Wednesday, 1 October 2014
SprayLine has been constructing double/double primerseals with crumb rubber modification on a variety of sites in Victoria. It is not typical practice to use two-layer primerseals, or polymer modification in primerseals, and methods to do so are not described by Austroads specification or guidance documents.
Austroads engaged the ARRB Group to inspect seven double/double primerseal sites in Victoria and rate their performance.
The broad concept of double/double primerseals was adapted by Sprayline from shoulder sealing practices observed in South Australia.
These were typically 14/7 or 16/7 sized primerseals that were placed on a variety of reasonable quality prepared pavements (sandstones, crushed rock, black sand, etc.), and used relatively low binder application rates. The binder typically used was cutback C170 with two parts of cutter for the first, larger aggregate size layer, and four parts for a second, smaller aggregate layer.
The seals were designed using the Update of the Austroads Sprayed Seal Design Method for double/double seals, or Update of Double/Double Design for Austroads Sprayed Seal Design Methods typically with
+0.1 L/m2 allowance for absorption, and +0.1 L/m2 allowance for texture.
The seals are placed right across all lanes and shoulders of the road, so the seal type is consistent throughout and construction is uniform.
As with the historical shoulder treatments in South Australia, unmodified binders such as C170 have been used for double/double primerseals, and are used with a small amount of cutter.
The seals all use crumb rubber modified binders, to improve the hardiness and stress resistance of the seal compared to a non-modified binder. The concentration of rubber can be chosen depending on the level of stress resistance required.
The nature of having a thin seal directly on top of the pavement means that the quality and lifespan of the seal is strongly related to the soundness of the pavement on which it is being placed. A good understanding of the pavement quality and properties, and an accurate measure of the traffic the road will carry, will contribute greatly to creating a fitting seal design for the task, and the ongoing successful performance of the seal.
It is important to broom the first layer before laying the second and a correct level of cutter must be used that assists good adhesion to the pavement surface so this brooming can be performed.
The level of pavement dryback and moisture can be an important factor also. It is desirable for the bitumen to wet the pavement surface. A too-damp pavement may see the new seal being removed, or rolled up, under brooming, so it is preferable to tend towards a drier, ‘dusty’ pavement.
The double/double primerseals inspected were found to be performing well, and providing a viable option for an initial seal over a pavement where stresses can be expected to be higher than ideal for a typical single/single primerseal.
Read more in the report: Double/Double Primerseal Inspections