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What are the disadvantages of pervious concrete?
- Can not be used on pavements with heavy traffic flow.
- Requires longer curing time.
- Difficult to find out water content in fresh concrete.
- Conventional concrete tests like slump test and compaction factor test are not applicable.
- Requires specialized construction practice.
How strong is pervious concrete?
In the laboratory, pervious concrete mixtures have been found to develop compressive strengths in the range of 500 to 4000 psi (3.5 MPa to 28 MPa), which is suitable for a wide range of applications. Typical values are about 2500 psi (17 MPa).
How do you treat pervious concrete?
The key step to curing pervious is covering the slab with plastic sheeting. “The cement in pervious needs good hydration to cure properly,” Fisher says. “The plastic sheeting maintains a moist, humid environment for the pervious concrete mixture to cure.
Does pervious concrete need rebar?
In most cases, pervious projects have neither condition, so bars are rarely used. If rebar is required or used in pervious concrete, it will need to be very resistant to corrosion, since air, water, and deicing chemicals will be moving through the pavement.
How long does pervious concrete take to cure?
The sheets should remain for a minimum of 7 days for straight cement mixes or 10 days for mixes that contain supplementary cementitious materials. There should not be traffic of any kind on the concrete until the curing period is complete.
Does pervious concrete clog?
However, it is a common myth that all porous pavement systems eventually clog. A good PPS system design, simple but effective site design, and careful construction inspection and field guidance can all but eliminate the clogging threat.
What are the benefits of pervious concrete?
- Eliminates untreated storm water and creates zero runoff!
- Directly recharges groundwater.
- Mitigates “first flush” pollution.
- Protects streams, watersheds, and ecosystems.
- Mimics the drainage and filtration of bioswales and natural soils.
- Reduces surface temperatures and heat island effects.
What is the biggest disadvantage of permeable concrete?
Disadvantages: Permeable paving is not ideal for high traffic/high speed areas because it has lower load-bearing capacity than conventional pavement. Nor should it be used on stormwater “hotspots” with high pollutant loads because stormwater cannot be pretreated prior to infiltration.
What PSI is pervious concrete?
Pervious concretes can develop compressive strengths in the range of 500 to 4000 pounds per square inch (psi) – suitable for a wide range of applications. Flexural Strength. Flexural strength of pervious concrete ranges between 150 and 550 psi.
What is pervious concrete why is it used instead of regular concrete?
Pervious concrete allows for expansive soils to absorb water and expand evenly across the area of the slab, unlike conventional concrete where more water infiltrates only along perimeter than the center causing differential expansion of the soil leading to cracks.
Is pervious concrete sustainable?
Pervious concrete pavement is a unique and effective means to address important environmental issues and support green, sustainable growth.
Where is pervious concrete used?
Pervious concrete has applications in sustainable construction, and is often used in sidewalks, parking areas, and greenhouses. Using pervious concrete is recognized as a Best Management Practice for Stormwater by the US Environmental Protection Agency (EPA), since it provides pollution control.
Who invented pervious concrete?
It can also reduce city temperatures and alleviate urban hot island phenomena [2]. Pervious concrete was first invented in Britain in 1824. It was first introduced into the United States in the mid-1970s and has since been rapidly developed [3]. The Sponge City was introduced in China in approximately 2012.
Does pervious concrete clog?
However, it is a common myth that all porous pavement systems eventually clog. A good PPS system design, simple but effective site design, and careful construction inspection and field guidance can all but eliminate the clogging threat.
What are the benefits of pervious concrete?
- Eliminates untreated storm water and creates zero runoff!
- Directly recharges groundwater.
- Mitigates “first flush” pollution.
- Protects streams, watersheds, and ecosystems.
- Mimics the drainage and filtration of bioswales and natural soils.
- Reduces surface temperatures and heat island effects.
Where is pervious concrete used?
Pervious concrete is traditionally used in parking areas, areas with light traffic, residential streets, pedestrian walkways, and greenhouses. It is an important application for sustainable construction and is one of many low impact development techniques used by builders to protect water quality.
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(approx. 20%) of interconnected voids. Like conventional concrete, it is
made from a mixture of cement, coarse aggregates, and water. However, it
contains little or no sand, which results in a porous open-cell structure
that allows water passes through readily. When pervious concrete is used
for paving, it can take in storm water at a rate of several hundred or more
inches of water per square foot per hour, which far exceeds the flow rate
needed to prevent runoff in even the most severe rain events. In slow
draining type C & D clay soils, rainwater is temporarily stored in a coarse
gravel layer underneath the pavement while it is allowed to naturally
percolate slowly into the underlying soil. Because the pavement and base
layer act as a detention area to capture and infiltrate water, it can
completely prevent the polluted site runoff that normally occurs with
impervious pavements. Properly designed the pervious concrete system can
eliminate the cost of the conventional storm water infrastructure with no
need for bio-swales or rain gardens. Pervious concrete pavement systems are
fully recognized as an EPA Storm water BMP which fully complies with the
San Francisco bay area C.3 provision for both infiltration and Treatment.
As the water percolates through the open cells of the pavement and crushed
rock base layer aerobic bacteria in the voids help to break down harmful
pollutants and chemicals. At Bay Area Pervious Concrete, we have
proprietary mix designs that we use to customize pervious concrete for the
variable conditions found throughout the San Francisco Bay Area. We can
design the optimal mix design for your site and application. Contact us for
more information. Maximize The Value of Your Property Are you faced with
losing valuable areas of your property to rain gardens, bio-swales or
detention ponds? Are you considering the enormous expense of underground
tanks? Frustrated because local stormwater regulations are impacting your
property in terms of both site design and costs? Looking for a straight
forward, cost effective, comprehensive solution that gives you back control
of your property? Well look no more, we’ve got you covered. Why Manage
Stormwater Runoff, When You Can Eliminate It Think of Pervious concrete as
a muti-purpose pavement. Unlike conventional asphalt or concrete pavements
which create stormwater runoff, pervious concrete provides the same a hard
and durable vehicle or pedestrian surface while also doubling as your
passive storm water management system. A properly designed system has no
runoff to manage, 100% of the water that falls on it stays in it. Roof
Run-Off Is it possible to also capture and infiltrate water from adjacent
roofs? Yes, you can. The soil infiltration rate, design storm and
pavement/roof area ratio will determine how much you can capture but in
many cases 100% of the roof area can be handled by the pervious concrete
system using a principle known as LATIS. Stormwater Super Powers: LATIS
This super power is a result of a principle known as LATIS, or “Large Area
Thin Infiltration System”. Conventional stormwater systems take water from
large impervious areas and concentrates it into a smaller but much deeper
footprint while a LATIS keeps the water spread out thin. Dealing with only
a few inches of rain water vs. several feet in a pond has a significant
positive impact on performance, particularly on low infiltrating clay
soils. Want to know more, check out the LATIS info. Lower Project Costs
Stormwater planning doesn’t require detention ponds, bio-swales, or
infiltration trenches when all of the hardscape can function as an
infiltration bed when using a pervious concrete LATIS. Because there is no
water surface flow or transporting of water to another location for
treatment, drainage pipes, curbs, gutters and slopes are not needed. Water
simply stays where it falls and soaks in under the slab. Pervious concrete
is the most affordable of the Low Impact Development Best Management
Practices (LID BMPs) due to the fact that hardscape is generally already in
the budget, and now the parking lot, roads or courtyard can function as
your complete stormwater BMP. When noting the Life Cycle costing for
projects, pervious concrete, like all concrete products, have a lower Life
Cycle Cost then asphalt. While asphalt has a lower installation cost, there
is far more surface maintenance in order to maintain the hard durable
surface over the course of its useful life. With concrete, it is installed
and can be left for decades. Some maintenance can be done to ensure surface
porosity, but it is rare, and not required. While learning to work with a
new material is a design challenge in and of itself, there are many
benefits to using pervious concrete. We will discuss this more in
environmental benefits of using pervious concrete section. Environmental
Benefits There are many benefits to using pervious concrete instead of
asphalt or traditional concrete, and we will discuss the big ones here. If
you think of additional benefits or applications, please email us here and
we may post it! Native Hydrology Fundamentally, what is most exciting about
pervious concrete, is that we are returning land to a pre-developed
hydrology, while maintaining the useful hard surface that we prefer to
live, work, play and drive on. The rain that falls can again soak into the
ground it landed on instead of flowing across a hardscape, picking up
contaminants, and getting sent through pipes to the nearest body of water.
Removes Pollution When rain soaks into the ground, not only does it
re-charge, or re-fill, the groundwater and local aquifers, but it also
filters out the stormwater contaminants. Instead of picking up the
pollution and sweeping it into surface streams and finally the ocean, the
pervious concrete and native soil filter out the heavy metals,
hydrocarbons, and other pollutants passively. In this way, we remove the
biggest pollution source of our freshwater in North America in a very
affordable way! Cool Pavement Pervious concrete is a cool pavement,
allowing an air and water exchange from the surface of the pavement to the
sub-soil beneath. Studies have shown that trees and landscaping thrives
with pervious concrete due to this and other effects. This also reduces the
heat island effect of dense development. Local Materials Pervious concrete
is composed of cement, local aggregate and water, with optional
ad-mixtures. It can be made using waste materials from industry like fly
ash, slag, micro silica and recycled aggregate, and so can increase its
recycled content. Water Recapture When planning for landscaping, it is
possible both to leverage the porosity of the pervious concrete as a means
of capturing irrigation water, and also, a way to pave safely closer to
existing trees without starving them of the necessary air and water, while
increasing usable hardscape area. Green Credits Due to these attributes,
pervious concrete qualifies for points with LEED, CAL Green and Green Point
Rated systems. For LEED, designing with pervious concrete falls under
Sustainable Sites, Innovation in Design and Water Efficiency. It is also an
EPA Stormwater Pollution Prevention Best Management Practice. If you would
be interested in more information, check out the current blog or contact
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Pervious Concrete – Advantages, Disadvantages & Application
What Is Pervious Concrete?
Pervious concrete has large voids that allow water or air to pass through it. The pores size varies from 2 to 8 mm, has a void content of 18 to 35 percent.
Further, this concrete has a compressive strength of 2.8–28.0 MPa (ACI 522, 2010). It is also known as porous concrete or water-permeable concrete.
Pervious concrete is often used in pavements. Porous concrete helps recharge groundwater, minimizing stormwater run-off by enabling it to seep into the ground.
Consequently, it offers an advantage in resolving critical environmental challenges. Hence, a better effort towards sustainable development.
While the first use of porous concrete dates back to the 1800s in Europe as pavement surface, it gained popularity in the US in the 1970s. Later, the demand for porous concrete increased after WWII as the cement supplies were badly affected. However, India realized the benefits of this concrete in the 2000s.
This article is dedicated to pervious concrete and discusses several aspects. First, we shall understand the advantages and working of this concrete.
The second is the composition, preparation, and execution process. At last, the article concludes with a discussion on the strength and cost of porous concrete.
Advantages Of Pervious Concrete
The primary advantage of pervious concrete is that it absorbs the stormwater. However, it serves multiple direct and indirect benefits as well.
1. Groundwater Recharge
The stormwater seeps through the pervious concrete and infiltrates through the ground. It ultimately adds up to the groundwater increasing groundwater level.
2. Reduction In Surface Run-off
The stormwater run-off reduces as the porous concrete surface lets the water seep through it to the ground. Hence, the surface run-off reduces.
3. Reduction Of Sewers
Due to the reduced stormwater surface run-off, the size and need of the stormwater sewers reduce. Therefore, offering savings in drainage system costs.
4. Development Of Trees
The stormwater infiltrating through the ground provides higher moisture content. Moreover, the voids of the pervious concrete allow the necessary air for roots to breathe. Consequently, offering a healthier environment for roots to grow into trees and plants.
5. Filtering Of Stormwater
The porous concrete act as a filter for the stormwater. The dirt gets trapped into the voids, and hence only clear water reaches the stream, pond, or lake.
Disadvantages Of Pervious Concrete
Can not be used on pavements with heavy traffic flow.
Requires longer curing time.
Difficult to find out water content in fresh concrete.
Conventional concrete tests like slump test and compaction factor test are not applicable.
Requires specialized construction practice.
Special design considerations need to be implemented.
Requires regular cleaning to maintain its permeability.
How Does Pervious Concrete Work?
Pervious concrete allows the water to flow through it. The permeability of concrete depends upon the percentage of voids in the concrete, and the size of voids. Typically, 15-25% voids are achieved in hardened permeable concrete.
The flow rates of pervious concrete are approximately 480 in./hr (200 L/m2/min). The flow rate can be much higher than the typical values.
This concrete doesn’t use fine aggregates; therefore, it has a grainy-textured surface. It offers higher friction to the vehicle tires.
Subsequently, driving on pervious concrete is easier in rain and snow. Also, it doesn’t allow water pooling, consequently offering any night-time glare.
Surface Texture of Porous Concrete
The stormwater contains contaminants that clog the voids of the pervious concrete. Hence, inhibiting its’ fundamental property of allowing water to pass through it.
Therefore, pervious concrete design uses stormwater sediment test data to minimize the clogging issue. However, the de-clogging is achieved by wetting the concrete and cleaning with vacuum suction or pressure washing.
This concrete has joints to account for changes due to temperature changes. The methodology to execute and finish a joint in permeable concrete is similar to conventional concrete.
How To Produce Pervious Concrete?
Cement, coarse aggregate (size should be 9.5 mm to 12.5 mm), and water make up pervious concrete which contains little to no fine aggregates. A small amount of sand added boosts the strength.
A void content of 15 to 25% is desired in pervious concrete. The higher the mortar content, the lesser the voids, hence lesser permeability.
The water-cement ratio in the mixture is 0.28 to 0.40. A correct water-cement ratio is essential as a lesser water-cement ratio offers higher strength. But too little water will weaken the concrete.
Therefore, check the concrete mix ratio before pouring it into the field.
Crushed aggregates, old crushed concrete blocks, recycled concrete, and clay-brick aggregate are suitable materials for pervious concrete. Using the old concrete reduces the concrete’s carbon footprint—also, the concrete density changes due to the aggregate type. The concrete density ranges from 1420 to 1840 kg/m3.
Porous concrete made of different types of aggregates
In addition to Portland cement, Geopolymers can be utilized as cementitious ingredients in the production of porous concrete. The qualities of pervious geopolymers are comparable to those of permeable concrete made of Portland cement.
Strength Of Pervious Concrete
The pervious concrete has voids that render lesser strength than the traditional concrete. However, the porous concrete attains enough strength to cater to the driveway and parking loads.
The use of different aggregate materials also affects the concrete strength. For the crushed limestone pervious geopolymer concrete (LS), the compressive strength is in the range of 11.9–
13.6 MPa. Recycled concrete (RCA), RBC, and RCB pervious geopolymer concrete yielded lower compressive strengths in 7.0–10.3, 2.8–3.8, and 2.9–6.6 MPa, respectively.
Pervious concrete cylinders specimen for compression testing
It is evident from the above data that the porous concrete is strong enough to support cars. However, with special mix designs, we can achieve a strength of up to 20 MPa. The concrete strength can be further increased by adding a subgrade and sub-base layer of coarse and fine aggregate.
The durability of porous concrete is also sufficient. It can stay in place for up to 20-40 years. The key to durability lies in the concrete mix and concrete laying technique.
The lower water-cement ratio results in lesser shrinkage consequently, delivering a longer life.
Cost Of Pervious Concrete
Even though the installation cost of pervious concrete is often higher than that of regular concrete, it can be a significantly more cost-effective solution over its lifetime.
The higher installation cost is due to the additional concrete. It is laid as a 6-inch thick layer compared to 4- inch thick traditional concrete.
Because pervious concrete reduces the need for additional drainage systems, it can minimize the overall costs of residential and commercial projects.
This concrete allows rainwater to drain directly to the underlying soil, reducing stormwater control requirements.
In addition, compared to other paving solutions, such as asphalt, porous concrete requires lesser repair and upkeep.
As seen in the below chart, the cost of permeable concrete per square foot is comparable to that of porous asphalt. At the same time, the cost of permeable pavers is way higher.
Cost Comparison of Permeable paver versus pervious concrete and porous asphalt in USD per square foot.
Application
Road pavement,
Parking areas,
Areas with light traffic,
Residential streets,
Pedestrian walkways,
External Walls,
Greenhouses.
Conclusion
Keeping the benefits of pervious concrete in mind, it is a viable product for urban areas. The porous concrete solves multiple problems at once and strengthens the vision of sustainable development.
The strength, durability, and lifetime cost are comparable to the conventional paving systems. Therefore, porous concrete is a good option. The US Environmental Protection Agency (EPA) has included permeable pavement in the industry best practices list.
However, the use of permeable concrete in India is still scarce. The main reason for hesitation in using permeable concrete is the unavailability of expertise.
Another reason is the limitation of study material in the Indian context. Once the difficulties are overcome, the use shall increase as the cost is less than the current prevailing permeable paving system.
Also Read
Types Of Concrete Used In Construction
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:: Pervious Pavement :: Pervious Concrete for Green, Sustainable Porous and Permeable Stormwater Drainage ::
Engineering Properties
Fresh Properties
The plastic pervious concrete mixture is stiff compared to traditional concrete. Slumps, when measured, are generally less than ¾ inches (20 mm), although slumps as high as 2 inches (50 mm) have been used. When placed and compacted, the aggregates are tightly adhered to one another and exhibit the characteristic open matrix.
For quality control and quality assurance, unit weight or bulk density is the preferred measurement because some fresh concrete properties, such as slump, are not meaningful for pervious concrete. Conventional cast-cylinder strength tests are also of little value, because the field consolidation of pervious concrete is difficult to reproduce in cylindrical test specimens, and strengths are heavily dependent on the void content. Unit weights of pervious concrete mixtures are approximately 70% of traditional concrete mixtures.
Concrete working time is typically reduced for pervious concrete mixtures. Usually, one hour between mixing and placing is all that is recommended. However, this can be controlled using retarders and hydration stabilizers that extend the working time by as much as 1.5 hours, depending on the dosage.
Hardened Properties
Density and Porosity
Subgrade
Figure 3. Typical cross section of pervious concrete pavement. On level subgrades, stormwater storage is provided in the pervious concrete surface layer (15% to 25% voids), the subbase (20% to 40% voids), and above the surface to the height of the curb (100% voids).
The density of pervious concrete depends on the properties and proportions of the materials used, and on the compaction procedures used in placement. In-place densities on the order of 100 lb/ft³ to 125 lb/ft³ (1600 kg/m³ to 2000 kg/m³) are common, which is in the upper range of lightweight concretes. A pavement 5 inches (125 mm) thick with 20% voids will be able to store 1 inch (25 mm) of a sustained rainstorm in its voids, which covers the vast majority of rainfall events in the U.S. When placed on a 6-inch (150-mm) thick layer of open-graded gravel or crushed rock subbase, the storage capacity increases to as much as 3 inches (75 mm) of precipitation (see Figure 3 and Hydrological Design Considerations).
Permeability
The flow rate through pervious concrete depends on the materials and placing operations. Typical flow rates for water through pervious concrete are 3gal/ft²/min (288 in./hr, 120 L/m²/min, or 0.2 cm/s) to 8 gal/ft²/min (770 in./hr, 320 L/m²/min, or 0.54 cm/s), with rates of up to 17 gal/ft²/min (1650 in./hr, 700 L/m²/min, 1.2 cm/s). Even higher rates have been measured in the laboratory.
Compressive Strength
In the laboratory, pervious concrete mixtures have been found to develop compressive strengths in the range of 500 to 4000 psi (3.5 MPa to 28 MPa), which is suitable for a wide range of applications. Typical values are about 2500 psi (17 MPa). As with any concrete, the properties and combinations of specific materials, as well as placement techniques and environmental conditions, will dictate the actual in-place strength. However, currently there is no ASTM test standard for compressive strength of pervious concrete. Testing variability measured with various draft test methods has been found to be high and therefore compressive strength is not recommended as an acceptance criterion. Rather, it is recommended that a target void content (between 15% to 25%) as measured by ASTM C 1688: Standard Test Method for Density and Void Content of Freshly Mixed Pervious Concrete, be specified for quality assurance and acceptance. As stated in ACI 522.1-13: Specification for Pervious Concrete Pavement, the tolerance for the specified void content is +/-5%.
Flexural Strength
Flexural strength in pervious concretes generally ranges between about 150 psi (1 MPa) and 550 psi (3.8 MPa). Many factors influence the flexural strength, particularly degree of compaction, porosity, and the aggregate-to-cement (A/C) ratio. However, the typical application constructed with pervious concrete does not require the measurement of flexural strength for design.
Shrinkage
Drying shrinkage of pervious concrete develops sooner, but is much less than conventional concrete. Specific values will depend on the mixtures and materials used, but values on the order of .002 have been reported, roughly half that of conventional concrete mixtures. The material’s low paste and mortar content is a possible explanation. Roughly 50% to 80% of shrinkage occurs in the first 10 days, compared to 20% to 30% in the same period for conventional concrete. Because of this lower shrinkage and the surface texture, many pervious concretes are made without control joints and allowed to crack randomly.
Durability
Freeze-Thaw Resistance
Freeze-thaw resistance of pervious concrete in the field appears to depend on the saturation level of the voids in the concrete at the time of freezing. In the field, it appears that the rapid draining characteristics of pervious concrete prevent saturation from occurring. Anecdotal evidence also suggests that snow-covered pervious concrete clears quicker, possibly because its voids allow the snow to thaw more quickly than it would on conventional pavements. In fact, several pervious concrete placements in North Carolina and Tennessee have been in service for over 10 years.
Note that the porosity of pervious concrete from the large voids is distinctly different from the microscopic air voids that provide protection to the paste in conventional concrete in a freeze-thaw environment. When the large open voids are saturated, complete freezing can cause severe damage in only a few cycles. Standardized testing by ASTM C 666 may not represent field conditions fairly, as the large open voids are kept saturated in the test, and because the rate of freezing and thawing is rapid. It has been shown that even after 80 cycles of slow freezing and thawing (one cycle/day), pervious concrete mixtures maintain more than 95% of their relative dynamic modulus, while the same mixtures showed less than 50% when tested at a more rapid rate (five to six cycles/day). It was noted that better performance also could be expected in the field because of the rapid draining characteristics of pervious concrete.
Research indicates that entrained air in the paste dramatically improves freeze-thaw protection for pervious concrete. In addition to the use of air-entraining agents in the cement paste, placing the pervious concrete on a minimum of 6 inches (150 mm), and often up to 12 (300 mm) or even 18 inches (450 mm) of a drainable rock base, such as 1-inch (25-mm) crushed stone, is normally recommended in freeze-thaw environments where any substantial moisture will be encountered during freezing conditions.
Sulfate Resistance
Aggressive chemicals in soils or water, such as acids and sulfates, are a concern to conventional concrete and pervious concrete alike, and the mechanisms for attack are similar. However, the open structure of pervious concrete may make it more susceptible to attack over a larger area. Pervious concretes can be used in areas of high-sulfate soils and groundwaters if isolated from them. Placing the pervious concrete over a 6-inch (150-mm) layer of 1-inch (25-mm) maximum top size aggregate provides a pavement base, stormwater storage, and isolation for the pervious concrete. Unless these precautions are taken in aggressive environments, recommendations from ACI 201 on water-to-cement ratio and material types/proportions should be followed strictly.
Abrasion Resistance
Because of the rougher surface texture and open structure of pervious concrete, abrasion and raveling of aggregate particles can be a problem, particularly where snowplows are used to clear pavements. This is one reason why applications such as highways are generally not suitable for pervious concretes. However, anecdotal evidence indicates that pervious concrete pavements allow snow to melt faster, requiring less plowing.
Most pervious concrete pavements will have a few loose aggregates on the surface in the early weeks after opening to traffic. These rocks were loosely bound to the surface initially, and popped out because of traffic loading. After the first few weeks, the rate of surface raveling is reduced considerably and the pavement surface becomes much more stable. Proper compaction and curing techniques reduce the occurrence of surface raveling.
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Pervious Concrete
Installation Process
The pervious concrete installation process consists of four basic steps:
Mixing
Placing
Compaction
Curing
The creation, placement, and curing of concrete are all done on-site, rather than in a factory under uniform conditions. Although pervious concrete can be mixed by the same suppliers and delivered by the same trucks as dense concrete, its unique physical characteristics require a contractor with specialized experience. The structural dissimilarities between pervious and impervious concrete call for different installation processes, neither of which is more difficult or time-consuming than the other. However, the quality and performance of pervious concrete are dependent upon the installer’s familiarity with the constructional impact of its properties. This can be gained only through hands-on experience with the substance, which is not a commonly-used building material in all locales.
To ensure quality control , NRMCA has established a certification program for contractors who place pervious concrete pavement. The contractor certification includes a written exam and components of performance and work experience. This project is funded by the Ready Mixed Concrete Research Foundation and is implemented in cooperation with state ready mixed concrete associations.
Mixing
Pervious concrete is made up of cement, course aggregates, and water. The mixing process requires tight control of ingredient proportions, more so than that of conventional concrete. Only a narrow range of water content provides sufficient levels of both strength and porosity, so measurements must be exact.
Pervious concrete generally has a void content of 15-25%, with a .35-.45 water to cementitious materials (w/cm) ratio. Since this small proportion of water evaporates quickly, the mixture should be discharged completely within one hour of its initial mixing. This time frame can be extended by 50% or more with the inclusion of moisture-sustaining admixture chemicals in the mix.
Placing
A stone reservoir is installed as a subbase for the pavement, creating large air voids and allowing for further water infiltration and storage. Prior to concrete placement, this layer is compacted to 92-96% of its maximum density, so as to provide maximum support while maintaining permeability. The level of compaction depends on the soil properties, pavement design, and infiltration requirements.
Soil with high clay content has low permeability, requiring a thicker stone reservoir to compensate for decreased water retention. Engineering fabrics are often used to separate the soil and stone layers. The subbase must be kept moist in order to prevent the lower portion of the pavement from drying out too quickly. The longer the drying time, the greater the strength and durability of the concrete.
Because of its thick, rocky consistency, discharge from concrete mixers is slower for pervious concrete than for dense. Since it cannot be pumped, it must flow, usually requiring worker assistance. The pouring of the mixture into the framework should be rapid and continuous. Once it’s in place, a vibrating screed is used to level off the fresh pavement, resulting in a smooth final surface. The vibrations increase the compaction, and therefore density, of the concrete.
Vibrating screeds also allow for the use of extremely thick, low-slump mixtures, which produce maximum strength and durability. Manual screeding methods lack the force necessary to smooth over such thickness.
Compaction
The final compaction or consolidation of the poured concrete mixture is accomplished with a steel-pipe roller immediately after screeding. This step must be completed within fifteen minutes of placement in order to avoid the hardening effects of evaporation. In the placement of conventional concrete, troweling operations are performed after compaction, resulting in a smooth surface finish. However, since this step tends to close off the uppermost voids of porous concrete, it is skipped in pervious installation. This sacrifice leaves the pavement fully permeable, and the rougher surface finish actually improves traction.
Although pavement-cracking does occur, it is not abundant, since pervious concrete contains minimal amounts of water. When cracks do arise, they do not significantly compromise the pavement, either structurally or aesthetically. However, if the prevention of random pavement-cracking is desirable, control joints can be placed soon after consolidation as a remedy. These can be formed with a rolling joint tool, or less ideally, with sawing equipment at a later time. Since pervious concretes shrink much less than impervious ones while drying, their individual joints tend to be larger in width. Control joints at ¼ the thickness of the slab generally provide good control of cracking, and are installed to line up with the joints of adjoining pavements at recommended intervals of twenty feet.
Curing
The final installation step is the protection of the pavement by curing. This is essential because the open pore structure and faceted surface of pervious concrete create a high potential for evaporation. Moisture is vital because it acts as the catalyst for the cement’s chemical reactions, which produce the concrete’s strength. The purpose of the curing procedure is to allow the concrete to gradually harden over an extended period of time, building up its maximum strength capacity through steady, controlled evaporation. Curing should begin as soon as possible after the placement, compaction, and jointing of the pavement. The process involves first fog-misting the concrete with a curing compound, and then covering it with plastic sheeting for a minimum of seven days. This maintains the pavement’s moisture, allowing it to induce hardening for an extended period of time.
Once the curing process is complete, pervious concrete can function well with minimal maintenance. Due to its considerable strength and durability, it requires fewer repairs than most other building materials, such as asphalt. In order to prevent the void structure from becoming clogged with debris, it may be necessary to regularly vacuum or pressure-wash the pavement. The latter method has been shown to restore as much as 80-90% of the concrete’s original permeability.
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