Friday, January 16, 2009

Eco-Friendly Concrete - Is There Such a Thing?

Concrete being poured, raked and vibrated into...Image via Wikipedia

New Eco-Friendly Concrete Leaks - In a Good Way by Mary K. Smith

Paved surfaces are so common that not many of us consider their effect on the environment and water quality. As more and more land is paved, more rain lands on pavement rather than soaking into the soil. Standard paving techniques create "impervious" surfaces, that is, surfaces that cannot be penetrated by water.

As impervious surfaces increase, erosion, flash floods, depletion of the water table and pollution also increase. The simple solution would be to stop installing impervious surfaces. But it is not that easy. We depend on roads, highways, sidewalks, tennis courts, etc.

What if a material existed that was as durable as pavement but would capture rain water and allow it to percolate through to the soil below? Such a material does exist. It's known as pervious concrete, or porous or permeable concrete.

Pervious concrete is very similar to standard concrete. It contains a mixture of coarse aggregate, cement and water, but it contains little or no sand. A thick paste is made from the cement and water. It binds the aggregate solids together, but with lots of voids and spaces between them. The end result is a system of very permeable, connected spaces, usually 15%-25% of the structure that drains very quickly. Pervious concrete may allow 3 to 8 gallons of water per minute to run through each square foot of material. The formulation can be changed however, to double that percolation rate if necessary.

This high porosity limits the strength of pervious concrete, but it is sufficiently strong to allow for many applications such as hardscaping, low-volume pavements, alleys and driveways, low-water crossings, parking lots, sidewalks and pathways, patios, etc.

Pervious concrete can be used to help recharge groundwater and reduce stormwater runoff. Pervious concrete can reduce the need for swales, retention ponds, and other techniques to manage stormwater.

One good example of permeable concrete comes from Chicago. Chicago contains 1,900 miles of public alleys, the equivalent of 3,500 acres. Few have drainage structures or a even connection to the sewer system. Localized flooding became a problem after years of neglect. To resolve the problem, the city created the Chicago Green Alley Program to promote best management practices in stormwater management. The goal of the program was to solve difficult drainage issues without costly sewer infrastructure improvements.

To reach this goal, the program used green building techniques such as recycled materials, reflective pavements, energy-efficient lighting and permeable paving. The result was to reduce by 80% the amount of runoff entering the stormwater sewer system. The impact reduced localized flooding, and reduced the urban heat island effect.

Pervious concrete, light colored and made of recycled materials, was chosen for the Green alley Program for it's sustainable properties and durability. Pervious concrete is the perfect choice, in this situation, as it allows stormwater runoff to percolate into the soil thus reducing the load going into the sewer system of Chicago.

Pervious concrete is becoming the preferred method of engineers and architects to manage stormwater. Developers have a distinct advantage when they can manage stormwater on confined commercial sites without retention ponds or detention facilities. Residential developers are also finding ways to make their projects greener and to reduce costs by using pervious concrete. They now use nature, and permeable concrete, to replenish the water table rather than pay for infrastructure to transfer stormwater. The result is lower costs for retention facilites while allowing extra green space.

In another example, a homeowner in Michigan, had a problem with minor flooding in his garage caused by a poorly constructed driveway. Prior to pervious concrete, the only solution would have been to rip out the entire driveway, re-grade and re-install. The solution chosen replaced 1/6 of the driveway near the garage with pervious concrete. The water flowing toward the garage is now captured by the permeable concrete and directed into the soil below. The cost was only a fraction of the cost of the traditional alternative.

Pervious concrete is being widely accepted as a stormwater management solution. It is now recommended as a Best Management Practice by the EPA on a regional and local basis. A national-level program has been put in place by the National Ready Mixed Concrete Association (NRMCA) to certify contractors in the installation of pervious concrete.

Mary Smith is a freelance writer. She heard about pervious concrete from the talented design team at Florida Engineering Solutions. They do structural engineering design, not construction. When you need prestressed - precast concrete design, remember FES - smart structural design.

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