Weathering of road tar. I

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Abstract
Crude tars from six sources have been used in a full‐scale experiment to determine the physical and chemical changes that occur in road tar used in wearing‐course macadam. Samples taken from the road at intervals up to two years have been studied in the laboratory. It has been shown that when tars weather, there is an increase in viscosity due to two main processes, evaporation and oxidation. In the oxidation mechanism, ‘C2 resin’, soluble in pyridine but not in benzene, is formed from material of lower molecular weight; it increases both the viscosity and the temperature coefficient of viscosity, so that in cold weather very high viscosities are attained. In the case of lower‐gravity tars, evaporation and oxidation account for similar amounts of hardening, but with coke‐oven tars, evaporation predominates.

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