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Microsurfacing and Cape Seals in the Darling Downs
microsurfacing. Microsurfacing is bituminous slurry surfacing containing poly modified emulsion binder, aggregate, GP cement, additives and water (Austroads 2018 has three known microsurfacing specialist contractors; Colas, Downer MICROSURFACING AND Microsurfacing is placed using a specialist Microsurfacing paving unit and can be placed in multiple layers. DTMR currently has a 3mm, 5mm, 7mm and 10mm microsurfacing mixes listed in its CAPE SEALS IN THE specification, MRTS13 Bituminous Slurry Surfacing (Department of Transport and Main Roads 201 DARLING DOWNS DISTRICTMicrosurfacing 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.
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.
Background
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
Fresh microsurfacing placed
Fresh microsurfacing placed relationship to crash rates, road user ride experience and vehicle travel costs and wear. The Department of Microsurfacing and Cape Seals Transport and Main Roads Darling Downs District has used microsurfacing An alternative treatment to address these pavement defects primarily as a rut filling, shape deformation filling or roughness treatment. DTMR Darling Downs of rutting and roughness is microsurfacing. Microsurfacing have also used microsurfacing and cape seals as a full width surfacing treatment. 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 a 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 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 628vpd, 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 120counts/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 vs a post-microsurfacing average of 89 counts/km. The treatment resulted in a 28.2% improvement in the average measured roughness. For context, a 50count/km is the Austroads standard for new roads signposted at 100km/hr (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
The Department of Transport and Main Roads Darling Downs District has used microsurfacing primarily as a rut filling, shape deformation filling or roughness treatment. DTMR Darling Downs A cape seal is the application of a Single/Single seal, followed by a microsurfacing layer. Usually a have also used microsurfacing and cape seals as a full width surfacing treatment. larger aggregate is used, for example a 14mm or 20mm (Austroads 2019). This process after its location where it was first implemented in the Cape Province of South Africa
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 628vpd, including 31% Heavy vehicles. In this section, continuous rutting and depressions were evident in the outer wheel path approximately 20 with limited other pavement defects in the locality. Roughness was recorded at an average of 120counts/km using the NAASRA (National Association of Australia State Road Authorities) roughness meter counts. Microsurfacing being placed in the Darling Downs District
Rutting on the Moonie Highway treated with microsurfacing
Microsurfacing being placed on the Moonie Highway
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 vs a post-microsurfacing average of 89 counts/km. The treatment resulted in a 28.2% improvement in the average measured roughness. For context, a 50count/km is the Austroads standard for new roads signposted at 100km/hr (Austroads 2018a). A full width reseal was completed over this section in March/April 2021.
Microsurfacing being placed on the Moonie Highway
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
Highway has an AADT of 1,450vpd, 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 133count/km, with a maximum roughness of
An infrequent failure mechanism experienced is this adherence issue occurring 190count/km. While these defects were significant, microsurfacing An infrequent failure mechanism experienced is this adherence issue occurring shortly after placement shortly after placement was completed with a 3-layer Microsurfacing Case Study #2 – Surat Development Rd treatment, including 2 narrower layers followed by a 3.6m full width layer. This section was then resealed 9 months later with a Two sections of Surat Developmental Road received microsurfacing treatment, which has an AADT of 265vpd, including 24.9% H robust geotextile seal to address eavy vehicles. This lower volume road exhibited fairly sound pavement, with predominately outer wheel path rutting. any crack migration. Roughness was reduced consistently Roughness averaged through this section at 105counts/km. through the treated section to 86 counts/km after microsurfacing. Microsurfacing, using three layers of 7mm mix, was comple Shape restored by microsurfacing treatment placed on the Surat Development Road This is a 35% improvement in ted in June 2019, with a combination of average roughness.full width and outer wheel path treatment. Here roughness was initially an average roughness of Shape restored by microsurfacing treatment placed on the Surat Development Road Shape restored by microsurfacing treatment placed on the Surat Development Road Case Studies: Microsurfacing 105counts/km, which after microsurfacing, reduced to an average of 79 counts/km resulting in a 24% reduction in roughness and Cape Seals as surfacing treatment ). A full width reseal was completed over this section in April 2021. The following case studies are included to investigate the scenarios in which microsurfacing and cape seals have been used as a surfacing treatment.
Final finish after three-layer treatment of microsurfacing on the Surat
Development Road
issue with the first layer of microsurfacing.
Microsurfacing Case Study #2 – Surat Development Rd
Two sections of Surat Developmental Road received microsurfacing treatment, which has an AADT of 265vpd, 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 105counts/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 105counts/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
Cape Seal Case Study - Cunningham Highway Warwick
This urban environment in the rural town of Warwick on the Cunningham Highway has AADT of 4,597vpd including 21.8% heavy vehicles. This section needed a suitable resurfacing treatment, as Final finish after three-layer treatment of microsurfacing on the Surat Development Road the current surfacing was at Final finish after three-layer treatment of microsurfacing on the Surat Microsurfacing Case Study #3 - Gore Highway Development Road the end of its life (17 years old). A cape seal was chosen after Microsurfacing Case Study #3 - Gore Highway This 1 km section of the Gore Highway has an AADT of 1,450vpd, including 43.35% some investigation, as a reseal heavy vehicles. was no longer suitable for this urban environment. The location has a history of noise concerns This 1 km section of the Gore Highway has an AADT of 1,450vpd, including 43.35% heavy vehicles. The pavement through this section was consistently exhibiting rutting, cracking, undulation and 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 and asphalt surfacing was not 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 affordable. There is also the issue opportunity to overlay due to flood heights. Rutting was approximately 40mm in depth and roughness averaged 133count/km, with a maximum roughness of 190count/km. While these defects of joining into driveways, services roughness averaged 133count/km, with a maximum roughness of 190count/km. While these defects were significant, microsurfacing was completed with a 3-layer treatment, including 2 narrower layers and kerbs in an overlay scenario. A cape seal was a new treatment for the Darling Downs District. The 2.5km section was resurfaced 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 robust geotextile seal to address any crack migration. Roughness was reduced consistently through the followed by a 3.6m full width layer. This section was then resealed 9 months later with a 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 treated section to 86 counts/km after microsurfacing. This is a roughness. with a cape seal in March 2020, 35% improvement in average roughness. with a 14mm S45R seal (crumb
Defects on the Gore Highway prior to microsurfacing treatment Defects on the Gore Highway prior to microsurfacing treatment Measured roughness before and after microsurfacing treatment on the Gore HighwayMeasured roughness before and after microsurfacing treatment on the Gore Highway
rubber binder) followed by a 7mm microsurfacing layer placed the same day. Cape seal being undertaken on the Cunningham Highway
Microsurfacing (surfacing treatment) Case Study - Warrego Highway
Sections on the Warrego Highway between Dalby and Chinchilla have an AADT of 3,195vpd, 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 of all traffic environments (approximate average of 1mm 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 Case Studies: Microsurfacing and Cape Seals as surfacing treatment standard box sizes attached to the microsurfacing paver. Dependant on the machine The following case studies are included to investigate the scenarios in which microsurfacing and utilised, width options that Darling Downs District have Case Studies: Microsurfacing and Cape Seals as surfacing treatmentcape seals have been used as a surfacing treatment. utilised include half box widths 1.5m and 1.8m, or full box width The following case studies are included to investigate the scenarios in which microsurfacing and Cape Seal Case Study - Cunningham Highway Warwick 3.2m and 3.6m.cape seals have been used as a surfacing treatment. This urban environment in the rural town of Warwick on the Cunningham Highway has AADT of • Microsurfacing works require sizeable stockpiles for Cape Seal Case Study - Cunningham Highway Warwick 4,597vpd including 21.8% heavy vehicles. This section needed a suitable resurfacing treatment, as aggregates, as productivity observed have reached 300-This urban environment in the rural town of Warwick on the Cunningham Highway has AADT of 4,597vpd including 21.8% 400m3/day. heavy vehicles. This section needed a suitable resurfacing treatment, as • 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. • The average cost of microsurfacing is approximately $700/m3 (all-inclusive rate), which does vary depending on the quantity, location and market factors.
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
Darling Downs District. The 2.5km section was resurfaced with a cape seal in March 2020, with a improve the reinstatement of crossfall. 14mm S45R seal (crumb rubber binder) followed by a 7mm microsurfacing layer placed the same - Microsurfacing treatments are limited in width by standard box sizes attached to the day. 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 productivi have reached 300-400m3/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. - The average cost of microsurfacing is approximately $700/m3 (all-inclusive rate), which does vary depending on the quantity, location and market factors. Cape seal being undertaken on the Cunningham Highway
Microsurfacing (surfacing treatment) Case Study - Warrego Highway Sections on the Warrego Highway between Dalby and Chinchilla have an 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 desi 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
Cape Seal being placed on the Cunningham Highway Warwick
seal application. These learnings will be incorporated in future surfacing treatments. Cape Seal being placed on the Cunningham Highway Warwick Overall, there have been several general learnings from microsurfacing and cape seal projects undertaken in Darling Downs District.
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