by John Sedenquist
Emulating nature, emerging post-construction urban site stormwater management practices seek to control, catch, contain, treat, and infiltrate runoff from stormwater and snowmelt. Ponds, swales and buffer zones, and permeable pavements are typical ways of controlling post-construction site runoff. Conventional construction methods use granular pervious fill materials to create these runoff detention controls.
Pervious cellular lightweight concrete is a permeable, engineered material with a portland-cement base, containing small, open, air cells uniformly distributed throughout the concrete. The use of PCLWC in urban post-construction stormwater management practices provides improved runoff detention over granular pervious fill or lightweight aggregate materials.
Recent innovation in foam liquid concentrate technology provides a new geotechnical fill solution for improving stormwater runoff detention management: the production of pervious forms of cellular lightweight concrete (PCLWC). PCLWC technology improves runoff detention, providing high detention capacity while imparting minimal impact on native soils, eliminating compaction testing and pre-loading for project area settlement mitigation, and significantly reducing carbon dioxide emissions and heavy metal contamination to area wetlands and waterways.
Cellular Concrete Geotechnical Fill Materials
Cellular lightweight concrete fills have a long and established track record of success, providing value-engineered solutions when standard fill materials are too heavy, site access is limited, or project schedules must be contracted.
Developed in Scandinavia, typical cellular lightweight concrete fill is an engineered, impermeable lightweight material with a portland cement base containing small, closed air cells uniformly distributed throughout the concrete. More precise control of the volume of these air cells, produced mechanically by means of special foam liquid concentrate agents, was introduced in the years following World War II, resulting in relatively stable air cells (bubbles) and controlled density for a broad range of load-reducing geotechnical fill needs — from 20 to 120 pounds per cubic foot (PCF).
Unlike typical cellular lightweight concrete geotechnical fills, PCLWC is a permeable, open-cell, lightweight concrete, able to stabilize soil without disturbing or redirecting natural water flow. The use of PCLWC in urban post-construction stormwater management practices provides improved runoff detention over granular pervious fill or lightweight aggregate materials.
Comparing PCLWC to Other Fills
Typical granular pervious fill materials have a detention capacity of about 2.4 gallons per cubic foot. If 1,000 gallons of stormwater runoff detention is required for the project, approximately 380 cubic yards of native soil must be replaced or topped with granular pervious fill material. By comparison, PCLWC has a detention capacity of 4.8 gallons per cubic foot. For every 1,000 gallons of detention required, only 210 cubic yards of native soil needs to be replaced or topped with PCLWC.
PCLWC is self-leveling and naturally achieves 100% compaction, eliminating settlement with no reduction in runoff detention capacity. It maintains its shape and detention capacity following placement and will not liquefy during a seismic event. Pre-loading for project area settlement mitigation is not required when using PCLWC. Unlike non-contained or non-homogeneous fill materials such as soil or aggregate, PCLWC provides the added benefit of gaining a 2-to-1 point-load-distribution edge.
An on-site foam generator adds fixed volumes of air at a fixed pressure to create the micro-bubbled foam, which, when blended with a cementitious slurry, enables the production of PCLWC.
Where native-soil project loads are a concern, typical granular pervious fill materials, which weigh approximately 110 pounds PCF, can be replaced with higher-cost, lightweight aggregate pervious fill, which weighs only about 90 PCF. To prevent settlement, lightweight aggregate pervious fill materials are often installed in lifts, with each lift achieving 85% compaction. In some urban settings where native soil loads are a concern, even lightweight aggregate pervious fill materials must be reduced. In these cases, as the lightweight aggregate pervious fill material is reduced its detention capacity is also reduced and may not meet project specifications.
The controlled density of PCLWC supports a broad range of load-reducing geotechnical fill needs — typically from 20 to 35 PCF, which is significantly lighter than lightweight aggregate fill options. PCLWC is 80% lighter than typical granular pervious fill options and requires 45% less volume, saving excavation and installation expenses and eliminating compaction and testing costs.
Reduction of Carbon Dioxide Emissions
The use of PCLWC for stormwater runoff detention significantly reduces project carbon dioxide emissions. Projects with volumes exceeding 10,000 cubic yards of detention fill use an on-site PCLWC batch plant that produces and pumps cementitious slurry, with trucking needed only for the delivery of cement. Such was the case in Queens, NY, during construction of Citi Field, the new home of the New York Mets. More than 17,000 cubic yards of PCLWC were batched on-site, eliminating the need for a fill-truck staging area outside the stadium and removing more than 1,000 trucks from the Van Wyck Expressway, Grand Central Parkway, and other borough streets.
For projects with volumes of 10,000 cubic yards or less, PCLWC production also happens on site, by treating slurry manufactured at a local ready-mix plant and delivered and discharged by transit mixer to the hopper of a job site concrete pump. Engineered foam liquid concentrate (which enables the production of PCLWC) is injected into the delivered slurry in the pump hose — not the transit mixer. This method expands the volume of the delivered slurry about 3.8 times (i.e., one seven-cubic-yard slurry load produces 27 cubic yards of PCLWC), eliminating up to 55% of the trucks (and the accompanying road traffic congestion) that would have been required had a granular pervious fill option been used.
Other Sustainable Attributes of PCLWC
Both PCLWC batching methods can incorporate ground granulated blast-furnace slag or fly ash in the slurry mix design without adversely affecting PCLWC performance. The use of these post-industrial byproducts in PCLWC production eliminates the need to landfill these materials and reduces the need for virgin materials in PCLWC production as well as the environmental impacts from the extraction and processing of these virgin materials.
The use of PCLWC for stormwater runoff detention significantly reduces project carbon dioxide emissions. On-site material batching requires trucking only for the delivery of cement, eliminating the need for a project fill-truck staging area and removing heavy, round-trip dump truck traffic from urban streets and thoroughfares.
Testing by Middle Tennessee State University documents the ability of PCLWC to enhance the environment by filtering contaminants that can adversely affect soil and water. Various chemicals and solids were placed on PCLWC specimens and then rinsed with increasing amounts of water — 0.5 to 30.0 inches — over a given surface area. Testing results showed PCLWC filtered 78% of the hydrocarbons and heavy metals. Testing for oil retention resulted in 97% of oil remaining on the PCLWC specimen.
PCLWC creates an ideal environment within its cell structure to aid in the natural breakdown of environmentally unfriendly materials. Hydrocarbons, such as oils, are a food source for many naturally occurring bacteria and fungi. The cell structure of PCLWC allows these microorganisms to feed on the retained oil and biodegrade it into simpler chemical components, which are released harmlessly into the atmosphere.
As communities struggle to strike a balance between economic growth and environmental issues, innovative construction materials can provide effective solutions for improving stormwater runoff detention and support sustainable urban development. Adopting PCLWC technology adds an important detention practice to your urban stormwater management toolbox.
About the Author:
John Sedenquist, COO, oversees operations for Cellular Concrete LLC (Allentown, PA).
