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LAMBS. THEDESIGN, CONSTRUCTION AND USEOF

LARGE AGGRBGATE MTXES FOR BASES

Manual 13

1997

ISBN I 874968-t5-2

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Published by

Sabita Ltd

PO Box 6946

Roggebaai 8012

Fint published in

1993


List of manuals published by Sabita

Manual I Construction of bitumen rubber seals

Manual 2 Bituminous products for road construction

Manual 3 Test methods for bitumen-rubber

Manual 4 Specifications for rubber in binders

Manual 5 Manufacture and construction of hot-mix asphalt

Manual 6 Interim specification bitumen-rubber

Manual 7 SURF+ Economic warrants for surfacing roads

Manual 8 Bitumen safety handbook

Manual 9 Bituminous surfacings for temporary deviations

Manual 10 Appropriate standards for bituminous surfacings

Manual I I Labour enhanced construction for bituminous surfacings

Manual 12 Methods and procedures - Labour enhanced construction for

bituminous surfacings

Manual 13 LAMBS - The design, construction and use of large

aggregate mixes for bases

Manual 14 GEMS - The design and use of granular emulsion mixes

Manual 15 Technical guidelines for seals using homogeneous modified

binders

Manual l6 REACT - Economic analysis of short-term rehabilitation

actions

Manual 17 The design and use of porous asphalt mixes

Manual 18 Appropriate standards for the use of Sand Asphalt

Manual 19 Technical guidelines for the specification and design of

bitumen rubber asphalt wearing courses

Considerable effort has been made to ensure the accuracy and reliability

of the information contained in this publication. However, neither Sabita

Ltd nor any of its members can accept liability for any loss, damage or

injury whatsoever resulting from the use of this information. The content

of this publication does not necessarily represent the views of any

members of Sabita Ltd.


Preface

In promoting its core goal of excellence in asphalt technology, Sabita

started a broad based Asphalt Research Programme in 1988 to meet the

needs of the road industry in southern Africa. Manual 13 was the result

ofresearch into the design and performance ofheavy duty asphalt

pavements, identified by the Asphalt Research Strategy Task Force. This

revised edition has been updated to take account of developments since

the first publication in 1993.

The project focused on the use of Large Aggregate asphalt Mixes for

Bases (LAMBS) as a cost-effective solution to an increasingly aggressive

pavement environment, especially in terms of traffic loading. The

original research and development work addressed the following major

issues:

'

'

'

'

'

the development ofan analytical design approach for large aggregate

mixes

the development of laboratory compaction and testing procedures

the establishment of design criteria

an investigation into the performance of large aggregate mixes

the constructability of these materials.

Practical experience gained in the implementation of the technology on

major contracts in KwaZulu Natal was captured by the LAMBS steering

and liaison committees. This included

a

a

o

onsite design, testing and construction aspects

design methods, specifically in the fields of spatial composition

review in the light ofrecent international trends, such as

SUPERPAVE TM.

The findings highlighted misconceptions on the definition of LAMBS.

Recommendations are made on mix properties to achieve performance

and engineering requirements taking volumetrics into consideration.

Schematics are used to clarify the voids structure, aggregate size,

structure and grading. The use of gyratory compaction equipment

allowed comparison to the proposed gradation limits of SUPERPAVETM

This experience has been captured in the revised LAMBS Manual 13,

taking the technology to a higher level of application.


Acknowledgements

Manual 13 was originally compiled by the Division for Roads and

Transport Technology of the CSIR and Sabita, with contributions by

road authorities, consulting engineers and road builders.

The revised manual was compiled on behalf of Sabita by F Hugo,

University of Stellenbosch, and Andr6 de Fortier Smith, Institute for

Transport Technology, University of Stellenbosch, under the guidance of

the LAMBS Steering Committees and its advisors, with

acknowledgement to:

A Thomson

R Butler

G Palframan

R Lindsay

S Pienaar

K Ducasse

J Nothnagel SA Department of Transport

D Rossmann SA Department of Transport

E Knottenbelt SA Department of Transport

P Myburgh Sabita

R Vos

Sabita

C Rust

CSIR

B Verhaeghe CSIR

J Newell

Protea Asphalt

G Kilian

Protea Asphalt

F Swift

Rand Roads

C Johnson Rand Roads

A Lewis AA Loudon and Partners

S Wilson AA Loudon and Partners

H Loots

Bitutek Laboratory

R Dunbar

J Venter

J Marais

Stewart Scott Inc

C Andrews Stewart Scott Inc

KwaZulu Natal, Department of Transport

KwaZulu Natal, Department of Transport

KwaZulu Natal, Department of Transport

KwaZulu Natal, Department of Transport

KwaZulu Natal, Department of Transport

KwaZulu Natal, Department of Transport

Bradford Conning and Partners

Bradford Conning and Partners


Scope

This manual gives guidelines based on the findings of the HDAPs project

for the design of Large Aggregate Mixes for Bases (LAMBS). Issues

such as quality control and construction procedures are only addressed in

so far as they may influence the design criteria which were developed.

The details ofthe findings ofthis project are contained in a series of

reportsl-15.

Manual l3 addfesses the following:

' general considerations regarding the design of LAMBS, with a

review of relevant engineering properties and appropriate

surfacings

. a detailed description of the LAMBS mix design method,

including schematics to emphasise mix properties to achieve

defined engineering and performance requlrements

. constructability of LAMBS

o quality control of LAMBS

. cost considerations regarding the use of LAMBS

. practical illustration of the method through a case study

. an overyiew of the performance of LAMBS.

LAMBS hot-mix asphalt bases are engineering property driven designs

making use of dynamic test equipment and are not specifically linked to

aggregrate gradations. The performance of this approach has been

evaluated through the HVS to provide APT results and confidence in the

approach.


CONTENTS

INTRODUCTION

l.l Background

DESIGN CONSIDERATIONS FOR LAMBS

2.1 Introduction

2.2 Defrnition of LAMBS

2.3 Appropriate surfacings for LAMBS

2.4 Relevant engineering properties of LAMBS

GUIDELINES ON MIX DESIGN

3.1 Introduction

3.2 Method

3.3 Design process

3.4 Selection of mix components

3.5 Sample preparation in the laboratory

3.6 Determination of asphalt properties

3.7 Evaluation of behaviour and performance

CONSTRUCTABILITY OF LAMBS

4.1 Introduction

4.2 Guidelines on constructabilitv

CONTROLLING THE QUALITY OF LAMBS

5.1 Introduction

5.2 Requirements in terms of engineering

properties

5.3 Density requirements

5.4 Performance related specifications

ECONOMIC CONSIDERATIONS

IN THE USE OF LAMBS

6.1 Introduction

CONCLUSIONS

REFERENCES

APPENDD(

l0

l0

t7

t7

22

22

23

29

30

33

37

42

42

46

46

46

47

48

49

50

54


CONTENTS

Figures and Tables

Figure I Schematic of an Open Graded Mix

Figure 2 Schematic of a Stone Mastic Asphalt

Figure 3 Schematic of a Dense Continuously Graded Mix

Figure 4 Schematic of a LAMB

Figure 5 Schematic of a Large Aggregate Gap Graded Mix

Figure 6 'SUPERPAVEru Grading Chart for 26.5mm

nominal mixes

Figure 7 SUPERPAVETM Grading chart for 37.5mm

nominal mixes

Figure 8 Outline of the suggested mix design method

Figure 9 The Hugo Laboratory compaction method

Figure 10 Indirect tensile test

Figure 11 Dynamic creep test

12

t3

t4

15

l6

26

27

29

33

34

37

Table I

Table 2

Table 3

Table 4

Table Al

Table A2

SUPERPAVETM Cdteria for 26.5mm

nominal mixes

SUPERPAVEru Criteria for 37.5mm

nominal mixes

Mixing and compaction temperatures for

preparation of laboratory samples

Design criteria for large aggegate mixes for

bases mix design

Design test results for Jan Smuts LAMBS

Comparison of the design properties with the

constructed asphalt properties

25

26

3l

38

55

56


INTRODUCTION

1.1 Background

l.l.l The needfor heavy-duty asphalt mixes

Traffic volumes, axle loads and tyre pressures are increasing

world-wide. On some major routes in South Africa traffic has

increased to levels beyond the current highest design class. In

addition, the state is being lobbied to increase the legal axle load

to between 9 and 10 metric tons, which will worsen the

aggressive loading conditions.

This situation, in conjunction with decreasing road funding in

South Africa, has put new demands on the engineering

properties and cost-effectiveness of asphalt mixes.

1.1.2 Perfonnance of large aggregate mixes

Large aggregate mixes have considerable structural and

economic advantages over conventional asphalt (or finer graded

asphalt). Full-scale field testing using the Heavy Vehicle

Simulator (HVS) has illustrated that these mixes can carry traffic

well in excess of 50 million E80s (equivalent standard axles)

with relative ease.

Lower binder demands and lower aggregate crushing needs cost

less than the conventional requirements of asphalt mixes.

1.1.3 The needfor a new design approach

The aim of traditional design methods, such as the Marshall

method, is to produce laboratory samples with empirical

strength characteristics which approximate those of an in-service

mix after it has been fully consolidated under traffic. However,

freshly compacted laboratory mixes cannot be accurately

compared with working samples. These change as a result of

short- and long-term ageing of the binder. The former occurs in

the production process and the latter is due to material

properties and environmental factors.

Furthermore, in conventional practice, the structural and mix

designs of the pavement occur independently of each other -

8


often the mix is designed after the structural design has already

been completed. Ideally these design procedures should be

interactive.

Current empirical mix design parameters cannot provide a basis

of evaluation for all possible major distresses in asphalt mixes.

The Marshall method, for example, cannot adequately indicate

whether or not a mix will be susceptible to permanent

deformation.

Advanced structural analysis procedures can be done using

estimated material characteristics to describe fundamental

engineering properties. It is ofcourse preferable to measure the

appropriate parameters. Future design procedures should

therefore provide a basis for doing this.


DESIGN CONSIDERATIONS FOR LAMBS

2.1

Introduction

For the purpose of this manual, heavy-duty roads pavements are

those which are expected to carry traffic volumes in excess of 50

million E80s during the design period. The runways of

high-volume airports and certain loading facilities could fall into

this category. Although LAMBS were developed with

heavy-duty pavements in mind, they can also provide

cost-effective solutions where traffic volumes do not exceed 50

million E80s.

Other applications of LAMBS include:

'

'

'

rehabilitation projects where an asphalt overlay is required

special loading conditions in new projects such as steep

gradients and intersections subject to slow moving or

stationary traffic

where, due to time constraints, the speed of construction is of

the essence, for example airport runways and busy roads.

2.2

Defrnition of LAMBS

LAMBS obtain their strength and resistance to deformation

primarily from aggregate interlock. This is readily achieved by

using large top size aggregates such as 37,5mm and 53mm.

LAMBS do not presuppose a specific grading - rather, the

approach optimises the properties of available resources in terms

of raw materials and plant. Because LAMBS are used in the

base layer of a pavement, factors such as skid-resistance,

ravelling, and noise generation do not have to be considered.

To clarify any misconceptions regarding the definition

of LAMBS, schematics of typical asphalt mixes were

prepared as shown in the figures which follow. The

intention of these schematics is to emphasize the mix

properties that are sought to achieve certain

performance or engineering characteristics, which

ultimately guide the design composition. The schematics

are in no way a true representation of the volumetric

composition of the mixes.

l0


The acronyms given to mixes such as LAMBS or SMA's are

used to characterise these mixes in terms of:

o voids structure,

. maximum aggregate size,

. grading,

' aggrcgate structure.

The order of the schematics is relevant. The mixes shown

move from open graded mixes with stone to stone contact,

through a phase where the mixes have maximum density and

exhibit a sensitivity to shear resistance due to lubrication and

loss of stone to stone contact, to mixes where the mortar is the

dominant controlling factor, specifically after loss of stone to

stone contact. Gaps may occur in the gradings on either side

of the maximum density line. The figures also indicate whether

the mixes may be construed as LAMBS and whether they

satisfy the gradation limits of SUPERPAVETM26. Typical

gradings of the mixes are plotted on the 0.45 power gradation

chart with control limits and a restricted zone. The significance

ofthese charts is discussed in section 3.3.

ll


Figure I Schematic of an Open Graded Mix

00

80

60

40

20

0

T

T

0.075 0.3 19.0 25

Binder

a

a

a

a

Porous Asphalt when particle size is small (< 19 mm)

Penetration Macadam when maximum particle size is

large (> 37.5 mm)

Not SUPERPAVE

NotLAMB

t2


Figure 2 Schematic of a Stone Mastic Asphalt (SMA)

00

80

60

40

20

a

a

SMA when maximum particle size is small (< 25mm)

SUPERPAVETM when grading within control points - this

the most common SUPERPAVETM mix.

LAMBS when maximum particle size is large(> 25 mm) -

this is the preferred LAMBS mix.

13


Figure 3 Schematic of a Dense Continuously

Graded Mix

00

80

60

40

20

0

0.075 0.3 2.36

19.0 25

. Not SUPERPAVETM

. pseudo LAMBS, unlikely to qualify as such.

Gradings approximating the maximum density line (and within the

restricted zone) frequently militate against stable aggregate structures,

adequate binder films and exhibit sensitivity to fatigue, moisture

susceptibility and compactibility.

t4


Figure 4 Schematic of a LAMB

100

80

60

40

20

0

0.075 0.3 2.36 25 37.5

a

a

LAMBS, this is not commonly used.

SUPERPAVEru - but not commonly used.

l5


Figure 5 Schematic of a Large Aggregate Gap

Graded Mix

00

80

60

40

20t

I

-.-/-'- 'Zt

0

0.075 0.3 2.36 25 37.5

. SUPERPAVETM but not commonlv used

. NotLAMBS

l6


2.3

Appropriate surfacings for LAMBS

The purpose of surfacing LAMBS is to ensure that the required

functional properties, such as ridability and skid resistance, are

met. In general, any conventional surfacing can be used with

LAMBS. In urban areas, for example, where functional

properties are important, porous asphalt surfacings have been

used successfully on LAMBS. In such cases, LAMBS may have

to be sealed with a layer of bitumen prior to overlaying with a

porous asphalt to prevent moisture ingress.

2.4

Relevant engineering properties of LAMBS

An analytical mix design procedure calls for the incorporation of

relevant engineering properties into the process. The most

important properties are:

'

resistance to permanent deformation

t stiffness

'

' resistance to fatigue cracking

. durability

t resistance to moisture damage.

resistance to low-temperature cracking

Workability, which is not usually defined as a structural design

parameter, cannot be overlooked because if it is poor, it will

militate against the achievement of the above mentioned

engineering properties. Workability can be addressed by the

selection ofaggregates and aggregate grading, and binder and

filler content.

t7


2.4.1 Resistance to permonent deformation

Permanent deformation is the accumulation of plastic strain in

the asphalt caused by the combined effect of consolidation and

shear movement resulting from repeated traffic loading.

Normally excessive consolidation is prevented by proper

compaction during construction, while the resistance of a mix to

shear movement can be controlled by proper mix design. Shear

deformation is normally the primary cause of permanent

deformation failure.

Mix design parameters which will enhance the resistance of a

mix to permanent deformation include the following:

' a large maximum stone size (37,5mm or 53mm)

. suitable gradings, ensuring stone interlock and contact

. careful selection ofaggregate and filler

- use crushed aggregates and avoid sands that have round

particles

- avoid fillers with a high percentage of very fine particles

. low binder content (with due regard to fatigue life and

resistance to moisture damage)

. a high-viscosity binder.

2.4.2 Stiffness

The dynamic creep test was developed by Brown and Cooperr6

and has been used extensively in LAMBS'research. The

dynamic creep modulus can be used to predict permanent

deformation in asphalt bases and interim criteria for its use have

recently been developed r7. The static creep test is

unsatisfactory.r3

The stiffness of an asphalt base layer is indicative of its

load-spreading characteristics to afford protection to subbases

and subgrades.

An improved method of measuring the stiffness of

asphalt by the Indirect Tensile Test (ITT) was

originally developed by the University of

Stellenboschrs and later refined at the CSIR's Division of Roads

and Transport Technology.

l8


Horizontal measuring devices are attached to the

sample itself, allowing for sample movement without

measuring erors. It has been shown that the resilient

modulus values measured by this method correspond

very well with in situ field values determined by back

calculation from in-depth deflection measurements.

Design factors that will contribute to high stiffness

arele:

'

high viscosity binders

. optimum binder content

' gradings conducive to compaction

. a high filler content (within limits)

. low air voids (within limits).

LAMBS with a high stiffness should be used when

the base layer is well supported. When LAMBS is

used as an overlay on a very flexible structure, the

stiffness may need to be limited.

2.4.3 Resistance to low'temperature cracking

Low-temperature conditions prevail in few geographic areas in

South Africa2o. Such conditions, in conjunction with normal

ageing of the binder, may lead to low-temperature cracking.

The ability of a mix to resist low-temperature cracking can be

assessed by its indirect tensile strength and tensile strain at

maximum stress in the ITT test. These properties, in turn, are

largely dependent on the type of grading and binder, and the

amount of filler and binder.

19


2.4.4 Resistance to fatigue cracking

Resistance to fatigue cracking can be controlled by limiting

tensile stresses in the layer. This can to some extent be achieved

by either providing stiff support or increasing the depth of the

layer, depending on cost and economic considerations. There is

some evidence of shear fatigue within thick asphalt layers and

care should be taken to avoid segregation at the contact zone

between layers2r.

Fatigue properties are also dependent on the

composition of the mix itself. Factors that will

influence fatigue life are:

t binder content

' voids

' gradings

Recent CSIR work on fatigue testing of asphaltic materials

involved the use of the four-point beam test22. This method is

widely used overseas and produces more repeatable results than

the trapezoidal beam test or the ITT test.

2.4.5

Durability

Although base layers are protected by the surfacing to some

extent, and weather and climate are not as critical as they are for

surfacings, some of these factors will affect the upper part of the

base layer.

Durability can largely be controlled by sound mix design, such

as using high-quality aggregate and proper gradings. Durability

can be improved by thicker binder films which are obtained bv

the use of large aggregate asphalt.

20


There is growing evidence that surface cracking is a primary

cause of distress in asphalt pavements. This can be caused by:

. rollers during construction compaction

. thermal stresses

. fatigue due to traffic load

. ageing

. delamination of layers

. stripping

. shrinkage.

To prevent stripping in wet conditions, the asphalt must either

be impermeable or porous with interlinking voids, to prevent a

build-up of excessive pore water pressure resulting from tyre

passes. Mix compositions leading to trapped water or minute

capillaries should be avoided. Segregation must be limited if

not totally eliminated as it allows ingress of water which could

lead to stripping.

2l


3 GUIDELINES ON MIX DESIGN

3.1 Introduction

Asphalt mix design is the selection of a mix composition

yielding an optimal balance of performance characteristics of the

material. Although not limited to large aggegate asphalt, the

method has been developed and evaluated primarily for asphalts

containing aggregate with a maximum size of 37,5mm and

53mm. It has been developed for heavy-duty traffic conditions,

such as high intensity of loading and slow speeds.

The method addresses the following engineering properties:

'

resistanoe to permanent deformation

' stiffness

' resistance to low-temperature cracking

o resistance to fatigue cracking

' durability

Properties which should receive particular attention are

resistance to deformation, measured by the dynamic creep test,

and stiffness, measured by the dynamic ITT. Resistance to lowtemperature

cracking and fatigue, as well as durability, are

controlled by specific criteria such as indirect tensile strength

and stiffness, film thickness and voids content. The design

method follows an elimination process to minimise unnecessary,

time-consuming mechanical testing.

3.2 Method

Although the method focuses on the design of mixes which are

resistant to permanent deformation and satisfy other pavement

engineering needs, the following should also be taken into

account when the final selection is made:

. constructability of the mix

' cost considerations.

22


The main variables that should be considered during the design

process are:

. binder content and type

. grading (aggregate type and shape)

. coarse (fractured faces) and fine aggregate angularity

. sand equivalence (clay content)

. filler/binder ratio

. VIM ,VMA AND VbE

all influenced by

- Amount of filler

- Degree of compaction

- In-service temperature

. Vo G^ (G.. = Maximum Theoretical Specific Gravity of

the Mix, Rice's Method)

Although the method focuses on the design of mixes which are

resistant to permanent deformation, the following should be

taken into account when the final selection is made:

. fatigue characteristics in terms of stress or strain regime

. specific constructability aspects

' cost considerations.

The three main variables that should be considered during the

design process are:

. binder content and type

. grading (aggregate type and shape)

. amount of filler.

3.3

Design Process

A volumetric mix design philosophy similar to SUPERPAVETM

is recommended. The first phase of the design process is the

determination of an optimum design aggregate structure. Three

trial or candidate aggregate blends with varying gradings are

compacted at the estimated optimum binder contents of these

blends. Optionally the candidate blends may be compacted at

two additional binder contents to determine the effect of binder

content on the volumetric properties of these blends. The blends

are then evaluated in terms of volumetric criteria specific to

VIM, VMA, Vbe and the filler/binder ratio. The candidate blend

which best meets these criteria has the optimum design

23


aggregate structure. The next phase ofthe design process is the

determination of an optimum binder content by evaluating the

volumetric and engineering properties of the optimum blend

compacted at four different binder contents.

Even though the grading of the mix is only one of many factors

influencing mix characteristics, it is an important design

consideration and a logical point to start the design process.

Trial blends should be selected in accordance with the schematic

gradings shown in Figures 6 and7, while avoiding the restricted

zone, defined relative to a graph established from a n-power

equal to 0.45 in the formula:

'

: ($)"

* 100

where:

p = percentage passing sieve size d (mm)

D = maximum stone size

n = a parameter established by Fuller to determine the

shape of the grading curve.

The n-power of 0.45 results in mixtures with minimum VMA as

indicated by Nijboef3 and Hugo2a. The gradings of

SUPERPAVETM mixes are further limited by control points

which govern the range within which the gradings may lie.

These were established in accordance with ASTM specification

D35 1525 and their values are given in Tables I and 2 for the

respective nominal gradings. LAMBS gradings are not

necessarily restricted to lie within these control points, but as

they move further away, the mortar reduces volumetrically and

the mix progressively opens.up. This could cause a loss of shear

strength disqualifying the mix as a LAMB.

The restricted zone has been defined to avoid mixtures that have

a high proportion of fine sand relative to total sand in the mix.

Gradations that pass through the restricted zone often result in

tenderness, which is manifested by a mixture which is difficult

to compact during construction and offers reduced resistance to

permanent deformation during its performance life. The

restricted zone also steers gradations away from the maximum

.A


density line. Gradations that follow this maximum density

gradation often have inadequate VMA to allow a sufficient

amount of binder for durability and fatigue resistance23'26.

These mixtures are sensitive to binder content and lack shear

resistance.

The following definitions with respect to aggregate size apply:

. Maximum Size: One sieve size larger than the nominal

maximum size.

o Nominal Maximum Size: One sieve size larger than the

first sieve to retain more than 10 percent.

For 25mm and 37mm nominal aggregates, the following

guidelines set out in Tables I and2 and Figures 6 and7, should

be used for establishing aggregate gradings.

Table I SUPERPAVETM Criteria for 26.5 mm

nominal mixes

Restricted Zone Boundarv

Sieve. mm

Control Points

Minimum

Maximum

37.5

100

26.5

90

100

19

t2.5

9.5

4.75

2.36

1.18

19

45

40

27

18

40

3l

24

0.6

0.3

t4

1l

l8

ll

0.15

0.075

I

I

25


tm

90

80

70

340 c

30

20

1o

0

2.34 4.75

Si€ve Size BeiB€d to 0.45 Power

Figure 6 SUPERPAVETM Grading Chart for 2,6.5 mm

, nominal mixes

Table 2 SUPERPAVETM Criteria for 37.5 mm

nominal mixes

Restricted Zone Boundarv

Sieve. mm

Control Points

Minimum

Maximum

53

100

37.5

90

100

26.5

l9

12.5

9.5

4.75

35

35

2.36

1.18

r5

4l

23

r6

27

22

0.6

t2

t6

0.3

l0

10

0.15

0.075

0

6

26


100

90

80

70

o

960

uo

o

3rc

d

30

20

10

0

Sieve Size Raised to 0.45 Fow er

Figure 7 SUPERPAVETM Grading Chart for 37.5 mm

nominal mixes

The design process entails the following steps:

'

determination of which grading/filler combinations are

achievable using material available from the quarry

. preparation oflaboratory samples using three achievable

candidate gradings compacted at the estimated optimum

binder contents of these gradings and optionally at two

other binder contents

'

'

determination of density and calculation of the voids

content, percentage voids in mineral aggregate, voids filled

with bitumen and film thickness

rejection of any grading/filler/binder combinations which do

not comply with the volumetric criteria and choosing an

optimum aggregate blend

. compacting the optimum aggregate blend at four different

binder contents and choosing a design optimum binder

content in terms of the resulting volumetric properties

'

determination of the indirect tensile stiffness, indirect

tensile strength and tensile strain at maximum stress of the

optimum blend at the different binder contents

. rejection of those binder contents which result in mixtures

which do not comply with the criteria for stiffness, indirect

tensile strength and strain at maximum stress

27


dynamic creep lesting of the remaining binder content

combinations

rejection of those binder contents which result in mixtures

which do not comply with the dynamic creep modulus

criteria

selection of the optimum grading/filler/binder mixture,

based on performance and behaviour, required

constructability aspects and cost considerations

28


This can be summarized as below:

Select three possible candidate gradings with quarry bin blendings using an n-value of0.45 but

taking due cogniance of the rcstrictive zones

v

Prcpare laboratory samples at the stimted optimum binder content and opionally two other binder

contents using either the Hugo-hamrer or gyratory mmpactor

. at the estimated optimum

. optionally at estimated optimum binder content - 0,5%

. optionally at estimated optimum binder content + 0.5%

v

Determine density and calculate % voids, % voids in mineral aggregate, % voids filled with

bituren and film thiclmess

Y

Chmre an optimum aggrcgate blend based on the volurnetric criteria

Y

Compacting the optimum aggregate blend at four different binder contents and choosing a design

optimum binder content in terms ofthe resulting volunrtric properties

. at the estirnated optimum

. optionally at estimated optimum binder content t 0,3%

. optionally at estimated optimum binder content + 0.6%

v

Determination of the indirect teNile stiffness, indirect teNile strength and tensile strain at maximum

stress of the oDtimum blend at the different binder mntents

Y

Rejection of those binder contents which result in mixtures which do not mmply with the critda

for stifftress, indiret tensile strength and tensile strain at maximum str€ss

v

Subject the remaining binder content combimtions to dymmic crep testing

v

Reject the binder contents which rcsult in mixtures which do not conform to the dynamic creep

cntenon

v

Select the application grading/binder/filler combination based on:

Engineering properties, Constructability, Economics

Figure 8 Outline of the suggested mix design method.

3.4 Selection of mix components

Aggregate, filler and binder properties should comply with the

guidelines given in TRH8re, TRH1427, SABS 1200'zE, the CSRA

Standard Specifications2e and/or the project specifications,

whichever is relevant.


In addition, the following should be taken into consideration

during the design stage of LAMBS:

. The use of rounded natural sand should be minimized.

The use ofcrusher sand is recommended to enhance

intergranular friction and thus resistance to deformation. A

minimum fine aggregate angularity of 45Vo is recommended2s.

. It is recommended that at least 9OVo (by mass of the

crushed aggregate) should have two or more fractured faces

. A minimum clay content (sand equivalent) value of 45 is

recommended25.

. A high-viscosity binder, preferably a 40150 pen bitumen, is

recommended unless the pavement structure is very flexible.

. Flakiness of the aggregate should be limited to 3OVo.

The design should be based on gradings readily achievable for a

specific project. The grading curve should ideally not reflect

small abrupt deviations, as these can lead to a decrease in

structural strength as well as segregation during construction.

Constructability, which should be taken into account during

design, is generally improved by the use ofhigher binder and

filler contents, and finer gradings curves. It should be noted that

semi-gap graded mixes tend to segregate easily when the binder

content drops too low.

At least four binder contents should be used with the optimum

design aggregate blend. For continuous gradings with maximum

stone sizes of 37.5mm, for example, the estimated optimum

bitumen content should generally fall in the range of 4Vo to 4,5Vo.

The first phase of the design process is to blend the quarry

grading (iterative computer programs are recommended) to

match the required candidate gradings as closely as possible.

Three candidate grading/filler blends should then be used to

manufacture the laboratory samples.

3.5

Sample preparation in the laboratory

If only one grading/filler combination can be used, at least six

samples per binder content should be prepared (if all

engineering tests arerequired). Three ofthese samples are then

used to determine the average indirect tensile stiffness, indirect

tensile strength and strain at maximum stress with a specific

30


inder content. If the samples pass this first stage, the remaining

three are used (or can now be prepared) to determine the

dynamic creep modulus.

If, as recommended above, a more comprehensive design matrix

is used, the number of samples may be decreased to four per

bitumen/ filler/grading combination, two of which are used for

the ITT and two for the dynamic creep test.

3.5.1 Mix preparation

The ideal mixing and compaction kinematic viscosity ranges for

bitumen are 170 + 20 x 10-6 m2ls and 280 t 30 x 10-6 m2/s

respectively. The mixing and compaction temperatures should

be determined from temperature-viscosity curves for each

bitumen type and the specific refinery. Typical mixing and

compaction temperatures for the generally available bitumens

are summarized in Table 3 (method C2, TMHI)3o. It is

suggested that the aggregate be heated to l0'C above the mixing

temperature prior to mixing. Generally 4,2kg of asphalt must be

prepared to produce samples 150mm in diameter, with a height

of 96mm, but this may differ significantly, depending on the

type of materials used.

After mixing, the uncompacted asphalt is spread out in a bowl to

a depth of about 60mm and placed in a force-draft oven, where

it is conditioned for four hours at the specified mixing

temperature. Half-way through the process, the material should

be remixed with a spatula25.

Table 3 Mixing and compaction temperatures for

preparation of laboratory samples.

PROCESS=

Mixing

P4Y:T,T$4.FI9I.:

.,,:, BITIJMEN :::

40/50 pen

60/70 pen

80/100 pen

fel FruRE

:i

160 +5'C

145 +5'C

140 +5"C

Compaction

40/50 pen

60/70 pen

80/100 pen

150 +5"C

135 t5'C

130 +5'C

JI


3.5.2 Laboratory compaction

Because of its availability, the Hugo compaction method is

recommended for the laboratory compaction of samples

(Figure 9). The automated Hugo hammer is preferred.

However, if manual compaction is applied, steps should be

taken to maintain consistency in compaction. LAMBS are

generally used for high traffic volumes and axle loadings,

therefore it is important to do the mix design with the best

available procedures. Provision should therefore be made for

gyratory compaction, where it is available. It is recommended

that the final mix design, when designed using the Hugo

compaction method, be correlated with the gyratory compactor.

In the Hugo compaction method, moulds, base plate and

hammer should be heated to compaction temperature before the

mixes are placed in the moulds. When the mix is ready, the

mould, collar and base plate are positioned on the compaction

pedestal. A piece of round filter paper is placed in the bottom of

the mould. The mix is placed in the mould, care being taken to

avoid segregation. It is prodded 15 times with a heated spatula

around the perimeter and the rest ofthe surface is prodded 10

times, leaving a slightly convex surface. A piece of filter paper

is placed on top of the mix and the heated hammer is placed in

position.

Samples are compacted at the temperature shown in Table 3. If

the temperature drops below the specified levels, the mix should

be discarded. Under no circumstances should it be reheated. The

10,438 kg hammer is used to apply I l0 blows to one side of the

sample.

It should be kept in mind that this compaction energy applied is

related to the volume of material to be compacted to a design

height of 96mm (a guideline at which the operator must aim)

and deviations from this require that the number of blows be

adjusted to ensure the same unit compaction energy per volume

material.

32

It should be noted that the applied energy is related to the

volume of material when the compacted height is 96mm. The

unit energy compaction should be maintained if another design

height is used. After each blow the hammer is rotated by one

segment. The hammer is turned over and the same process


epeated. The sample is then cooled prior to removal from the

mould.

10,438 kg

110 blows per side

Turn 30o after each blow

Figure 9 The Hugo laboratory compaction method.

3.6 Determination of asphalt properties

Three sets of criteria are used in the process of elimination

during the design procedure:

. voids content, percentage voids in mineral aggregate (VMA)

and film thickness

. indirect tensile stiffness, indirect tensile strength and tensile

strain at maximum stress

' dynamic creep modulus.

The procedures and criteria for the above tests are discussed

below.

3.6.f Vohmetricproperties

Methods of obtaining the properties required to conduct the

design process are given in TRH8te and TMHI3o. The required

properties are:

' bulk relative density

' voids content in mix (VIM)

' voids in mineral aggregate (VMA)

' voids filled with bitumen (Vbe).

JJ


3.6.2 Indirect tensile testing

Figure 10 Indirect tensile test.

The indirect tensile test (diametral configuration) is used to

determine the following properties:

' stiffness

' indirect tensile strength

t tensile strain at maximum stress.

The use of these properties in structural analyses, distress

prediction and the control ofcertain failure modes are discussed

below.

The stiffness of the asphalt is determined by the dynamic

indirect tensile test using a locally developed measurement

technique, in which the LVDTs are attached to the sample.

(See Figure l0).

The test conditions for the dynamic indirect tensile test are:

'

load type

. load magnitude

' temperature

o assumed Poisson's ratio

'

10 Hz haversine wave

3OVo to 5OVo of indirect tensile

strength

25'C

0,4.

80 conditioning cycles prior to testing

34


The resilient moduli (stiffness) must be calculated by the following

formulae3l:

Enr = P(unr+O.27ytMll

Enr = P(unr+O.nYtAIJt

onr =

onr =

3.59 AHI/AU - 0.2i1

3.59 AHr/AVr - 0.T1

where:

Enr = instantaneous resilient modulus (MPa)

Enr = total resilient modulus (MPa)

onr = instantaneous Poisson's ratio

l)nr = total Poisson's value

P = dynamic load (N)

[ = thickness of sample (mm)

AHr - instantaneous recoverable horizontal deformation

(mm)

AV1 = instantaneous recoverable vertical deformation (mm)

AHr - total recoverable horizontal deformation (mm)

AVr - total recoverable vertical deformation (mm)

Test results are given as the average stiffness of five readings.

Two stiffness values are given - with an assumed and a

calculated Poisson's ratio. When the assumed Poisson's value is

used to calculate the stiffness, it is a function ofthe horizontal

deformation. When calculated Poisson's ratios are used.

however. the stiffness is a function of the vertical deformation.

The latter method is generally used, with the actual stiffness

normally lying between the two values.

In the static indirect tensile test, the maximum force generated

during the failure of the sample in a displacement controlled

mode is measured. New measurement techniques allow accurate

calculation of the tensile strain at maximum stress.

Test conditions for the static Indirect Tensile Test are:

. displacement rate : 50mm per minute

. temperature : 25nC

35


The indirect tensile strength is calculated as follows32:

or = zP"nhEtD

where:

6r = indirect tensile strength (MPa)

Pur = ultimate applied load required to cause the sample to

[ =

D =

fail (N)

thickness of sample (mm)

diameter of sample (mm)

The tensile strain at maximum stress is calculated as follows32:

. For l00mm diameter samples (l2.7mniloading strip)

€-, = AHx(0.00019o+0.000062y(0.0100o+0.0027)

. For 150mm diameter samples (19.05mm loading strip)

€., = AHx(0.fi)008u+0.000028y(0.0067o+0.0018)

where:

t s = tensile strain at maximum stress

AH = horizontal deformation at the maximum load (mm)

D = Poisson's ratio

3.6.3

Dynamic creep test

The dynamic creep test is used to assess the deformation

characteristics ofthe asphalt. It is preferred to the static creep

test, mainly because it takes the dynamic effect of traffic loading

into account and yields results with superior repeatability.

Initial results have also shown that the dynamic creep modulus is

superior in predicting rutting of asphalt layers. (See Figure 1l).

36


Figure 11 Dynamic creep test

The conditions for the dynamic creep test are:

'

loading type

. load magnitude

' temperature

' duration oftest

. conditioning cycles before testing

The dynamic creep modulus is defined as follows:

0,5 Hz square wave

1@ kPa

40"c

3 600 cycles (two hours)

30 cycles.

1""" = "" I

where:

Eoc = dynamic creep modulus (MPa)

o = maximum uniaxial stress

e

= total vertical strain after 3 600 cvcles

3.7 Evaluation of behaviour and performance

The results of the tests described in the previous section must be

evaluated and related to the mix performance and behaviour. As

mentioned earlier, not all the grading/filler/bitumen

combinations need to be evaluated for all the distresses

37


and criteria, as some would have been eliminated in the design

process. The interpretation ofthe test results obtained is dealt

with in this section. As the focus is on distress types related to

bases, distress mechanisms such as bleeding, skid resistance and

ravelling are not considered.

3.7.1 Volumetricconsi.derations

Using the percentage voids and VMA criteria (Table 4), specifrc

grading/fi ller/bitumen combinations may be discarded, thus

eliminating unnecessary mechanical testing.

The calculation of film thickness as described in Appendix B of

TRH 8 should strictly be applied using the fraction percentage

by volume passing and not the fraction percentage by mass

passing. This approach would account for aggregates with

varying specific densities. An aggregate gradation with a denser

fines fraction will generally have a higher calculated film

thickness than the same aggregate gradation with a denser stone

fraction.

Table 4 Design criteria for large aggregate mixes

for bases mix design

ffirv .,, ' "-Hl' $, ffi

Percentage voids

Density

VMA (binder content)

VMA (percentage)

Vbe (percentage)

Film thickness

Stiffness @ 25" Cll0Hz

Stiffness @ 25" Cll0Hz

rTs @ 25'C

Dynamic creep modulus on

laboratory briquettes

ffffift1: -,".,, r,,,,:

4Vo min.,6Vo max.

Aim for max. density

Aim for dry side of min. VMA vs binder

curye

l2Vo min. for 25mm Nom max., llVo min.

for 37.5mm Nom max.

Aimfor 65Vo min.75Vo max.

Aim for min. 8 microns (depends on

aggregate typ€)

min. 2000 MPa

For stiff layer:

For flexible layer: min. 1000 MPa

max. 2500 MPa

Min. 0.8 MPa (800 kPa)

Min. 10 MPa

If the voids content exceeds an upper limit, resistance to ageing,

fatigue and water damage normally decrease significantly,

whereas mixes with too few voids tend to become unstable as

the voids are filled with bitumen.33

38


A lower limit is specified for the VMA. Mixes with VMA

values lower than this limit may either be deficient in bitumen or

have too low a voids content.

The other volumetric criteria shown in Table 4 provide an

indication of the performance to be expected. However, they

must be confirmed by mechanical testing. For example, mixes

on the dry side of the VMA curve should be more resistant to

permanent deformation than those on the wet side. Mixes with

thin film thicknesses and high voids contents may be prone to

ageing and fatigue cracking. The maximum bulk density also

serves as a guide to optimum binder content as far as

performance is concerned. Finally, the limiting values of the

Vbe should ensure adequate durability.

3.7.2 Stiffness

The stiffness of the asphalt forms the link between the asphalt

mix and structural designs of the pavement. Adequate asphalt

stiffness will contribute to the conffol of distress in all the

pavement layers. For example, a stiff asphalt base improves the

load-spreading ability of the pavement, leading to more effective

protection of the underlying layers and subgrade.

The interaction between fatigue and stiffness should be

considered. Where high strains are likely to occur in the base

due to poor support, the mix should be designed with a lower

stiffness.

A base material with a lower stiffness will result in less stress in

the base, as well as an improved fatigue life. The structural

design process should therefore be iterative and interactive with

the mix design process. This is the reason for the specification

of two sets of stiffness criteria in Table 4.

3.7.3 Resistance to fatigue cracking

Asphalt fatigue occurs when the stiffness of a mix is

progressively reduced by repeated stress applications, ultimately

leading to micro and subsequently to macro cracking. Both the

structural design and the mix design will influence the fatigue

characteristics ofthe asphalt layer. For heavy duty pavements it

is recommended that the layer thickness and support should be

optimised to provide the economic solution. The mix design

39


process can then be directed to obtaining a mix with resistance

to permanent deformation.

3.7.4 Resistance to low-temperature cracking

Low-temperature cracking occurs when the tensile stresses

caused by reductions in temperature and traffic loading exceed

the fracture strength of the asphalt. Methods of predicting this

type of distress are available, but they require a range of tests

which are outside the scope of routine mix design. The tensile

strength of asphalt can indicate possible susceptibility to lowtemperature

cracking, the minimum of 800 kPa should be

adhered to.

3.7.5 Mohture damage

Moisture damage is caused by a loss of adhesion or bond

between the binder and aggregate in the presence of moisture.

The tensile strength ratio ofthe indirect tensile strength of

conditioned samples to that of normal samples should be greater

than 0,8 for the mix to be resistant to water damage under

normal loading and environmental conditions.3a

To assess the susceptibility of the mix to moisture damage, it is

recommended that the procedure as outlined in ASTM test

method D 486735 be followed. Six specimens zue compacted to a

void content corresponding to void levels expected in the field,

usually between 6-87o. This set is divided into two subsets of

three specimens which have approximately equal void contents.

One subset is maintained dry while the other is partially

saturated with distilled water and moisture conditioned.

Specimens are saturated by applying a partial vacuum as

calculated below for five minutes.

Partial vacuum (mm Hg) = 508 - 0.0806 x height above sea level

(m)

40

Applying the procedure outlined in ASTM D 4867, a saturation

level of between 55-8OVo should be aimed for. The saturated

specimens are soaked in distilled water at 600C for 24 hours and

subsequently at25oC for the one hour prior to testing.

Optionally, the specimens may be frozen at -180C for 15 hours

before thawing. The tensile strengths of both sets are determined


using the tensile splitting test and the effect of the moisture is

indicated by the tensile strength ratio. After splitting, the

specimens are visually inspected for moisture damage35.

The risk of moisture damage occurring is likely to be much

higher when water is trapped in the asphalt and excessive pore

pressures can develop under traffic. In crushed stone bases this

has been clearly demonstrated under HVS loading. In general it

was found that the bases with permeability greater than 50mUh,

as measured by using the Marvil apparatus, showed accelerated

increase in distress. Until further research has indicated

otherwise, it is suggested that this value also be used as a guide

for asphalt bases36.

3.7.6 Resistance to permanent deformation

Deformation of asphalt is due to the combined effects of postconstruction

consolidation and shear strain. Shear deformation

is the more severe mode and should be countered by proper mix

design. Excessive consolidation is normally prevented by

proper compaction during construction.

A limit of l0 MPa for the dynamic creep modulus is a simple,

yet effective, approach. It has been found that the dynamic

creep modulus can be used to predict resistance to deformation

accurately in the field. More sophisticated, but also more

complex, methods of predicting the exact amount of deformation

may become available in the future.

3.7.7 Final design

As mentioned earlier, asphalt mix design is the selection of mix

components to achieve a desirable balance between properties.

If two binder contents produce mixes with similar high dynamic

creep moduli, it is advisable to choose the mix with the higher

binder content, for improved fatigue performance and ageing.

Constructability is improved by higher binder and filler

contents, and finer gradings. The final selection should be based

on these considerations.

4l


CONSTRUCTABILITY OF LAMBS

4.1

Introduction

The following guidelines generally apply to the construction of

all asphalt layers. Where the guidelines particularly apply to

LAMBS, special mention is made.

It has been shown in the consfuction ofseveral trial sections

and large contracts that large aggregate mixes can be

constructed with relative ease.a A number of variables were

addressed during the construction oftrial sections, including:

'

'

'

'

'

'

'

type of plant (batch vs drum)

hauling distance

length and paving thickness ofsections

construction equipment (eg vibratory vs static steel wheel

breakdown rollers)

gradings (eg continuously graded, semi-gap-graded)

maximum stone sizes (37,5mm vs 53mm)

binder source, tyPe and content.

Subsequent construction projects using LAMBS were studied in

order to validate or revise findings in the earlier construction

trials. Where appropriate, the discussions of the respective

topics have been revised.

4.2 Guidelinesonconstructability

Problems related to constructability can be avoided by taking

appropriate measures. Some recommendations regarding the

manufacturing and construction of LAMBS are given below.

These have resulted from experience gained by industry37 and

from research projects, trial sections and implementation

projects.

4.2.1 Stockpiling

Careful attention should be given to the stockpiling of the

aggregate. This is the first phase in the manufacturing process

and even limited segregation occurring in the stockpile will have

an increasingly negative effect in the other processes.

42


4.2.2 The coldfeed system

The coldfeed system need not be modified for LAMBS, but

when 53mm stones are used, some modifications to the gates of

the 53mm cold storage may be required. Due to the larger

grading spectrum, there must be an adequate number of coldfeed

bins to accommodate the various aggregates, especially in tle

case of drum plants.

4.2.3 The screen house

Modifications to the screen sizes may be required to

accommodate the large-sized aggregate. In many cases all that

is necessary is the removal ofthe top screen - provided that the

quality of the large-aggregate stockpile is carefully monitored.

The landing deck may require strengthening and the screen

tensions must be checked.

4.2.4

The pugmill mixing process

The flights in the dryer and drum mixers may have to be

reinforced to minimise increased wear and tear. In the case of a

batch plant, the clearance between the paddle tips and liners in

the pugmill may have to be increased.

The capacity of the pugmill is important and the motors must

have adequate power. It is advisable to install a belt-driven

device in case overloading occurs. A longer mixing time may

be necessary to coat the larger aggregate.

4.2.5

Drum mixers

Regarding wear and tear, drum mixers are preferred to batch

plants for the production of LAMBS, mainly because they have

no elevators, screen house or pugmill. However, because it may

be more difficult to control the grading in drum mixers, the

following procedures are suggested:

. the lifters/flights in the drum need special attention

'

'

the mixing time may be extended by changing the angle of the

drum

the bitumen injection pipe can be shortened or lengthened to

achieve the required coating

t it is preferable but not essential to have an aftermixer


. it is recommended that a secondary discharge chute be

installed at the exit of the drum, thus creating storage for a

reasonable amount of asphalt and decreasing the vertical drop

of the asphalt.

4.2.6 Inading onto trucks

4.2.7 Silos

4.2.8 Paving

Segregation may occur when the asphalt is dumped onto a truck.

This can largely be prevented by loading the front of the truck

first, then the rear and finally the centre.

Silos should preferably be cylindrical. Segregation may occur if

the asphalt is dropped a significant height in tall silos. To

reduce this effect, a secondary stage within the silo is suggested.

The silo must not be continually emptied and a fair volume of

material must remain to reduce the height of the drop.

Whenever possible the layer thickness should be maximised

since it improves uniformity and degree of compaction. The

thickness of the lift being paved should be at least 2 times the

maximum aggregate size; for example, a l20mm layer is

preferred rather than 2 x 60mm layers.

It should be remembered that for dynamic creep control tests,

the cores should be at least 96mm thick. Difficulties experienced

with surface tolerances should not be allowed to drive or force

thin layers to be paved. The flow control gates and slat feeders

must be set to ensure that the required amount of material is fed

to the augers and that a uniform height of asphalt is maintained

at the front of the screed. Handwork should be limited to the

absolute minimum during the paving process and backspreading

should be avoided.

4.2.9 Compaction

44

Compaction is generally regarded as the single most important

factor affecting the durability and resistance to permanent

deformation and fatigue cracking of the asphalt. Adequate

compaction is one of the most cost-effective methods of

ensuring the quality of the layer.


The advantage of TAMBS over conventional asphalt is that they

retain their temperature longer, which can assist the compaction

process. LAMBS have proved to be easier to compact than

some thin asphalt layen.

It is recommended that a static three wheel roller of

approximarcly l2t be used for the breakdown compaction. A

22tto2iTtprcumatic-tyred roller should be used for the

intermediate compaction phase, and the static three wheel roller

again for the finishing. Vibratory rollers should be used with

cafe.

4.2.10 Tlial scctbns

Trial sections of sufficient length for final assessment of the mix

and for honing the contractot's operation are recommended.

45


CONTROLLING THE QUALITY OF LAMBS

5.1

Introduction

This manual presents a design method for large aggregate mixes

for bases, using fundamental principles and relevant engineering

properties. As such, it is a significant improvement on current

empirical mix design methods. It is, therefore, strongly

recommended that quality assessment be based on these

performance related criteria and that current end-result

assessment methods be dropped.

5.2

Requirements in terms of engineering properties

Dynamic creep modulus and indirect tensile strength are the

recommended fundamental engineering properties to be

assessed. (Table 4). Cores should be 150mm in diameter.

5.3

Density requirements

It has become cleafa'26 that the degree of compaction of asphalt

mixes should not be considered to be a fixed value under all

circumstances. It depends on factors such as

'

'

environmental temperature regime

anticipated traffic volumes, tyre pressures and axle loadings

This has given rise to the development of controlled compaction

procedures such as the SUPERPAVETM gyratory compactors.

From an exploratory study38, it has been found that the Hugo

hammer's compactive effort is sometimes too light compared to

the SUPERPAVETM specification and other times it appears to

be too high. This is of course due to the fixed nature of its

compactive effort compared to the variable effort with the

gyratory compactor. Currently, other methods of achieving the

same end results are being explored.

In view ofthese findings, it is suggested that the final layer has a

density equal to or greater than9S%o of the laboratory

determined bulk relative density of the approved production

mix, using the Hugo compaction method as described in Section

3.4.

46


Another option is to assume that the ultimate field density at the

end of pavement life will be97Vo of maximum theoretical

density, according to TMHI, method C4. Then the prescribed

construction field density shall be greater than or equal to this

value (97Vo) minus the numerical value of the percentage design

voids in the approved production mix. Cores should be 150 mm

in diameter.

5.4 Performance-relatedspecifications

In accordance with current trends world-wide, performance

related specifications and guarantees by the contractor should be

developed as soon as possible.

47


6 ECONOMIC CONSIDERATIONS IN THE USE OF

LAMBS

6.1 Introduction

The LAMBS technology emanating from the research project

has extended the traffic loading range previously covered by

asphalt pavements from more than 12 million E80s to in excess

of 50 million 880s. Asphalt pavements have thus entered the

load category generally considered by SA practice to be the

domain of rigid pavements.

LAMBS now offer practical, competitive alternatives and costeffective

solutions together with proven technology. Specific

issues to be considered are:

. construction costs and time

'

'

user delay costs

maintenance and rehabilitation costs over the life cvcle of the

pavement.

LAMBS' conduciveness to the provision of a safe, durable, long

lasting pavement should also be considered.

48


CONCLUSIONS

This manual presents a design method for LAMBS using

fundamental principles and relevant engineering properties. It

has been updated in the light of experience gained with recently

completed LAMBS projects. During the execution of these

projects it was possible to explore the behaviour of LAMBS

under gyratory compaction. In addition the SUPERPAVETM

design procedure was explored to determine appropriate

improvements to the LAMBS manual. LAMBS is an

engineering property based hot-mix asphalt which is not

dependant on specific gradations, is evaluated through

dynamically loaded test equipment and has been assessed using

the HVS to provide APT results.

It has become clear that the Hugo hammer's compactive effort is

sometimes too light compared to the SUPERPAVETM

specification and other times it appears to be too high. It should

however be noted that, despite the difference between the results

of the gyratory compactor and the Hugo hammer, the proper use

of the Hugo hammer and diligent application of the guidelines in

this manual, should well result in the satisfactory design and

construction of LAMBS. The philosophy of using a method

which did not require sophisticated and expensive equipment

and which was relatively simple yet accurate, was retained.

Construction projects have shown that LAMBS can be

constructed with relative ease, providing the guidelines in the

manual are followed. It is believed that the changes to the

manual will enhance the design and construction of LAMBS.

The factors to be considered in the economic analysis and life

cycle costing of LAMBS when comparing them with alternative

materials, remain appropriate.

The work from which this manual was compiled and subsequent

LAMBS projects have shown that LAMBS are a cost-effective

and durable option for roads carrying very heavy traffic.

49


8 REFERENCES

l. Van der Merwe CJ, Grobler JE, Hugo F. HDAPs:

Recommendations on mix design and related aspects of an

amended research programme. Output report HDAP/I,

Southern African Bitumen and Tar Association, February 1989.

2. Van der Merwe CJ, Hugo F, Grobler JE. HDAPs: An

investigation into the laboratory compaction of largeaggregate

asphalt. Output report HDAP/2, Southern African

Bitumen and Tar Association, July 1989.

3. Rust FC, Grobler JE. An investigation into the physical

properties of laboratory compacted samples. HDAP/3,

Southern African Bitumen and Tar Association, August 1990.

4. Grobler JE, Rust FC, HDAPs: Constructability of large-stone

asphalt. Task report HDAP/4, Southern African Bitumen and

Tar Association, January 1992.

5. Rust FC, Grobler JE. Experimental design and detailed

planning. HDAP/5. Southern African Bitumen and Tar

Association, August 1990.

6. Rust FC, Grobler JE. Repeatability of selected compaction

methods (Test results obtainedfrom Hugo and Marshall

compaction). HDAP/6, Southern African Bitumen and Tar

Association, June 1990.

7 . Rust FC, Grobler IE. The design and construction of the Much

Asphalt trial sections. HDAP, Southern African

Bitumen and Tar Association, August 1990.

8. Grobler JE, Rust FC. Field vs laboratory properties - Much

Asphalt triqls. HDAP|8, Southern African Bitumen and Tar

Association, August 1990.

9. Grobler JE, Rust FC. Intbrim recommendations on mix design.

HDAP/9. Southern African Bitumen and Tar Association .

October 1990.

10. Grobler JE, Rust FC. The design and construction of the

Dundee trials. HDAP|IO, Southern African Bitumen and Tar

Association, October 1990.

I l.

50

Grobler JE, Rust FC. Laboratory vs field properties - Dundee

trials. HDAP/I l, Southern African Bitumen and Tar

Association, November l99l


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t3.

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Grobler JE, Rust FC, Prozzi J. Evaluation of the Dundee trials

using the HyS. HDAP/I2, Southern African Bitumen and Tar

Association. December 1991.

Grobler JE, Rust FC. Suitability of interim design methodfor

large-stone asphah. HDAP/I3, Southern African Bitumen and

Tar Association, November 1991.

Grobler JE, Rust FC. HDAPs : Performance of large-stone

asphalt mixes. HDAPll24 Southern African Bitumen and Tar

Association, December 199 l.

Rust FC, Grobler JE. HDAPs : Recommendations on mix

designfor large-stone asphalt. Output report HSAP/I5,

Southern African Bitumen and Tar Association, December 1991.

Cooper KE, Brown SF. Development of a simple apparatus for

the measurement of the mechanical properties of asphab mixes.

Paper presented at the Eurobitume Symposium, Madrid, 1989.

Grobler JE . The establishment of design criteria for the

permonent deformation of asphalt. Project rcportg2l32l, South

African Roads Board, March 1993.

Dunaiski PE, Hugo F. A proposed methodfor measuring the

lateral displacements during the indirect tensile test on asphalt

briquettes using linear variable dffirential transducers. Paper

presented at the Fourth International Symposium of the Role of

Mechanical Tests on the Characterization, Design and Quality

control of Bituminous Mixes, Budapest, October 1990.

National Institute for Transport and Road Research. Selection

and design of hot-mix asphalt surfacing for highways. Draft

TRH8, Pretoria, CSIR, 1987.

Hugo F, Kennedy TW. Surface cracking for asphalt mixtures in

South Africa. hoceedings of the Association of Asphalt Paving

Technologists, Volume 54, San Antonio, Texas, 1985.

Groenendijk J, Vogelzang CH, Miradi A, Molenaar AAA and

Dohmen LJM. Lintrack papers submitted to TRB 1997. Delft

University of Technology, Delft, 1996.

Tayeball AA, Deacon JA, Coplanz JS, Harvey JT, Monismith

CL. Fatigue response of asphalt aggregate mixes: Part I -

Test method selection. SHRP project A-003A, University of

California, Berkeley, November 1992.

51


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25.

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Nijboer LW Plasticity as afactor in the design of dense

bituminous road carpets, Elsevier Publishing Company,

Amsterdam, 1948.

Hugo F, A criticsl review of bituminous mixtures at present

being usedfor surfacing pavements, with proposals for the

composition and design of mixtures suitable for South African

conditions, MSc Thesis, University of Natal, 1970.

ASTM D3515 - 89, Standard Speciftcations for hot mixed, hot

laid Bituminous Paving Mixtures, 1997 Annual Book of ASTM

Standards.

McGennis RB, Anderson RM, Kennedy TW and Solaimanian.

Background of SUPERPAVEM Asphalt Mixture Design and

Analy s is, Report No FHWA-SA-95-003. US Department of

Transportation. February 1995.

National Institute for Transport and Road Research. Guidelines

for road construction materials, Draft TRH 14, Pretoria, CSIR,

1985.

South African Bureau of Standards. SABS 1200 Standardized

specifications for civil engineering constuction, SABS,

Pretoria, 1988.

Committee of State Road Authorities. Standard specifications

for road and bridge works, CSRA, Department of Transport,

Pretoria. 1987.

National Institute for Transport and Road Research. Standsrd

methods of testing road materials TMHl, CSIR, 1986.

ASTM D4123-82 (Reapproved 1987). Standard test methodfor

Indirect Tensile Test for Resilient Modulus of Bituminous

Mixtures.

Kennedy T'W, Anagnos JN. Procedures for the static and

repeated - Load Indirect Tensile Tests. Research report 183-14,

Centre for Transportation Research, University of Tems at

Austin, August 1983.

Brown SF, Cooper KE, Preston JN and Bell CA, Development

of a new procedure for bituminous mix design. Paper presented

at Eurobitume Symposium, Madrid, 1989.

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34.

35.

36.

37.

38.

Von Quintis HL, Kennedy TW MMAS mixture properties

related to pavement performance. Proceedings of the

Association of Asphalt Paving Technologists, February 1989.

ASTM D4867-88, Standard Test Methodfor effect of moisture

on Asphalt Concrete Paving Mixtures, l99Z Annual Book of

ASTM Standards.

Viljoen CEL and YanZyl NJW. Z&e "Marvil" permeability

Apparatus for in situ testing of surfacings and basecourse

layers. NITRR Technical Note TP/l8l/83, Pretoria, 1983.

Newell ID. LAMBS: Constructability of Large-aggregate Mixes

for Bases. Notes from a symposium on LAMBS held in

Pietermaritzburg, January 1993.

Hugo F, Van de Ven M and Smit A de F. N2 North Coast

-Contracts Investigation: LAMBS. A Limited Comparative Study

between the Hugo Hammer and the SUpERpAVEM Gyratory'

Compactor. University of Stellenbosch.

53


APPENDTX

WORKED EXAMPLE

l.INTRODUCTION

This appendix discusses a simplified example of the application of the

mix design method. Depending on the magnitude of the project where

LAMBS are to be used, the number of mix variables (eg filler content and

n-values) may be increased or even reduced. In this case study, only one

grading and one filler content were considered in the design process.

2. JAN SMUTS AIRPORT CASE STIJDYAI

The new LAMBS mix design method was first used on one of the

taxiway pavements at Jan Smuts Airport. A pavement comprising

100mm asphalt and a 250mm crushed stone base had to be reconstructed

within a limited period of time with material which had to provide an

improvement on the structural capacity of the existing pavement. A

further requirement was a high resistance to rutting under aircraft wheel

loads (typically 200 kN with I 500 kPa tyre pressures). It was also

necessary for the pavement to be usable by such aircraft within hours of

construction.

Regarding the selection of components, only one grading of a continuous

nature was evaluated. This grading was obtained from a straight

crusher-run and fell within the CSRA grading envelope (dense gravel

asphalt base). The grading could be regarded as fine, with an n-value

varying between 0,4 and 0,45. Quartzite aggregate was used and no

additional filler was added to the straight crusher-run gradings. A60nO

penetration grade bitumen was used. Time constraints limited the initial

selection of component combinations to be evaluated. If this selection of

material components had not met the design requirements, more

combinations would have been evaluated.

Samples using the above components were then manufactured according

to the prescribed methods with four bitumen contents (3,57o to 5Vo).

Additional samples were manufactured from those complying with the

voids content criteria, to determine their engineering properties. The

results of the design process are given in Table A1.

54


Table Al Design test results for Jan Smuts LAMBS.

' 'content

sit";u*,: lrr],ffi

i : .(Eil.

. ' .o1o' \.

4,O

4,5

5,0

2 448

2 451

2 468

2 452

4,1

3,2

r,8

1.6

2 550

2700

2350

| 250

I 150

I 100

10,4

n-'

Note that the 5Vo bitumen content samples failed the voids criterion.

For the three remaining bitumen content samples in Table Al, the

stiffness and indirect tensile properties were determined and compared

with criteria of 2 000 and 800 kPa respectively. All the samples met the

criteria at this stage and, therefore, no combinations were eliminated.

The final phase of the elimination process consisted of dynamic creep

testing. Testing commenced with the 3,5Vobitumen content samples

followed by the 4Vo bitumen content samples. It was clear thatthe 4,57o

bitumen content samples would not reach the limit of l0 MPa and they

were, therefore. not tested.

Based on this simplified elimination process, bitumen contents of 3,5Vo

and 4Vo were both considered satisfactory. The small difference in

dynamic creep modulus was not considered significant. Therefore, the

appropriate bitumen content to be selected had to be based on the desired

properties, constructability and cost.

It was finally decided that 4Vo bitumen would be used for the following

reasons:

'

'

'

'

'

'

constructability would be easier (owing to constraints imposed

by night work and time limits)

economic factors were not highly significant

slightly higher stiffness values were obtained

fatigue resistance would be enhanced

resistance to bitumen ageing would be reduced due to an

increased film thickness

resistance to permanent deformation would be acceptable.

A comparison of the design values with the properties of cores (after

construction) showed some interesting results (Table 2): the laboratory

55


compaction effort simulated the field densities relatively well; and the

short-term conditioning procedures during mix preparation permitted a

good estimation of the field stiffness and indirect tensile strength.

Table A2 Comparison of the design properties with the

constructed

It is significant that the Hugo compaction method simulated the field

properties very well, which would not have been the case if the Marshall

method had been used.

3 REFERENCE

Al.

Grobler JE, Rust FC, MolenaarP. A proposed design method

for large-stone asphalt and the implementation thereof in the

rehabilitation of an airport pavement. Twelfth Annual

Transport Convention, Pretoria, 1992.

56


NOTES


NOTES


NOTES


NOTES


Ordinary

AJ Broom Road Products (Pty) Ltd

Akasia Road Surfacing (Pty) Ltd

Bitumen Construction Services (Pvt) Ltd

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More Asphalt

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Roadsmart (Pty) Ltd

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Shell SA Marketing (Pty) Ltd

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|-:

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Beosumbar & Associates cc

CP Jonsson & Associates

DMV Harrismith (Pty) Ltd

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Salphalt (Pty) Ltd

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As at October 2OO5

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