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Transcript of SHRP-91-510
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SHRP-A /UWP-91-510
Chemical Properties of Asphalt sand Their Relationship to
Pavement Performance
Raymond E. Robertson
Western Rese arch InstituteLaramie, WY
Strategic Hig hway Researc h Pr ogramNationa l Research Counc il
Washington, D .C. 1991
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SHRP-A /UWP-91-510Contract A-002A
Product Code 1001, 1007
Program Manager: Edward T . HarriganProject Manager: Jack S . YoutcheffProgram Area Secretary: Juliet Narsiah
March 1991Reprinted November 1993
key words:adhesion
agingasphaltchemical mod efailure mod els
fatigu e cr ackingimer molecularmetalsmoistur e damageoxidation
pavem ent p er for manc eper manent defo r mationpolarit yruttingther mal c r ackingvirgin asphalt
Strategic High way Research ProgramNati onal R esearch C ounci l2101 Constitution Avenue N.W.
Washington, DC 20418
(202) 334-3774
This manual represents the views of the author o nly, and is not necessarily reflective of the views of the
National Research Council, the views of SHRP, or SHRP's sponsor. The results reported here are notnecessarily in agreement with the results of other SHRP research activities. They are reported to stimulatereview and discussion within the research community.
50 /NAP /1193
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Acknowledgmen ts
The research described herein was supp orted by the Strategic H ighway ResearchProgram (SHRP). SHRP is a unit of the National Research Council that was authorizedby section 128 of the Surface Transportation and Uniform Relocation Assistance Act of1987.
Th is pr oj ect w as conducted by the Weste rn R esearch In stitute in coo perati on wi th thePenn sylvania Tran spo rtation In sti tu te, the Texa s Tran spo rtation In sti tute, and SRIIn ternational. Raymond E. Robe rtson was the p rincipal in vestiga tor. The suppo rt andenco uragemen t of Dr . Edwa rd T. Harr igan, SHRP Asphalt Prog ram Manage r, and Dr.Jac k Youtche ff, SHRP Technical Cont ract Mana ger, are gra tefully ac knowled ged.
The experimental work cited herein was conducted by Dr. J. F. Branthaver , Dr . K.Ensley, Dr. J. Duvall, H. Plancher, P. M. Harnsberger, S. C. Preece, F. A. Reid, J.Tauer, M. Aldrich, A. Gwin, M. Catalfomo, G. Miyake, and J. Wolf. Their support inthe preparation of this report is gratefully acknowledged. The review of this manuscriptby Dr . J. Clain e Petersen, former pr incipal inves tigato r for A-00 2A; Dr . Jan F.Branthaver at W RI; and Dr. David Anderson at PTI, co-principal investigator forA-002A; and Dr. Edward T. Harrigan and Dr. Jack Youtcheff of SHRP is herebyacknowledged.
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Con ten ts
1.0 Introdu ction ................................................... 11.1 Objectives of the SHRP Asphalt Program ........................ 11.2 Purpose of Report ......................................... 2
2.0 Chemistry .................................................... 32.1 Molecular Level ........................................... 3
2.1 .1 Typic a l Spe cies in Virgin As phalt ........................ 32.1.2 Oxidation to Form New Molecules ....................... 52.1.3 Metals ............................................ 92.1.4 Polarity ........................................... 9
2.2 Intermolecular Level ...................................... 102.3 Chemical Mode of Asphalt .................................. 17
3.0 Spec u lation on Relationship of Chemistry to PavementPerformance ................................................ 203.1 General Comments ....................................... 203.2 Aging ................................................. 213.3 Speculation on the Relationsh ip of Composit ion
to Various Failure Models ................................. 233.3.1 Rutting and Permanent Deformation ..................... 243.3.2 Thermal and Fatigue Cracking ......................... 253.3.3 Adhesion and Moisture Damage ........................ 26
4.0 Summary .................................................... 29
V
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Li st of St ructu res
1 M olecular Fragment of an Aliphatic Hydr ocarb on ........................ 4
2 M olecular F r agment of an A ro matic Hy dro car bon ........................ 4
3 M olecular Fragmem of an A ro matic Hydr ocarb on ........................ 4
4 M olecular Fragment of an Aliphatic and Ar omatic Hy drocar bon ............. 4
5 M olecular Fragmem of a Pyridine ................................... 5
6 Molecular Fragmem of a Thi ophene .................................. 5
7 M olecular Fragmem Sh owing Benzyl Carb on ........................... 6
8 M olecula r Fragmem of a Ket one .................................... 6
9 M olecular Fragmem of a Carb oxylic Aci d .............................. 6
10 Molecular Fragmem of a S od ium Carb oxylate ........................... 7
11 M olecular Fragment of a Calcium Carb oxylate .......................... 7
12 M olecular F ragment of a Ca r boxylic Anhy dride ......................... 7
13 M olecular Fragment of a Phen ol .................................... 7
14 M olecular Fragment of a H omolog .................................. 8
15 M olecular Fragment of a H omolog .................................. 8
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16 Molecular Fragment of a Homolog .................................. 8
17 Molecular Fragment of a Quinolone .................................. 8
18 Molecular Fragment of a Sulfoxide .................................. 9
19 Metalloporphyrin ............................................... 9
Vll"""
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List of Figures
1A Randomized Molecules......... " " " * " " ......... .............. 10
1B Organized Molecules . o o 10
2 Three Dimensional Molecular Matrix .............................. 11
3 S.E.C. Chromatograms ........................................ 14
4 Effects of Temperature on Aging Kinetics of an Asphalt Sensitiveto Molecular Structuring ....................................... 17
5 Performa nce ................................................ 21
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F, _C_ ZVE S_Y
The che mistry of petrole u m asphalt at th e molecular and
intermolecular levels is discussed herein and a combination of
interpretation and speculation placed on how chemical data may explain
performance characteristics in roadways. At the molecular level
historical studies have shown that there are at least hundreds of
thousands of uniqu e mol e cular species that exist within any particular
asphalt. Asphalts are known to b e comprised of molecular species that
vary widely in polarity and molecular w e ight. Th e major obj e ctiv e of
this portion of th e work is to explain behavioral characteristics of
asphalts in t e rms of ch e mistry. Polarity is a very major contributor to
th e performance characteristics. Polar materials tend to associate
strongly into a matrix which is dispersed in less polar and non-polar
materials. In g e neral, th e mechanical or structural properties of
asphalt ar e r e lat e d to th e inte rm olecular structuring among polar
components. These polar interactions may arise by involvement of any of
numerous different ch e mical species. Th e exact nature of th e chemical
specie is l e ss important than th e overall ass emb lag e of a s e t of polar
materials to form a matrix within th e non-polar continuous medi um . Th e
matrix gives elastic charact e r to th e asphalt while the continuous non-
polar phase gives a viscous component to asphalt. It is speculat e d that
ex cessive structuring leads to brittl e cements which tends to crack
while too little structuring leads to mat e rials which defo rm und e r
stress. Oxidation adds to the ag e hardening and brittleness of asphalt
cement by contributing additional polar materials to the structured
zones within the binder. At very low temp e ratur e it appears that the
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non-polar mate r ials also tend to organize into a very rigid material,
and that this rigid mat e rial shrinks at low t e mperatur e . It is
sp e culated that this type of shrinkage is largely responsible for low
t e mpe ratur e cracking.
A second major obj e ctiv e is to translat e r e s e arch me thods into t e st
methods. Numerous types of research m e thods have been employed to
differentiate among asphalts with an emphasis on identifying methods
which distinguish among asphalts in the same fashion that they ar e
distinguished by their performance characteristics.
Th e e xp e cted advantages of this work, both technical and economic,
ar e to dev e lop methods for s e l e ction of asphalts that will perform in a
predictable fashion th e reby alleviating the current co n unon problem of
prematur e road failure. Throughout t h e progr am , both us e rs and
produc e rs of p e trol e um asphalt have worked closely with SHRP contractor
personn e l both to give advic e on progr am dir e ction and to maintain a
sense of what is practically implementable in each sector.
As with any new development questions arise as to how much
sp e cialization will be required for implementation of new test methods.
Certainly n e w test methods could become extraordinarily complex, but it
is still another obj e ctiv e to assur e that there is no mor e complexity
than is absolu te ly necessary. Problems of impl em entation, ther e fore,
should be minimiz e d and espec'ially so when one considers that each new
test method will enter a round robin test progr am in both state highway
and user laboratories. The round robin effort, in fact, has already
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b e gun. F i n a ll y , t his w o rk i s su ppo rt ed by pu bli c f u n d ing and the
r e sults that ar e impl e mented into new t e st m e thodology will be public
domain.
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1. 0 INTRODUCTION
i.i Objective s o f the SHRP A s phalt Pr o gram
Th e SHRP asphalt research area is a highly focused endeavor which
ultimately must d e v e lop specifications for binders and asphalt-aggregate
mixtures that relat e to th e p e rformanc e of both binders and mi x tures in
pav eme n t s. To accomplish this th e fund a mental material properties must
b e known first. Then fund am ental properties are to be used to develop
me aningful sp e cifications. The current specifications for petrol e um
asphalt are little mor e than a quality control exe rcis e for th e r e finer's
vacuum crude tow e r operations, hence there is little or no distinction
among asphalts other than a s e t of viscosities at two elevated
t e mp e ratur e s and a mix plant oxidation susceptibility. That is, all of
many asphalts may b e classifi e d as, for e xampl e , AC-20, and all will be
applied as if they are a single material. Yet, their perfo rm ance
charact e ristics in pavements often vary quite significantly. Th e results
of using poorly classified asphalts have been rude and costly surprises
in th e form of various types of early failures with increasing frequency.
It behooves the user, th e refore, to d e t erm ine what properties of asphalt
binders are r e flected in pavement p e rformance and then select binders
which have d e sirabl e p e rformanc e prop e rties. From a practical viewpoint,
no doubt many petrol eum residua will require modification to prepare
asphalts that have desirable p e rformanc e characteristics.
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1.2 Purpo s e o f Rep o rt
Th e purpose of this report is to d e scrib e th e current status of th e
SHRP chemical studies of petrol e um asphalt. It is intended to b e
instructiv e to th e non-chemist and further is a speculative effort to
correlate some of th e known ch e mical properti e s of asphalts with pavement
performance characteristics. It is generally believed that performance-
bas e d specifications for binders will b e mainly physical property tests.
The obj e ctive of the composition studies is to develop correlations
between th e chemical and physical properties. Further, the composition
studies ar e to define a s e t of practical analytical methods to describe
composition. This then will allow definition of acceptable composition
for a given p e rformanc e which in turn is instructive to the producer in
th e manufacture of asphalt. If any given asphalt has a compositional
deficiency, th e producer can determine how to modify th e asphalt and
remedy th e deficiency. It has become obvious that deficiencies vary from
on e asphalt to another. It is important to note that ma ny ch e mical
properties will not correlate with performance so it is a ma jor goal to
distinguish th e chemical properties that do relate.
Referenc e s ar e made to ongoing research which has been reported to
SHRP, but in many cases has not been published. Many statements are
based on current SHRP research and have no literature referenc e s.
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2.0 CH EMISTRY
It is convenient to divide the chemistry of petroleum asphalt into
two parts. The first part is the ch e mistry at the molecular level and
th e second part is the chemistry of interaction among all molecular
species in asphalt. Much of the physical nature of asphalt can best b e
describ e d, in t e rms of composition, as an a sse mbly or matrix of molecular
species (building blocks) into large multi-molecular units within the
asphalt. N ume rous techniques exist to e xamin e asphalt at both th e
molecular and int erm ol e cular lev e ls. In the following subsection the
ch e mistry of asphalt at th e m ol e cular l e v e l is r e vi e wed without r e f e r e nc e
to e x p e ri me ntal methods. This is to illustrate what types of "building
blocks" ar e pres e nt. The che mi stry of interaction am ong asphalt
molecules is r e viewed in th e second subsection with reference to som e of
th e t e chniqu e s being used curr e ntly in th e SHRP progr am .
2.1 M o lecular Level
Extensi ve research on petrole um composition at th e molecular l e v e l
has be e n done by many workers for many years. At this point, much of it
is e ssentially textbook material b u t worth r e vi e wing h e r e .
2.1.1 Typical Species in Virqin Asphalt. At the molecular level
most of th e total mass of neat (tank) asphalt is a mixture of a wide
variety of high boiling hydrocarbons. Some are aliphatic (waxy
mat e rials), some ar e aromatic (more like air-blown asphalts) and some
molecules hav e both aliphatic and aromatic carbon. E xam pl e s of such
molecular fragm e nts ar e shown in Structures 1 (aliphatic type) and 2
(aromatic type).
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H H HI I I
H_ C _"C" C_"C "c /C_ C /
_CH2_ ICH2 _x_CH3_ I I II IHIC-_c /C_c /C _c _C_H
x Typically = 15 or More Carbons I I IH H H
Structure 1 Structure 2
Structure 2 is more co mmonly represented by Structure 3. Both s tructures
1 and 3 are only representative molecular fragments of larger molecules
that make up asphalt s . Structure 1 is s hown as a straight chain of
carbon atoms, but typically in asphalt there ar e numerous co mbinations of
aliphatic carbon chains which have one or more branches.
Structure 3
Structure 4 shows a molecule which i s a mixture of aliphatic and ar o matic
carbon.
2
____C H 2 (CH)x- CH3
Structure 4
While the major mass of asphalt is hydrocarbon, a large proportion of
molecules also contain one or more heteroato ms; nitrogen, sulfur, oxygen
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and metals. Nitrogen, in the form of a pyridin e is shown in Structure 5.
CH2-(CH2)x" - CH 3
Structure 5
Sulfur, as a benzothiophene, is shown in Structure 6. Structure 6 on the
right is a shorthand notat i on of the one to the le f t. These are
identical molecular fragments.
HI
--H or
/
Structure 6
Bo th su l fur and ni tr o ge n ma y a ppe a r i n a ny o f a vari e ty o f sites withi n
molecules. Hence, it is easy to see that tens of thou s ands of di ff erent
molecul a r species may be present in a s phalt con s idering that every
different arrangement of elements constitutes a different molecule. It
also follows that any definition of properties ba s ed on specific
molecular species would be a mon u mental task. It is much more e f fective
to classi f y the chemistry in ter ms of the molecular type s .
2.1.2 Oxidation to Form New Molecules. Certain types of carbon in
asphalt are susceptible to o x idation. An aliphatic carbon ne x t to a n
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aromatic ring is known a s a b e nzyl carbon and is an example of a r e adily
o x idizab l e site. Structure 7 is an e x ample of a mol e cular fragm e nt
showing a b e nzyl carbon in bold type (adjacent to th e aromatic ring).
CH2-(CH2 )x
Structure 7
Sites such as these oxidize to form ketones as shown in Structure 8.
C_ (c H2)x--
Structure 8
More severe oxidation m ay result in formation of carb ox ylic acid s and
lo ss o f part of the molecule such as shown by both versi o ns of
Structure 9.
rStructure 9
Carbox y l ic acids, whether present in th e origina l crude or for me d up o n
o x idati o n, may be c o nv e rt e d to sodium (Na) salts (Structur e 1 0) o r
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calcium (Ca) salts (Structure 11 ) by appropriate reaction with sodi u m or
calci u m inorganic compounds.
O O OII II II
Struct u re i0 Structure 11
Carboxy l ic acid anhydrid e s (typica ll y call e d anhydrides) may be formed
upon o x idation wh e n two b e nzyl carbons are pr e sent on adjac e nt aromatic
rings. An ex ampl e is Structur e 12.
0% c/O_c // 0
(CH2)xCH3
Structure 12
Anothe r type of o x yg e n containing m o l e cule which may b e present in
asphalt is a class known as ph e nols wh e r e o x ygen is attach e d dir e ctly to
an aromatic ring. This class is illustrat e d by Structure 1 3.
OH
CH3-(CH2) x'- CH2,_ CH 3
Structure 13
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Again n u merous variations, or iso me rs, containing t h e basic phenol unit
may e x ist. I n addition to isomeric combinations o f all mo le cular types
shown above, homologs of each also typically exi s t. An example of a
homologous s e ries is shown by Structures 1 4, 15, and 1 6.
0II
C_ cH2 (CH2) x CH3
-- -- Structure 14
OII
C_c H2--(CH2)x+I-CH3 Structure 15
OII
C_ cH2--(CH2) x+2--C H3 Structure 16
In the above case , each varies only by one aliphatic carbon. Each is a
unique molecule, although the properties a mo ng all of th e s e will be quite
similar.
Another important class of c o mpo und s typically found in aged
asphalts is quin o lones a s shown in Structure 1 7.
Structure 17
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Many s ulfur comp ou n ds are al so s u s cept ib le t o o xidation and
typically f o rm sulfoxides. A sulfoxide containing fr a gment is shown in
Structure 18.
OII
-(CH2)xCH2 -S -CH2 -CH 2 -
structure 18
2.1.3 Metals. There are also metals present in asphalts, again in
varying amounts and distributions. The most common metals are vanadium,
nickel, and iron although other metals may also be present. Typically
metals are present as organo-metallic materials, specifically as
porphyrins. An ex a mple is shown by Structure 19.
CH3CH 2 H CH3
CH3@__ CH2CH3
3NZ'N (
cH cH2 H3CH3 H CH2CH3
Str ucture 19
By now it is obvious that hundreds of thousands of unique molecules
may be found in any given asphalt. Further, the second, third and so
forth asphalts will contain hundreds of thousands of different molecules.
2.1.4 Polarit y. All of the naturally occurring heteroatoms,
nitrogen, sulfur, oxygen, and metals contribute to polarity within these
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molecules. Likewise, o xidation products formed up o n aging are p o lar and
further contribute to th e polarity of the entire s ystem. Polarity, which
is th e separation of charge within a molecule, can be seen by the
following exampl e . Th e dipole moment (separati o n of charge) of pyridine
(C5H5 N) is 2. 1 9 d e byes (in th e gas phase) wherea s the dipole moment o f
benzene (C6H 6) i s zero. B e nzene is th e all carb o n anal o g of pyridine.
Polarity also exists in all other heteroat o m containing species.
Polarity is imp o rtant in asphalt because it tends to cause m o lecule s t o
organize themselves into preferred orientati o ns. Historically, the s e
have been referred to a s formations of micelles, coll o ids, etc., alth o ugh
th e s e t e rms have been mi s us e d. A more current understanding of molecular
orientation within asphalts is given in t h e following Subsecti o n.
2.2 Interm o l e cul ar Lev e l
At the int erm ol e cular level, polar m o lecule s including tho s e in
a s phalt, have an o ther behavioral characteri s tic. Thi s is attraction of
o n e polar molecule for another as a result of their s eparated charges, or
dip o les. Figur e 1 illustrates this sch e matically.
(- + )C -
Figure 1A Figure IB
1 0
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In part A the polar mol e cul e s are randomized, but in part B th e molecules
ar e well orient e d with respect to each other. Part B r e pres e nts a mor e
stable th e rmodynamic state. It is important to note here that it makes
little difference which of the man y polar molecules shown earlier is
involved. Any one of th e m any types of polar mol e cu le s may fil l th e
mo le cular sch em atics shown in Figure 1 A or B. The primary require me nt is
that some sort of charge separation is pres e nt in the molecules. It is
obvious that a mu l ti- molecular structure may form as illustrated
sche matica l ly in Figure 2, although the individua l m o le cular co mponents
will vary from one to the ne x t so that no sp e cific reg u larity e x ists
within the organized zone (see note I).
G 0C+ -) C+ -) C+ - 7
0 0F igure 2(Note 1 ) For si mp l icity, this schematic shows positive and negative
charg e s at the ends of molecules. The associations among molecules ar e
combinations of e l e ctrostatic and other short range forces. Th e actual
charges are b e st d e fined in term s of asymm e tric e lectron den s ity and ar e
not true plus a nd minus charges as would be the case with ions. Neither
are charges neces s arily distribut e d e nd to end. F ig u r e 2 is only
intended as a convenient illustration.
1 1
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Du r i ng t h e o r g an i zat io n , so me a moun t of thr e e -d i men sional, i n t e rmolecul a r
structure forms. Historically, this has be e n called the mic e ll e , or
colloid. While it is not a true mic e lle, it is an organized set of
mol e cul e s. Th e set does hav e som e pr e f e rr e d organizational structure as
compared to that shown in Figure IA where it is only randomized
molecules. Th e structur e is held together by el e ctrostatic and other
short rang e forc e s which are weak compar e d to covalent ch emi cal bonds.
Sh o rt rang e ( n on - coval e nt) forc e s rang e from about 3 to I0 Kcal / mol e ,
whil e coval e nt bonds are much strong e r. For comparison, carbon-carbon
covalent bonds, the bonds that hold organic molecules together, are 80 or
great e r Kcal / mol e and carbon-hydrogen covalent bonds are typically i 00
Kcal / mo le . It follows th e n that the o rganized (intermolecular) structur e
may be subject to rearrangement or may be scrambl e d either from physical
stress or by raising its temperature. However, this will occur without
changing mol e cular composition. All of th e molecular speci e s remain th e
sam e . On th e other hand, t h e physical properti e s will be different.
Whe n th e mol e cul e s ar e rando mi z e d, th e y can mov e about with r e spect to
e ach other more easily than when they ar e more organiz e d. Th e mor e
highly organiz e d structur e has mor e resistance to motion or d e formation.
Said differently, structured asphalt is more of a springlik e material,
more viscous, and is stiffer. Th e ability to form an organized, or self-
asse mbl e d, structure d e p e nds upon th e strengths of th e attractions and
upon th e n u mb e r of sites wh e r e int e rmol e cular attractions occur.
Ox idation has a pronounc e d e ff e ct on th e organiz e d structure. As
o x idation occurs, new sites that ar e gr e at e r polarity ar e fo rme d and ar e
formed in larg e r amounts than in the virgin asphalt. So th e propensity
to self-as sociat e will increase. Also, the rate of association depends
1 2
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upon the nu mb e r o f s it e s an d the magnit ud e s o f the a tt r act i on s . The
stronger the attractions and the more there are, the greater the driving
force to associate. But, association is inhibited by the high viscosity
of a sphalt. Hence, the overall process is slow. For example, in one
experiment, virgin asphalt was observed to double in viscosity over a
period of a few years. The sample was protected so that oxidation did
not contribute anything. After this extended storage, the sample was
heated to mix plant temperature while protecting it from oxidation. The
viscosity returned to near its original value. The rates and magnitudes
of stiffening of asphalt after oxidation, with and without aggregate
present, and at various temperatures, are being studied at the present
time in the SHRP asphalt progr a m.
The degree of association varies from one asphalt to another and
several methods can be used to dete r mine type and magnitude of
association. One method being used at the present ti me is size exclusion
chromatography. It is illustrated in Figure 3 which shows three size
exclusion chromatography (SEC) experiments, all plotted on the same axes.
Asphalts AA G, AAK, and AAM (hereafter noted as G, K, and M) are all
similarly classified petroleum asphalts. All three were separated by S EC
in the same fashion. The S EC process separates materials (in this case,
asphalts) into components according to size at the molecular or multi-
molecular level. It matters not whether the size excluded entity is a
single molecule or an associated group of molecules. In the current
work, a system was chosen that causes the least possible disturbance to
the association. The SEC separates asphalt by apparent molecular size so
that if there are associated groups of molecules in the whole asphalt
they will also exist and therefore be separated as an associated group by
SEC. The SEC profiles sh o wn in Figure 3 are plots of the fraction of the
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SEC CHROMATOGRAMS
3 0 AAM /_\
-- \ ' _AAG25 /
" == _ //1 _
\
20 II
.,. , \15: \
*_ 1 0 f'\ \/ _ /(J I _a \
I \u_ I
\0
0 20 40 60 80 100 120
Large SmallMolecular Molecular
Size Size
Fi 9ure 3
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wh ol e a s pha l t o n t he y -ax is and t he mo l e cula r or mul t i - mo l e cular si ze on
the x-a x is. The three materials illustrated are virgin asphalts. Note
that asphalt G has a very small fraction that is in the large molecular
size, and a significantly larger fraction that is smaller molecular size
(the right side of the plot). Asphalt K is almost the reverse. Further,
these two are bimod a l indicating that there is a relatively small amount
of intermediate size materi a l in either. Asphalt M, however, is more of
a continu u m. That is, it is not bimodal. Asphalt M does have a
significant amount of large molecular species present as does K. Upon
further examinations of M and K by vapor-phase-osmometry molecular weight
determination in pyridine, it was discovered that the large molecular
species in K are comprised of many smaller molecules where a s in Asphalt M
the largest fraction (left of plot) is comprised of truly large
molecules. The l a rge molecules in M will not dissociate, for example,
with an increase in temperature, whereas the large ones in K will
dissociate. If nothing else is apparent, note that these three asphalts
a ppear to be very different from e a ch other when exa mined by SEC. Yet
conventional tests would indic a te that all three are si milar.
Another effect within asphalts is the behavior of the molecules
which have very little association. These are not necessarily smaller
molecules, but are the less polar and therefore less associated portions
of the asph a lt (to the right of the SEC plot). Ion exchange
chromatography (IEC) and supercritical fluid chromatography (SFC) h a ve
both been used to show that these molecules a lso vary from one asphalt to
another. Often called a maltene phase, this less-associated material
behaves as a "solvent" for the polar materials and will behave as a
dispersing agent. It will tend to reduce association of the polars.
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At this point, it is worth considering a more integrated description
of asphalt. It appears to be a material containing polar molecules that
associate strongly into organized units that are dispersed in a less
polar and continuous phase. The association of polars appears to depend
upon composition of polars and upon th e rmal history. Some speculation
can be made on the effects of the differing chemistry within the
continuous n e utral phase. First, it is apparent from the SEC curves that
asphalt G has a large fraction of small molecular species which implies
that th e l e ss polar and non-associat e d materials dominate its behavior.
It would b e expected that G would be able to acco mmodate additional
amounts of polars with minor changes in its physical properties. For
e x ampl e , asphalt G should be able to tolerate significant oxidation
b e for e any major viscosity change occurs and this is what is observed.
Asphalt K should be and is th e reverse. Oxidation of K should increase
th e alr e ady predominant polar effect and raise viscosity rapidly with
oxidation. This likewise is observed.
The propensity to oxidize has been observed by another method being
us e d in the SHRP asphalt studies. Oxidation in a pressurized vessel is
b e ing stu d i e d as a m e thod to simulat e long-te rm aging of asphalt in
pav e ment. After 400 hours of oxidati o n at 60C (140F) und e r pre s sure,
asphalt G had an aging index of 17 while K had an index of 23. When th e
temperature was raised to 11 3C (235F) and both oxidized at atmospheric
pressure for 72 hours, asphalt G had an aging index (measured at 6 0C) of
only 1 8 while K jump e d to 530 under th e same conditions. At the elevated
t e mp e ratur e , both asphalts ar e mor e dissociated and oxidize rapidly.
Upon cooling both to 6 0C, G can acco mmodate its own oxidation products
whereas th e newly formed polars in K strongly dominate i t s behavior. The
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po lar s i n aspha l t K a s s o c i ate v er y s t rongly a nd rais e the v i sc osit y qu it e
substa n tially. The same trends have been observed with other asphalts.
While on the subject of differing rates of o x idation of any given asphalt
at different temperatures, note in Figure 4 how the viscosity ch a nge
inc r e a ses with incre a sing oxid a tion temper a ture. This indic a tes that
maximum ro a d service temperatures must be taken into account in the
selection process for asphalts.
EFFECTS OF TEMPERATURE ON AGINGKINETICS OF AN ASPHALT SENSITIVE
TO MOLECULAR STRUCTURING
106 -
60C POV Aging
105
"_ 1 0 4
103 I I I I0 100 200 300 400
Oxidati o n Time, hr
Figure 4
2.3 Chemical Mo d e l o f A s p h alt
Th e model that is emergin g f rom this w ork is b u il t up o n e a rl ier
mo de l s and ha s b een r e fin e d du r ing th e SHRP program. I n this cas e mod e l
means nothi n g mor e than a cl e ar und e rstandi n g of th e b e havioral
charact e ristic s of a s phalt. Th e mod e l must b e abl e to e x plain all of th e
obs er v e d b e havioral charact e ristics a s we ll as all of th e variations i n
b e havior fr o m on e a s phalt t o an o th e r. He r e , it i s wo rth whil e to r ev i e w
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the manu fa ctu r e of as ph a l t. P et r o l e u m asp ha l t i s typ ically a high
boiling vacuum distillation residuu m which is prepared f rom nu merous
petroleum stocks. Some asphalts are produced by alternate methods but
generally very similar portions of the crude oil ends up as asphalt. The
chemistry and physical properties, therefore, vary quite significantly
from one asphalt to a nother and each reflects the nature of the crude oil
used to prepare it. The most consistent description, or model, of
petroleu m a sphalt is as follows. Asphalt is a collection o f pol a r and
non-polar molecules. The polar molecules tend to associate str o ngly to
form organized structures throughout the continuous phase of the non-
polar materials. Nuclear magnetic reson a nce data and the rm odyna mic data
indicate that the associations are not more than about 40 molecules, but
some have smaller asse mblies and a gain it varies f rom one asphalt to
another. Some show very little association. The non-polar phase, on the
other hand, has the a bility to dissociate the organized structure, but
again it varies from one asphalt to another. As temperature is raised,
the associations of polar molecules decre a ses and the materi a l becomes
more dissociated and therefore less viscous. As temperature is reduced,
the opposite occurs. Recent observations indicate that the non-polar
phase also organizes, but at very low te mperature, temperatures below
0C. Further, asphalt is susceptible to oxidation which increases both
the amount of polarity and the n u mber of polar sites pre s ent among
asphalt molecules. This further contributes to the ability of an asphalt
to organize, but again varies from one asphalt to another.
It is important here to point out that the variations in behavioral
characteristics of asphalts must be measurable. And ag a in the objective
of the SHRP program is to elucidate methods which can distingui s h among
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asphalts and ev e ntually reduc e thes e methods to highway laboratory
practice. Whil e th e quantitation of all important aspects of th e model
has not b ee n completed, it is int e r e sting to speculate on how the
variations in chemical properties may be reflected in the pavement
p e rform a nc e .
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3. 0 SP ECULATION ON R ELATI ONSHIP OF CH EMISTRY T O PAVEM ENT P ERFO RMANCE
3.1 General C o_tn ent s
Whil e it is unusual to d e scrib e p e rformanc e in terms of failure,
nev e rtheless i t is instructive to do so here since it is g e nerally
und e rstood that all pav em ents will eventually fail unless th e y ar e
r e built periodically. Performanc e must b e d e fin e d as sufficient time to
failure or to r e constructio n to justify th e us e of any particular
met h odology an d any g iv e n set o f ma teri a ls. S uff i ci e n t s er v i c e li f e is
determined by cost of construction, traffic density, harshness of the
environ ment, soil (support) conditions, and nu merous other factors. The
determination of acceptable service life is not the subject of this work.
However, service lifetimes of a few months to a few years, which are
experienced all too frequently, are unacceptable, whereas i0 to 20 years
of service gives more acceptable life cycle costs. Ne i ther is
methodology the subject of this work, so it will be assumed that
construction methodology is both consistent and adequ a te. The focus will
be upon variations in the quality of construction materials.
The s i gnificant fa ilure modes in asphalt p a vement that may be
related to m a terials are g eneral l y a g reed to be
I. Permanent deformation
2. Rutting
3. F a tigue crack i ng
4. Low temperature cracking
5. Moisture damage
6. Total loss of adhesion
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Some speculations on the relationships of these failure m o des t o the
chemical properties of asphalts ar e given in the s e ctions following the
section on aging.
3.2 Aging
Aging is often included in the above lis t, but aging is actually a
conditioning st e p which may be ben e ficial o r detrimental. It may b e
detrimental when excessive hardening or stiffening is ob s erved in an
already adequate pavement. On the other hand, aging may be beneficial
when a soft mi x ture hardens into an adequate pavement. A simplified
view of performance in terms of m e chanical properties is that a pavement
may be too soft and therefore rut and def o rm, or ma y be too stiff and
brittl e and therefore subject to cracking, either under traffic l o ad or
under th erm al s tress. Figure 5 illustrate s these points schematically
and also include s the effects of temp e rature, time, and oxidation.
PERFORMANCE
Little Well HighlyAss o ciati o n Balanced Associated
I ISOFT BRIT TLE
Perma n e n t Fatig u e and Low
Deformatio n , Temperat u reRutting Cracki n g
Time, Aging, Cooli n g
Moist u re, Heati n g
Figure 5
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Oxidation, or o x idativ e hardening, imparts permanent hard e ning in
asphalt whil e the hardening from reduced temperature and from molecular
organization ar e r e versibl e . That is, as an asphalt cement oxidizes,
whether in bulk or in a mixtur e , the cement becomes and stays stiffer at
any given s e t of conditions. However, warming a cold asphalt in the
absence of any other effects will soften it to its original value and
h e ating a c e ment to an elevated t e mp e rature such as a mi x -plant
temperature will reverse the effects of organizational hardening.
It is important to note that hard e ning resulting from mol e cular
organization can be reversed periodically by recycling pavement.
However, the recycling process removes only the organizational
hardening, but since pavement suffers oxidative hardening also, this
portion is not removed during the recycled process.
Th e e v e nts that lead to aging in a pavement are very slow because
th e driving forc e s for orientation (dipole of each molecule) ar e sm a ll
and th e whole m e dium is quite viscous at road service temperature.
Nonetheless, slowly th e molecules will shuffle about and eventually find
the b e st orientation, known as the th e rmodynamic stable state or
eq u ilibrium, and in so ori e nting themselves become a better pack e d and
bound together system. Aggregates, no doubt, have a distinct effect on
this orientation but also differ in their ability to cause orientation.
The result is an increasingly stiffer (more rigid) material until
th erm odynamic equilibrium is achi e ved.
In a roadway, achievement of the rm odynamic e quilibri u m is a moving
targ e t ! There ar e constant changes in composition which r e sult from
oxidation of asphalt. During oxidation polarity changes, hence, the
"b e st packing" changes. Further, te mp e ratur e changes constantly in
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pavement and th e rmodynamic equilibrium varies with temperatur e . Also,
traffic t e nds to disorient molecular species, especially under heavy
traffic loads and this further effects achievement of the rm odynamic
e quilibri um. It is not clear wheth e r traffic loads speed up orientation
by providing additional energy for molecules to move or if orientation
is slowed by k ee ping the syst e m "stirred."
Oxidative aging has on e interesting and saving feature to it. While
some asphalts, such as asphalt G, may oxidize and acco mmodate their own
oxidation products without major changes in viscosity, other asphalts do
show s ubstantial increases in viscosity upon oxidation. As shown in
Figure 4 e arli e r, virtually all asphalts eventually quench their own
oxidation and in so doing qu e nch th e ir own increase in viscosity as a
result of oxidation. Also, as was noted earlier in Figure 4, this
ph e nomenon varies with th e ma x imum t em peratur e of oxidat ion.
Compositional variations among asphalts dictate th e amount of viscosity
increase that is observed as a result of oxidation at different
t e mpe ratur e s. While th e chemistry of quenching is not fully understood,
it is a measurable chemical property of an asphalt and does relate to
th e visco e lastic prop e rti e s, and th e r e fore relates to th e p e rformanc e
charact e ristics of th e asphalt.
3.3 Speculati o n on the Relatio n s hip o f C om p os iti o n t o Vari o u s Failu r e
Mo de s
Th e me asurable chemical properties that ar e believed to relate to
th e me chanical or structural strength of a pavement ar e several. Again
from th e model, it is b e li e ved that asphalt is a set of "hardcore"
agglomerat e s (structured u nits) that consist of polars disp e rs e d in a
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less polar to nonpolar phase. The r e sult is a mat e rial which has an
e lastic behavior as a r e sult of the network form e d by the polar
molecules, but it is also a material with a viscous behavior that can
flow or cr ee p becaus e various parts of the network can move with respect
to each other under prolonged stress. The relative contributions of th e
e lastic and viscous behavior vary with composition.
3.3.1 Ruttinq and Permanent Deformation. Again, consider asphalts
G, K, and M. Th e ir SEC pl o ts ar e shown in Figure 3. Asphalt G is
la r g e ly th e disp e rsing phas e , not much e lastic in character. On e would
ex p e ct it to b e v e ry compatibl e as is o b se rv e d in oxidation and
compatibility index studies. Asphalt G is somewhat ins e nsitiv e to
oxidation. It does not harden well which makes it a mat e rial that will
rut or deform, especially at higher temperatur e s. It has little of th e
compositional feature of large molecular size to give it elasticity.
Asphalt G is mostly th e non-associated material which do e s tend to
organize and harden at low temperature. Th e refore G would b e expected
to be v e ry stiff (susc e ptible to cracking) at very low temperature. In
fact, th e se ar e observed behaviors for G in roadways. Contrast this
with th e S EC plot for asphalt K. Asphalt K has a large amount of
agglo me rat e d mat e rial and would b e e xp e c te d t o hav e a greater elastic
modulus at high temperature than asphalt G and not have t he pr o pensity
to rut; and this is also what is obs e rved. Asphalt M, with its truly
high molecular weight, should and do e s behave somewhat like asphalt K at
high temperature. Asphalt M is also a more homogeneous material than K
or G. This feature should impart a relatively low temperature
susceptibi l ity to M, and this likewise is observed.
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If heretofore th e relationship of composition to performanc e
properties has n ot b ee n obvious, consider some recent results from cross
blending of disp e rs e d and dispersing phas e s of asphalts. Th e s e
separated phases were generated by S EC. Major changes in properties
were observed in an asphalt prepared by blending fractions of different
asphalts. In one case, the dispersed phase of one asphalt was mixed
with th e dispersing phase of a second asphalt. The resulting asphalt
was more than i000 times th e viscosity of either original mat e rial ! The
dispersing phase of the s e cond asphalt is not a good "solvent" for the
dispersed phase of th e first. By appropriate cross bl e nding, one can,
within r e ason, achi e v e a wide variety of properti e s. Viscosities can be
pushed up or down at will. Tan deltas (a measure of th e relative
contribution of th e elastic and viscous moduli) can be varied
significantly. This area is not yet fully understood and is under
intensiv e study at the present time. Obviously, a clear understanding
of this phenomenon is needed for specification purposes, since many
asphalts are sold as mixtures from different crude oils.
3.3.2 Thermal and Fatigue Cracking. Cracking is another serious
failure in roadways, and again can be related to binder composition. If
th e mol e cular network (agglomerate, micell e , colloid, or what e v e r term
is pr e f e rr e d) becomes too rigid, th e ability of an asphalt to d e form
elastically will be lost. Instead, the asphalt fractur e s and likely
will b e s e parated to a point that healing cannot occur. Th e constant
working of very rigid matri x will ev e ntually suffer fatigue and crack.
Th e potential to crack is compounded by yet another organizational
feature. At lo w temperature, th e more neutral materials b e gin to
organize into a more structured form as can b e seen by diff e r e ntial
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scanning calorimetry (DSC). Now, th e asphalt is quite brittle and
subject to cracking under str e ss. To mak e matters worse, some cements
shrink significantly at low temperature as a result of the organization
of low polarity and nonpolar components. This aggravates cracking.
Again, it varies with composition from on e asphalt to another. All
other things being equal, th e more linear aliphatic materials show the
most pronounced tendency to shrink with decreasing temperature. Hence,
cracking at low t e mp e ratur e would b e expected if shrinkage occurs, and
is e x pect e d to b e most clos e ly related to the compositional feature of
ali p hatic / aro matic ratio when all other characteristics are equal.
Aliphatic / aro matic ratios can be det e rmined by NMR, but it is probably
more practical to either measure shrinkage directly or predict it from
very rapid DSC measurements.
3.3.3 Adhesion and Moisture Damaqe. Adhesion and moisture damage
go together only to a point. While loss of adhesion certainly is a
s e rious moisture damag e problem, other forms of moisture damage may also
occur. At this point, consider only adhesion. By definition, it must
involve both asphalt and aggregate. While SHRP has co mmissioned studies
to investigate th e int e raction of asphalts and aggr e gat e s, the
e x am ination of adhesion, p e r se, is not within the scope of the binder
composition studies. Nev e rth e less, som e interesting observations can be
made considering only th e binder.
Adhesion of components in asphalt to aggregate appears to be
governed as much at the molecular level as at th e inter-molecular level.
Sp e cific functional groups (molecular types) seem to be very important.
Ce rtainly overall polarity, that is, separation of charge within the
organic molecules, promotes attraction of polar asphalt components to
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the also polar surface of aggregate. Several workers within the
Strat e gic Highway Program have shown that aggregates vary quite
substantially, and in some cases aggregate behavior varies with
e nvironm e nt. Some aggregates hav e positive sites, some negative, and
some show variation in polarity with moisture content, temperature, etc.
For this discussion, aggregate will b e considered to be simply a highly
polar surfac e . Adhesion arises because of th e interaction of th e polars
in asphalt with th e polar surface of an aggregate. But polarity alone
in asphalt may not b e sufficient to achieve good adhesion in pavem e nt
b e caus e asphalt is affected by its environment. Asphalt has th e
capability of incorporating and transporting water. More on this in the
moisture d am ag e section. Absorption of water, like all other behavior,
vari e s with asphalt composition including changes in composition as a
result of oxidation. Incorporation of wat e r is m e asurabl e as are th e
e ffects of the invasion of water into th e asphalt aggr e gat e mixtures.
At the molecular level in asphalt it has been observed that basic
nitrogen compounds (pyridines) tend to adhere to aggregate surfaces
tenaciously. Carboxylic acid salts, while quite polar, tend to b e
r e mov e d from aggregate more easily, but this varies with the type of
salt. Monovalent cation salts, such as sodi u m or potassi um , of acids
tend to b e removed from aggregate quite easily. Calci u m or other
divalent salts of acids ar e much more resistant to the action of water.
From a practical viewpoint, it would b e hoove the user to assure th e
acids in asphalts are not in th e form of monovalent salts. The e xam pl e s
of pyridin e s and sodi u m salts are som e what th e extremes and it should b e
obvious that th e r e is a spectrum of tenacity of adhesion am ong th e
organic mol e cul e s in asphalt. Th e e valuation of this spectr um is being
don e els e wh e re within the progr am .
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Moisture damage without apparent loss of adhesion is another co mmon
problem in pavement. Certainly, highway design is a major factor in
reducing the availability of water, but to b e clear, water is
omnipresent in roadways, so the e ffects of moisture are unavoidable.
For this discussion, consider that design i s adequate to remove most
water a nd also to resist its invasion in to pavement. Still, some water
will come in contact with asphalt in pavement and will affect its
p e rformanc e when the water soaks into th e concrete. Water, like
aggregate, is a highly polar material and to some extent is transported
into th e asphalt by virtue of attraction of polar water molecules to
polar asphalt components. Upon invasion into the asphalt, water will
effect the mechanical properties, typically softening it. From a
chemical viewpoint, the action of water is somewhat like the dilution of
asphalt with a low molecular weight solvent. This typically results in
r e duc e d strength and further results in rutting or other deformation.
Aged or oxidized asphalts, which have greater amounts of polars
(oxidation products), tend to incorporate water to a greater extent than
n e w asphalts. This would be expected from polarity considerations. The
probability of moisture invasion, from a ch e mical viewpoint, increases
with pavement ag e . But aged pavements are also harder than their
counterpart n e w pavements, so the effects of moisture and oxidation ar e
somewhat counter to each other. The co mbined effects are better
measur e d in t erm s of the mechanical properties of the mi x. As usual,
th e behavior varies with composition.
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4. 0 SU _a 4 ARY
The chem is try o f pe tr o l eu m a s ph alt a t th e mol ec ular and
intermolecular levels has been discussed. It is clear that hundreds of
thousands of molecular species exist within any particular asphalt. The
polarity a mong asphalt molecules varies widely and the physical
properties are governed by the balance of polars and nonpolars
components. Polars tend to associate while less polar and nonpolars
cause dissociation. Several specul a tions on the effects of chemical
composition on pavement performance have been offered. In general, the
mechanical or structural properties of asphalt are related to the
intermolecular structuring among the polars. These interactions may
arise by involvement of any of numerous different chemical species. The
exact nature of the chemical specie is less important than the
distribution of charge within the specific molecule. Excessive
structuring leads to brittle cements which tend to crack, while too
little structuring leads to materials which deform under stress.
Oxidation adds to age hardening and brittleness of asphalt cement by
contributing to the structures zones within binder.
Finally, the word asphalt should be used in the sa me sense a s the
word glue. As much as there are significant differences a mong glues,
e.g., carpenters glue, airplane glue and rubber cement perform very
differently from each other, so are there tremendous differences among
asphalts. The differences among asphalts are as great as the diversity
of the crude oils used to manuf a cture them. The objective of developing
new specifications to describe and achieve consistent behavior, and
therefore consistent performance, is of high priority from the user's
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cost standpoint. But th e user must also realize that the cost-effective
supply line (crud e oil) consists of a wide variety of mat e rials, and
there is little chance of significantly limiting the source of supply.
That is, th e s e l e ction of a very limited s e t of crude oils to
manufacture asphalts is im possibl e . It is therefore obvious that
achievement of consistent performanc e with asphalts frequently will
require modification of materials that are produced today. A principal
valu e of th e composition studies is to develop an understanding of what
compositional features ar e needed to produce ma terial with the desired
properti e s. Then this information can be used to select and / or modify
asphalts to obtain binders that will p e rform in a cost-effective manner.
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Aspha lt Adviso ry Comm ittee George WestShell Oil Co mpany
Thomas D. Moreland, chairmanMoreland Altobelli Associa tes, lnc . Liai sons
Gale C. Page, vice chai rman Avery D. AdcockFlorida Department of Transportation United States Air Force
Peter A. Bellin Ted FerragutNieder sachsi sches Landesa mt Federal H ighway Ad ministration
fiir Strass7enbauDonald G. Fohs
Dale Decker Federal Highway Administ ration
National As phalt Pa ving AssociationFredrick D. Hejl
Jose ph L. Goodrich Transpor ta tion Research BoardChevron Research Company
Aston McLaughlin
Eric Harm Federal Aviation Ad minis tration
Illinois De partment of Tra nsportationBill Weseman
Charles Hug hes Federal Highw ay Adminis tration
Virgin ia H ighway & Transpor tation Research Council
R obert G . J enkins Exp ert Task GroupUniversity of Cincinnati
Ernest Bas tian, Jr.Anthony J. Kriech Federal Highw ay Administr ati onHe ritage Group Co mpany
Wayne BruleRichard Langlois New York State Depar tment of Tra nsportationUn ivers ite La rval
Joseph L. G oodrichRichard C. Meininger Chevron Res earch Co mpanyNational Aggregates Association
Woody Hals te adNicholas Nahas ConsultantEXXON Chemical Co .
Gayle KingCharles F. Por ts Bitu minous Materials Co ., lnc .APAC, lnc .
Robert F. LaForce
Ron Re ese Colorado Department of TransportationCalifornia Department of Tra nsportation
Mark Plummer
Donald E. Shaw Marathon Oil CompanyGeorgia -Pac ific Corporation
Raymond PavlovichScott Shuler Exxon Che mical Co mpa nyThe Asp halt Institute
Ron Reese
Harold E. Smith Californ ia Depar tment of TransportationCi ty of Des Moines
Scott Shuler
Thomas J. Snyder Colorado Paving AssociationMarathon Oil Company
Richard H. Sullivan
Minnesota Department of Transportation
A. Hal eem Tahir
American Association of State Highw ay andTransportation Officials