INTRODUCTION TO PERVIOUS CONCRETE
Pervious concrete, also known as permeable concrete or porous concrete, is a special type of concrete with a high porosity that allows water to drain through it. Unlike regular concrete, which is used mainly for structural applications, pervious concrete functions as a drainage material as well as a paving material. Its pore structure enables rainwater and melted snow to pass through it and drain away to the sub-base or directly into the ground below.
Pervious concrete has a considerable amount of air voids between the coarse aggregate that give it a high void ratio and porosity. The void content can range from 15-35%, compared to regular concrete which has only 5% or less voids. This high void ratio allows water to rapidly pass through the concrete into the subgrade, resulting in little or no runoff. By allowing rainwater to percolate directly through its surface instead of running off, pervious concrete helps reduce stormwater runoff and flooding while recharging groundwater.
Pervious concrete is most commonly used in low-traffic urban areas such as sidewalks, driveways, patios, fire lanes, pedestrian walkways, and even parking lots. It provides an attractive alternative to impermeable surfaces such as asphalt for such areas as it mimics the natural movement of water while also delivering structural support. Some of the key benefits of using pervious concrete include reduced stormwater management costs, groundwater recharge, filtration of pollutants, and reduced heat island effect compared to conventional pavements.
COMPOSITION AND MIX DESIGN
The composition of pervious concrete is similar to traditional concrete but it contains little or no fine aggregates (sand) which creates more voids. A typical mix design involves the following ingredients:
Coarse aggregates: Angular crushed stone between 9.5 mm to 12.5 mm size with less than 5% passing a 4.75 mm sieve. Larger aggregates create more void spacing.
Cement: Type I or blended hydraulic cements are typically used. The cement content is lower compared to conventional concrete.
Water: Just enough water is added to coat the aggregates without filling the voids. The water-cement ratio ranges from 0.27 to 0.32.
Admixtures (optional): Water reducers and viscosity modifiers may be used to improve workability and strength.
For a standard mix design, the proportions are:
Aggregates: 1750-2000 kg/m3
Cement: 100-150 kg/m3
Water: 45-60 kg/m3
Void ratio: 15-35%
Properly blending the ingredients to achieve adequate strength while maintaining high porosity is crucial. The fresh mix needs to be workable enough to be consolidated and leveled without segregation of materials.
CONSTRUCTION AND INSTALLATION
Pervious concrete is placed and compacted in much the same way as conventional concrete but requires some different techniques to preserve the void structure:
Site preparation: The subgrade needs to be stable and compacted to support traffic loads. It should have an infiltration rate of at least 10 inches/hour. Gravel storage layers below further enhance drainage.
Formwork: Slipform pavers or fixed forms made of metal or wood are used. Forms need to be level and tightly joined.
Placement: The concrete is discharged directly from the truck through a chute or conveyor belt and spread evenly using rakes, shovels or screeds.
Compaction: Vibratory rollers or plate compactors are used to consolidate the mix without closing the voids. Over-working leads to loss of porosity.
Initial curing: Fine misting of water or protective covers help retain moisture for proper hydration and strength gain without surface scarring.
Jointing: Saw-cutting contraction joints within 12-24 hours favorably influence crack development.
Edge restraints: Concrete borders, steel edging or asphalt prevents spalling along edges due to traffic and weather.
Maintenance: Periodic vacuuming removes sediment to ensure long-term infiltration.
Proper construction is crucial to achieve adequate strength and minimize maintenance needs of pervious concrete. Its successful use relies on attention to mix proportioning, placement techniques and timely curing practices.
PROPERTIES AND PERFORMANCE
Compared to conventional concrete, the engineering properties of pervious concrete are generally lower on account of the high void content. It provides satisfactory performance for many non-structural applications. Some typical properties include:
Compressive strength: Varies from 4-28 MPa depending on mix design and curing. Adequate for light vehicular traffic.
Flexural strength: 0.5-4 MPa
Permeability: Over 1000 mm/hr infiltration rate permits rapid drainage.
Freeze-thaw resistance: Resistance depends on air void structure and adequate curing. Can withstand 50-150 cycles.
Abrasion resistance: Wear surface withstands light traffic reasonably but resists impacts less than conventional pavements. Periodic vacuum cleaning maintains permeability.
Skid resistance: Coefficient of friction ranges from 0.30 to 0.50 based on aggregate used.
-Durability: Service life of 15-25 years reported with proper mixture, installation and maintenance. Failures usually due to clogging of voids leading to flooding or structural issues involving subgrade failure.
Well-designed and constructed pervious concrete provides comparable if not better performance than conventional pavements for low-traffic applications, at a lower overall cost and environmental impact when stormwater and flooding costs are considered. Ongoing research further improves mixture designs and promotes the use of pervious pavements.
CONCLUSION
Pervious concrete is an innovative alternative to traditional impervious concrete paving that provides both structural support and hydraulic functionality to capture and treats stormwater runoff. By allowing rainwater to pass directly through its surface into the underlying layers, pervious concrete mitigates flooding, replenishes groundwater supplies, filters pollutants and reduces the urban heat-island effect compared to conventional pavements. With proper mix proportions, construction techniques and maintenance practices, it delivers satisfactory engineering properties for pedestrian and light vehicular traffic applications. Its benefits have led to the increased adoption of pervious concrete for sustainable stormwater management and transportation infrastructure development. Ongoing research efforts aim to address limitations and further propagate its usage globally.
Andrew Stroud