University of Technology
SydneySchool of Civil and Environmental
Engineering Applied Geotechnics (49118)
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Insitu Stabilisation
Scott Young Stabilising Manager
Downer Australia
Greg WhiteCEO
AustStab1
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Aim
To introduce the fundamentals of stabilisation and show applications and advantages in pavement design, construction and rehabilitation.
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Agenda
• Introduction• Types of stabilisation• Design outline• Binders used in stabilisation• Construction• Unsealed roads• Sustainability• Further Research
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Stabilisation is the introduction of additional material to a pavement with the purpose of improving the engineering characteristics.The additional material can either be aggregates or binders.
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The process
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The process
Truck Classification
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Failed Pavement
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Types of Failure
Advantages of Stabilisation
•Re-use existing pavement materials, this reduces landfill
and the need to use diminishing quarry resources
•Strengthen existing pavements•Improve the permeability of pavements, reducing the main
cause of pavement failure – water ingress•Drastically reduce construction time and lane closures
•Reduce greenhouse gases and construction energy usage
•Reduce the cost of construction because of lower material inputs, raw material transport and energy use
•Subgrade improvement in greenfields sites to long term strength gains and wet weather construction access, and
•Improves the wearing characteristics of unsealed pavement
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Types of StabilisationCategory of
stabilisation
Indicative
laboratory
strength after
stabilisation
Common binders
adopted
Anticipated performance
attributes
Subgrade CBR>5%
(subgrades and
formations)
Addition of lime
Addition of chemical
binder
Improved subgrade
stiffness
Improved shear strength
Reduced heave and
shrinkageGranular 40% < CBR <
+70%
(subbase and
basecourse)
Blending other
granular materials
which are classified
as binders in this
context
Improved pavement
stiffness
Improved shear strength
Improved resistance to
aggregate breakdown
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Types of Stabilisation (continue)
Category of
stabilisation
Indicative
laboratory
strength after
stabilisation
Common binders
adopted
Anticipated performance
attributes
Modified 0.7 MPa < UCS<
1.0 MPa Addition of small
quantities of
cementitious binder
Addition of lime
Addition of
chemical binder
Improved pavement
stiffness
Improved shear strength
Reduced moisture
sensitivity, i.e. loss of
strength due to increasing
moisture content
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Category of
stabilisation
Indicative
laboratory
strength after
stabilisation
Common binders
adopted
Anticipated performance
attributes
Lightly Bound UCS 1.0 – 2.0 MPa
Addition of small
quantities of cementitous binder
Addition of lime
Similar to Modified
Bound UCS > 2.0 MPa
(Basecourse) Addition of greater
quantities of
cementitious binder
Addition of a
combination of
cementitious and
bituminous binders
Increased pavement
stiffness to provide tensile
resistance
Greatest stiffness and
hence load carrying capacity
Types of Stabilisation (continue)
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Typical Pavements using Stabilisation
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• Once the mix design (ie the percentage of what binder is to be added to the host material) has been determined, and the design strengths calculated (eg CBR, UCS and/or Elastic Modulus), the pavement design commences.
• Common methods of pavement design include:
EmpiricalMechanisticFinite Element Modelling
Pavement design
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Pavement Design Approaches
� Several ways in which practitioners design pavement thicknesses:
� CIRCLY - mechanistic approach, has limitations for light trafficked roads <1 x 105
� Design Charts - Austroads empirical approach, can be quite conservative &
not recommended
� Existing Conditions - based on insitu pavement material available
� Experience - local knowledge & past performance may yield reasonable design
solutions
� AustStab - empirical approach, tailored specifically to stabilised materials
� New approaches - elastoplastic, viscoelastic, viscoplastic material modes and
usually combined with FEM, structural numbers
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Mechanistic Approach
� Austroads 1992, 2004 & 2008 pavement design guides used a mechanistic approach
� Analysis tool is CIRCLY (layered elastic model)
� Design life is based on repeated traffic loading and climatic conditions are taken into consideration in a simplistic way
� Mechanistic model is too conservative for local roads
Subgrade
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Granular design chart
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AustStab Design Guide for Cement Stabilised
Pavements for Lightly Trafficked Roads
Binders
Most stabilisation in Australia of pavement materials uses the following binders• Lime• Cementitious• Bitumen• Dry Powdered Polymers• Other granular material
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Preliminary binder SelectionPrior to selection of a binder a pavement material is tested for particle size distribution and Atterberg limits.
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Laboratory Testing
Reasons for laboratory testing•Determine most appropriate binder•Determine optimum binder content•Provide the parameters required for empirical or mechanistic pavement design (Modulus, CBR, UCS,PSD)
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Typical Testing
•Unconfined compressive strength (UCS)•CBR•Modulus•Lime demand•Particle size distribution•Atterberg limits
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Cementitious Stabilisation
Cementitious stabilisation is used to• Strengthen existing pavements•Improve low quality material to make suitable for base and subbase•Reduce need to increase base thickness to achieve design strength•Dry out wet pavements
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Cementitious Stabilisation
Primary reaction is the binder reacts with water in the soil to form cementitious material. This reaction is independent of the type of soil.Cementitious binder is made up of one or more of the following constituents:
GP Cement SlagGB Cement LimeFly Ash
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Cement
Historically Portland Cement was used in stabilisation. Cement
is produced by mixing calcium carbonate, alumina, iron oxide
and silica and then calcining and sintering this mixture.
The product hydrates in the presence of water to form hydrated
silicates and aluminates and calcium hydroxide.
If there is clay present in the soil the Ca(OH)2 will react with it.
The hydrated cement via inter particle bonding produces a
strong and durable pavement.
GP cement is often blended with slag or flyash and is called
GB cement.
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Problems with Cement
Cement gains strength quickly and has a relatively high shrinkage
The resultant stabilised pavement is prone to•Reduced working time in the field•Higher shrinkage•Block cracking
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Cementitious Blends
In recent years the use of supplementary binders has been the preferred option in stabilisation.
Common Blends•Slag/lime•Cement/flyash•Slag/cement•Cement/lime•Triple blends
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Supplementary materialsFlyAsh
By product of burning of coal in electricity
generation
Recovered from flue gas.
Has high percentages of silica and alumina.
Granulated ground blast furnace slagBy product of iron manufacture, these glass particles
react with water particularly in the presence of an activator to form calcium-alumina-silica hydrate similar to those produced in the hydration of cement.
Normally use an activator such as lime or cement.
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Advantages of Cementitious Blends
•Increased working time•Reduced shrinkage•Minimal cracking•Slower strength gain over time•Cheaper cost•Uses recycled products (slag/flyash)
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Lime Stabilisation
Lime is produced by the calcining of limestone.
Types of LimeQuick lime CaOSlaked lime Ca(OH)2
Agricultural lime Crushed limestone (<2mm)
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Chemical reactions
�Burning: • CaCO3 + heat (>1000oC) -> CaO + CO2
�Hydrating: • CaO + H2O -> Ca(OH)2 + Heat
�Pozzolanic reaction:�Ca++ + OH - + Soluble Clay silica -> Calcium Silicate Hydrate (CSH)�CA++ + OH - + Soluble Clay Alumina -> Calcium Aluminate Hydrate (CAH)
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Flocculation
Realignment of clay particles
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Lime reacts with most clays
Clays have pozzolans that react with the lime to form calcium silicates and aluminates.
For the reaction to be stable there must be an alkaline environment (pH > 12.3)
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Lime demand test
To determine minimum lime content the lime demand test is used.
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Strength gain using lime
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Lime stabilisation of subgrades
In Australia there are many roads that are built of poor subgrades often with CBR <3%
•Affected by water•Can be expansive•Poor compaction base
Result of lime stabilisation
•Dry out pavement•Establish all weather working platform•Reduces permeability•Reduces pavement thickness
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Bituminous stabilisation
Bituminous stabilisation can be carried out using bitumen emulsion or foamed bitumen
Current practice is to use foamed bitumen due to
•Cost•Temperature dependence
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Behaviour of Bitumen Stabilised Material
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Advantages of BSM
•Increase in strength of pavement (substitute for asphalt)•Improved durability and moisture sensitivity•Lower quality aggregates can be utilised•Environmental advantages•Not sensitive to material variability•Greatly reduced traffic delays•Able to remedy many types of pavement failures•Reduces construction traffic42
Foamed bitumen
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Foamed bitumen coats fines
Often requires foaming agent
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Expansion ratio vs half life
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Lime as secondary binder
•Stiffens bitumen•Anti-stripping agent•Usually 1-2%•Improves bond strength•Reduces moisture sensitivity•Assists dispersion of bitumen
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Foamed Bitumen stabilisation –
particle size distribution
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Granular stabilisationGranular stabilisation is the blending of one or
more materials with a pavement material to
improve its engineering properties.
Typical uses:
•Mixing of materials from various parts of a
source deposit•Mixing imported material with insitu pavement
•Mixing in water•On site mixing plant combining different off site products
•Mixing recycled products with existing pavement
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Design for granular stabilisation
The principle properties affecting stability of base and subbase are as for quarry products
•Internal friction –particle size distribution•Cohesion – from clay fraction
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Example of blending two materials
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Example of blending two materials
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Dry Powdered Polymers (DPP)
DPP has been shown to “waterproof”the pavement material by finely coating the fine material.
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Construction of Stabilised Pavements
Initial site preparation
•The full length of the pavement to be stabilised should be inspected and samples taken of different types of pavements.•Often the pavement will be premilled to break down existing seals and oversized material.•Remove thick bituminous or stabilised patches.•Search for and adjust services
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Pre-milling
Spreading binders
• Use load calibrated mechanised spreaders
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• Verify binder application
o Use trays or matso Load cell measurement
• For heavy applications two spreading passes are required to ensure uniform distribution and hence uniform strength gain.
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If quicklime is used, slaking is required prior to mixing
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Application of liquid binders
• Conventional water truck with spraybar• Preferably by direct pumping into the mixing chamber of the stabiliser
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Adding water
Practise is to add water directly into the mixing chamber. Ensures proper mixing and accurate and even distribution of correct water content to facilitate compaction.
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Two Pass Mixing
Two pass mixing is required to ensure the adequate mixing of binder. • First pass should be 75 – 90% of final depth.
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Joints
Overlap at start of work by 1.5m.This is required due to size and shape of drum.
Transverse Joints
Longitudinal Joints
• Overlap at least 100 mm• Joints should be clear of wheel path.
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Compaction
•Commence as soon as possible after mixing•Completed within working time of binder
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First compaction
- Padfoot roller- Most effective for lower levels- Grader used to eliminate foot marks
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Compaction (Continue)
Steel Drum
- Most effective for upper levels
Multi tyred roller
- Used as final run to knead the surface and close pores
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Check Density
• Accelerometer attached to vibrating roller• Trial section to ascertain passes required• Proof rolling• Devices such as clegg hammer• Nuclear densometer • Sand replacement
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Levelling and Trimming
Trimming by grader will give correct levels and grades.
Trimmed material should not be used to fill in low spots of compacted material, this will cause delamination.
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Curing
Curing of any stabilised layer
•Light and frequent water spray•Bituminous surfacing•Constructing next layer
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Unsealed RoadsBinders - Cement blends
- Lime
- Polymers
Depth 150 mm
Results
•Reduces maintenance by over 100%
•Reduces dust (loose material down by over
300%)
•Reduces effect of water
•Environmentally friendly
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Before After
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Sustainability Principles
•Source materials close to construction site•Avoid significant natural vegetation removal•Use gravel pits that do not affect native landscape•Reduce foot print of material source•Avoid encroachment on water table•Avoid possible erosion•Reduce use of water•Reuse materials as much as possible
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Advantages of Stabilisation
•Direct cost benefits•Social benefits•Environmental
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Direct cost benefits
Stabilisation is often the only practical means of rehabilitating an existing failed pavement.Fortunately the cost of stabilising is at least 30% often over 50% cheaper than the alternative - remove and replace with new material.Although whole of life costs should be used, it has been found that the life, maintenance costs and rehabilitation costs are similar for conventional pavements.
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Social benefits
• Insitu stabilisation is much faster process with minimal excavation and little material brought in or taken away from site.•Less chance of rain disruption causing extended delays•Lanes reopened on same day.
In higher trafficked countries road agencies often charge for downtime of road lanes. This is a real cost to the community.
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Environmental Benefits
Existing failed pavements retain a very useful proportion of their asset value
Addition of approximately 5% of binder restores and often exceeds the pavement’s original engineering properties.
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Primary Environmental Benefits
•Reduced energy in excavation and trucking to/from site•Not using ever rarer land fill sites with materials that have value•Reduces drastically need for increasingly rare quarry resources•Reduced gas emissions from these operations•Use of recycled products in binders
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Total Costs
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Further Research
1. Optimisation of bitumen foaming agents for use in Foamed Bitumen Stabilisation.
2. Review of CBR uniformity obtained in the field post lime stabilisation & development of reliable design parameters.
3. Assessing the performance of lightly trafficked roads in local government to characterise & model the failure mechanism of pavements designed to have a vertical modulus of between 1,000 & 2,000MPa.
4. Assessing the benefits associated with two pass mixing in terms of uniform strength gain.
5. Developing mix design protocol for the process of ‘mellowing’, where an initial lime pretreatment is undertaken in advance of traditional cementitious stabilisation treatments.
6. Characterisation of field working time (using density and UCS) for various materials stabilised with various powder binders.
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Summary
�Stabilisation can be used in one form or another in nearly every pavement construction or rehabilitation situation, giving:� Time and lack of disruption benefits� Benefits to the environment� Cost benefits
In addition to the environmental and time benefits, rehabilitation using stabilisation is usually the most economical alternative.
