|Unsealed roads – A new approach|
A binder is a material used for the purpose of improving the properties of a subgrade or pavement material. In Australia, most binders used for road stabilisation are in a powder format and the main liquid binder is bitumen. In some situations, additives are used with binders to enhance the construction process or provide additional long term properties to the stabilised materials. For unsealed roads either a binder or liquid dust suppressants are used. Whilst some dust suppressants are known to provide short term solutions to reduce the generation of dust from moving vehicles, they are invariably applied from the surface and not mixed by a stabilisation machine. Applying dust suppressants from a moving water cart only benefits the supplier as more of their product is used compared to the appropriate application by a mixing process with the liquid incorporated inside the mixing chamber. The mixed dust suppressant is then compacted with rollers and trimmed to the appropriate crossfall.
Stabilisation binders are typically categorised in terms of their main constituent. AustStab categorises binders in the following format:
Cementitious: Consists of a wide range of cement and blends, and includes Portland (GP) and blended cements (GB) cements or contains a combination of pozzalan material and cement and/or lime , such as fly ash and lime and slag (ground granulated blast furnace slag) and lime
Lime: Hydrated lime [Ca(OH)2] or quicklime [CaO]. (Click here for more information)
Bitumen: Class C170 bitumen is typically used in stabilisation and in Australia either foamed bitumen or emulsion is used. Bitumen stabilisation may incorporate a supplementary binder such as lime or cement.
Chemical: Proprietary products primarily include polymers and salts.
In specifications, the binder is specified as a percentage of the mass of the stabilised material and not as a volume.
The increasing desire by communities to have all weather access to their unsealed roads results in innovative solutions by Shire engineers. The use of lime or cementitious binders for unsealed roads has gradually increased over the last decade in NSW and the practice has been adopted by over 20 Shires.
In 2007 AustStab received Federal funding to construct trials and measure the performance of stabilised unsealed roads using either lime or a synthetic insoluble powder polymer. These trials have been constructed in south west NSW and the following page provides some of the details to the trial and the ongoing performance. The title of the research project is Recycling unsealed roads to reduce dust & maintenance using insitu stabilisation. One of the great benefits of this project is that the existing road material is typically used and this places less reliance on using quarried material to replace the fine grain material that has been blown away by moving vehicles. Reducing dust on unsealed roads benefits both drivers and agriculture production adjacent to the roads.
The joint Federal Government and industry research project aims to:
• substantially reduce dust generation from unsealed roads
• reduce maintenance frequency and hence maintenance costs to Shires
• provide safe all weather access on unsealed roads
• incrementally improve the structural strength of the road
The Federal funding is from the Department of Infrastructure, Transport, Regional Development and Local Government under the AusLink low volume roads initiative. AustStab and its members are providing both financial and in-kind support to the development of the mix design , trials and performance study.
The binder mix design approach has been based on the binder type. If lime has been proposed for the road, the minimum application rate is determined using the lime demand test. Once this minimum has been established, use this application rate and plus 2% to carry out 28-day UCS testing to AS 1141.51 with standard compactive effort and 4 hour soak prior to testing. The target UCS value sought in the mix design is between 1.5 to 2.0 MPa . To ensure safe use of lime in the laboratory, please use hydrated lime with a minimum Available Lime Index (ALI) of 85%. Lime with a lower ALI may be used in a laboratory program provided the application rate can be adjusted by the contractor as shown in AustStab’s Lime Practice Notes (Click here for more information)
For cementitious binders, the UCS testing is carried out at 2 to 3 application rates typically starting at 2%. Similar to lime, a UCS value of 1.5 to 2.0 MPa is sought after 28-days curing.
Please note that all application rates are measured by a percentage by mass of the maximum dry density of the untreated sample. Do not use volumetric conversions for application rates.
When sampling the material please note that the minimum stabilisation depth is 150 mm and material at pavement and formation level will need to be extracted and combined in the laboratory to manage the mixing of these materials in the field in the same proportion.
The project is underway and this mix design summary may change during the project period based on some of the performance outcomes. A copy of the laboratory mix design report used as a guide can be downloaded here by clicking here (File size 1.7 Mb).
If you wish to confirm the current test method during the trial period, please contact AustStab by email. (NOTE: This service is limited to Shire staff and Australian engineers providing a consulting service to Shires.)
AustStab has developed a specification suite to the stabilisation of unsealed roads using lime, cementitious materials and synthetic insoluble dry powder polymers. To access these specifications, please contact the association by clicking here.
Please note that hydrated lime has been used in the laboratory and quicklime will typically be used on site. The application rate for quicklime is 0.76 times the rate for hydrated lime provided that the Available Lime Index is greater than 85% and 80% for the hydrated lime and quicklime respectively. Where this does not occur, the lime application rate is determined as detailed in the AustStab guidelines. Lime slurries is not recommended for this work.
At intersections or on tight curved roads, a doubling of the application rate is recommended to ensure sufficient surface durability due to turning and breaking traffic.
The construction of all the trial sites has been documented such that performance can be measured against what was constructed versus the design goals. A copy of the construction report can be downloaded here by clicking here (File size 3.03 Mb).
Five trials sites were constructed between the 30th May and 1st September 2008 and they will be monitored against a control section for several years. These trials are listed in the table below and the downloadable PDF files provides GPS coordinates and directions to get to the trial sites. When arriving at the start of the trial site, information about the binder and width of stabilisation can be found in a summarised A4 page attached the fence line.
|Town/Shire||Road||How to get there?||Trial Information|
|Griffith||Barber & McNamara Roads||Click here||Click here|
|Wombat (Harden)||Woodlands Road||Click here||Click here|
|Jerilderie||Old Corowa Road||Click here||Click here|
|Jerilderie||Four Corners Road||Click here||Click here|
|Temora||Back Mimosa Road||Click here||Click here|
Other trials are also being conducted in NSW and for information about these trials, please contact AustStab.
At this stage site inspections are being carried out every three months and this consists of the follwing attributes of the sites:
- Visual observations of any surface distress and logging the location and type of distress
- Surface profile at 4 to 5 sections and at the control section
- Rainfall data per month and documenting any flooding events
- Visual recording of the dust emitted from a vehicle passing at 80 kph
- Collection of loose material from the surface and PSD analysis
Loose stabilised material collected at two locations at each site are also being cured at 1, 3, 6 and 12 month durations to establish the long term strength gain of the binder.
A interim performance report is expected to be published in late 2009.
Reports and Presentations
Stabilisation of Unsealed Roads, IPWEA NSW Division Annual Conference 2010 presentation.
The unsealed roads research team consists of:
Research project manager: Greg White, Chief Executive Officer, AustStab
Financial manager: Warren Smith, President AustStab
Construction manager: Andrew Middleton, Stabilised Pavements of Australia
Mix design specialist: Peter Sheen, Testrite
AustStab would like to thank Griffith, Harden, Jerilderie and Temora Shires for their support in the trials and their cooperation in the development of the control sections at each site.
Contractors and binder suppliers who have provided financial and in kind support to the trials are:
Blue Circle Southern Cement
Downer EDI Works
Independent Lime &Cement
Stabilised Pavements of Australia
For general inquires about the trials, please contact Andrew Middleton by email or on 0408 021 272