How Pervious Concrete and Permeable Pavements Help Reduce Water Runoff

Dean Frank, P.E., LEED Green Associate

What is Pervious Concrete?• Considered an open graded system.• Consists of:

• Cement,• Coarse aggregate,

• Little or no fine aggregates,• Water,

• Admixtures.

• No slump (or zero slump).• Point-to-point aggregate contact.

• Aggregate adhered together with paste bridge.

Why Do We Need Pervious Pavement?• Recharging groundwater.

• Reducing stormwater runoff:• Relieve stress on poor/underdesigned infrastructure.

• Clean stormwater through filtration.

• Meet U.S. EPA stormwater regulations:• Pervious concrete is among the Best Management Practices

(BMPs) recommended by the EPA for the management of stormwater runoff.

• Increase land use/efficiency for development.

• Contributes to sustainability goals.

Streets and Roads

Parking Lots

Sidewalks Paths

Alleys

Pervious concrete is generally NOT used in heavily loaded pavement applications (e.g. highways, delivery lanes, truck stops, etc.), but can be if designed and placed correctly

Pervious Pavement Profile:A System For Water Management

Pervious Concrete 6” – 10”

Choker Course (optional) 1” – 2”

Open-Graded Base 6” – 24”+

Filter Fabric (optional)

Subgrade

Pervious Concrete Pavement –Storm Water Cleansing

Reduce:-Total Suspended Solids-Metals (eg. Cu, Zn, Pb)-Nutrients

Pervious Concrete

Aggregate Subbase

Subgrade

Water Table

Decorative Pervious Concrete Pavement

Edgewater ParkMinneapolis, MN

Source: Cemstone

Source: Bunyan Industries

Sidewalk in China

Benefits of Pervious Concrete

• Eliminates costly storm water management practices.

• Provides for more efficient land development.

Pervious Concrete Pavement -Environmental Considerations

• Percolation recharges groundwater.

• Water resources are conserved.

• Less need for irrigation.

• Adjacent vegetation receives more rainwater.

• Runoff reduced.

• Cooler surface has less impact on air temperature.

Benefits of Pervious Concrete –Efficient Land Use

• Closed existing detention pond.

• Paved with Pervious Concrete.

• Water storage for retention or on-site use.

Prime Retail Outlets Williamsburg, VA

Converting a detention pond to usable parking space

Soil Considerations With Pervious Concrete

• Top of subgrade should be above wet season water table by at least 2 feet.

• Permeable soils desirable (Perm. > ~0.2 in/hr):• Increase subbase depth for lower permeability soils.• Consider special design details on expansive clay soils.

• Compact subgrade to 92% - 95% of std. proctor.

• Keep heavy axle loads off subgrade during construction.

Clay Soils

• Increase Storage Capacity of Pervious Layers.• Increase Infiltration Rate of Soil (if possible).• Use Integrated Best Management Practices.• Impervious Liner Over Expansive Clays:

• Use In Areas With Potential Drought Periods and Low Water Tables (i.e. Drying/Wetting Cycles),

• Not Necessary In Areas With Semi-Saturated Clay.

Peak Water Table Considerations

Pervious Concrete

Aggregate Open-Graded Base

Subgrade

Wet Season Water Table

Two (2) Foot Minimum

Properties of Pervious Concrete

Air Void Content:

• 15-35% typical - Field studies show 20-25% average.

• Size of aggregate influences.

• Void content lower than 15% does not provide sufficient interconnectivity for rapid percolation.

Higher void content Higher Percolation Lower Compressive Strength

Typical Unit Weights:• 100 to 120 lbs/ft3

Properties of Pervious Concrete

28-day Compressive Strength1: Typically 500 to 3,000 psi

• 2,000 psi is typical and applicable for most applications.

• Mixture proportioning and compaction affect strength:

• Small amounts of fine aggregate can increase strength (careful not to fill voids).

• Thickness of paste layer surrounding aggregates impact strength (relates aggregate size, cement content and w/c).

• Compressive strength typically NOT used as acceptance criteria

• Air void structure (~15 – 25%) and unit weight are used.

• Flexural Strength: MR = 2.3f’c2/3 = 366 psi (~2,000 psi compressive).

Note 1: High variability associated with strengthtests of pervious concrete.

Mixture Design

Cement

• 450 – 550 lbs. Portland Cement is typical (and desireable).

• Type I or II, typical (ASTM C150 Standard Specification for Portland Cement).

• ASTM C595 Standard Specification for Blended Hydraulic Cements and ASTM C1157 Standard Performance Specification for Hydraulic Cement may be used.

• Fly Ash / Slag cement substitute acceptable (~10-15% cement replacement).

Mixture Design

Water

• w/cm ratio: 0.30 – 0.36 typical:• Need sufficient water to display a

wet, metallic sheen on the aggregate.

• Too much water in mix will lead to drainage of paste (clogging of pores).

• Water may be added to truck on site.

• Recycled water requirements – ASTM C94 Standard Specification for Ready-Mixed Concrete (or AASHTO M-157).

Dry

OK

Wet

Mixture Design

Coarse Aggregate

• Texture and Porosity Affected by:• Aggregate Size• Aggregate Grading• Aggregate Angularity• Paving Equipment

• Grading between ⅜ and ¾ inch• Aggregate Moisture Content:

• Should be in saturated surface – dry (SSD) condition

Mixture DesignAdmixtures• Water-reducing admixtures (High- and Mid-Range):

• Used depending on w/c ratio.• Retarding admixtures for cement hydration control:

• May act as lubricants for discharge, and• Used in hot weather placements.

• Accelerating admixtures extend working time for a mix:• Used in cold weather placements.

• Super Absorbent Polymers (SAPs)• increases workability and speed of installation• holds significant water which allows for full hydration (i.e. improves curing)

• Air-entraining admixtures:• Not commonly used, but• Used successfully in areas susceptible to freeze/thaw.

Mixture Design

Fibers

• Improves ability of mixture to hold together.

• Increases permeability.• Comply with ASTM C1116

Standard Specification for Fiber-Reinforced Concrete.

Mixture Design

• Target void content of 15% to 25% as measured by ASTM C1688 - Standard Test Method for Density and Void Content of Freshly Mixed Pervious Concrete.

• During production and placement, produced density of fresh pervious concrete shall be within ±5 lbs. of the specified density based upon the approved mix design.

Fresh Concrete Surface

Example with Rounded Aggregates

Hardened Concrete Surface

Example with Angular Aggregates

Hardened Concrete Surface

Example with Sealed Surface From Excessive Surface Paste(Due to Mixture Design & Improper Compaction)

Designing Cross-Sectional Thickness

Design for:• Traffic Loading Conditions• Soil Conditions (Strength, Permeability)• Hydrologic Conditions

Source: Charger Enterprises, Inc.

Source: Pervious Concrete California

Source: http://www.ceer.iastate.edu

Designing Cross-Sectional Thickness

Two-Step Design Procedure:• Structural Design: For Traffic Loading Conditions• Hydrologic Design: For Environmental Conditions

• Select the greater required thickness

Source: Charger Enterprises, Inc.

Designing Cross-Sectional Thickness: Structural Design

• Can be designed similar to conventional concrete pavement, but with lower flexural strength.

• Calculated pervious concrete thickness is generally:

• 6” for a typical parking lot (autos and occasional truck) or

• 7 to 10” if more trucks are present or if poor soils are present.

• Heavy truck and/or heavy turning areas may be a candidate for conventional concrete pavements.

Cross-Section Design – Goals:Hydrologic Design

• Hydrologically functional landscape that mimics the natural hydrologic regime.

• Mimic the predevelopment site hydrology by using pervious concrete pavement system that stores and allows infiltration of stormwater.

Cross-Section Design:Hydrological Considerations

• Rainfall Intensity.• Rainfall Duration.• Soil Permeability:

• Underdrain may be used for soil infiltration rates of less than 0.2” per hour.

• Release Rate.

Cross-Section Design:Hydrologic Design

• Permeability of Pervious Concrete:• 3.5 gal/ft2/hr is typical• Accommodates rainfall events of 340 in/hr!• Permeability far exceeds significant storm events.

• Storage - Four potential areas:• Infiltration into subgrade.• In the subbase.• In the pervious pavement.• On top of the pavement (ponding to top of curb).

Cross-Section Design:Site Layout/Planning - (Warm Climates)

Filter Fabric

T0

T1

T2

Pervious Concrete ~20% Voids

Aggregate Base ~ 40% Voids

Subgrade

Stormwater

Curb

Water may be stored aboveor below the top of the

pervious layer during a storm event.

Cross-Section Design:Site Layout/Planning - (Cold Climates)

Filter Fabric

T1

T2

Pervious Concrete

Aggregate Base ~40 % Voids

Subgrade

Curb

Other Design Considerations: Curbs

Allow Run-Off

Hold Run-Off

Prevent Debris From Clogging Slab

SWALE

Other Design Considerations:Slopes

Water

Other Design Considerations:No Adjacent Dirt Parking

Dirt Tracking On To Pervious Concrete Pavement

Top of Pavement Elevation Higher Than Grass/Landscaping

Other Design Considerations:Grading

Other Design Considerations: Islands

Impervious Asphalt

Pervious Concrete

Concrete Transition Strip

Other Design Considerations: Transitions

Stormwater Storage Options:Additional Storage

Freeze-Thaw Resistance –Mix Design

• Pervious concrete can perform well in F-T environments.

• In hard-freeze areas where subgrade stays frozen, aggregate base should be used as storage (24+ inches thick in some cases).

• Other freeze environments where subgrade is not continually frozen, typical aggregate thicknesses may apply.

Freeze-Thaw Resistance –Mix Design

• Smaller aggregate provides greater resistance to F-T.

• Small addition of sand (7%) may also increase resistance.• However, it also reduces voids and permeability slightly.

• Adding fibers improves durability slightly.

Ref: Pervious Concrete Mixture Proportions for Improved Freeze-Thaw Durability, Kevern, et al.

Site Preparation - Compaction

Don’t over compact, subgrade must remain permeable.

Construction –Place Filter Fabric (If Specified)

Geotextile fabric is sometimes specified.

Construction –Place/Compact Open Graded Base

Construction - Setting Forms

Use string lines to set form elevation.

Wood or steel forms can be used.

Construction –Surface Texture

• Important to keep the voids open.

• Do NOT use trowels unless mix is designed appropriately.

• Do NOT seal the surface.

• Avoid roller marks.

Construction –Mixer Discharge

• Pervious concrete flows down chutes at slower rate.

• Minimize number of chutes.

• Will restrict the discharge radius.

• Place in successive semi-circular arcs.

• Squared-off at end of each truck’s discharge.

Construction –Strike-Off Methods

Motorized Roller Screed(1-step placement method)

Vibratory Truss Screed(2-step placement method)

Construction – Jointing:Why Joint Slab?

• Concrete shrinks as it sets, hardens and dries out.

• Result in cracks.• Joints control location of cracks.

• Shrinkage not as great as in conventional concrete.

• Panels should be made as square as possible.• Cut in perpendicular to the curb.• Length to width ratio should not exceed 1.5.

• Maximum joint spacing 20 feet.• Joint depth = ¼ to ⅓ pavement thickness.

Construction – Jointing:Types of Joints

Control Joint Isolation Joint Construction Joint

Construction – Jointing:Tooled Joints

Joint Roller

Tooled Joint

Construction – Jointing:Saw Cut Joints

• Discouraged for the inexperienced.

• Can damage both edges.

• Dust can clog voids.

• However, joint sawing has been done successfully:

• Vacuum dust or wet slurry.

• Care must be taken to keep pavement from drying out during the saw cutting.

Construction –Curing Process

• Maintain adequate moisture and temperature.• Continue strength and durability.• Water will evaporate from concrete unless moisture

is maintained.• Curing uninterrupted for 7 days.

Construction –Curing Additional Precautions

• Exposure time of pervious concrete should be minimized.

• Should not exceed 20 minutes.• For high temperature, windy conditions or low

humidity (also in cold weather), exposure time should be reduced further.

• Or other precautions like fog sprays should be used to protect the surface from drying.

Construction –Moist Cure

1. Cure with 6 mil plastic (minimum),

2. Secure edges of plastic,

3. Cover within 20 minutes or less,

4. Continue curing, uninterrupted, for at least 7 days.

Maintenance for Long-Term Performance

www.perviouspavement.org/downloads/pervious_maintenance_operations_guide.pdf

www.perviouspavemennt.org

Maintenance

• Maintenance different than other pavements.

• Goal: Keep permeability close to design. May not restore all voids.

• Owner’s responsibility.

• Ensures longevity of system.• Prevent clogging during

construction.

• Limit debris from construction. Clogging From Construction

Maintenance• Pervious concrete

pavement can function well with no cleaning. Is not mandatory.

• Will work better if sand, dirt, leaves and other debris are removed.

Maintenance

Frequency of Cleanings:

• Depends on amount of sediment deposited.• Inspect pavement periodically:

• Recommended annually for most conditions.

• Observe drainage during a significant storm event.• Trash, leaves or paper may degrade into small

particles that may migrate into the voids and appears to have removed debris from surface.

Maintenance Methods

Maintenance Methods –Power Washing

Maintenance Methods –Vacuuming

Maintenance –Miscellaneous Considerations

• Sweeping pervious may be ineffective.• Small sediment gets trapped in coarse surface.• Do not sweep from impervious onto pervious areas.

• Do not flush impervious areas onto pervious areas.• Chemicals to clean pervious not recommended.

• Owner should establish a maintenance plan.

• Periodic evaluation of permeability.• Contractor should assist the owner with developing this

maintenance plan.

• Use methods that do not damage (ravel) the surface.

References - ACI 522R-10

• Provides technical information on pervious concrete’s:

• application,• properties, and• construction methods.

www.concrete.org

References - ACI 522.1-13

• Materials and Construction Specification for Pervious Concrete Pavement

www.concrete.org

References - ACI 522.1 - 13: Specification for Pervious Concrete

Pavements

• Performance Specification.• Provides Guidelines for:

• Quality Assurance:• Materials• Testing and Acceptance

• Placement Methods

• Does not provide recipe for pervious concrete mix design.

References –National Pervious Concrete Pavement

Association (NPCPA)

www.npcpa.org

References – NRMCA Concrete Answers Series

http://www.perviouspavement.org/

www.nrmca.org

Link

Summary

• Pervious concrete pavement can:• Reduce stormwater runoff

• Recall: It is a recommended BMP by EPA!• Recharge groundwater.• Clean stormwater through filtration.• Increase land use/efficiency for development.

• Pervious concrete may be used:• over both permeable and impermeable soils, and• in areas with freeze potential.

SummaryStreets and Roads

Sidewalks

Paths

Alleys

Parking Lots

Acknowledgment

Special Thank You to the National Ready Mixed Concrete Association (NRMCA)

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