A lot of words get thrown around when someone is talking about a roadbed or parking lot, and in common terms, we use things like blacktop and asphalt, concrete and cement, pretty interchangeably. They really are not the same things, though. Read on…
Asphalt (the dark side), a/k/a bitumen a/k/a, asphalt concrete a/k/a flexible pavement is the black gummy but not looking stuff the interstates are mostly surfaced with, about 94% of them in fact. Asphalt has been around since right after the Civil War. Bitumen, which the primary ingredient in asphalt mix, is a semi-solid form of petroleum or “pitch”, and is mostly a byproduct of refining, although some occur naturally. It usually has to be put over gravel, substrate, or even polystyrene in areas where frost heaves are common. Asphalt is an economical surface material that is fairly forgiving and is somewhat water permeable, depending on the substrate or soil, rock, or materials that are used to underlay it.
Asphalt is a little safer in snow and ice conditions due to tire grip, and you are less likely to slide (hydroplane) in rain. It is easier to repair if there is damage as it can be filled or patched. It does not, however, hold up well to repeated freezing-thawing conditions or extreme rain over time with heavy traffic. Asphalt is a petroleum product, is recyclable, but off-gasses a lot (that rotten egg smell you notice when passing road work). It does require re-sealing every year or two and other regular maintenance. In hot conditions, asphalt can become hot and sticky and will run somewhat, causing road breakdown and tar tracking.
Concrete (the light stuff) is a mix of Portland cement, aggregate (crushed rock), sand and water. Portland cement is a primary ingredient and determines how “stiff” the concrete becomes. It is a hydraulic limestone that is heated and ground, then mixed with gypsum. Concrete used for pavement has been around about 100 years longer than asphalt. It is seen most often in urban environments, partly to help with the runoff of chemicals in local waterways or where tracking black tar can be problematic.
Concrete is more environmentally friendly and more durable. Vehicles typically use 15-20% less fuel when they run on concrete surfaces, and the lifespan of concrete is far longer than asphalt for roadways. It can be textured, patterned, or colored, and costs slightly more than asphalt. It is often more expensive when it does need repairs as a whole section may have to come out to fix an issue.
Tarmac is a term we usually hear used for airport runways. Tarmac actually is a very old composite of “road metal” or crushed gravel mixed with tar. First used by the Romans to build roadways, it was a highly durable material that used local materials for surfacing roadbeds where gravel did not stay on the surface well without sinking. It is interesting the term is used for airports, which are busy constantly, with heavy impacts and high heat generated from tires, as tarmac is only effective on roadways with low vehicle traffic. Most commercial airport runways are concrete or a composite of concrete and asphalt.
Modern tarmac is actually a mix of coal tar and ironworks slag. Historically it developed from Macadam, the first attempt to step up from gravel roadbeds in the early settling of America’s East. Sharp crush white gravel was laid and compacted, then sprayed with stone dust to set it in place. Later forms used cement dust or even bitumen as binders on the white surfaces. It was out of those attempts that modern asphalt mixes developed that bound better and stayed put longer with less maintenance on roadways. Those early attempts using the same materials, only mixed, were called blacktop, a term still in common use. Asphalt came into use to refer to the more complicated mix and composite materials that were formulated particularly to weather, substrate, and local soil conditions, as well as temperatures of the material when applied due to distance from the plant.
