The technical literature up to 1994 dealing with the use of antioxidants to decrease the rate of age hardening in bitumen used for roading chipseals was reviewed. Laboratory studies showed that in some cases bitumen treated with antioxidants have reduced hardening rates due to oxidation compared to control bitumens.
Much of the work reported does not adequately consider interfering effects such as volatisation or dispersion of the antioxidants during mixing and accelerated aging. The additives' initial effect on bitumen viscosity is also often not properly taken into account. Virtually no research has been done on the chemistry of the antioxidant-bitumen system to attempt to understand why some additives are more effective than others or to explain the general lack of effectiveness compared to antioxidants in other materials subject to autoxidation.
Lead dialkyl dithiocarbamate (LDADC) compounds showed promise in laboratory studies. One of these compounds was used in field trials in Australia, and after five years the rate of hardening was reduced by 31% at 5% concentration. This reduction was translated to an estimated increase in seal life of 64%, or seven years. In laboratory studies hydrated lime (Ca(OH)2) has decreased the rate of oxidation and viscosity increase of roading bitumen, but its effect appeared to be limited in the field.
Basic research on the chemistry of bitumen antioxidant systems is lacking even though, given the potential benefits of increased seal life, such research would be justified.
Keywords: Aging, antioxidants, autoxidation, bitumen, chemistry, hardening, New Zealand, roads