The usual definition of pavement? A hard, durable and impervious surface suitable for motorized and foot traffic.
But that standard definition is becoming less accurate each year with the growth in popularity of the pervious segment of the paving industry.
This increasingly popular approach to paving lays down a surface that is penetrable, so that rainwater can bleed through it instead of pooling hazardously or running off in an erosive rush. Some government storm water guidelines are helping drive project owners toward porous pavement, but common sense also is leading developers to the same conclusion.
The technology has been around for 35 years since being developed by the science-based Franklin Institute in Philadelphia. It was a heretical theory that pavement might do a better job of handling rainwater by letting the water pass through it rather than forcing the water to run off. The longstanding unchallenged theory had been that water flowing freely under a paved surface undermines it and eventually destroys its integrity.
In the 1970s, when storm water management became a major issue for federal and state environmental protections agencies, runoff became a focus of concern. At that time — and to a large extent still today — the conventional response to handling rainfall was to channel the water from paved surfaces to collection basins, from which the storm water eventually could be absorbed or released into streambeds. The idea was to contain runoff, rather than to reduce it.
Porous pavement introduced the revolutionary concept that runoff actually can be reduced by allowing it to pass through pavement. In that way the rainwater can more quickly recharge the subsoil, instead of pooling and streaming on the top of soil. After all, it is the moving water atop the ground that produces flooding, soil erosion and damage to engineered facilities. Data shows that pavements letting rainwater bleed through them not only reduce the peak discharge from storms, they also lessen the total volume of discharge.
Even so, some developers and builders are slow to change their approach. Keeping water in its place — that is, away from subgrade materials in a roadbed — is deemed by most builders to be the safest construction course.
But not everyone is skeptical. Both concrete and asphalt industry spokesmen report that interest in porous pavement continues to build, especially in the last five years.
“I see it getting stronger and stronger as a choice for builders,” said Kent Hansen, director of engineering at the National Asphalt Pavement Association. His office is the contact for builders and project owners inquiring about porous pavement. The volume of e-mails to his office about the alternative is increasing.
“I get more Google alerts about porous pavements each year. The messages keep coming through that people are looking at it and using it. They ask for specifications. They’ll ask things like where porous pavement works and where contractors have done it successfully.
“I think the only thing that has changed is people understand it a little better.”
Regulation Could Equal Proliferation
Professional understanding indeed is increasing, but so is the insistence of government regulators that storm water be better managed. Dan Huffman sees regulatory pressure as the bigger incentive driving developers and builders into the porous pavement camp.
“Storm water regulations at all levels — federal, state and local — are increasingly encouraging pervious surfaces by simply limiting impervious surfaces,” said Huffman, who is managing director of national resources for the National Ready Mixed Concrete Association.
“Increasingly it is accepted that generally increasing hard surfaces is negative for some things, including storm water.”
Huffman has been following the development of pervious concrete over the last decade and said “the trend is extremely positive.”
Neither Hansen nor Huffman have hard data showing increased use of porous pavement by the building industry, but anecdotal evidence continues to mount. Each segment of the pavement industry has reported that the use of porous pavement is spreading to different regions of the country.
Once considered a warm weather paving option — Florida contractors have employed it for a quarter of a century — both concrete and asphalt applications of the porous technology have been used successfully in four-season climate zones. Porous asphalt pavement has stood up well for decades in Michigan and Pennsylvania. Pervious concrete has withstood temperature variations unscathed in places like Vermont and eastern Canada. Chicago street department officials currently are testing both materials.
“We are not aware of a place anywhere in the country where there is a suspected deterioration due to freeze-thaw cycling,” Huffman said of the concrete product.
Environmental pressure from the government unquestionably is being brought to bear on the issue, but “green” developers probably would be drawn to porous pavement without regulatory guidelines. That’s because practical application of the technology fits hand in glove with the philosophical goals of such developers. A case in point is Pringle Creek Community in Salem, Ore., a 32-acre (13-ha) development. The National Association of Home Builders has awarded the development its first Green Land Development of the Year award.
Pringle Creek developers totally bought into use of the new pavement. According to its developers, Pringle Creek has more porous asphalt than any other residential development in the country and all its streets are formed of pervious concrete. Those features and an inordinate amount of green space mean that 90 percent of rainfall will be soaked up rather than run off.
“The green building movement is the hot button now and everybody wants to talk about it,” Huffman said. “What it does is it allows us to come in the door and get accepted. That is really why there are so many people pursuing it.”
Part of the resistance to using the new pavement is the absence of proven performance standards. Huffman notes that without a standard of measurement for acceptable porous pavement, local regulatory agencies are reluctant to approve of projects using the material. This reluctance leads builders to shun the product rather than risk running into delays in the project approval process.
“It is not that the technology is not sound. It is very sound,” Huffman said. “But local regulators are not quick enough to understand and provide for it. While we have federal support, the trickle down to the local agency is still slow in coming.”
Setting a Standard
Some of this slowdown would be eliminated if an American Society for the Testing of Materials (ASTM) standard for the product existed. It does not. The ASTM dates from 1898 and was formed to standardize industry expectations for products or services. Now international in its scope, the society continues to be the gorilla on the shoulder of every new product introduced to market.
In June, an ASTM International subcommittee began to explore porous asphalt pavement with the goal of establishing guidelines for design and construction of the pavements. Presumably the organization’s exploration will expand to include pervious concrete pavements, too.
“There is no ASTM standard for this concrete,” Huffman said. “What is amazing is that we are doing as well as we are in the absence of the standard.”
Because more than 90 percent of pavement laid is asphalt, the larger opportunity for expansion of porous pavement is in the asphalt industry. That segment of the industry also was the first to introduce the product and not much has changed about it in the decades since, except for the mix used for the surface.
The product involves laying a geotextile over uncompacted subgrade followed by No. 2 stone topped with a thin layer of No. 57 stone. The asphalt mix that covers it has been changed over the years in response to changed engineering in vehicles. Unmodified asphalt once was used, but that seldom is the case now.
“What we have gone to is a polymer-modified asphalt to make it tougher,” Hansen said. “It also increases the amount of asphalt we can put in the mix, so we can keep the asphalt binder high and keep durability high. That has been the biggest change.”
Asphalt contractors basically use the same equipment to lay porous and non-porous pavement. More care is taken by equipment operators to not disturb the stone bed; rolling the paved surface takes less effort inasmuch as the goal is to avoid over-compaction. After all, Hansen noted, “you do want a certain amount of voids in the mix.”
Contractors also need to be concerned about plugging the “holes” in porous pavement during construction by contaminating the surface with gritty material like sand or dirt. In the Pringle Creek project, for example, developers took the unusual step of covering a first course of asphalt with geotextile material to reduce contamination, then removing the material before laying the final course.
Some contractors use conventional asphalt for project roadways specifically to meet fire department access requirements during construction, but subsequently place porous pavement in runoff and parking areas.
Pervious concrete, on the other hand, is a more specialized product that requires different equipment than that used in standard concrete work. The concrete is poured relatively dry using coarse aggregate and little sand. Typically, the void structure of pervious concrete is 20 to 25 percent, which allows up to 8 gal. of water to pass through a square foot of it every minute.
Huffman, the Ready Mixed Concrete Association expert on the product, said that equipment manufacturers have yet to produce all the types of machines needed for large-scale application of pervious concrete.
“I’m aware of a number of people out there designing equipment to respond to this opportunity,” Huffman said. “We are at a stage where the most common means of placement is using various vibratory screeds, but for larger jobs we need large pieces of equipment and slip forms. We don’t need the big highway paver right now.”
Not Always the
It also should be noted that porous pavement is, in the words of Hansen, “not always the best thing to use. Everything has its place.”
For example, some tight clay soils or impervious rock strata will thwart the absorption of rainwater into subsoil areas, and high groundwater levels also preclude porous pavement from being effective. Permeable pavement is not suitable for applications where high compressive strength is the top priority. But for many pavement applications, the porous product meets every criterion.
Proponents of porous pavement believe that cultural resistance to paving will reach a point where developers will be unable to win approval for pavement projects because of cumulative environmental damage. That is the point, say proponents, where porous pavement will come into its own.
That time may be nearer than generally is recognized. The North Carolina General Assembly in its last session took the unusual step of limiting the use of conventional pavement in parking lots. Specifically, the legislation requires that at least 20 percent of new parking areas — with the exception of parking decks or covered parking lots — be constructed of porous pavement. The standard applies to both asphalt and concrete parking lots.
“To my knowledge, this is the first state that has done anything like this,” said Bill Arent, executive vice present of the Carolinas Ready Mixed Concrete Association. The budget line item was inserted late in the session and Arent said he does not know the impetus for it. He said the legislation did not happen as a consequence of industry lobbying.
The regulation is scheduled to become effective in October 2008. Additional legislation will be sought in the General Assembly next session to give the regulation an enforcement provision so that the mandate will have teeth. At this moment, widespread opposition to the regulation is not expected.
“Regardless of the type of pavement used,” Arent said, “permeable pavement is a great idea. It just makes a lot of sense.” CEG
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