Question: Can pervious concrete stand up to freeze-thaw?

Answer: In most areas of the United States, if it’s installed correctly.

Pervious concrete is one of the hottest topics in land development today.

As owners, architects, land developers, and concrete professionals become more familiar with its benefits, the interest in pervious concrete continues to grow, reports experts at the Portland Cement Association.

Pervious concrete pavements can meet the requirements of new EPA regulations that call for decreasing the amount of surface water runoff and initially treating the runoff. But can it take Mother Nature’s ability to bust apart rock with the power of water freezing found in the northern United States?

What it is
Pervious concrete is made up of cementitious materials, water, admixtures, and narrowly graded coarse aggregate. Little or no fine aggregate is used in the mixture.

With just enough cement paste to coat the aggregate, a system of interconnected voids (from 15 to 35 percent) creates a highly permeable concrete that drains very quickly.

If water passes directly through the concrete, the amount of surface water runoff is reduced dramatically.

It can also be used as part of a system to reduce the pollution in storm water.

Pervious pavements have been used for years throughout the warmer climates of the United States with excellent results. However, in climates prone to severe freeze-thaw cycles, many owners, architects, land developers and concrete professionals are skeptical that pervious concrete pavements can be made to resist freeze-thaw damage. Research on this topic is currently underway.

Resistance of any concrete to freezing and thawing depends on the permeability, the degree of saturation, the amount of freezable water, the rate of freezing, and the average maximum distance from any point in the paste to a free surface where ice can form safely. The rate of freezing in most applications is dictated by the local climate. Entrained air may also help protect the paste.

Drainage key to success
Perhaps the most important aspect in designing pervious concrete pavements for freeze-thaw areas is avoiding, or at least limiting, saturation, especially during the time of year when freezing can be expected.

Pervious concrete pavements can be designed to control the degree of saturation and the average maximum distance to a free surface. Proper sub-base design and preparation are key to pulling rainwater, ice, and snowmelt away from the pavement and ensuring suitable drainage.

The paste should also be protected by using air-entraining admixtures to create a sufficient air-void system. The National Ready Mixed Concrete Association (NRMCA 2004) has developed guidelines for using pervious concrete in areas prone to freeze-thaw conditions.

NRMCA recommendations
Dry freeze and hard dry freeze: Dry freeze areas are those parts of the country that undergo 15 or more freeze-thaw cycles annually but receive little precipitation during the winter. Many parts of the Western United States at higher elevations come under this category.

If the ground stays frozen during a long continuous period of average daily temperatures below freezing, the area is referred to as hard dry freeze area.

Since pervious concrete is unlikely to be fully saturated in this environment, no special precautions are needed to assure pervious concrete performance. However, a 4" to 8" layer of clean aggregate base below the pervious concrete is recommended as an additional storage area for the water.

Wet freeze: This includes areas of the country that undergo 15 or more freeze-thaw cycles annually and receive precipitation during the winter. The middle part of the eastern United States falls into this category. Since the ground does not stay frozen for long periods, it is unlikely that the pervious concrete will be fully saturated. No special precautions are needed, but a 4" to 8" layer of clean aggregate base below the pervious concrete is recommended.

Hard wet freeze: Certain wet freeze areas where the ground stays frozen as a result of a long continuous period of average daily temperatures below freezing are referred to as hard wet freeze areas. In these areas, the pervious concrete may become fully saturated because frozen soil has very low water permeability. The frost penetration depth (depth at which the temperature is at 32 F) varies throughout the country and pervious concrete pavement must be designed for freeze-thaw resistance. The NRMCA suggests the design consider the following factors:

1. Calculate the frost penetration depth in your area. In the Washington, DC, area, for example, it is about 30".

2. Calculate 65 percent of the frost penetration depth. The Federal Aviation Administration (FAA) says that the top 65 percent should contain non-frost-susceptible materials and the bottom 35 percent may be in frost-susceptible subgrade. The FAA uses the 65 percent limitation to prevent frost heave.

In this case, the key factor is water infiltration. This is about 19.5" for the 30" frost penetration depth.

3. Provide pervious concrete pavement plus aggregate base equal to the number calculated. For example, using the a 19.5" calculation, a 6" pervious concrete pavement over a 13.5" aggregate base would be sufficient. The aggregate base must consist of clean, well-draining and open-graded aggregate base with less than 1.5 percent of the material being finer than 0.02".

If the frost depth is very deep, for example, 100" in North Dakota, additional measures can help reduce the chances of a fully saturated pervious concrete pavement. A perforated PVC pipe can be placed in the aggregate base to capture water and drain it away.

Pervious concrete in a freeze-thaw environment should always be air-entrained to provide additional protection.

High groundwater table: Pervious concrete is not recommended in freeze-thaw environments where the ground-water table rises to a level less than 3" from the top of the surface or where substantial moisture can flow from higher ground.

More research needed
While the benefits of pervious concrete pavements are well known, concerns over the freeze-thaw resistance may prevent many designers from using pervious concrete in colder climates.

Several pervious concrete pavement projects in dry and wet freeze areas have shown good field performance over several years and research on freeze-thaw resistance of pervious concrete pavement continues across the United States.

The limited experience with pervious concrete pavements in hard wet freeze areas means that care must be taken in its specification and installation. Pervious pavements should be placed by experienced installers and the structure and surrounding details should be designed to accommodate the anticipated water flow and drainage requirements.

For more information on pervious concrete design and requirements, contact the Portland Cement Association at www.cement.org.

Published in the January/February 2007 issue of Contractor Tools and Supplies magazine.

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