Effect of concentration on the rate of formation of carbonyl compounds during oxidation of roading bitumen

Abstract

The formation of carbonyl compounds during oxidation of bitumen (asphalt) used in road surfacings was studied at 50 °C, with the object of determining the effect of bitumen reactive species concentration on the reaction rate. A reaction temperature of 50 °C was selected as a realistic in-service temperature. Solutions of bitumen in tetrachloroethene, decahydronapthalene and a mixture of the two solvents, at concentrations up to 89.7 g L−1 (5.9%), were oxidised under an atmosphere of pure oxygen at atmospheric pressure. The extent of the reaction was measured in terms of carbonyl group formation from the increase in area of the peak centred at 1698 cm−1 in the infrared spectrum. The reactions were carried out over a long time-frame to ensure that the true, slow (linear) stage had been reached. Because only the initial concentrations of bitumen were known, and the concentration of reactive species changed in an unknown way as the reaction proceeded, rates of oxidation were compared at the same relative extent of reaction so that the initial concentration ratios were preserved. The overall mean, apparent order of reaction with respect to bitumen concentration was 1.16 ± 0.03 and did not show any statistically significant difference due to the solvent used or any trend as the reaction proceeded from the fast (exponentially decreasing) to slow (linear) stages.