What Is Pervious Concrete?
The pervious concrete is a special type of concrete with high porosity which allows water to transmit through it. it is also a type of lightweight concrete as the weight is noticeably reduced by eliminating fine sand.
It is mostly used for laying porous pavements that allow the runoff water from precipitation or other sources to pass through it and thus reducing runoff from the site and aiding in groundwater recharge too.
Read More: What is Lightweight Concrete, Foam Concrete
History of Previous Concrete:
The pervious concrete found its first usage in Europe for the surfacing of pavement and load-bearing walls. This happened in the 1800s.
As the amount of cement used was less as compared to the conventional concrete, it gave cost efficiency. This was the main factor for the introduction of pervious concrete.
After a while, in the 1920s, it regained popularity for 2-story buildings in Scotland and England. After World War II, pervious concrete developed in full fledges owing to the scarcity of cement.
During the 1970s, it became widely accepted in the United States too. While in India, it was introduced much later in the 2000s.
Other Names Concrete
No fines Concrete
Pervious concrete is made up of cement, coarse aggregates, and water. Concrete paste binds the large aggregates together forming large voids which allow the passage of water.
- No fines concrete
- Porous concrete
- Permeable concrete
Typically, fine aggregates are omitted or very little amount of sand is added to enhance its strength. Hence, it is also called no fines concrete.
If necessary, chemical admixtures are added to the pervious concrete.
Reason for Porosity:
The reason for the porosity of pervious concrete is the presence of a large number of interconnected voids. These voids facilitate the passage of water within the concrete.
This concrete sets rapidly, hence the usage of retarders is quite common. Air entraining admixtures are even used to reduce the damage caused by freezing and thawing.
The usage of admixtures should be monitored as per the instructions given by manufacturers.
This concrete does not produce thrust to the sides of the formwork because the concrete does not flow. Hence, the formwork from the sides can be removed earlier as compared to conventional concrete. The time of formwork is prolonged for structural members, as the strength of pervious concrete is low.
Curing is important for the strength-gaining process of concrete. Sufficient hydration is ensured by curing. The pervious concrete is mostly cured with the help of plastic sheeting.
Advantages of Pervious Concrete
- To manage the stormwater runoff effectively.
- To reduce the size and number of storm sewers.
- To recharge the groundwater.
- To prevent the pollution of groundwater by aiding in the filtration of water before infiltration.
- To eliminate the harm to sensitive ecosystems.
- To reduce the heat island effect.
- To enhance land use by omitting the need for retention ponds.
- To eliminate the surface ponding.
- To induce the heat-insulating properties in the walls of a building.
- To enable easy plastering owing to its rough surface texture.
- To eliminate the hydrocarbon pollution coming from asphalt pavement surfacing.
- To reduce the cost of construction.
Pervious Concrete Problems
- Cannot be used for the pavements where heavy traffic flow is expected.
- Requires specialized construction practice.
- Quite sensitive to the water content.
- Difficult to determine the water content in fresh concrete.
- Tests like slump test or compaction factor test are not applicable.
- Requires much longer curing time.
- Requires special consideration during the design in the soils susceptible to frost and expansive soils.
- Periodic cleaning of pervious concrete pavement is required to maintain its permeability.
Properties of Pervious Concrete
Pervious concrete mainly consists of cement, coarse aggregates, and water. Sometimes admixtures are added too.
The size of the coarse aggregates should range from 10 mm to 20 mm. Fine aggregates can be added in a meager amount to increase strength or avoided.
Aggregate cement ratio for concrete is generally kept 6:1 to 10:1.
Aggregates having sharp edges are also avoided as local crushing may take place when loaded.
2. Water-Cement Ratio
The water-cement ratio of concrete is generally kept between 0.28-0.40. The right quantity of water is crucial in pervious concrete.
Adding more water will affect its strength but very low water may cause surface failure, as aggregates will not be properly smeared by cement paste.
A higher water-cement ratio will cause cement paste to flow to the bottom during compaction and the voids at the bottom will be filled up.
If the water-cement ratio is proper, then the mixture has a wet metallic appearance.
The mixture is checked in the field, as pervious concrete is sensitive to the water content.
To check the consistency of the mix, no standard method like slump test or compaction factor test is applicable for pervious concrete. Visual examination by an experienced person and trial and error methods are used to determine the water-cement ratio for concrete.
The density of pervious concrete mainly depends upon the grading of aggregate.
For the aggregates with same specific gravity, the density of (one-size aggregate) concrete is about 10 times less as compared to that of well-graded aggregate.
By using low-weight aggregates, concrete of density about 640 kg/m3 can also be obtained. While the concrete with normal weight aggregate forms the concrete with a density of 1600-2000 kg/m3.
4. Flow Rate
The permeability or flow rate of concrete depends on the type of materials used in its mixing and placing operations.
Flow rate for water through pervious concrete is generally 288 in/hr to 770 in/hr.
5. Void Ratio
Because of the omission of fine aggregates, the void ratio is increased. It is about 15-25 % in pervious concrete.
Large voids may also lead to entrapped air. The quantity of entrapped air may be checked by the Rapid Air System. In this test, the concrete is stained black and then the section is observed under a microscope.
Compaction of concrete should be done to increase the compressive strength.
However, over compaction should be avoided as it may reduce the permeability even though strength is increased. So, vibration is applied for a very short time for pervious concrete.
Higher compaction is not recommended as it can result in the high local density of the concrete. Mechanical compaction is also not required.
Simple rodding may suffice the compaction needs of the pervious concrete.
7. Compressive Strength
The compressive strength of this concrete is quite low as compared to normal concrete. However, it is sufficient for use in buildings up to certain stories and other applications.
The compressive strength of pervious concrete varies between 14 N/mm2 to 140 N/mm2.
Normally, this type of concrete is not reinforced. But, if the need arises, then the reinforcement is given a coating of cement paste of about 3 mm thick. This improves bond strength and prevents corrosion too. The coating is primarily applied via shotcreting.
8. Flexural Strength
The flexural strength of pervious concrete is in between 10 N/mm2 to 38 N/mm2.
Factors affecting flexural strength include
- Degree of compaction
- A/C ratio
The flexural strength is not of much significance in the applications of pervious concrete.
it does not exhibit segregation. Hence, it can be dropped from a considerable height.
The value of drying shrinkage of pervious concrete is quite low as compared to conventional concrete. This is because of the large volume of aggregates as compared to cement paste, which restrains contraction.
The value of drying shrinkage of pervious concrete made with river gravel reduces by half as compared to the traditional concrete. It is of the order 120 * 10-6 to 200 * 10-6.
The low value of drying shrinkage is because of Presence of a very thin layer of cement paste between aggregates Point to point contact between aggregates
Though the value is low, the rate of drying shrinkage is much more as compared to conventional concrete. This is because a large amount of cement is exposed to air initially.
In 10 days, the drying shrinkage of conventional concrete is about 20-30 % while that of pervious concrete is about 50-80 %.
The drying shrinkage in this concrete is totally completed in about a month.
11. Thermal Conductivity
Thermal conductivity of the pervious concrete is much less as compared to the conventional concrete.
Thermal conductivity depends largely on the type of aggregates used in the concrete.
12. Sulphate Resistance
Aggressive chemicals like acids and sulphates if present in either soil or water prove detrimental to the concrete- whether it is conventional concrete or pervious concrete. Even their attack mechanisms are similar.
The open structure of this concrete makes it more susceptible to the damage by sulphates.
13. Abrasion Resistance
Pervious concrete has a rough surface texture and open structure. Hence, the abrasion resistance of pervious concrete is not much.
This is a major reason as to why pervious concrete is not suitable for highways.
14. Frost Resistance
Frost resistance of pervious concrete is quite good because of the absence of capillary.
However, it decreases if the pores are fully saturated. Also, the rapid disintegration of concrete takes place in such a case.
Application of Pervious Concrete
1. Stormwater Management
It helps in regulating the runoff and is considered the best technique for stormwater management. The primary use of pervious concrete is in pavement construction.
This concrete is even recognized by the U.S. Environmental Protection Agency- EPA as a technique that can give the best first flush pollution control and stormwater management.
Drainage systems take up huge spaces and are costly too. Pervious concrete retains the water at the site, therefore, reducing the size of the storm sewers. The water is allowed to infiltrate into the soil and thus, the amount of runoff is decreased.
This concrete acts as an infiltration basin and allows the water to infiltrate, groundwater recharge is enhanced too. The depleting levels of groundwater can thus be controlled.
Another advantage of pervious concrete pavement for stormwater management is that it filters the stormwater prior to its infiltration. Thus, the pollutant load is prevented from entering the groundwater.
This concrete also allows the passage of air along with water. This aids in the development of trees, as they require a sufficient amount of air too.
Laying a strip of pervious concrete alongside the pavement can thus be helpful.
2. External Walls
Its usage as external load-bearing walls in single-storey, as well as multi-storey buildings, is complemented by its insulating property. As its coefficient of thermal expansion is relatively lower, it offers heat insulation.
Also, plastering can be easily done on the walls of pervious concrete owing to its rough texture.
Low capillary action in pervious concrete resists the dampness inside the building even though the outer walls get wet.
It is also suitable at places where fine sand is not easily available.
3. Sustainable Construction
As the colour of the pervious concrete is light, it also aids in energy savings. The concrete reflects the light and thus need for lighting at night is reduced.
As per U.S. EPA, about 90 % of surface pollutants are carried by the runoff of the first 1.5 inches of rainfall.
Traditionally designed stormwater drains do not divert this water to treatment plants but into local water bodies. As a result, algal growth is enhanced in the water body and aquatic life may be harmed too.
Its treatment for making the water potable again is expensive.
There are three major sources of pollution recognised widely.
One pollutant is sediment, which is carried away in the runoff. Dirt and debris form the constituents for sediment.
The other pollutant is heavy metals. They are introduced to the runoff from the brake linings of cars.
The third pollutant is formed by the hydrocarbons. One of the sources of hydrocarbon is the oil dripping from the vehicles onto the pavement. However, its major source is asphalt. In the urban runoff, 90-95 % of the hydrocarbon comes from the binder and sealant used in asphalt pavements as per the studies.
These form serious environmental issues, which are addressed by EPA, regional watershed authorities and local authorities by regulating stormwater management more strictly.
Pervious concrete is thus becoming a popular solution to this concern. The reasons are listed below:
- It reduces the amount of untreated runoff water from reaching the storm sewers as it enhances infiltration at the site.
- Aquifer levels are refilled too as groundwater recharge is facilitated directly.
- More water is channeled towards the trees and landscaping, this reduces the need for irrigation water.
- Pollutants are prevented from contaminating the watersheds and thus the sensitive ecosystems are not harmed.
- Hydrocarbon pollution coming from asphalt pavements is also eliminated.
4. Parking Areas
Pervious types concrete has found its application especially in pavements of the parking areas and parks.
As the pervious concrete absorbs the water, the stagnant water puddles are reduced. This eases the walkway in parks and ensures safety for drivers on the pavement.
5. Residential Street and Light Traffic Areas
The urban heat-island effect is reduced by the usage of pervious type concrete.
This is owing to the fact that the concrete is light in colour and has an open-cell structure. Hence, less heat is absorbed by pervious concrete and thus less heat is radiated into the environment.
The open void structure of the pervious concrete also helps in keeping the surface cool owing to the cooler temperature of the soil below.
Read More: Self Compacting Concrete Test & Result
Maintenance of Pervious Concrete
Maintaining the permeability of pervious concrete is essential for it to function well. For this, regular cleaning should be done.
The surface of the concrete is wet and then vacuum sweeping is done for cleaning the surface.
Pervious Concrete Costs
The Cost of Pervious Concrete is reduced considerably as sand is eliminated.
While preparing lean mixes, the cement used is as little as 70-130 kg/m3 of concrete.
With the reduced use of cement, the cost is lowered significantly.
Pervious Concrete Costs is around 2 to 7 dollars per Square feet in USA.
Pervious concrete is a special type of lightweight concrete in which fine sand is eliminated. Only one size of coarse aggregates is mostly used which creates large voids, thus increasing permeability.
Pervious concrete is sensitive to water content. Light compaction is sufficient for this concrete, over compaction may reduce the permeability.
The compressive strength of pervious concrete is less as compared to conventional concrete. Normally, reinforcement is not provided.
The drying shrinkage is less and faster in pervious concrete. Thermal conductivity is low for pervious concrete but its sulphate, as well as abrasion resistance, are poor.
Pervious concrete is mainly used for stormwater management and pavement for lighter traffic. It also helps in sustainable construction.
Maintenance of pervious concrete is easy too as vacuum sweeping after wetting the surface may suffice.
Thus, pervious concrete has found a very significant place in stormwater management along with environmental considerations.
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