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Bituminous pavement materials have low viscosity at elevated temperatures. This allows effective compaction during the laying process. In laying unchipped materials compaction is necessary to densify the mix whereas in laying chipped materials compaction is necessary to embed the chippings and also densify the matrix. Since wearing courses are normally 40 mm thick the time available for these aims to be met is limited. This time period is a function of the hot‐mix and underlying material properties and ambient parameters prevailing when it is laid.