Contractor (September 2009) Pervious Concrete - Drain Away Your Stormwater Worries

01 Sep 2009

Rob Gaimster
Cement & Concrete Association of New Zealand (CCANZ)

Imagined or otherwise, it seems that New Zealand’s weather conditions are becoming more extreme. What is very real however, is that as urban development spreads, the effects of heavy rain and associated stormwater, is an increasingly important issue.

To meet more stringent environmental regulations, engineered solutions for roads, driveways and car-parking facilities, designed to effectively manage stormwater runoff, are becoming almost mandatory in new developments across Europe and North America.

Key to these pavement stormwater management solutions is pervious (or permeable) concrete; umbrella terms that describe either cast-in-situ porous concrete or permeable concrete block paving.

In conventional pavements, stormwater is allowed to run across the surface to gutters, where it is then directed into pipes for quick removal, as it is undesirable to allow too much water into conventional sub-grade material. In both porous concrete and permeable concrete paving systems, stormwater is deliberately directed down into a specially designed underlying permeable sub-base (either coarse graded aggregate and/or hydraulically bound coarse graded aggregate) where it can be released slowly into the ground or drainage network, or harvested for non-potable applications such as garden irrigation.

In porous concrete a high flow rate of water occurs through interconnecting voids within the hardened concrete. These voids are created by using little or no sand in the concrete mix, and carefully proportioning water and cementitious material to create a paste that binds aggregate particles. Typically, 15 and 25 percent voids are achieved in the hardened concrete. Similarly, permeable concrete block paving promotes water flow down from the surfaces through gaps between each concrete paving block.

By managing stormwater runoff through pervious or permeable concrete systems the quality of ground, stream and river water can be controlled against contaminants through filtration, peak flood flows reduced, and the external environment improved and its use optimised.

While the high porosity and low mortar content of porous concrete results in a lower strength compared to a conventional concrete mix, strengths in excess of 20 MPa are achievable, and suit a wide range of pavement applications. The surface appearance of porous concrete is aesthetically pleasing, akin to the always popular exposed aggregate finish. For permeable concrete block paving, the same extensive range of colours and shapes available in conventional precast concrete paving products can be similarly produced, and offer the same performance characteristics of slip and skid resistance and durability.

The recommended maintenance regimes for pervious systems is important, with a general emphasis placed on ensuring the surface area is free from debris such as leaves and topsoil. Vacuuming and pressurised washing is also advised. While clogging of permeable concrete block paving is inevitable, it is anticipated that the expected loss of infiltration will only be 10 percent of the initial rate over a service life of 20 years.

While developments and uptake of pervious concrete systems primarily stem from Europe and North America, interest in New Zealand is gathering momentum. For instance, researchers from the University of Auckland’s Faculty of Engineering, supported by the Auckland Regional Council and the North Shore City Council, have trialled a 200 square metre section of permeable concrete block paving on Birkdale Road on the North Shore.

The effectiveness of the surface was monitored for over two years, with results showing it decreased stormwater runoff at peak flow by an average of 75 percent during most storms, and reduced the volume of total runoff by about 40 percent when compared to runoff from asphalt on the same road. Storms with less than 7mm rainfall produced only a slow trickle of runoff that was too low to measure accurately. Runoff from the permeable concrete block pavement also had on average 70 to 80 percent less sediment, copper, and zinc.

Despite the challenging trial conditions on a busy bus route, overall the permeable concrete block paving performed well, with results informing on-going investigations, as well as contributing to an updated ‘Stormwater Management Practice Note’ and ‘Pervious Pavement Guidelines’ from the North Shore City council on how to best design and install the system.

From a porous concrete perspective, the industry is collaborating to develop an enhanced understanding of mix proportioning, design, construction and appropriate maintenance practices best suited for New Zealand’s environment.

As concerns mount over the detrimental impact increased urbanisation is having on our waterways, on-going local trials and the refinement of design and construction specifications will see a likely rise in the uptake of robust and adaptable pervious concrete systems, whether cast in-situ porous concrete or permeable concrete block paving.

The benefits of these systems demonstrate that concrete offers sustainable solutions for our built environment across all levels of construction activity.

Article appeared in Contractor magazine.