Runways represent the beginning and end of our time in the air on any flight. However, while experiencing the thrill of taking off or landing somewhere new, this leaves us little time to think about what the area we’re landing on really is. Most runways have either a concrete or asphalt surface, but what are the differences?
What exactly are asphalt and concrete?
Let’s begin by determining exactly how concrete and asphalt are formed. The first consists largely of a set of rocks known as aggregates. They are available in both coarse (larger) and fine (smaller) sizes. Aggregates make up 60 to 75% of a given concrete mix and are bound together by a paste that makes up the remainder.
This paste consists mainly of water (14-21% of the entire mixture) and cement (7-15%). However, 8% of concrete is air, which provides some degree of workability when freshly mixed. When the binder paste hardens and gains strength, the concrete takes its solid, rock-like shape.
By the way, while asphalt contains aggregates like concrete, it has a different binding agent. It is known that this binder is a petroleum-based hydrocarbon mixture. It is also darker than concrete and typically has a darker brown or even black appearance.
Differences in terms of runways
But what do these different compositions mean for their use as runway surfaces? From a cost perspective, concrete is the more expensive and time-consuming surface to install (and subsequently repair). Although generally the stronger compound, it is more prone to cracking. This further increases the frequency and cost of repairs.
On the other hand, concrete is more environmentally friendly due to the petroleum-based binder of asphalt. In addition, it does not deform in hot conditions as in asphalt which is more flexible. An interesting point to note is that there are reinforced concrete reinforcements under asphalt runways.
These are deepest in areas subject to more pressure, such as the ends of runways where large and heavy aircraft most frequently land. In general, airport architects consider several aspects when it comes to choosing a runway surface, including climate, traffic and raw material access. Each has its own values and both are legitimate surfaces.
More unusual runway surfaces
Not all runways have the luxury of being paved with smooth asphalt or concrete. Indeed, many runways around the world consist of the original surface of the region without such compounds being layered on top. For example, in 1969, Boeing had to develop the ‘Unpaved Lane Kit’ to allow the first generation 737s to land on gravel landing strips.
Meanwhile, conditions in Antarctica mean that planes carrying research scientists there must use the blue ice runways. Due to the lower levels of friction on these surfaces, incoming aircraft rely on the use of reverse thrust to stop themselves on the runway.
Located in Scotland’s Outer Hebrides, Barra airport deserves respect for its unique runway structure. Specifically, it is the only airport in the world where planes land on a beach.