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Which is better, concrete or asphalt?

Apr, 20 2011


The debate goes on: Which is better, concrete or asphalt? While there is no cut-and-dried answer, a smart public works official will consider the following questions before selecting a material for the next road project: Which pavement option is better for my specific application? Which will be more cost-effective for me? Which will last longer, decreasing lifetime maintenance costs? Which do I and my team have experience working with? Do I want to fully replace, or rehabilitate?

Before a public works (or street maintenance, or road repair) department can decide whether to use concrete or asphalt in a specific application, the department manager must arm himself with information to help him make the best choice.

“There are three main factors to consider: initial cost, time to first rehabilitation, and cost of total reconstruction or major rehabilitation at the 40- to 50-year mark,” said Dave Newcomb, P.E., PhD, vice president of research and technology with the National Asphalt Pavement Association (NAPA), which is based in Lanham, Md. Similar to calculating the depreciation of a vehicle or other asset (think GASB 34), the cost of the installation—whether it's concrete or asphalt—along with the long-term maintenance must be considered.

“If we get communities to base roadway design on total life of pavement and total capacity, concrete and asphalt are comparable in many ways,” said Scott Haislip, director of streets and roads with the American Concrete Pavement Association (ACPA), headquartered in Skokie, Ill.

Crunching the Numbers

Cost is often the No. 1 factor that public works departments must consider, but basing assessments solely on upfront costs can be dangerous. Cost of the raw material may fluctuate daily or may be difficult to acquire locally (think of rising oil costs or shortages of portland cement). Both ACPA and NAPA encourage public works departments to look at the long-term costs of maintenance and rehabilitation when deciding which material to specify. Because estimates show that 25% of roads in cities across the United States must be either rehabilitated or completely replaced in the next two years, public works officials must take a close look at how those costs are going to affect them in the long run.

Haislip said that asphalt is more common for a department to specify than concrete. “Asphalt over concrete—that's the standard,” he said. “That's what they've been doing for so long.”

But because it's the “standard” doesn't necessarily make it the best option for all applications. Concrete may be better in some instances, the first of which is new construction. In urban metropolitan areas, where new roads are being built and existing roadways are being expanded, using concrete may be a good way to minimize future inconvenience. “Funding is a big challenge, however,” said Haislip. “People are looking for short-term fixes, which have a political impact.”

Another place to use concrete paving is where underground utilities need to be redone. When separating sewer and drainage pipes (no more combined sewers), the public works department must dig out the roadway, which could present a good opportunity to go in and place longer-life pavements.

Asphalt also can be specified as a long-term solution. “Some pavement types must be completely removed and reconstructed when they reach the end of their design life, but total removal and reconstruction due to structural failure of a thick asphalt pavement is rare,” said Margaret Cervarich, vice president of marketing and public affairs with NAPA. “With a thick asphalt structure, it is possible to obtain a ‘Perpetual Pavement,'one that can last indefinitely, with the only rehabilitation being milling of the surface followed by an asphalt overlay.” Perpetual Pavement is a relatively new type of asphalt concrete pavement designed to resist structural fatigue distress for at least 50 years. Maintenance is limited to replacement of the surface course.

Overlays to the Rescue

Overlays are hot on both sides of the debate. Because public works departments may not have the funds to place new roads—either concrete or asphalt—many turn to overlays, which can be placed over either. The vast majority of roads in the United States are asphalt, but that does not limit what can be placed over it.

Four types of concrete overlays are currently in use: whitetopping, ultra-thin whitetopping (UTW), bonded overlay, and unbonded overlay. “Ultra-thin whitetopping is gaining in popularity because it can be done so easily,” said Haislip. UTW is used in situations where the existing asphalt pavement is full-depth (asphalt surface on asphalt base), and is ideal for normal traffic loads on residential streets and low-volume roads. It also can be used in asphalt intersections where pavement shoving and rutting are problematic.

Asphalt also offers many options. “There have been significant advances in asphalt overlay technology,” said New-comb. “When rehabilitating either a concrete or an asphalt road, an asphalt overlay is fast and cost-effective to construct. The result provides a smooth, safe, durable, quiet surface for many years.” There are several options to consider when rehabilitating an asphalt road: recycling, rubblization, mill and fill (the most popular today), thin overlays, modified mixes, open-graded friction courses, stone-matrix asphalt, Superpave, and Perpetual Pavements.

So which pavement option is better? Officials planning improvements on their local roads must take multiple factors into consideration, and have several resources to turn to. The Federal Highway Administration's procedure for performing life-cycle cost analysis is described in publication FHWA-SA-98-079, “Life-Cycle Cost Analysis in Pavement Design.” This publication, along with a host of others, can be combined with classroom knowledge and hands-on experience to give you the best pavement for your needs. Where environmental sensitivities are involved, the Building for Environment and Economic Sustainability (BEES) program from the National Institute of Standards and Technology can provide an even-handed comparison. The BEES software is available for free download at www.bfrl.nist.gov/oae/software/bees.html.
Source: Hanley Wood

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