Salmon listing requires new stormwater management strategy

By BRUCE T. CHATTIN
Washington Aggregates and Concrete Association

With the recent listing of Puget Sound Chinook and the uncertainty of what the new ground rules will be, one thing is for certain - things will change and the use of impervious surfaces and the current methods to manage stormwater appear to be themselves endangered. This was the clear message of the Governor's task force report, "Extinction is Not an Option," published in late 1998. The undesirable effects and harm caused by stormwater runoff and lack of infiltration of impervious surfaces is harmful to fish, water quality and habitat. Traditional hard paving methods will need to be reviewed.

Since March of 1994, the ready mixed concrete industry locally and nationally has renewed its interest in the very old technology of "no fines" concrete. Also known as K-Crete, EnviroCrete, EcoCrete and pervious/porous pavement - this pavement surface traditionally reserved for greenhouses, soil stabilization, erosion control and subgrade drainage, appears to be a bona fide solution for current surface stormwater management needs.

The Washington State Aggregates and Concrete Association constructed test panels in 1994 to evaluate the use of "no fines" pavement potential in Western Washington, and evaluated how such as material could be used commercially. Testing included evaluation of the acceptable Florida models and construction methods. While, this isn't Florida, the concept transfers well to the Northwest where temperatures and freeze-thaw cycles are nominal and stormwater management concerns are equally as acute. With the recent attention towards salmon, habitat and stormwater management, the association and its local members have been preparing the product for more widespread use.

"No fines" concrete quickly soaks up liquid.

According to the concrete industry, the potential benefits for the use of "no fines" concrete fits well with the direction public works departments and developers are considering to meet the new standards for stormwater management. The immediate benefits of using "no fines" concrete result in no water leaving the site, eliminates the need for stormwater collection and detention systems, and increases the pervious to impervious ration ratio of a property. The multiple layer pavement and drainage system will naturally provide water retention and will essentially mimic the drainage and filtration action of naturally unpaved surfaces.

Tests conducted indicate the use of 5/8-inch crushed material is desirable, as the use of any smaller aggregates produces a strain on already marginal supplies of 3/8-inch pea gravel. One of the considerations for a viable porous pavement is not to strain existing aggregate products by using or creating specialty blends of aggregates.

Permeability tests conducted with 5/8-inch aggregates indicate more than sufficient drainable capacity in a well graded 5/8-inch aggregate mix with approximately 20 percent voids for anticipated precipitation in 24-hour, and 10- and 25-year storm events received in the Northwest. Recent testing is evaluating the use of recycled aggregates for secondary drainage/filter sections and other filtering options.

The concrete industry has been initially reluctant to push the pavement into the market because of the special considerations in its manufacture and equally important construction. Manufacture of the mix design places new considerations in batching, discharge and aggregate gradations. Placement and curing of the pavement requires expertise and few local contractors have the experience to place the new material. This can be overcome with proper instruction and a commitment to quality. The concrete association and local members are working on a specification and recommended guidelines to provide specifiers direction and clear construction practices to insure successful applications.

Concrete industry quality control personnel and contractors are working on a complete "system" that will allow for maximum stormwater management and many new applications. The potential for "no fines" concrete applications are seen in parking lots, driveways, sidewalks and low-speed residential-volume roads or any situation where drainage review is required and/or near-zero runoff is desirable. Industry representatives are considering design practices on use of the material for whole site pavement management and for perimeter or edge drainage for parking lots and road shoulders.

"No fines" or "Portland cement pervious pavement" is acknowledged and suggested as a best management practice in the Puget Sound Stormwater Management Manual (February 1992), Chapter III-3-63.

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