TECHNICAL
MICROSURFACING AND CAPE SEALS IN THE DARLING DOWNS DISTRICT
Bernie-Anne Freeman Department of Transport and Main Roads
Recently the Department of Transport and Main Roads Darling Downs District (DTMR) have undertaken alternative prior works and sealing treatments to provide value for money and sustainable outcomes on the Department’s road network. Microsurfacing has been utilised as a rutting filling, shape correcting and roughness improvement treatment prior to resealing works, as well as a full width surfacing treatment. Also, as an alternative surfacing treatment, DTMR has also completed some cape seals. A cape seal is a Single/Single seal (usually using a 14mm or larger aggregate) followed by a microsurfacing layer. This treatment has benefits when restricted by height in an urban overlay situation due to its thickness. As a non-structural surfacing alternative to asphalt, microsurfacing provides adequate surface texture for all speed environments, at a reduced depth, resulting in less material required. Rutting and roughness are two defects commonly found on the Department’s road network. Rutting is a longitudinal depression on the pavement
Fresh microsurfacing placed.
surface usually in the wheel path. Roughness is irregularities from the intended longitudinal profile of the road surface (Austroads 2018a). Roughness can be considered in two categories: low amplitude, high frequency roughness (‘chattery’) or high amplitude, low frequency roughness (characteristic of black soils). Aside from rutting and roughness being measured road characteristics reportable for asset review, addressing rutting defects is in the interest of road authorities from a safety and asset preservation perspective. Water ponding on the road surface has the potential to result in aquaplaning, causing a safety issue. Water ponding in wheel paths is subject to repetitive pressure from tyres, which may cause cracks to form, allowing further water ingress into the pavement leading to deterioration. Addressing road roughness is of interest, due to its relationship to crash rates, road user ride experience and vehicle travel costs and wear.
Microsurfacing and Cape Seals An alternative treatment to address these pavement defects of rutting and roughness is microsurfacing. Microsurfacing is bituminous slurry surfacing containing poly modified emulsion binder, aggregate, GP cement, additives and water (Austroads 2018b). As of February 2021, Australia has three known microsurfacing specialist contractors: Colas, Downer and Fulton Hogan. Microsurfacing is placed using a specialist Microsurfacing paving unit and can be placed in multiple layers. DTMR currently has 3mm, 5mm, 7mm and 10mm microsurfacing mixes listed in its specification, MRTS13 Bituminous Slurry Surfacing (Department of Transport and Main Roads 2018). Microsurfacing differs from slurry seals, as microsurfacing uses polymer modified emulsion bitumen, larger aggregates and is placed in layers, to ensure suitability for heavier trafficked applications. The Department of Transport and Main Roads Darling Downs
ENGINEERING FOR PUBLIC WORKS | JUNE 2021
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