EVALUATION OF ASPHALT—AGGREGATE BOND
AND STRIPPING POTENTIAL
by
SUBRATA KUMAR DAS, B.S.
A THESIS
IN
CIVIL ENGINEERING
Submitted to the Graduate Faculty
of Texas Tech University in Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
IN
CrVIL ENGINEERING
December, 2004
ACKNOWLEDGEMENTS
The author would like to express his appreciation and gratitude to major
professor, Dr. Sanjaya P Senadheera for his guidance, help, and encouragement
throughout this research work. The author would like to express his sincere appreciation
and gratitude to his committee member Dr. Richard William Tock for his invaluable help
and advice throughout the project. The author also wishes to thank Dr. Shabbir Hossain
for his cooperation and assistance.
The author grateflilly acknowledges financial support of this work by the Texas
Department of Transportation (TXDOT). Special thanks to Ms. Sangeetha Arunagiri and
Mr. Jeremy for their assistance with the lab testing.
Finally, the author wishes to thank his wife for her continuous encouragement and
support during the course of this research work.
ABSTRACT
One of the primary deterioration mechanisms of asphalt concrete is the de-
bonding of asphalt from aggregate in the presence of moisture. This distress mechanism
is also referred to as stripping. It is a very complex process of moisture damage that is
caused by several factors, including the characteristics of asphalt cement and aggregate as
well as environmental conditions. This research evaluated the interfacial bonding
mechanisms and stripping potential of aggregate-asphah binder combinations commonly
used by Texas DOT in maintenance seal coats. This evaluation was conducted using two
test procedures; Net Adsorption Test and TechMRT Pull-Out Test. Results from this
evaluation will help improve the performance of seal coats and HMA pavements in
Texas.
HI
TABLE OF CONTENTS
ACKNOWLEDGEMENT ii
ABSTRACT iii
LIST OF TABLES vii
LIST OF FIGURES viii
CHAPTER
l.INTRODUCTION 1
Problem Statement 3
Research Objective 4
Thesis Outline 4
II. LITERATURE REVIEW 5
Introduction 5
Properties of asphalt 5
Rheology 5
Chemical Properties 6
Properties of aggregate 9
Mineralogy 9
Chemical Composition 11
Physical Properties 12
Properties of water 14
Cohesion-adhesion theories 14
Chemical Reaction 15
Molecular Orientation 16
Mechanical Intedocking 16
Surface Energy 17
Stripping mechanism theories 17
Displacement 18
Detachment 18
Spontaneous Emulsification 18
Pore Pressure 19
IV
Osmosis 19
Microbial Activity 19
Testing of stripping potential 20
III. MODIFIED NET ADSORPTION TEST 24
Introduction 24
Experimental Design 25
Sources of Aggregate 25
Sources of Binders 26
The Modified Net Adsorption Test Procedure 27
Apparatus 28
Reagents 30
Preparation of Aggregate 30
Preparation of Stock Solution 31
Test Procedures 31
Preparation of Test Samples and Control 31
Initial absorbance ofthe Stock Solution 32
Measurement of Initial adhesion 32
Measurement of Final adhesion 32
Calculation Procedure 32
SHRP Procedure 32
UUJ Procedure 33
Discussion of test data 34
Performances of Aggregates with different Binders 34
Limestone aggregate with Diffeent Asphahs 35
Lightweight aggregate with Different Asphalts 38
Rhyolite aggregate with Different Asphalts 40
Sandstone aggregate with Different Asphalts 43
Siliceous Gravel with Different Asphahs 44
Evaluations of Each B inders with Different aggregates 47
PG64-22 47
AC15-5TR Seal Coat Binder 48
AC 5 with2% Latex Seal Coat Binder 50
AC 10 with 2% Latex Seal Coat Binder 52
AC 15-P Seal Coat Binder 54
IV TECH-MRT PULL-OUT TEST(TMPT) 57
Introduction 57
Original t-238 test method 58
Tech-mrt pull-out test (tmpt) 58
Experimental plan 61
Material Selection 61
Test Factorial 63
Procedure 63
Discussion of Results 64
Normal Conditioning 64
Discussion on Percent area coated ofthe Aggregates 69
Discussion on Tensile Stress and Pseudo Energy 72
Discussion on Elongation of Different Combinations 72
Extreme Conditioning 72
Discussion of Test Data under Extreme Conditioning 74
Influences of Elevated Temperatures 74
Discussion on the influences of elevated temperatures... 77
V. CONCLUSIONS AND RECOMMENDATION 78
REFERENCES 82
VI
LIST OF TABLES
2.1 Mineral Types and their Stripping propensity 10
3.1 Sources of Aggregate used for the Laboratory Testing Program 26
3.2 Sources of Binders used for the Laboratory Testing Program 27
3.3 Grading of Aggregate as per the IRISH standard for NAT 31
3.4 SHRP Evaluation Criteria for Aggregate-Binder Adhesion 34
3.5 Comparison of Different Limestone -Binders combinations 36
3.6 Comparison of Different Lightweight -Binders Combinations 38
3.7 Comparison of Different Rhyolite-Binders Combinations 41
3.8 Comparison of Different Sandstone-Binders Combinations 43
3.9 Comparison of Different Siliceous Gravel-Binders Combinations 45
3.10 Adhesion of PG64-22 Binder 47
3.11 Adhesion of AC 15-5TR Seal Coat Binder 49
3.12 Adhesion of AC 5+2%Latex Seal Coat Binder 51
3.13 Adhesion ofAC10+2%Latex Seal Coat Binder 53
3.14 Adhesion of AC 15P Seal Coat Binder 54
4.1 Sources ofMaterials used for the TechMRT Pull-Out Test 62
4.2 Comparison of percent coated area of different combinations 65
4.3 Comparison of Maximum tensile stress (psi) of different combinations 70
4.4 Comparison of Pseudo Pull-Out Energy (psi-ft) of different combinations 70
4.5 Comparison Binder Pull-Out Thread Length for Binder-Aggregate Combinations...71
4.6 Comparison of results under normal and extreme conditionings 73
4.7 Comparison of results with PG64-22 and aggregates prepared at 1 lO^F and 140°F .75
4.8 Comparison of results with AC15-5TR and aggregates made at 110°F and 120°F ...76
vn
LIST OF FIGURES
1.1 Estimated Percentages of Pavements Experiencing Moisture-Related Distress^ 3
2.1 Examples of important chemical functionalities present in asphah molecules 8
3.1 Mechanical Shaker 29
3.2 Spectrophotometer 29
3.3 Limestone aggregate with Field/Plant Asphalts 37
3.4 Lt.Wt. aggregate with Field/Plant Asphalts 39
3.5 Rhyolite aggregate with Field/Plant Asphalts 42
3.6 Sandstone aggregate with Field/Plant Asphahs 44
3.7 Siliceous Gravel with Field/Plant Asphalts 46
3.8 Adhesion of PG 64-22 Seal Coat Binder 48
3.9 Adhesion of ACI 5-5TR Seal Coat Binder 49
3.10 Adhesion ofAC5 with 2%latex Seal Coat Binder 51
3.11 Adhesion of AC 10 with 2% Latex Seal Coat Binder 53
3.12 Adhesion of AC 15P Seal Coat Binder 55
4.1 Apparatus 60
4.2 Comparison of PG 64-22 (Plant) -Aggregate Combinations 67
4.3 Comparison of AC 15-5TR (Plant) -Aggregate Combinations 67
4.4 Comparison of AC 5+2%L (Field)-Aggregate Combinations 68
4.5 Comparison of AC 10+2%L (Plant)-Aggregate Combinations 68
4.6 Figure 4.6 Comparison of AC 15P (P) -Aggregate Combinations 69
viu
CHAPTER 1
INTRODUCTION
1.1 Introduction
Asphalt concrete roads constitute more than 90% ofthe paved road network in the
United States, and $15 billion a year is spent on asphah pavements about one-sixth ofthe
total highway operation expenditures in the United States of America (FHWA 8/9/4 ).
Asphalt pavements are typically designed for 20 years or more. In many cases, even
when satisfactory design methods, good mix design, compaction and adequate drainage
are adopted, frequent failures do occur and water is often considered as one of the
primary reasons for such failures. Water poses an ever-present threat to the longevity of
asphalt pavements (Peter E. Graf, 1986). It plays a significant role in the deterioration of
an asphalt concrete.
Surface treatments such as seal coats (or chip seals) have been used as a standard
pavement preventive maintenance technique to reduce the rate of deterioration of the
pavement structure. A typical seal coat surface treatment, which consists of a thin asphalt
binder film followed by a layer of stone chips, has a lower percent contact area between
asphalt and aggregate compared to hot mix asphalt (HMA). Thus, seal coats are more
vulnerable to aggregate loss caused by loss of adhesion. Hence the compatibility between
asphalt and aggregate is critical for its long-term performance. Therefore asphah-
aggregate combinations that are susceptible to bond loss due to moisture damage and
other factors need to be identified using more effective, performance-based laboratory
tests.
According to H.J.Fromm, the stripping of aggregates from asphalt initiates at the
bottom ofthe pavement and gradually moves up (1974). This may be because the top of
the pavement is highly compacted so if chance prevails, water will enter the pavement
from the granular base and deteriorate the pavement. In some cases, water can also
infiltrate through the surface cracks and ground water seepage from the sides due to
hydraulic gradient. Water enters the interface of asphalt and aggregate in various ways.
Osmotic potential difference causes diffusion across the bitumen film, seepage through
the voids, wear and tear of the binder film, existing moisture in the pores of aggregates,
diffusion from the pores to the interface, and direct contact with the aggregate due to its
partial coating are some of the common ways by which moisture can migrate into the
interface. Evidence suggests that damage will be minimal if stripping is restricted to
coarser aggregates but if stripping occurs to finer aggregate particles then the impact will
be severe because it constituents the basic matrix ofthe mixture (T.W. Kennedy, FL.
Roberts, and K.W. Lee, 1982; Mark A Taylor and N. Paul Khosia, 1983).
Moisture sensitivity of the aggregate-binder bond and the development of test
methods to evaluate it has been a research topic since 1930's. Major research efforts were
undertaken in the 1970's and 1980's under the National Cooperative Highway Research
Program (NCHRP) (Umaru Bagampadde, Ulf Isacsson and Badru M. Kiggundu, 2003).
Various laboratory test methods were developed in this period but most of them failed to
effectively simulate the actual field conditions. In the late 1980's, the efforts of Strategic
Highway Research Program (SHRP) produced some outstanding research in
understanding the fimdamental properties of asphah-aggregate interactions including both
chemical and physical processes. The report SHRP-A-341 explains methodologies to
enhance asphalt-aggregate bond and reduce its water susceptibility (Christine W. Curtis,
Keith Ensley, Jon Epps., 1993). The Net Adsorption Test(NAT) was developed and
recommended to highway agencies to select compatible materials with a strong affinity
for each other, and low susceptibility to water damage (Christine W. Curtis, Keith
Ensley, Jon Epps., 1993).
1.2 Problem Statement
Seal Coating and Hot Mix Asphalt (HMA) furnish pavements with durability,
improved surface friction, smoothness and stability. It can be anticipated that the life of
permeable pavement would be shorter than that of an impermeable one. This is due to the
fact that the asphalt mix may degrade and deteriorate when in contact with water for an
extended period, and finally destabilizes the pavement. Figure l.I, shows the
T
MONTAW "M0BTOQlW0r»'!MIM«9OTAx'^ ^/ ^MAINE'S
-.~~J—-—-t •• ""-v'^j-., / H •'./' I SOUTH «I««TA : \ 7 J , ' „ , ' 5 4 i / i ,
10.20 CJ 20-30 0 30.50
Figure 1.1 Estimated Percentages of Pavements Experiencing Moisture-Related Distress
[Hicks, 1991]
percentage of asphalt pavements experiencing moisture induced distresses. It appears that
the problem which is also referred to as stripping, occurs all over the country. Asphalt
stripping could initially cause raveling, i.e. debonding of aggregates from the pavement
surface due to loss of bond between asphalt and aggregates. Gradually this process could
lead to complete stripping ofthe surface layer.
1.3 Research Objective
The objective of this research was to evaluate the compatibility of various asphalt
type-aggregate source combinations. The asphalt-aggregate combinations tested in this
study are those commonly used by TXDOT districts in seal coats. These combinations
were evaluated using two test procedures, i.e. Modified Net Adsorption Test and
TechMRT Pull-Out Test to identify asphalt-aggregate pairs that show resistance to water
damage and, hence, improve the performance and durability of seal coats and HMA
pavements in Texas.
1.4 Thesis Outline
This thesis is divided into five chapters. Chapter 1 is an introduction, which deals
with the problem statement, objectives and organization of the report. Chapter 2 is a
review of the literature, and it describes the previous research on compatibility between
asphalt cement and aggregate and its water sensitivity. Chapter 3 discusses the procedure
for the Modified Net Adsorption Test, its results and discussion. Chapter 4 discusses the
procedure of Tech-MRT Modified Pull-Out Test (TMPT), its results and discussion.
Finally, Chapter 5 presents the conclusions and recommendations ofthe whole study.
CHAPTER 2
LITERATURE REVIEW
2.1 Introduction
This section is a review of the fimdamental aspect of asphalt-aggregate
interactions on asphalt-aggregate adhesion and stripping. Interaction between asphah
aggregate can take place due to several mechanisms and these include physical and
chemical aspects of asphah-aggregate bonding that promotes adhesion.
2.2 Properties of Asphah
2.2.1 Rheology
Rheological properties of an asphalt binder have a major influence on bonding
between asphalt and aggregate. During mixing, placing and compacting the mix, the
viscosity of the asphalt is the key issues of concern. During mixing of asphalt and
aggregates, an asphalt binder with high viscosity may not effectively wet the aggregate
surface. But during the service period, high viscosity can be beneficial against stripping
because high viscosity asphalts usually carry high concentration of polar functionalities
that provide more resistance against stripping (Umaru Bagampadde, 2002; Hicks, G. R,
1991).
2.2.2 Chemical Properties
Asphalt is dark brown to back in color and its properties depend a great deal on
refinery operations. A typical chemical composition of an asphah is 85% Carbon, 10%
Hydrogen, 1-5% Sulphur, 0.3-1.1% Nitrogen, 2-0.8% Oxygen and other minerals up to
1500 ppm. Asphahs are complicated colloidal systems of two types of hydrocarbon
materials. Asphaltenes (5-25%) and Maltenes, which includes Resins (15-30%),
Aromatics (40-65%)) and Saturates (5-20%) (http://www.westernresearch.org, Sundaram
Logaraj, 2002). Asphah is often defined as a colloidal suspension of asphaltenes in oils
with resins acting as agents to prevent coagulation of the asphaltenes, which is a polar
condensed aromatic and the heaviest fraction is asphah (Young W. Jeon and Christine W.
Curtis, SHRP-A-003B). Maltenes consist of resins, which are polar aromatic, aromatics
and saturates are aliphatic hydrocarbons and alkyl. Stiffer binders offer greater resistance
to moisture susceptibility but stiffening provided by high asphaltene contents can be
detrimental (westernresearch, 09/10/04). It has also been reported that asphalt with higher
percentage of maltene in asphalt and oxidized asphah could play a significant role against
stripping. Acidic ingredients are the main constituents of asphahs, and basic ingredients
are not found in significant amounts.
It has been reported that acid concentration on the aggregate surface can be 15 to
60 times greater than the nonadsorbed asphalt fraction. Previous researchers (H. Plancher,
S. M. Dorrence and J.C. Petersen, 1977) found asphah mainly consists of five oxygenated
functional groups and a nitrogenous functional group believed to contain basic pyridine-
type nitrogen.
Oxygenated functional group types are ketones, carboxylic acids, dicarboxylic
anhydrides, 2-quinoline types, and sulfoxides. Based on their resuhs, the relative
affinities of the functionalities of asphahs for the aggregate surfaces are ranked as
follows: carboxylic acids > dicarboxylic anhydrides > 2-quinolone types > sulfoxides
>nitrogen > ketones. This shows that the carboxylic acid type is the one most selectively
adsorbed to the aggregate surfaces from the asphalt. The interaction of asphalt was
stronger with limestone aggregate than with quartzite, which is considered to be an acidic
rock. Carboxylic acid has the strongest affinity for limestone whereas nitrogen
compounds (e.g. quinoline) had greater attractions for quartzite and granite than with
limestone. But the moisture sensitivity test showed contrasting results. Carboxylic acid
compounds performed devastatingly in the moisture sensitive tests. It was easily stripped
off by water than any other compound. This indicates that the carboxyl group has a
stronger tendency to form hydrogen bonding with water molecules. Ketones and nitrogen
compounds were selectively retained by aggregate surfaces during moisture damage tests.
The authors (Christine W. Curtis, Keith Ensley, Jon Epps., 1993) observed that sulfoxide
functionalities acts like carboxylic acids, highly sensitive to water. But some other
authors (H. Plancher, S. M. Dorrence and J.C. Petersen, 1977) could not correlate their
behavior because sulfoxides fimctional group performed differently with different
aggregates during water sensitivity tests, and therefore the authors indicated that the
surface chemistry of aggregate could be the reason which dictates the level of interaction.
Functional groups of asphahs susceptible to water were ranked based on the water
sensitivity test resuhs as follows: carboxylic acids > dicarboxylic anhydrides > sulfoxides
> nitrogen > 2-quinolone types > ketones. Authors indicated that ketones, some
carboxylic acids, dicarboxylic anhydrides, and sulfoxides are derived from oxidative
aging whereas most ofthe carboxylic acids and 2-quinolone types are naturally occurring
in asphahs.
- ; ' • - yy, '^ o. " H
'•f N O
N
a r o m a t i c H) i y p e (1 •
O
H
p y r r o l i c \V] P y r ^ d l n i c H) S u l f i d e P | S u l f o x i d e [.21
-̂ ^ :->5 O
^ ' V /
A n h y d r i d e {2)
4.21 Fo4-i-««-,-3 « n c !x ;d«J<ve •UtiiriKi
Figure.2.1. Examples of important chemical ftinctionalities present in asphah molecules
(Young W. Jeon and Christine W. Curtis, SHRP-A-003B)
- c: • O - H
C Cl f b o X y f i c o c i d n,.2i
w « , ' * " w « -
K e t o n e {7,]
2.3 Properties of Aggregates
Aggregates play a significant role in overcoming pavement distress by
contributing to the stability ofthe mix. Various studies have shown that the properties of
aggregates play a major role in stripping and adsorption when compared to the asphah
binder (H.J.Fromm, 1974; Kandhal, PS., 1992).
2.3.1 Mineralogy
The mineralogical and chemical properties of aggregate influence hs surface
energy and chemical reactivity. Author (westernresearch, 09/08/04) has ranked road
aggregates based on their chemical nature from highly basic to highly acidic as follows:
marble (highly basic, high CaCos content and lowest silicate content) > Limestone >
Basalt > Dolomite > Sandstone > Granite > Quartzite (highly acidic, high silicate content
and lowest CaCos). Aggregates consist of numerous minerals and each mineral has
specific properties and crystalline structure. Aggregates are studied under microscope for
the qualitative and quantitative analysis. Natural mineral aggregates are categorized into
five main groups and those are summarized in the Table 2.1 (Umaru Bagampadde, 2002).
Table 2.1 Mineral Types and their Stripping propensity (Umaru Bagampadde, 2002)
Category
Silica
Ferro-magnesiu m
Limestone
Feldspar
Clays
Mineral Type
Quartz - Si04
Olivine - (MgFe)2Si04 Augite -(Ca, Mg, Fe)(Si, Al)206 Hornblende - (Ca, Na)23(Mg, Fe^^ Fe^^ A1)5(A1, Si)8 022(OH) 2 Biotite - K (Mg, Fe^^)3 (Al, Fe3+)- Si30io(OH)2
Calcite - CaCOs Dolomite -CaMg (C03) 2
Albite - NaAlSisOg Orthoclase -KAlSiaOg Anorthite -CaAl2Si208
llHte Kaolinite Montmorillonite
Rock
Granite Rhyolite Sandstone Quartzite
Gabbro Diabase Andesite Basah Diorite Mica
Limestone Chalk Dolomite
Rhyolite Granite Quartzite Gneiss Sandstone Diabase Gabbro
Dust Baghouse fines
Comment
Poor adherents as water attaches easily due to H-bonding.
Olivine and aughe form insoluble Mg and Ca sahs while biotite gives soluble K sahs. Hornblende is intermediary in character.
Generally good adherents but may be friable. Undergo strong acid-base and electrostatic interactions with bitumen. Some have soluble salts.
Some stripping potential due to Na and K soluble sah formation. Anorthite forms insoluble Ca sahs that are resistant to stripping.
Fine coatings (<4 |j,) and readily take up water.
Reference
(Rice, 1958; Majidzadeh et al., 1968; Stuart, 1990)
(Rice, 1958; Majidzadeh et al., 1968; Stuart, 1990)
(Curtis, 1990; Stuart, 1990)
(Scott, 1978; Stuart, 1990)
(Clough, 1961; Ishai et al., 1972, Balghunaim, 1991, Kandhal et al., 998)
10
2.3.2 Chemical Composhion
The interfacial activity occurring between charged mineral aggregate surfaces and
asphah cements can be of fundamental importance to the stripping of asphalt from
aggregate (Hyon H. Yoon and Arthur R. Tarrer, 1988). The surface charge of the
aggregates affects the interaction between asphalt and aggregate. At the interface,
electrons transfer from the asphah relics to the aggregate surface, based on the ability of
aggregates to accept these electrons (C.W Curtis, R.L. Terrel, L.M. Perry, S. Al-Swailmi
and CJ Brannan, 1991). Based on Electrokinetic studies, authors (VP Wagh and AR.
Tanner, 1990) used the streaming Potentiometer to evaluate the effect of asphalt coating
on the aggregate surface charge density.
Mineral aggregates have distinct polarities or electrochemical properties. The
functional groups of asphalt that are adsorbed on an aggregate surface come mainly from
the acid fraction of the asphalt (Hyon H. Yoon and Arthur R. Tarrer, 1988). In the
presence of water, the acid molecule (e.g. R-COOH) is broken into an anion (R-COO")
and a proton (H*). This causes the polarity of the asphah surface at the interface to be
negatively charged. The aggregate in contact with water is also negatively charged with
varying degrees depending upon the nature of the mineral and the chemical reaction due
to the pH ofthe water at the interface. Consequently, the two negatively charged surfaces
at the interface are repulsed causing stripping.
Aggregate surface chemistry can be explained based on their ionic strength or
acid-base interaction. Thus they can be classified as acidic, basic or neutral.
11
The acid-base interactions include all interactions of electron donor-acceptor type bonds
including hydrogen bonds (DingXin Cheng, Dallas N Little, Robert L. Lytton, James C,
Holste, 2002). The isoelectric points (lEP) were used to characterize aggregates, from the
material reference library (MRL) of the Strategic Highway Research Program (SHRP).
Quartz based rocks were found to be acidic, silicate rocks as neutral, and carbonate rocks
as moderately to strongly basic. Aggregates often are considered inert and not reactive,
but in terms of their dissolution behavior, acidic rocks dissolve at basic pH and basic
rocks suffered at acidic pH (C.W Curtis, R.L. Terrel, L.M. Perry, S. Al-Swailmi and CJ
Brannan, 1991). Therefore the pH value ofthe contacting water plays a significant role in
the stripping phenomena but percent contribution to the overall stripping phenomenon is
not yet known. Previous research showed that the pretreatment of aggregate surfaces
could be very usefiil because it can modify the aggregate surface chemistry that can
significantly enhance the asphah-aggregate interactions and mkigate the stripping affects.
Investigators in the past noted that acidic aggregates showed more serious stripping than
basic aggregates.
2.3.2 Physical Properties
Surface texture of aggregates ranges from smooth to rough. Scanning Electron
Microscopy (SEM) is a tool used to check the surface textures of various aggregates
(C.W Curtis, R.L. Terrel, L.M. Perry, S. Al-Swailmi and CJ Brannan, 1991).
Investigators in the past suggested that smooth, glassy surfaces of aggregates can
promote stripping phenomenon while rough surfaces can better retain asphalt due to the
irregularity and thus remain unaffected by stripping affects. But in some instances,
12
however, surface roughness may work against adhesion (Hicks, G R, 1991; Majidzadeh,
K and F.N. Brovold, 1968). Rough surfaces may have uneven coating of asphah on it.
Thin film of asphah at the sharp edges of aggregate can be prone to water damage, which
can gradually affect the whole mass. The mineralogical composhion of aggregates plays
a significant role in its surface texture.
Many investigators believe that porosity and absorption has a strong influence on
adhesion and stripping phenomenon. In general, oils in asphah penetrates and gets
absorbed into the pores of aggregates while asphahenes that remained on the surface of
aggregates become harder over the course of time. This phenomenon enhances the
adhesion of asphah to aggregate. A smooth, crystalline surface whh low surface
roughness means low porosity and surface area. Mechanical interlock and adhesion is
considered to improve with large areas of interfacial contact and surface roughness.
Authors (Hyon H. Yoon and Arthur R. Tarrer, 1988) provided evidence that the depth of
penetration of asphalt cement depends on the size of pore, viscosity and the surface
tension of the asphalt. Their test resuhs indicated higher stripping intensity for dolomite
when compared with limestone because of hs low pore size.
Dust and dirt on aggregate hinders adhesion. Sometimes, aggregate dust also
coats on coarse/fine aggregates. When dust is present, the asphalt binder coats the dust
and increases asphah viscosity. This minimizes contact between asphah and aggregate
surface and reduces the asphah-binder film thickness and coated area. Therefore an
appropriate level of viscoshy is needed to maximize the wetting area and for better
asphaltt-aggregate adhesion.
13
2.4 Properties of Water
In 1998, Yoon and Tarrer indicated that a significant change in the pH of water in
contact whh the aggregate surface could change the stripping potential. They used series
of boil tests to assess the sensitivity of stripping to changes in the pH of water, contact
with the aggregate surface. Both basic aggregates (e.g. limestone, dolomite etc.) and
acidic aggregates (e.g. granhes, quartz etc.) caused an increase in the pH of water and
these changes severely affected the stripping phenomenon. Thus h is evident that the
stripping potential is increased with those types of aggregates that increase the pH of
water in contact with aggregate.
Limestone is known to be basic aggregates and they typically perform well
against stripping despite the fact that it increases the pH ofthe contacting water. Previous
research (Hyon H. Yoon and Arthur R. Tarrer, 1988) explained that other than the pH of
water, the surface chemistry should also be considered when characterizing stripping
potential. The interaction between asphah and the aggregates containing different types
of metal ions and the polar species of asphah can be crucial in the stripping phenomenon.
Limestone contains alkaline earth metals, which interact strongly with asphah
components such as carboxylic acids to form alkaline earth salts. The adsorption is strong
since the formations are insoluble and strong enough to resist water even at a higher pH.
2.5 Cohesion-Adhesion Theories
Moisture damage in asphalt mixes is a complex process that involves two
14
primary interfacial phenomena, i.e. cohesion and adhesion. First, water can interact
physically/chemically whh the asphah cement to cause a reduction in cohesion whh an
associated reduction in stiffness and strength of the mixture (Hicks, G R., 1991;
Kunnawee Kanhpong and Hussain U. Bahia, 2003/ Second, water can get between the
asphalt film and the aggregate and break the adhesive bond between the asphah film and
the aggregates, thus "stripping" the asphalt from the aggregate (Hicks, G. R., 1991;
Kunnawee Kanhpong and Hussain U. Bahia, 2003). Stripping or moisture damage can be
reduced when strong adhesive bonds are present. In 1988, Al-Ohaly and Terrel indicated
that the adhesion of asphalt to aggregates improves whh microwave energy treatment
when compared whh the conventional technique of heating the asphah mixtures.
Microwave energy has the capability to heat the medium evenly and polarizes the field
through which it passes. It was also observed that plain aggregate in oven didn't show
uniform temperature and thus adhesion to aggregates were not uniform. To better explain
the adhesion phenomena, four main theories have been presented as follows (Hicks, G.
R , 1991).
2.5.1 Chemical Reaction
The long-term adhesion between asphah and aggregate depends on the chemistry
of the asphah-aggregate bond. It is believed that the surface chemistry of aggregates
governs the absorption of asphalt Selective adsorption occurs at the interface where the
asphalt wets aggregates. The acidic components of asphah react with basic components
of aggregates to form water insoluble sahs.
15
Author (Hicks, G. R., 1991) stated that the chemical reaction between most asphalts and
acidic aggregates (e.g., quartz, granite) is weaker when compared whh basic aggregates
(e.g., hmestone, dolomhe etc.). Studies also showed contradiction whh the concept that
aggregates can be absolutely acidic or basic in nature (Majidzadeh, K and FN. Brovold,
1968).
2.5.2 Molecular Orientation
Molecular Orientation theory states that the asphah molecules orient themselves
in the direction of the mineral aggregate ions to satisfy the maximum capachy of energy
demands ofthe aggregate surface (Hicks, G. R., 1991; Kunnawee Kanitpong and Hussain
U. Bahia, 2003).
Water molecules are dipolar and are more polar than the asphah molecules.
Consequently, water molecules better satisfy the energy requirements of aggregate
surfaces.
2.5.3 Mechanical Interlocking
Asphah is forced into the pores and irregularities of the aggregate surface,
providing the mechanical interlock (Kunnawee Kanhpong and Hussain U. Bahia, 2003;
Rice, J.M., 1958). Mechanical adhesion is affected by several properties ofthe aggregate,
including (a) surface texture, (b) poroshy, (c) surface coatings on the aggregate, (d)
surface area, and (e) particle size (Hicks, G. R., 1991).
16
2.5.4 Surface Energy
Adhesion is a thermodynamic phenomenon related to the surface energy of the
materials involved, asphah, water, air and aggregate (Kunnawee Kanitpong and Hussain
U. Bahia, 2003). In 1958, Rice, J.M. used a parameter cahed adhesion tension to compare
the strength of various asphah-aggregate bonds. Adhesion tension is described as the
change in energy due to the wetting of aggregates by asphalt, h varies with asphalt -
aggregate pairs since this phenomenon depends on the chemistry between them. Usually
the adhesion tension for asphah-aggregate interface is much lower than the adhesion
tension for water-aggregate interface. This shows that most aggregates have a higher
affinhy for water than for asphalts. Thus water can rupture the bond between asphah-
aggregate if h comes in contact with it. The rate of stripping depends upon the magnitude
ofthe free energies involved.
2.6 Stripping Mechanism Theories
Previous studies explain several mechanisms of how water enters through the
asphalt film and deteriorates the existing pavement. Investigators (H.J.Fromm, 1974;
Mark A Taylor and N. Paul Khosia, 1983) observed from the field specimens that
initiation of stripping occurs at the bottom of the pavement and gradually moves up
affecting the coarse aggregates. In 1983, Mark A Taylor and N. Paul Khosia reported that
asphalt at the bottom of the pavement remains in tension due to the traffic loadings and
often coarse aggregates of granular base entraps water or moisture and remains exposed
to water for a prolonged period.
17
2.6.1 Displacement
Water penetrating to the aggregate surface through a break in the asphah film is
called stripping by displacement. Initial incomplete coatings of aggregate or rupture in
asphalt film are the reasons, which causes this effect. Tensile stresses caused by traffic
loading or lack of compaction can be the causes of rupture of asphalt film at the sharp
edges of aggregate or at the corner of angular pieces of aggregate (H.J.Fromm, 1974;
Mark A Taylor and N. Paul Khosia, 1983). This mechanism can be explained based on
thermodynamic and chemical reactions. The author (H.J.Fromm, 1974) also explained
that the stripping can be healed by complete removal of water from the aggregate surface.
2.6.2 Detachment
The peeling off of asphah film whhout any break from the aggregate surface by a
thin layer of water is called detachment (Mark A Taylor and N. Paul Khosia, 1983). The
rationale for the detachment mechanism is supported by the interfacial surface energy
theory. Water has low surface tension than asphah therefore it has better capabihty of
wetting the surface of aggregate. Asphah is known for their high molecular weight and
low polarity than water, hence asphah develops weak bond whh aggregate. On the other
hand, water has high polarity and strongly bonds with aggregate due to stronger
molecular orientation forces at the interface.
2.6.3 Spontaneous Emulsification
In spontaneous emulsification, water and asphah combines to form an inverted
emulsion, where asphah represents the continuous phase and water represents the
discontinuous phase (Mark A Taylor and N. Paul Khosia, 1983). H.J.Fromm(1974)
explains that water penetrates aggregate in the form of droplets and breaks the bond
between asphah and aggregate, certain agents such as clays can further weaken the bond
between them. Therefore emulsion formation is possible depending upon the type of
asphah and additives. Involved author observed that self-healing process i.e. re-adherence
of severely stripped aggregate occurs when exposed to dry environmental condhion
where the moisture has evaporated completely from the surface ofthe aggregate.
2.6.4 Pore Pressure
Entrapped water in the voids of asphah concrete mixes can develop high pore
water pressure due to traffic loading, which can break the film of asphah and aggregate.
This is a mechanical process of breaking the asphalt-aggregate bond.
2.6.5 Osmosis
Osmosis is diffusion of water molecules through a film of asphah. This is a very
slow process and occurs if there is absorbed water and dissolved sahs in the pores ofthe
aggregate. This is not an important factor of stripping based on the laboratory tests resuhs
of H.J.Fromm, 1974.
2.6.6 Microbial Activity
Investigators (Brown, L R , Darnell T.R., 1987; Brown, L. R., Pabst, JR. G S.,
and Marcev, J. R., 1990 and Parker Jr, F., Benefield L, 1991) have presented
experimental evidence that microbial metabolic reactions occur at the interface,
19
producing by-products that causes deterioration in the adhesion bond between asphalt and
aggregate. Diverse groups of microorganisms are capable of degrading asphalt and other
hydrocarbon and can play significant role as a catalyst in microbial activhy. Authors
(Brown, L.R., Darnell T.R., 1987) indicated that the development of blisters at the
interface was mainly due to microbial activhy.
2.7 Testing of Stripping Potential
The development of tests to determine the water senshivhy of asphalt concrete
mixtures began in the 1930s (Terrel, R. L., and J. W. Shute, 1989). Since then numerous
tests have been developed in this direction to evaluate the factors affecting the adhesion
between asphah and aggregate. Investigators (Mark A Taylor and N. Paul Khosia, 1983)
indicated that the reliability of many of these tests is too low to get wide acceptance, but a
few have achieved that distinction over the year. The correlation among laboratory tests
and field results are generally poor, and research is in progress to find better test
procedures that can represent field information better. Some of the better-known tests
procedures to evaluate moisture susceptibihty of asphah-aggregate bond are mentioned
below.
Authors (Brown, L. R., Pabst, JR. G. S., and Marcev, J. R., 1990) have experimental
evidence that aggregate pretreated whh lime, mercury chloride or organofiinctional silane
can significantly reduce microbial growth that causes serious stripping effect in flexible
pavement. An experimental protocol was developed to evaluate this phenomenon
20
(Brown, L. R., Pabst, JR. G S., and Marcev, J. R, 1990). This test procedure involves
pretteating some aggregates whh silane, air-drying overnight and putting them in 50 ml
of medium contained in 6-oz serum stopper prescription bottle, incubated at 30°C on a
rotary shaker. The degree of stripping was evaluated visually after removing the
aggregates from the cuhure and towel drying them. Author (Parker Jr, F., Benefield L.,
1991) also examined the adverse effects of microorganisms on asphah aggregate bond.
Samples were prepared and condhioned for 7 months with solutions to simulate and
suppress microbial activhy under both anaerobic and aerobic condhions and then tensile
and bond strength were measured and compared whh control specimen to evaluate the
microbial activity.
The indirect tension test was introduced in 1978 to predict the moisture
susceptibility of asphalt concrete mixes. Tensile strength and instantaneous tensile E-
modulus of the specimens are calculated by measuring the maximum load and the
corresponding horizontal displacement. The susceptibihty of asphalt concrete to moisture
damage is predicted based on the tensile strength as a result of moisture damage.
The Environmental Condhioning System (ECS) was developed by the
Department of Civil Engineering at the Oregon State University to evaluate the moisture
damage in an asphah concrete mix (Saleh Al-Swailmi and Ronald L. Terrel, 1993). The
basic principle of ECS involves simulating the field condhions and evaluating the
strength loss and other damage caused to the specimens prepared in the laboratory or
obtained from the field. The modulus ofthe asphah concrete and changes in permeabihty
are monitored during conditioning and stripping is evaluated at the conclusion ofthe
21
condhioning procedure. This method fails to recognize adhesion and cohesion failures
separately and is thus unable to evaluate the fimdamental properties of asphalt-aggregate
bonding.
In 2003, Kunnawee and Hussain developed an approach to assess adhesion and
cohesion failures separately, by using two separate mechanical systems to measure them
independently. The dynamic Shear Rheometer (DSR) was used to measure the
cohesiveness of asphahs and the
22
debonding of asphalt films from the aggregates surfaces in asphalt mixttires resulting
from water (Kunnawee Kanitpong and Hussain U. Bahia, 2003). The Net Adsorption
Test (NAT) was developed as a simple laboratory test to determine precisely the adhesive
nature of asphalts to mineral aggregates either in the presence or absence of moisture. In
1986, the Net Adsorption Test was developed as part ofthe Strategic Highway Research
Program (SHRP), It is based on the basic principle of adsorbing asphalt from a asphalt-
toluene solution and then the potential of desorbing in the presence of water. This test is
capable of evaluating the mix before and after water conditioning whereas most of the
other tests as described above can only evaluate after the water conditioning of the mix.
The other tests evaluate the total effect of all mechanisms simultaneously and provide an
overall evaluation without clearly separating the cause of failure due to each mechanism.
Thus the NAT is considered to be an effective test method that evaluates the
performances of asphalt-aggregate combinations.
23
CHAPTER 3
THE MODIFIED NET ADSORPTION TEST
3.1 Introduction
As discussed in earlier chapters, moisture is a major threat to flexible pavements
and there are very few laboratory test methods available which accurately quantifies the
effects of moisture on the bond between asphalt and aggregate. In the late 1980's, The
Stt-ategic Highway Research Program (SHRP) developed a very simple and fast test
method that quantitatively measures the adhesion failures and moisture sensitivity of the
asphah-aggregate bond. The testing protocol was released as SHRP Standard Test
Method # 1013 - Measurement of Initial Asphah Adsorption and Desorption in the
presence of Moisture (1990) originally designed for Hot Mix Asphalt (HMA). This test
method produced enough evidence that suggest that adhesion failure is not entirely
responsible on bitumen which was a common belief held in the industry. Rather, it
suggested that aggregate characteristics could be the controlling influence on the
adhesion phenomenon. This provided a better fundamental understanding of how asphalt-
aggregate bond works. But the original version ofthe SHRP's Net Adsorption Test had
limitations with the specifications, laboratory equipments and expressing the test results
(A.R.Woodside, W.D.H. Woodward, T.E.I. Russell and R.A. Peden, 1993). These
Authors also stated that although the SHRP method is effective in illustrating the
moisture sensitivity of the bond, it does not give a clear representation between the
amount of bitumen initially adsorbed and the quantity desorbed after the addition of
24
water. Further research on this test method at the University of Ulster, heland, by
Woodside et al (1993) resulted in a modified version of this method by making it better
illustrate the moisture sensitivity ofthe bond. Authors (Geraldine Walsh, I.L. Jamieson,
Margaret O'Mahony, 1995) added some more work and modified it further to extend its
application for the seal coats purpose. Thus the modified NAT by the Irish method is
more effective, gives better representation of results and clearly differentiates between
adsorption of asphalt in presence and absence of water when compared with SHRP
method of expressing test results.
3.2 Experimental Design
The experimental design for the modified NAT used two factors: asphalt type,
aggregate type. The parameter calculated from test results were percent initial adsorption,
percent final adsorption and percent net adsorption. Five different types of aggregate
from different sources and five different types of plant asphalt cement were chosen for
the primary lab testing protocol and other five more combinations were chosen to
compare the performance of field samples of asphalts.
3.2.1 Sources of Aggregate
The five sources of aggregates used in the laboratory testing program are listed in
table 3.1. These materials were selected based on their usage in Texas Seal Coat Work.
25
Table 3.1 Sources of Aggregate used for the Laboratory Testing Program
Aggregate Type
Limestone
Lightweight
Rhyolite
Siliceous Gravel
Sandstone
Source
Vulcan Materials Brownwood, TX
TXI Streetman, TX
Trans-Pecos Materials (Hoban Pit)
Midland, TX
Delta Materials Marble Falls, TX
Martin Marietta Materials Sawyer, OK
3.2.2 Sources of Binders
Different sources of asphalt binders were tested under the laboratory testing
program. Natural occurring asphalt binders are called plant binders and some field
samples of asphalt binders at the time of constructions were collected and tested under
this program. The sources of the binders used in the laboratory testing program are
indicated in the table as follows:
26
Table 3.2 Sources of Binders used for the Laboratory Testing Program
Asphalt Binder
AC 15-5TR
AC 5 with 2% latex
AC 10 with 2% latex
AC 15P
PG 64-22
Type
Plant
Plant
Field Sample
Plant
Field Sample
Plant
Field Sample
Plant
Field Sample
Plant
Sources
Trumbull Asphalt Houston, TX Alon USA
Big Spring, TX Trumbull (Wright Asphalt)
Lufkin, San Jacinto U.S. 59 N-B shoulder
Date: 06/18/02 Alon USA
Big Spnng, TX Eagle Asphalt (Corpus Christi)
Odessa, Midland FM 1379 intersection
Date: 8/23/02
Alon USA Big Spring, TX
Eagle Asphalt (Corpus Christi) Lubbock, Floyd
FM 602, South of U.S 62 Date: 06/26/02 Koch Materials Fort Worth, TX
Eagle Asphalt (Corpus Christi) San Antonio, Kendacc
Ranch Road 473 W Date: 08/14/02
Lion Oil Co. El Dorado, AR
3.3 The Modified Net Adsorption Test Procedure
The Test method, as previously mentioned involves the adsorption of asphalt to
the aggregate from an asphalt-toluene solution. The test also involves desorption phase in
27
the presence of water. The moisture sensitivity of the adhesive bond between asphalt and
aggregate were examined in this research using the modified NAT and the results were
evaluated based on method suggested by SHRP and also by Woodside et al at the
University of Ulster.
3.3.1 Apparatus
• Shaker Table: Equipped with 8 holders capable of holding 500 ml Erlenmeyer
flasks.
28
Fig 3.1 Mechanical Shaker
• Spectrophotometer: Capable of providing a continuous 410 nm wavelength with
an accuracy of+/- 2nm, holding standard 10 mm path length cuvettes.
Fig 3.2 Spectrophotometer
29
Spectrophotometer cuvettes capable of 4.5 ml and 10 mm path length.
Erienmeyer Flasks with the capacity of 500 ml.
Volumetric Flasks with the capacity of 25 ml and 1000ml.
Filter paper: Whatman No. 42, 125 mm in diameter.
250 ml graduated glass cylinder.
10 ml pipettes
Analytical Balance with precision up to 3 decimal.
Aggregate Drying Oven capable of maintaining 135 °C.
3.3.2 Reagents
Toluene: UV graded / Spectroanalysed grade
Distill water
3.3.3. Preparation of Aggregate
Each aggregate-asphalt combination needed four samples with fifty grams of the
aggregate graded according to standard grading practice (Table 3.3). They were dried
uncovered at a temperature of 135 °C in an oven for approximately 15 hrs. The sample
was removed from the oven at least 15 minutes prior to being placed in a 500 ml
Erlenmeyer flask to prepare the stock solution. The following table gives the quantities of
aggregate for each size to make up the 50 g test sample.
30
Table 3.3 Grading of Aggregate as per the IRISH standard for NAT (Geraldine Walsh,
I.L. Jamieson, Margaret O'Mahony, 1995)
Sieve Size
2.36 mm
1.18 mm
600 ^ m
300 ^ m
150nm
75 p, m
Percent Retained
8.0
25.0
17.0
23.0
14.0
6.0
Wt. Retained (g)
4.3
13.5
9.1
12.4
7.5
3.2
Total 50
3.3.4. Preparation of Stock Solution
In a IOOO ml volumetric flask, 0.5 gram of each asphalt type (+/- 0.001 g) was
weighed and dissolved in 500 ml of toluene to produce a I gram/liter concentration of
stock solution.
3.3.5 Test Procedure
The test takes nearly 24 hrs to complete and the test procedure can be divided into
four main steps. These steps are as follows:
3.3.5.1 Preparation of Test Samples and Control
Each aggregate sample was placed in each ofthe four labeled 500ml Erienmeyer
flasks. Out of the four Erienmeyer flasks, three are considered as test samples and the
31
fourth flask was used as the control sample. 140 ml of a freshly-prepared stock solution
(asphalt/toluene) was added to each of the three test flasks, while the control flask only
contained 140 ml of toluene to ensure that no material on the aggregate would interfere
with the adsorption measurements. All four flasks were placed on a mechanical shaker
operating at 300 rpm.
3.3.5.2 Initial absorbance ofthe stock solution
Approximately four milliliters of the prepared stock solution was diluted with
pure toluene in a dry, clean 25 ml volumetric flask, and the initial absorbance of the
asphalt/toluene solution was obtained at a wavelength of 410 nm using a
spectrophotometer.
3.3.5.3 Measurement of Initial Adhesion
After 6 hours on the shaking table, 4 ml of solution from each flask was filtered
and diluted with toluene in a clean dry 25 ml volumetric flask. About 4 ml of the filtered
solution was then transferred into the spectrophotometer cuvette, and absorbance at 410
nm was determined. The shaker table was reactivated and operated ovemight after the
addition of 2 ml of distilled water to each ofthe 250 ml Erienmeyer flasks.
3.3.5.4 Measurement of Final Adhesion
After a 15-17 hour period on the shaker table, a final absorbance was determined
using the same procedure explained in 3.3.5.2 above.
3.3.6 Calculation Procedure
3.3.6.1 SHRP Procedure
Initial Adsorption (Ai) i.e. the amount of bittimen initially adsorbed onto the
aggregate surface:
32
Ai = VC*(A,-A2)/(WA,)
Net Adsorption (An) i.e. the amount of bitumen remaining on the aggregate after water is
added water is added:
A„= VrC (Ai-Aj)/ (WAi)
%) Net Absorbance reading (%An) i.e. the percent of bitumen remaining on the aggregate
after the test
%An = (An/A,)* 100
3.3.6.2 UUJ Procedure
Maximum Absorbance reading (Amax) ie. theoretical absorbance reading when all
the bitumen in the solution has been absorbed by the aggregate:
An,ax = VC*(A,-0) / (WA,)
Initial Absorbance (%Ai) i.e. re-evaluation ofthe SHRP data to determine the percentage
of bitumen initially adsorbed onto the aggregate surface
%Ai = (A, / Aniax) * 100
%Net Adsorption (%An) i.e. re-evaluation of SHRP data to determine the percentage of
bitumen remaining on the aggregate after the test
%An = (An / A^ax) * 100
Where:
A, = Initial adsorption, mg / g
V = volume of solution in the flask, 140 ml
W = weight of aggregate, in grams
C = Initial concentration of bitumen in solution, Ig /1
33
Al = Initial absorbance reading
A: = Absorbance reading after 6 hours
A3 = Absorbance reading after 16-17 hours
An = Net adsorption, mg / g
Vr = Volume of solution in the flask at the A3 is obtained, 136 ml.
3.4 Discussion of Test Data
The criteria selected by SHRP to evaluate the performance of aggregate-binders
adhesion based on percent Net Adsorption results (%An SHRP) are summarized in Table
3.4.
Table 3.4 SHRP Evaluation Criteria for Aggregate-Binder Adhesion (Geraldine Walsh,
I.L. Jamieson, Margaret O'Mahony, 1995)
Percent Net Adsorption %An SHRP
>70
55-70
<55
Expected Performance of Aggregate-Binder Bond
Good
Marginal
Poor
3.4.1 Performance of Aggregates with Different Binders
The Test results are summarized in the following tables for both the SHRP
method and the revised University of Ulster method to better evaluate the performance of
aggregate-binder bond.
34
3.4.1.1 Limestone Aggregate with Asphahs
Limestone is a sedimentary rock that is being used by many TxDOT districts in
both seal coat and hot mix asphalt (HMA). Limestone was sampled from the Vulcan
materials Brownwood pit and it was tested with five different plant asphalts as follows:
PG 64-22 from Lion Oil, AC I5-5TR from Alon USA, AC 15-5TR from Trumbell, AC
15P from Koch Material, and AC 10 with 2% latex from Alon USA. Field samples of
asphalts collected from the construction site were also tested with limestone as follows:
AC 5 with 2% latex from Eagle, AC 15-5TR from Trumbell, and AC 15P from Eagle
Asphalt. These field samples performed differently when compared with the
performances of plant asphalts with limestone. Table 3.5 shows the results of limestone
with plant and field asphalts on the basis of modified NAT. Figure. 3.3 summarizes the
percent initial adsorption and percent final adsorption based on revised the University of
Ulster Method and percent net adsorption based on the actual SHRP Method for
limestone aggregates with different types of asphalts.
35
Table 3.5 Comparison of Different Combinations of Limestone with Binders
ASPHALT
PG 64-22 (P-Lion Oil) AC 5+2% (F-Eagle) AC 10+2% (P-AlonUSA) AC 15P (P-Koch) AC 15P (F-Eagle) AC 15-5TR (P-AlonUSA) AC 15-5TR (P-Trumbell)
AC I5-5TR (F-Trumbell)
SHRP
A,
1.55
1.62
1.39
1.79
1.39
2.21
1.70
1.54
An
0.69
1.10
0.95
1.14
0.94
1.81
1.37
1.21
%An
69.84
68.03
67.99
63.99
67.84
82.21
81.20
78.63
University of Ulster Method A
VC (A,-0)/(WAi)
2.80
2.80
2.80
2.80
2.80
2.80
2.80
2.80
A, (A./An,ax)* 100
55.18
57.72
49.82
63.76
49.60
78.76
60.71
55.16
An (An/An,ax)* 100
24.59
39.21
33.80
40.73
33.63
64.75
48.93
43.38
F - Field Asphalt, P- Plant Asphah
36
90
75
After 6 hours
60
45
30
15 —
After 24 hours % Net (SHRP)
Q P G 64-22 (P)
g A C 15P(F)
AC 5+2%L (F)
S A C 15-5TR (P-Aln)
Q A C 10+2%L (P-Aln)
E A C 15-5TR (P-Tr)
lAC 15P (P)
I AC 15-5TR (F-Tr)
Fig. 3.3 Limestone Aggregate with Field/Plant Asphalts
Based on the results shown in Figure 3.3, limestone performed better with plant
asphalts than with the field sample. Previous research showed that most carboxylic acids,
which adhere strongly to limestone, are more often found in naturally occurring asphalts
than in oxidized asphalts. The bond between limestone and PG 64-22 asphalt proved to
be highly water sensitive. Although the initial adhesion of PG 64-22 on limestone is quite
high, the final adhesion is very low. This behavior can be predicted on the basis that
acidic material (e.g. carboxylic acids) strongly adsorbs onto some aggregate surfaces like
limestone and can react with water to form carboxylate salts and free carboxylic acid. At
the mterface ofthe asphalt aggregate bond, the authors (H. Plancher, S. M. Dorrence and
J.C. Petersen, 1977) found higher concenfrations of free carboxylic acid molecules than
carboxylate salts for limestone when compared with other aggregates.
37
Plant sample asphalts could be ranked based on their performance of adherence to
limestone aggregate as follows: AC 15-5TR (Alon USA) > AC 15P (Koch Materials) >
PG 64-22 (Lion Oil). These asphalts could also be ranked based on water sensitivity as
follows: AC 15-5TR (Alon USA) > ACIO with 2% latex (Alon USA) > AC 15P (Koch
Materials).
3.4.1.2 Lightweight Aggregate with Different Asphalt
Lightweight aggregate that is artificially prepared in the United States from shale
has the distinguishing properties of low weight, rough surface, and high porosity. This
aggregate was tested with four different plant asphalts and one field sample asphalt
collected from the construction site as summarized in Table 3.6. The results of
lightweight aggregate with different binders are compared and presented in Figure 3.4
Table 3.6 Comparison of Different Combinations of Lightweight with Binders
ASPHALT
PG 64-22 (P-LionOil) AC-5TR (P-AlonUSA) AC 5+2%L (F-Eagle) AC 10+2%L (P-AlonUSA) AC 15P (P-Koch)
SHRP
A,
1.51
1.46
1.71
1.22
1.98
An
0.89
0.95
1.38
0.84
1.29
%A„
59.10
65.55
80.87
68.68
65.58
University of Ulster Method
'^max
VC (A,-0)/(WAi)
2.80
2.80
2.80
2.80
2.80
Ai (Ai/An,ax)*100
53.79
52.15
61.00
43.64
70.70
An (An/A„ax)*100
31.79
34.00
49.33
29.85
46.21
F - Field Asphalt, P- Plant Asphalt
38
QPG 64-22 (P) • AC 15-5TR (P-Aln) ^ A C 5+2%L (F) HAG 10+2%L (P-Aln) BAG 15P (P)
Fig.. 3.4 Lightweight Aggregate with Field/Plant Asphahs
Figure 3.4 shows that the initial adhesion onto lightweight aggregate is
remarkably good compared to the initial adhesion onto other aggregates because of its
high porosity. But most of the combinations of this aggregate with plant asphalts
performed badly in the presence of water. The worst was its performance with PG 64-22
and AC 15P, which showed desorption of almost 30% in the presence of water. Some of
the possible reasons for this are as follows: the pore pressure developed due to the high
porosity of the aggregate, and there is an affinity for the aggregate surface to form
hydrogen bonding in the presence of water or for the formation of soluble salts with Na
or K present at the aggregate surface, etc. Lightweight aggregate overall performed well
with the field AC 5 with 2% latex, hi this case, the loss of adhesion in the presence of
water was the least. The comparison graph shows that although the initial adhesions with
39
the plant asphalt AC 15P (Koch materials) is highest (70%), but it loses adherence of
25% (approx.) in presence of moisttire. The comparison graphs also show that
lightweight aggregate performed badly even with other fresh asphalts, i.e. PG 64-22
(Lion Oil), and AC 15-5TR (Alon USA), in the presence of moisttire. Thus the moisture
sensitivity of this aggregate is higher with plant asphalts than with field asphalts, and so
the latter option is a better choice for long-term performance of pavements against
stripping.
3.4.1.3 Rhyolite Aggregate with Different Asphalts
Rhyolite is gravel aggregate that is being used by some districts in Texas for seal-
coating and even for HMA. The typical physical characteristic of this aggregate is high
surface area with very low porosity. The results of NAT with rhyolite aggregate are
presented in Table 3.7, and Figure 3.5 shows the comparison of percent initial, percent
final, and percent net adsorption of various combinations with rhyolite aggregate.
40
Table 3.7 Comparison of Different Combinations of Rhyolite with Binders
ASPHALT
PG 64-22 (P-LionOil) AC I5-5TR (P-AlonUSA) AC5+2%L (P-AlonUSA) AC 5+2%L (F- Eagle) AC 15P (P- Koch) AC 10+2%L (P-AlonUSA) AC I0+2%L (F-Eagle)
SHRP
A,
1.52
1.79
1.73
1.86
1.50
1.22
1.48
An
1.03
1.34
1.00
1.56
1.13
1.03
1.27
%An
67.71
75.14
57.67
84.17
75.39
84.15
85.97
University of Ulster Method
Amax
VC (Ai-0)/(WA|)
2.8
2.8
2.8
2.8
2.8
2.8
2.8
A, (Ai/Amax)* 100
54.16
63.78
61.92
66.32
53.74
43.69
52.88
An (An/Amax)*100
36.81
47.95
35.71
55.82
40.48
36.77
45.53
F - Field Asphalt, P- Plant Asphalt
41
90
75
60 a '-fl a u ® 45
<
30
15
After 6 hours
L M_ D PG 64-22 (P)
• AC IS-P
After 24 hours % Net (SHRP)
AC 15-5TR (P-Aln) D AC 5+2%L (P-Aln)
AC 10+2%L (P) @ AC 10+2%L (F)
AC 5+2%L (F)
Fig. 3.5 Rhyolite Aggregate with Field/Plant Asphalts
Figure 3.5 shows that most ofthe binders except plant asphalt PG 64-22 and AC 5
with 2% latex performed well based on the criteria selected by SHRP. But it shows a
different picture if the percent initial and final adsorption is compared between them.
Field sampled asphalts performed better than fresh asphalts in terms of initial and final
adsorption.
Percent net adsorption of rhyolite with these asphalt combinations was found to
be in the acceptable range of >70% except for the asphalt binder PG 64-22, which was
just under 70%. The possible reason is its higher surface area, which influenced the
strong adhesion of asphalt and aggregate even though the susceptibility to desorption of
organics by water is greater than any other aggregates.
42
3.4.1.4 Sandstone Aggregates with Different Binders
Sandstone rocks are both acidic and basic in nature and have the physical
characteristic of rough surface with moderate porosity. Sandstone was tested with four
different types of plant asphalts and a field asphalt collected from the site of pavement
seal coating. The results of all the combinations with sandstone are tabulated below.
Table 3.8 Comparison of Different Combinations of Sandstone with Binders
ASPHALT
PG 64-22 (P-LionOil) AC I5-5TR (P-AlonUSA) AC 5+2%L (F- Eagle) AC 10+2%L (P-AlonUSA) AC 15P (P- Koch)
SHRP
Ai
1.46
1.59
1.47
1.15
1.37
An
1.13
1.17
1.18
0.83
0.98
%An
77.35
73.8
80.68
72.06
71.47
University of Ulster Method
A
VC(Ai-0)/(WAl)
2.8
2.8
2.8
2.8
2.8
Ai (Ai/Amax)* 100
52.25
56.66
52.37
40.89
48.82
An (An/Amax)* 100
40.4
41.82
42.25
29.46
34.88
F - Field Asphalt, P- Plant Asphalt
43
90 T
75 — -
60 — B i
a. I. 045 _.
s o
ls
% Net (SHRP)
DPG 64-22 (P) • AC 15-5TR (P-Aln) nAC5+2%L(F) • AC 10+2%L (P) BAClSP (P)
Fig. 3.6 Sandstone Aggregate with Field/Plant Asphahs
Figure 3.6 shows that sandstone formed good initial bonding with plant asphalts
such as PG 64-22, AC 15-5TR, and AC 15P but underwent loss of adhesion with AC 15P
m the presence of water. Plant asphalt AC 10 with 5% latex performed worst of the
asphalts. It also shows that field asphalt AC 5 with 2% latex performed better than the
plant asphalts in the presence of water. Sandstone aggregate is considered tohave both
acidic and basic characteristics; therefore, the performance of initial adhesion and final
adhesion with most ofthe tested asphalts produced similar results.
3.4.1.5 Siliceous Gravel Aggregates with Different Binders
Siliceous aggregates are known for their hardness, polished surface, and very low
porosity. Siliceous gravel aggregate was tested with four different plant-sourced
44
commercial asphalts and one field sample binder collected from the construction site of
the pavement seal coating. The results are summarized in Table 3.9 as follows:
Table 3.9 Comparison of Different Combinations of Siliceous Gravel with Binders
ASPHALT
PG 64-22 (P-LionOil) AC 15-5TR (P-AlonUSA) .\C5+2%L (F- Eagle) .\C 10+2%L (P-.AlonUSA) .\C 15P (P- Koch)
SHRP
A,
1.25
1.15
1.47
1.12
1.12
An
0.96
0.99
I.I3
0.72
0.71
"oA„
76.76
85.68
77.03
63.93
63.06
University of Ulster Method
^ m a x
VC(A,-0)/(WAl)
2.8
2.8
2.8
2.8
2,8
A, (Ai/Amax)* 100
44.70
41.07
52.59
40.1
40.08
An (An/Amax)* 100
34.28
35.22
40.51
25.62
25.27
F - Field Asphalt, P- Plant Asphalt
45
PG 64-22 (P) • AC 15-5TR (P-Aln) nAC5+2%L(F) •AC10+2%L(P) •AC15P(P)
Fig. 3.7 Siliceous Gravel with Field/Plant Asphalts
The comparison chart shows that the percent net adsorption values of three
combinations were good and that two combinations were marginal based on the criteria
set by SHRP. But the picture is very different when the percent initial and final
adsorptions were also taken into consideration. Siliceous aggregate performed worst
when compared with other aggregates for the laboratory testing in terms of percent initial
and percent final adsorptions. The percent initial and final adsorptions of the siliceous
gravel with all the plant asphalts were very low, but the results with field AC 5 with 2%
latex were satisfactory. This combination shows an initial adsorption of more than 50%
and a final adsorption of 40%, which helps the pavement perform better against sttipping.
46
In general, siliceous gravel is called a hydrophilic aggregate since it has a higher affinity
to form hydrogen bonding with water. Sttidies show that the surface charges on this
aggregate are acidic, and very little basic ingredients are found in nattirally occuring
asphalts and hence hinder the formation of long-lasting bonds at the polar sites. Perhaps,
then the field sample asphalt is somewhat oxidized and thus it has more basic
functionalities suitable to interaction with the surface of siliceous gravel aggregate to
fonn better water resistant bonds.
3.4.2 Evaluation of Each Binders with Different Aggregates
3.4.2.1 PG 64-22
This asphalt binder was non-modified and sampled from the plant of Lion Oil
Co., El Dorado, AR. The results with five different types of aggregates are listed in Table
3.10
Table 3.10 Adhesion of PG 64-22 Binder
ASPHALT
Limestone
LT.WT
Rhyolite
Sandstone
Siliceous gravel
SHRP
A,
1.05
1.51
1.52
1.46
1.25
An
0.74
0.89
1.03
1.13
0.96
%An
69.84
59.10
67.71
77.35
76.76
University of Ulster Method
A • ' ' •max
VC(A|-0)/(WAl) 2.8
2.8
2.8
2.8
2.8
Ai (Ai/Amax)* 100
55.18
53.79
54.16
52.25
44.70
An (An/Amax)* 100
24.59
31.79
36.81
40.40
34.28
47
Fig. 3.8 Adhesion of PG 64-22 Binder
Test results show similar percent initial adsorption (approx. 50%) on most
aggregates but the percent final adsorption varied considerably among them. Limestone
performed badly among all aggregates with a percent final adsorption approx. 30%.
Lightweight aggregate also showed low adherence in the presence of water. One of the
reasons could be the rheological characteristics of PG 64-22, i.e. high viscosity and hence
low wettability on the aggregate surfaces.
3.4.2.2 AC 15-5TR Seal Coat Binder
This is a Seal Coat binder used by TX DOT. h is an AC 15 binder modified using
5% tire rubber. The material was sampled from Alon USA, Big Spring, TX. The
48
performances of aggregates with plant asphalt AC 15-5TR (Alon USA) are tabulated and
results are compared in the chart as follows:
Table 3.11 Adhesion of AC 15-5TR Seal Coat Binder
ASPHALT
Limestone
Lightweight
Rhyolite
Sandstone
Siliceous gravel
SHRP
Ai
2.21
1.46
1.79
1.59
1.15
An
1.81
0.95
1.34
1.17
0.99
%An
82.21
65.55
75.14
73.80
85.68
University of Ulster Method
^max
VC(Ai-0)/(WAl)
2.8
2.8
2.8
2.8
2.8
A, (Ai/A„^O*100
78.76
52.15
63.78
56.66
41.07
An (An/Amax)*100
64.75
34.00
47.95
41.83
35.22
Fig. 3.9 Adhesion of AC 15-5TR Seal Coat Binder
Overall, the adhesion performances ofthe AC 15-5TR binder with the aggregates
49
were very good. Figure 3.9 shows that the perfomiance of this binder with limestone is
the best. Although the percent net adsorption value of siliceous gravel aggregate is very
high. Its percent initial and final adsorptions are both very low. The possible reasons are
its physical characteristics such as polished surface, very low porosity and chemical
properties such as the surface charge of the aggregate being unable to attract the acidic
functionalities present in asphalts cement. Sandstone and lightweight aggregate showed
moderate initial adherence with this binder and the reasons could be illustrated based on
their physical and chemical properties. The physical properties of sandstone support
adhesion because it has a rough surface texture and moderate porosity. But this aggregate
is mainly acidic in nature and there are very few basic polar sites available in plant
asphalt to attract them to form strong long lasting bonds. Rhyolite showed good initial
adsorption because of its roughness and greater surface area but it desorbs approx. 16 %
in the presence of water. The reasons could be formation of Na / K soluble salts and
higher affinity to form hydrogen bonds in the presence of water.
3.4,2.3 AC 5 +2% Latex Binder
This asphalt binder was modified with 2% latex, collected from two different
sources of plant and field. Rhyolite was the only aggregate tested with both field sample
and plant asphalt AC 5 with 2% latex. The results of net adsorption tests for the vanous
combinations are tabulated and a companson graph is plotted as follows:
50
Table 3.12 Adhesion of AC 5 +2% latex Seal Coat Binder
ASPHALT
Limestone+ AC 5 (Field) LT.WT+ AC 5 (Field) Sandstone+ AC 5 (Field) Siliceous gravel+ AC 5 (Field) RhyoIite+ AC 5 (Plant) Rhyolite+ AC 5 (Field)
SHRP
A,
1.62
1.71
1.47
1.47
1.73
1.86
An
1.10
1.38
1.18
1.13
1.00
1.56
%A„
68.03
80.87
80.68
77.03
57.67
84.17
University of Ulster Method
'^max
VC (A,-0)/(WAl)
2.8
2.8
2.8
2.8
2.8
2.8
Ai (Ai/An,ax)*100
57.72
61.00
52.37
52.59
61.92
66.32
An (A„/A„,ax)*100
39.21
49.33
42.25
40.51
35.71
55.82
90 After 6 hrs
Fig. 3.10 Adhesion c
After 24 hours
Rhyolite & AGS + 2%L(Plant)
% Net (SHRP)
y
Limestone BLightweight HSandstone gSiliceous gravel •Rhyolite & AC 5+2%L (P) •Rhyolite
f AC 5 +2% Latex Seal Coat Binder
51
The chart shows that AC 5 with 2% latex field binder performed good with
lightweight, rhyolite, siliceous gravel and sandstone aggregate but not with limestone
aggregate. Field asphalt binders possibly lack in concentration of carboxylate ions which
is found mainly in the fresh plant asphalt, which causes strong adhesive bond with
limestone. The chart, which compares the performances of rhyolite with plant as well as
field asphalts AC 5 with 2% latex, shows clearly that rhyolite performed much better
with plant asphalts than with the fresh one. The reason being oxidized asphalts produces
much more polar basic functionalities which are good for the acidic aggregates such as
rhyolite, sandstone, siliceous gravel for the formation of long lasting bonds and less
stripping in the presence of water.
3.4.2.4 ACIO+2% Latex
This is a binder modified with 2% Latex, sourced from the manufacturer Alon
USA and Big Spring, TX and also one sample was collected from the site of seal coating
at Lubbock, Floyd FM 602, south of U.S 62 with the source manufacttirer Eagle Asphalt
(Corpus Christi). Except rhyolite, aggregates were only tested with the plant sampled
source. The results are tabulated and a comparison chart is presented as follows:
52
Table 3.13 Adhesion of ACIO +2% Latex Seal Coat Binder
ASPHALT
Limestone & AC10+2%L(PIant) Lightweight & ACIO(Plant) Sandstone & AClO(Plant) Siliceous gravel & AC10+2%L(Plant) Rhyolite & AC 10+2%L (Plant) Rhyolite & AC 10+2%L (Field)
SHRP
A,
1.39
1.22
1.14
1.12
1.18
1.48
An
0.95
0.84
0.83
0.72
1.03
1.27
%An
67.99
68.68
72.06
63.93
87.89
85.97
University of Ulster Method
Amax VC (Ai-0)/(WAI)
2.8
2.8
2.8
2.8
2.8
2.8
Ai (Ai/Amax)* 100
49.82
43.64
40.89
40.10
42.02
52.88
An (An/Amax)* 100
33.80
29.85
29.46
25.62
36.81
45.53
Fig. 3.11 Adhesion of AC 10 with 2% latex Seal Coat Binder
53
Figure 3.11 shows that limestone perfomied better when compared with the
perfonnances of sandstone and siliceous gravel. The result also compares the
perfonnances of rhyolite with both plant and field sample asphahs. Rhyolite aggregate
perfomied better with field sample asphalt AC 10 with 2% latex than with plant asphalt
AC 10 with 2% latex. Siliceous gravel perfomied badly with fresh asphalt with a percent
initial adsorption of 40% and percent final adsorption of approx. 26%. Based on the
perfonnances it can be stated that limestone is a better choice with plant asphalt ACIO
with 2% latex and rhyolite, sandstone, siliceous gravel and Lightweight aggregate could
be the choice with the field sample asphah AC 10 with 2% latex.
3.4.2.5 AC 15P Seal Coat Binder
All the aggregates were tested with the AC 15P binder, sampled from the plant of
Koch Materials, Fort Worth, TX and the results are tabulated in Table 3.14 with a
comparison chart Fig. 3.12 as follows:
Table 3.14 Adhesion of AC 15P Seal Coat Binder
ASPHALT
Limestone
LT.WT
Rhyolite
Sandstone
Siliceous gravel
SHRP
A,
1.79
1.98
1.50
1.37
1.12
An
1.14
1.29
1.13
0.98
0.71
%An
63.99
65.58
75.39
71.47
63.06
University of Ulster Method
•'^max
VC (A,-0)/(WAl)
2.8
2.8
2.8
2.8
2.8
Ai (A,/Amax)* 100
63.76
70.70
53.74
48.82
40.08
An
(An/Amax)* 100
40.73
46.21
40.48
34.88
25.27
54
S Limestone I Lightweight O Rhyolite I Sandstone I Siliceous Gravel
Fig. 3.12 Adhesion of AC 15P Seal Coat Binder
The results show that percent net adsorption for the various combinations are
marginal to good according to the criteria standardized by SHRP as mentioned eariier in
this chapter.
The acidic aggregates, siliceous gravel performed badly with the plant asphalt AC
15P. The percent initial adsorptions on limestone and Lightweight aggregates are more
than 60%) but they lost more than 20% of the binder in the presence of water. The reason
behind high percent initial adsorption for limestone is understood to be strong acid base
and electrostatic interactions but desorption ofthe binder is because some ofthe strongly
adsorbed acidic materials form soluble salts in the presence of water. The rough surface
and high porosity of Lightweight aggregate attracts asphalts and thus causes high percent
55
initial adsorption but it also losses substantial amount of asphalts in the presence of water
mainly due to the capillary pressure and viscosity effects and also sometimes due to the
weak bond formation at the interface.
56
CHAPTER 4
Tech-MRT Pull-Out Test
4.1 Introduction
Surface treatments such as seal coats (chip seal) are widely used as a pavement
preventive maintenance treatment. A seal coat provides a layer of new aggregate exposed
to traffic, which can furnish better durability and wear characteristics, improve surface
friction, and reduce the rate of deterioration of the pavement structure. Seal coat, which
consists of a sprayed asphalt layer, followed by a layer of stone chips show problems
such as aggregate loss caused by poor bonding of the aggregate to the asphalt. With the
loss of aggregate from the surface, the roadway loses its friction and also causes a
negative perception among road users. Therefore, the aggregate-binder compatibility
issue is critical for seal coats. A test was developed in Australia to assess the extent of
initial adhesion between aggregate and a bituminous binder under both wet and dry
conditions (Test Method T238). This test procedure was originally designed to test the
compatibility between aggregate and cut back asphalt at room temperature. Therefore,
modification of this test method was required to test asphalts other than cutbacks at
different temperatures. This Modified procedure is the Tech-MRT Pull-Out Test,
intended to evaluate both the cohesion and adhesion bonding mechanisms between
asphalt and aggregate.
57
4.2 Original T-238 Test Method
Australian Standard Test method T-238 sets out the procedure for the assessment
of the extent of initial adhesion between cover aggregates (with or without treatment pre-
coating) and cut back binder with or without modifiers under wet or dry test conditions.
Bitumen was cutback to give the desired viscosity at the testing temperature. The target
viscosity ofthe binder used for the test is 15000 stokes. The aggregate sample for the test
shall consist of about 20 particles representing the dominant size constituting the
aggregate. Clean and dry condition, dusty, saturated, surface dry and saturated surface
w et condition were used for the preparation of aggregate test samples. Preparations of
sample involve heating the binder until it becomes fluid and pour sufficiently in a metal
tray with 3 mm thick layer. Each sample of aggregate prepared shall be tested after curing
under soaked or un-soaked condition. The test involves checking the percentage of coated
area and categorizing them into three main classes i.e. completely stripped, partially
stripped and not stripped.
4.3 Tech MRT Pull-Out TEST (TMPT)
Tech MRT Pull-Out Test was developed based on the Australian Standard Test
T238: (Initial Adhesion of Covered Aggregates and Binders). Tech MRT puUout test
(TMPT) evaluates the initial adhesion between aggregate and any type of binder at higher
temperattire to simulate field conditions. The standard test procedure involves preparing a
sample with 72g of asphalt on a metal tray of 6 inches / 6 inches at standard temperattire
58
100°F. The standard test condition is clean and dry aggregate; curing not under water but
at room temperature. Asphalt is heated at 320°F until it becomes fluid before pouring on a
metal tray to prepare the sample. An aggregate particle with a nail on top of it is placed in
the asphalt at the same temperature and kept aside for 24 hrs at the room temperature.
After 24 hours the sample is evaluated under various test conditions. Based on the results,
the bond sttength and binding compatibility ofthe combination is determined.
Apparatus
• Metal tray container size: 6 in X 6 in and 3mm deep
• Aggregate: Predominant sized aggregate used in seal coat
• 2.5" long galvanized nails
• Water bath (18 in by 12 in and 6 in deep) capable of maintaining water at a
temperature between 0° F and 500°F.
• Force gauge. Pliers, long nose
• Long nose Pliers
Heat resistant gloves and tongs.
Spatula or palette knife
• Duck Tape
•
•
59
' , V * . ?* <' ' ' *» >.•" :: •«
^ *% |T^~
j^0y' '-"^,
''^ !„;•• ' " ** , , ' 7 , 11 .
Fig 4.1 Apparatus
60
4,4 Experimental Plan
The Tech-MRT Pull-Out Test (TMPT) was conducted usmg asphalt binder type,
aggregate type and moisttire as the primary variables. The response variables were
percent area of aggregate coated with asphalt on aggregates, tensile sttess, elongation
length of asphalt and the energy required to pull out the aggregates. The binder-aggregate
combinations were chosen based on field data and net adsorption test results. Five
different types of aggregate from sources as indicated in the Table 4.1 and five asphalts
types as indicated in Table 4.1 were chosen for the primary lab testing protocol other than
the 5 special combinations chosen to compare the field results.
4.4.1 Matenal Selection
Different types and grades of aggregate were selected for the TMPT. Aggregates
for this test were selected based on the researchers experience with compatibility and the
modified NAT results. The aggregate types selected for lab testing were limestone,
rhyolite, siliceous gravel, sandstone and lightweight aggregate. The sources of materials
used in the laboratory testing program for the TechMRT Pull-Out Test are listed in Table
4.1.
61
Table 4.1: Sources ofMaterials used for the TechMRT Pull-Out Test
Sl.No
1
2
3
4
5
Asphalt Binder
AC 15-5TR (Plant)
Alon USA
Big Spnng, TX
AC 5 2% +2% latex(Field Sample)
Eagle Asphalt (Corpus Christi)
Odessa, Midland
FM 1379 intersection
Date: 8/23/02
AC 10+2% latex (Plant)
Alon USABig Spring, TX
AC 15P (Plant)
Koch Materials
Fort Worth, TX
PG 64-22 (Plant)
Lion Oil Co.
El Dorado, AR
Aggregates
Limestone
Vulcan Materials
Brownwood, TX
Lightweight
TXI
Streetman, TX
Rhyolite
Trans-Pecos Materials
(Hoban Pit)
pecos, TX
Sandstone
Martin Marietta Materials
Sawyer, OK
Siliceous
Delta Materials
Marble Falls, TX
62
4.4.2 Test Factorial
Samples were prepared under different environmental conditionings. Some
samples were prepared at different temperattires to check its influences on pavement.
Some samples were also prepared at 120°F, 140°F and pulled out at room temperattire to
simulate the field conditions based on the field database. The regular test condition of
aggregate was "washed, cleaned and dried states". Under Normal Conditioning, samples
were prepared at 1 IO°F and kept at 24 hours at room temperattire before the pulled out of
aggregate. Asphalt-aggregate combinations were also tested for the Extreme
conditioning, to check the effect of adverse environments on the bonding under freeze-
thaw conditions. Under this conditioning, samples were soaked under water for forty
eight hours, twenty four hours at under room temperatures after the three repetition of
sixteen hours in water and eight hours of freezer.
4.4.3 Procedure
The evaluations of test results were based on the percent aggregate area coated
with asphalt, tensile stress, elongation length of asphalt string and energy required to pull
out the aggregates. The percent coated area by asphalt on aggregate surfaces was
evaluated by visual observation. The elongation length, which is the length of asphalt
string formed when aggregate is pulled out from the asphalt, is used as a measure of
cohesion failure ofthe interface. The string developed due to the pull out ofthe aggregate
from the base asphalt is measured as elongation length in inches, which measures
63
cohesion failure. This parameter helped to evaluate the possibility of retuming of the
aggregate to its original position on distress. The strength of the aggregate-asphalt bond
was evaluated based on tensile stress and pseudo energy.
4,5 Discussion of Results
4,5.1 Normal Conditioning
The results shown in Table 4.2 through 4.5 are obtained under normal
conditioning. The aggregates used were clean and dry. The samples were prepared at
I00-110°F, un-soaked in water and kept open in air for 24 hours before the pullout of
aggregates with the force gage. The peak load and the coated area were measured with
the force gage and square scale respectively. The equations used to calculate tensile stress
and pseudo energy are given below:
^ ., c. r -\ Peak Load (lb) Tensile Stress (psi) -Coated Area (sq.in)
Energy (psi.Fl) = f"'\\°'"'^"'\ *Elonga,ion(ft) ^•^ Coated Area{sq.in)
64
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66
Lightweight Rhyolite Sandstone Limestone
H <25%Coating Intact
S 50-75% Coating Intact
125-50% Coating Intact
I >75% Coating Intact
Figure 4.2 Comparison of PG 64-22 (Plant) -Aggregate Combinations
•a
I 100^ H <u a. 75
vn 50
u OXI
2 5 -
Lightweight Rhyolite Sandstone Limestone Siliceous
H <25% Coating Intact
B 50-75% Coating Intact
0 25-50%Coating Intact
S >75% Coating Intact
Figure 4.3 Comparison of AC 15-5TR (Plant) -Aggregate Combinations
67
Lightweight Rhyolite Sandstone Limestone Siliceous
H <25% Coating Intact
B 50-75% Coating Intact 0 25-50% Coating Intact
S >75% Coating Intact
Figure 4.4 Comparison of AC 5+2%L (Field) -Aggregate Combinations
<u • ^ ^
0)
H Ji "B. B «
c/3
R exi u et) &X)
100 -p
75 - • •
50 - •
25 - - • -
0
Lightweight Rhyolite Sandstone Limestone Siliceous
M <25% Coating Intact
S 50-75% Coating Intact
25-50% Coating Intact
>75% Coatmg Intact
Figure 4.5 Comparison of AC 10+2%L (Plant) -Aggregate Combinations
68
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50 --
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Lightweight RhyoUte Sandstone Limestone Siliceous
M <25% Coatmg Intact
a 50-75% Coatmg Intact
25-50%Coating Intact
I >75% Coating Intact
Figure 4.6 Comparison of AC 15P (P) -Aggregate Combinations
4.5.1.1 Discussion on Percent area coated ofthe aggregates
It is generally accepted that the greater the coated area, the better the performance
of the asphalt-aggregate bond. Table 4.2 compares the asphalt- aggregate initial adhesive
bond for various binder-aggregate combinations. Combinations that performed well on
percent coated area of asphalt on aggregate surfaces are as follows: limestone aggregate
with plant asphalt AC 15-5TR; Lt.Wt, rhyolite and sandstone with field asphalt ACS with
2%latex; lightweight with plant asphalt ACIO with 2%latex and these results are also
supported by the results of Modified NAT. hiitial adherence of most Plant asphaU is high
with limestone and lightweight aggregate, while Field asphalt shows high adherence with
rhyolite, sandstone and siliceous gravel.
69
Table 4.3 Comparison of Maximum Tensile Stress (psi) of different combmations
Max. Tensile Stresses (psi)
AC 15-5TR
AC 15P
AC 10+2''oLatex
AC 5+2% Latex
PG 64-22
Source
Plant
Plant
Plant
Field
Plant
Lightweight
45.3
51.8
47.1
56.3
Limestone
62.2
54
59.9
56.5
31.0
Sandstone
50.6
49.5
74.7
47.1
117.0
Rhyolite Gravel
32.8
58.6
56.5
86.1
Siliceous Gravel
35.2
Table 4,4 Comparison of Pseudo Pull-Out Energy (psi-ft) of different combinations
Pseudo Pull-Out Energy (psi-ft)
AC 15-5TR
AC 15P
AC 10+2%Latex
AC 5+2% Latex
PG 64-22
Source
Plant
Plant
Plant
Field
Plant
Lightweight
9.1
6,71
7.5
7.6
2.3
Limestone
7.9
10.1
1.3
Sandstone
13.3
10.6
8.4
7.6
Rhyolite Gravel
9.9
9.3
7.7
2.0
Siliceous Gravel
10.6
9.4
7.3
9.7
70
Table 4.5 Comparison Binder Pull-Out Thread Length (in) for Binder-Aggregate
Combinations
Pull-Out Thread Length (in)
AC 15-5TR
AC 15P
AC 10+2%Latex
AC 5+2% Latex
PG 64-22
Source
Plant
Plant
Plant
Field
Plant
Lightweight
2,9
2.4
1.6
0.3
Limestone
2.2
Sandstone
3.1
2.9
1.6
3.0
Rhyolite Gravel
3.1
3
1,8
0.3
Siliceous Gravel
3.5
7.9
3.3
1
71
4.5.1.2 Discussion on Tensile Stress and Pseudo Energy
Tensile Stress and pseudo Energy measures the interfacial bond strength between
asphalt and aggregate. This is a simultaneous qualitative measurement of cohesive and
adhesive strength between asphalt and aggregate based on the numerical value.
Limestone performed very well with Plant AC 15-5TR and Plant AC 15P and moderately
with plant AC 10 with 2%latex. As expected, limestone performed pooriy with Field
asphalt samples of AC 5 with 2%latex and rhyolite. Siliceous gravel performed well with
Field AC 5 asphalt with 2%latex. Lightweight aggregate performed well with plant
asphalts PG 64-22 and AC 15P and hence again supports modified NAT results.
4.5.1.3 Discussion on Elongation of different combinations
Elongation is measured as the maximum length of asphalt string before it breaks
from the aggregate surface. This parameter helps to evaluate the cohesive failures
between asphalt and aggregate bonding. Higher elongation is considered to be better for
the performance of pavement against stripping because longer the elongations better the
chances of retuning a loose aggregate to its original position upon distress. Table 4.5
shows that the combination of aggregate with Plant asphalts AC-15P and ACIO with
2%latex has much higher elongation than the combinations with PG 64-22.
4.5.2 Extreme conditioning:
The following results are obtained under both normal and extreme
conditions.Clean and dry aggregates were used to prepare samples at 110°F, un-soaked in
72
water and kept open in air for 24 hours at room temperature before the pullout of some
aggregates under normal conditions. The sample is then put under water for 16hours and
kept in freezer for 8hours and this is repeated for three cycles. Finally, the sample is
placed under water for 48 hours before the aggregates are pulled out after extreme
conditioning. The percent coated area by asphalt on aggregate surfaces is evaluated based
on visual observation and the observations of tensile stress and pseudo energy are
measured qualitatively by using equation.
Table 4.6 Comparison of results under NORMAL and EXTREME condifionings
a. Dusty Limestone and AC 15-5TR
Coated Area
Tensile Stress (Ib/in^)
Elongation (in)
Pseudo Energy (IbFt/in )
Normal Conditioning
40(no stripping)
24.2
1.6
2.7
Extreme Conditioning
0(no stripping)
20.9
0.6
1.3
b. Clean Siliceous Gravel and AC5 with 2%latex (Field)
Coated Area
Tensile Stress (lb/in )
Elongation (in)
Pseudo Energy (IbFt/in^)
Normal conditioning
100(not stripped)
23.6
2
3.9
Extreme Conditioning
0(no stripped)
17.9
0.8
1.3
73
4.5.2.1 Discussion of Test Data under Extreme Conditioning
Aggregate-binder combinations for this test were chosen based on their field use.
The comparison of results with both conditioning procedures reveals that there are
substantial reductions in elongation under extreme conditions than compared to normal
conditioning and hence predicts the possibility of cohesion loss in binder. Tensile
strength did not change significantly even with extreme conditioning, but the percent
coated area in both cases showed remarkable differences.
4.5.3 Influences of Elevated Temperatures
Table 4.6 compares results of samples prepared under elevated and normal
temperatures . The aggregates used were clean and dry. The samples were prepared at
140''F or 120°F, un-soaked in water and kept in open air for 24 hours before the aggregate
is pulled out. The percent asphalt coated area on aggregate surfaces was evaluated by
visual observation and the observations of tensile stress and elongation were measured
qualitatively.
74
Table 4.7 Comparison of results with PG64-22 and aggregates prepared at 110°F &140°F
a. Limestone and PG64-22 (Lion Oil)
Percent Coated Area
Tensile Stress (lb/in")
Elongation (in)
Energy (IbFt/in')
Samples prepared at 110-100 °F
72.7(partial stripping), 18.8 (no Stripping)
31.0
negligible
1.3
Samples prepared at 140-130°F
100(no stripping)
109.5
0.2
2.5
b. Lightweight and PG64-22 (Lion Oil)
Coated Area
Tensile Stress (Ib/in^)
Elongation (in)
Energy (IbFt/in^)
Samples prepared at 110-100°F
44(partial stripping), 46 (no Stripping)
56.3
negligible
2.3
Samples prepared at 140-130°F
100(no stripping)
97.7
0.3
2,6
75
Table 4.8 Comparison of results with AC 15-5TR (Alon USA) and aggregates prepared at
110°Fandl20°F
a. Limestone and AC 15-5TR (Alon USA)
Percent Coated Area
Tensile Stress (Ib/in" )̂
Elongation (in)
Energy (IbFt/in")
Samples prepared at 110-100 °F
100 (no Stripping)
36.9
2.2
6,7
Samples prepared at 120-110°F
15(partial stripping), 84.5 (no Stripping)
62.1
1.4
7.1
b. Lightweight and AC 15-5TR (Alon USA)
Coated Area
Tensile Stress (lb/in )
Elongation (in)
Energy (IbFt/in^)
Samples prepared at 110-100°F
100 (no Stripping)
30.2
4.1
10.2
Samples prepared at 120-110°F
25(partial stripping), 38(no Stripping)
45.3
2.9
9.2
76
4.5.3.1 Discussion on the influences of elevated temperatures
Table 4.8 compares the performances of plant asphalt AC 15-5TR with limestone
and lightweight aggregates at normal and higher temperatures. Although there is an
increase in tensile strength at higher temperatures, it is accompanied by a decrease in
elongation and pseudo energy required to pull out the aggregates when compared with
normal temperature.
PG 64-22 is the asphalt with no modifier among all asphalts tested in the
laboratory. As a consequence the initial adhesion or the percent coated area at normal
temperature with most aggregates were poorer when compared with other asphalts.
Previous researchers found that the viscosity of asphalt should be low enough for proper
wet ability and void control. It is also evident from the test results as mentioned in Table
4,7 that the performances of asphalt PG64-22 with aggregates were enhanced with the
increase of temperature. There was a very significant increase in tensile strength and
percent coated area at 140°F compared to 110 °F
77
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
The use of appropriate combinations of asphalt and aggregate can result in better
pavements and thereby translate into in significant savings in pavement life-cycle costs.
This smdy demonstrated that surface chemistry and mineralogy of aggregate play a
significant role in the adhesion of asphalt and closely related stripping phenomenon.
Surface chemistry and mineralogy determine the number of bonding sites and the
intensity of chemical reactivity for bitumen fiancfionalities at the aggregate surface.
Research suggests that asphalt adheres better to the "basic" aggregates than "acidic"
aggregates, but there are exceptions. It was observed from experimental results that
adhesion of asphalt binder sampled from the binder plants exhibited better adhesion with
limestone, which is a basic aggregate, with than acidic aggregates such as rhyolite and
siliceous gravel. Acidic aggregates, such as rhyolite and siliceous gravel, formed better
bonds with field sampled asphalts. Although these acidic aggregates showed a lower
percent initial adherence with field asphalts compared to basic aggregates and plant
asphalts, they showed higher resistance to water-induced damage. One of the reasons, as
explained by previous investigators, is that acidic aggregates form strong and water
resistant bonds with the basic polar sites largely available in the oxidized asphalts, but
negligible in naturally occurring asphalt
Further work is required to determine the polar fiinctionalifies of asphalts used in
the laboratory testing program with the help of reliable tests method to better explain the
78
fiandamental aspects of interactions with aggregates.
This study demonstrated that sandstone aggregate performed similar by with all
asphalts except plant binder ACIO with 2% latex. Sandstone exhibits the physical
characterisfics of rough surfaces and moderate porosity, which help to adsorb asphalt in
the pore. Sandstone aggregate is also known to have both acidic and basic polar sites.
Therefore the initial adherence should not be very high or vary with the type of asphalt,
but desorpfion rates could be different depending upon the types and nattare of asphalts.
Microbial activity at the interfacial bonding was evident by many previous
invesfigators (Brown, L.R., Darnell T.R., 1987; Brown, L. R., Pabst, JR. G, S., and
Marcev, J. R., 1990 and Parker Jr, F., Benefield L., 1991), but their acfivity in the
presence of different types of asphalts and aggregates with different conditionings are not
known yet. It is recommended that future studies try to evaluate the affect of microbial
activity on interfacial bonding.
Absorption of asphalt to aggregate is influenced by the porosity of aggregates.
Porous aggregate exhibit larger surface areas and hence show resistance to debonding
from asphalt binder. This study observed that lightweight aggregate, which is highly
porous, resulted in better initial adhesion with all binders. Test results show that the
inifial adhesion of siliceous gravel with binders was worst than other aggregates.
Siliceous gravel has a smoother glassy surface with low porosity, and, hence, devoid of
most mechanical interiocks with bitumen. Based on this study, the use of rougher surface
aggregate is recommended, since it has more surface area, and, hence, facilitates more
adhesion when compared to a smoother aggregate with low surface area.
79
This study also found a loss of cohesive strength of interstitial asphalt binder at
the interfacial bonding between plant binder AC15-5TR-limestone and field asphalt
binder AC5 with 2%latex-Siliceous gravel in the presence of moisttire. The cohesive
property of bitumen could be crucial in the prevention of stripping, since it determines
the surface tension of bittimen on aggregate surfaces. Therefore, ftirther laboratory work
IS required to run additional different combinations to help identify asphalts-aggregates
pairs which perform better against cohesion failures.
Viscosity is another fundamental physical property of binders which plays a
significant role in wetting aggregate surfaces. Results using Tech-MRT Pull-Out Test
(TMPT) showed that PG 64-22 binder had a lower adherence with aggregates than other
asphalt binders when samples were prepared at normal (110°F) temperatures. However,
TMPT'S performance increased enormously when the samples were prepared at higher
temperamre than the normal temperature (110°F). This was also evident from our study in
which binders with high viscosity, such as PG 64-22, needed to be prepared at higher
temperature in order to get better wet-ability and adherence on aggregate surfaces. Our
test results showed that the initial adsorption of PG 64-22 on different aggregate surfaces
were similar with the exception of siliceous gravel aggregates, which as noted earlier
have physical characteristics of glassy, polished surfaces and very low porosity. Our test
results also indicated a poorer performance for the asphalt binder AC15-5TR and
limestone aggregate having high surface dust content. This might be explained based on
the rafionale that dust and dirt increases the viscosity of asphalt binder and decreases the
wet-ability and mechanical interiocks on aggregate's surfaces. Previous sttadies also
80
support the concept that viscosity should be low enough to limit any voids which play an
important role in stripping phenomenon (Bagampadde et al. 2003).
The Modified Net Adsorption Test method proved to be a reliable test method for
evaluating the adherence performances of asphalts with uncoated aggregates in the
presence and absence of moisttare. However, further work is needed to devise a reliable
test method which can evaluate the performances of pre-coated aggregates with different
binders. If this were possible a better correlation could be established between differences
in the performance of coated and uncoated aggregates with different binders for use in
seal coat and HMA applicafions.
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85
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