Austroads' latest report describes the development of a laboratory sprayed seal binder cracking test which can be used to measure the relative fatigue cracking resistance of thin binder films.
Cracking is one of the major causes of deterioration of sprayed seals in Australia and New Zealand but there is no well‑established laboratory test which can be used to rank the low temperature cracking performance of different binders.
The binder films tested have a thicknesses comparable to the thicknesses of the binder films in sprayed seals (1–3 mm).
Initial studies involved optimising the experimental equipment and test conditions used in a prototype sprayed seal binder cracking test which was developed during Austroads project TT1820 Maximising the Performance of Sprayed Seals.
Once the test conditions were optimised, the results obtained in the test were verified by comparing the results obtained for a series of four bitumen samples and 12 polymer modified binders (PMBs) with the conventional asphalt fatigue life results obtained after each of the materials was incorporated into a single type of 10 mm dense graded asphalt mix. A very good correlation was found between sprayed seal cracking test fatigue life results and conventional asphalt fatigue life results.
The results obtained in sprayed seal binder cracking tests and asphalt fatigue tests during the study and Austroads project TT1823 Binder Characterisation Properties for Enhanced Performance were also compared with the results obtained in two prospective binder quality control tests (i.e. force ratio and stress ratio tests) which could be included in the Australian PMB specification in the future.
Based on these comparisons, both quality control tests appear to be suitable for ranking the low temperature cracking performance of Australian PMBs when they are used in sprayed sealing and asphalt applications. Stress ratio tests appear to be more versatile than force ratio tests as they are able to effectively characterise a wider range of Australian PMBs.
The results obtained in the study indicated that sprayed seal binder cracking tests were suitable for characterising the fatigue cracking performance of thin films of unaged (i.e. fresh) binders. Further work is needed to find suitable test conditions to characterise the properties of laboratory aged binders.
Report link: Development of a Sprayed Seal Binder Cracking Test