In the alley-center infiltration approach, permeable material (asphalt, concrete, or pavers) is placed in the center of the alley and high-albedo concrete is placed along the outer edges. This way, water soaks through the center of the alley, but truck tires roll on top of the stronger pavement.
Each of Chicago's four pervious-pavement designs can be tweaked to fit just about any alley in the city. This is an example of full-alley infiltration using permeable pavement. Source: Chicago DOT
The Right Ingredients
The Chicago DOT (CDOT) turned to local materials consultant S.T.A.T.E. Testing LLC of East Dundee, Ill., to convert national ASTM standards to Illinois DOT specs, which are based on American Association of State Highway & Transportation Officials standards.
“We tried to make this as palatable as possible for contractors to bid,” says Cynthia Williams, quality assurance manager for CDOT.
S.T.A.T.E. Testing experimented with mix designs to meet project requirements, which include reflectivity, a minimum permeability rate of 3 gallons per square foot per minute for all surface materials, and a compressive strength of at least 1800 psi for pervious concrete.
The challenges were many.
“On the hot-mix asphalt side, we struggled with drain-down,” says Jay Behnke, PE, president of S.T.A.T.E. Testing. “Conventional hot-mix is a pretty tight mixture [of rock and sand] that stays put, and the liquid asphalt doesn't drain down off the aggregate. We figured we needed 25% air voids to get permeability, but with that many holes in the mix the asphalt was leaking away.”
The company added ground tire rubber to the mix instead of fibers, and learned it acts just like polymer. Unlike cellulose fiber—which is usually added to hold the asphalt on the aggregate—ground tire rubber doesn't require paving contractors to buy special plant equipment. Another bonus: it's a recycled product.
On the concrete side, the alleys required a pervious surface that could withstand load-bearing vehicles such as garbage trucks. By using aggregates made from hard, solid rock, S.T.A.T.E. Testing was able to achieve strengths in excess of 3200 to 3500 psi, which is comparable to standard concrete. The next challenge was laying the concrete.
“Alleys are inverted so back yards don't get flooded,” says Behnke. “How do you construct an invert with a paver?”
Luckily, West Fargo, N.D.-based Lura Enterprises Inc. had recently developed a concrete roller screed for this type of work. Aconstant velocity joint in the center of the screed allows it to flex, enabling workers to finish a slab so that both sides are flat but slope to the center. It also strikes off pervious concrete with ease. The weight of the lightweight screed—called the Lightning Screed—can increase to 220 pounds per 20-foot section by adding water to the tube.Survey Says
There's definitely a price difference between constructing conventional alleys and green alleys.
“With traditional alleys, you're usually just doing an overlay,” says Behnke. “With green alleys you have to dig down deeper because you need to put in the drain.”