|MadSci Network: Engineering|
Joints are placed in sidewalks and most other large slabs of concrete to provide for the expansion and contraction of the concrete. Concrete is a brittle material; chemically it is a type of ceramic, and ceramics like glass or tile, do not stretch much before they break. Thus, if you don't put expansion joints in concrete at regular intervals, the concrete will break and form cracks wherever it needs to. We don't think about it, but each expansion joint is simply a crack in the concrete. When crude oil is refined, it is heated to high temperature, and most of the hydrocarbons are vaporized and turned into gasoline, motor oil, etc. A fraction of the crude oil consists of very large molecules, typically compounds with multiple ring structures. These compounds are black, tar-like materials which appear solid at room temperature, but soften as they are heated. This material is typically called asphalt. Sidewalks and streets are often covered with a layer of asphalt mixed with small stones which we typically think of as "asphalt." Since the hydrocarbon is much more expensive than the stone, the added stone reduces the cost of the mixture, plus it helps to give structure and strength to the mixture, with the asphalt binding everything together and making the surface water repellant. When a new street or sidewalk is paved, you might notice on a hot summer day that your kick stand from your bike will sink or make an impression into the asphalt paving. Although the asphalt appears pretty solid, it still retains some flexibility, particularly if it can move slowly. So, new asphalt can more easily expand and contract as it heats and cools, making expansion joints less necessary. I will let you in on one other little secret. Asphalt reacts with oxygen in the air and becomes more brittle with time, making older asphalt more likely to crack. The average asphalt road lasts about 7 years before it needs to be repaved, whereas the average concrete road lasts maybe three or four times longer. So, maybe expansion joints are not put into asphalt paving in part because it won't last long enough to justify their use.
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