The U.S. porous paving industry has enjoyed solid growth from the late 1990s through 2008 as a result of the Clean Water Act and the storm-water regulations promulgated to effect its intent.
Aside from the recent construction lag related to the economic downturn of 2008, all signs point to an even stronger market growth. The porous paving market doubled in the last 10 years to approximately $110 million in 2008, but is expected to triple in the next 10 years.
The growth is driven by the regulatory demands upon contractors and owners to meet water-quality and quantity-reduction standards. It also appears to be brought on by a root-level desire by design engineers and architects to reduce the deleterious effects of impermeable surfaces upon our waterways. Regulations, design and developers are responding to the groundswell of interest by the public to integrate green-building concepts into their business and into their life.
Plenty to go around
While the U.S. porous paving industry is enjoying success, recycling facilities have struggled. Very successful municipal residential recycling programs have created a surplus of materials and have reduced the bulk value of such materials. In particular, post-consumer waste glass has languished, with only 20% of the total recycled volume finding its way to reuse. By most estimates, 80% of the materials that are recycled at the curb are lost to the landfills in the end. While many states prohibit the placement of glass in landfills, much of the waste glass is brought to the landfill as daily cover.
A small amount of cullet has been used for quite some years in portland cement paving, but suppliers kept quantities small to avoid an alkali-silica reaction and the lesser known alkali-carbonate reaction, which is known to weaken concrete strength. Cullet also has been used as filler in asphaltic concrete pavements, but the volume must be kept low to avoid binder stripping from the smooth surface of the glass.
Presto Products–Geosystems of Appleton, Wis., and the Kaul Corp. in Denver had each been involved in porous paving solutions for decades. They recognized the opportunity for use of the abundant supply of waste glass cullet and, in 2007, began development of FilterPave. The goal was to create a porous paving product from the ground up that would utilize recycled materials.
Rather than using glass as filler in existing hard surface-bound products, the team set out to develop a pavement consisting primarily of waste glass and sought out a binder specifically formulated for this purpose. The team approached BASF Corp., and with their help a custom elastomeric binder was formulated to successfully bind the glass cullet.
The binder is referred to as elastomeric because it can flex and still retain its binding characteristics. With such flexibility, yield strength occurs at 100% elongation. This characteristic results in a strong pavement that resists the catastrophic failure mode of concrete and more accurately mirrors the strength properties of asphalt, while remaining porous.
While raw waste glass is abundant, a porous glass pavement requires appropriately sized glass aggregate to assure maximum strength. Testing began in late 2007 in Boulder, Colo., at a university parking lot to test in-situ strength. Initially, the aggregate was a 3?8-in. maximum particle size and a 2-in.-deep polyethylene containment structure was placed over 10 in. of open-graded base course. The glass was placed within the structure, and the binder was over-sprayed on the crushed glass from the surface. It was found that the particles needed to be completely coated with binder prior to placement. So, subsequently, the lot was relayed using a drum roller to mix the binder with the glass particles, and better results were observed.
Eventually, the polyethylene matrix structure was dropped from the product development when the binder had improved to the extent to where the desired strength was sustainable without the matrix. The same lot in Boulder still is in service today and, while the lot contains a patchwork of test areas from subsequent ideation testing, it is functioning as a successful porous pavement with heavy daily use.
Dozens of projects are now in service and performing well in New York, Michigan, Wisconsin, Montana, Colorado, Texas, Florida, Illinois and Missouri. Cold weather and frost heave do not tend to be problems with porous pavements, since by their very nature, they drain well, and the glass porous paving system is no exception. Presto Geosystems plans to continue rolling out the program to new contractors in unclaimed territories throughout 2010.
From point A to C factor
Early on, the team worked with Glass Plus Inc., Rhinelander, Wis., to alter the gradation and processing of the glass aggregate. Aspect ratios of no more than 2-to-1 were allowed, and the glass was required to be free of shards as well as free of deleterious matter such as paper or plastics. In 2009, specially formulated additives were placed into the glass-cullet processing regimen to increase bond strength to the binder. Suppliers across the U.S. who can deliver glass cullet processed to the correct specifications will be brought on as the market grows so that supply will be located closer to project sites to minimize the transportation carbon footprint.
As the gradations evolved, the porosity and C factor testing results for the final product changed as well. Now, with the glass aggregate specification completed, the final porosity ranges from 32% to 38%. This yields a very attractive C factor approaching zero. Ordinary lawn will sheet drain 20% of the water falling on it and, therefore, has a C factor of 20. This bonded glass product has 50% more porosity than pervious concrete and twice that of porous asphalt and can be used to completely nullify concerns for runoff from any paving surfaces for which it is chosen.
Twinkle for their eye
While the team knew immediately that FilterPave was aesthetically pleasing, the response to the early projects was nonetheless surprising to the laydown crews. At each installation, crowds of onlookers gather to marvel at the product, and responses to the glass pavement were at times overwhelming as they worked to complete the installation. The surface of the pavement twinkles in the sunlight and gleams in the street lights at night. The multitude of waste-glass colors creates a depth to the material showing through the pigment that is added to create one of five standard colors.
Also developed by the team were custom methods for laydown of the material. Early in the development process in spring 2008, the team approached a major national manufacturer of volumetric concrete mixers to create a custom device for demonstration purposes. The 15-cu-yd hoppers supplied aggregate to a belt that dispensed accurately measured quantities to an auger. That auger also received the two-part elastomeric binder and delivered the properly batched mixture to a waiting raking and finishing crew. Unlike typical ready-mix concrete or asphaltic concrete hauling, the FilterPave mixer would only batch the materials necessary and could be started and stopped with nearly zero waste of material.
Since the flexible porous pavement was placed much like concrete, many of the hand tools utilized were similar. Vibratory screeds, vibrastrikes, fresnos and trowels were the typical tools most called upon. This new porous material was a favorite of the finishing crews, due to the very low density of the mixture and the ease of finishing. Also useful was a very small rotary trowel. This extended-handle device was perfect for finishing off the surface, and one crew was capable of placing and finishing 2,000 sq ft per hour.
The recycled glass pavement has been used to pave pathways at nature centers, an arboretum and public pedestrian areas as well as parking lots and sidewalks. It has been used for public and private driveways, and the porous glass pavement was an ideal material for a boat launch along the Long Island sound. The material can work well in conjunction with adjacent impervious concrete and asphalt areas and has been used as an architectural feature utilizing the various available colors.
Draining made of glass
A well-designed porous pavement system can outlast traditional pavement because it is more resilient to the forces of nature. Porous pavement can eliminate underground storm-water systems, saving the owner money, and can help meet storm-water quality and quantity project goals. Since water drains immediately upon melting, porous pavements do not have the effect of pooling water under ice and insulating ice from melt. Typically, porous pavements use less than half the ice control, reducing the effect on our environment, reducing costs and improving public safety. Porous pavement systems also assist in low-impact development goals by reducing the amount of surface disturbance resulting from underground sewer and storm-pond construction.
A porous pavement that is made from nearly 100% post-consumer waste glass has several advantages. It helps meet the U.S. Green Building Council’s LEED and other green-building initiatives. It helps increase recycling efforts at the curb by raising awareness of beneficial reuse. It reduces the need to harvest the earth for yet more natural resources, lowering a site’s carbon footprint both because of its substitution of waste material for aggregate and for the fact that it requires less base thickness than typical porous pavements.