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Microsurfacing and Case Seals In The Darling Downs District
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 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
Fresh microsurfacing placed.
District has used microsurfacing primarily as a rut filling, shape deformation filling or roughness treatment. DTMR Darling Downs have also used microsurfacing and cape seals as a full width surfacing treatment.
A cape seal is the application of a Single/Single seal, followed by a microsurfacing layer. Usually a larger aggregate is used, for example a 14mm or 20mm (Austroads 2019). This process was named after its location where it was first implemented in the Cape Province of South Africa.
Case Studies: Microsurfacing as shape and roughness correction
The following case studies are included to investigate the scenarios in which microsurfacing has been used within the DTMR Darling Downs as a shape and roughness correction treatment.
Microsurfacing Case Study #1 - Moonie Highway
Located on the Moonie Highway 60km west of Dalby, this 13km section of road has an Annual Average Daily Traffic (AADT) of 628 vpd, including 31% heavy vehicles. In this section, continuous rutting and depressions were evident in the outer wheel path approximately 20-35mm in depth, with limited other pavement defects in the locality. Roughness was recorded at an average of 120 counts/km using the NAASRA (National Association of Australia State Road Authorities) roughness meter counts.
In April 2019, a 7mm mix microsurfacing treatment was placed in two layers to restore pavement shape and minimise roughness. The eastbound direction initially averaged roughness of 124 counts/km versus a post-microsurfacing average of 89 counts/km. The treatment resulted in a 28.2% improvement in the average measured roughness. For context, a 50 count/km is the Austroads standard for new roads signposted at 100km/hr
Microsurfacing being placed in the Darling Downs District.
Rutting on the Moonie Highway treated with microsurfacing.
(Austroads 2018a). A full width reseal was completed over this section in March/April 2021.
One isolated failure was observed in this section shortly after placement, which exhibited a crescent-moon shaped failure which was deemed an adherence issue with the first layer of microsurfacing.
Microsurfacing Case Study #2 – Surat Development Rd
Two sections of Surat Development Road received microsurfacing treatment, which has an AADT of 265 vpd, including 24.9% heavy vehicles. This lower volume road exhibited fairly sound pavement, with predominately outer wheel path rutting. Roughness averaged through this section at 105 counts/km.
Microsurfacing, using three layers of 7mm mix, was completed in June 2019, with a combination of full width and outer wheel path treatment. Here roughness was initially an average roughness of 105 counts/km, which after microsurfacing, reduced to an average of 79 counts/km resulting in a 24% reduction in roughness). A full width reseal was completed over this section in April 2021.
Microsurfacing Case Study #3 - Gore Highway
This 1 km section of the Gore Highway has an AADT of 1,450 vpd, including 43.35% heavy vehicles. The pavement through this section was consistently exhibiting rutting, cracking, undulation and pumping of fines. Investigations showed that this has a shallow white rock pavement, with no opportunity to overlay due to flood heights. Rutting was approximately 40mm in depth and roughness averaged 133 count/km, with a maximum roughness of 190 count/km. While these defects were significant, microsurfacing was completed with a 3-layer treatment, including 2 narrower layers followed by a 3.6m full width layer. This section was then resealed 9 months later with a
An infrequent failure mechanism experienced is this adherence issue occurring shortly after placement.
Shape restored by microsurfacing treatment placed on the Surat Development Road.
Final finish after three-layer treatment of microsurfacing on the Surat Development Road.
robust geotextile seal to address any crack migration. Roughness was reduced consistently through the treated section to 86 counts/km after microsurfacing. This is a 35% improvement in average roughness.
Case Studies: Microsurfacing and Cape Seals as surfacing treatment
The following case studies are included to investigate the scenarios in which microsurfacing and cape seals have been used as a surfacing treatment.
Cape Seal Case Study - Cunningham Highway Warwick
This urban environment in the rural town of Warwick on the Cunningham Highway has AADT of 4,597 vpd including 21.8% heavy vehicles. This section needed a suitable resurfacing treatment, as the current surfacing was at the end of its life (17 years old). A cape seal was chosen after some investigation, as a reseal was no longer suitable for this urban environment. The location has a history of noise concerns and asphalt surfacing was not affordable. There is also the issue of joining into driveways, services and kerbs in an overlay scenario. A cape seal was a new treatment for the Darling Downs District. The 2.5km section was resurfaced with a cape seal in March 2020, with a 14mm S45R seal (crumb rubber binder) followed by a 7mm microsurfacing layer placed the same day.
Microsurfacing (surfacing treatment) Case Study - Warrego Highway
Sections on the Warrego Highway between Dalby and Chinchilla have an AADT of 3,195 vpd, including 33.76% heavy vehicles. Here a freshly stripped seal was under speed reduction and needed intervention during winter. Due to widening works and the stripping seal, the surface texture presented exhibited varying textures, resulting in a more challenging seal design to be undertaken in cold weather. Microsurfacing was considered a lower risk alternative to a reseal and was undertaken with a single layer.
Learnings
Observations in the summer following placement on both surfacing case studies has shown migration of binder through the microsurfacing to the surface, resulting in the occasional flushed wheel path appearance. Learnings from these experiences include reducing the initial binder spray rate and selecting a binder with a higher softening point to reduce the risk of migration in a cape seal application. These learnings will be incorporated in future surfacing treatments.
Overall, there have been several general learnings from microsurfacing and cape seal projects undertaken in Darling Downs District.
• Due to the nature of microsurfacing operations, this treatment provides best value for money in long continuous sections, for example full lane width for the length of a traffic setup. • Surface texture appears suitable for all traffic environments (approximate average of 1mm
Defects on the Gore Highway prior to microsurfacing treatment.
Measured roughness before and after microsurfacing treatment on the Gore Highway.
Cape seal being undertaken on the Cunningham Highway.
of texture measured), which is achieved by a hessian drag as a part of the microsurfacing operation. The surface texture is affected when a single layer is placed too thick, causing larger particles to settle and finer material to makeup the surface material. To mitigate this issue, multiple passes are recommended when rut depth increases. Multiple passes will also improve the reinstatement of crossfall.
• Microsurfacing treatments are limited in width by standard box sizes attached to the microsurfacing paver.
Dependant on the machine utilised, width options that
Darling Downs District have utilised include half box widths 1.5m and 1.8m, or full box width 3.2m and 3.6m.
• Microsurfacing works require sizeable stockpiles for aggregates, as productivity observed has reached 300400m3/day. • To ensure a neat finished product, field application includes the start of the run to be taped and end protected by plastic taped down. This allows for any excess material to be removed at the end of the run.
• Microsurfacing is best used over stable pavement where cracking is limited. If completed over cracked pavements, this has the potential to reflect through the microsurfacing layer over time, especially if cracks are mobile.
Acknowledgements
Department of Transport and Main Roads Darling Downs District Delivery Team
References
• Austroads. (2019). Guide to
Pavement Technology Part 4K, 1.3 Edition. Sydney: Austroads
Ltd
• Austroads. (2018b). Guidelines and Specifications for
Microsurfacing, 1.1 Edition.
Sydney: Austroads Ltd • Austroads. (2018a). Guide to
Asset Management – Technical
Information, Part 15: Technical Supplements, 3rd Edition.
Sydney: Austroads Ltd • Department of Transport and
Main Roads. (2018) MRTS13
Bituminous Slurry Surfacing,
Queensland, The State of
Queensland
ABOUT THE AUTHOR
A Senior Civil Engineer and Project Manager, Bernie-Anne Freeman has developed her technical engineering and leadership skills working on a range of design, construction and maintenance infrastructure projects. She currently enjoys leading the Darling Downs road resurfacings team at the Queensland Department of Transport and Main Roads where she designs and delivers pavement maintenance, asphalt, microsurfacing and sprayed seal works. Bernie-Anne has been selected for a global leadership program for women in STEM which will take her to Antarctica.
