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Why “Blacktop” Asphalt Paving Rules on Public Roadways

By November 10, 2021No Comments

Asphalt is cheaper to install, and easier to repair, than cement concrete. But both require regular maintenance to ward off deterioration – including potholes.

According to the Federal Highway Administration (FWHA), about 94 percent of all surfaces used for transportation – roads, streets, and highways – are paved with asphalt. The characteristics and properties of bitumen-based asphalt, also known as blacktop, are quite different than those of concrete, even if the latter is used in a small portion of roadway construction. A big contributing factor to why blacktop is used far more than concrete is in the maintenance and repair required for each.

Simply put, a blacktop patch of a pothole is easy to apply and fairly common. It is also, mile for mile, less expensive to build. A pothole filled with the right asphalt repair material, done under the right conditions, is also relatively inexpensive. Repairing the deterioration in concrete slabs is an entirely different matter.

And to be fair, both blacktop asphalt and grey concrete are subject to the same assault of time, temperatures, traffic, and poor maintenance. To use a road is to abuse a road. The weight of vehicles wears down both. Extreme temperatures, both hot and cold, affect both types of materials. Even simple sunshine can break down road material – more so on the darker surface of asphalt than lighter, greyer concrete.

Blacktop asphalt is made up of a stone aggregate mixed with a petroleum byproduct, bitumen, heated and pressed into place over a gravel base such that there are no gaps to, initially, allow in water or air. But over time gaps do develop, allowing in moisture that can ultimately undermine it. Repairing those cracks with crack sealer, or small potholes with blacktop patch material, will prolong the utility of the pavement overall. A stitch in time (i.e., pothole filler) saves nine, as the saying goes.

When concrete bears traffic it stays pretty solid until either the subsurface deteriorates from erosion or the freeze-thaw cycles, sometimes from flooding or excessively heavy loads, or buckling that can come from very warm weather. In the summer of 2021, when triple-digit weather hit the Pacific Northwest for several weeks, the expansion of some concrete slabs on Interstate highway 405 in Tukwila, Washington caused them to pop out of place. Entire sections of those panels had to be removed and replaced with new concrete. The pouring, dry, and set times for that takes hours, often overnight – a lot longer than a filling ruts and potholes in asphalt.

Patching blacktop comes with its own complications. For starters, many municipalities and departments of transportation use a temporary patch material in winter, when a significant portion of potholes form, because the standard asphalt hot mix is likely not available at that time of year (asphalt plants close during winter). The alternatives include a cheap hole filler material, a temporary “cold mix” material that has to be replaced by permanent pothole filler a few months later when temperature warm up. The winter alternative is a polymer modified asphalt repair patch mix  that can be used in ice, snow, and cold temperatures and still provide a permanent fix.

There isn’t really an equivalent to blacktop patching with a concrete repair material for roadways. In driveways, on sidewalks, with patios, and concrete building foundations epoxy injections are possible. On a highway, the cracks need to go through an arduously sawing, sandblasting, air blasting, installation of a backer rod, and application of a sealant (either a hot, poured elastic material, or silicone).

For the most part, a blacktop asphalt pavement is an uninterrupted ribbon of a smooth ride when in good condition. The seams between concrete slabs nonetheless used on some roads and highways, and on on- and off-ramps, are more jarring to motorists and their passengers. It’s easy to see why asphalt is the preferred material by far.

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