Semi-automatic evaluation of the degree of bitumen coverage on bitumen-coated aggregates

EN 12697-11 is the standard providing test methods for evaluating affinity between aggregate and bitumen and its influence on the susceptibility of the mixture to stripping. Among the methods, the rolling bottle test has a number of advantages in terms of rapidity, low costs, and suitability for routine testing. However, since affinity is assessed by visual registration of two independent operators, results may be altered by a large amount being inevitably subjective. The authors suggest a semi-automatic procedure to overcome potential limits and shortcomings of the method and obtain accurate results. Different mixtures were analyzed and compared, using common natural and recycled aggregates and a 70/100 pen bitumen. The procedure was successfully validated with a manual pixel inspection and confusion matrixes were created. The results showed that the procedure lead to a more reliable registration compared to the standard method and it is suitable, with different accuracies, for both light and dark aggregates.