The Constructor

Pervious Concrete: Futuristic Solution to Urban Runoff?

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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 streets, pedestrian walkways, and greenhouses.

Fig: A pervious concrete street in Rosemary Beach, Florida. This concrete was 10 year at the time of photograph.

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 compaction 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 recompacted. Typically pervious concrete has ‘water to cementitious materials’ (w/cm) ratio of 0.25 to 0.35 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 segregation, and too little water will lead to balling in the mixer and very slow mixer unloading. Whereas some loose stones can always be expected on a pervious concrete surface, too low a water content 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. The most 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 vibratory or manual screeds, though vibratory screens are preferable. After striking off the concrete, the riser strips are removed and the concrete compacted by a manually operated roller that bridges the forms. Rolling consolidates the fresh concrete to provide strong bond between the paste and aggregate, and creates a smoother riding surface. Excessive pressure when rolling should be avoided as it may cause the voids to collapse. Rolling should be performed immediately after strike off.

Fig: A view of a cube that shows how pervious concrete works.

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 tooled with a rolling jointing tool. This allows joints to be cut in a short time, and allows curing to continue uninterrupted. 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. Plastic sheeting is typically used to cure pervious concrete pavements. Immediate Application in India: In India, the issue of urbanization of cities left many metro cities in floods. For example, floods faced in Mumbai are perfect example of how much pervious concrete can be utilized. In Mumbai, due to additional growth of areas, one of the areas where a river which used to take water out of city and pour it in sea was blocked. After this, the outcome was loud and clear. Mumbai faced its worst floods ever which not only handicapped the city but also killed many. As the scope of urban cities will increase, pervious concrete can provide a solution to this rapid growth of cities. Disadvantage: 1. Many pavement engineers and contractors lack expertise with this technology. 2. Porous pavement has a tendency to become clogged if improperly installed or maintained. 3. Porous pavement has a high rate of failure. 4. There is some risk of contaminating groundwater, depending on soil conditions and aquifer susceptibility. 5. Some building codes may not allow for its installation. 6. Anaerobic conditions may develop in underlying soils if the soils are unable to dry out between storm events. This may impede microbiological decomposition.
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