Paving the Way

By Alan Rider

Houston-based TrueGrid's systems use a series of interlocking, recycled plastic paver blocks on top of a drainage substrate. The grids create a pavement that is 98 percent permeable, and can also be filled with dirt or grass. Photo courtesy TrueGrid.

Like the air we breathe, pavement is ubiquitous. So much so that virtually everyone takes it for granted. Planners, however, probably should not. While a wide range of projects — from complex multilane surface streets to the simplest walkways — involve some form of paving, the reality is that many planners rely on engineers and construction pros to work out the details. But it doesn't have to be that way.

While planners may never achieve the level of granular knowledge or hands-on experience of those two groups, the fact is that gaining a solid understanding of the basics of pavement can give planning professionals a definite leg up when it comes time to lay it down.

A plethora of pavement

The importance of this familiarity cannot be overstated. There are more than 2.5 million miles of paved roads in the U.S., to say nothing of the parking lots, plazas, sidewalks, bike paths, and other hardscapes.

Billions of dollars are spent annually for their construction, maintenance, and repair. The Fixing America's Surface Transportation Act, passed by Congress just three years ago, provided more than $42 billion for highway and bridge improvements in 2016 alone. That number doesn't account for money spent on local surface streets, and private developments, for which there is no good overall estimate.

Consider, too, that this process also requires large inputs of materials and energy, and generates its share of pollutants, including greenhouse gases.

All of which means there's a lot riding on decisions about surfaces. Perhaps more importantly, it also requires planning professionals to recommend pavement options based on more than initial cost alone.

Planning's place

While the type of paved surface is often affected by factors beyond their control — like a client's budgetary constraints — the selection process is increasingly falling under the planner's purview, says land-use planner and landscape architect Rick Satre, AICP.

"Pavement is almost always a key component in a project," he says. "You name it and we've likely used it, from traditional impervious asphalt and concrete to water-permeable pervious asphalt and concrete, to stamped and colored concrete, to all kinds of pavers and even reinforced turf for fire lanes."

Where the responsibility for choosing a type of pavement ultimately lies depends, in part, on the material under consideration.

"Historically, and even today, a traditional impervious pavement falls more under the engineering umbrella than planning," says Branch Engineering principal Renee Clough, AICP, PLS, PE. "Now, though, as pervious options are starting to be utilized more, the selection and design of the pavement is moving into a gray area between planning and engineering." And experienced planners know there's nothing that says they must limit themselves to one type of pavement in a given project, says Phil Farrington, AICP, director of planning for CDC Management.

"We were leading planning efforts for a new regional hospital and we ended up using concrete for the primary roadways and loading dock and asphalt for secondary roads and parking lots. We even looked at using pervious asphalt for a riverfront trail that was part of the project."

Knowing when to use what — and why — can come in handy for planners.

Building blacktop

Though it may seem hard to believe, asphalt has been a common paving material for more than 2,000 years. The earliest examples, with naturally occurring sources of asphalt combined with stone, sand, and gravel, can be traced back to Babylon in the year 625 BC, according to the National Asphalt Pavement Association.

In the U.S., Newark, New Jersey, has the honor of being the first city to use it as a paving material in 1870. Generically known as blacktop, the first hotmix asphalt production facilities in the U.S. opened in the 1920s, and it has been going strong ever since.

Today, asphalt is said to cover nearly 95 percent of the paved roads in the U.S. Among its advantages are the fact that it's reasonably durable, relatively inexpensive, and can be completed quickly.

Modern asphalt uses roughly 95 percent aggregate — gravel and sand — held together by a bituminous binder derived from petroleum distillation. This last fact has contributed to rising costs in recent years as the price of crude oil has fluctuated.

Asphalt does have some advantages in the grand scheme of things, says Audrey Copeland, vice president of engineering, research, and technology at the National Asphalt Paving Association.

"When it comes to life-cycle costs, asphalt pavements are a sound investment," she explains. "Not only is the initial cost generally lower, but because the surface can be easily maintained and repaired to protect the road structure, there should never be the need to completely tear out and replace it."

Asphalt can be made a greener choice with the inclusion of recycled materials, including the grinding down and incorporation of the worn-out pavement itself (known as RAP, for recycled asphalt pavement), granulated rubber from old tires, or even reprocessed asphalt shingles.

"Asphalt pavements are sustainable in part because of their ability to incorporate this reclaimed material," says Copeland. "In fact, asphalt pavement itself is recycled at a greater rate than any other material in the United States."

Concrete thinking

Concrete pavement dates back every bit as far as asphalt. The ancient Romans used the first lime-based concrete to pave portions of their extensive road network, not to mention to build famous — and still-standing — structures like the Colosseum.

In the U.S., the first use of concrete pavement was in Bellefontaine, Ohio, in 1893. The original concrete pavement is still in service today, which is a testament to one of the material's key strengths, namely its longevity.

Like asphalt, concrete consists of a high percentage of aggregates, both coarse gravel and fine sand, bound together with fluid, typically lime-based cement that hardens over time. It has the advantage over asphalt pavement when it comes to lifespan.

"Concrete pavements are typically designed to last 20 years, but they often far exceed their design life," says Gerald Voigt, president and CEO of the American Concrete Pavement Association.

Concrete pavement is said to be better in hot climates, offer more design options like textures and colors, and require less overall maintenance than comparable asphalt surfaces. At the end of its lifespan, it too can be reused as the base for new concrete or asphalt paving.

Those factors, plus the use of recycled materials like fly ash from coal-burning power-plants, can make concrete a greener choice. Downsides include a higher initial energy use and increased emissions of CO2 and dust in the creation of the cement that holds the pavement's aggregate together. It can also be more expensive and labor-intensive to construct, which explains why its use is often limited to highways, main thoroughfares, and other high-traffic applications.

On the upside, concrete pavement offers an advantage beyond its longer lifespan: "Concrete doesn't require rehabilitation or reconstruction as often," says Voigt, "so it consumes less raw materials and energy over time, and generates fewer emissions along the way as well."

asphalt pavement with the top several inches milled off, a process that can be completed more quickly than total replacement. Roller-compacted concrete competes with asphalt in terms of the speed and cost of construction, but it has a rougher surface texture than traditional concrete paving.

Full-depth reclamation, a process that dates back 30 years, is another recycling option. It's considered a sustainable practice because it preserves everything in place. Reclaimers can strip off up to 18 inches of old concrete, spread cement, and remix the material before it's cured and covered with new concrete or asphalt.

Finally, there's what's known as the Next-Generation Concrete Surface. This texturing treatment, which consists of a special longitudinal grooving technique, can be applied to new or existing concrete pavement. Extensive testing shows it can improve the ride quality and skid resistance, while also making the surface markedly less noisy — up to 75 percent quieter by some measures — than traditional concrete surfaces.

Down the road

While it may seem not much ever changes in the paving industry, the truth is that communities have more innovative pavement choices than ever before. Here are just a few of the latest developments:

NEW BINDERS

These materials can make pavement greener by supplementing, or perhaps eventually replacing, energy-intensive Portland cement and petroleum-derived bitumen with naturally occurring materials. For example, researchers from several major universities are working on what's becoming known as bioasphalt, using binders made from a remarkably wide range of materials from algae to vegetable oils. Beyond the environmental benefits, researchers say these binders could also eventually be significantly less expensive.

WATER-PERMEABLE PAVEMENTS

Larger aggregate sizes allow water to percolate through to the subsoils below. Water is filtered by naturally occurring microorganisms as it moves through the underlying base rock before being returned to the water table. Pervious pavement can reduce runoff , and therefore the pollutants that get washed into sewers and waterways. Higher initial costs can be offset by reducing stormwater management system needs.

GRID SYSTEMS

Houston-based TrueGrid's systems use a series of interlocking, recycled plastic paver blocks installed on top of a traditional drainage substrate with voids that can then be filled in with naturally occurring aggregates. Th e grids create a pavement that is 98 percent permeable while holding the gravel in place, making it largely maintenance free and reducing other common problems like dust, sinkage, and mud. Ideal for overflow parking or pathways, the material stays cooler in the summer compared to traditional pavements and can even be filled with dirt and planted with grass.

The system also needs no restriping like traditional parking lots, as stall markers can be built into the grids. The manufacturer says communities can expect a 20- to 60-year life cycle, especially when used with self-binding gravels that have finer particles that bond together over time with the downward pressure of foot or tire traffic. Installation costs are said to be a third to half as much as asphalt.

CONCRETE PAVERS

Another popular hardscape option, these come in all shapes and sizes, from brick-like blocks to larger slabs. Designed to provide eye-catching detail to smaller-scale paving projects like plazas or walkways, they also allow stormwater to percolate through the voids between blocks, offering many of the same benefits as permeable asphalt and concrete. New bricklaying machines that take the jumble of bricks fed into a hopper and output perfect pavement offer the promise of making this otherwise labor-intensive choice much quicker and more affordable. Materials like EnviroBond — which mixes sand with a plant-based glue — help extend pavers' lifespan by preventing the filler from washing out during heavy rains.

TEXTURED/COLORED CONCRETE

This is another specialty option. Decorative textures can add aesthetic appeal, while pigmented concrete can be used to call attention to areas like crosswalks and bike lanes to help them stand out from the roadway, even after painted-on striping begins to fade.

RECYCLED PLASTICS

These materials are finding their way into paved surfaces thanks to a nearly unlimited supply of raw materials. The Dutch firm VolkerWessels has designed large prefabricated panels that are put down over a traditional sand base and speed construction, reduce maintenance, and extend lifespan compared to traditional paving. Th e materials are said to be able to withstand extreme temperatures, from minus-40 to 176 degrees Fahrenheit. Built-in voids in the bottoms of the panels can be used for cables, pipes, traffic loop sensors, and even heating elements to ward off snow and ice.

The Italian firm Corecom has created a product it calls EVIzero that uses a polyolefin material processed into small, nearly clear chips that can take on the color of the rock beneath them, thereby allowing walkways and bike paths to blend in with their surroundings.

PHOTOVOLTAIC PANELS

Solar panels are being incorporated into road surfaces by an Idaho-based company called Solar Roadways. Heating elements to melt ice and snow and LED screens for pavement directional markings like turn-lane arrows can be incorporated right into the hard-as-steel textured glass pavement.

The fledgling company is starting small with bicycle paths and shopping center parking lots, but envisions the day the technology can be applied to roadways to further reduce dependence on fossil fuels. Th e technology is currently about three times as expensive as asphalt, but those prices will likely come down, much like what has happened with the photovoltaic solar panels.

Add all these together and there are more paving options than ever before. All of which mean planners need to up their pavement knowledge before recommending what to lay down on a given project.

Freelance writer Alan Rider frequently hits the road from his home base in Northern California.

The Big Picture

The Italian firm Corecom uses a recycled polyolefin material made into nearly clear chips that allow walkways to blend in with their surroundings. Photo courtesy Corecom.

The work of urban planners affects their communities for decades to come. That is where the process of life-cycle assessment and life-cycle cost assessment come in as an opportunity to pull back and look beyond the short-term details for the long view.

Both these life-cycle measures are processes through which planners can take into account the total impact of a given paving project, from "the cradle to the grave," as the saying goes. While not every project will require a detailed LCA or LCCA, it's a good idea to look at the future to guide your decision making today. This process can be broken down into four distinct phases:

Material Acquisition

We don't often give a lot of thought to where the materials we specify come from, but it's worth considering. From the mining of virgin rock for roadbeds to the energy consumed and emissions produced in manufacturing asphalt or cement, our choices come with an environmental cost.

Construction, Maintenance, and Rehabilitation

Considering these phases of the life cycle get most of our attention, it's not uncommon to make decisions based on short-term factors like construction costs. Adopting a LCA/LCCA perspective can bring out less obvious plusses and drawbacks, from transportation of raw materials to traffic disruption from pavement repairs.

Use

The most obvious consideration here is projected life span, but it's not the only one. Noise generation (especially for main arteries near residential areas), permeability (for improved drainage), fuel economy gains (from decreased rolling resistance), and thermal management (heat retention/radiation that contributes to the urban heat island effect) are just some of the factors that influence pavement choices.

End of Life

Though this seems obvious, there are nuances here as well. Whether it's a relatively small section of bad road or an entire parking lot, remember the three Rs apply: reuse (using the existing roadbed as a foundation for new surface overlay), recycle (whether it's recycled in place or removed to be recycled elsewhere, most worn out pavement can be ground up and used again), and removal (the least ideal option that involves taking old pavement to the local landfill).