-
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
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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
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CIV 4200 Civil Engineering Project II 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.
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CIV 4200 Civil Engineering Project II July 21, 2015
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.
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CIV 4200 Civil Engineering Project II July 21, 2015
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
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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
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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
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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
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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.11 Marshall mix design test results for . % binder
content. ....................................... 42
Table A.12 Marshall mix design test results for . % binder
content. ....................................... 43
Table A.13 Marshall mix design test results for . % binder
content. ....................................... 44
Table A.14 Marshall mix design test results for . % binder
content. ....................................... 45
Table A.15 Marshall mix design test results for . % binder
content. ....................................... 46
Table A.16 Marshall mix design test results for 6.0% binder
content. ......................................... 47
Table A.17 Otta Seal Trial Contracts.
...........................................................................................
48
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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
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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
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Waiswa & Ssekamwa July, 2015 2
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.
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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
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Waiswa & Ssekamwa July, 2015 4
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.
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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
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Waiswa & Ssekamwa July, 2015 6
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
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Waiswa & Ssekamwa July, 2015 7
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.
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Waiswa & Ssekamwa July, 2015 8
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:
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Waiswa & Ssekamwa July, 2015 9
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.
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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
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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
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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.
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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),
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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,
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(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.
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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.
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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
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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.
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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.
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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.
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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
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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
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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.
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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).
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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).
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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).
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Waiswa & Ssekamwa July, 2015 27
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.
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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
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Waiswa & Ssekamwa July, 2015 29
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
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Waiswa & Ssekamwa July, 2015 30
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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Waiswa & Ssekamwa July, 2015 31
References
Cook, J. R., Petts, R. C., & Rolt, J. (2013). Low Volume
Rural Road Surfacing and Pavements; A
Guide to Good Practice. London, United Kingdom: Africa Community
Access
Programme.
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
Development & Poverty Reduction. 20th Conference of ASEAN
Federation of Engineering
Organisations, (pp. 1-2). Phnom Penh, Cambodia.
Giang, T. V., Pett, R., & Duc, N. H. (2007). Rural Road
Surfacing Research for Sustainable Access
and Poverty Reduction in South East Asia. Journal of the Eastern
Asia Society for
Transportation Studies, 7, 1944.
Hongve, J. (2006, October). Bituminous Surfacing Options for Low
Volume Roads Constructed
by Labour-Based Methods.
International Focus Group on Rural Road Engineering. (2002).
Otta Seal. Technical Information.
Joshi, S. G., & Jha, A. K. (2013, July 30). Otta Seal
Experience in Nepal. TRB 2013 Annual
Meeting, pp. 1-2.
Lennox, R., & MacKenzie, M. (2008). Eco-Road Building for
Emerging Economies: An Initial
Scan for Promising Alternative Technologies. Department for
International Development.
Gaborone, Botswana: Global Transport Knowledge Partnership.
Mathers, S. J. (1994). The Industrial Mineral Resource Potential
of Uganda. British Geological
Survey, ODA/BGS Technology Development and Research Programme.
Nottingham,
United Kingdom: National Environment Research Council.
Ministry of Works and Transport. (2010). Road Design Manual
(Vol. 3 Pavement Design; Part I
Flexible Pavements). Kampala, Uganda.
Mt. Elgon Labour-based Training Centre. (July, 2015). A Final
Completion Report on the 25
Practical Field Training Exercises in Low Cost Sealing. Mbale:
MELTC.
Norwegian Public Roads Administartion. (1999). A Guide to the
Use of Otta Seals (3000 ed., Vol.
Publication No. 93). (C. Overby, Ed.) Oslo, Norway: Allkopi
AS.
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Waiswa & Ssekamwa July, 2015 32
Overby, C., & Pinard, M. I. (2007, October). The Otta Seal
Surfacing. An Economic and Practical
Alternative to Traditional Bituminous Surface Treatments.
Overby, C., & Pinard, M. I. (2008). Appropriate Standards
and Specifications for Surfacing of
Low-Volume Rural Roads. The 12th International Conference of
International Association
for Computer Methods and Advances in Geomechanics (IACMAG). Goa,
India.
Overby, C., & Pinard, M. I. (2012, November 12). The Otta
Seal Surfacing. A Practical and
Economic Alternative to Traditional Bituminous Surface
Treatments.
Pinard, M. I., & Obika, B. O. (1997). Optimal Use of
Marginal Aggregates for Achieving Cost
Effective Surfacing on Low Volume Roads in Developing Countries.
Paper Presented at
the Low Volume Roads Session of the 1997 13th IRF World Meeting
in Toronto, Ontario,
Canada. Gaborone, Botswana.
Pinard, M. I., & Overby, C. (2006, October). Note on
Pavement and Surfacing Technologies for
Low-Volume Roads. 1-2. Retrieved November 6, 2014
Pinard, M. I., & Overby, C. (2013). Review of Low-Cost Seal
Technology Options on Low Volume
Roads in Uganda. Mbale: InfraAfrica Consultants.
Roads Department; Ministry of Works, Transport &
Communications. (1999). The Design,
Construction and Maintenance of Otta Seals (Vol. Guideline No.
1). (M. I. Pinard, Ed.)
Gaborone, Botswana.
Roads Department; Ministry of Works, Transport &
Communications. (2002). The Use of Silcret
and Other Marginal Materials for Road Surfacing (Vol. Guideline
No. 8). (C. Overby,
Ed.) Gaborne, Botswana.
Southern and Eastern African Mineral Centre. (2008, January).
Opportunities in Mining
Investment in Uganda. (M. W. Gebremichael, G. R. Nkini, E. C.
Kimaro, & L. Moshi,
Eds.) Mineral Potential of SEAMIC Member Countries, 10(1), 20.
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2014
The Central Materials Laboratory. (2000). Laboratory Testing
Manual. (S. S. Rutajama, & C.
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Tuffour, Y. A., & Braimah, A. (2014, July). Suitability of
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Construction. Journal of Engineering, 04(07), 46-53. Retrieved
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Mungyereza.
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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.
Dry Mass before Washing = 2392g
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
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Table A.3 Sieve analysis results for Kajjansi borrow pit.
Dry Mass before Washing = 2448g
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.
Dry Mass before Washing = 2460g
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
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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