FINITE ELEMENT ANALYSIS OF
ARIZONA'S THREE-LAYER OVERLAY SYSTEM OF RIGID PAVEMENTS
TO PREVENT REFLECTIVE CRACKING
BY
NAN JIM CHEN, P.E., Ph.D.
JOSEPH A. DI VITO, P.E.
GENE R. MORRIS, P.E.
OF
ARIZONA TRANSPORTATION RESEARCH CENTER
ARIZONA DEPARTMENT OF TRANSPORTATION
FOR PRESENTATION AT THE 1982 ANNUAL MEETING OF
ASSOCIATION OF ASPHALT PAVING TECHNOLOGISTS
FEBRUARY, 1982
ABSTRACT
Asphalt-rubber has been used as a Stress Absorbing Membrane Interlayer
(SAMI) for overlay of flexible and rigid pavement for more than ten years in
Arizona to retard reflective cracking with satisfactory performance.
This paper discusses several field experiments of asphalt-rubber
overlays on rigid pavement with a three-layer system which consists of an
asphalt concrete leveling course, a SAMI and a wearing course. This
three-layer overlay system has provided a very smooth ride on an interstate
highway for two years without a single visible reflective crack. The
Average Daily Traffic (ADT) is approximately 100,000 with nine percent
commercial vehicles.
Analytical studies utilizing the Finite Element Method (FEM) indicate this
3 layer system is far superior to simple overlays without interlayers.
St~esses in an overlay above an existing joint are reduced signficantly (up
to 95% or more) due to the presence of a SAMI, and overlay life is extended.
As a result of the analysis, the three-layer system is recommended for
overlay of a rigid pavement, especially when the rideability of the existing
pavement needs to be improved. The leveling couse not only improves rideability
of the final overlay, but also the performance of SAMI.
I NTRODUCTI ON
Reflection cracking has been defined as the cracking of a resurface
or overlay pavement above underlying cracks or joints. Reflection cracks
may be induced by environmental conditions or traffic loads or a combina
tion of the two. It is a worldwide problem occurring in virtually all
types of pavements unless steps are taken to prevent it. The problem is
especially critical in overlays of rigid pavements and this paper is
directed towards that problem.
Several treatments have been used to reduce reflective cracking in
asphalt concrete overlays with varying degrees of success. Generally
they fall into one of the following categories:
1. Treatments to existing concrete pavements (sucn as breaking into
small sections) prior to any rehabilitative work,
2. Stress or strain relieving interlayers,
3. Cushion courses or special overlay treatments,
4. Increased overlay thickness.
Arizona's three-layer asphalt-rubber overlay system has been used on
several experimental projects where Portland Cement Concrete Pavements
(PCCP) were overlaid. The asphalt-rubber materials have been discussed tho
roughly in other reports (1 through 9) and will not be discussed in this paper.
The 3-layer system incorporates a thin flexible overlay placed in two
layers with a low modulus asphalt-rubber interlayer between. It consists
of an open-graded mix 1/2" to 3/4" thick placed as a leveling course
directly on the cracked, rigid pavement followed by an asphalt-rubber
flush-in with cover material. The final course is another open-graded
mix 1/2" to 3/4" thick. The result is a thin, flexible overlay that
resists reflective cracking for a much longer time than conventional
overlays. Figure 1 is a simplified cross-section of this system.
-1-
ASPHALT - RUBBER'FLtjSHi!lN i"\lO.rvu'(1 !'--No;;..
BLOTTED WITH COVER MATERIAL
!~'ACFC OR LEVELING COURSE o ~ (:;.1>..\ G. fl I'. () I::: u \l <'l-.;:: - r.t l "-
PORTLAND CEMENT CONCRETE PAVEMENT
BASE MATERIAL
FII'URE 1/.1 TYPICA(rnoss S~G1'iON OF A..RH\lNA'S/TtI~E-L.A.Y,f;R_ V/ \.AsPHALT-~EIt'OVml,AY....sySTErl~ ~ .
-2-
As with most of the previously mentioned treatments, trial and error
applications preceded mathematical modeling. The same is true for asphalt
rubber and the need currently exists to prepare analytical studies to
verify field performance. The intent of this research was to establish
an appropriate mathematical model to verify empirical observations. The
analytical study consisted of a finite element analysis of stresses and
strains with different layer configurations and is explained in detail
in Part II. Part I will address the design and performance of the various
experimental projects that received this specialized overlay treatment.
PART I - PROJECT PERFORMANCE
All pavements overlaid with the three-layer system were plain,
undowe1ed pavements exhibiting varying degrees of structural cracking
and load failure. Three of the projects are in the Flagstaff area at
an approximate elevation of 7000 feet, while the three other projects
are in the Phoenix valley area. Comparison of the two climate extremes
has been informative. Flagstaff winter temperatures have been as low as
-30oF while Phoenix summer temperatures can be as high as +1150 F.
Table 1 provides a description of each of the projects and the dates
overlaid with the three-layer system. A detailed description of each
project follows.
1. 1-17 at Bell Road - This section of Interstate 17 in north
Phoenix was not originally scheduled to receive the three-layer
overlay system. In June, 1974, an open-graded Asphalt Concrete
Friction Course (ACFC) was placed and asphalt-rubber was flushed
into the mat with cover material applied. The treatment served
very well until another adjacent resurfacing project in 1980
called for an ACFC to be placed over this experimental section,
-3-
TABLE 1. SUMMARY OF FIELD PROJECTS WITH THE THREE-LAYER ASPHALT-RUBBER OVERLAY SYSTEM
Three-Layer Project System Placement Length and Width Traffic ADT and %
Date Commercial Vehicles
l. Southbound 1-17 at Bell Road Summer, 1974 1500 feet 38' wide 50,000+, 9%
(Phoenix area)
2. Eastbound and Westbound 1-40 Summer, 1974 4 miles 38' wide 6,000, 20%
Riordan to 1-17 (Flagstaff area)
3. Westbound 1-40 at Butler T.1. Summer, 1974 3000 feet 38' wide 7,500,28% , ..,. (Flagstaff area) ,
4. Mesa Country Club Drive Fall, 1976 4000 feet 68' wide 25,000, 5%
S.R. 87 (Phoenix area)
5. Westbound 1-40 at East Fall, 1977 1000 feet 38' wide 7,500, 28%
Flagstaff T.1. (Flagstaff area)
6. Southbound 1-17 at Durango Fall, 1979 1500 feet 48' wide 95,000+, 9%
Curve (Phoenix area)
making it a three-layer overlay. So far, it has not exhibited any
reflective cracks. Before 1980, the original treatment controlled
90% of the reflection of all joints and cracks.
2. 1-40, Riordan to 1-17 - A three-layer system was placed on this
four-mile divided interstate highway project in 1974 in an
attempt to control severe structural failure related to frost
heave and thaw weakening. Cracking and joint faulting had
progressed to the point where the highway had to be inspected
daily for broken chunks of concrete. Structural failure (cracking)
had occurred in a very large percentage of the travel lane. A
typical view of cracking and skewed joints prior to overlay is
shown in Figure 2. In addition, severe faulting was evident
throughout the section. Prior to overlay, the integrity of the
structural section was restored with sub-slab grouting. Struc
tural cracks were grouted and sealed with asphalt-rubber as was
the longitudinal edge joint. The PCCP was originally built 9"
thick, unreinforced, with centerline tie bars. Shoulders were
constructed of asphalt concrete. Since the three-layer layer has
been in place, approximately 25% of the travel lane transverse
joints in the PCCP have reflected through as cracks in the overlay.
The cracks that have reflected are narrow, have not spal1ed and
have not required any major maintenance. There is little
reflective cracking in the passing lane. Approximately 10%
of the longitudinal edge joint has cracked through the overlay.
However, neither the tied centerline joint nor the structural
cracks have appeared on the surface. All surface cracks have
remained narrow and have not spal1ed, very likely due to the
-5-
TYPICAL VIEW OF CRACKING AND SKEWED JOINTS PRIOR TO OVERLAY
SHOULDER
131 lSi 111 12'
PASSIN~ LANE - - - -
12' TRAVEL LANE , ~ -r- ... ..
} ..............
1
10' SHOULDER
FIGURE 2. TYPICAL VWI OF CRACKING AND SKEl-IED JOINTS PRIOR TO OVERLI\Y
-6-
1.0
extended flexibility of the asphalt-rubber at cold temperatures.
While not completely controlling joints and cracks, the three
layer system has performed well in rehabilitating this section of
highway that really should have been reconstructed.
3. Butler Traffic Interchange, 1-40 - Except for sub-slab grouting,
this section was exactly like the Riordan experiment and was
placed at the same time, summer 1974. Performance has also been
similar to the Riordan project indicating that magnitude of
vertical deflection may not be critical.
4. Mesa Underpass on S.R. 87 - This experiment was originally
established to test different application rates of the asphalt
rubber flush-in on the three-layer overlay system. It was
constructed in the fall 1976. The lower ACFC layer was placed
using two different gradations to test the effect of voids in
the mat versus reflective crack prevention. After close
observation for four years, the entire project had no reflective
cracks. Since no changes were apparent in traffic patterns or
loading, or any other factor, it is believed that asphalt
rubber stiffness properties may have changed or the life of
fatigue crack propagation has been reached. Some surface
instability was observed in the areas where 0.8 to 1.2 gallons
per-square-yard (gpsyd) of asphalt-rubber was placed. Applica
tion rate was originally specified as 0.4, 0.6 and 0.8 but some
areas received as much as 1.2 gallons per-square-yard. At date
of this report, the surface has approximately 10% reflective
cracking. Again, these cracks are hairline and have required no
maintenance. Application rate (in the range of 0.4 to 0.8 gpsyd)
-7-
seems to have no effect on reflective cracking prevention nor
did the variable gradation of the first-layer. Figures 3 and
4 were typical pavement conditions before overlay and five years
after overlay.
5. East Flagstaff, 1-40 - The three-layer overlay system was
placed as one of six different overlay test sections and is the
only one of the six to perform well. All others have already
been dismissed as unacceptable. The three-layer system has
prevented 90% of all cracks and joints from reflecting through
to the surface. It was placed in late summer, 1977 and to date,
has shown very good results.
6. Durango Curve on 1-17 - The overlay of this urban Phoenix
freeway section of rigid pavement took place in November, 1979.
The three-layer system was to be compared with the very expensive
alternative of grinding and grooving to restore ride and skid
number. At the time of design, it was estimated to be 1/3 the
cost of grinding. In addition, it could be constructed in a
much shorter time period. After two years of service there are
no reflective cracks of joints whatsoever. (An 18" transverse
hairline crack was observed during January, 1982 inspection.)
The only surface defects noticeable are two small areas (less
than three square yards each) exhibiting rutting and shoving.
They are the result of inadvertent excessive applications of
asphalt-rubber during construction, a problem not attributed to
design nor one that should occur during normal project construc
tion. Figures 5 and 6 were pavement conditions before overlay
and two years after overlay.
-8-
FIGURE 3. MESA UNDERPASS ON S.R. 87 BEFORE OVERLAY IN 1976
rIGURE 4. MESA UNDERPASS ON S.R. 87 FIVE YEARS AFTER OVERLAY (JANUARY, 1982)
-9-
FIGURE 5. DURANGO CURVE ON 1-17 BEFORE OVERLAY IN 1979
FIGURE 5. DURANGO CURVE ON 1-17 TWO YEARS AFTER OVERLAY (JANUARY, 1982)
-10-
The three projects in the Flagstaff vicinity have all demonstrated
similar performance. Some reflective cracking is evident but it is essen
tially confined to the transverse joints in the travel lane. This indicates
reflective cracking is related to the combination of thermal and load
associated stresses. The fact that structural cracks did not reflect
through demonstrates loading alone is insufficient to cause reflective
cracking of the three-layer overlay system.
Also, the passing lane of the three projects is virtually crack
free which indicates thermal stresses alone are insufficient to cause
cracking. The tied centerline joint has not cracked through, further
supporting these conclusions.
The Phoenix area experiments demonstrate the three-layer overlay
system will prevent reflective cracking of joints and cracks for up to
four years in mild climatic environments. After this length of
service, approximately 10% cracking is apparent. The Durango curve
project has proven the system is very stable under the extreme conditions
of high traffic volume with heavy loadings combined with extreme high
summer temperatures.
In all cases, the system has maintained good surface friction
qualities (high skid numbers) and good ride quality. The system is far
less expensive than grinding to smooth PCCP and much less disruptive
to traffic during construction.
-11-
PART II - ANALYTICAL STUDIES
To explore the basic reason why asphalt-rubber prevents reflective
cracking of rigid pavement and to determine the structural adequacy of
the system, an analysis was conducted of the theoretical behavior of the
overlay structure. Several Finite Element Method (FEM) computer programs
were utilized for the analysis of stresses and strains with different
layer configurations. The primary computer program used for this study
was a slightly modified static analysis program for solid structures -
SOLID SAP by Wilson (10). The slight modification of this program was
in the calculation of the effective stresses (11), which is essentially
the "root mean square" shear stresses, and defined by using the normal
and shear stresses in an orthogonal Cartesian coordinate system as
They were considered to be a realistic determinant for fracture (cracking)
under the triaxial stress state existing in the overlay pavement. In
order to reduce the cost of computer time, a linear elastic plane strain
analysis was assumed 11ith 685 nodes and 620 elements. Among the field
experiments, there are several overlay configurations with different
existing conditions. The finite element analysis 11as simplified by
studying a nine-inch concrete pavement (PCep) with a O.3-inch jOint
overlayed by different thicknesses of asphalt concrete and a SAMI. The
general configuration of the pavement overlay system is shown in Figure?
Seven different overlay designs were analyzed ranging from a very thin
(1 5/8") three-layer design to a seven-inch overlay. Information on input
thickness and layer arrangement is given in Table 2. Mechanical properties
-12-
I ..... W I
TL
15CM 15CM +TL iL SURFACE (S-) (611
)
COURSE 0.69 MPa (100 PSI) 'V'v
MOVING TRAFFIC
\ ~ /SAMI
'AI'
~LEVELING COURSE ...
• ~O. 76 CM JOINT 22.8 CM PCeP-"",
(0.3 11 ) (9111
)
"''/~'' ... ~~, ''<1/ SUSGRAOE
lOADING HI G1 FI EI 0 1 C' SI AI CASE A S C o E F G H I
TL (CM) 40 35 30 25 20 15 10 5 o -5 -10 -15 -20 -2.5"30 -35-40
UN) 16 14 '2 10 8 6 4 2 o .. 2 .. 4 ·6 -8 -to -12 -14 -16
FIGURE 7. GENERAL CONFIGURATION OF THE OVERLAY SYSTEr~ ON RIGID PAVEMENT
I I-' ... I
OVERLAY DESIGN THICKNESS
CASE I
CASE 2
CASE :3
CASE 4
CASE 5
CASE 6
CASE 7
SURFACE SAMI _EVE LING COURSE COURSE CM (IN.) CM UN.) CM (IN.)
I
0.95 :0.315 ,
1.59 10.625 1.59 '0.625 I I , I I 3.18 11.25 0.95 ,0.315 0.0 K>.O
I
I I I
4.13 ~ 1.625 0.0 10.0 0.0 10.0 ,
I I i 9.21 0.95 '0.315 0.0 13 .625 1°.0 I
I I
:0.0 10.16 :4.0 0.0 10.0 0.0 I
I I I 16.83 16.625 0.95 ,0.315 0.0 ,0.0
I
11.18 :1.0 0.0 I I 10.0 0.0 10.0
I I I
TABLE 2. ARRANGEr'IENT OF OVERLAY THI CK~,ESS
TOTAL
CM ( IN.)
I
4.13 11.625 I
I 4.13 ,1.625
I
I i
4.13 11.625 I
I
10.16 J4.0
10.16 14.0
I 11.18 11.0
I 11.18 11.0
I
of different pavement layers used for analysis are listed in Table 3.
From the results of very limited tensile and shear tests on viscoelastic
properties of asphalt-rubber, the ranges of creep compliance values can
be roughly determined as shown in Figure 8. The elastic moduli of SAM!
were selected accordingly for finite element analysis. For the study of
moving traffic, 2000 psi was selected and 10 psi chosen for the thermal
study.
The thermal effect on an overlay was studied by using two different
temperature profiles from the surface of the overlay down to the subgrade,
with the higher one used as a stress free temperature. A profile of
temperature differences is given in Figure 9. A 50-degree (oF) tempera
ture change on the surface of the overlay was used for analysis.
Moving traffic on the overlay was represented by a 12" long, 100 psi
load for the FEM computer analysis. The location of this loading was
moved every two inches from one side of the joint to the other (see
Figure 7) and influence lines of stresses above the existing joint were
plotted and evaluated.
The stress~s which were investigated in this study are located appro
ximately 0.09" right above the existing joint (center of the lowest layer
of mesh within overlay used for finite element analysis) and those stresses
are not the average stresses in different layers. A higher stress indi
cates that reflective cracking may start to propagate sooner than that
with a lower stress.
The major findings of this study are as follows:
(a) Three-layer overlay system - The three-layer overlay analyzed,
consisted of a thin (5/8") asphalt concrete leveling course, a
3/8" SAM! and a thin (5/8") asphalt concrete surface wearing
-15-
..... en ,
ELASTIC SHEAR POISSOO MODULUS MODULUS RATIO
E G J.I M Po KSI MPa KSI
I 1 SURFACE AND
2070:300 828 1120 0.3 LEVELING COURSE I I
SAMI i 1
(TRAFFIC) 13.8 12 5.52 10.8 0.35 I i
i I
SAMI 0.027610.004 0.06910.01 0.35
(THERMAL) I I I
I I
11040 :1600 PCCP 27SOO14OOO 0.2 I I
I I
1 SUBGRADE 69 110 27.6 14 0.48
I I
TABLE 3. ELASTIC PROPERTIES OF OVERLAY cmiPON~;m FOR ANALYSIS
THERMAL COEF.
a of
0.0000125
0.0000150
0.0000150
0.0000039 I
0.0000100
10 0 .....
• 0 .....
~ rI)
0 "" ·0 L'.t ...... co
4 co :::>
"" ~
I-
~ -' ~
~ 0 a..
~ V1 ...... -<
~ 0 C": C>
't w u..
en w .... L>
~ 0 Z < - ~ -- -' "(' a..
~ ~O W C5 u
~ 0 a..
:> ~ '" 0 LeI
~ 0:: ..... U ......
...... LL
't 0
{;J
~ '"' TO
z < 0::
"(' ...... ~
CO
w
~ "" OJ :::>
'"' ·0 ~
LL .....
--.... o o .... ~ If) "'" o '0 '0 ·0 '0 _ ..... _- ..... --
-17-
CHANGE OF TEMPERATURE (OF) 10 20 30 40 50
-:2 0 --• z ..... - ..... (.)
10 ~ 0:: :;) fI)
~ 0
20 ...J ..... II)
..... 0 z
..... i:!
30~ c c
15
40
FIGURE 9. THlPERATURE PROFILE USED FOR THERtlAL ANALYSIS
-18-
course. The major advantage of this system over a 2-layer (SAM!
and AC) system is that the leveling course will provide a very
smooth surface for the SAM! to behave as a stress absorbing
layer. Without this leveling course, very often, the asphalt
rubber will flow into joints or large cracks and a continuous
stress absorbing layer can not be achieved. Results of the analy
tical study indicate stresses in the leveling course above the
joint are very high but due to the existence of the SAM!,
stresses in the surface course are very low. See Figures 10
and 11. Both shear stress (S12) and effective stress (Seff) are
at a maximum when the simulated traffic loading is completely
on one side of the concrete slab. The very low stress level in
the surface course indicates a very low stress intensity factor
which in turn will provide a longer service life for the overlay
versus reflective cracking.
(b) Overlay Thickness - Overlay thickness from 1 5/8" to 7" were
studied, with and without a SAM!. The typical influence lines of
shear and effective stresses within the overlay are indicated in
Figures 12 and 13. The stresses in the overlay reduced signifi
cantly because of the existence of the SAM!. For an overlay
without a SAM!, stresses reduce significantly due to increasing
thickness (refer to Figures 14 and 15). However, for an overlay
with a SAM!, stresses were only slightly reduced due to the
increasing overlay thickness (Figure 16). This indicates, from
an economical point of view, a thick overlay may not be justified
for a reflective overlay design when a SAM! is utilized.
-19-
MPQ PSI 800
5
4 600
0.. 400 ..... ..... ~ 2 () « 200 0: I ()
I.IJ > 0 0 m « (/) (/) IJJ 0: t; 0: -« -3 I.IJ :J: (/) -4
-600 •
~ 2 -5
-800
LEVELING COURSE (5/8 11
)
SURFACE COURSE (51 II)
HI G F' E' 0' C' Sl do
f-!A~SrlC ~O~E ~F~G~H~~ TRAFFIC LOADI NG CASE
FIGURE 10. INFLUE~CE LINES OF SHEAR STRESS ABOVE THE CRACK TIP DUE TO f10VING LOAD (THREE-LAYER SYSTEt1)
-20-
I
'" ..... I
MP01
PSI !!: 1800 ...... 12
~ ~ IISOO o
IAJ 10 ~
= 1400
fn m ~ 811200
iii ~ 1000 --...... o ~
LEVELING COURSE {5/8 111
}
.h 0.-.... SURFACE COURSE
SAMI (5/8 11
,
~ ~ .6::V--I&.. LuJ
• O' ~ • i •
X
~ A S C D E F G H I HI G' P E' DD C' SI A'
TRAFFIC LOADING CASE FIGURE 11. INFLUENCE LINES OF EFFECTIVE STRESS ABOVE THE CRACK TIP DUE TO MOVING LOAD
(THREE-LAYER SYSTEM)
I N N I
0.. .... .... t§ c 3 L&.I >-0 m C en en L&.I a:: .... en a:: c L&.I l: en
MPOII PSI 4 600
3\ 400
2l I I 2001
I 5/881 OVERLAY
I I WITH SAMI (IN AC)
/ WITij SAMHIN SAMI)'
o~ I ~ 0==8:$ Q ~, H' G' F' E' 0' c i S' AD 0
I A S C 0 E F G
-'r ~ TRAFFIC LOADING -200 CASE
1-400 -3
i -4r-soo
FIGURE 12. TYPICAL INFLUE~ICE LINES OF SH~AR STR~SS ABOV': HIE C~,1\CK TIP DUE TO nOVI!~G LOAD
f1. MAli PSI I 1= I SIS'" OVERLAY ~ 9 (,)
~ 11200 (,) 8 1&1 > 1
11000 0 m 4 U) 6 U)
1800 1&1 1 0:: N 5 w l;; 1
2L'V· I ,WITH SAMI (IN SAMI) 1&1 > ....
1200l ~/ /WITH SAMI (IN AC) t5 1&1 I I&. I&. 1&1 ot 0
• ABCOEFGH I HI G' F' F OB C' fJ A' X 4
TRAffiC LOADING CASE :I FIGURE 13. TYPICAL I~lFLUErlCE LINES OF EFFECTIVE STRESS ABOVE THE CRACK TIP DUE TO :lOVING LOAD
, GO .;0-,
MPal PSI
4lsoo
Q" -D- 3. 400 ~ (.) c( 2 a:: (.) 200
hi I
~
I 5/S 11 OVERLAY
7- OVERLAY HI G' F' E' 01 C' BI AI m
ct 01 i , ". ",
m en _I w a:: 1-200 D-en -2 0: ~ W ~ m
•
~ :2
AB CDE FG
TRAFFIC LOADING CASES
H I
FIGJRE ;4. I\[FLUr:~CE LINES OF SHEIIR STRr::S5 iIROV':: THe S!I'\CK TIP D:':E TO noV!"[" [.DAO (WITHOUT SAI·n)
MPOI PSI
ft1. .1400
r:: 9 ~
11200 I I \ I 5/8111 OVERLAY
'0 ~ 8 « 0 hi :> 0
~ 6 U)
I 800 m I
~ N U"I Q:': 5 I
t; kI 41 600 :> ..." ..... 0 liJ iI.. I!.L 1&1
• .., .., A 8 C 0 E F G H H' G' F' E' 0' C' 8' tJ: X I
c( TRAFFIC LOADING CASE :2
FIGURE 15. INFLUENCE LINES OF EFFECTIVE STRESS ABOVE THE CRACK TIP D~~ TO nOVIrlG LOAD (WITHOUT SA;,I)
MPo P51 1400
9 5EFF
8 1200
Q" ..... .... 1 1000 ~ 0 6 <r 800 It: 0 5 5 12 lIJ 4 ~
3 m 400 <r en 2 y;ITH SAM I 1&1 200 5EFF :-en en I ~ lIJ 5'2 It: 0 t; 0 I 2 3 4 5 6 1 (IN)
• • • • • X 5 10 15 (eM) <r ::I OVERLAY TH ICKNESS
FIGURE 16. EFFECTS OF SAt1l ON SHEAR liND EFFECTIVE STRESSES
-26~
(c) Thermal Effects - The elastic moduli of asphal t concrete (AC)
and asphalt concrete friction course (ACFC) for thermal study
were as'sumed to be the same as that for traffic study, i. e. ,
300 ksi, this value may be too high for slow loading such as
thermal changes. Unfortunately, no effect was being made to
study the visco-elastic behavior of AC and ACFC. Results of
this study indicate the horizontal tensile stress near an
existing joint is reduced significantly with the low modulus
interlayer, especially for thin overlays. The SAMI does not
reduce the overall horizontal stress in an asphalt concrete
(AC) or asphalt concrete friction course (ACFC) due to thermal
expansion or contraction. However, it will significantly reduce
stress concentration above the existing joint, minimizing
reflection cracking. For an overlay without a SAMI, horizontal
tensile stress reduces significantly due to increasing thickness
as shown in Figure 17. For an overlay with a SAM!, horizontal
tensile stress is only slightly reduced due to the increasing
overlay thickness. !n a three-layer overlay system, horizontal
tensile stresses are very high in the leveling course and may
cause cracks. Stresses in the surface course are very uniform
and the influence of existing cracks can hardly be detected.
This indicates a SAM! can effectively retard or eliminate
reflection cracking due to thermal expansion or contraction of
a jointed rigid pavement.
(d) Effects of Movi ng Tra ffi c - Throughout the ana.lytica 1 study,
it was assumed there was no load transfer capability across the
joint. This condition would exist when vertical differential
-27-
-~M :2 0: W ::r: ..... -~ ..J 0: W > 0 Z .... en en W 0: ..... UJ
laJ ..J ..... UJ Z laJ .....
• N .... 0: 0 ::r:
• X « :2
PSI
3000
15 2000
10
1000
5
500
250
0 0 0 •
I 3 4 5 6 7 (IN ) • 5
OVERLAY
, ,
10 15 (eM)
THICKNESS
FIGURE 17. EFFECTS OF SAm ON THERHAL HORIZONTAL TENSILE STRESS
-28-
movement of a joint is relatively small under loading. Sufficient
vertical movement can occur that aggregate interlock and dowels
may not have an effect in reduction of shear stress. This
study has revealed that both shear and effective stresses are
at a maximum when traffic loading is completely on one side of
the concrete slab. Results also indicate that a SAMI can reduce
horizontal thermal stress, effective stress and shear stress
in the overlay which many researchers believe to be the major
factors causing reflective cracking.
SUMMARY AND CONCLUSIONS
Based on the field performance of several projects and analytical
studies, the following conclusions can be made:
1. The three-layer system can be used for reducing reflective
cracking when placed over a rigid pavement. The leveling course
provides a smooth surface for a SAMI to behave as a continuous
stress absorbing layer. The stresses in the surface course are
extremely low and insure a much longer service life.
2. When a SAMI is used in the three-layer overlay system, asphalt
rubber application rate is important because excessive rates
promote instability of the system. A specified rate of appro
ximately 0.60 gallon-per-square-yard yields good results.
3. Field performance indicates that load stress alone or thermal
stress alone are insufficient to cauSe major reflective
cracking. The combination of these stresses in cold climatic
regions may be sufficient to cause reflective cracks.
4. Field performance indicates that those cracks that do reflect
through are narrow, do not spall and require little if any
maintenance.
-29-
5. A three-dimensional finite element analysis with improved time
and temperature dependent material properties will provide
better results.
10 to 20 times.
However, computer time required may increase
The primary purpose of this study is to report
a successful application of asphalt rubber as a SAM! in rigid
pavement overlay and attempt to provide analytical explanations
for the apparent success of this new approach in pavement
rehabilitation.
6. "Shearing" action is more inducive to reflective cracking of
overlays than "Bending" action when a traffic loading is moving
from one side of a joint to the other.
7. SAM!s can not only reduce stresses caused by thermal changes
but also vertical shear stresses due to moving traffic loading.
8. It is intuitively obvious that reduced stress level will result
in increased service life of overlay. The prediction of service
life with or without SAM! could be accomplished by analyzing
the fatigue behavior of the systems.
9. When a SAM! is utilized, the thickness of overlay becomes less
critical. This may result in very economical approaches to
overlay design.
10. A better understanding of SAM! properties is needed through
continued laboratory testing.
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REFERENCES
1. C. H. McDonald, "Asphalt-Rubber Compounds and Their Applications for
Pavement" 21 st Ca 1 i forni a Streets and Hi ghway Conference, 1969.
2. G. R. Morris and C. H. McDonald, "Asphalt-Rubber Stress-Absorbing
r~embranes: Field Performance and State of the Art" TRB, Transpor
tation Research Record 595, 1976, pp. 52-58.
3. G. Cooper and G. R. Morris, "Reclaimed Rubber in Seal Coats and
Resea rch in Rubber/ Aspha lt Products" Proceedi ngs of Ari zona Confer
ence on Roads and Streets, Tucson, University of Arizona, 1974.
4. B. A. Vallerga, G. R. Morris, J. E. Huffman and B. J. Huff "Applica
bility of Asphalt-Rubber r~embranes in Reducing Reflection Cracking"
Symposium - Prevention and Control of Reflective Cracking, Proceedings
AAPT, Volume 49, 1980, pp. 330-353.
5. G. B. Way, "Prevention of Reflective Cracking in Arizona" presented
at 59th Annual Meeting, Transportation Research Board, 19BO.
6. E. L. Green, "The Chemical and Physical Properties of Asphalt
Rubber Mixtures" Arizona Department of Transportation Report
ADOT-RS-14(162), July 1977.
7. R. A. Jimenez, "Testing r1ethod for Asphalt-Rubber" Arizona Department
of Transportation Report ADOT-RS-15(164), January 1978.
B. R. A. Jimenez, "Testing of Asphalt-Rubber and Aggregate Mixtures"
Arizona Department of Transportation Report FHWA/AZ-79/lll, 1979.
9. R. A. Jimenez, "Testing Asphalt-Rubber with the Schweyer Rheometer"
Report on Grant Number ENG-7B-10395, National Science Foundation, 1980.
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10. E. L. Wilson, Solid SAP - A Static Analysis Program for Three
Dimensional Solid Structures, Structural Engineering Laboratory
U. C. SESM 71-19. University of California, Berkeley, September,
1971, revised December, 1972.
11. N. F. Coetzee and D. L. Monismith, "Analytical Study of Minimization
of Reflection Cracking in Asphalt Concrete Overlays by Use of a
Rubber-Asphalt Interlayer" TRB, Transportation Research Record 700,
1979, pp. 100-108.
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