Spraying behaviour of polymer and crumb rubber modified binders (NACoE project P111)

Bitumen sprayers in Australia are designed to apply a satisfactory transverse distribution of binder across the pavement from a series of nozzles. These sprayers are calibrated annually to assure this transverse distribution is being achieved, and the calibration is often undertaken with an oil at ambient temperatures that mimics the properties of a conventional bitumen at spraying temperatures. However, the use of polymer modified binders (PMBs) including crumb rubber modified (CRM) binders is increasing in sprayed seals and the effects of these higher viscosity binders on the transverse distribution from bitumen sprayers are unknown. Anecdotal evidence indicates that construction issues such as ‘tramlining’, where the sprayed bituminous binder resembles sheets of corrugated iron, is known to occur with CRM binders. An experimental program has been undertaken as part of the NACoE research program to explore the viscosities and spraying behaviour of PMB and CRM binders from a single nozzle, and to compare them to conventional bitumen (and related calibration fluids). The output of different single nozzles was measured in the laboratory to assess the potential effect of the higher viscosity binders on transverse distribution. This work developed a relationship between the viscosity of the binder and the spray width out of a single nozzle for homogenous binders such as bitumen and PMB, whilst CRM binders featuring suspended rubber in the binder behaved differently and did not fit this relationship. This paper will present the results of the experimental work, and related simulations of bitumen sprayer outputs that demonstrates the implications of these findings on transverse distribution.