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PROJECT REPORT ORIGIN PR/INT/202/00
LITERATURE REVIEW
STABILISED SUB-BASES FOR HEAVILY TRAFFICKED ROADS
DFID Project Source References
Subsector: Transport
Theme: T2
Project Title: Design of stabilised sub-bases for heavily trafficked roads
Project Reference: R6027, R8010
Copyright Transport Research Laboratory, UK and the Bureau of Research and Standards,
Department of Public Works and Highways, Philippines.
This document is an output from a co-operative research programme between the Departmentfor International Development (DFID), of the UK and the Department of Public Works andHighways (DPWH), Philippines. The project was funded from both the DFID Knowledge andResearch Programme which is carried out for the benefit of developing countries, and from theresources of the Bureau of Research and Standards of DPWH. The views expressed are notnecessarily those of DFID or DPWH.
The Transport Research Laboratory and TRL are trading names of TRL Limited, a member of the TransportResearch Foundation Group of Companies.
TRL Limited. Registered in England, Number 3142272. Registered Office: Old Wokingham Road, Crowthorne,Berkshire, RG45 6AU, United Kingdom.
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The information contained herein is the property of the Transport Research Laboratory and theDepartment of Public Works and Highways, and does not necessarily reflect the views or policies of DFIDor DPWH. Whilst every effort has been made to ensure that the matter presented in this report is relevant,accurate and up-to-date at the time of publication, neither the Transport Research Laboratory nor theDepartment of Works and Highways accept liability for any error or omission.
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CONTENTS
1 INTRODUCTION............................................................................... 1
2 STABILISATION IN ROAD PAVEMENTS............................................... 2
2.1 The role of the sub-base................................................................... 32.1.1 The role of a stabilised sub-base in a flexible pavement....................... 3
2.1.2 The role of a stabilised sub-base in a concrete pavement...................... 4
3 TYPES OF STABILISATION.................................................................5
3.1 Mechanical Stabilisation................................................................... 5
3.2 Cement Stabilisation ....................................................................... 5
3.2.1 Soil Cement............................................................................ 6
3.2.2 Cement Bound granular Material (CBM) ........................................ 6
3.2.3 Lean concrete ......................................................................... 6
3.3 Lime Stabilisation ..........................................................................7
3.4 Bitumen or Tar stabilisation ..............................................................8
3.5 Other types of stabilisation................................................................ 8
3.5.1 Blastfurnace slag...................................................................... 8
3.5.2 Pozzolanas ............................................................................. 9
3.5.3 Non-pozzolanic chemical soil stabilisers ......................................... 9
4 ELASTIC MODULUS.......................................................................... 9
5 TESTING AND MIX DESIGN..............................................................10
5.1 Suitability of materials for stabilisation ................................................10
5.2 Mix design..................................................................................12
5.2.1 Post Construction - Strength.......................................................13
5.2.2 Durability .............................................................................14
5.2.3 Construction equipment ............................................................14
5.2.4 Pre-construction trials ..............................................................15
6 PROBLEMS ASSOCIATED WITH STABILISATION .................................15
6.1 Construction ................................................................................15
6.1.1 Quantity of stabiliser................................................................15
6.1.2 Mixing.................................................................................16
6.1.3 Compaction and limited time ......................................................16
6.1.4 Rapid setting..........................................................................16
6.1.5 Curing time...........................................................................166.1.6 Variability.............................................................................16
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6.1.7 Testing.................................................................................16
6.2 Durability ...................................................................................16
6.2.1 Carbonation...........................................................................16
6.2.2 Sulphate and salt damage...........................................................17
6.2.3 Cracking ..............................................................................17
6.2.4 Break-up ..............................................................................17
7 CURRENT STABILISATION PRACTICE AROUND THE WORLD...............17
7.1 UK Practice.................................................................................17
7.1.1 Concrete pavements.................................................................17
7.1.2 Bituminous pavements ..............................................................18
7.2 TRL ORN31 Practice.....................................................................18
7.3 USA Practice ...............................................................................19
7.3.1 Designs for concrete pavements ..................................................19
7.3.2 Designs for flexible pavements....................................................20
7.4 Australia.....................................................................................20
7.4.1 Austroads Pavement Design Guide...............................................20
7.5 South Africa ................................................................................21
7.6 The Philippines.............................................................................21
8 PAVEMENT DESIGN FOR HEAVILY TRAFFICKED ROADS....................22
9 CONCLUSIONS................................................................................24
10 RECOMMENDATIONS FOR PILOT TRIALS IN THE PHILIPPINES. ...........25
11 ACKNOWLEDGEMENT.....................................................................26
12 REFERENCES / BIBLIOGRAPHY.........................................................27
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EXECUTIVE SUMMARY
Stabilisation is the process of mixing a stabiliser, for example cement, with a soil or
imported aggregate to produce a material whose strength is greater than that of the
original unbound material. The use of stabilisation to improve the properties of amaterial is becoming more widespread due to the increased strength and load spreading
ability that these materials can offer. Stabilisation technology is extremely relevant for
heavily trafficked pavements where its' benefits are beginning to be appreciated.
This report describes the basic types of stabilisation, indicates when it should be used,
and discusses the main advantages and disadvantages of its use. The role of the sub-
base and other pavement layers are also discussed for both flexible and rigid
pavements.
An extensive literature review of international publications was carried out and this
report describes some of the latest research and design methodology associated withstabilised materials used for sub-bases on heavily trafficked roads. As well as
references to the literature it also contains an extensive bibliography of work on this
subject.
Many of the pavement design manuals from other countries were examined. These
include manuals from the UK, USA, Australia and South Africa; many of which
include in their specifications the design of asphalt pavements with stabilised sub-bases.
In these design manuals, stabilised sub-bases are used with either stabilised or granular
roadbases. This report discusses advantages and disadvantages of these designs. The
various pavement design manuals also showed that stabilised sub-bases are often used
under concrete pavements, which is presently not the case in the Philippines where agranular sub-base is still specified. The benefits of this form of construction are also
discussed.
The report notes that few of these design manuals produce savings in pavement
thickness from the use of stabilised sub-bases even though they are frequently
recognised to have higher strengths than unbound granular materials. They are merely
substitutes. Their use also permits the use of lower-grade, marginal materials after
suitable stabilisation, which may reduce haulage of high quality unbound materials and
depletion of resources. The report concludes that there is a role for stabilised sub-bases
in the Philippines, especially for heavily trafficked pavements where they could
improve performance and hence reduce maintenance costs.
Finally, the report outlines technical recommendations for pilot trials of stabilised sub-
bases in the Philippines. These trials would be constructed under the auspices of the
Bureau of Research and Standards of the DPWH and monitored under a DPWH/DFID
jointly funded research project being undertaken by staff from BRS and TRL.
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STABILISED SUB-BASES FOR HEAVILY TRAFFICKED
ROADS
1
INTRODUCTION
The main objective of stabilisation is to improve the performance of a material by
increasing its strength, stiffness and durability. The performance should be at least
equal to, if not better than that of a good quality natural material.
This report describes the basic types of stabilisation, the main advantages and
disadvantages of the technique and the latest research and design methodology for such
materials.
The term ‘heavily trafficked roads’ varies between design standards and countries. In
this report, as an approximate guide, the term is applied to roads with a design life of
more than 10 million equivalent standard axles (ESA).
The term ‘stabilisation’ is the process whereby the natural strength and durability of a
soil or granular material is increased by the addition of a stabilising agent. . In
addition, it may provide a greater resistance to the ingress of water. There are many
types of stabiliser that can be used, each with their own advantages and disadvantages.
The type and quantity of stabiliser added depends mainly on the strength and
performance that needs to be achieved.
The addition of even small amounts of stabiliser, for example up to 2 per cent cement,
can modify the properties of a material. Larger amounts of stabiliser will cause a large
change in the properties of that material, for example 5 to 10 per cent of cement added
to a clean gravel will cause it to behave more like a concrete.
The strength of a stabilised material will often continue to increase for a period of
several years from the time it is constructed, as shown in Figure 1 (Croney, 1998).
Figure 1 Rate of increase of strength with age for cemented material (After
Croney, 1998)
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The strength of a stabilised material will depend on many factors. These include:
• the chemical composition of the material to be stabilised;
• the stabiliser content;
•
the degree of compaction achieved;• the moisture content;
• the success of mixing the material with the stabiliser;
• subsequent external environmental effects.
When small quantities of stabiliser are added, the material is often described as
‘modified’ rather than ‘bound’. There are no fixed criteria for these definitions, but a
limit of 80kPa (indirect tension) or 800kPa (Unconfined Compressive Strength after 7
days moist curing) for a reasonably graded material is suggested by NAASRA (1986).
2
STABILISATION IN ROAD PAVEMENTS
There are many different reasons for using stabilisation, ranging from lack of good
quality materials to a desire to reduce aggregate usage for environmental reasons.
Ultimately the main reason for using stabilisation will usually be cost savings. The
engineer is trying to build a problem-free pavement that will last for its intended design
life for the most economic price. The cost savings associated with stabilisation can take
many forms including reduced construction costs, reduced maintenance costs
throughout the life of the pavement or an extension of the normal pavement life.
The location of suitable materials for road construction will become increasingly
difficult as conventional high-quality materials are depleted in many areas. The costs of hauling materials from further away may also increase, thus compounding the problem.
One solution is to stabilise locally available materials that presently may not conform to
existing specifications.
From the point of view of bearing capacity, the best materials are those which derive
their shear strength partly from friction and partly from cohesion. For stabilisation to
be successful, the material should attain the desired strength (i.e. be capable of
sustaining the applied loads without deformation) and should retain its strength and
stability indefinitely.
Not all materials can be successfully stabilised, for example if cement is used as the
stabiliser then a sandy soil is much more likely to yield satisfactory results than a soft
clay (Watson, 1994). The material to be stabilised must be tested to ensure that it is
compatible with the intended stabiliser – the subject of testing will be discussed later in
this report. It is also recommended from experience that layers which are less than
150mm thick should not be stabilised (Lay, 1986/88).
Netterberg (1987) reports that unless proven by experience or durability testing, a
material should not be improved too much. For example a material for use as a base
layer should only be stabilised if it could be used unstabilised for a sub-base layer.
Another recommendation from the same report is to “discount any increase in strength
of more than 100 per cent.”
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Capping and sub-base layers can usually be stabilised without significant problems.
One of the main problems with stabilised layers is that they crack to a greater or lesser
degree. This cracking is caused by changes in moisture content and temperature and
cannot be avoided. The amount of cracking will depend on many factors, but generally
a stronger material will produce wider cracks at a greater crack spacing than a weaker
material.
A cement stabilised granular base directly under an asphalt surfacing will frequently
result in reflection cracking as shrinkage cracks in the base propagate through the
asphalt surfacing. If cracks are left unsealed, then water penetration can lead to further
deterioration, particularly if the underlying sub-base is not stabilised.
Stabilisation of the sub-base under a granular base, however, can have many benefits
without causing reflection cracking in the surface of an asphalt pavement. It is reported
that a thickness of 125-150mm of granular cover over a stabilised sub-base is generally
sufficient to substantially delay or stop reflection cracking (NAASRA, 1987).
2.1 The role of the sub-base
The sub-base is an important layer in both flexible and rigid pavements. It mainly acts
as a structural layer helping to spread the wheel loads so that the subgrade is not over-
stressed. It also plays a useful role as a separation layer between the base and the
subgrade and provides a good working platform on which the other paving materials
can be transported, laid and compacted. It can also act as a drainage layer. The
selection of material and the design of the sub-base will depend upon the particular
design function of the layer and also the expected in-situ moisture conditions (TRL,
1993).
Stabilised sub-bases can be used for both flexible and rigid road pavements, althoughthe reasons for doing this can vary. In order to identify the benefits of stabilising sub-
bases, it is necessary to examine the role of the sub-base for each pavement type.
2.1.1 The role of a stabilised sub-base in a flexible pavement
A stabilised, and therefore stiffer, sub-base provides greater load spreading ability and
hence reduces stresses imposed on the subgrade. When stabilised the sub-base provides
much of the structural rigidity in the pavement, and also assists during the compaction
of the upper granular layers and hence increases their ability to withstand deformation.
If the sub-base is stabilised, reflection cracking in an asphalt surface layer can be
minimised by having an unbound granular roadbase. This unbound roadbase providesnot only a large proportion of the structural load spreading but also assists in delaying
or preventing reflection cracking from the shrinkage and movement of the stabilised
layer. The granular roadbase is subjected to relatively high traffic stresses and crushed
aggregate is often used to withstand attrition and to assist in achieving a high value of
elastic modulus, limiting the horizontal tensile strains at the bottom of the bituminous
surfacing.
The use of a stabilised sub-base with a granular base is often referred to as an ‘upside-
down pavement’ (Lay 1986). It is reported (LCPC, 1997) that a typical mode of
deterioration for this type of pavement, based on experience from France, is slight
rutting attributed to the unbound granular layer and eventually fine transverse cracking
which occurs after much trafficking.
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2.1.2 The role of a stabilised sub-base in a concrete pavement
For a concrete pavement, the term ‘sub-base’ refers to the layer immediately below the
concrete slab. In a concrete road, the high elastic modulus of the concrete layer causes
most of the traffic-induced stresses to be taken in the concrete layer in the form of
bending stresses.According to O’Flaherty (1994), there is a common misunderstanding about the main
function of the sub-base beneath a concrete slab. He states that the main function of the
sub-base is to ensure uniform support to the concrete, counteracting the effect of
unsatisfactory subgrade support, rather than increasing the structural stability (i.e.
strength) of the pavement.
If the subgrade could be relied upon to provide uniform support throughout the life of
the pavement then a sub-base may not be required and the slab could be cast directly on
the prepared in-situ soil, providing it is good quality and naturally uniform. This
uniform support appears to be crucial, especially where the subgrade is either weak or
expansive because the non-uniform support will eventually lead to the fatigue failure of the pavement.
It has been found that substitution of the top layer of a weak subgrade by a stronger
unbound granular layer has little influence on the stresses at the bottom of the slab
(TRRL, 1978). For example, a gravel sub-base 150mm thick on a weak subgrade will
only reduce the tensile stress by about 10 per cent in a thin slab and less in a thicker
slab.
For a concrete pavement with a granular sub-base, the two major modes of damage
are:
1.
tensile stresses at the base of the concrete layer due to inadequate strength and/orthickness of the concrete and
2. lack of bearing capacity – mainly at joints or cracks where pumping and erosion of
the support can aggravate the problem.
Use of a stabilised sub-base, provided it has adequate strength and durability, can help
to alleviate this second mode of damage. The problem of ‘pumping’ mainly occurs on
roads built on subgrades with a high fines content. With a granular sub-base, fines in
the subgrade or sub-base can go into suspension if water is present and this fine
material can be pumped out of a joint or crack under the passage of heavy wheel loads.
This eventually leads to a void under the slab, resulting in slab cracking, rocking orfaulting. Use of a stabilised sub-base can frequently prevent pumping by a) stopping or
reducing water penetration to underlying layers and b) ensuring that there are no free
fines available immediately beneath the concrete slab.
Stabilised sub-bases provide a uniform, stable and permanent support for concrete slabs
throughout their design life. They can also aid construction of the concrete slabs by
providing a low permeability surface, which minimises water loss from the fresh
concrete and also provide a hard layer beneath the slabs to aid compaction.
The stress generated in a concrete slab partly depends on the stiffness ratio between the
slab and the underlying support. In many countries, including the UK, the national
design standards specify that all rigid pavements must be constructed with a cemented
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sub-base of adequate stiffness. “This type of sub-base erodes less than an unbound
material and is less water-susceptible should join sealants fail” (UK DOT, 1995).
3 TYPES OF STABILISATION
There are a number of different types of stabilisation, each having its own benefits and potential problems. The types described below are those most frequently used,
however, it must be noted that not all of them are appropriate for all situations.
3.1 Mechanical Stabilisation
The most basic form of mechanical stabilisation is compaction, which increases the
performance of a natural material. The benefits of compaction, however, are well
understood and so they will not be discussed further in this report.
Mechanical stabilisation of a material is usually achieved by adding a different material
in order to improve the grading or decrease the plasticity of the original material. The
physical properties of the original material will be changed, but no chemical reaction is
involved. For example, a material rich in fines could be added to a material deficient in
fines in order to produce a material nearer to an ideal particle size distribution curve.
This will allow the level of density achieved by compaction to be increased and hence
improve the stability of the material under traffic. The proportion of material added is
usually from 10 to 50 per cent.
Providing suitable materials are found in the vicinity, mechanical stabilisation is usually
the most cost-effective process for improving poorly-graded materials. This process is
usually used to increase the strength of a poorly-graded granular material up to that of
a well-graded granular material. The stiffness and strength will generally be lower than
that achieved by chemical stabilisation and would often be insufficient for heavily
trafficked pavements. It may also be necessary to add a stabilising agent to improve the
final properties of the mixed material.
3.2 Cement Stabilisation
Any cement can be used for stabilisation, but Ordinary Portland cement is the most
widely used throughout the world.
The addition of cement to a material, in the presence of moisture, produces hydrated
calcium aluminate and silicate gels, which crystallise and bond the material particles
together. Most of the strength of a cement-stabilised material comes from the physicalstrength of the matrix of hydrated cement. A chemical reaction also takes place
between the material and lime, which is released as the cement hydrates, leading to a
further increase in strength.
Granular materials can be improved by the addition of a small proportion of Portland
cement, generally less that 10 per cent. The addition of more than 15 per cent cement
usually results in conventional concrete. In general, the strength of the material will
steadily increase with a rise in the cement content. This strength increase is
approximately 500 to 1000 kPa (UCS strength) for each 1 per cent of cement added
(Lay 1986/88). The elastic modulus of an unbound natural gravel or crushed rock will
be in the range 200-400 MPa. When stabilised, this will increase to a range of approximately 2,000 to 20,000 MPa.
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Cement stabilised materials can be mixed in-situ or mixed at a plant and transported to
site. To achieve stronger cement bound materials, i.e. greater than about 10 MPa cube
strength at 7 days, the materials should generally be plant mixed (DETR, 1998).
One of the main problems with stabilising a material is mixing in the cement. The
particle size of ordinary Portland cement is quite well defined with a range of 0.5-100microns and a mean of 20 microns (Ingles & Metcalf, 1972). The larger particles of
cement never completely hydrate, and it has been found that the same amount of a
more finely ground cement will produce higher strengths. Finely ground cements are,
however, expensive to produce and it has been suggested (Ingles & Metcalf, 1972) that
the larger particles of cement could be replaced with smaller particles of an inert filler.
The greater bulk would aid the distribution process so that the same amount of active
cement would be available throughout the material. Thus producing an equally
effective binder, which could be cheaper than ordinary cement.
The use of cement as a stabiliser is more widespread than lime. This is due to many
reasons, but the main factors are likely to be the cost and the higher strengths that areattainable using cement. Other factors include availability, past experience and the
more hazardous nature of lime. The price of cement is often similar to that of
quicklime or hydrated lime, however cement can be used on a wider range of materials
and the strengthening effect of cement is much more than that of an equal amount of
lime. Hence either higher strengths are possible using an equal amount of cement
instead of lime or the same specified strength can be achieved using a lower quantity of
cement than lime. The effects of lime and cement on the 7-day strength of various soil
types was presented graphically by Sherwood (1993) and Dumbleton (1962), as shown
in Figure 2.
There are three main types of cement-stabilised materials:3.2.1 Soil Cement
Soil cement usually contains less than 5 per cent cement. (Lay, 1986). It can be either
mixed in-situ (usually up to 300mm layer at a time) or mixed in plant. The technique
involves breaking up the soil, adding and mixing in the cement, then adding water and
compacting in the usual way. Croney (1998) recommends that a minimum strength
should be 2.5 MPa (7 day cube crushing strength) or, if this material is used to replace
sub-base then the strength requirement should be increased to 4 MPa.
3.2.2 Cement Bound granular Material (CBM)
This can be regarded as a stronger form of soil-cement but uses a granular aggregate(crushed rock or natural gravel) rather than a soil. The process works best if the natural
granular material has a limited fines content. This is almost always mixed in plant and
the strength requirement is 5-7 MPa (7 day cube crushing strength), (Croney, 1998).
3.2.3 Lean concrete
This material has a higher cement content than CBM and hence looks and behaves
more like a concrete than a CBM. It is usually made from batched coarse and fine
crushed aggregate, but natural washed aggregate (e.g. river gravels) can also be used.
The UK specification for this material gives a normal strength of 6-10 MPa or a higher
strength of 10-15 MPa (7 day cube crushing strength).
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Hydrated lime is used extensively for the stabilisation of soil, especially soil with a
high clay content where its main advantage is in raising the plastic limit of the clayey
soil. Very rapid stabilisation of water-logged sites has been achieved with the use of
quicklime.
There is little experience with lime stabilisation for road pavements in the UK wherethe process is intended primarily for treating wet, heavy clays. Small quantities
(typically 1-3 %) are used to reduce the plasticity of the clay. It is reported that such
small quantities usually result in a small increase in CBR strength although no
significant increase in compressive or tensile strength should be expected (Paige-Green,
1998). Paige-Green reports that typically, a minimum of 3 to 5 per cent stabiliser is
necessary to gain a significant increase in the compressive and tensile strength.
Although the use of lime stabilisation is widespread, the reported performance of the
technique is often variable. In fact, many parts of Australia stopped using lime
stabilisation in the 1970’s due to some major problems. More recently the technique
has regained favour and is being used in on-going road trials; e.g. Killarney RoadTrials and Freestone Creek to Eight Mile Intersection (Evans 1998). However, Evans
concluded that “…it may be prudent to continue to assume that lime stabilised
subgrades do not contribute greatly to pavement strengths.”
The strengthening effect of cement is significantly greater than the equivalent quantity
of lime unless the host material contains a significant quantity of clay, and so,
generally, to achieve the higher strengths necessary for heavily trafficked roads,
cement appears to be a more practical stabiliser.
3.4 Bitumen or Tar stabilisation
Bitumen and tar are too viscous to use at ambient temperatures and must be made intoeither a cut-back bitumen (a solution of bitumen in kerosene or diesel), or a bitumen
emulsion (bitumen particles suspended in water). When the solvent evaporates or the
emulsion ‘breaks’, the bitumen is deposited on the material. The bitumen merely acts
as a glue to stick the material particles together and prevent the ingress of water. In
many cases, the bituminous material acts as an impervious layer in the pavement,
preventing the rise of capillary moisture.
In a country where bitumen is relatively expensive compared to cement and where most
expertise is in cement construction, it appears more reasonable to use a cement
stabiliser rather than a bitumen/tar based product.
3.5 Other types of stabilisation
Materials in this group do not, on their own, produce a significant cementing action
and may need to be used in conjunction with cement or lime (O’Flaherty, 1985).
3.5.1 Blastfurnace slag
This is a by-product of the iron industry. It cannot be used on its own as a stabiliser but
when it is ground into finer particles the product, known as ground granulated
blastfurnace slag (ggbfs), can be used as a cement replacement, with up to 85 per cent
of the cement replaced with the slag.
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3.5.2 Pozzolanas
Pozzolanas possess little or no cementitious properties in themselves but will in certain
circumstances chemically react with lime to form compounds possessing cementitious
properties. Natural pozzolanas are mainly of volcanic origin; artificial pozzolanas are
products obtained from heating natural products. Examples of artificial pozzolanas are pulverised fuel ash (pfa) which is obtained from the burning of coal in power stations
and rice husk ash (Sherwood, 19930, (Montgomery, 1991).
3.5.3 Non-pozzolanic chemical soil stabilisers
These chemical stabilisers mostly take the form of strongly acidic, ionic, sulphonated,
oil-based products. A cementitious reaction does not usually occur, but due to many
factors including ionic exchange, the absorbed water can be reduced leading to better
compaction and increased strength. The material must have an appropriate clay content
for the stabiliser to have a beneficial effect. When correctly utilised, these products can
be very cost effective (Paige-Green, 1998).
Products containing chemicals such as sodium chloride and ligno-sulphonates purely
‘stick’ the material or soil particles together, while other products such as those
containing enzymes act biologically to achieve the same effect.
Although non-pozzolanic stabilisers are usually cheaper, they are usually not as
effective as traditional stabilisers such as cement or lime, which produce significantly
greater strengths.
4 ELASTIC MODULUS
In a pavement engineering context, one of the most fundamental engineering properties
of any material is the elastic modulus. The term ‘elastic modulus’ is defined as the ratio
of stress to strain and is a measure of the material’s stiffness properties. In addition to
the modulus of a material, it is also important to know its strength because a material
may be very stiff, but not very strong and could crack or break under heavy traffic.
The modulus of elasticity of a cemented material can be measured by several different
methods including: dynamically (Ed) using electrodynamic excitation of long beams of
150mm section or statically (Es) by loading 150mm diameter cylinders fitted with
extensiometers. Croney (1998) reports that comparative studies have consistently
shown the dynamic modulus to be higher than the static value. An approximate
conversion is given below (for values of Ed >5):
Ed = 10 + 0.8Es (in GPa)……….. (Croney, 1998)
There is also much discussion about whether to use dynamic or static modulus values
in pavement calculations and often the average of both values is used.
A relationship between dynamic modulus and compressive strength at 28 days is shown
below in Figure 3.
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Figure 3 Relationship between dynamic modulus and compressive strength (at 28
days) for some cement treated materials (Croney, 1998)
Materials cemented with pozzolanic stabilisers such as lime and cement, perform in a
more elastic, semi-brittle manner under traffic than unbound materials. Ideally,
knowledge of a material’s stiffness modulus and shear strength are required todetermine an appropriate thickness for the overlying pavement layers. The number of
factors involved in knowing these variables are high, for example the shear strength
will depend on factors including the effective stress which is dependant on the stress
history, etc. To simplify matters, index tests are often used. Historically, the CBR has
been used but it is now often thought to be useful only for modified materials where the
strength of the materials measured in the CBR test would not exceed 100 per cent. The
Unconfined Compressive Strength test is considered a more useful guide to the elastic
modulus and many correlations exist, for example TRH13 (CSRA, 1986) and
Austroads (1992). In the move towards mechanistic design there is a driving force to
use more direct measurements. Such testing however may be beyond the resources of
many laboratories.
5 TESTING AND MIX DESIGN
5.1 Suitability of materials for stabilisation
Before stabilising a material, especially a soil, it must be tested to ensure the
compatibility and the effectiveness of the intended stabiliser. These initial tests will
vary between countries, but often take the form of determining the particle size
distribution, liquid and plastic limits, soil acidity and sulphate content. One such
chemical test is for the Initial Consumption of Lime (ICL). The test is used when limeor cement is added to a clayey soil. For strength gains to occur, the chemical reactions
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require a high pH (>12.4) to be maintained, which is the ICL value. This will vary
considerably for different soils. After sufficient lime has been added to satisfy the ICL
of the soil, additional lime will be required for the formation of cementitious
compounds. Hence, further testing is still required to establish the optimum stabiliser
content for the required strength. The test for soil acidity and sulphate content is
carried out to indicate any potential problems with the hydration of the cement or possible chemical attack of the hydrated cement. Typical specifications are given in
Table 1 and Table 2.
Table 1 Typical specifications for cement stabilisation of a granular material to
form capping in UK (Watson, 1994).
Test specifications
Maximum liquid limit (LL) 45
Maximum Plasticity Index (PI) 20
Maximum organic matter content 2 %
Maximum total sulphate content 1 %
Saturation moisture content (chalk) 20 %
Grading
sieve size
% passing
(by mass)
125 mm
90 mm
10 mm
600 um
63 um
100
85-100
25-100
10-100
0-10
Table 2 Guide to the type of stabilisation likely to be effective (From TRL ORN
31, 1993 adapted from NAASRA, 1986)
Soil properties
More than 25% passing
the 0.075mm sieve
Less than 25% passing
the 0.075mm sieveType of
Stabilisation
PI
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5.2 Mix design
Before stabilisation is used in road construction, a laboratory testing programme must
be carried out on the material in order to determine a) the amount of water and b) the
amount of stabiliser to be added to achieve the specified strength. Care must be taken
to avoid excess quantities of stabiliser because this can cause wide shrinkage cracksduring curing which can lead to extensive reflection cracking through overlying
asphalt.
One test method suggested, (Croney 1998) is to first calculate the amount of water to
be added, by determining the optimum moisture content (OMC) that will give the
maximum density, and then adding approximately one per cent to this value. This
addition is necessary because the OMC of the cement and material will differ from that
of the material alone because the fine grained cement will demand proportionately
more water than the unbound material.
The amount of stabiliser needed to achieve the specified strength can then be
determined using cubes made up with various cement contents which are cured for a
fixed perod; usually 7 or 14 days before testing, usually by crushing. For example, a
suggested Unconfined Compressive Strength requirement for a stabilised sub-base is 4
MPa at 7 days (Croney, 1998). This value is further qualified as the average strength
of five cubes with a minimum value of 2.5MPa for any individual cube (MCHW 1000,
1998).
In general, the strength of the material will steadily increase with a rise in the cement
content. This strength increase is approximately 500-1000 kPa (UCS strength) for each
1 per cent of cement added (Lay 1986/88). Some additional stabiliser may be necessary
to take account of the variability in mixing that will occur on site. For example, an
extra 1 per cent of cement is proposed in TRL ORN31 (1993).
It should be noted that in the UCS test the results can be affected by both the size and
shape of the sample tested, e.g. a cube or cylinder specimen. The results are often
converted to those for a 150mm cube by multiplying the result with a correction factor.
Some correction factors are given in Table 3.
Table 3 Conversion Factors for UCS Test (after Sherwood, 1993).
Specimen shape and size Correction factor
(to 150mm cube)
Cube - 150mm
Cube - 100mm
Cylinder - 200 mm x 100 mm diameter
Cylinder - 142 mm x 71 mm diameter
Cylinder - 115.5 mm x 105 mm diameter
Cylinder - 127 mm x 152 mm diameter
1.00
0.96
1.25
1.25
1.04
0.96
The effect of cement content on strength will vary depending on the type of material to
be stabilised. This can be seen in Figure 4 (NAASRA 1986).
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Figure 4 Effect of cement content on strength of various soils stabilised with
Ordinary Portland cement and cured for 7 days at 25oC . (NAASRA, 1986 and
Metcalf, 1977)
5.2.1 Post Construction - Strength
To ensure adequate strength during construction, the quality of a cement stabilisedmaterial is usually determined by strength tests on the material after it has been allowed
enough time to sufficiently harden (usually 7 days). The strength can be tested in many
ways, but some of the most popular tests are the Unconfined Compressive Strength
(UCS) test, sometimes known as cube crushing, and the California Bearing Ratio
(CBR) test. As mentioned above, many practitioners now prefer to use the UCS test.
For strength and performance testing, NAASRA (1986) reports that: ‘It should be
noted that the CBR test is not relevant to cement-bound materials and it cannot be used
for design purposes. The unconfined compressive strength (UCS) test has been
extensively used to determine the relative response of materials to cement stabilisation.
However, the UCS has little direct application to pavement design and it is better to usesome form of tensile strength testing as this will have a bearing on pavement design.
Cemented materials are relatively brittle, and fail in tension under relatively low strain.
The critical strain usually decreases with increasing modulus. Hence modulus is more
relevant to performance than UCS’.
South Africa has recently introduced tests to determine the tensile strength of stabilised
materials, particularly for stabilised sub-bases beneath concrete pavements (Paige-
Green, 1998). In the test, a load is applied to the curved surface of a cylindrical
specimen until failure occurs. A flexural test (3 point beam test) can also be carried
out. Minimum limits for the Indirect Tensile Strength (ITS) of cemented materials have
been set in the latest of the South African series of Technical Recommendations forHighways (COLTO, 1996).
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5.2.2 Durability
As well as ensuring that an adequate strength and stiffness has been achieved by the
stabilisation process, it is also necessary to ensure that this strength is maintained over
the design life of the pavement. It should be noted that the UCS and CBR tests do not
actually measure the durability of the stabilised material. This can be determined bydurability testing which could take the form of either a soaked CBR test or a wet/dry
brushing test (South Africa). In more temperate climates a freezing/thawing test may
also be appropriate. A recent revision to the South African wet/dry brush test has been
recommended by Paige-Green (1998), who proposed that the mechanical wet/dry brush
test should be used as it removes some of the operator variability that was apparently
present with the previous test. After this testing has been carried out, if any doubt
remains about the durability of the material then a further carbonated UCS test could be
carried out (de Wet & Taute, 1985).
5.2.3 Construction equipment
Stabilisation may take the form of mix-in-plant or mix-in-situ. Mix-in-plant is most appropriate where imported granular materials are being used and mix-in-situ is more
appropriate for the stabilisation of native soils.
In-plant mixing may take place on or off site, but an important requirement for
stabilised materials such as cement-bound material, CBM, (ie where the water content
is much lower than for concrete) is that the plant must have a positive mixing action to
thoroughly mix the constituents – “a simple tumbling action is not sufficient” (Watson,
1994).
In-situ mixing plant consists of a rotovator which uses rotating tines to break and mix
the soil. Machines in highway construction are generally much more powerful than agricultural machinery and hence are capable of stabilising clay and granular materials
up to 350mm thick. Some are also capable of breaking bound material. Agricultural
rotovators may be used for thinner layers up to 150mm in conjunction with suitable soil
types (Watson, 1994).
In the United States, the process of in-situ stabilisation of soils is used far more than in
Europe. A wide range of multiple and single pass plant have been developed which has
led to a cost saving which often cannot be realised in smaller countries.
Lay (1986/88) reports on equipment called ‘stabilisers’ that are capable of cutting into
in-situ material up to depths of 500mm, extracting the material which is then mixed
with stabiliser from a hopper and then replaced. The amount of additive placed is afunction of the mechanical operation and the speed of travel. Lay quotes (Grahame and
Goldsborough, 1980) as containing further information.
Stabilisation of deep lifts, up to 400mm thick, are now possible due to the recent
development of;
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• Large mixing and pulverising machinery, such as the CMI RS500 (Australia)
• Large capacity purpose-built binder spreaders with automated spread control,
• High performance compaction equipment; and
•
Slow setting binders.Useful information about equipment can be found in NAASRA (1986, 1998) Chapter
9: Construction.
If a thick layer e.g. 300mm is to be stabilised, then problems in achieving adequate
compaction could require that the material is placed in two lifts. An Australian design
manual (Queensland, 1990) recommends use of a cement slurry to bond the two layers
together. This publication also reports that the second layer must never be stabilised
using ‘in-situ’ stabilisation methods even if the first layer was stabilised ‘in-situ’, since
this method will usually cause damage to the first layer. The manual also recommends
that the first layer of a two part layer process is never less than 150mm thick, so that it
can support the plant that will lay the second layer.
5.2.4 Pre-construction trials
A field trial should be carried out ahead of the main work in order to determine the
actual strength and density that can be achieved using the same plant that will be
involved in the main contract. Paige-Green (1998) recommends the use of proof rolling
on trial sections that incorporate density or strength testing after each roller pass. These
trials can identify the optimum number of roller passes that are necessary and also
provides an indication of the target density or strength that is required for quality
control testing after rolling.
6
PROBLEMS ASSOCIATED WITH STABILISATION
Previous sections of this review have identified the advantages of using stabilised
pavement layers. However, the use of stabilisers can result in an increase in the cost of
construction and will only be cost effective if the increased cost can be traded off
against the improved performance of the road.
Also before selecting stabilisation techniques, the engineer must be aware of the
potential problems of stabilisation as well as its advantages. This section discusses
some of the more common problems in relation to cement and lime, the most used
stabilisers. Most of the problems can be avoided or reduced with careful material
selection and testing.
The problems listed below are in approximate order of occurrence, rather than
seriousness.
6.1 Construction
6.1.1 Quantity of stabiliser
It is important that the correct amount of stabiliser is added to the material. If too much
of the stabiliser is added, it can cause excessive shrinkage cracks. Too little stabiliser
will produce a material with insufficient strength or durability.
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6.1.2 Mixing
The stabiliser and material must be thoroughly and evenly mixed throughout the full
depth of the layer. For in-situ stabilisation, this is best achieved with a pulvimixer,
rotavator or a disc harrow, however an experienced grader operator can also obtain
good results. A common problem is that an incorrect depth of material is mixed, thusaltering the rate of application of stabiliser. Paige-Green (1998) recommends that
specialist equipment is used for mixing rather than agricultural equipment.
6.1.3 Compaction and limited time
It is essential that the correct degree of compaction is achieved if the material is to
reach the required strength. Compaction must be completed within the limited time
periods set in the specifications, which is often only a few hours for cement.
6.1.4 Rapid setting
A number of problems have been reported where a lime stabiliser has reacted very
quickly with certain materials (typically calcretes and tillites containing amorphoussilica, aluminium and/or high clay contents), causing a rapid set to occur and thus
preventing satisfactory compaction.
6.1.5 Curing time
It is essential to cure the material under correct conditions so that an adequate initial
strength is achieved before trafficking. For curing to occur a moist environment must
be provided by light water spraying, the application of curing membranes or the
placement of the next layer of material. If the periodic water-spraying method is used,
then care must be taken to ensure that the surface does not dry out between sprayings
as carbonation can occur (Netterberg and Paige-Green, 1984), (Netterberg 1987). The
curing period, usually 7 days before use by construction traffic, can cause delays which
should be planned for.
6.1.6 Variability
Small changes in the chemical composition of the material to be stabilised, or exposure
to harmful compounds after hardening can have large influences on the strength of
cement or lime stabilised materials. These compounds include organic matter,
sulphates, sulphides and carbon dioxide. Sulphate attack can cause volume changes
(swell) of the material. Work in the USA (Mitchell, 1986) and the UK (Dept. of
Transport, 1976) have placed limits on the total water-soluble sulphate content of the
material to be stabilised at 0.5 per cent and 1.0 per cent, respectively.
6.1.7 Testing
The amount of quality control testing that is required for stabilised materials is much
greater than for granular materials and this will add extra time, effort and cost to the
construction process.
6.2 Durability
6.2.1 Carbonation
Carbon dioxide in the atmosphere can attack the stabilised layer resulting in large
strength reductions over time. The influence of carbonation can be minimised by
ensuring that the stabiliser content of the material exceeds the initial consumption of
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Sub-base material specifications:
For pavements with a design life up to 12 million standard axles (msa), a cement bound
material (CBM2) or wet lean concrete (C10) is specified whereas for designs greater
than 12 msa, a cement bound material (CBM3) or wet lean concrete (C15) is specified.
The range of material categories and strength requirements are given in Table 4.Cement stabilisation of the subgrade can be used instead of importing a granular
capping material, as long as this stabilised layer has a minimum equivalent CBR of 15
per cent. It is also specified that compaction must take place within 2 hours of the
addition of cement.
Table 4 Strengths of UK cemented materials and moduli used for calculations
(DETR, 1998 & Croney, 1998)
Material category (in UK)
Minimum 7 day
Cube Compressive strength
(MPa) (= N/mm2)
*(Ref 1)
Modulus of elasticity
for use in structural analysis
(GPa)
**(Ref 2)
Average of 5 Individual Dynamic
(Ed)
Static
(Es)
Mean
CBM 1 Soil-cement (granular)
(silty PI ‹ 10)
(clay PI › 10)
4.5 2.5 18
7
1
10
4
0
14
5
0.5
CBM 2 Cement-bound material 7 4.5 23 13 18
CBM 3 Normal lean concrete 10 6.5 27 19 23
CBM 4 Stronger lean concrete 15 10 30 23 27
C7.5 Wet lean concrete 5.5
C10 Wet lean concrete 8
C15 Wet lean concrete 13
* Ref. 1: DETR, 1998: MCWH Series 1000, **Ref. 2: Croney and Croney, 1998.
7.1.2 Bituminous pavements
For flexible construction, weak cemented sub-bases may be used: CBM1, CBM2, or
C7.5, see Table 4, but, as reported by (Chaddock, 1997), current specifications require
these materials to be constructed with the same thickness as unbound granular sub-basematerials.
7.2 TRL ORN31 Practice
This design guide is for bituminous-surfaced roads in tropical and sub-tropical
countries. The design of concrete pavements is not included. The design catalogues for
various pavement types allow for stabilisation of the roadbase, sub-base and capping
layers using cement or lime.
The materials recommended in the guide are roadbase (CB1 and CB2) and sub-base
(CS), with unconfined compressive strength (UCS) values as shown in Table 5.
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Table 5 Properties of cement (or lime) stabilised materials
Material code Description Unconfined Compressive
Strength - UCS (MPa)
CB1 Stabilised roadbase 3 – 6
CB2 Stabilised roadbase 1.5 – 3
CS Stabilised sub-base 0.75 – 1.5
Specifications for these materials (CB1, CB2, CS) also include grading envelopes,
maximum values for Liquid Limit (LL), Plasticity Index (PI), and Linear Shrinkage
(LS) as well as recommended values for the coefficient of uniformity (i.e. the ratio of:
sieve size that 60 per cent material passes to sieve size that 10 per cent of material
passes).
For cement-stabilised materials, the amount of cement to add is determined by
laboratory trials according to BS 1924, using initial values of 2, 4, 6 and 8 per cent cement. Cubes or cylinders are then made and cured for set times before a strength test
is carried out. The UCS test is usually used to determine the optimum cement content.
The procedure for lime stabilised materials is similar, but a longer curing time is
allowed. For stabilised sub-base material, the CBR test can be used as an alternative to
the UCS requirement. A minimum value of CBR 70 per cent after seven days moist
curing is recommended.
In the design charts given in ORN31 the traffic loading is given in several categories
up to 30 million standard axles. It is important to note that where a stabilised roadbase
is shown, the surfacing is a thin surface dressing and not asphaltic concrete (Chart 8).
This is mainly to reduce the effects of reflection cracking. In Charts 1 to 6, a stabilised
sub-base is allowed but there is always an overlying granular roadbase, again to reduce
the possibility of reflection cracking.
7.3 USA Practice
The main design manuals used in the USA are the AASHTO Design of Pavement
Structures (AASHTO, 1993) and part II rigid pavement Design (1998).
Initial cement contents are recommended for the various soil types (classified under
AASHTO designation M145-82) as follows:
A1-A3 soils (granular materials): 3.5 – 7.0 % (by weight)
A4-A7 soils (silt clay materials): 7.0 – 10.0 % (by weight)
These are expected to give 7-day strengths of at least 2 MPa. The cement contents
given above only form a start point from which laboratory testing is required to achieve
the required strength.
7.3.1 Designs for concrete pavements
Extensive research on base support for concrete roads has been carried out in the USA
(Darter et al, 1995). This showed that the support provided to the concrete slab by the
underlying layer (called the base or sub-base) was found to have a very significant effect on the performance of the pavement. Amongst the findings it was reported that:
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i “On a soft subgrade (27 kPa/mm) changing from an aggregate base to a treated
base produces a large increase in the load carrying capacity (in this case 13 to
26 million ESALs)”.
i For an untreated granular base, increasing its thickness does not affect traffic
life. This is supported by earlier findings from the AASHO road test (1962)which concluded that “the effect on performance of varying the thickness of the
sub-base between 3 and 9 inches was not significant”. For a treated base,
however, with a modulus of approximately 6900 MPa, the thickness has a very
significant effect.
7.3.2 Designs for flexible pavements
The AASHTO (1993) pavement design manual has adopted the use of elastic modulus
as the standard materials quantification measure. However instead of using a wholly
mechanistic approach, the elastic modulus of each layer is correlated with a strength
coefficient to develop designs using the Structural Number approach. For the sub-base,
the manual also offers correlations between elastic modulus and CBR, R-value andTexas triaxial test results. To utilise benefits in terms of utilising a higher structural
number coefficient for a stabilised sub-base compared with a granular sub-base their
elastic modulus would be required. It may still not be possible to interpolate a
structural number coefficient because of the range of elastic moduli given in the
manual.
7.4 Australia
“State Road Authorities have been stabilising heavily trafficked roads to about 400mm
in depth for many years and Local Government Authorities are typically stabilising at
depths in the order of 150-200mm” (Pike 1998). The design method for a stabilised
pavement typically greater than 200mm is documented in the comprehensive Austroads
Pavement Design Guide (1992).
7.4.1 Austroads Pavement Design Guide.
The Australian guide to pavement design (Austroads, 1992) uses the mechanistic
approach to road design, which it emphasises has been developed for Australian
conditions. Pavement materials are characterised by the modulus of elasticity either
directly or through correlation with other tests. Eight test methods are given for
characterising stabilised pavement materials. These are ranked in order of preference
from flexural testing to presumptive values, being the most and least preferred ,respectively.
Stabilised sub-bases, below either a stabilised or crushed stone base material, are
utilised extensively in the manual as optional pavement materials. There is a substantial
saving in sub-base thickness when cemented instead of granular materials are used.
Should the cemented sub-base layer fail through fatigue, the manual permits a
continuance of the service life of the sub-base as a granular layer when estimating the
total traffic loading that the pavement will survive. Although a number of example
designs are given in the manual, it is necessary to compute the suitability of alternative
designs and select on their relative merit. To do this, a computer program is required
to calculate the various stresses and strains in the trial pavement.
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7.5 South Africa
The stabilisation of different pavement layers is widely used in South Africa. The
standards include the Technical Recommendations for Highways series especially
TRH13: Cementitious Stabilisers in Road Construction (1986) and TRH 14 Guidelines
for Road Construction Materials (1985). As shown in Table 6, there are four classes of stabilised material C1-C4, where C1 is the strongest. The specification limits become
less strict as the material is used further below the road surface. C1 materials are
seldom used because of their tendency to form wide shrinkage cracks (Paige-Green,
1998). Material Class C2 (usually cemented crushed stone) is used for a high quality
sub-base. The lower strength materials C3 and C4 (cemented natural gravels) are used
for lower layers or for bases on low volume roads.
Table 6 Strength requirements for stabilised materials (TRH 14, 1985)
Laboratory soaked UCS (MPa) after 7 days
100% mod AASHTO 97 % Mod AASHTO
Stabilised
Material
Classification Minimum Maximum Minimum Maximum
Minimum
ITS*
(kPa)
C1 6 12 4 8 -
C2 3 6 2 4 400
C3 1.5 3 1 2 250
C4 0.75 1.5 0.5 1 200
Note *ITS = Indirect Tensile Strength (COLTO, 1996)
7.6 The Philippines
The Philippines has a materials and construction manual: Standard Specifications for
Public Works and Highways, Volume 2 (DPWH, 1995). Most of the materials tests are
based on the American AASHTO methods. It should be noted that the manual does not contain pavement design information. Included in the manual are several specifications
for the use of stabilisers in the roadbase. These are:
1. Lime stabilised - Road Mix Base course (Item 203)
2. Cement stabilised - Road Mix Base course (Item 204)
3. Cement stabilised - Plant Mix Base course (Item 206)
Included in the specifications is a strength requirement. The appropriate strength test is
dependent upon the type of material, which is either:
a)
For gravelly soils: CBR test. Material passing the 19mm sieve shall have aminimum soaked CBR of 100 per cent (AASHTO T193), obtained at maximum dry
density (AASHTO T180).
b) For fine textured soils: UCS test. Seven day compressive strength = Minimum of
2.1 MPa (ASTM 1633).
In the 1995 specifications the use of stabilised materials for sub-bases is not specified
for either flexible or concrete pavements. However, the new Interim Pavement Design
Guide (DPWH, 1998) allows stabilised materials to be used for the base or sub-base in
asphalt pavements. In the pavement design catalogue, assumptions are made for the
layer coefficients of the materials, their elastic modulus and equivalent CBR values.
For stabilised sub-bases, an elastic modulus of 700,000 psi is assumed, although this
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seems high. Existing specifications for these materials are used, as given in DPWH,
1995. The new Interim Pavement Design Guide does not include the use of stabilised
sub-bases under concrete pavements.
8 PAVEMENT DESIGN FOR HEAVILY TRAFFICKED ROADS
The definition of ‘heavily trafficked roads’ varies between different design standards.
For example in South Africa ‘heavily trafficked roads’ are those which carry in excess
of 12 million standard axles (Freeme et al, 1987). In this report, as an approximate
guide, it has been assumed that ‘heavily trafficked roads’ are those with a design life of
more than 10 million equivalent standard axles.
For any pavement, it may be desirable to stabilise the base or sub-base in order to
protect the subgrade such that it can withstand the vertical loads imposed by traffic.
This is particularly true for heavily trafficked pavements, where high traffic loads or
volumes inevitably mean that stronger and thicker pavement layers are required.
Examination of the major pavement design guides from around the world has shown
that the use of stabilisation is widespread. All of the design guides studied allowed
stabilisation of at least one pavement layer and most of the guides reported that the use
of stabilisation became more beneficial for higher traffic levels.
Most pavement design manuals for heavily trafficked roads are based on a mechanistic
approach which models the pavement as a multi-layered elastic structure. The
stresses/strains at various points in the structure that result from the applied loads are
compared to establish stress/strain criteria. It is then necessary to calibrate these models
with observed performance data, i.e. empirical correlations, hence the procedure is
commonly referred to as mechanistic-empirical design.
The use of stabilised sub-bases in several design manuals is compared in Table 7. It can
be seen that pavements with granular sub-bases and stabilised sub-bases can be
specified in almost all of the design manuals listed for traffic levels up to 100 million
ESA.
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Table 7 Comparison of Pavements with Stabilised Sub-bases.
Country: USA UK
(a)
UK
(b)
Australia South
Africa
Philippines
Design Guide Source: AASHTO TRL
ORN31
DETR
HD26/94
Austroads CSRA
TRH4,
TRH13
DPWH
Interim
design guide
Year 1993 / 98 1993 1994-98 1992 1985 / 86 1998
ASPHALT
Does the specification
include: Granular base with
stabilised sub-base?
Y Y N Y Y Y
Maximum traffic for above
pavement design (million
ESA)
50 30 n/a 100 50 30
CONCRETE
Design guide includes
concrete?
Y N Y Y Y Y
Sub-base type allowed:
i) Granular material
ii) Stabilised material
Y
Y-
N
Y
N
Y
N
Y
Y
N
Maximum traffic for above
pavement design
(million ESA)
>500 - 400 300 50 30
As previously discussed, the stabilisation of the sub-base layer beneath a concrete
pavement can minimise problems caused by poor materials, difficult construction
conditions and, in some cases, low standards of construction quality control where
inadequate slab support can lead to premature cracking. The Philippines design manual
does not specify the use of stabilised sub-bases beneath concrete pavements (Table 7).Although it may not be possible to justify them at low levels of traffic, further study
could determine whether stabilised sub-bases would be economically beneficial at
higher levels of trafficking.
Apart from the pavement design manuals and specifications described earlier, there are
relatively few published reports concerning the use of stabilised materials for heavily
trafficked pavements. One of the few reports on this subject (Freeme et al, 1987) gives
details of accelerated loading trials in South Africa using the Heavy Vehicle Simulator
(HVS) on pavements with stabilised bases and sub-bases. One of the major results of
this study was the confirmation of the in-situ moduli (i.e. layer stiffnesses) for
cemented materials of different strengths and in different states of deterioration. It wasfound that weakly cemented materials, having UCS strengths of less than 3 MPa, can
break down quite rapidly into small blocks under trafficking. The report includes tables
of the moduli of strongly cemented and weakly cemented materials in their new (i.e.
uncracked) state and then at varying stages of their life. These values may be useful for
general mechanistic design of road pavements with stabilised layers. It was also
reported that many of the weakly cemented materials cracked and some of them
appeared to break down into a near-granular state. The report estimates that the
uncracked state for weakly cemented materials lasts for only approximately 10 per cent
of the life of the pavement.
A new form of erosion was also reported whereby the top of the stabilised base waseroded by mechanical interaction with the asphalt surfacing. This loose material was
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being broken down into fines and pumped out from cracks in the asphalt. It must be
noted that most of the pavements in this study had a cemented base and cemented sub-
base. It is likely that a stabilised sub-base with a strong unbound granular base would
not suffer from this type of deterioration and that the break-down of a stabilised
material could be avoided by using a higher cement content.
It was also reported that the thickness of the cemented layers must be sufficient to cope
with overloaded axles as well as cumulative repetitions of legal axle loads.
In the Philippines the amount of traffic will continue to increase, as will the demands
for high strength pavements that are able to carry even greater traffic. It can be argued
that no particular form of pavement construction is necessarily the best. The choice in
any situation will depend on factors such as the funding that is available for the project,
the local cost of the different forms of construction, the likely future maintenance
levels, the volume and composition of traffic, subgrade conditions, climate, and the
design life of the road pavement.
Before a new road is built, a detailed cost benefit analysis should be carried out todetermine the most appropriate form of construction. The use of a stabilised material
can help with whole-life cost reduction, but care should be taken to ensure that the
material, its construction and the environment are suitable. For the sub-base layer, the
decision whether to use unbound granular materials or cement-bound materials will
depend principally on the availability of good quality aggregates. If they are readily
available, their use will usually be cheaper than the alternative of stabilising a lower
quality material.
9 CONCLUSIONS
Stabilised sub-bases are now used by many road authorities for the design of heavily
trafficked roads. The primary benefits include the material’s increased load spreading
ability, which is highly relevant to the Philippines with its increasing traffic levels, and
the material’s increased ability to resist water penetration and hence to be more durable
in areas with less effective drainage. The use of stabilised sub-bases in the Philippines
is now included in the recent publication of the Interim Pavement Design Guide
(DPWH, 1998) which allows the use of stabilised sub-bases under asphalt surfaced
roads for design traffic levels up to 30 million ESA.
The stabilisation of pavement materials is a fairly straightforward operation and with
good construction techniques the properties of poor materials can often be significantlyimproved. It is essential that the amount of stabiliser to be used with a material is first
established in the laboratory and that there is an appropriate level of construction
supervision and quality control to ensure that similar strengths are achieved in the road.
Cement stabilised materials, in particular, offer the possibility of both increasing
pavement performance whilst utilising materials that may not generally meet accepted
sub-base specifications. However, increasing the cement content to achieve a higher
strength or to improve the material will also increase the possibility of reflection
cracking and hence the pavement designer must seek a balance between these two
conflicting factors.
The performance of both rigid and flexible road pavements in the Philippines wouldalmost certainly be improved by the use of stabilised sub-bases. What is not presently
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normally accepted material specifications. In this case construction costs should be
reduced and performance may well be enhanced.
5. Carry out FWD testing after construction to determine in-situ moduli. Other tests,
including DCP tests, coring and testing of cored samples may also be required. All
tests should be repeated periodically to establish the change in strength with time.6. Compare results and performance with control section. From these results it should
be possible to determine the theoretical future load carrying capacity of the
pavement by comparing the stresses and strains or Structural Number of the
experimental pavement to existing criteria (LR 1132 and AASHTO). These
estimates would then be compared to actual performance measured during the
monitoring period. It should be noted that this analysis can only be done on a site
specific basis where traffic volumes and load are carefully monitored.
11 ACKNOWLEDGEMENT
The work described in this report forms part of the Knowledge and Research (KAR)
programme of TRL (Director Mr S W Colwill), and part of the Research and
Development Division programme of Bureau of Research and Standards (Director Raul
C. Asis) of DPWH, Philippines. Any views expressed are not necessarily those of
DFID or DPWH.
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