Maintaining local roads is a perennial challenge for councils. Here, AUS-SPEC Manager Nandini Mehta outlines four treatment options to prolong the lifespan of pavement.
Sprayed preservation surfacing treatments are generally applied to low-volume sealed roads where primary distress is caused by environmental factors from binder oxidation. If road surfaces are left untreated, microcracking and potholes can develop. This could result in localised pavement failure.
A need was identified at various IPWEA events to develop a generic specification for pavement preservation treatments to help councils effectively maintain their inventory of road assets. In response, AUS-SPEC, the national local government specification system, in collaboration with IPWEA and industry partners, has released a new specification and technical note:
AUS-SPEC Specification
1147 Sprayed Preservation Surfacing
Description: A generic specification for the supply of materials and application of sprayed preservation treatments to prolong the life of existing wearing pavement surfaces.
AUS-SPEC TECHnote
GEN025 Sprayed Preservation Surfacing Treatments
Description: A technical note to determine when and where to apply sprayed preservation surfacing treatments to extend the life of sealed road networks.
Why does pavement decay?
Pavements deteriorate for many reasons, but predominantly from lifetime vehicle loading and environmental elements. Council roads are generally lightly trafficked and surfaced with either sprayed seals or asphalt, or unsealed wearing courses. Any deterioration of bitumen roads is usually due to the breakdown of the surfacing, primarily caused by oxidation and seal cracking. This permits water ingress to the base gravel, causing material decay.
Prevention practices
Early failures can be due to a breakdown in the construction process of materials, site preparation or placement practices. These issues can be avoided if construction methods, placement techniques and base gravel materials conform to proper specifications, like those available from AUS-SPEC.
Quality construction and material practices apply to all types of pavements. For flexible sealed pavements, quality construction and material practices include sprayed seals, slurry seals, microsurfacing, enrichments, asphalt overlay and pavement preservation treatments. Improved construction methods, appropriate treatment methods and proper specifications can all contribute towards improved pavement performance, safer roads and delayed (usually costly) rehabilitation.
Planned pavement preservation will reduce ageing, increase waterproofing and restore serviceability without decreasing pavement capacity or strength. The net result is lower whole-of-life costs.
Types of treatment
In local roads the standard preservation treatment is an aggregate reseal. Local roads generally see low volume traffic and usually suffer from oxidation of the bitumen binder rather than polishing of the aggregate, which tends to manifest on high volume roads as the limiting wear factor.
The standard reseal intervention for wetter and cooler climates is a maximum of 12 years and 15 years for dry climates. In wet climates the bitumen seal is more exposed to cracking defects and potholing.
To protect and prolong pavement life, the following four sprayed preservation treatments should be considered by councils for low volume roads:
Treatment type 1: enrichment
This is a sprayed treatment that incorporates bitumen and proprietary additives being applied to bituminous surfacing in order to provide a protective barrier against oxidation. Enrichment treatments are typically non-sand filled. The nominal residual application rate range is 0.30 to 0.60l/m2 . Slow and medium setting grades of emulsion may be diluted with water to improve coverage and flow between aggregate particles.
Enrichment should be used on a structurally sound 14mm or 20mm single seal because the process adds a little bitumen on each cycle, which eventually leads to flushing. More enrichment cycles are possible if a 20mm single seal is initially selected. However, note that a 20mm aggregate single seal requires more binder and produces more road noise.
Pavement life cycle with treatment strategies:
Wentworth Shire Council in Western NSW, with its hot dry climate, does spray runs up to 12km by half-road on very hot days, then return half-road runs three hours later when the emulsion has dried. This process is repeated every five years up to three times, with a full reseal in year 20.
Treatment type 2: rejuvenation
A similar process to enrichment, rejuvenation is generally applied to local asphalt roads at a rate of 0.35-0.50l/m2. Binders are proprietary products and typically water-based. The first application is intended to be applied before year eight to maintain the viscosity and elastic properties of the existing binder and halt the ageing of the bitumen binder. The process should only be used where the existing road pavement is structurally sound.
Site selection is important as this treatment might reduce road surface friction.
According to Steve Paff of Tweed Shire Council, some councils use rejuvenation products that do not improve the asphalt condition visually. However, these products are selected to slow the rate of deterioration, ravelling and binder oxidation.
Tweed Shire Council applied rejuvenation treatment on a section of road at Flametree Terrace, Banora Point. This road was built as part of a subdivision estate in 1984. The present (2018) condition of the road, 34 years later, is depicted in the photo below:
As can be seen in the present-day photo (above) the cumulative effect of these treatments has resulted in an asphalt surface that is still quite good. This is a great result considering the asphalt is 34 years old, which is double (or an extra 10 to 15 years past) the normal replacement life of most asphalt surfaces.
By comparison, another section of the road (below) left out of the 1996 and 2013 treatments shows more evidence of deterioration.
Another benefit of rejuvenation is the reduction in permeability of the asphalt surface, which further safeguards against material decay.
Treatment type 3: polymer modified emastic (PME)
PME is typically water-based and composed of mineral fillers combined with a bituminous material. It is applied at a nominal rate of 1.00 l/m2 . PMEs are generally applied to asphalt 60 surfaces from year eight to replace the fine aggregate matrix lost from the surface as a result of binder oxidation.
When applied to spray seals, PME treatments also help prevent aggregate loss and improve surface texture. This reduces traffic noise in residential areas.
To date, Flametree Terrace has benefitted from the following proprietary treatments:
Treatment type 4: microsurfacing
This treatment is appropriate for inhibiting ravelling and oxidation, as well as improving surface friction, filling minor irregularities and rutting. It also provides for greater durability and flexibility.
In urban subdivisions it is suitable as a pavement treatment, especially so for improving road shape and ride, or as an alternative resurfacing option to spray seals.
Councils wanting to prolong the life of their pavements through microsurfacing can use:
AUS-SPEC Specification
1146 Slurry Seals and Microsurfacing Description: A generic specification to prolong pavement life through standardised design, supply, mixing and placement of slurry seals, surface correction and wearing surface applications on road pavements, carparks, cycleways and footpaths.
Summary
To achieve the best outcome, appropriate surface preservation treatments should be selected based on the condition and age of the pavement. Councils can use AUS-SPEC generic specification and technical documents to apply the right treatment at the right time to both preserve and prolong the life of their road assets.
For country councils in particular, the use of enrichment and rejuvenation of bitumen binder can offset the detrimental effects of using high plasticity base gravels with adverse shrinkage cracking.
About AUS-SPEC
AUS-SPEC is the national specification system for the design, construction, maintenance and operation of local government assets. It is developed by industry, for industry, and is managed by NATSPEC, a national not-for-profit organisation partly owned by IPWEA.
AUS-SPEC comprises various specification packages designed to promote uniformity and best practice in local government asset management. To learn how your local council can save money, improve quality and reduce risk, visit
www.natspec.com.au today.
The article was first published in the July/August edition of inspire magazine. Read the original and more here. #AssetManagement#RoadConstructionMaintenance#RuralEngineering