Frequently asked questions

AustStab receives numerous questions from students and practitioners, and this site answers some of the commonly asked questions that have presented over the last decade.  It draws on information from

QUESTION: When would I use granular stabilisation?
ANSWER: When the distance to the source of a binder is cost prohibitive or the road may be left unsealed after treatment.

QUESTION: Can granular stabilisation be used for unsealed roads?
ANSWER: Yes.  Granular stabilisation is ideal for unsealed roads as it introduces an improved particle size distribution along with uniform moisture throughout the depth of the layer leading to longer performance rather than rip and grader blend.

QUESTION: Can you stabilise granular materials with a high PI?
ANSWER: Yes!.  A small amount of cohesion is important for optimising compaction, but high PI (> 10) levels may cause swell and shrinkage leading to cracking.  Typically small amounts of lime is used to reduce the PI of the material.

QUESTION: What is the difference between a dust suppression and stabilisation binder?
ANSWER: The generation of dust on unsealed roads is caused by the loss of fine particles (typically those particles less than 0.075mm)
from the loosening of the pavement materials at the surface and the disturbance of the wearing course caused by the action of traffic and climatic conditions, typically due to wind action.  Dust suppressants tend to glue the fine particles together where as a stabilisation may or may not bind the smaller and larger particles together when applied in a stationary of mobile
mixing plant.  Dust suppressants are usually sprayed on the surface on a regular basis whereas stabilisation binders are mixed into the material and typically have a life exceeding 5 years.

QUESTION: Can I use the formation or subgrade material for insitu stabilisation of a granular material?
ANSWER: Yes provided the laboratory mix design has taken into consideration the reduction in strength due to the formation material
being incorporated in the mixing process.

QUESTION: What binder should I use?
ANSWER: The best type of binder is based on the type of pavement material to be reached (ie shrinkage, modified or unbound) and the
ability of the binder to react with the pavement material and what you wish to attain with your mix design and pavement design.  A good start is to use the following chart.

QUESTION: What are the possible applications of lime stabilisation?
ANSWER: The applications of lime stabilisation in Australia are: 

increase subgrade stiffness,
reduce the PI of insitu pavement material,
enhance volumetric stability for the top layer of select material,
modify subbase layers to improve stiffness of the pavement,
produce a temporary construction platform for civil works reduce permeability, and
to recycle/rehabilitate an existing pavement

QUESTION: What is the difference between quicklime and hydrated lime?
ANSWER: The word lime is a generic term used to describe either quicklime or hydrated lime but should not limestone or agricultural
lime). Quicklime is short for CaO and is manufactured from limestone.  When water is added to quicklime and chemical reaction takes place and hydrated lime, Ca(OH)2 is formed.

QUESTION: What is an insoluble synthetic dry powdered polymer (ISDPP)?
ANSWER: Insoluble dry powder polymers are powder based polymers that have been manufactured such that their particle size is fine for intimate mixing with clay particles and insoluble to prevent water changing their structure during their contact with water over many years.One of the most common DPP’s in Australia is Polyroad.  This stabilising binder consists of a polymer thermally bound to an inert fine carrier, typically fly ash, which is then mixed with hydrated lime.

QUESTION: What structural design process could I use for cement stabilisation of lightly trafficked roads?
ANSWER: Figure 8.4 of the Austroads pavement design guide may be used for modified cemented materials where the design outcome is to not have a bound materials.The layered elastic analysis approach is generally conservative for cemented materials because the standard equivalent axle load approach is too conservative based on experience and performance of such lightly bound roads in use in Australia.At this stage AustStab recommends the use of the following design chart from the AustStab guideline titled Interim pavement design guide for a cement stabilised base layer for light traffic.

QUESTION: What is the typical compliance measures used for insitu stabilisation?
ANSWER: The common compliance measures for road stabilisation at base course are: 

Relative compaction
Stabilised layer thickness
Application rate of binder
Surface profile
Ride quality (for roads with posted traffic speeds exceeding 70 kph)

QUESTION: What is the maximum spread rate for powder binders?
ANSWER: The maximum spread rate from experience is considered to be 20 kg/m2.  This is applicable to conventional spreading and the use of integrated spreaders as built on some Wirtgen reclaimers.When using quicklime, the spreadsheet should be reduced to at least 15 kg/m2 to allow adequate slaking of quicklime by water spraying.

QUESTION: Can you incorporate the seal into the mixing process?
ANSWER: Yes.  It is common to use the seal in the process as the aggregates in the seal can add strength to the recycled materials.  If the thickness of the seal exceeds 50 mm, the seal should be incorporated in the laboratory mix design task.

QUESTION: Can you stabilise at night?

Yes.  When stabilising at night the following considerations should be given:

a) Colder temperatures at night impact on binder type and some slow setting binders may not get sufficient strength gain for reopening traffic in the morning.
Conversely in very hot climates it may be prudent to work at nights to reduce the drying shrinkage effects from the curing of cemented materials.
b) The use of spot lights still creates a darker surrounds from the operators cabin, and limits their ability to meet the surface compliance.  Special consideration should be made, especially for final trimming to ensure you get the best results.
c) Services need to be found during the day as utility companies are unlikely to attend repairs at night unless prior arrangement has been made.
d) Two sets of crew may be required to speed up the works and reduce the period of main noise generation from 8pm to midnight.
e) Vibration and noise issues need to be communicated to residents/business, especially when reversing beepers are used for construction equipment.  Contractors may resort to heavier rollers and static compaction to minimise vibration.
f) Traffic management needs to be considered for emergency vehicles, residents working night shift and some  organisations may operate a 24 hours a day and may still need to have limited access to their site.

QUESTION: How much should the contractor overlap on longitudinal runs?
ANSWER: The minimum overlap adopted by contractors is 100mm.  If the overlap is greater than 100 mm then the contractor should
ensure that the binder application is not doubled in the overlapped section.Caution is required when using cementitious binders if the overlap does not included more binder and mixing occurs in previously compacted cemented material.  In this instance, the mixing should be within the working time of the cemented material.

QUESTION: Should I stabilise full width or half width during one working shift?
ANSWER: In some cases there are no alternative routes and local traffic has to have access to the road during construction.  In this instance half road width stabilisation is carried out per shift.  In summer months with long day light hours, both sides of a road can be stabilised and compacted within the same day.

QUESTION: How do you prevent padfoot roller marks from appearing in the final surface?
ANSWER: Padfoot rollers are beneficial when used to compact stabilised pavements as they gain compaction in the bottom part of the
layer relatively quickly.  However, it is important not to get padfoot marks in the finished product.Once the bottom of the pavement approaches full compaction, as indicated by the padfoot roller walking out of its depressions, it should be removed and the top section of the pavement cut back and forth with a grader to the bottom of the depressions.  The compaction is then completed with a smooth drum roller (and possibly a rubber-tyred roller).  This process should be done expediently and while the material is fresh, to prevent a false pavement layer from being constructed.

QUESTION: What is mellowing for lime stabilisation of clay materials?
ANSWER: Effective mixing of lime into soils is a two pass process.  The initial pass is used to distribute the lime throughout the soil and thereby allow the mellowing operation to take place, followed by some cation exchange and some pozzolanic reactivity.

QUESTION: The contract specified 12 kg/m2 of lime to be spread and the contractor has only spread 9 kg/m2.  Is this satisfactory?
ANSWER: Most specifications will request the contractor to spread x  kg/m2 of lime and this lime is typically referred to as hydrated lime rather than quicklime.  However when lime is used from the same lime source, the amount of quicklime that is spread is 0.76 times the amount of hydrated lime.  When water is sprayed over quicklime a chemical reaction takes place and the quicklime is converted to hydrated lime of similar effectiveness.  This process is called slaking.There is no known problems if the contractor spreads too much quicklime except that it may be difficult to slake the quicklime when the spread rate exceeds about 15 kg/m2.

QUESTION: Can you recompact a cement stabilised material if the compaction compliance is nonconforming?
ANSWER: If the working time of the cementitious binder has not been exceeded when remixing commences and recompaction has been completed.

QUESTION: What happens when you add more water during the mixing stage of insitu stabilisation with cementitious binders?
ANSWER: Adding too much water (ie a moisture content greater than OMC) during mixing can reduce the final strength of the cemented material.  The reduction in strength is dictated on the amount of water, the temperature of the pavement material and the binder application rate.

QUESTION: How much asphalt should be placed over a cemented stabilised layer?
ANSWER: In local roads as little as 25 mm of asphalt has successful worked on cemented materials.  In heavy trafficked roads, 50 to 100 mm of asphalt is used provided crack sealing is carried out when they reflect to the surface to prevent water ingress below the pavement.To minimise reflective cracking for heavy duty pavements and take advantage of the second stage or cracked phase of life, Austroads recommends 175 mm of asphalt over a cemented material.Before placing the asphalt layer, AustStab recommends for local roads a Sami seal be placed over the cement stabilised layer to minimise the probability of reflective cracking appearing in the surface.

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