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In this task, you are given a question and a context passage. You have to answer the question based on the given passage.

What material did airport designers use to build the longer runways required by the increase in jet aircraft traffic?, Context: Airport construction boomed during the 1960s with the increase in jet aircraft traffic. Runways were extended out to 3,000 m (9,800 ft). The fields were constructed out of reinforced concrete using a slip-form machine that produces a continual slab with no disruptions along the length. The early 1960s also saw the introduction of jet bridge systems to modern airport terminals, an innovation which eliminated outdoor passenger boarding. These systems became commonplace in the United States by the 1970s.
Output:
reinforced concrete