When a roof is redone, the old shingles are often taken to a landfill and dumped. US highway agencies have investigated recycling asphalt shingles in pavement for about 25 years, initially using manufacturing waste and later examining shingles removed from existing roofs. These tear-off shingles are harder to reuse because they come from different rooftops and have weathered for years in sun and rain. Engineers therefore needed field evidence to assess how they would perform in real traffic conditions.
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