Performance Assessment of Otta Seals on Low Traffic Volume Roads

MAKERERE UNIVERSITY

COLLEGE OF ENGINEERING, DESIGN,

ART & TECHNOLOGY

SCHOOL OF ENGINEERING

DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING

CIV 4200 Civil Engineering Project II

Performance Assessment of Otta Seals on Low Traffic Volume Roads

By

Name Registration number Signature

Waiswa Ronald 11/U/452 ....

Ssekamwa Dennis Derrick 11/U/472 ....

This Final Year Project Report has been duly submitted to the Department there

above in partial fulfilment of the requirements for the programme leading to the

following award: Bachelor of Science in Civil Engineering.

MAIN SUPERVISOR CO-SUPERVISOR

Assoc. Prof Umaru Bagampadde Eng. David Kaddu

Signature... Signature..........

Date: 21st-July-2015 Date: 21st-July-2015

JULY 2015

CIV 4200 Civil Engineering Project II July 21, 2015

Waiswa & Ssekamwa July, 2015 I

DECLARATION

We, Ronald Waiswa Regn No. 11/U/452 and Dennis Derrick Ssekamwa Regn No. 11/U/472

declare that this report submitted in partial fulfillment of the requirements of the

programme/degree leading to the following award:

Bachelor of Science in Civil Engineering (BSc. CIV), at Makerere University is our own and

has never been submitted/reproduced for any award/qualification in any university or institution

of higher learning whatsoever; and we hereby declare to be held responsible for everything herein.

..

Declaration signature Date: Tuesday, July 21, 2015

...

Declaration signature Date: Tuesday, July 21, 2015


Waiswa & Ssekamwa July, 2015 II

DEDICATION

We dedicate this report to our parents/guardians whose efforts towards our academic excellence

have proved worthwhile. We also appreciate our supervisors for the guidance and support rendered

to us throughout the entire project implementation process. To our friends and the Civil

Engineering classmates, with whom we have shared the joyful and tough moments of the course.


Waiswa & Ssekamwa July, 2015 III

ACKNOWLEDGEMENT

We appreciate and thank the Almighty God for the life and strength that has enabled us get this far

in the project. We would also like to take this opportunity to thank the people who have enabled

us go through this final year project which has exposed us to great challenges and experiences in

the engineering field. We further register our sincere appreciation to our departmental supervisors

Assoc. Prof. Umaru Bagampadde and Eng. David Kaddu, for their guidance and patience during

this period.

We cannot of course, forget to thank Mr. William Ntume and Fred Mukasa, the in-charge of the

Structures Laboratory and Highway Laboratory respectively at Makerere University who guided

our practical work, acquisition of material and apparatus used in laboratory tests. We thank Eng.

Patrick Etoko, Mr. Samson Obela and Ms. Prosy Nabukalu, of Kireka Central Materials

Laboratory who gave us practical assistance in carrying out certain tests. We also thank our

colleagues and all our classmates for the moral support given to us.


Waiswa & Ssekamwa July, 2015 IV

ABSTRACT

To seal the low traffic volume roads in Uganda, there has been an advocacy of using low cost seals

such as penetration macadam, single and double surface dressing, slurry seals, quarry-dust seals,

sand seals, cold premix and Otta Seals. Of importance to this research, is the Otta Seal which

comprises essentially a 16-32 mm thick bituminous surfacing constituted of an admixture of

graded aggregates ranging from natural gravel to crushed rock in combination with relatively soft

(low viscosity) binders, with or without a sand seal cover.

This report therefore considers a research on the performance assessment of Otta Seals on low

traffic volume roads in Uganda. The performance was typically assessed by developing different

designs of the mix using marshall mix design tests. The Otta Seal developed in this research

constituted of natural gravel of size


Waiswa & Ssekamwa July, 2015 V

Contents DECLARATION............................................................................................................................ i

DEDICATION............................................................................................................................... ii

ACKNOWLEDGEMENT ........................................................................................................... iii

ABSTRACT .................................................................................................................................. iv

List of Figures .............................................................................................................................. vii

List of Tables .............................................................................................................................. viii

List of Acronyms .......................................................................................................................... ix

Chapter One .................................................................................................................................. 1

INTRODUCTION......................................................................................................................... 1

1.1 Background ......................................................................................................................... 1

1.2 Problem Statement.............................................................................................................. 2

1.3 Main Objective .................................................................................................................... 2

1.4 Specific Objectives .............................................................................................................. 3

1.5 Justification ......................................................................................................................... 3

1.6 Benefits ................................................................................................................................. 3

1.7 Scope of the Study ............................................................................................................... 3

Chapter Two .................................................................................................................................. 5

LITERATURE REVIEW ............................................................................................................ 5

2.1 Development of the Otta Seal............................................................................................. 5

2.1.1 Origin ............................................................................................................................. 5

2.1.2 Aggregates ..................................................................................................................... 5

2.1.3 Bitumen .......................................................................................................................... 6

2.1.4 Design ............................................................................................................................ 6

2.2 Use of the Otta Seal ............................................................................................................. 8

2.2.1 Regional ......................................................................................................................... 8

2.2.2 Global ............................................................................................................................. 8

2.3 Otta Seal Requirements ...................................................................................................... 8

2.3.1 Aggregate Strength ........................................................................................................ 9

2.3.2 Gradation (Particle Size Distribution) ......................................................................... 10

2.3.3 Fines Content ............................................................................................................... 13


Waiswa & Ssekamwa July, 2015 VI

2.3.4 Plasticity Index............................................................................................................. 13

2.3.5 Binder Specifications ................................................................................................... 13

2.4.1 Mechanism of Performance ......................................................................................... 14

2.4.2 Marshall Mix Design Tests .......................................................................................... 15

Chapter Three ............................................................................................................................. 16

MATERIALS AND METHODS ............................................................................................... 16

3.1 Materials ............................................................................................................................ 16

3.1.1 Aggregates ................................................................................................................... 16

3.1.2 Bitumen ........................................................................................................................ 16

3.2 Methods .............................................................................................................................. 17

3.2.1 Laboratory Tests .......................................................................................................... 17

3.2.2 Analysis of Results ...................................................................................................... 18

Chapter Four ............................................................................................................................... 20

PRESENTATION, ANALYSIS AND DISCUSSION OF RESULTS .................................... 20

4.1 Aggregate Strength ........................................................................................................... 20

4.2 Gradation Tests ................................................................................................................. 20

4.3 Fines Content ..................................................................................................................... 20

4.4 Plasticity Index .................................................................................................................. 21

4.5 Gravel Wastage from Screening ...................................................................................... 22

4.6 Marshall Mix Design Tests ............................................................................................... 22

4.7 Discussion........................................................................................................................... 25

Chapter Five ................................................................................................................................ 28

CONCLUSIONS AND RECOMMENDATIONS .................................................................... 28

5.1 Conclusions ........................................................................................................................ 28

5.2 Recommendations ............................................................................................................. 29

References .................................................................................................................................... 31

Appendices ................................................................................................................................... 34

Appendix A .............................................................................................................................. 34

Appendix B .............................................................................................................................. 49


Waiswa & Ssekamwa July, 2015 VII

List of Figures

Figure 2.1 Origin of the Otta Seal (Otta Valley, Norway). (Source: NPRA, 1999). ...................... 5

Figure 2.2 General grading envelope for Otta Seals. (Source: NPRA, 1999). ............................. 12

Figure 4.1 Gradation curves superimposed on the Otta Seal gradation envelope. ....................... 21

Figure 4.2 A Plot of Bulk Density against Binder Content. ......................................................... 26

Figure 4.3 A Plot of Stability against Binder Content. ................................................................. 26

Figure 4.4 A Plot of Flow against Binder Content. ...................................................................... 26

Figure 4.5 A Plot of Void Content against Binder Content. ......................................................... 26

Figure 4.6 A Plot of Voids filled with Binder against Binder Content. ....................................... 26

Figure B.1 Air drying samples collected from the different borrow pits...................................... 49

Figure B.2 Washing samples in preparation for wet sieving. ....................................................... 49

Figure B.3 Riffling process. .......................................................................................................... 49

Figure B.4 Arranging test sieves for sieve analysis. ..................................................................... 49

Figure B.5 Preparation of the samples for liquid and plastic limit analyses................................. 50

Figure B.6 Tamping the samples in preparation for the 10% FACT analysis. ............................. 50

Figure B.7 Liquid limit determination. ......................................................................................... 50

Figure B.8 Conduction of the 10% FACT analysis. ..................................................................... 50

Figure B.9 Measuring about 1.2kg of the gravel for the marshall mix design tests. .................... 51

Figure B.10 Mixing of MC-3000 bitumen and natural gravel to make marshall mix design

specimens. ............................................................................................................................. 51

Figure B.11 Oven drying the samples and monitoring the temperature at 135. ....................... 51

Figure B.12 Adjusting the marshall compaction apparatus for compaction. ................................ 51

Figure B.13 Marshall Compaction apparatus adjusted to give 75blows/face. Note the reading on

the meter gauge. .................................................................................................................... 52

Figure B.14 Placement of collars to extrude the specimens from the molds. ............................... 52

Figure B.15 Placing the marshall hammer in position to start compaction. ................................. 52

Figure B.16 Extrusion apparatus................................................................................................... 52

Figure B.17 Extruded marshall mix design specimens. ................................................................ 53

Figure B.18 Measurement of specimen height using a slide caliper. ........................................... 53

Figure B.19 Specimens in a water bath maintained at 40 for 1 hr. ........................................... 53

Figure B.20 Specimens being subjected to stability and flow analyses. ...................................... 53


Waiswa & Ssekamwa July, 2015 VIII

List of Tables

Table 2.1 Average aggregate spread rates for an Otta Seal. (Source: IFG, 2002) .......................... 7

Table 2.2 Average binder spray rates for an Otta Seal. (Source: IFG, 2002) ................................. 7

Table 2.3 Otta Seals in various countries (as per 2007). (Source: Overby & Pinard, 2007) .......... 9

Table 2.4 Aggregate strength requirements for Otta Seals. (Source: NPRA, 1999) .................... 10

Table 2.5 Otta Seal aggregate grading requirements. (Source: ORN 3, 2000) ............................. 11

Table 2.6 Material requirements for Otta Seals. (Source: NPRA, 1999) ..................................... 12

Table 4.1 Aggregate Strength test results. .................................................................................... 20

Table 4.2 Fines Content of the gravel samples. ............................................................................ 21

Table 4.3 Plasticity Indices of the samples. .................................................................................. 21

Table 4.4 Gravel Wastage from Screening. .................................................................................. 22

Table 4.5 Summary results of the marshall mix design tests. ....................................................... 23

Table 4.6 Results for particle density determination of natural gravel. ........................................ 24

Table A.1 Sieve analysis results for Mutundwe borrow pit. ......................................................... 34

Table A.2 Sieve analysis results for Kireka borrow pit. .............................................................. 34

Table A.3 Sieve analysis results for Kajjansi borrow pit............................................................. 35

Table A.4 Sieve analysis results for Kanyanya borrow pit. ......................................................... 35

Table A.5 Atterberg results for Mutundwe borrow pit. ................................................................ 36

Table A.6 Atterberg results for Kireka borrow pit. ...................................................................... 37

Table A.7 Atterberg results for Kajjansi borrow pit. .................................................................... 38

Table A.8 Atterberg results for Kanyanya borrow pit. ................................................................. 39

Table A.9 Ten Percent Fines Aggregate Crushing Test (10% FACT) results. ............................. 40

Table A.10 Marshall mix design test results for . % binder content. ....................................... 41






Table A.16 Marshall mix design test results for 6.0% binder content. ......................................... 47

Table A.17 Otta Seal Trial Contracts. ........................................................................................... 48


Waiswa & Ssekamwa July, 2015 IX

List of Acronyms

10% FACT - Ten Percent Fines Aggregate Crushing Value

AADT Average Annual Daily Traffic

ACV Aggregate Crushing Value

AIV Aggregate Impact Value

BS British Standard

DANIDA Danish International Development Agency

DFID Department for International Development

IFG International Focus Group on Rural Road Engineering

MC Medium Curing (a type of Cutback Bitumen)

MELTC Mount Elgon Labour-based Training Centre

MoWT Ministry of Works and Transport

NPRA Norwegian Public Roads Administration

ORN Overseas Road Note

SEAMIC Southern and Eastern African Mineral Centre

TRL Transport Research Laboratory

UBoS Uganda Bureau of Statistics

UNRA Uganda National Roads Authority

VMA Voids in Mineral Aggregate


Waiswa & Ssekamwa July, 2015 1

Chapter One

INTRODUCTION

1.1 Background

The roads constructed in Uganda have been either left unpaved (in which case they comprise what

we call gravel roads) or paved. From a recent study carried out by the Uganda Bureau of Statistics

last year, it was noted that, the national roads network remained un-changed for the period 2010

to 2011. However, there was a notable increase in the length of paved roads from 3,264km in

2011 to 3,317km in 2012 representing a two percent increase, while unpaved roads increased from

16,736km in 2011 to 17,683km in 2012, representing a six percent increase (UBoS, 2013).

Despite this development, the current unpaved network of roads still pauses a big challenge

majorly in terms of maintenance (due to gravel losses and various structural distresses),

rehabilitation (due to general pavement failure and functional distresses) and serviceability (due

to dustiness in hot weather and/or muddy surfaces during wet seasons) to the general public. Now

because these gravel roads are openly exposed to distress-causing environmental factors, they

tend to suffer early deterioration. This effect puts a high demand on the limited maintenance

resources of road agencies, reduces access to a level that severely curtails social and economic

development, and can put lives at risk in rural communities. Besides maintenance costs, there are

also concerns about gradual gravel resource depletion and environmental degradation associated

with mining of gravel from deposits to replenish gravel lost on roadways. Due to these

issues, such roads will ultimately call for sealing operations to improve on their serviceability,

extend their service lives and reduce on the gravel losses so as to preserve the limited gravel

resources for sustainability. Since most of these gravel roads are low-volume rural roads, and carry

very little traffic ranging between 50 100 /, their upgrade using conventional chip

seals, is generally not economically justifiable by the level of traffic. This requires consideration

of low-cost surfacing options. One type of such surfacing options that can provide an economic

and practical alternative to traditional surfacings, like the Chip Seal, is the Otta Seal. The Otta

Seal, according to Tuffour and Braimah (2014), is simply a bituminous chip seal constructed to

upgrade low-volume gravel roads to surfaced standards using natural gravel or crushed rock

containing some amount of fines for the seal coat. The natural gravel used may be an assortment

of laterites and quartzite or simply one of either. However, the biggest challenge with this relatively

new type of technology is the lack of information regarding its design, construction and



performance due to an empirical approach to its design and to some extent, the reluctance of the

concerned road authorities/agencies to embrace new technology. The purpose of this study

therefore is to assess the performance of this seal (Otta Seal) under different designs so as to draw

information and criteria regarding its construction for optimum performance.

1.2 Problem Statement

The gravel roads found in the country include, among others; highly trafficked and low-volume

ones. The low-volume roads are mostly found in rural and semi-urban areas of the country. To

avoid loss of gravel material from such roads which, according to Pinard and Overby (2006), such

loss rates range in the order of 30-50 mm per annum, depending on factors such as traffic, climate

and terrain; such unpaved or gravel roads may be upgraded by bituminous surfacing to reduce

gravel loss and prolong their life. However, because most of these low-volume roads carry very

little traffic (in most cases substantially less than 50veh/day), their upgrade using conventional

chip seals is generally not economically justifiable by the level of traffic, (Tuffour & Braimah,

2014). This requires consideration of low-cost surfacing options. One type of such surfacing

options that can provide an economic and practical alternative to traditional surfacings, like the

Chip Seal, is the Otta Seal (mainly because of the approach to its construction which calls for the

use of locally available materials). Unfortunately, lack of information regarding this relatively new

type of bituminous surfacing has suppressed its more widespread use, despite its excellent

performance in a number of countries, mainly in Africa but also in some parts of Asia and South

America, (Overby & Pinard, 2012). This information lacking includes mainly the criteria to adopt

in the selection of natural gravel material and binder amounts to be used in its construction. The

selection being based on aggregate suitability (i.e. minimum requirements of the aggregates) and

binder (bitumen) content/proportion (i.e. percentage of bitumen) required for optimum

performance. This research was therefore conducted to assess the performance of Otta Seals under

different designs so as to draw information and criteria regarding its construction for optimum

performance.

1.3 Main Objective

To Assess the Performance of Otta Seals on Low Traffic Volume Roads in the Rural and Semi-

urban Areas of the Central Region of Uganda.



1.4 Specific Objectives

I. To evaluate the aggregate strengths of natural gravels from parts of the central region in

line with Otta Seal requirements as opposed to the use of conventional stone chippings;

II. To characterize the same gravel samples (suitability assessment) in relation to Otta Seal

requirements (characterization will employ the conduction of physical tests i.e. sieve

analysis and Atterberg limits);

III. To determine the performance of natural gravel mixed with bitumen (using marshall mix

design tests); and

IV. To compare findings from the study on the use of Otta Seals with established practice

elsewhere.

1.5 Justification

The construction of Otta Seals on low traffic volume roads has proved to be a more economic and

practical alternative to traditional bituminous surfacings like the Chip Seal. Because the approach

to its design is basically empirical, inadequate designs have been often developed during its

construction; such incidences are a key factor contributing greatly to the emergence of pavement

distresses and deterioration. Therefore performance assessment of Otta Seals on such low traffic

volume roads in order to come up with an optimum design is of paramount importance.

1.6 Benefits

The research will be of use to the concerned road agencies (District Local Governments in

particular) that are majorly responsible for such low volume roads. It will avail them with the

information on what criteria to follow in the selection of gravel material and binder content for

Otta Seal construction.

1.7 Scope of the Study

The research involved the collection of four (4) gravel samples from four (4) borrow pits situated

within the areas of Kanyanya, Kireka, Kajjansi and Mutundwe in the central region of Uganda;

and also called for the acquisition of MC-3000 type of cutback bitumen from Mt Elgon Labor-

based Training Centre in Mbale. It further involved the conduction of suitability tests which were

strength tests, characterization tests (in terms of particle size distribution, plasticity index and fines



content) on the natural gravel and an assessment of the performance of the Otta Seal constructed

using such gravel material passing the requirements. The performance assessment tests were

carried out using marshall mix design tests.



Chapter Two

LITERATURE REVIEW

2.1 Development of the Otta Seal

2.1.1 Origin

The Otta Seal is a particular type of bituminous surfacing that was originally developed in the early

1960s by the Norwegian Road Research Laboratory. It derives its name from the location in

Norway where it was first used-the Otta Valley, (Roads Department of Botswana, 1999). NPRA

(1999) and Joshi and Jha (2012) describe Otta Seals as consisting essentially of a 16-32 mm thick

bituminous surfacing constituted of an admixture of graded aggregates ranging from natural

gravel to crushed rock in combination with relatively soft (low viscosity) binders, with or without

a sand seal cover.

In agreement with the above, Hongve (2006), further states that the Otta Seal technology has since

been applied successfully inter alia in a number of Sub-Saharan countries Uganda inclusive on

secondary roads.

Figure 2.1 Origin of the Otta Seal (Otta Valley, Norway). (Source: NPRA, 1999).

2.1.2 Aggregates

As any other seal, the Otta Seal employs the use aggregates together with bitumen or asphalt

cement as a binder. Due to low cost considerations, the most commonly used type of aggregates

are the natural gravels albeit crushed stone is sometimes used. According to Lennox and

Mackenzie (2008), natural gravel is defined as a naturally occurring material which can be



excavated and used successfully in certain conditions as a low traffic volume road surface if

complying with certain grading (aggregate sizes) and plasticity requirements, and watered and

compacted. Petts, et al. (2006) define natural gravel as any naturally occurring granular material,

including laterite gravel, used as a road surfacing material. However, Cook, et al. (2013) state that

gravel is simply a naturally-occurring, weathered or naturally transported rock within a specific

coarse particle size range. It can be seen that all the definitions given by the different researchers

relate to the same thing.

2.1.3 Bitumen

The type of bitumen used in Otta Seals is very critical for its performance and a good result requires

that both the binder type and application rates are tailored to the gravel/aggregate properties. The

following binder properties are considered:

Be soft enough to initially coat the fines in the aggregate;

Be soft enough to rapidly move up through the matrix of aggregate voids by the action of

rolling and traffic;

Remain soft long enough to continue moving up through the matrix of aggregate voids

over a period of 4 to 8 weeks; and

Be able to be applied in large enough quantities in one spray operation.

The type of binder used in addition to the above properties should also:

Be viscous enough to provide sufficient stability after the initial curing of the seal;

Be durable enough to give the expected service life;

Be able to be used with the available equipment and skills;

Be environmentally friendly to the greater possible extent; and

Be economical in use.

2.1.4 Design

The Otta Seal option using natural gravel, has a simple design. It is however, noted (Overby &

Pinard, 2007) that its design is entirely empirical, thats, it is based on experience in the selection

of both an appropriate type of binder and an aggregate application rate. Fortunately, guidelines are



now available to guide practitioners in its design. Its construction is similar in principle to that of

a conventional bituminous surface treatment. Nevertheless, many of its construction activities can

be undertaken using Labour-based methods -a major advantage in terms of employment provision.

The design is generally amenable to changes during construction. This is in contrast to the design

of a chip seal which has a Rational Approach (i.e. it relies on confirmatory trials on site) and is not

easily amenable to design changes during construction. The following subsections explain the key

constituencies of an Otta Seal:

2.1.4.1 Aggregates

Depending on the grading of the natural gravel, the spread rates are as follows in Table 2.1 with

the spread units modified to/2:

Table 2.1 Average aggregate spread rates for an Otta Seal. (Source: IFG, 2002)

Aggregate Spread rate (/) Dense grading Medium grading Coarse grading

16-20 13-16 13-16

2.1.4.2 Binder

The viscosities of binders used in construction should reflect the grading of aggregate employed

and the level of traffic at the time of construction. Normally MC-800, MC-3000 or 150/200

penetration grade bitumen are used. Different binders are used with different aggregate grading

and traffic levels to come up with an Otta Seal. The spray rates are of course, not calculated by

design and must be chosen empirically through road trials. The range of spray rates for different

levels of traffic that can be employed during trials is given in Table 2.2.

Table 2.2 Average binder spray rates for an Otta Seal. (Source: IFG, 2002)

AADT (/) Binder Spray rate (/) 500 1.6-1.8

If MC-3000 cutback bitumen is used as the binder in the construction, the spraying temperature

ranges between135 155.



2.2 Use of the Otta Seal

2.2.1 Regional

The technology has been so far applied in various parts of the country. Such areas where it has

been observed are: Busamaga in Mbale District (applied on Busamaga-Magada-Bumuluya Model

Road in particular), Lamwo, Kapchorwa, Sironko, Wakiso, Nakasongola etc. Although theres

noticeable progress of the technology in some parts of the country especially in the Eastern region

where trials are being conducted by Mt. Elgon Labour-based Training Centre (MELTC), its

applicability is not well evidenced in other parts. This is mainly attributed to the empirical nature

of its design, lack of information regarding its properties, design, construction and performance

and the reluctance of the construction industry to embrace new technology.

2.2.2 Global

According to Overby and Pinard (2007), the technology has been field tested and successfully

applied in a number of countries and under a variety of conditions. However, the technology is

still not fully understood by practitioners because it does not comply with conventional thinking

on road surfacings, and there is still some uncertainty regarding its acceptance as a standard type

of surfacing. Nevertheless, as indicated in Table 2.3, its use is becoming more widespread as more

and more experience is gained.

2.3 Otta Seal Requirements

Any Otta Seal construction necessitates the availability and usage of both bitumen and aggregates

as the two major components. The aggregates referred to in this context are the natural gravels

which majorly consist of laterites and quartzite. Like the chip seals, such natural gravels used in

this type of seal will have to meet certain minimum requirements as published in the Guide to the

use of Otta Seals by NPRA (1999). Taking into consideration the availability, quality, and haulage

distances of gravel material located in any borrow pit, the so selected material is only considered

fit for use in Otta Seal Constructions basing on the following criteria:



Table 2.3 Otta Seals in various countries (as per 2007). (Source: Overby & Pinard, 2007)

Country Length (km)

Norway 12000

Sweden 4000

Iceland 2000

Kenya 500

Botswana 2400

Zimbabwe 80

South Africa 30+/-

Bangladesh 20+/-

Australia 2.2 (two trials)

Namibia Trials

Nepal Trials

Tanzania 100+/-

Chile 100+/-

Ghana 20+/-

2.3.1 Aggregate Strength

The 10% FACT Value (for both dry and wet conditions) is the only strength index used in

assessing the quality of aggregates, with the minimum value being dictated by the level of traffic

at the time of construction, (Tuffour and Braimah, 2014). However, it should be taken into account

that even though this is an acceptable and often used parameter of assessment, the Aggregate

Impact Value test (AIV) has advantages as a substitute for the ACV or 10% FACT test for

controlling aggregate strength on site as stipulated in the Roads Department of Botswana (2002).

The 10 percent Fines Aggregate Crushing Value (10% FACT) is defined as the force in

required to crush a sample of size between 9.5 and 13.2 mm aggregate so that 10 percent of the

total test sample after crushing passes a 2.36mm sieve. The greater the percentage of weak/soft

particles in the material being tested, the lower the 10% FACT Value.

The aggregates used for Otta Seals have this value of strength relatively lower in magnitude as

compared to those required for the conventional Chip Seals, (see Table 2.4). As the table shows,

the required aggregate strength for Otta Seals is lower than that of a Chip Seal. Such lower values

Due to overlaying/strengthening operations, the number of kilometres of Otta Seals has been reduced to 3,500 km in

2007.



can be tolerated simply because the graded particle matrix results in less internal pressure caused

by stone to stone contact.

Table 2.4 Aggregate strength requirements for Otta Seals. (Source: NPRA, 1999)

Aggregate Strength

Requirements

AADT at the time of construction BS Test Designation

< 100 > 100 Min. Dry % FACT 90 110 BS 812 Part 111:1990 Min. Wet/Dry Strength Ratio 0.6 0.75

Also, the soft binders used in this type of seals are, in addition, able to surround, coat and hold in

place any particle that may break during rolling (NPRA, 1999).

Note: The shape of particles also affects the strength with cubical or rounded materials being

stronger than flaky or elongated materials, (Roads Department of Botswana, 2002). The

explanation to this phenomenon is that; as compared to cubic and/or rounded particles, flaky

particles tend to impede compaction. These particles tend to lock up with adjacent aggregate

particles more readily than cubic and/or rounded particles and resist reorientation into a denser

configuration hence hampering compaction. They additionally have a tendency to fracture along

their weak, narrow dimension when subjected to compaction loading. If they are present in large

amounts, this fracturing can effectively change the aggregate gradation as larger flat and elongated

particles are broken into smaller, more cubic particles. The resulting greater number of fine

particles will fill existing void spaces and reduce the VMA. This reduction in VMA can result into

little room available for the asphalt binder to coat the aggregate particles, which can result in

mixture instability (rutting and shoving).

2.3.2 Gradation (Particle Size Distribution)

The essential feature of the particle size distribution of aggregates for Otta Seals is the wide range

allowed for each particle size so that a variety of natural gravels can be processed and used for the

construction. The grading of the material is based on the level of traffic expected, (Overseas Road

Note 13, 2000). In saying expected, ORN is not clear about the time when to consider the traffic.

Is it at the time of construction or rather at a certain point in time after construction? The answer

to this has been given by the NPRA (1999) where it is argued out that the preferred aggregate

grading will, to some extent, depend on the traffic volume at the time of construction, as well as



during the two months immediately following the sealing operation, as this contributes

significantly in forming the Otta Seal.

The preferred maximum size is 16mm but 19mm can be accepted in the first seal where a double

seal is to be constructed. However, the use of such aggregates beyond 16mm up to 19mm or more,

should preferably be avoided where possible as this may lead to loss of stones during service,

(NPRA, 1999). Of course in the field, gravel materials are processed by removing such over-size

aggregates and excessive fines if present. These large aggregates generally do not bond and

embed well in seal mats and, hence, are easily dislodged from their placement positions by

traffic, leading to pothole development. Excessive fines are removed so as to reduce the plasticity

of the soil and also improve the adhesion of binder to aggregate surfaces. The processing may also

involve reducing the contents of fractions that are over-represented.

According to the ORN 3 (2000), roads carrying light traffic (< 100 /), a 'coarse' grading

should be chosen while a 'dense' grading should be applied to one carrying> 100 /. Table

2.5 shows the grading envelope according to the ORN 3:

Table 2.5 Otta Seal aggregate grading requirements. (Source: ORN 3, 2000)

Percentage Passing

Sieve (mm) Dense Coarse

19.0 100 100

16.0 79-100 77-100

12.0 61-100 59-100

9.5 42-100 40-85

4.750 19-68 17-46

2.360 8-51 1-20

1.180 6-40 0-10

0.600 3-30 0-3

0.300 2-21 0-2

0.150 1-16 0-1

0.075 0-10 0-1

Because most low-volume roads carry minimal traffic usually within the range of 50

100 /, NPRA (1999) provides a more generalized grading envelope cutting across the

Dense and the Coarse Particle Size Distributions (see Table 2.6).

Aggregates should be screened to remove stones greater than 19mm



Table 2.6 Material requirements for Otta Seals. (Source: NPRA, 1999)

Sieve Sizes (mm) Overall Grading Requirements (% Passing) 19 100

16 80-100

13.2 52-100

9.5 36-98

6.7 20-80

4.75 10-70

2.00 0-48

1.18 0-38

0.425 0-25

0.075 0-10

The graph in Figure 2.2 has been developed from Table 2.6 after NPRA (1999) to depict the

general grading envelope for Otta Seal aggregates:

Figure 2.2 General grading envelope for Otta Seals. (Source: NPRA, 1999).

Thus, any gravel material can only be considered fit for Otta Seal construction if its gradation

curve falls/lies somewhere within the grading envelope in the above graph and is as smooth and

parallel to the envelopes as possible.



2.3.3 Fines Content

In addition to the overall grading, the quantity and nature of the dust component (percentage

passing 0.075 mm) must be controlled. Very dusty aggregate results in problems with adhesion of

the bitumen to the stone particles. This amount of fines (< 0.075) should preferably not

exceed10%. A higher fines content may result in construction problems, as the binder tends to

coat the finer particles before the larger ones, and may lead to a less durable surfacing with inferior

surfacing characteristics. However, aggregates with fines contents up to 15% have performed well

on some projects (in Botswana), and no surfacing defects have yet been recorded due to excessive

fines contents on any of these projects (NPRA, 1999).

2.3.4 Plasticity Index

According to the MoWT (2010) Road Design Manual, the plasticity Index of the fines content is

not to exceed 12% irrespective of the level of traffic expected to use the road although other authors

like the International Focus Group on Rural Road Engineering (2002) and Tuffour and Braimah

(2014) recommend a maximum value of 10%. This limit appears to be dictated by the need to

minimize the clay content in order to reduce binder demand because such particles require high

amounts of binder for wetting during bituminous construction due to their large surface area.

2.3.5 Binder Specifications

Penetration grade bitumen

The hardest type of bitumen that can be used for Otta Seal considering hot climatic conditions is

the 150/200 penetration grade.

Cutback bitumen

Cutback bitumen in the MC3000-and MC 800 viscosity ranges are the most common binders used

in the Otta Seal.

The range of acceptable binder viscosities for Otta Seals is provided by the following standard

types:

o MC 800 cutback bitumen (softest),



o MC 3000 cutback bitumen (medium), and

o 150/200 penetration grade (hardest).

An Otta Seal will in most cases require a cutter that produces a medium curing (MC) cutback

bitumen. And for this case, MC-3000 is always chosen simply because of its relative hardness as

compared to MC-800, ease of handling and availability on local markets. MC-3000 is obtained by

cutting back 80/100 or 150/200 penetration grade types of bitumen using power paraffin as a cutter.

The amount of the cutter required is between 8 12% of total mixture for the case of 80/100 base

bitumen and 5 8% of total mixture for the case of 150/200 base bitumen, (NPRA, 1999).

Note: The durability of the binder can be improved by replacing 3% points of the cutter with

flux oil where the 80/100 penetration grade is used as base bitumen.

Flux oil

Fluxing is the addition of heavy oil that affects the long-term viscosity and durability of the binder.

It is such a process that slows down the hardening process of the bitumen and, within certain limits

will produce a more durable seal. Engine oil, used or unused, is suitable for this purpose. Fluxing

should be carried out in moderation, as theres a risk of disintegration of the seal if the binder is

excessively fluxed, (Overby, 1999).

The spraying temperature for MC-3000 cutback bitumen ranges between 135 155 but an

optimum temperature of 145 is usually considered on site. It should however be noted that

cutback bitumen should not be held at spraying temperature for periods exceeding twelve (12)

hours. This causes loss of light oil fractions and an undesirable change in binder properties,

(NPRA, 1999).

2.4 Performance Assessment

2.4.1 Mechanism of Performance

The mechanism of performance of an Otta Seal is such that graded aggregate is placed on a

relatively thick film of comparatively soft binder which, on rolling and trafficking, can work its

way upwards through the aggregate interstices. In this manner, the graded aggregate relies both on

mechanical interlocking and bitumen binding for its strength a bit like a bituminous premix,



(NPRA, 1999). After 4-8 weeks of trafficking and rolling, the seal has an appearance like that of

a premix. However, it should be noted that the construction procedures of an Otta Seal are different

from those of a premix (cold mix asphalt) and/or hot mix asphalt although they may have a similar

appearance and structure in the end.

Because of this property of having an appearance and/or structure a bit like that of a bituminous

premix, the performance of an Otta seal can be assessed by employing the Marshall Mix Design

Tests.

2.4.2 Marshall Mix Design Tests

In this method, the resistance to plastic deformation of a compacted cylindrical specimen of

bituminous mixture is measured when the specimen is loaded diametrically at a deformation rate

of 50/. There are two major features of the Marshall method of mix design.

(i) density-voids analysis and (ii) stability-flow tests. The Marshall Stability of the mix is defined

as the maximum load carried by the specimen at a standard test temperature of 60C. The flow

value is the deformation that the test specimen undergoes during loading up to the maximum load.

Flow is measured in 0.25 mm units. In this test, an attempt is made to assess the performance of

the Otta Seal under different percentages of asphalt cement (bitumen). An optimum amount of the

binder is then obtained that gives the best performance of the seal.



Chapter Three

MATERIALS AND METHODS

3.1 Materials

3.1.1 Aggregates

A total of four (4) gravel samples distributed within the Central Region of Uganda were considered

in this study. These samples were obtained from the areas of Kanyanya, Kireka, Kajjansi and

Mutundwe

This is contrary to what was stated in the proposal of collecting five (5) gravel samples from five

places, the fifth place being Wakiso. The reason as to why material from Wakiso was left out was

because the borrow pits there had too much fines from the preliminary survey carried out and

therefore could not pass the gradation requirements from just visual inspection. This material could

be considered maybe for base and sub-base design in any road construction but not for Otta Seal.

The criteria for choosing these places was based on material availability. Permission to access

these gravel pits situated within these areas was sought from the concerned personnel responsible.

These gravel samples were collected in situ and brought to the laboratory at the University and

some to the Central Materials Laboratory in Kireka.

These four (4) samples were each subjected to suitability tests 3.2.1, for Otta Seal requirements

as described in 2.3. Only ONE sample of these passed the general requirements and was used in

the performance assessment tests. Samples that did not meet these requirements were discarded.

An idea was held such that in case no sample met all the requirements, blending would be done to

have the test sample for performance assessment tests meet the minimum Otta Seal requirements.

3.1.2 Bitumen

About 1.5 (net volume) of MC-3000 cutback bitumen was required for the tests. However, to

cater for storage losses, haulage spills, evaporation and any other unaccounted-for losses, a total

of about 10 was obtained. This was acquired from Mt. Elgon Labor-based Training Centre in

Mbale district on requisition.



3.2 Methods

3.2.1 Laboratory Tests

3.2.1.1 Aggregate Strength

The Ten Percent Fines Aggregate Crushing Value (10% FACT) defined there above was

determined by measuring the load required to crush a prepared aggregate sample such that 10%

of the material passed a specified sieve after crushing. The specified sieve in this case was the

2.36mm sieve. This test was carried out on the samples using particles which passed the 14mm

sieve and were retained on the 10mm sieve in accordance with BS 812 Part 111:1990.

3.2.1.2 Gradation Tests

The particle size distribution of each gravel material source was determined using wet sieving

method in accordance with BS 1377 Part 2:1990 on representative portions of the gravel samples.

This was done after the materials had been oven dried at 105 for 24 hours.

3.2.1.3 Fines Content

The fines content of each gravel sample was determined using dry sieving method in accordance

with BS 1377 Part 2:1990. The percentage of material passing the 0.075 sieve was read from

the gradation curves and stated.

3.2.1.4 Plasticity Index

The plastic and liquid limit tests were carried out in accordance with BS 1377 Part 2:1990 on the

fines fraction (below 0.425mm) of the unscreened samples.

3.2.1.5 Screening

This procedure was used to bring the grading of the natural gravel samples to the grading of Otta

Seals. It basically involved the removal of over-size fractions i.e. particles that exceeded the size

of 19mm.

3.2.1.6 Gravel wastage from screening

The quantity of gravel material reduced whenever the material was subjected to screening. For the

case of this study, it was important to establish the extent of material wastage arising from the

screening process. The masses of the unscreened samples were weighed and then the materials



wasted from these samples after screening also weighed. The wasted masses were expressed as

percentages of the original sample masses.

3.2.1.7 Marshall Mix Design Tests

A total of twenty one (21) test specimens were prepared. Seven (7) different percentages of

bitumen content giving seven (7) aggregate-bitumen combinations were adopted. The percentages

were varied from 3.0% through 6.0% in steps of 0.5% around an assumed value of 4.0%,

evaluated from findings of trial contracts by Overby (1999). For each aggregate-bitumen

combination, three (3) test specimens were prepared. The average values of the parameters being

tested for the three (3) test specimens of each aggregate-bitumen combination were taken for that

particular combination. About 1.2 of natural gravel was required for each test specimen. It will

again be noted here that the natural gravel that was used in these tests, was the one from Kanyanya

borrow pit. This is because it is the only one sample that met all the Otta Seal requirements and so

there was no such need for blending, see Tables 4.1, 4.2, 4.3 & 4.4 and Figure 4.1.

The procedure/steps for preparing the test specimens and conducting the experiments for the

Marshall Mix design tests were carried out in accordance with ASTM 1559-89: Standard Test for

Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus.

3.2.2 Analysis of Results

3.2.2.1 Aggregate Strength

These were presented in a tabular form using Microsoft Excel.

3.2.2.2 Gradation Tests

The obtained gradation curves were superimposed on the gradation envelopes of the Otta Seal and

compared.

3.2.2.3 Fines Content

These were obtained by reading off the percentages of masses of material passing the 0.075

sieve from the gradation curves. They were then presented in a tabular.

3.2.2.4 Plasticity Index

The plasticity indices were obtained by getting the differences between the liquid limits and the

plastic limits of the samples. The results were then tabulated.



3.2.2.4 Gravel Wastage from Screening

These results were presented in a tabular form.

3.2.2.5 Marshall Mix Design Tests

These results were presented in tabular forms and graphs/charts using MS-Excel.



Chapter Four

PRESENTATION, ANALYSIS AND DISCUSSION OF RESULTS

4.1 Aggregate Strength

The strength test results that were carried out in accordance with BS 812 Part 111:1990 are shown

in Table 4.1.

Table 4.1 Aggregate Strength test results.

S/N Place/Source of

Gravel

Wet strength

(kN)

Dry strength

(kN)

Wet/Dry strength

ratio

1 Kanyanya 365 474 0.77

2 Kireka 227 310 0.73

3 Kajjansi 286 473 0.61

4 Mutundwe 314 382 0.82

As can be seen from Table 4.1, all of the samples passed this requirement except that the one from

Kajjansi showed a small deviation in the wet/dry strength ratio. The Kajjansi material had the value

of this ratio as 0.61 which was less than 0.75 but more than 0.60. This could make such material

only applicable to roads where the traffic expected at the time of construction is less than say 100

/ (see Table 2.4 under 2.3.1). However, with this exception, all the samples had their

strengths (TFVs) and wet/dry strength ratios well above the minimum requirements. For a detailed

table of results, see Table A.9 in Appendix A.

4.2 Gradation Tests

The gradation tests carried out in accordance with BS 1377 Part 2:1990 on representative portions

of the gravel samples using wet sieving gave the following results.

Figure 4.1 shows the gradation curves superimposed on the Otta Seal gradation envelope. As can

be seen from the figure, all the gravel samples met the gradation requirement for Otta Seal. This

can be observed as the specific gradation curves all fell within the Otta Seal gradation envelope,

See Tables A.1 through A.4 in Appendix A.

4.3 Fines Content

The fines content values i.e. percentage of material passing the 0.075 sieve for the different

borrow pit materials can be read off the gradation curves from Figure 4.1. Table 4.2 shows these

results.



Figure 4.1 Gradation curves superimposed on the Otta Seal gradation envelope.

From Table 4.2, it is observed that all the samples had their fines content values well below the

maximum of 10% as stated by Overby (1999). Therefore all the samples met this suitability

requirement.

Table 4.2 Fines Content of the gravel samples.

Source Kireka Mutundwe Kajjansi Kanyanya

Fines Content (%) 5.18 2.21 2.15 3.33

4.4 Plasticity Index

The plasticity indices were obtained by getting the differences between the liquid limits and the

plastic limits of the samples. The results are given in Table 4.3.

Table 4.3 Plasticity Indices of the samples.

S/N Place/Source LL (%) PL (%) PI (%)

1 Kanyanya 23 14 9

2 Kireka 44 25 19

3 Kajjansi 34 14 20

4 Mutundwe 34 14 20

0

10

20

30

40

50

60

70

80

90

100

0.063 0.630 6.300 63.000

Perc

enta

ge P

assi

ng

(%)

Sieve Size (mm)

Legend

Otta Seal Envelope

Kireka

Mutundwe

Kajjansi

Kanyanya



It can be observed from Table 4.3 that none of the samples except that from Kanyanya passed the

plasticity index requirement. Samples from the areas of Kireka, Kajjansi and Mutundwe showed a

bit of plasticity whilst the one from Kanyanya was not all that very plastic. The PI according to

MoWT, was not to exceed 12%; Kanyanya had a PI of 9% hence passing the requirement. For

more about this analysis, refer to Tables A.5 through A.8 in Appendix A.

4.5 Gravel Wastage from Screening

These results were obtained by deducting the masses of representative portions after screening

from the masses before screening. The results are given in Table 4.4:

Table 4.4 Gravel Wastage from Screening.

It can be observed from Table 4.4 that the material from Kanyanya and Mutundwe suffered the

highest wastages while the ones from Kireka and Kajjansi suffered less with Kireka suffering the

least. Although it is technically adequate, this observation renders the material from Kanyanya as

being not economical since most of the hauled aggregates would be wasted.

4.6 Marshall Mix Design Tests

The Marshall Mix design tests were carried out on twenty one (21) test specimens. Seven (7)

different percentages of bitumen content giving seven (7) aggregate-bitumen combinations were

adopted. The percentages were varied from 3.0% through 6.0% in steps of 0.5% around an

assumed value of 4.0% evaluated from findings of trial contracts by Overby (1999). For each

aggregate-bitumen combination, three (3) test specimens were prepared. The average values of the

parameters being tested for the three (3) test specimens of each aggregate-bitumen combination



were taken for that particular combination. Detailed test results have been appended in Tables A.10

through A.16 in Appendix A. Table 4.5 gives a summary of the results.

Table 4.5 Summary results of the marshall mix design tests.

As can be seen from Table 4.5, (see also Figure 4.2), the bulk density of the mix initially increased

with an increase in binder content. This is because the binder was absorbed into the pores of the

aggregates and hence increased the overall mass of individual particles hence bulk density up to

an optimum value. When more bitumen was added, it was simply adsorbed onto the aggregate

surfaces causing a poor aggregate-aggregate interlock and relative motion between the individual

aggregates. The less dense bitumen replaced the more dense aggregates hence reducing the overall

bulk density.

The stability (see Table 4.5 and Figure 4.3) initially increased with binder content because of the

increased locking-up of aggregates and stiffness of the mix due to the absorbed bitumen that

enhanced aggregate-aggregate binding. More bitumen added was rather adsorbed onto the

aggregate surfaces which caused relative motion between the aggregates and hence a weaker

matrix.

The flow (see Table 4.5 and Figure 4.4) increased with binder content because addition of bitumen

reduced the internal friction between the aggregate particles and enhancing the possibility of

relative motion between them. As more of the binder was added, the mixture became weaker and

more compressible which increased its susceptibility to deformation.



The void content (see Table 4.5 and Figure 4.5) decreased with an increase in binder content

because the binder added filled the air voids present. Bitumen was added up to a point of reaching

saturation; where the mixture could not take in any more bitumen.

The percentage of voids filled with binder (see Table 4.5 and Figure 4.6) increased with binder

content because initially the air pores absorbed bitumen and all got filled up. The more binder that

was added was adsorbed onto the aggregate surfaces and thus existed between the aggregate

interstices. All of this thus increased the overall percentage of voids filled with binder.

Particle density determination was carried out and the specific gravity of the gravel was found to

be about 2.985 as can be seen from Table 4.6.

Table 4.6 Results for particle density determination of natural gravel.

Parameters (g) Set A Set B

Mass of jar (m1) 1030.38 1348.45

Mass of jar + gravel (m2) 1699.22 1966.69

Mass of jar + water + gravel (m3) 2704.82 2988.12

Mass of jar + water (m4) 2270.70 2579.81

Gs = (m2-m1)/{(m4-m1)-(m3-m2)} 2.85 2.94

Average Gs 2.895

The specific gravity of MC-3000 bitumen was also determined as 0.998.

To come up with the optimum binder content, the charts were plotted. The data values being

extracted from Table 4.5.

The procedure from here was then to determine the optimum binder content. The original marshall

procedure determines the optimum binder content as the average of the following three values:

1. Binder content at maximum mix density (3.95% from Figure 4.2);

2. Binder content giving a void content halfway between the upper and lower requirements

for the mix in consideration (3.80% from Figure 4.3).

Note: The nominal spray range for the MC-3000 in an Otta Seal construction between

1.6 2.2/2 for an aggregate spread rate of 16/2 (see table 2.1 and 2.2 under 2.3.1).



Taking the density of bitumen as approximately 1000/3 and that of aggregates as

2895/3, and considering the so stated ranges of material application, the following

minimum and maximum binder contents were obtained:

Minimum binder content =(1.61031000)/2

(161032895)/2 100 = 3.45%

Maximum binder content =(2.21031000)/2

(161032895)/2 100 = 4.75%

Now from the graph in Fig.4.3, 3.45% binder content corresponds to a void content of

12.9% while 4.75% corresponds to a void content of 11.5%. Therefore the upper and

lower bounds are respectively 12.9% and 11.5% with the halfway value being 12.2%.

3. Binder content at maximum stability (3.95% from Figure 4.5).

The optimum binder content was therefore calculated as follows:

=3.95 + 3.80 + 3.95

3= 3.90%

This value obtained above thus represents the optimum binder content for which an Otta Seal gives

optimum performance.

4.7 Discussion

In an effort to come up with an optimum binder content for an Otta seal, we performed performance

assessment tests on natural gravel mixed with differing binder amounts. The performance was

assessed by way of conducting the Marshall Mix design tests.

The natural gravel (which comprised four different samples) was first of all subjected to suitability

tests, in which case three samples failed to pass all the requirements. Only the sample from

Kanyanya passed all the requirements. The suitability tests were: strength tests, particle size

distribution, plasticity index and fines content. In terms of strength, all the natural gravels showed

adequate strength, for even AADT exceeding 100 vehicles (see Table A.9 in appendix A).



Figure 4.2 A Plot of Bulk Density against

Binder Content.

Figure 4.3 A Plot of Stability against Binder

Content.

Figure 4.4 A Plot of Flow against Binder

Content.

Figure 4.5 A Plot of Void Content against

Binder Content.

Figure 4.6 A Plot of Voids filled with Binder

against Binder Content.

In general therefore, strength is unlikely to be

an issue of concern for most natural gravel

deposits in the central region of the country.

This is attributed to the fact that the coarse

fractions of gravelly soils geologically tend

to consist of fragments of hard rock or rocks

that have quartz as the predominant mineral

(Tuffour and Braimah 2014).



In terms of particle size distribution, all the samples showed a good positive response with quite

an appreciable degree of parallelism to both the upper and lower bound of the Otta Seal gradation

envelope. This Otta Seal gradation envelope is general and it so happens that any gradation curve

falling within it renders such material fit for use in Otta Seal construction. It should be

remembered however, that the positive response exhibited here, was after screening out the over-

size fractions i.e. > 19. Even though this screening process was helpful in reducing such over-

size fractions which tend not to bind properly with smaller fractions and instead end up

contributing to raveling, it has the disadvantage of reducing the quantity of gravel material

available for use. For the gravels evaluated in this study, practically all needed screening, with

one from Kanyanya experiencing as much as over 40% reduction in quantity after the process.

All the samples had their fines content values well below the maximum value of 10%. The material

from Kireka had the biggest value of all (5.18%) followed by Kanyanya (3.33%), then Mutundwe

and Kajjansi which had almost comparable results below 3%. This shows that the samples had

less fines (clayey particles), in general, and hence not very plastic as can be evidenced from the

plasticity index results. Maximum PI recorded was 20%.

Regarding plasticity, it can be seen from Table 4.4 that the PI values ranged between 0 and 20%

with only the material from Kanyanya passing the requirement with a PI of 9%. Material from

other sources did not pass the requirement for plasticity index owing to the high clayey contents

they contained. PI happened to be the key criterion in the selection of which sample to use in the

Marshall Mix design tests. All the other requirements except PI were met by all the samples.

Analysis of results from the performance assessment tests revealed that the optimum binder

content to be used in the construction of an Otta Seal is 3.9%. This optimum value agrees well

with the findings of Overby (1999) where the binder spray rate ranges between 1.6/2 and

2.2/2 for a range of aggregate spread rates between 16/2 and 20/2 for dense grading.

It will be noted however, that the engineering properties of natural gravel depend on the area where

it is formed as it is not a conventional material. The material is subject to decomposition due to

natural processes such as weathering. Suitability tests thus ought to be carried out prior to the

construction of an Otta Seal.



Chapter Five

CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

Based on the findings from this study, the following conclusions have been made:

I. Given that the samples from Kanyanya, Mutundwe, Kajjansi and Kireka exhibited the

following dry strengths; 365, 314, 286, 227 respectively as compared to the

minimum required dry strengths of 90 (for < 100) and 110 (for >

100), it is safe to say that gravel from the given borrow pits has adequate strength which

is well above the required strength of gravel to be used in Otta Seal design. Also samples

from Kanyanya, Kireka and Mutundwe which had wet/dry strength ratio as

0.77, 0.73, 0.82 passed the required wet/dry strength ratio of 0.6 (for < 100)

and 0.75 (for > 100) unlike the sample from Kajjansi that failed with a wet/dry

strength ratio of 0.61.This implies that the strength of gravel from Kajjansi is reduced

greatly with the addition of water.

II. From the gradation tests it can be seen that the borrow pits had well graded materials

according to the Unified Soil Classification System. Given that the coefficient of Curvature

(Cz) for the borrow pits are 2.2, 1.2, 1.7 2.9 for Kireka, Kajjansi, Kanyanya and

Mutundwe respectively which lie between 1 and 3 hence they can be classified as

well/dense graded which means that there was no excess of particles in any size range

(Craig, 2004). Since all the samples had 35% or less of total sample passing sieve No. 200

(0.075) it can be concluded that they were of granular material according to AASHTO

classification. According to this type of classification, material have group classifications

of: A-2-6(0), A-2-4(0), A-2-6(0) and A-2-7(0) for Mutundwe, Kanyanya, Kajjansi and

Kireka respectively. From the plasticity index results it can be concluded that samples from

Mutundwe, Kajjansi and Kireka have medium plasticity since their PIs were within the

range of 10-20 while that of Kanyanya has low plasticity since its plasticity was in the

range of 5-10 (Das, 2006).

III. From the marshall mix design tests it can be seen that gravel mixed with bitumen (MC-

3000) behaved differently depending on the percentage of binder used and grading of the

gravel with the optimum binder content obtained as 3.90% specifically for gravel from



Kanyanya which had passed all the relevant tests. For the case of stability, as the binder

content was increased, stability of the mix also increased till an optimum value after which

it dropped, while flow of the mix increased with increase in binder content. This implies

that for an optimum performance the gravel has to be well graded hence gradation of

samples has to be predetermined before these tests are carried out. When well graded gravel

is used inherent stability possessed will make it easy to achieve the necessary stability of

the seal without having to resort to hard binders. The optimum binder content will also

vary differently because different gravel samples are of a different nature and their

characteristics change depending on the environmental conditions hence the value obtained

cannot be taken as a conclusive optimum binder content to be used for all gravel samples.

IV. From the findings by Overby (1999) in Botswana, where the aggregate spread rates range

between 16/2 and 20/2 for dense graded aggregates and an average bitumen spray

rate of 1.9/2; the optimum binder content is observed to range between 3.28% and

4.10%. Bitumen spray rates of about 1.6/2 and aggregate spread rates amounting to

16/2 have been evidenced on the Busamaga-Magada-Bumuluya model road

constructed by Mt. Elgon Labour-based Training Centre in Mbale district. Of important

notice also, are the different contractors who have implemented nineteen (19) low-cost

sealing contracts whereof one of the seals to be constructed was the Otta Seal. These

contracts which were also referred to as practical field training exercises were executed in

twenty three (23) districts of the Teso, Lango and Acholi sub-regions, (MELTC, 2015).

Varying lengths of the Otta Seal were constructed, see Table A.17 in Appendix A. The

bitumen spray and aggregate spread rates were averaged at values of 1.7/2 and 16/2

respectively for all of the contractor firms. Looking at this, we found out that our obtained

value for the optimum binder content i.e. 3.90% happens to be in good agreement with

such values for established practice elsewhere.

5.2 Recommendations

Although an optimum binder content for an Otta Seal has been arrived at in this study,

more research should be conducted to relate such optimum values to specific gradations of



the available gravel material for instance dense, medium or coarse grading in relation to

the level of traffic expected at the time of construction.

MC-3000 cutback bitumen has a low flash point of about 66 and is highly flammable

due to the high contents of volatile organic solvents. This poses a safety hazard in handling

it during Otta Seal construction. Basing on this therefore, there should be further research

in the use of other soft binders say for example the emulsions that are relatively cheaper

and safer to use in the construction of Otta Seals.

Concerning the issue with university laboratories, more necessary equipment should be

installed to avail students with an opportunity of conducting their tests within the proximity

of the university. This in turn helps in reducing on the overall project as outsourcing tends

to prove pretty expensive; and

The college is also urged to liaise with the university council on the issue of final year project

financing. Final year projects prove costly both in monetary terms and time. The university should

set a platform on how to secure funds and sponsor students final year projects to reduce on the

cost implications such students suffer.



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Craig, R. F. (2004). Craig's Soil Mechanics (7th ed.). London, United Kingdom: Spon Press,

Taylor & Francis Group.

Das, B. M. (2011). Principles of Foundation Engineering, SI (7th ed.). (H. Gowans, & T. Altieri,

Eds.) Stamford, California, United States: Global Engineering: Christopher M. Shortt.

Dr. Gourley, C., Greening, A., Dr. Jones, D., & Petts, R. (2002). Paving the Way for Rural

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Appendices

Appendix A

Table A.1 Sieve analysis results for Mutundwe borrow pit.

Dry Mass before Washing = 2444g

Sieve Size (mm)

Mass Retained (g)

Mass passing (g)

Percentage passing (%)

20.000 0 2444 100.00

14.000 464 1980 81.01

10.000 514 1466 59.98

5.000 852 614 25.12

2.360 308 306 12.52

1.180 66 240 9.82

0.600 96 144 5.89

0.425 14 130 5.32

0.300 20 110 4.50

0.212 20 90 3.68

0.150 20 70 2.86

0.063 16 54 2.21

Table A.2 Sieve analysis results for Kireka borrow pit.


Sieve size (mm)

Mass Retained (g)

Mass passing (g)

Percentage passing (%)

20.000 0 2392 100.00

14.000 352 2040 85.28

10.000 550 1490 62.29

5.000 814 676 28.26

2.360 340 336 14.05

1.180 116 220 9.20

0.600 42 178 7.44

0.425 10 168 7.02

0.300 10 158 6.61

0.212 8 150 6.27

0.150 10 140 5.85

0.063 16 124 5.18



Table A.3 Sieve analysis results for Kajjansi borrow pit.


Sieve Size (mm)

Mass Retained (g)

Mass Passing (g)

Percentage Passing (%)

20.000 0.00 2448.00 100.00

14.000 232.00 2216.00 90.52

10.000 692.00 1524.00 62.25

5.000 1216.00 308.00 12.58

2.360 202.00 106.00 4.33

1.180 36.00 70.00 2.86

0.600 8.00 62.00 2.53

0.425 2.00 60.00 2.45

0.300 1.77 58.23 2.38

0.212 1.77 56.46 2.31

0.150 1.72 54.74 2.24

0.063 2.00 52.74 2.15

Table A.4 Sieve analysis results for Kanyanya borrow pit.


Sieve Size (mm)

Mass Retained (g)

Mass Passing (g)

Percentage Passing (%)

20.000 0 2460 100.00

14.000 240 2220 90.24

10.000 508 1712 69.59

5.000 1046 666 27.07

2.360 384 282 11.46

1.180 108 174 7.07

0.600 40 134 5.45

0.425 12 122 4.96

0.300 10 112 4.55

0.212 10 102 4.15

0.150 10 92 3.74

0.063 10 82 3.33



Table A.5 Atterberg results for Mutundwe borrow pit.

Test

No

Ave

rag

e

gm

1

gm

2

gm

3

gm

1-m

2

gm

2-m

3

%

(m1-

m2)

/(m

2-m

3)*1

00

Test

No

Aver

age

pene

trat

ion

mm

Cont

aine

r No .

gm

1

gm

2

gm

3

gm

1-m

2

gm

2-m

3

%

(m1-

m2)

/(m

2-m

3)*1

00

Ave

rage

Moi

stur

e co

nten

t%

34

105

C

34%

Pla

stic

Lim

it

%

14

%

20%

32.5

533

.20

34.3

93

Performance Assessment of Otta Seals on Low Traffic Volume Roads

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