Pervious concrete
Encyclopedia
Pervious concrete is a special type of concrete
with a high porosity
used for concrete flatwork applications that allows water
from precipitation
and other sources to pass directly through, thereby reducing the runoff
from a site and allowing groundwater recharge. The high porosity is attained by a highly interconnected void content. Typically pervious concrete has little or no fine aggregate and has just enough cementitious paste to coat the coarse aggregate particles while preserving the interconnectivity of the voids. Pervious concrete is traditionally used in parking areas
, areas with light traffic, residential street
s, pedestrian walkways
, and greenhouse
s. It is an important application for sustainable construction
and is one of many low impact development
techniques used by builders to protect water quality
.
by the U.S. Environmental Protection Agency (EPA) for providing first flush
pollution
control and stormwater
management. As regulation
s further limit stormwater runoff, it is becoming more expensive for property owners to develop real estate
, due to the size and expense of the necessary drainage
systems. Pervious concrete reduces the runoff from paved areas, which reduces the need for separate stormwater retention ponds and allows the use of smaller capacity storm sewers. This allows property owners to develop a larger area of available property at a lower cost. Pervious concrete also naturally filters storm water and can reduce pollutant
loads entering into stream
s, pond
s and river
s.
Pervious concrete functions like a storm water infiltration basin
and allows the storm water to infiltrate the soil
over a large area, thus facilitating recharge of precious groundwater supplies locally. All of these benefits lead to more effective land use. Pervious concrete can also reduce the impact of development on tree
s. A pervious concrete pavement allows the transfer of both water and air to root systems allowing trees to flourish even in highly developed areas.
will depend on the specifics of the pavement design and a layer of open graded stone may have to be placed over the soil. Engineering fabrics are often used to separate fine-grained soils from the stone layer. Care must be taken not to over-compact soil with swelling potential. The subgrade should be moistened prior to concrete placement, and wheel ruts from the construction traffic should be raked and re-compacted.
Typically pervious concrete has a water to cementitious materials (w/cm) ratio of 0.28 to 0.40 with a void content of 15 to 25%. The mixture is composed of cementitious materials, coarse aggregate and water with little to no fine aggregates. Addition of a small amount of fine aggregate will generally reduce the void content and increase the strength, which may be desirable in certain situations. This material is sensitive to changes in water content, so field adjustment of the fresh mixture is usually necessary. The correct quantity of water in the concrete is critical. Too much water will cause paste drain down, and too little water can hinder adequate curing of the concrete and lead to a premature raveling surface failure. A properly proportioned mixture gives the mixture a wet-metallic appearance or sheen.
A pervious concrete pavement may be placed with either fixed forms or slip-form paver. A common approach to placing pervious concrete is in forms on grade that have a riser strip on the top of each form such that the strike off device is actually 3/8-1/2 in. (9 to 12 mm) above final pavement elevation. Strike off may be by mechanical or manual screeds, though mechanical screens are preferable. After striking off the concrete, the concrete is compacted to provide a strong bond between the paste and aggregate, and to create a smoother riding surface. Excessive pressure when rolling is avoided as it may cause the voids to collapse. Rolling is be performed immediately after strike off.
Jointing pervious concrete pavement follows the same rules as for concrete slabs on grade, with a few exceptions. The rules of jointing geometry, however, remain the same. Joints in pervious concrete are saw cut after the curing stage or tooled with a rolling jointing tool prior to curing. Proper curing is essential to the structural integrity of a pervious concrete pavement. Curing ensures sufficient hydration of the cement paste to provide the necessary strength in the pavement section to prevent raveling. Curing should begin within 20 minutes of concrete discharge and continue through 7 days. Covering pervious concrete pavement with 6 mil. plastic sheeting is the standard method of curing, however, this contributes to a substantial amount of waste sent to landfills. Alternatively, preconditioned absorptive lightweight aggregate as well as internal curing admixture (ICA) have been used to effectively cure pervious concrete without waste generation.
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
with a high porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
used for concrete flatwork applications that allows water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
from precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
and other sources to pass directly through, thereby reducing the runoff
Surface runoff
Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...
from a site and allowing groundwater recharge. The high porosity is attained by a highly interconnected void content. Typically pervious concrete has little or no fine aggregate and has just enough cementitious paste to coat the coarse aggregate particles while preserving the interconnectivity of the voids. Pervious concrete is traditionally used in parking areas
Parking lot
A parking lot , also known as car lot, is a cleared area that is intended for parking vehicles. Usually, the term refers to a dedicated area that has been provided with a durable or semi-durable surface....
, areas with light traffic, residential street
Street
A street is a paved public thoroughfare in a built environment. It is a public parcel of land adjoining buildings in an urban context, on which people may freely assemble, interact, and move about. A street can be as simple as a level patch of dirt, but is more often paved with a hard, durable...
s, pedestrian walkways
Sidewalk
A sidewalk, or pavement, footpath, footway, and sometimes platform, is a path along the side of a road. A sidewalk may accommodate moderate changes in grade and is normally separated from the vehicular section by a curb...
, and greenhouse
Greenhouse
A greenhouse is a building in which plants are grown. These structures range in size from small sheds to very large buildings...
s. It is an important application for sustainable construction
Construction
In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...
and is one of many low impact development
Low impact development
Low-impact development is a term used in the United States to describe a land planning and engineering design approach to managing stormwater runoff. LID emphasizes conservation and use of on-site natural features to protect water quality...
techniques used by builders to protect water quality
Water quality
Water quality is the physical, chemical and biological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which...
.
Stormwater management
The proper utilization of pervious concrete is a recognized Best Management PracticeBest management practice for water pollution
Best Management Practices is a term used in the United States and Canada to describe a type of water pollution control. Historically the term has referred to auxiliary pollution controls in the fields of industrial wastewater control and municipal sewage control, while in stormwater management ...
by the U.S. Environmental Protection Agency (EPA) for providing first flush
First flush
First flush is the initial surface runoff of a rainstorm. During this phase, water pollution entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm...
pollution
Water pollution
Water pollution is the contamination of water bodies . Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds....
control and stormwater
Stormwater
Stormwater is water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt that enters the stormwater system...
management. As regulation
Regulation
Regulation is administrative legislation that constitutes or constrains rights and allocates responsibilities. It can be distinguished from primary legislation on the one hand and judge-made law on the other...
s further limit stormwater runoff, it is becoming more expensive for property owners to develop real estate
Real estate
In general use, esp. North American, 'real estate' is taken to mean "Property consisting of land and the buildings on it, along with its natural resources such as crops, minerals, or water; immovable property of this nature; an interest vested in this; an item of real property; buildings or...
, due to the size and expense of the necessary drainage
Drainage
Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.-Early history:...
systems. Pervious concrete reduces the runoff from paved areas, which reduces the need for separate stormwater retention ponds and allows the use of smaller capacity storm sewers. This allows property owners to develop a larger area of available property at a lower cost. Pervious concrete also naturally filters storm water and can reduce pollutant
Water pollution
Water pollution is the contamination of water bodies . Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds....
loads entering into stream
Stream
A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...
s, pond
Pond
A pond is a body of standing water, either natural or man-made, that is usually smaller than a lake. A wide variety of man-made bodies of water are classified as ponds, including water gardens, water features and koi ponds; all designed for aesthetic ornamentation as landscape or architectural...
s and river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
s.
Pervious concrete functions like a storm water infiltration basin
Infiltration basin
An infiltration basin , is a type of best management practice that is used to manage stormwater runoff, prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay...
and allows the storm water to infiltrate the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
over a large area, thus facilitating recharge of precious groundwater supplies locally. All of these benefits lead to more effective land use. Pervious concrete can also reduce the impact of development on tree
Tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
s. A pervious concrete pavement allows the transfer of both water and air to root systems allowing trees to flourish even in highly developed areas.
Construction
An experienced installer is vital to the success of pervious concrete pavements. As with any concrete pavement, proper subgrade preparation is important. The subgrade should be properly compacted to provide a uniform and stable surface. When pervious pavement is placed directly on sandy or gravelly soils it is recommended to compact the subgrade to 92 to 96% of the maximum density. With silty or clayey soils, the level of compactionSoil compaction
In Geotechnical engineering, soil compaction is the process in which a stress applied to a soil causes densification as air is displaced from the pores between the soil grains. When stress is applied that causes densification due to water being displaced from between the soil grains then...
will depend on the specifics of the pavement design and a layer of open graded stone may have to be placed over the soil. Engineering fabrics are often used to separate fine-grained soils from the stone layer. Care must be taken not to over-compact soil with swelling potential. The subgrade should be moistened prior to concrete placement, and wheel ruts from the construction traffic should be raked and re-compacted.
Typically pervious concrete has a water to cementitious materials (w/cm) ratio of 0.28 to 0.40 with a void content of 15 to 25%. The mixture is composed of cementitious materials, coarse aggregate and water with little to no fine aggregates. Addition of a small amount of fine aggregate will generally reduce the void content and increase the strength, which may be desirable in certain situations. This material is sensitive to changes in water content, so field adjustment of the fresh mixture is usually necessary. The correct quantity of water in the concrete is critical. Too much water will cause paste drain down, and too little water can hinder adequate curing of the concrete and lead to a premature raveling surface failure. A properly proportioned mixture gives the mixture a wet-metallic appearance or sheen.
A pervious concrete pavement may be placed with either fixed forms or slip-form paver. A common approach to placing pervious concrete is in forms on grade that have a riser strip on the top of each form such that the strike off device is actually 3/8-1/2 in. (9 to 12 mm) above final pavement elevation. Strike off may be by mechanical or manual screeds, though mechanical screens are preferable. After striking off the concrete, the concrete is compacted to provide a strong bond between the paste and aggregate, and to create a smoother riding surface. Excessive pressure when rolling is avoided as it may cause the voids to collapse. Rolling is be performed immediately after strike off.
Jointing pervious concrete pavement follows the same rules as for concrete slabs on grade, with a few exceptions. The rules of jointing geometry, however, remain the same. Joints in pervious concrete are saw cut after the curing stage or tooled with a rolling jointing tool prior to curing. Proper curing is essential to the structural integrity of a pervious concrete pavement. Curing ensures sufficient hydration of the cement paste to provide the necessary strength in the pavement section to prevent raveling. Curing should begin within 20 minutes of concrete discharge and continue through 7 days. Covering pervious concrete pavement with 6 mil. plastic sheeting is the standard method of curing, however, this contributes to a substantial amount of waste sent to landfills. Alternatively, preconditioned absorptive lightweight aggregate as well as internal curing admixture (ICA) have been used to effectively cure pervious concrete without waste generation.
Testing and inspection
Pervious concrete can attain a compressive strength ranging from 400 psi to 4000 psi (2.8 to 28 MPa) though strengths of 600 psi to 1500 psi (2.8 to 10 MPa) are more common. Pervious concrete, however, is not specified or accepted based on strength as there are not yet standardized test methods. More important to the success of a pervious pavement is the void content. Acceptance is typically based on the density (unit weight) of the in-place pavement. An acceptable tolerance is plus or minus 5 lb/cu.ft. (80 kg/m3) of the design density. This should be verified through field testing. The fresh density (unit weight) of pervious concrete is measured using the test method described in ASTM C1688. Slump and air content tests are not applicable to pervious concrete. If the pervious concrete pavement is an element of the storm water management plan, the designer should ensure that it is functioning properly through visual observation of its drainage characteristics prior to opening of the facility. Questions have been raised about the freeze thaw durability of pervious concrete. Pervious concrete in freeze thaw environments must not become fully saturated. Saturation of installed pervious concrete pavement can be prevented by placing the pervious concrete on a thick layer of 8 to 24 inches (200 to 600 mm) of open graded stone base. Laboratory testing has shown that entrained air may improve the freeze thaw durability even when the pervious concrete is in a fully saturated condition. However, the entrained air content cannot be verified by any standard ASTM test procedure.Maintenance
EPA recommends that pervious concrete pavement be cleaned regularly to prevent reduction in permeability. Cleaning can be accomplished through vacuum sweeping or pressure washing.See also
- Green infrastructureGreen infrastructureGreen Infrastructure is a concept originating in the United States in the mid-1990s that highlights the importance of the natural environment in decisions about land use planning. In particular there is an emphasis on the "life support" functions provided by a network of natural ecosystems, with an...
- Low impact developmentLow impact developmentLow-impact development is a term used in the United States to describe a land planning and engineering design approach to managing stormwater runoff. LID emphasizes conservation and use of on-site natural features to protect water quality...
- Permeable pavingPermeable pavingPermeable paving is a range of materials and techniques for paving roads, cycle-paths, parking lots and sidewalks that allow the movement of water and air around the paving material. Although some porous paving materials appear nearly indistinguishable from nonporous materials, their environmental...
- Urban runoffUrban runoffUrban runoff is surface runoff of rainwater created by urbanization. This runoff is a major source of water pollution in many parts of the United States and other urban communities worldwide.-Overview:...
Further reading
- US EPA. Office of Research and Development. "Research Highlights: Porous Pavements: Managing Rainwater Runoff." October 17, 2008.