6.2.20 Pervious Concrete

6.2.20 Pervious ConcreteDescription: Pervious concrete is a mixture of coarse aggregate, Portland cement and water that allows for rapid infiltration of water and overlays a stone aggregate reservoir. This reservoir provides temporary storage as stormwater infiltrates into underlying permeable soils and/or out through an underdrain system.

LID/GI Consideration: Pervious concrete is suitable for many types of development, from single-family residential to high-density commercial projects. Though not often used for heavily traveled roads, it is well suited for alleys, driveways, and parking lots. Pervious concrete provides volume reduction due to infiltration, groundwater recharge, and an ability to blend into the normal urban landscape relatively unnoticed. It also allows a reduction in the cost of other stormwater infrastructure, which may offset the greater upfront capital cost of pervious (versus impervious) concrete. This is considered to be a green infrastructure IMP.

Augusta Stormwater Management Manual

Version: July 20206.2.20- 1

6.2.20 Pervious Concrete

TOC

KEY CONSIDERATIONS STORMWATER MANAGEMENT SUITABILITY

DESIGNCRITERIA: ` Refer to American Concrete Institute (ACI) 522 Report on pervious concrete for design considerations

` Construction recommended by National Ready Mixed Concrete Association (NRMCA) certified personnel as specified in ACI 522.1

` Typically used for low volume auto traffic areas, or for overflow parking applications

` Not permitted on sites with low permeability soils, wellhead protection zones, or water supply aquifer recharge areas

ADVANTAGES/BENEFITS: ` Provides reductions in runoff volume, stormwater runoff, and impervious area

` Helps minimize size of detention areas ` High level of pollutant removal ` Reduces standing water on pavement

` May help to reduce stormwater management costs

DISADVANTAGES/LIMITATIONS: ` High cost compared to conventional pavement ` Geotechnical analysis of soils is required ` Not recommended for areas with heavy traffic or trucks

` Not recommended under tree canopy

ROUTINEMAINTENANCEREQUIREMENTS: ` Keep concrete free of trash, debris, and dirt ` Use street sweepers or vacuum trucks to clean pervious concrete as needed

` Keep grass surrounding the area trimmed, remove grass clippings from area and do not allow bare soil to occur

` Occasional pressure washing may be necessary

Residential Subdivision Use: Yes High Density/Ultra-Urban: Yes Roadway Projects: Yes (Typically low traffic areas)

RUNOFF REDUCTION CREDIT

` 100% of the runoff reduction volume provided (no underdrain)

` 75% of the runoff reduction volume provided (with an upturned underdrain)

` 50% of the runoff reduction volume provided (with underdrain)

POLLUTANTREMOVAL

80% Total Suspended Solids

50% Phosphorus

65% Nitrogen

60%Metals - Cadmium, Copper, Lead, and Zinc removal

NA% Pathogens – Fecal Coliform

Runoff Reduction

TSS Removal

u Channel Protection

u Overbank Flood Protection

u Extreme Flood Protection Retrofit

suitable for this practice

u may provide partial benefits

Low Land Requirement

High Capital Cost

High Maintenance Burden

IMPLEMENTATION CONSIDERATIONS




TOC

6.2.20.1 GeneralDescriptionandStormwaterManagementSuitabilityPervious concrete (also referred to as enhanced porosity concrete, porous concrete, Portland cement pervious pavement and pervious pavement) is a subset of a broader family of pervious pavements including porous asphalt and various kinds of grids and paver systems. Pervious concrete is generally regarded to have a greater ability than permeable asphalt to maintain its porosity in hot weather and thus is provided as a limited application control. Please see Figure 6.2.20-1 for typical applications.

Pervious concrete consists of a specially formulated mixture of Portland cement, uniform, open-graded course aggregate, and water. The concrete layer has a high permeability, such that the underlying permeable soil layer allows rapid percolation of rainwater through the surface and into the layers beneath. The void space in pervious concrete is in the 15 to 22% range, as opposed to 3 to 5% for conventional pavements. The permeable surface is placed over a layer of open-graded gravel and crushed stone. The void spaces in the stone act as a storage reservoir for stormwater.

Pervious concrete is often designed primarily for stormwater quantity (i.e., the removal of stormwater volume). However, it can provide stormwater quality control when used as an integral part of the stormwater management system plan. Pervious concrete systems can be designed to capture and infiltrate the water quality volume (WQv) and the channel protection volume (CPv), as well as provide an infiltration option for overbank protection and extreme flood protection.

Modifications or additions to the standard design have been used to pass flows and volumes in excess of the water quality volume, or to increase storage capacity and treatment. These include, but are not limited, to:

` Placing a perforated pipe near the top of the crushed stone reservoir to pass excess flows after the reservoir is filled

Figure 6.2.20-1. Typical Pervious Concrete System Applications(Photos by Bruce Ferguson, Don Wade)




TOC

` Providing surface detention storage in a parking lot, adjacent swale, or detention pond with suitable overflow conveyance.

` Connecting the stone reservoir layer to a stone-filled trench.

` Adding a sand layer and perforated pipe beneath the stone layer for filtration of the water quality volume.

` Placing an underground detention tank or vault system beneath the layers.

In order to meet the WQv and accommodate extended detention requirements for the CPv, the minimum drawdown time shall be 24 to 48 hours. Longer drawdown is acceptable as necessary to infiltrate, bypass or detain and release larger storm events with a maximum drawdown time of 5 days. Undue compaction, which could affect the soils’ infiltration capability, shall be avoided.

Pervious concrete systems are typically used in low-traffic areas such as:

` Parking pads in parking lots

` Overflow parking areas

` Residential street parking lanes

` Recreational trails

` Golf cart and pedestrian paths

` Emergency vehicle and fire access lanes

6.2.20.2 PlanningThe following criteria must be considered when evaluating the suitability of pervious concrete for a development site:

Physical Feasibility Requirements ` Site Slope – Slopes shall be a maximum of 6%

` Minimum Depth to Water Table – A separation distance of 2 feet is required between the bottom of the rock storage area of the pervious concrete and the elevation of the seasonally high water table.

` Soils – Native soils can be used if they have at least 0.5 inch/hr infiltration ability.

` Hotspots – Pervious concrete is not permitted for hotspot runoff.

` Trout Stream – In cold-water streams, the use of pervious concrete with an underdrain shall be evaluated for the potential amount of stream warming that occurs in the practice.

` Damage to Existing Structures and Facilities – Consideration shall be given to the impact of water exfiltrating the pervious concrete.

` Proximity – The following is a list of specific setback requirements for the location of pervious concrete:




TOC

` 10 feet from building foundations

` 100 feet from private water supply wells

` 200 feet from public water supply reservoirs (measured from edge of water)

` 1,200 feet from public water supply wells

The data listed below are necessary for the design of pervious concrete and shall be included with the SWMP:

` Existing and proposed site, topographic and location maps, and field reviews

` Impervious and pervious areas

` Roadway and drainage profiles, cross-sections, utility plans, and soil report for the site

` Design data from nearby storm sewer structures

` Water surface elevation of nearby water systems as well as the depth to seasonally high groundwater

` Setback distances from building foundations, water supply wells, reservoirs, etc

See Appendix B for more information on required elements for the SWMP.

6.2.20.3 DesignThe following criteria shall be considered minimum standards for the design of pervious concrete:

` Pervious concrete systems are permitted where the underlying in-situ subsoils have an infiltration rate greater than 0.5 inches per hour. Infiltration rates for in-situ soils shall be determined through geotechnical investigation prior to design of the system. During construction and preparation of the subgrade, over compaction of soils shall be avoided. Refer to ACI 522 for compaction recommendations.

` Pervious concrete systems are typically used in applications where the pavement receives tributary runoff only from impervious areas. The ratio of tributary impervious area to the area of the pervious concrete system shall be a maximum of 1:1. Pervious concrete systems shall be sized for a minimum drawdown time of 24 hours and a maximum drawdown time of 5 days.

` Tributary runoff from adjacent pervious areas is not recommended. However, if it is necessary for runoff to come from adjacent pervious areas, it is important that those areas be fully stabilized to reduce sediment loads and prevent clogging of the pervious surface. Pretreatment using dry enhanced swales or vegetated filter strips for removal of course sediments is recommended, and continued maintenance of these areas will be required (see Sections 6.2.15 and 6.2.18). A minimum of 4 feet of clearance is required between the bottom of the gravel base course and underlying bedrock or the seasonally high groundwater table.

` Pervious concrete systems shall be sited at least 10 feet down-gradient from buildings and 100 feet away from drinking water wells.

` To protect groundwater from potential contamination, runoff from designated hotspot land uses




TOC

or activities must not be infiltrated. Pervious concrete shall not be used on manufacturing or industrial sites where there is potential for high concentrations of soluble pollutants and heavy metals. In addition, pervious concrete shall not be considered for areas with high pesticide concentrations. Pervious concrete is also not suitable in areas with karst geology without adequate geotechnical testing submitted and approved with the SWMP.

` Pervious concrete systems can be used independently or in conjunction with other stormwater management system components to effectively infiltrate, bypass, or detain and time-release all required storm events.

` For the purpose of sizing downstream conveyance and structural control systems, pervious concrete surface areas can be estimated as 35% impervious.

` For treatment control, the design volume shall be, at a minimum, equal to the water quality volume. The water quality storage volume is contained in the surface layer and the aggregate reservoir. The storm duration (fill time) is normally short compared to the infiltration rate of the sub-grade, so storage duration of two hours can be used for design purposes. The total storage volume in a layer is equal to the percent of void space times the volume of the layer.

` The cross-section typically consists of two layers, as shown in Figure 6.2.20-2. The aggregate reservoir can sometimes be avoided or minimized if the sub-grade is sandy and there is adequate time to infiltrate the necessary runoff volume into the sandy soil without by-passing the water quality volume. A description for each of the layers is presented below:

` Pervious Concrete Layer – The pervious concrete layer consists of an open-graded concrete mixture usually ranging from depths of 6 to 12 inches depending on required bearing strength and pavement design requirements. Pervious concrete can be assumed to contain 18 percent voids (porosity = 0.18) for design purposes. Thus, for example, a six-inch thick pervious concrete layer would hold 1.08 inches of rainfall. Refer to ACI 522R (most current version) for recommendations on mix proportioning for pervious concrete. See the Georgia Concrete & Products Association (GCPA) specifications (referenced) as well.

` Reservoir Layer – The reservoir gravel base course consists of washed, bank-run gravel, 1.5 to 2.5 inches in diameter with a void space of about 40% (GADOT No.3 Stone). The depth of this layer depends on the desired storage volume, which is typically the WQv at a minimum. Typical depths for the reservoir layer range from 2-4 feet. Aggregate contaminated with soil shall not be used. A porosity value (void space/total volume) of 0.40 shall be used in calculations unless aggregate-specific data exist.

` Filter fabric can be used in certain applications, as site conditions warrant. General guidance for the use of filter fabrics is below. Actual use shall be under the guidance of a Georgia licensed engineer.

` Geotextiles consisting of permeable materials shall line the sides of the aggregate base to prevent migration of adjacent soils into it and subsequent permeability and storage capacity reduction. Geotextiles are not recommended under the aggregate base in an infiltration design because they can accumulate fine particulates that inhibit infiltration.




TOC

` Geomembranes consisting of impermeable materials shall be used to accomplish the following:

` Provide a barrier on the side and bottom of the aggregate base in a detention design to prevent infiltration into the subgrade typically due to soil instability, the presence of stormwater hotspots, or potential for groundwater contamination. Geomembrane barriers reduce the credit for TSS removal from 80% to 70%.

` Line the sides of the aggregate base whenever structure foundations of conventional pavement are 20 feet or less from the permeable pavement (to avoid the risk of structural damage due to seepage). The use of geomembranes for this purpose will not reduce credit for TSS removal in the system.

` Geogrids may be used on top of subgrade soils for additional structural support, especially in very weak, saturated soils. All manufacturer requirements must be followed in design and installation of geogrids.

` Pervious concrete systems shall not be used in fill soils. The underlying soil shall have an infiltration capacity of at least 0.50 in/hr, as initially determined from the NRCS soil textural classification, and subsequently confirmed by field geotechnical tests. The minimum geotechnical testing is one test hole per 5,000 square feet, with a minimum of two borings per practice (taken within the proposed limits of the facility). Test borings are recommended to determine the soil classification, seasonal high ground water table elevation, impervious substrata, and an initial estimate of permeability. Often a double-ring infiltrometer test is done at subgrade elevation to determine the impermeable layer, and, for safety, one-half the measured value is allowed for infiltration calculations.

` The pit excavation shall be limited to the width and depth specified in the design. Excavated material shall be placed away from the open trench to avoid jeopardizing the stability of the

Figure 6.2.20-2 Pervious Concrete System Section(Modified From: LAC 2000)




TOC

trench sidewalls. The bottom of the excavated trench shall not be loaded so as to cause compaction, and shall be scarified prior to placement of reservoir base material. The sides of the trench shall be trimmed of all large roots. The sidewalls shall be uniform with no voids, and scarified prior to backfilling. All pervious concrete systems shall be protected during construction and constructed after upstream areas are stabilized.

` An observation well consisting of perforated PVC pipe 4 to 6 inches in diameter shall be placed at the downstream end of the facility and protected during site construction. The well shall be used to determine actual infiltration rates for use in final design of the pervious concrete system.

` A warning sign shall be placed at the facility that states: “Pervious concrete is used on this site to manage stormwater. Do not resurface with non-pervious material. Call the property manager for more information.”

` Details of construction of the concrete layer are beyond the scope of this Manual. However, construction by NRMCA-certified personnel, following the guidelines of ACI 522R and ACI 522.1, is recommended.

Design ProceduresStep 1. Determine if the development site and conditions are appropriate for the use of pervious concrete.Consider the application and site feasibility criteria in this Chapter. In addition, determine if site conditions are suitable for pervious concrete. Create a rough layout of the pervious concrete dimensions, taking into consideration existing trees, utility lines, and other obstructions.

Step 2. Determine the goals and primary function of the pervious concrete.

Consider whether the pervious concrete is intended to:

` Meet the runoff reduction criterion (RRv) or 80% TSS removal criterion (WQv). For information on the sizing of an IMP utilizing the runoff reduction approach, see Step 3A. For information on the sizing of the IMP utilizing the TSS removal approach, see Step 4A. Note: Minimum infiltration rates of the surrounding native soils must be acceptable and suitable when the pervious concrete area is designed for purposes of compliance with the runoff reduction criterion.

` Be “oversized” to include partial credit for storage capacity for other stormwater requirements (channel protection volume [CPv ])

` Provide a possible solution to a drainage problem

` Enhance landscape and provide aesthetic qualities

Consider if the IMP has any special site-specific design conditions or criteria. List any restrictions or other requirements that may apply or may affect the design.

The design of the IMP shall be centered on the restrictions/requirements, goals, targets, and primary function(s) of the IMP described in this Section. By considering the primary function, as well as topographic and soil conditions, the design elements of the practice can be determined (i.e., planting media, underdrain, inlet/outlet, overflow, etc.).




TOC

CompleteSteps3A,3B,and3C fora runoffreductionapproach,orskipStep3andcompleteSteps 4A and 4B for a TSS removal approach. Refer to Chapter 5 for detailed informaiton on compliance calculations.

Step 3A. Calculate the RRv.

Calculate the RRv using the following formula:RRv=(P)(RV)(A)/12

whereRRv = runoff reduction volume (ft3)P = runoff reduction rainfall (1.0 inches)Rv = volumetric runoff coefficient, which can be found by Rv= 0.05+0.009(I)

and where I = new impervious area of the contributing drainage area (%)

A = Area draining to the practice (ft2)12 = Unit conversion factor (in/ft)

Using the runoff reduction credit information provided on the first page of this IMP specification, find the appropriate runoff reduction percentage (RR%) provided by the practice. Then determine the minimum Volume of the practice (VP) as follows:

(VPMIN)≥RRv/(RR%)where

RR% = runoff reduction percentage, or credit, assigned to the specific practice;VPMIN = Minimum storage volume required to provide RRv (ft3)RRv = runoff reduction volume (ft3).

Step 3B. Determine the storage volume of the practice and the pretreatment volume.

To determine the actual volume provided in the pervious concrete, use the following equation:VP=(VBL)*(N)

where VP = volume provided (temporary storage) VBL = volume of base layer N = porosity

To determine the porosity, a qualified licensed professional shall be consulted to determine the proper porosity. Most gravel has a value of 0.40.

Provide pretreatment by using a grass filter strip or pea gravel (sheet flow), or a grass channel or forebay (concentrated flow), as needed. Where filter strips are used, 100% of the runoff shall flow across the filter strip. Pretreatment may also be desired to reduce flow velocities or assist in sediment removal and maintenance. Pretreatment can include a forebay, weir, or check dam. Splash blocks or level spreaders shall be considered to dissipate concentrated stormwater runoff at the inlet and prevent scour. Forebays shall be sized to contain 0.1 inch per impervious acre of contributing drainage.




TOC

Refer to Section 6.2.4 for design criteria for a grass channel and Section 6.2.15 for vegetated filter strips.

Step 3C. Determine whether the minimum storage volume was met.

When the VP ≥ VPMIN, then the runoff reduction requirements are met for this practice. Proceed to Step 5.

When the VP < VPMIN, then the IMP must be sized according to the WQv treatment method (see Step 4).

Step 4A. Calculate the WQv.

Calculate the WQv using the following formula:WQv=(1.2)(Rv)(A)/12

whereWQv = water quality volume (ft3)1.2 = target rainfall amount to be treated (inches)Rv = volumetric runoff coefficient, which can be found by Rv = 0.05+0.009(I)

and where I = new impervious area of the contributing drainage area (%)

A = area draining to this practice (ft2)12 = unit conversion factor (in/ft)

Step4B. Ifusingthepractice for80%TSSremoval,determinethe footprintof theperviousconcrete and the pretreatment volume required.

The peak rate of discharge for the water quality design storm is needed for sizing of off-line diversion structures (see Section 5.26). If designing off-line, follow steps (a) through (d) below:

a. Using WQv, compute CN

b. Compute time of concentration using TR-55 method.

c. Determine appropriate unit peak discharge from time of concentration

d. Compute Qwq from unit peak discharge, drainage area, and WQv

To determine the minimum surface area of the pervious concrete, use the following formula: Af=(WQv)(dpc + drl)/[(kpc * dpc * tpc)+(krl * drl * trl) ]where Af = surface area of pervious concrete (ft2) WQv = water quality volume (ft3) dpc = pervious concrete depth (ft) drl = reservoir layer depth (ft) kpc = coefficient of permeability for pervious concrete(ft/day) krl = coefficient of permeability for reservoir layer (ft/day) tpc = drain time of pervious concrete (days) trl = drain time of reservoir layer (days)


Version: July 20206.2.20- 10


TOC

Step5.Sizeunderdrainsystem(ifapplicable).

An underdrain system is required if the in situ soil infiltration rate is less than 0.5 in/hr. Underdrain should be a 4-6-inch perforated PVC pipe (AASHTO M 252) in an 8-inch gravel layer.

6.2.20.4 Inspection,ProtectionandMaintenanceRequirementsAll IMPs require proper construction, protection, and long-term maintenance or they will not function as designed and may cease to function altogether. The design of all IMPs includes considerations for maintenance and maintenance access. A legally binding IMP Maintenance Agreement shall be completed. For Augusta policies, additional guidance, and forms pertaining to IMP protection, inspection and maintenance requirements, see Chapter 8 of this Manual.

Requirements DURING Construction ` Areas where IMPs will be located shall be readily identifiable and protected from unwanted encroachments during construction, both on SWMPs and at the construction site. Physical protection measures can include, but are not limited to, orange fencing, wood or chain link fencing, and signage.

` Avoid undue compaction, which could affect the soils’ infiltration capability.

` For infiltration-based IMPs, protection of IMP locations during construction of a land development will ensure that native soils that surround (or are within) the stormwater treatment area will remain uncompacted and therefore continue to meet the design parameters that were specified in the approved SWMP.

` During construction, care shall be taken to avoid tracking sediments onto any pervious concrete to avoid surface clogging. Any area of the site intended ultimately to be a permeable pavement area shall not be used as the site of a temporary sediment basin.

` A lack of IMP protection will most certainly reduce or destroy the stormwater functionality of the IMP once it is installed, often leading to costly corrective actions required by AED.

Protection Requirements AFTER Construction ` Providing signage for the IMP will:

` Allow for easy identification and location of the IMP.

` Serves as a general education tool, making those responsible for property, landscape or IMP maintenance and the general public aware of the water quality features of the IMP and to avoid encroachment.

` Design the layout of the pervious concrete such that maintenance equipment can easily achieve access.


Version: July 20206.2.20- 11


TOC

Inspection RequirementsWhile AED requires an annual inspection, more frequent inspections help identify problems earlier and keep maintenance costs lower. The following are recommended frequent inspection items:

Pervious Concrete Typical Routine Maintenance Activities and ScheduleActivity Schedule

` Ensure that contributing area, facility, inlets and outlets are clear of debris.

` Ensure that the contributing area is stabilized and mowed, with clippings removed.

` Remove trash and debris.

` Check to ensure that the pavement surface is not clogging (also check after moderate and major storms).

` Ensure that activities in the drainage area minimize oil/grease and sediment entry to the system.

As needed

` Make sure that there is no evidence of deterioration or cracking of the concrete.

` Inspect inlets, outlets and overflow spillway to ensure good condition and no evidence of erosion.

` Repair or replace any damage to the concrete.

` Ensure that flow is not bypassing the facility.

Monthly

` Vacuum sweep pervious concrete surface followed by high pressure hosing to keep pores free of sediment.

` Flush the underdrain system and check for signs of clogging.

Annually or based on inspection

` Utilize organic de-icers on the pavement surface. During temperatures below freezing

For pervious concrete, inspections of the following elements are important for proper function of the IMP, and inspection frequency and diligence are critical:

` Inspect the pervious concrete and underdrain for clogging or sediment build-up.

` Inspect the property that drains to the pervious concrete for erosion, exposed soil or stockpiles of other potential pollutants.

Maintenance RequirementsPervious concrete is a special mixture with a high void space that allows water to infiltrate into the subsoil through the pavement surface and base layers. This aggregate base layer acts as both a structural layer and a container to temporarily hold stormwater runoff until it can infiltrate into the subsoil or drainage system.

There are some common problems to be aware of when maintaining pervious concrete. They include, but are not limited to, the following:

` Sediment build-up on surface

` Settling

` Cracking


Version: July 20206.2.20- 12


TOC

Routine maintenance should be performed on pervious concrete to ensure that the area is functioning properly. Pervious concrete should be observed monthly to ensure the practice is functioning properly. Maintenance activities, including street vacuum or low-pressure washer to remove debris and sediment, should be conducted at least annually, or as needed.

In addition to routine maintenance, pervious concrete has seasonal and intermittent maintenance requirements. In the winter months, pervious concrete can be plowed, however, the snow plow should be equipped with snow shoes which can allow the blade to glide across uneven surfaces. De-icing materials such as sand, ash, salt, or other products should be avoided if possible. They will harm the concrete and other materials and may cause clogging. Organic de-icers are recommended.

Pervious concrete may also include an underdrain or a trench outlet. If the practice includes an underdrain or a trench, additional maintenance will be required. Periodic testing may be necessary to make sure that the underdrain or trench is not clogged. The underdrain or trench can be tested by pouring water into clean out and observing how the water exits the practice. The observation well for the underdrain should be checked to make sure water is draining out of the practice.

Specifically, the following maintenance actions are of critical importance to the long-term performance of the IMP:

` Conduct mowing and trash removal as needed to prevent obstacles to the intended drainage and maintenance of the pervious concrete

` Remove grass clippings and other landscaping debris

` No bare soil is permitted in the area draining to the IMP

` Keep outlets clear of debris to prevent clogging, and clear if necessary

` Prevent clogging of the aggregate through maintenance with vacuum trucks, street sweepers and pressure washers as needed

Documents

6.2.20 Pervious Concrete