Do modified porous asphalt mixtures have a longer service life?

The service life of 26 Porous Asphalt (PA) test sections laid on three locations in the late eighties and early nineties on Dutch Motorways is investigated. The bitumens used were bitumen 70/100 and bitumens modified with EVA, SBS, rubber, drainage inhibitors and chemical modified bitumen. The reference, Dutch standard PA, contains 4.5% pure bitumen 70/100. PA mixtures with polymer modified bitumen (PMB) were investigated with 4.5% and 5.5% bitumen (nominal, m/m on 100% mineral). The end of service life of PA is caused by loss of stones from the surface, called ravelling. To investigate this the test sections were monitored (laboratory tests and visual inspections). The used laboratory tests (Cantabro Test, Indirect Tensile Test and Cyclic Direct Tensile Test) were not suitable to explain the behaviour of PA in the test sections. The use of 4.5% PMB seems to give no life extension in comparison with standard PA. Modifications allow the use of a higher bitumen content without segregation problems, which in general increases the service life. PA with 5.5% bitumen (PMB or pure 70/100 with drainage inhibitors) generally acts better. To gain insight in the ravelling phenomenon first steps are made to develop a model which describes ravelling of PA mixtures. In this paper some theoretical backgrounds are given.