When it Rains, it Drains

Pervious concrete puts storm water back in the ground.

Several Utah government entities and Staker Parson Companies are teaming to bring a new and innovative concrete pavement technology to the Utah marketplace. Pervious concrete pavement is a durable and environmentally sound surfacing material that reduces stormwater runoff, replenishes water tables and aquifers, and mitigates surface pollutants.

An upcoming project that will feature pervious concrete pavement is the Wasatch Touring building located at 100 South 700 East in Salt Lake City, Utah. Over the past several years, trees planted in front of this building have struggled for survival due to a lack of water and oxygen to the trees’ root systems. By paving the area with pervious concrete, water and oxygen will percolate from the concrete surface down through the pavement and into the soil and roots below. The rule of thumb when designing pavement around trees is to keep the ground open to the full circumference of the canopy created by the trees. With pervious systems the ground only needs to be open enough to accommodate the growing trunk of the tree because the water can simply pass through it. Added benefits of pervious concrete pavement include stormwater runoff control and a reduction in landscape maintenance costs.

Whereas typical concrete mixes are engineered to contain a matrix of sand and aggregates of various sizes, pervious concrete mixes are designed with minimal sand and fine aggregates. This results in a product with nearly twenty percent air voids between the course aggregate particles. Water flows freely through these voids and into the subsurface below. Visually, the surface of pervious concrete has the appearance of a Rice Krispy Treat. As in all concrete mixes, the achievement of strength and durability is highly dependent on maintaining the engineered water/ cement ratio. The water/cement ratio of pervious concrete is less than .3 which facilitates an accelerated set.

The ground beneath the pervious pavement at Wasatch Touring contains clay that is fifteen feet thick. Staker Parson plans to install a perforated drain pipe through the clay lens and then fill the pipe with drain rock. The system will allow monitoring by researchers at the University of Utah. Work on the project will be underway by Earth Day, April 22, 2009.

Staker Parson Companies has completed several other pervious concrete projects in Utah and Idaho. The Swaner Nature Preserve in Park City, which was completed in the fall of 2008, also features an innovative stormwater management system utilizing pervious concrete. The Preserve site is on the edge of a wetland with a high water table. Stormwater collected on the building’s roof is directed to a pervious concrete sidewalk where it is filtered through the pervious concrete and an 18” thick recharge bed of 1.5” aggregate. The recharge bed also provides adequate stormwater retention. Filtered stormwater is then returned to the wetland via a French drain. The owners are pleased with the durability of the pervious pavement after its first Park City winter. Pervious concrete that is properly designed, manufactured, placed, and cured is a durable and environmentally friendly paving surface.

The Swaner Nature Preserve was built by Big-D Construction and is the first Platinum LEED (Leadership in Energy and Environmental Design) certified building to be constructed in Utah. Staker Parson provided a menu of concrete mixes designed to help the project earn LEED points. LEED is the standard by which green buildings are measured. This project was also used as an educational opportunity to teach paving contractors how to properly place and cure pervious concrete.

As stormwater retention regulations continue to become more stringent, owners, designers, developers, and contractors should consider the benefits of a pervious pavement system. This unique surface and subsurface recharge bed eliminates the need for expensive stormwater detention ponds, skimmers, pumps, drainage pipes, and other stormwater management systems. Pervious concrete puts stormwater back in the ground where it belongs.