Engineered to Leak

Nancy Anderson
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Weather extremes across the United States this month grabbed the attention of Engineers and architects everywhere. Damage from tornadoes, hail, lightning, and floods will undergo investigation everywhere this disastrous combination of natural events has exerted itself. While some property damage may be preventable by employing the new structural codes and construction techniques, nothing appears to survive in a direct hit from a large tornado.
Back down on the ground our Civil Engineers and concrete mix designers have found some hope for us in helping to control the damage from heavy rains that cause localized flooding. Most of us are familiar with the concept that much of the flooding in urban areas is due to the amount of structure and “impervious paving” that covers the ground surface. When man’s urban sprawl replaces the natural vegetation and bare earth, where is the water to go? There are no natural channels of absorption, run-off, or uptake. In other words, the rain just cannot “soak in.”

The “hope” I mentioned is a paving material that is “Engineered to leak.” Engineers and architects are beginning to view “pervious concrete” as the preferred method of managing stormwater. The ability to manage stormwater on confined commercial sites without retention or detention facilities gives developers an advantage. Residential developers are also beginning to find ways to use pervious concrete to make their projects greener while reducing costs. Rather than pay for specialized infrastructure to move stormwater to retention facilities, these developers can allow nature to replenish the water table directly. The reduction or even elimination of retention facilities allows these developers to lower costs while providing extra room for green space.

Pervious concrete is a mixture of coarse aggregate, cement, and water. It contains little or no sand. The cement and water create a thick paste binding the aggregate particles together, but with many voids and spaces between them. This creates a system of highly permeable, connected voids, usually 15%-25% of the structure that drains very quickly. Pervious concrete allows 3 to 8 gallons of water to pass through each square foot of material per minute, although the formulation can be adjusted to double that amount if needed.

The strength of pervious concrete is limited due to the high porosity, but it has sufficient strength for many applications such as hardscaping, low-volume pavements, alleys and driveways, low-water crossings, parking lots, sidewalks and pathways, patios, etc. Pervious concrete also has the capability to intentionally aid the recharging of groundwater and reduce stormwater runoff. By doing so, it reduces the need for retention ponds, swales and other stormwater management techniques.

Acceptance of pervious concrete has been widespread. The EPA now recommends pervious concrete as a Best Management Practice for the management of stormwater runoff on a regional and local basis. In addition, the National Ready Mixed Concrete Association (NRMCA) now has a National-level program in place to certify contractors in the installation of pervious concrete.
Still keep your ears open for those tornado sirens, but if you are planning the pavement of any area on your current projects, consider this pervious application. Design that parking lot to leak!

You can do this!

By K.B. Elliott

K. B. Elliott is a freelance writer for Working many related positions in the Detroit area for over 30 years gives him a unique perspective on the process. To read more of his blogs, please go to, and be sure to check out the postings for jobs in nearly any industry at Nexxt

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