6. Porous vs. Non-porous asphalt, (Rose, 2010)

4.1.2 Footprint Size

The traditional solution to stop stormwater from leaving a site is a detention basin. These

detention basins are necessary to catch all the runoff from the surrounding impervious

surfaces. In addition to the surrounding structures, parking lots can be some of the largest

August 3, 2012 23

generators of stormwater entering the basin. However, in the case of a porous asphalt parking

lot, the aggregate base below the parking lot serves as the detention basin. The land once

developed for the basin is left undeveloped, and the parking lot which once contributed to the

stormwater runoff problem is now part of the solution.

Porous asphalt pavement is able to solve two problems simultaneously. First, it reduces the

footprint of the project, allowing more land to remain undeveloped in its natural state or

developed in an alternative manner. It removes the need to alter nature, and create a

manmade pond. Second, porous asphalt removes the additional cost of purchasing the land

required for the detention pond. Porous asphalt pavements have the ability to collect

stormwater not only from the parking lots, but from the adjacent structures as well. By dealing

with stormwater onsite, the overall footprint of a project can be greatly reduced.

4.1.3 Runoff Quantity and Quality

When constructed properly, porous pavement systems can provide an excellent system for

the removal of pollutants. Two long term monitoring pavements in Maryland and Virginia

provide an estimate of porous asphalt pavements ability to remove pollutants. The studies have

observed that 82% to 95% of sediment is removed as well as 65% of total phosphorus, and 80%

to 85% of total nitrogen (Office of Water, EPA, 1999). These sediments are removed by a

combination of layer of biofilm in the aggregate base and also the filter fabric below the base.

4.1.4 LEED Rating System

When designed and installed properly, porous asphalt pavement qualifies for several credits

in the Leadership in Energy and Environmental Design (LEED) Green Building Rating System

(USGBC, 2011).

The LEED rating systems were developed by the U.S. Green Building Council (USGBC) in

2000. As an internationally recognized mark of excellence, LEED provides building owners and

operators with a framework for identifying and implementing practical and measurable green

building design, construction, operations, and maintenance solutions (USGBC, 2011).

Currently, many different LEED Rating systems are growing and evolving to encompass

many types of buildings. Once thought of only for commercial buildings, LEED is now branching

August 3, 2012 24

out into many different types of buildings. Credits are available for attainment with porous

asphalt pavement in the following LEED rating systems:

LEED-NC New Construction

LEED-EB Existing buildings and upgrades

LEED for Schools

LEED for Neighborhood Development

LEED for Homes

LEED for Retail - New Construction and Major Renovations

With this framework in place, there are several applications where porous asphalt

pavement can be integrated into a project for the fulfillment of credit requirements. The LEED

rating system contains five main categories to attain credits; Sustainable Sites, Water Efficiency,

Energy and Atmosphere, Materials and Resources, and Indoor Environmental Quality.

Specifically, the use of porous asphalt pavements can be advantageous in Sustainable Sites and

Water Efficiency (USGBC, 2011).

Within the category of Sustainable Sites, three credits are available pertaining to the use of

porous asphalt pavement. Two credits are awarded involving stormwater, one being for

quantity, the other for quality. These are both answered and fulfilled by the use of porous

asphalt pavement (USGBC, 2011).

Porous asphalt pavement significantly reduces the quantity of stormwater runoff from

impervious surfaces. The requirements for this credit call for the implementation of a

stormwater management plan that prevents the post-development peak discharge rate and

quantity from exceeding the predevelopment peak discharge rate and quantity for the 1- and 2-

year 24-hour design storms. Because porous asphalt can adequately provide the ability to meet

these criteria, this credit is easily obtained (USGBC, 2011).

The second credit in sustainable sites is for the quality of the released stormwater. The

credit requires the implementation of a stormwater management plan that reduces impervious

cover, promotes infiltration and captures and treats the stormwater runoff from 90% of the

average annual rainfall. The first suggestion in the LEED reference guide in obtaining this credit

August 3, 2012 25

is “Use of alternative surfaces (e.g. pervious pavements)”. Porous asphalt pavement treats all

stormwater, and is extremely effective at removing solids (USGBC, 2011).

Within the category of Water Efficiency, two points are available under the Water Efficient

Landscaping category. The credit states in order to obtain the points, the building must

“Reduce potable water consumption for irrigation by 50% from a calculated midsummer

baseline case”. One way this might be accomplished is from the use if harvested rainwater

collected through a porous pavement.

As asphalt can be reused with each cycle of road paving, asphalt pavements are 100%

recyclable. This does not qualify the pavement for credits associated with the building, unless

the pavement contains previously used material, but on a future remodel could gain credits

recycled material.

4.1.5 Natural Drainage

Conventional practice in site design is to construct a detention basin to collect surface

runoff from the entire site. Once collected, this water is infiltrated into the soil to replenish the

groundwater as it would on natural undisturbed ground. Porous asphalt removes this

collection step, and gets the water back into the groundwater reserves.

While also replenishing the groundwater, porous pavements have been shown to reduce

pollutant concentrations in water (Office of Water, EPA, 1999). The aggregate slows the

stormwater sufficiently enough to allow sedimentation to occur. In addition, studies have

found beneficial bacteria growth on aggregate bases. These beneficial bacteria are able to treat

the water as it travels through the recharge basin.

4.1.6 Reduced Costs

Initially, porous asphalt appears to be more expensive than a standard HMAC parking lot.

This is due to the additional design, construction, and materials. For this reason, it is often less

desirable for many projects because of the additional cost. However, when looking at the

project as a whole, this type of pavement becomes much more favorable due to the reduced

costs in other areas of the project.

August 3, 2012 26

Porous pavement does not cost more than conventional pavement. On a yard-by-yard basis,

the asphalt cost is approximately the same as the cost of conventional asphalt. The underlying

stone bed is usually more expensive than a conventional compacted sub-base, but this cost

difference is generally offset by the significant reduction in stormwater pipes and inlets (Adams,

2003).

The greatest cost difference however occurs when the added cost of a stormwater

detention system is factored into the price. When the cost savings provided by eliminating the

detention basin are considered, porous pavement is always an economically sound choice. On

those jobs where unit costs have been compared, the porous pavement always has been the

less expensive option (Adams, 2003).

4.2 Disadvantages

There are, however, some disadvantages of this pavement type. In general there is a lack of

technical expertise in these types of pavements. Clogging potential is of concern due to the

open structure of the pavement. There is also, as noted previously, a potential risk of

groundwater contamination as well as a potential for toxic chemicals to leak into the system.

Finally, there is a potential for anaerobic conditions to develop in underlying soils if the systems

is unable to dry out between storm events (United States Environmental Protection Agency,

2009).

4.2.1 Clogging

One possible disadvantage to the use of porous asphalt pavement is the clogging potential.

However, porous asphalt is many times more permeable than any soil it may be constructed

over. As a result, the functionality of the system is not compromised by less than total clogging

of the surface. Dr. Roseen of the UNH Stormwater Center stated "if 99% clogging were to occur,

the infiltration rate would still be greater than 10 inches per hour, which is greater than most

sand and soil mediums." (Roseen, Briggs, Ballestro, & Pochily, 2007)

August 3, 2012 27

4.2.2 Slopes

The stone recharge bed operates ideally when completely even. Even slight slopes will

cause greater concentrations of infiltration in certain areas. Horizontal movement along the

border of the bed and the native soil can be detrimental to the long term life of a recharge bed.

For these reasons, it is not recommended to use porous asphalt in areas where a slope of

greater the 6% is necessary. In these cases, it is better to either terrace the project and

continue to use porous asphalt pavement, or simply use full depth HMAC on the required grade

for these areas.

4.2.3 Bed Design

Due to the need for a large recharge basin directly underneath the pavement surface,

porous asphalt pavement is not a universal solution. If environmental restrictions do not allow

an underground aggregate base, then this pavement type cannot be used. Without the

recharge basin, the pavement will not function properly. This factor can be a limiting agent in

determining which sites are acceptable. In scenarios where this room is not available, standard

HMAC would more than likely be a better fit for the project.

August 3, 2012 28

5 Case Study Performance Observation

As previously stated, porous asphalt pavements have been used primarily for parking lots.

However, they have been used around the country in applications on both small and large

roadways. The following are three case studies from around the country. Each describes a

separate and unique use for the porous asphalt pavement.

5.1 Oregon Neighborhood Streets

In August of 2005, the Westmoreland neighborhood in the city of Portland, Oregon paved

four blocks of neighborhood streets using porous asphalt. The city has a combined sewer

system where stormwater runoff mixes with sanitary sewage during heavy rainfall events.

Some of the combined runoff and sewage goes to the wastewater treatment plant and some

overflows to the Willamette River. The goal of this project was to divert as much stormwater

away from the sewer system, therefore minimizing the amount of combined sewage entering

the local river (Environmental Services City of Portland, 2005).

With porous pavement, most if not all of the stormwater filters through the street surface

into layers of aggregate below the street, and then into the ground. This paving project has to

date shown only positive results. These include the elimination of any additional outflow into

the combined sewer system. In addition, this construction created a more natural stormwater

management system that allows stormwater to be absorbed, filtered and cleaned before

recharging groundwater (Environmental Services City of Portland, 2005).

5.2 University of Rhode Island Parking Lot

In 2003, the University Of Rhode Island (URI) constructed two parking lots for a total of

1,000 new parking spaces. Due to the fact that the lots were located within a fragile ecosystem,

an addition of that much impervious cover was not possible. In addition, the lots were not only

located within the Pawcatuck sole source aquifer, but also within the town of South

Kingstown’s groundwater protection overlay district, and the Wellhead Protection Area (WHPA)

for the University’s wells. For these reasons, water quality was of utmost importance. Porous

asphalt pavement was chosen to handle these challenges. The porous asphalt helps to control

August 3, 2012 29

runoff of pollutants to surface waters and protects groundwater supplies (University of Rhode

Island, 2008).

The porous asphalt layer in the two URI parking lots is 2.5 inches thick with a slope of less

than two percent on the surface to allow for maximum seepage through the pavement.

Located below the layer of porous asphalt is approximately a 1-inch layer of choker course

(AASHTO No. 57. Located under the choker layer is a uniformly graded, clean crushed rock

(AASHTO No. 2) recharge bed with 40% void space. The crushed rock layer is 36 to 42 in. deep

to protect against potential frost damage, and it is designed to store the volume of water

associated with a 100-year storm (University of Rhode Island, 2008).

When last published, no problems have been reported on this project. Minor scuffing has

occurred, which is caused by wheels turning under a stationary vehicle, but this is not unique to

porous parking lots. Additionally, research conducted has shown that the scuffing does not

compromise the drainage capabilities of the porous lots (University of Rhode Island, 2008).

5.3 Arizona SR-87

This project is a unique project on a major arterial in the Phoenix metro area. Therefore it

has unique design features not previously described.

In 1986, Project F045-l consisted of widening and reconstructing 1.47 mi of state

Route 87. The work included removing existing asphalt concrete pavement and existing

portland cement pavement, constructing new bituminous pavement, curb, gutter and sidewalk,

installing new traffic signals and other incidental work. Approximately 0.67 mi of the

northbound lanes of the project was paved using an open-graded porous pavement (Meir &

Elnicky, 1988).

The aggregate base consisted of two courses, the base course and the choker course. The

base course was a 48 inch. layer of crushed Portland cement concrete that had been removed

from the previous roadway. This layer was placed in two 2 ft. lifts. The choker course was then

placed in one 8 inch lift, and stabilized with 1.8 percent of AC-40 asphalt cement. The open-

graded asphalt concrete surface course was then placed in two 3 in. thick lifts and compacted

with steel wheel rollers (Meir & Elnicky, 1988).

August 3, 2012 30

On May 10, 1987, traffic was moved onto the porous pavement. Shortly thereafter, rutting

of the pavement surface was detected and an investigation of the pavement condition

commenced. Deformation measurements, nuclear density tests, and cores were taken along

wheel paths in the affected pavement area. After evaluating these data, the decision was made

to examine the deformation by opening a trench across a portion of the pavement to observe

movements in the separate pavement layers. Following an examination of the exposed

pavement layers, it was concluded that the units which were hauling asphalt treated base had

de-compacted the untreated base. Subsequently, highway traffic then re-compacted this

course causing pavement deformation (Meir & Elnicky, 1988). The final report for this project

was issued in 1991. Though it began with a rough start, no cracking or significant surface

deformation has occurred during the four years of service. NAPA reported the pavement has

performed well for 20 years (National Asphalt Pavement Association, 2009).

August 3, 2012 31

6 Additional Research Needs

6.1 Mix Design

To this date, the majority of the mix designs and design guides for porous asphalt

pavements stem from those of standard HMA pavements. In order to truly recognize the

potential benefits of porous asphalt, a detailed analysis of the mix design would be necessary.

More emphasis should be put on proper PG selection and aggregate characteristics for this

specific type of pavement. Many of the negative aspects of porous asphalt, including the

questions of strength and resistance to deflection and deformation could be investigated and

potentially solved with a look at the mix design for porous asphalt pavements.

6.2 Applications

In addition to more research in the design process, more research needs done on actual

testing and construction of porous asphalt projects. Included in these future projects should be

the parking lots and service roads already being constructed, but also roadways of every size

and capacity. This technology will never advance past the current state without innovative

projects, which push the known boundaries of porous asphalt. In order to know if this is an

adequate roadway surface, it needs to be constructed and evaluated in a real world situation.

The amount of knowledge gained from an actual road test would be invaluable. Regardless of

failure or success, the true usage limits of porous asphalt will never be fully recognized unless

these projects and trials are constructed and the results witnessed firsthand.

August 3, 2012 32

7 Conclusion

There are at least 105 million and maybe as many as 2 billion parking spaces in the United

States. Compromising at 500 million parking spaces in the country, these spaces occupy some

3,590 square miles, or an area larger than Delaware and Rhode Island combined (Kimmelman,

2012).

A third of these spaces are in parking lots. One study found there are eight parking spots

for every car in the country. Houston is said to have 30 parking spaces per resident. In

“Rethinking a Lot,” a new study of parking, Eran Ben-Joseph, a professor of urban planning at

M.I.T., points out that “in some U.S. cities, parking lots cover more than a third of the land area,

becoming the single most salient landscape feature of our built environment.” (Kimmelman,

2012)

In many new development projects, two-thirds of the new impervious surfaces are related

to the automobiles. Lost recharge, depleted groundwater levels, low stream baseflows, eroded

streambanks, and degraded water quality all are effects of this extensive paving program. Flood

and drought are both worsened by a development program of "sealing the earth's surface."

With the use of porous asphalt, parking lots can assist in the goal of better stormwater

management. There is not always something very exciting about a parking lot, but a parking lot

designed to maintain the hydrologic balance that existed before development is worth notice.

Porous pavement is not a universal solution to all stormwater problems. It is not intended

to be the same uniform product everywhere it is used. Instead it is simply another form of

asphalt pavement, free to be adapted and formed to meet individual project specification

August 3, 2012 33

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