Nokuthula Mazibuko, SANRAL
PAPER
SPEAKER PRESENTATION
Abstract
Flushing of a chip seal occurs when the binder rises to the top of the seal layer relative to the stone, mainly due to the penetration of the seal aggregate into soft substate, orientation of the stone, binder rise, over-application of binder, or use of soft binder. A flushed seal poses a safety risk to the road user due to the loss of macro-texture of the surfacing, which plays a key role in providing skid resistance in wet conditions.
On the N2 section 9, the surfacing was treated with texture slurry and resealed using a 20/7 double seal with a S-E1 polymer modified binder in both the tack coat and penetration coat, and with the final application of a diluted cat 65 spray grade emulsion. During an extreme hot spell, severe bleeding with bitumen bubbles appeared in certain sections of the road, which also caused pick- up of the seal in the wheel tracks.
The phenomenon of binder rise with bubbles was first reported in 1972 and later investigated in New Zealand. The investigations concluded that the small bubbles appearing even outside the wheel path were not caused by volatiles. The bitumen bubbles were caused by vaporisation of moisture in the base or at the bottom of the seal, which either enters the layer from above or the bottom layer was saturated prior the seal being placed.
This study presents the investigation process and results of moisture contents obtained in good and failed areas. It further highlights the effects of vaporisation due to increases in moisture content and temperature in the cores extracted from the abovementioned road section. The results will assist in improving the current specification by providing a measurable parameter before seal work can commence after rain (COTO 2020, A10.1.3.5).
About the author
Nokuthula Mazibuko is a Candidate Engineer with SANRAL, based in the Gqeberha office. Her current focus is preliminary assessment and design work for the R62 section 7 between Joubertina and Kareedown in the Eastern Cape. Nokuthula began her career as a Graduate Engineer at Aurecon on completing her Bachelorās Degree in 2014 and gained experience in various locations, including 18 months on the site of the SKA project in Carnarvon working on engineering solutions for road and drainage structures. Nokuthula graduated cum laude with a Masterās in Civil Engineering at the University of Stellenbosch in 2020.