Pervious Concrete A Solution to Stormwater Runoff
Lionel Lemay, Sr. Vice President, Sustainable Development, National Ready Mixed Concrete Association. and
Colin Lobo, Sr. Vice President, Engineering, National Ready Mixed Concrete Association
Pervious concrete is a mixture of hydraulic cement, coarse aggregates (stone), water and admixtures. American Concrete Institute (ACI) Committee 522 describes pervious concrete as a "near zero-slump, open-graded material consisting of hydraulic cement, coarse aggregate, little or no fine aggregate, admixtures, and water.”1 The cement and water in pervious concrete forms a paste that uniformly coats and bonds the aggregates together when properly compacted. An optimized quantity and consistency of paste is used in the mixture to allow the fresh pervious concrete mixture to flow adequately when placed and to obtain point-to-point contact of the coarse aggregates, creating an interconnected void system to allow stormwater to infiltrate through the pavement system when installed. A typical pervious concrete mixture will contain between 15 and 25% void content within the concrete. The drainage rate of pervious concrete pavement will vary with aggregate size and density of the mixture, but will generally fall into the range of 0.2 cm/s (288 in./hr) to .54 cm/s (770 in./hr)2.
Pervious concrete, in some form, has been used in construction for more than 100 years. Today, in the United States, it is being used primarily as a pavement material for parking areas, drives, walkways and some lightly trafficked residential streets. Parking lots and roadways are major sources of stormwater pollution. When precipitation runs off a conventional, impervious parking lot or roadway, it carries with it a wide range of pollutants, such as debris, metals (brake linings) and hydrocarbons (oils and grease). Polluted stormwater runoff eventually finds its way into streams, rivers and lakes.
A major reason for the interest in pervious concrete is being driven by the benefits it provides for Low Impact Development (LID). In the United States, federal regulations require states, counties and municipalities to adopt procedures that address stormwater runoff and associated pollutants. These regulations generally require that property owners collect and treat stormwater on site as opposed to allowing it to runoff to adjacent sites or into municipal stormwater management systems. Infiltration systems, such as pervious concrete pavements, help satisfy these requirements. The US Environmental Protection Agency (EPA) recognizes pervious concrete as one of the best management practices for stormwater management.
A typical pervious concrete pavement system consists of a layer of pervious concrete to support traffic loading over a layer of aggregate base. The pervious concrete allows water to percolate quickly into the aggregate base layer, often called the storage layer, before slowly infiltrating into the subgrade soil. The aggregate base layer is typically a clean, single-sized aggregate between 18 mm and 40 mm (3/4 in. and 1-1/2 in.) nominal size. In some cases, perforated pipes are incorporated into the aggregate base layer to increase storage capacity or collect water from other surfaces on the site including roofs, walks and conventional impervious pavements. This supplemental drainage may also be necessary to increase capacity of systems on fine-grained soils that reduce the rate of percolation of stormwater. Perforated pipes in the base layer collect excessive stormwater and diverts it to supplemental retention areas. Pervious concrete pavement systems are often combined with other stormwater infiltration systems such as rain gardens and bio-swales (see figure 3).
Pollution treatment: In a parking lot or roadway, pollution from vehicles and other sources, in the form of hydrocarbons, heavy metals and sediment, accumulate on the surface and are a major source of stormwater pollution in most watersheds. According to the United States Environmental Protection Agency (USEPA), about 90% of the surface pollutants are carried off by the first 12 mm (½ in.) to 25 mm (1-in.) of rainfall. In a conventional impervious parking area the polluted rainfall then flows to a stormwater drain, and eventually to streams, lakes or rivers. A pervious concrete system treats these pollutants and allows clean water to pass through the pavement, into the aggregate base layer, and then into the native soil beneath the pavement. The pervious concrete, aggregate base and subgrade soil filters and treats the rainfall in the same manner as the site naturally did before the parking lot or roadway was constructed.
Recharging aquifers: Some areas use aquifers for their drinking water and pump the water to local communities. Many areas of the United States and other parts of the world are dealing with water shortages and diminishing groundwater supplies, yet by increasing the area of impervious surfaces, an abundance of precipitation is allowed to run into storm sewer systems bypassing the natural process of rainfall recharging groundwater. Pervious concrete pavements allow rainfall to percolate back into the soil where it falls, thereby replenishing aquifers naturally.
Encourage tree growth: Impervious pavements keep the water and air away from tree roots. Many communities have adopted regulations that prohibit the construction of impervious pavements within the drip line of trees. This can prove difficult in places where trees are within, or near, roads, parking areas or other hard surfaces. A practical solution is to use pervious concrete for these hard surfaces. This allows rainwater and air to pass freely through the pavement to sustain the tree roots.
Reducing urban heat islands: Research at Lawrence Berkeley National Laboratory at Berkeley, California, shows that temperatures in residential zones rise by as much as (1.7ºC) 3°F and in downtown areas by as much as (3.9ºC) 7°F in some cities in the United States and even higher in some other cities around the world. This is primarily because of the predominant use of dark-colored roofing and pavements.1 This is known as the urban heat island effect. The research has shown that use of light- and heat-reflective materials for roofing and pavements, along with the planting of trees, could lower the average summer afternoon temperature significantly and thus reduce the need for air conditioning. Pervious concrete absorbs and stores less heat than conventional impervious pavements. The lighter color or albedo of pervious concrete pavements reflects radiant thermal heat while the open void structure permits the earth’s cooler temperatures from below, along with water evaporation, to cool the pavement thus helping to reduce urban heat islands2.
LEED Recognition Leadership in Energy and Environmental Design (LEED®) is a point rating system devised by the United States Green Building Council (USGBC) that evaluates the environmental performance of buildings. The system, which is used all over the world, is credit-based, allowing projects to earn points for environment–friendly strategies employed during the design and construction process. Projects receive points for the environment– friendly strategies of each component of design and construction, such as the building’s energy and water use, among other criteria. Using concrete in new buildings and major renovation projects, including the use of pervious concrete, can facilitate the process of obtaining points in LEED® 2009 for New Construction as follows:3
For unusual high loadings or for specific designs, some engineers have used mechanistic design procedures such as those described in StreetPave (or Pervious Pave) software developed by the American Concrete Pavement Association.4 When using this software, engineers often use a conservative assumed compressive strength for pervious concrete of 10 MPa (1,500 psi). One can calculate the tensile strength based on the relationship between compressive strength and flexural strength as developed by Crouch et al (see figure 4).5 This information, along with traffic loading and base material characteristics are input into the software program to determine the minimum pavement thickness.
For hydrological design, the pavement system must be designed to rapidly percolate stormwater through the pervious concrete, store water in the aggregate base layer, then infiltrate the water into the ground below or release the water at a controlled rate through outlets. A common design for a pervious concrete detention system is 150 mm (6 in.) of pervious concrete over 150 mm (6 in.) or more of clean open-graded aggregate. In some cases, a non-woven geotextile filter fabric or aggregate choker layer might be used to separate the aggregate base from the subgrade soil.
Key parameters for design are the pervious concrete permeability, the aggregate base layer void content, the infiltration rate of the in situ soil below the aggregate base layer, the design storm data, and topography and infiltration characteristics of surrounding landscape. A properly designed pervious system will be drained of water and ready to receive another design storm soon after a storm event, usually with about five days. It is important to ensure that water draining into the system does not overwhelm the designed capacity for stormwater management. Location of stormwater drains or flow of water from surrounding landscaping should not exceed the volume capacity of the system or deposit soil or other debris to clog the system. Relative elevations, slopes and grades should be considered in the design.
The next step is to add a volume of paste such that the final void content of the compacted mixture is around 20%. The void content is calculated from measured density of fresh pervious concrete in accordance with ASTM C1688. Two aspects are important regarding the paste – the volume should be well-balanced to sufficiently coat and bond the aggregate particles; and the paste consistency should be appropriate to facilitate ease of discharge from mixers and the installation process. Excessive paste or paste with a wet consistency can cause paste run-down and clog the pavement at the base. Insufficient paste or a consistency that is too dry can result in surface raveling. Water-cementitious materials ratio (w/cm) in the range of 0.26 to 0.35 is commonly used to control the consistency of the paste. Admixtures used for conventional concrete, including hydration stabilizing admixtures and viscosity modifying admixtures, as well as chemical admixtures specially formulated for pervious concrete are used to facilitate the development of robust pervious concrete mixtures.
Strength, or structural consideration, is a secondary importance for most low traffic applications of pervious concrete. Durability, specifically in terms of resistance to raveling, which assures a long service life is of greater importance than strength. Guidance on proportioning pervious concrete mixtures is provided by ACI Committee 5221 and by NRMCA8.
Time, moisture control, proper compaction and curing are critical to installing pervious concrete pavement. All equipment including curing sheets should be ready and adequate crew size should be available prior to the placement. The base should be soaked prior to placement.
Most commonly, pervious concrete is placed with formwork. Larger placements using slipform pavers or laser screeds may not need formwork. The general steps include spreading & the concrete, strike-off, compaction, cross-rolling, jointing and curing.
Two methods are common – a two-step or one-step process. In the two-step process, 9.5 to 12.5 mm (3/8 to 1/2 in.) strips are attached to the top of the forms and placed pervious concrete is struck off to the elevated level while ensuring the strike-off does not cause sealing of the surface. The strips are removed and the pervious concrete is compacted using a heavy weighted roller. In the one-step process, pervious is struck off and compacted to the design elevation using motorized roller screeds in one operation. The motorized roller screed is configured to achieve most of the full compaction during this phase of placement. To achieve adequate compaction, it is critical to maintain a consistent head of pervious concrete ahead of the roller-screed. Additional attention is necessary to compact edges and at construction joint locations. The use of roller-screeds and the one step process is more common due to time saving and reduced effort required.
Cross-rolling is accomplished using smaller weighted rollers in a lateral direction to the original rolling. Cross-rolling should achieve the final surface finish and should ensure that all roller marks are removed. It should not be relied on for compacting the surface. A special roller with a welded flange is used to score contraction joints in the pervious pavement.
The pavement should be covered with curing plastic sheets within 20 minutes after the placement to prevent the surface from drying out. Cross-rolling can be performed on top of the plastic. Soy-bean based spray-on curing compounds have been used to improve the curing process before plastic sheet is spread and to minimize discoloration caused by the plastic sheet. Curing sheets should be properly overlapped and anchored to prevent loss of moisture for a period of at least seven days. All placement operations should ensure adequate compaction, especially at edges, ensure that the surface is not sealed and that curing will allow the paste to gain adequate strength to maintain aggregate bond and minimize raveling. Traditional pigmented curing compounds are not recommended as an alternate for curing pervious concrete as coverage of the porous surface is not assured. More recently other products have been developed to avoid the use of plastic sheeting.
ACI Committee 522 has published a specification for pervious concrete that is used as the basis for most project specifications.10 The specification addresses the qualification of contractors by requiring NRMCA certification. The specification requires that pervious concrete mixtures be accepted on the basis of its fresh density measured by ASTM C1688. The fresh density should be within ±80 kg/m3 (5 lbs/ft3) of the target density of the mixture in the submittal that documents achieving the desired void content.
A test panel is required that verifies the thickness, density and final characteristics. Cores taken from the test panel establish the requirements for density and thickness of cores for the remainder of the project. The density of cores from the pavement should be within ±5% of the density of cores from the test panel. Tolerances are defined for the thickness of the pavement as constructed. At this time, there are no acceptance criteria for pervious concrete for strength, The danger with specifying strength requirements for pervious concrete is that attempting to comply with a strength requirement may result in an excessive strength that will reduce the voids and permeability of the pavement. Another caution is that the void content in the pavement should not be expected to be similar to that determined on the fresh concrete by ASTM C1688 because of the considerably different compaction when the pavement is installed.
Colin Lobo, Sr. Vice President, Engineering, National Ready Mixed Concrete Association
Introduction
 |
| Figure 1: The cement and water in pervious concrete forms a paste that uniformly coats and bonds the aggregates together when properly compacted but creates an interconnected void system that allows water to pass through it. |
Pervious concrete, in some form, has been used in construction for more than 100 years. Today, in the United States, it is being used primarily as a pavement material for parking areas, drives, walkways and some lightly trafficked residential streets. Parking lots and roadways are major sources of stormwater pollution. When precipitation runs off a conventional, impervious parking lot or roadway, it carries with it a wide range of pollutants, such as debris, metals (brake linings) and hydrocarbons (oils and grease). Polluted stormwater runoff eventually finds its way into streams, rivers and lakes.
 |
 |
| Figure 2: Pervious concrete is primarily used as a pavement material for parking areas, drives, walkways and some lightly trafficked residential streets | |
A major reason for the interest in pervious concrete is being driven by the benefits it provides for Low Impact Development (LID). In the United States, federal regulations require states, counties and municipalities to adopt procedures that address stormwater runoff and associated pollutants. These regulations generally require that property owners collect and treat stormwater on site as opposed to allowing it to runoff to adjacent sites or into municipal stormwater management systems. Infiltration systems, such as pervious concrete pavements, help satisfy these requirements. The US Environmental Protection Agency (EPA) recognizes pervious concrete as one of the best management practices for stormwater management.
A typical pervious concrete pavement system consists of a layer of pervious concrete to support traffic loading over a layer of aggregate base. The pervious concrete allows water to percolate quickly into the aggregate base layer, often called the storage layer, before slowly infiltrating into the subgrade soil. The aggregate base layer is typically a clean, single-sized aggregate between 18 mm and 40 mm (3/4 in. and 1-1/2 in.) nominal size. In some cases, perforated pipes are incorporated into the aggregate base layer to increase storage capacity or collect water from other surfaces on the site including roofs, walks and conventional impervious pavements. This supplemental drainage may also be necessary to increase capacity of systems on fine-grained soils that reduce the rate of percolation of stormwater. Perforated pipes in the base layer collect excessive stormwater and diverts it to supplemental retention areas. Pervious concrete pavement systems are often combined with other stormwater infiltration systems such as rain gardens and bio-swales (see figure 3).
 |
| Figure 3: A typical pervious concrete pavement system consists of a layer of pervious concrete to support traffic loading over a layer of aggregate base. Pervious concrete pavement systems are often combined with other stormwater infiltration systems such as rain gardens and bio-swales. Courtesy of 2020 Engineering. |
Benefits of Pervious Concrete
Cost savings: Impervious pavement surfaces do not allow stormwater to pass through them. Therefore, the surfaces must be sloped to allow for proper drainage of water to stormwater collection facilities or separate discharge basins commonly known as detention ponds. Detention basins, along with associated storm drains and underground storm sewer piping, add a substantial cost to project development. Integrating the stormwater system and the parking lot or roadway into a single mitigation strategy can result in significant cost savings. Pervious concrete forms a rigid pavement surface for vehicle and pedestrian traffic, while allowing the passage to stormwater directly through the pavement. By allowing rainfall to pass through the pavement system, and into the soil below, the entire pavement area can be used as a detention system, which can often reduce or eliminate the need for separate drainage structures.Pollution treatment: In a parking lot or roadway, pollution from vehicles and other sources, in the form of hydrocarbons, heavy metals and sediment, accumulate on the surface and are a major source of stormwater pollution in most watersheds. According to the United States Environmental Protection Agency (USEPA), about 90% of the surface pollutants are carried off by the first 12 mm (½ in.) to 25 mm (1-in.) of rainfall. In a conventional impervious parking area the polluted rainfall then flows to a stormwater drain, and eventually to streams, lakes or rivers. A pervious concrete system treats these pollutants and allows clean water to pass through the pavement, into the aggregate base layer, and then into the native soil beneath the pavement. The pervious concrete, aggregate base and subgrade soil filters and treats the rainfall in the same manner as the site naturally did before the parking lot or roadway was constructed.
Recharging aquifers: Some areas use aquifers for their drinking water and pump the water to local communities. Many areas of the United States and other parts of the world are dealing with water shortages and diminishing groundwater supplies, yet by increasing the area of impervious surfaces, an abundance of precipitation is allowed to run into storm sewer systems bypassing the natural process of rainfall recharging groundwater. Pervious concrete pavements allow rainfall to percolate back into the soil where it falls, thereby replenishing aquifers naturally.
Encourage tree growth: Impervious pavements keep the water and air away from tree roots. Many communities have adopted regulations that prohibit the construction of impervious pavements within the drip line of trees. This can prove difficult in places where trees are within, or near, roads, parking areas or other hard surfaces. A practical solution is to use pervious concrete for these hard surfaces. This allows rainwater and air to pass freely through the pavement to sustain the tree roots.
Reducing urban heat islands: Research at Lawrence Berkeley National Laboratory at Berkeley, California, shows that temperatures in residential zones rise by as much as (1.7ºC) 3°F and in downtown areas by as much as (3.9ºC) 7°F in some cities in the United States and even higher in some other cities around the world. This is primarily because of the predominant use of dark-colored roofing and pavements.1 This is known as the urban heat island effect. The research has shown that use of light- and heat-reflective materials for roofing and pavements, along with the planting of trees, could lower the average summer afternoon temperature significantly and thus reduce the need for air conditioning. Pervious concrete absorbs and stores less heat than conventional impervious pavements. The lighter color or albedo of pervious concrete pavements reflects radiant thermal heat while the open void structure permits the earth’s cooler temperatures from below, along with water evaporation, to cool the pavement thus helping to reduce urban heat islands2.
LEED Recognition Leadership in Energy and Environmental Design (LEED®) is a point rating system devised by the United States Green Building Council (USGBC) that evaluates the environmental performance of buildings. The system, which is used all over the world, is credit-based, allowing projects to earn points for environment–friendly strategies employed during the design and construction process. Projects receive points for the environment– friendly strategies of each component of design and construction, such as the building’s energy and water use, among other criteria. Using concrete in new buildings and major renovation projects, including the use of pervious concrete, can facilitate the process of obtaining points in LEED® 2009 for New Construction as follows:3
- Sustainable Sites Credit 6.1: Stormwater Design¯Quantity Control
- Sustainable Sites Credit 6.2: Stormwater Design¯Quality Control
- Sustainable Sites Credit 7.1: Heat Island Effect¯Non-Roof
- Water Efficiency Credit 1: Water Efficient Landscaping
- Water Efficiency Credit 3: Water Use Reduction
- Material and Resources Credit 4: Recycled Content
- Materials and Resources Credit 5: Regional Materials
Design Criteria
Pervious concrete systems are designed for two main criteria: 1) to support anticipated traffic loading, and 2) to handle the volume of stormwater for a specific design storm. Generally, the stormwater criteria govern the design thickness for the pervious system. Support of traffic loading is typically designed using rules of thumb. 150 mm (6 in.) of pervious concrete can typically support car parking areas and pedestrian traffic. 200 mm (8 in.) of pervious concrete is typically used for heavier traffic loading experienced on residential streets or commercial truck parking areas. |
| Figure 4: Relationship between flexural strength and compressive strength of pervious concrete |
For unusual high loadings or for specific designs, some engineers have used mechanistic design procedures such as those described in StreetPave (or Pervious Pave) software developed by the American Concrete Pavement Association.4 When using this software, engineers often use a conservative assumed compressive strength for pervious concrete of 10 MPa (1,500 psi). One can calculate the tensile strength based on the relationship between compressive strength and flexural strength as developed by Crouch et al (see figure 4).5 This information, along with traffic loading and base material characteristics are input into the software program to determine the minimum pavement thickness.
For hydrological design, the pavement system must be designed to rapidly percolate stormwater through the pervious concrete, store water in the aggregate base layer, then infiltrate the water into the ground below or release the water at a controlled rate through outlets. A common design for a pervious concrete detention system is 150 mm (6 in.) of pervious concrete over 150 mm (6 in.) or more of clean open-graded aggregate. In some cases, a non-woven geotextile filter fabric or aggregate choker layer might be used to separate the aggregate base from the subgrade soil.
Key parameters for design are the pervious concrete permeability, the aggregate base layer void content, the infiltration rate of the in situ soil below the aggregate base layer, the design storm data, and topography and infiltration characteristics of surrounding landscape. A properly designed pervious system will be drained of water and ready to receive another design storm soon after a storm event, usually with about five days. It is important to ensure that water draining into the system does not overwhelm the designed capacity for stormwater management. Location of stormwater drains or flow of water from surrounding landscaping should not exceed the volume capacity of the system or deposit soil or other debris to clog the system. Relative elevations, slopes and grades should be considered in the design.
Pervious Concrete Mixtures
Pervious concrete mixtures should be proportioned with special attention to ensure the mixture can be placed and compacted with the primary goal being to ensure that the installed pavement will allow for the percolation of water. The first step requires an evaluation of the aggregate – the aggregate void content is determined from its measured dry rodded density. Ideally, the void content of aggregate should be in the range of 38 and 42%. Nominal maximum aggregate size of 9.5 mm (3/8 in.) is most common, but 12.5 mm (1/2 in.) is also used for pervious concrete mixtures. The aggregate grading should be evaluated to ensure that excessive fines are not present as this will require an increase in the paste volume to coat aggregate surfaces. In some cases, a small quantity of sand may be used when the aggregate has a high void content or when higher strength is required for higher traffic load applications. |
| Figure 5: Ideal consistency of fresh pervious concrete mixture |
Strength, or structural consideration, is a secondary importance for most low traffic applications of pervious concrete. Durability, specifically in terms of resistance to raveling, which assures a long service life is of greater importance than strength. Guidance on proportioning pervious concrete mixtures is provided by ACI Committee 5221 and by NRMCA8.
Construction
The success of pervious concrete pavements is very closely linked to proper procedures used for construction. Comprehensive details on equipment needed and construction procedures are covered in the NRMCA text used for certification.9 Typical concrete production is expected with special attention to monitoring aggregate moisture and gradation and control of the water content in the mixture. It is not uncommon to add water at the jobsite to achieve the required consistency, but this should be avoided if possible. Delivery of concrete should be scheduled to ensure a constant availability of concrete during the placement without allowing delivery trucks to stack up at the jobsite.Time, moisture control, proper compaction and curing are critical to installing pervious concrete pavement. All equipment including curing sheets should be ready and adequate crew size should be available prior to the placement. The base should be soaked prior to placement.
Most commonly, pervious concrete is placed with formwork. Larger placements using slipform pavers or laser screeds may not need formwork. The general steps include spreading & the concrete, strike-off, compaction, cross-rolling, jointing and curing.
Two methods are common – a two-step or one-step process. In the two-step process, 9.5 to 12.5 mm (3/8 to 1/2 in.) strips are attached to the top of the forms and placed pervious concrete is struck off to the elevated level while ensuring the strike-off does not cause sealing of the surface. The strips are removed and the pervious concrete is compacted using a heavy weighted roller. In the one-step process, pervious is struck off and compacted to the design elevation using motorized roller screeds in one operation. The motorized roller screed is configured to achieve most of the full compaction during this phase of placement. To achieve adequate compaction, it is critical to maintain a consistent head of pervious concrete ahead of the roller-screed. Additional attention is necessary to compact edges and at construction joint locations. The use of roller-screeds and the one step process is more common due to time saving and reduced effort required.
 |
| Figure 6. Sequence of construction from left to right – placing and compacting with motorized roller screed; cross rolling (over plastic sheet); edge compaction with roller; jointing; securing plastic sheets to forms for curing. |
Cross-rolling is accomplished using smaller weighted rollers in a lateral direction to the original rolling. Cross-rolling should achieve the final surface finish and should ensure that all roller marks are removed. It should not be relied on for compacting the surface. A special roller with a welded flange is used to score contraction joints in the pervious pavement.
The pavement should be covered with curing plastic sheets within 20 minutes after the placement to prevent the surface from drying out. Cross-rolling can be performed on top of the plastic. Soy-bean based spray-on curing compounds have been used to improve the curing process before plastic sheet is spread and to minimize discoloration caused by the plastic sheet. Curing sheets should be properly overlapped and anchored to prevent loss of moisture for a period of at least seven days. All placement operations should ensure adequate compaction, especially at edges, ensure that the surface is not sealed and that curing will allow the paste to gain adequate strength to maintain aggregate bond and minimize raveling. Traditional pigmented curing compounds are not recommended as an alternate for curing pervious concrete as coverage of the porous surface is not assured. More recently other products have been developed to avoid the use of plastic sheeting.
Tests and Specification
ASTM has established a subcommittee that is working on developing testing standards for pervious concrete. At this time, test methods have been standardized for measuring the density of fresh pervious concrete and calculating the void content – ASTM C1688. A second method measures the infiltration rate of installed pavements – ASTM C1701. This method is intended to be used to evaluate the degree of clogging of pavements in service and not for acceptance of newly constructed pavements. Other standards under development include a practice for preparing specimens for compressive strength tests, a test method for measuring the density and void content of hardened concrete specimens, and a test method for evaluating the raveling potential of pervious concrete mixtures.ACI Committee 522 has published a specification for pervious concrete that is used as the basis for most project specifications.10 The specification addresses the qualification of contractors by requiring NRMCA certification. The specification requires that pervious concrete mixtures be accepted on the basis of its fresh density measured by ASTM C1688. The fresh density should be within ±80 kg/m3 (5 lbs/ft3) of the target density of the mixture in the submittal that documents achieving the desired void content.
A test panel is required that verifies the thickness, density and final characteristics. Cores taken from the test panel establish the requirements for density and thickness of cores for the remainder of the project. The density of cores from the pavement should be within ±5% of the density of cores from the test panel. Tolerances are defined for the thickness of the pavement as constructed. At this time, there are no acceptance criteria for pervious concrete for strength, The danger with specifying strength requirements for pervious concrete is that attempting to comply with a strength requirement may result in an excessive strength that will reduce the voids and permeability of the pavement. Another caution is that the void content in the pavement should not be expected to be similar to that determined on the fresh concrete by ASTM C1688 because of the considerably different compaction when the pavement is installed.
Contractor Qualifications
Construction of pervious concrete differs considerably from conventional concrete pavement. It requires different tools and procedures. Improper tools or practices can adversely affect the functioning and service life of pervious concrete pavements. To address this, a contractor certification program was developed by NRMCA in 2004. The text covers the general applications and benefits, general concepts of design, mixture proportioning and evaluation, tools and equipment, construction procedures and maintenance of installed pavements.9 Individuals are certified at three levels – technician, installer and craftsman. The training and certification is administered by local sponsoring groups. For all levels, the individual has to pass the exam. The installer and craftsman level are awarded based on achieving other certification criteria that include a performance evaluation and documentation of work experience. Details of the certification program are available on the certification section of the NRMCA website at www.nrmca.org. NRMCA certification of contractor crew is required in the most project specifications in the US.Maintenance
Pervious concrete pavements, as with all pavements, should be maintained throughout its lifetime to uphold optimum performance. The pavement should be inspected several times per year to make sure unexpected sediment sources, surface debris or leaves have not reduced or compromised the infiltration function or capacity. Areas of pavement exposed to falling leaves or other landscape debris should be cleaned as needed. The most effective means of is to vacuum the surface using street cleaning equipment. For especially clogged surfaces, pressure washing vegetation and debris from pavements before vacuuming. The frequency of the vacuuming is dependent on the design of the system and how much sediment is allowed to accumulate on the surface. When raveling occurs in a pervious pavement in service, there is no repair strategy recommended. Deteriorating pavement sections should be saw-cut and removed and filled in with new pervious concrete.References
- 1 ACI Committee 522, Report on Pervious Concrete - 522R-10, American Concrete Institute, Farmington Hills, Michigan, 2010.
- 2 Tennis, P. et al., Pervious Concrete Pavements, Portland Cement Association, Skokie, Illinois and National Ready Mixed Concrete Association, Silver Spring, Maryland, 2004.
- 3 Rosenfeld, A. H. et al. Painting the Town White¯and Green, MIT Technology Review, February/March 1997.
- 4 Kevern J., et al, Hot Weather Comparative Heat Balances in Pervious Concrete and Impervious Concrete Pavement Systems, Second International Conference on Countermeasures to Urban Heat Islands Proceedings, September 2009.
- 5 U.S. Green Building Council, November 2008, LEED 2009 New Construction and Major Renovations, Washington, DC, 2009.
- 6 StreetPave Software, American Concrete Pavement Association, Skokie, Illinois, 2005.
- 7 Crouch, L.K., et al, Estimating Pervious PCC Pavement Design Inputs with Compressive Strength and Effective Void Content, 2006 Concrete Technology Forum Proceedings, National Ready Mixed Concrete Association, Silver Spring, Maryland, May 2006.
- 8 Pervious Concrete – Guideline to Mixture Proportioning and Research Report, Publication #2PE002, National Ready Mixed Concrete Association, Silver Spring, MD, February 2009.
- 9 Text Reference for Pervious Concrete Contractor Certification, Publication #2PPCRT, National Ready Mixed Concrete Association, Silver Spring, Maryland, March 2011.
- 10 ACI Committee 522, Specification for Pervious Concrete Pavement, ACI 522.1-11, American Concrete Institute, Farmington Hills, Michigan.
NBMCW September 2011
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