Thud, thud, thud, go automobiles over many a brand-new concrete highway which looks as smooth as a table. Reason for this phenomenon, which has exasperated many a driver, is the bulging of the substance filling the expansion joints between the big rectangles of concrete. Like most solids, concrete expands in hot weather, contracts in cold. Hence the joints require a resilient substance which will fill the gap completely no matter how the concrete behaves. Most substances tried by road-builders have had two faults—they bulged in hot weather, left openings for seepage in cold....
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