Stabilised Sub Bases

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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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Literature Review: Stabilised Sub-Bases for Heavily Trafficked Roads

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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1

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 - 115.5 mm x 105 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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