UNSW May 2011 lecture on AustStab template.ppt€¦ · •Able to remedy many types of pavement...

The University of

New South WalesSchool of Civil and Environmental

Engineering Engineering

Pavement Engineering

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Insitu Stabilisation

Scott Young Stabilising Manager Downer

Greg White,

CEO AustStab

Aim

To introduce the fundamentals of stabilisation and show applications

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stabilisation and show applications and advantages in pavement construction and rehabilitation.

Agenda

• Introduction• Types of stabilisation• Design outline• Binders used in stabilisation

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• Binders used in stabilisation• Construction• Unsealed roads• Sustainability

Stabilisation is the introduction of

additional material to a pavement with

the purpose of improving the engineering 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

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Failed Pavement

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Types of Failure

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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•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

•Improvement 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

shrinkageshrinkage

Granular 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

• Improved pavement

stiffness

• Improved shear strength

• Reduced moisture

sensitivity, i.e. loss of • Addition of

chemical bindersensitivity, 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 • Addition of greater • Increased pavement

Types of Stabilisation (continue)

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

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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

Pavement design

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strengths calculated (eg CBR, UCS and/or Elastic Modulus), the pavement design commences.

• Common methods of pavement design include:

EmpiricalMechanisticFinite Element Modelling

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Granular design chart

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AustStab Design Guide for Cement Stabilised

Pavements for Lightly Trafficked Roads

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Binders

Most stabilisation in Australia of pavement

materials uses the following binders

• Lime

• Cementitious• 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•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•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 subbasesuitable 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 Slag

GB Cement Lime

Fly 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.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 toThe 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•Slag/lime

•Cement/flyash

•Slag/cement

•Cement/lime

•Triple blends

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Supplementary materials

FlyAsh

By product of burning of coal in electricity generation

Recovered from flue gas.

Has high percentages of silica and alumina.

Granulated ground blast furnace slagGranulated ground blast furnace slag

By 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•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 LimeTypes of LimeQuick lime CaO

Slaked 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.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•Poor compaction base

Result of lime stabilisation

•Dry out pavement

•Establish all weather working platform

•Reduces permeability

•Reduces pavement thickness35

Bituminous stabilisation

Bituminous stabilisation can be carried out

using bitumen emulsion or foamed bitumen

Current practice is to use 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•Environmental advantages

•Not sensitive to material variability

•Greatly reduced traffic delays

•Able to remedy many types of pavement

failures

•Reduces construction traffic

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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•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 depositsource deposit

•Mixing imported material with insitu pavement

•Mixing in water

•On site mixing plant combining different off site

products

•Mixing recycled products with existing

pavement44

Design for granular stabilisation

The principle properties

affecting stability of base

and subbase are as for

quarry productsquarry products

•Internal friction –

particle size distribution

•Cohesion – from clay

fraction

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Design targets for granular stabilisation

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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.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

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Spreading binders

• Use load calibrated mechanised spreaders

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• Verify binder application

o Use trays or matso Load cell measurement

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• For heavy applications two spreading passes are required to ensure uniform distribution and hence uniform strength gain.

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

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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

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shape of drum.

Longitudinal Joints

• Overlap at least 100 mm• Joints should be clear of wheel path.

Compaction

•Commence as soon as possible after mixing•Completed within working time of

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•Completed within working time of binder

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

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Multi tyred roller

- Used as final run to knead the surface and close pores

Check Density

• Accelerometer attached to vibrating roller• Trial section to ascertain passes

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• Trial section to ascertain passes required• Proof rolling• Devices such as clegg hammer• Nuclear densometer • Sand replacement

Levelling and Trimming

Trimming by grader will give correct levels and grades.

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levels and grades.

Trimmed material should not be used to fill in low spots of compacted material, this will cause delamination.

Curing

Curing of any stabilised layer

•Light and frequent water spray•Bituminous surfacing

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•Bituminous surfacing•Constructing next layer

Unsealed RoadsBinders - Cement blends

- Lime

- Polymers

Depth 150 mm

Results

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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

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

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•Reduce foot print of material source•Avoid encroachment on water table•Avoid possible erosion•Reduce use of water•Reuse materials as much as possible

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

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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.

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

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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.

Environmental Benefits

Existing failed pavements retain a very useful proportion of their asset value

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Addition of approximately 5% of binder restores and often exceeds the pavement’s original engineering properties.

Primary Environmental Benefits

•Reduced energy in excavation and trucking to/from site•Not using ever rarer land fill sites with materials that have value

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materials that have value•Reduces drastically need for increasingly rare quarry resources•Reduced gas emissions from these operations•Use of recycled products in binders

Total Costs

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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

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� Benefits to the environment� Cost benefits

In addition to the environmental and time benefits, rehabilitation using stabilisation is usually the most economical alternative.