The recycling and stabilising process
Recycling road pavement is the process of mixing existing road material insitu with a road stabiliser machine whilst incorporating moisture if required, then re-compacting the material to improve the road profile and road performance.
Stabilising is a form of recycling, but also incorporates an additive to provide additional strength and lasting quality to the insitu materials. Stabilising increases the life of the pavement material which can reduce the road’s long-term maintenance costs and provide a safer road surface for motorists.
Existing materials can be modified with 1 – 6% additive, providing a cost effective alternative to importing other pavement material. This allows for larger areas of road to be widened and/or rehabilitated from the same funding allocation.
Additives on stabilising projects include cement, cement/slag blends, cement/flyash blends, cement/flyash/lime blends, hydrated lime, quick lime and foamed bitumen.
During preconstruction, tests are performed on the material to be stabilised, determining which additive and application rate should be used. This determines which additive will maximise the quality of the pavement ensuring longer lasting life of the asset.
Road recycling is a process that has been widely adopted for rehabilitation of road assets throughout the pavement industry. Road recycling is not only a cost efficient process but generally offers construction time savings and sustainable solutions.
Environmental benefits of road recycling often include the following aspects:
- Significant reduction in the use of virgin quarry materials
- Minimal generation of construction waste through the use of insitu material
- Reduced trucking movements as a result of limited importing or exporting of materials form site
- Reduction in emissions, energy use and noise due to shorter construction duration
Plant-Mixed Pavement Layers Stabilised using Foamed Bitumen
Foamed bitumen stabilisation (FBS) involves insitu or plant mix stabilisation of pavement materials with bitumen as the primary binder. The process is used to improve the strength of granular materials while retaining a flexible pavement. The foamed bitumen content generally ranges from 3% to 4% by mass of the material being bound. Lime is added as a secondary binder to provide initial strength, improve the strength of the mix and help disperse bitumen through the mix. Secondary binders other than lime are not permissible
Transport and Main Roads Specification MRTS09 is about the stabilised pavement layer using foamed bitumen. It comprises a mixture of unbound granular pavement material, foamed bitumen, and a secondary stabilising agent that is placed in a single layer with a thickness that is no less than 125 mm and no more than 250 mm.
Suitability of the application
Foamed bitumen stabilisation is suitable for use in the following situations:
- In a weak granular pavement to improve strength.
- Rehabilitation of previously cementitious stabilised pavements where the addition of further cementitious binder is not feasible.
- An alternative to full depth asphalt in low to moderate trafficked roads.
- Improving a pavement material’s resistance to moisture effects
Consider the following factors before selecting the foamed bitumen stabilisation option:
- Cost – Depending on binder content the initial cost is higher than cementitious stabilisation but lower than asphalt.
- Pavement design for heavily trafficked roads – For heavily trafficked roads a very thick pavement is required. Incorporating an asphalt overlay will reduce the thickness of the FBS course.
- Utility depth – As a thick pavement is required for heavily trafficked roads, utility depth should be considered during the pavement option selection. The current design model results in a thicker pavement when compared to asphalt.
- Vibration – For the compaction of insitu stabilised pavements, heavy vibratory rollers are required for deep lifts. In areas where structures are adjacent to the pavement, vibratory compaction may not be acceptable.
- Moisture – Where the insitu moisture content of the unstabilised pavement course is greater than 70% of the unstabilised material optimum moisture content (OMC), compaction of the FBS material is difficult and the specified compaction limits may not be achievable.
- Plasticity and particle size distribution (PSD) of the granular material – There are requirements for maximum Plasticity and PSD of the material to be stabilised. Refer to relevant specification.
- Existing bound pavement – The preferred practice for stabilisation of an existing bound layer is to prepulverise the full thickness of the bound layer to prevent reflective cracking in the FBS course. If the thickness of the FBS course is less than the pulverised depth, a suitable work practice is required to ensure full compaction of the pulverised material.
Benefits of Foamed Bitumen Stabilisation
- Moisture resistant pavement – offers better resilience to flooding.
- Strong and flexible pavement – foamed bitumen improves the stiffness and load bearing capacity.
- Reduces shrinkage cracking.
- Better fatigue resistance than using a cement stabilised base.
- Using lime as a secondary additive provides longer working times during construction.
Some useful terms related to this specification are as below:
ESA (Equivalent Standard Axles): Standard Axles is a single axle with dual wheels loaded to a total mass of 8.2 tonne (80 kN) and with tyres inflated to a pressure of 750 kPa.
- Foamed bitumen: Class 170 bitumen which is temporarily brought into a foamed state by the addition of water and foaming agent(s).
- Foaming agent: A chemical additive added to bitumen to improve its foaming characteristics.
- Hydrated Lime: It is included in the stabilisation process to improve the dispersion of the foamed bitumen and increase the early strength of the stabilised material. Hydrated Lime is a granular form of lime consisting primarily of calcium hydroxide (Ca(OH)₂
- Reclaimed materials: Unbound gravel that has been milled or excavated from existing pavement from job Site and screened which could be reincorporated into the foamed bitumen stabilisation process.
- Reclaimed Asphalt Pavement (RAP): Asphalt that has been milled or excavated from existing asphalt pavements, or returned from job sites.
- Half life: A term used to define the expansion properties of the bituminous stabilising agent. It is defined as the time taken for the foamed bitumen to settle to one half of the maximum volume of the bitumen in its foamed state. It is measured from the start of bitumen foaming. Half-life is the time taken (measured in seconds) for the foamed bitumen to settle to one half of the maximum expansion volume.
Some of the documents for more detailed information is listed below:
- Transport and Main Roads Specifications MRTS09 Plant-Mixed Pavement Layers Stabilised using Foamed Bitumen
- Testing of Materials for Foamed Bitumen Stabilisation – Technical Note
Some information below related to foamed bitumen pavement stabilisation which might be good to know:
- Foamed bitumen stabilisation is the process of mixing bitumen (in a foamed state) as the primary stabilising agent and a secondary stabilising agent usually hydrated lime and sometimes blended cement. The purpose of the bitumen is to achieve a strong yet flexible pavement layer compared to other stabilisation treatments.
Generally the aim is to provide a modified material with the following properties:
- Initial modulus > 500 to 700 MPa, to provide enough strength for early trafficking
- three day cured modulus > 2500 to 4000 MPa, to provide a strong but flexible pavement
- three day soaked modulus > 1500 to 2000 MPa, to provide a strong but flexible pavement able to withstand inundation
- retained modulus > 0.5, to provide a strong flexible pavement that does not lose greater than 50% of strength when inundated
- Foamed bitumen stabilisation is a road construction technique whereby bitumen is used to bind the existing or imported granular material to produce a flexible pavement material for use in base and subbase pavement layers, and in particular for road rehabilitation.
Foamed bitumen is a mixture of hot bitumen, water and air. For instance, in Queensland typical quantities are 97.5% bitumen, 2% water and 0.5% foaming agent. The foaming agent is added to improve the quality of the foam. When hot bitumen (160 to 200 °C) comes in contact with cold water (15 to 25 °C) the mixture expands to greater than 10 times its original volume and forms a fine mist or foam.
The foamed bitumen is sprayed into the mixing drum where it coats the surface of the some fine particles (typically less than 0.075 mm in diameter), making agglomerations of loosely packed mortar that adhere to the larger particles. Foamed bitumen mixes are generally regarded as cold-mixes, as they are placed and compacted at ambient temperatures.
It is recommended to refer to Austroads Technical Report – Review of Foamed Bitumen Stabilisation Mix Design Methods
Some other documents which are related to foamed bitumen stabilized pavement can be downloaded from the links below for reference: