Microsurfacing and Cape Seals in the Darling Downs District

Microsurfacing and Cape Seals in the
Darling Downs District
Author 1 (Bernie-Anne, Freeman)
Department of Transport and Main Roads

Abstract:
Recently the Department of Transport and Main Roads Darling Downs District have been
undertaking alternative prior works and sealing treatments, in order to provide value for money and
sustainable treatments on our 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, we have undertaken a cape
seal. A Cape seal is a Single/Single seal (usually using a 14mm or 20mm aggregate) followed by a
microsurfacing layer. This treatment has benefits when restricted by height in an urban overlay
situation. 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 quantity
required.

Keywords:       Microsurfacing, Cape Seals, Reseal, Road Surfacing

Background

Rutting and roughness are two defects evident on the Department of Transport and Main Roads
Darling Downs District’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 our
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
standpoint. Water ponding on the road surface has the potential to result in aquaplaning, causing a
safety issue. Water ponding in road wheel paths, are subject to repetitive pressure from vehicle
tyres. This pressure can cause cracks to start to form, allowing further water ingress into the
pavement leading to deterioration. Addressing road roughness is of interest, due to its relationship
to crash rate, ride experience for the travelling public 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 per February 2021,
Australia has three known microsurfacing specialist contractors, being Colas, Downer and Fulton
Hogan. Microsurfacing is placed using a specialist microsurfacing unit and can be placed in multiple
layers. A 5mm, 7mm and 10mm microsurfacing mixes are available. 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 (DTMR) 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 microsurfacing layer. Usually a
larger aggregate is used, for example a 14mm or 20mm (Austroads 2019). This process is 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 investigate the scenarios in which microsurfacing have been
used within the Darling Downs District, 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 are evident in the outer wheel path of about 20-35mm depth. Very limited
other pavement defects exist. Roughness is 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 placed in two layers was used to restore
pavement shape and minimise roughness. In the eastbound direction initially average of 124
counts/km vs after microsurfacing, average of 89 counts/km (28.2% improvement in average
roughness). For some context 50count/km is the Austroads standard for new roads, signposted at
100km/hr (Austroads 2018a). A full width reseal is planned over this section in March 2021.
One isolated failure was observed in this section shortly after placement, which exhibited a crescent-
moon shaped failure which deemed an adherence issue with the first layer of microsurfacing.
Microsurfacing Case Study #2 – Surat Development Rd
Two sections on 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 here in June 2019, with a
combination of full width and outer wheel path treatment. Here roughness was initially an average
roughness of 105counts/km and was reduced after microsurfacing treatment to an average of 79
counts/km, a 24% reduction in roughness.
Microsurfacing Case Study #3 - Gore Highway
This 1 km section on the Gore Highway, has an AADT of 1450vpd, including 43.35% heavy vehicles.
Pavement through this section is consistently exhibiting rutting, cracking, undulation, pumping fines.
Investigations show this is a shallow white rock pavement, with no opportunity to overlay due to
flood heights. Rutting is approximately 40mm in depth and average roughness is 133count/km, with
a maximum roughness of 190count/km. While these defects were significant, microsurfacing was
completed here 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
migrating cracks. Roughness was reduced consistently through the treated section with initial
average roughness of 133 counts/km reducing to 86 counts/km after microsurfacing.
Case Studies: Microsurfacing as surfacing treatment
The following case studies are included investigate the scenarios in which microsurfacing and cape
seals have been used within the Darling Downs District 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
4597vpd including 21.8% heavy vehicles. This section needed a suitable resurfacing treatment, as
the current seal was at end of life. A cape seal was recommended after some investigation, after a
reseal was no longer suitable for this urban environment with a history of noise concerns, and
asphalt surfacing was not recommended due to price and issues with 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 3,195vpd,
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 stripped seal, the surface texture
presented exhibited varying textures, resulting in a more challenging seal design to undertake in cold
weather. Microsurfacing was considered a lower risk alternative to a reseal and was undertaken with
a single layer placed.
Learnings
Observations in the summer following placement on both surfacing case studies, have shown
isolated 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 spray

rate of binder and selecting a binder with a higher softening point to reduce risk of migration in a
cape seal application.
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 (approx. average of 1mm of
        texture measured), which is contributed by the 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 closer to a straight.
    -   Microsurfacing treatments are limited in width by standard box sizes attached to the
        microsurfacing unit. Dependant on machine utilised, width options that Darling Downs
        District have utilised include half box widths 1.5m, 1.8m, or full box width 3.2m and 3.6m.
    -   Microsurfacing works requires sizeable stockpiles for aggregates, as productivity observed
        has reached 300-400m3/day.
    -   To ensure a neat finish product, field application includes the start of the run to be taped,
        and end protected by plastic taped down, so at the end of the run any excess material can
        be removed without inference with underlying pavement.
    -   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.
    -   Average cost of microsurfacing is approximately $690/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