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Use of Rubber Aggregate in a Strain Relieving
Interlayer For Arresting Reflection Cracks in Pavements
by
Bob M. GallaVlay (1) and B. D. LaGrone (2)
A paper prepared for presentation at
International Symposium on
"The Use of Rubber in Asphalt Pavements"
10-12 Nay 1971
Salt Lake City, Utah
(1) Professor of Civil Engineering, Texas A & H University
(2) Technical Director, U. S. Rubber Reclaiming Co., Vicksburg" Miss.
" Gallaway & LaGrone 2
ABSTRACT
One of the most troublesome problems encountered in
design and maintenance of asphalt concrete pavements is that
of reflection cracking. Reflection cracks are generally
the result of some type of foundation movement 'or shrinkage
problem and can reduce the effectiveness of the pavement by
loss of .?truct~}'~_l strength of the total pavement structure,
by allowing intrusion of water into the pavement and down
into the supporting structure, and/or by reducing the long
lasting smooth-riding quality of the surface.
It has been suggested that a Strain Relieving Interlayer
(SRI) utilizing vulcaDized rubber aggregate might be used as
a crack arrestor in highway design to alleviate mechanical
distress in the surface course due to foundation movement
and for thermal distress due to the mismatch of thermal
expansion of the Qase ~~d surface courses.
An SRI formulation containing approximately equal parts
by volume of vulcanized rubber aggregate, mineral filler, and
residual asphalt emulsion has been found to yield a water
proof material with high elongation and 10\-1 air voids that
can be placed in a thin-layered membrane.
The SRI formulation has been tested in a laboratory
analog of a layered high':lay system. It has been determined
that the allowable foundation movement before cracks reflected
Go.llaway 6; LaGrone 3
to the surface course would be 300% greater for a pavement
system utilizing a 1/8 inch layer of the SRI compOSition and
4401~ greater for a 1/4 inch layer of the SRI composition.
Strain relieving interlayers of more than 1/2 inch are not
recommended due to possible stability problems.
A procedure for field installation of the strain
Relieving Interlayer has been developed and a test section
installed, Further development of the SRI concept is . .
underway. Performance of the field test at this stage
indicates that a method of alleviating reflection cracks in
both new construction and asphaltic concrete overlays is
now available.
'J'iidespread use of rubber aggregate in Strain Relieving
Interlayers would not only result in an improved highway
but would also provide a functional means of disposing of the
nillions of discarded tires which are now creating a very
sericus solid waste disposal problem in all sections of this
country.
Gallaway & LaGrone 4
The primary problem areas in highway design are founda
tion movements, thermal distress, ~ormal wheel loads, traction
loads (braking), chemical degradation of binder (asphalt
aging), and structural fatigue. The most important consid
eration in a given situation depends upon a nUBber of factors,
however foundation movement is often a most troublesome
problem, since the least control can be exerted to influence
foundation behavior.
,In the many years of field testing various additives for
improving performance of asphalt concrete pavements, one of
the most common problems encountered with the test pavements
has been reflection cracking.
These reflective cracks \vhich are ge:J.erally the result
'of some type of foundation movement or volume change mechanism
can reduce the overall effectiveness of the pavement (or
overlay). The prevention of this cracking would accomplish
three goals: (1) retain structural strength of the total
pavement structure, (2) prevent intrusion of "rater into the
pavement structure and on into the supporting structure, and
(3) provide a long-lasting smooth riding surface.
Numerous methods and procedures have been used to construct
bituminous pavement surfaces in an effort to minimize reflec
tion cracking in the surface course and thus provide a longer
period of serviceability for the new total pavement structure._
The search for a practical procedure to alleviate reflection
'.
Gallaway &; LaGrone
i !
cracking is still going on.
5
The commonly used practice of making supporting structures
more rigid, such as the use of unreinforced portland cement
stabilized bases, has proven to be ineffective in the presence
of severe foundation movements. The net effect of this so-
called stabilization process is to concentrate large base
cracks at four to six foot intervals compared to more and
smaller cracks without this type stabilization. In many cases
the cracks ultimately reflect through the surface course in
either case.
Severe foundation problems have been well documented and
are not limited to any particular ge,')pgraphical area.
Fi6 ure 1 shows severe cracking of an asphaltic concrete
overlay caused by movement'of the underlying pavement, or lack
of bridging over underlying cracks or joints.
Figure 2 shows a similar type of condition encountered
where the foundation movement was a result of swelling and
contracting of the base soil ',d th changes in moisture level.
Because rubber is a very tough elastic material it appears
to have a logical place in alleviating cracking problems in
asphaltic concrete surfaces and various forms of rubber have
been used with some degree of success for this purpose.
McDonald (1) recently reported the utilization of a
rubberized asphalt cement for chip seal applications in airport
maintenance and has observed virtually no cracking of the
surface.
Gallaway & LaGrone 6
i Natural rubber, latex, synthetic rubber of various types,
reclaimed rubber and ground vulcanized rubber have all been
used in asphalt concrete test sections with varying degrees
of success. It is however very difficult to design a
bituminous mix utilizing enough rubber to impart the desired
strain properties to the pavement and still maintain adequate
stabili ty.
For many years ground vulcanized rubber has been utilized
in resilient play surfaces such as running tracks, event
approaches and tennis courts without encountering reflection
cracking problems in any of these surfaces(2). U.S. Rubber
Reclaiming Co. was therefore very receptive to a proposal
submitted by Consulting and Research Services whose principals
s~ggested that a study be made to determine the feasibility
0: ;.;.tilizing vulcanized rubber aggregate with asphalt to serve
as a strain relieving interface for lay'"1'ed pavement systems.
Rocketdyne, Division of Harth jl..3erica~ Rockwell Corp.,
~as developed a SRL (Stress Relieving .:'~':;"I) device for
atte:'luation of thermal stress in solid r::'opellant rocket motors
to preclude debonding. Since the layered design of rocket
motors and high·,rays are similar, and since flexible pavements
a~d solid propellants are also similar i~ that they are both
particulate composites*, it was hypothesize~ that the SRL
ccncept could be used as a crack arresto:t' in high',{ay design to
* Solid propellant consists of a binder plus oxidizer where. the oxidizer is harder than the binder, just as aggregate is harder" when compared to asphalt binder in bituminous
concrete.
" .
Gallaway & 1aGrone 7
alleviate the mechanical distress in the surface course due
to foundation movements and thermal distress due to the
mismatch of thermal expansion of the base and surface courses.
Figure 3 shows a graphic demonstration of the similarity of
the rocket motor and highway as mentioned above. In order
for the SRI (Strain Relieving Interface) to be effective,
the SRI material used must have high elongation so that strain
imposed by foundation movement can be accomidated. It must
also maintain adequate stability so that rutting and ·shoving
will not occur.
~t was felt that a feasibility study was needed to
determine if, when, and to what extent ground vulcanized
rubber mixed with asphalt would serve as a strain relieving
interface for layered pavement systems.
Analog Test of Pavement Strain Relievin~ Interface
Necessary material characterization followed by tests
using appropriate analogs of a pavement system were performed
to quantitatively evaluate the SRI concept. This work was
undertaken by Bynum, Gallaway and associates and was sponsored
by U. S. Rubber Reclaiming Company, Inc. of Vicksburg, I-fississippi.
The first step was that of developing a formulation and
determining the fundamental properties of a composite material
that could be used in a thin layer as a strain relieving
interlayer. Details of the procedure involved in the develop
ment of a mix design to meet the specific requirements for a
strain relieving interface have been reported earlier by Bynum,
et al (3).
Gallaway & LaGrone 8
Various combirations of ground vulcanized rubber, mineral , i
filler, and asphalt (in both cement and emulsion form) were
investigated. A formulation containing approximately e~ual
parts by volume of ground vulcanized rubber, sand, and
residual asphalt in emulsion form was found to yield a water
proof material that exhibited high elongation, low air voids
and, at the same time, could be placed in a thin layer.
The rubber used in this formulation had a 1/8 inch
maximum particle size and was produced from ground scrapped
automobile tires. The fiber and wire which are a normal part
of such tires was removed in the reclaiming operation •
. In the final mixtures tested, sand was added to the
rubber on a volume basis to bring the combined rubber-sand
mixture up to optimum density as determined by the Goode and
Lufsey Chart (4). Data are presented in Table 1.
An asphalt emulsion was used in the final formulation.
with the idea that this would facilitate easier field installa-
tion with conventional e~uipment, i.e., the slurry seal type
machine. A cationic emulsion, Grade CSS-1h, was used for
laboratory tests as well as for initial field tests, however
anionic emulsions have subsequently been used with less
difficulty and showed more promising results.
Three strain relieving interlayer formulations were
tested in a specially designed analog of a leyered highway
system to study the extent of foundation movement that can be
ac~ated by the SRI without encountering pavement cracking.
The formulations used are shown in Table 2.
Gallaway & LaGrone 9
Highway Analog
The highway analog used in this study consisted of an
aluminum jig (Fig. 4) having movable plates in a guide. With
the plates in the butted position, the SRI was bonded to the
jig plates and then asphaltic concrete blocks were bonded to
the SRI. The area of the SRI was 4 x 8 inches, and the two
plates butted together at midlength of the 8 inch dimension.
The compactive effort required to reduce the specimen
air voids to the desired level of about 2% was determined to
be approximately 195 pSi. A plot of air voids versus unit
pressure is shown in Figure 5. Details of the procedure used
to develop this curve as well as other experimental technics
used in this study have been described by Bynum, et al (5).
Briefly the SRI mix was poured into the jig with the
simulated base plates locked in the butted position, then
cured, and compressed to yield the desired thickness and air
void content. The analog was completed by heating a flexible
asphalt concrete block to 250°F then placing it on the cured
SRI layer which was at ambient temperature (75°F) to simulate
the road base temperature. A wood jig was fitted around the
asphalt concrete block and a pressure of 80 psi applied to
simulate the field comyaction temperature and pressure of a
hot mix installed on the strain relieving interface. The jig
was then placed in the Instron tester and the two base plates
were separated at selected strain rates.
Figure 6 shows a plot of deformation required to cause
cracking in the asphalt concrete surface vs interface thickness.
Gallaway & LaGrone 10
Approximately 0.20 inches defor~ation of the base could i
be accommodated by a 1/4 inch layer of the SRI material before
any cracking was noted in the surface course. This compares
to approximately 0.038 inches for the control surface block
without the SRI.
Hiss et al (6) recently reported average joint movements
of .069 to .280 inches for a wide range of conditions encoun
tered in Ne,v York State. These movements were experienced
over a temperature range of from 6-95°P.
It was felt that the 300-440% increase in allowable
foundation movement obtainable by utilizing the SRI would be .
sufficient to substantially alleviate reflection cracks in
most instances and that further field tests were warranted.
Field Testing
During tl1e course of this study several patches of the
SRI mix were placed on existing pavement in a parking lot
adjacent to the lab to get an idea of the handling character
istics of the SRI material in field work. The material cured
in less than ~~ hour and showed good durability. These patches
wi thout any overlay were subjected to reOgular traffic conditions
and showed no ill effects after undergoing numerous cycles of
automobile breaking ancJo acceleration.
Loyd James, City Engineer at College Station, Texas, had
observed some of these tests being conducted by Bynum and
associates and having a very serious reflection cracking problem
Gallaway & LaGrone
on streets in one section of their city, he expressed an
interest in trying a test section.
1 1
The leading officials of the City of College Station
readily recognized the signifi~~ce of the SRI concept. The
development of the SRI system would not only provide a
method of making a more lasting repair of old cracked streets
with a minimum disruption of local traffic but would also
result in a functional means of disposing of the millions
of discarded tires that are becoming an ever increasing solid
waste problem. They therefore encouraged James to go ahead
with a field test of the Strain Relieving Interlayer. Details
of this field test were reported recently by James (7,8).
The street chosen for the test was built in 1963 of
unreinforced concrete and was located in ~~ area of the city
that encountered severe foundation movement due to the swelling
and contracting of the base soil with changes in moisture
level. The concrete slab was five inches thick set on top of
a two inch sand cushion. Transverse expansion joints were
located on twenty foot centers; there was a longitudinal shear
key at the centerline. Smooth steel bars served as load trans
fer devices at the joints; reinforcing bars were used to dowel
the pavement to the curb. There was no other reinforcemeEt.
The street had deteriorated significantly since con
struction. Cracks up to 3 inches '<ride had formed with some
vertical displacemeEts of up to 2 inches. Figure 7 & 8.
A hydro-h~~er was used to break the pavement into slabs
approximately 3 ft. wide and 10 ft. long - Figure 9. This was
Gallaway & LaGrone 12
done to level out the displaced slab and allow complete
seating of the concrete slabs.on the subgrade. It was also
felt that the potential movement of the full slab in this
particular area would be in excess of the capability of
the SRI. Sand was spread over the broken concrete to fill
any voids and the street was flooded and rolled with a
tracked front end loader and a 50 ton pneumatic roller.
The prepared surface was swept with a street sweeper prior
to laying the SRI.
Sand was brought to the job site in bulk with the rubber
aggr,egate delivered in 50 pound bags as a matter of expediency
for this small job. Nixing of the two materials was accom
plished with a front end loader. When the SB..'1.d and rubber
were adequately mixed the material was loaded ir:to the hopper
of a slurry seal machine for application.
Considerable effort was required to work out a field
application procedure for applying the SRI. Premature breaking
of the asphalt emulsion was first encountered and this caused
large lumps of the SRI mix to form and caking on the walls
of the mixing box occurred. Considerable hand labor was
required to spread the material and even then a thin layer
could not be obtained. A procedure was finally worked out
where the SRI was applied in an acceptable manner at the
normal slurry seal rate.
It was later found that the addition of SO!!le larger
mineral aggregate to the mix resulted in a SRI composition
that was much easier to lay with the conventional slurry seal
Gallaway & LaGrone 13
equipment (Figure 10). Over 1600 square yards of this mix
have been laid in less than one hour with no hand labor
required.
The original plans were to cover the SRI with a 1 inch
overlay of standard plant mix asphalt concrete. There was
some delay in getting this work done however, and it was
decided to cover the area with a standard 3/8 inch pea gravel
seal coat.
This was the first attempt at making a field installation
of the SRI mat·erial and several unanticipated problems were
encountered that necessitated compromise in some areas. Even
so, with the cooperation and ingenuity of several engineers
and technicians a practical method was worked out for applying
the SRI and after several months of service, the test section
is giving excellent service.
It is quite obvious that alterations of the mix design
used for a Strain Relieving In±erlayer will be necessary from
one area to the next so that local materials can be utilized.
Care should be taken to assure an optimum mix ,vi th the rubber
aggregate and mineral aggregate. If the mix is altered signi
ficantly the strain relieving characteristics will no doubt
be affected.
Conclusions:
It has been shown that vulcanized rubber aggregate can be
used as a Strain Relieving Interlayer in a layered highway
system to alleviate reflection cracks.
Gallaway & LaGrone 14
Laboratory data indicates that a 1/4 inch layer of the
SRI could be expected to accommodate approxi~ately 0.20 inches
of base movement with a minimum of cracks reflecting through
the surface course.
Field installation procedures have been developed and
further field testing is needed to substcr,tiate our findings
to date.
Use of the SRI is suggested in areas where severe
foundation movement or cracking of the base course is expected.
It may also provide a means of placing relatively thin (2 to
3 i~ches) overlays of asphalt concrete hot mix over old
concrete highways without encountering the characteristic
reflection cracks at the exp~~sion joints. In cities this
can be of tremendous importance since thic~ overlays eventually
exceed the level of the curb and create hazardous driving
conditions as well as drainage problems. (~igure 11)
Use of vulcanized rubber aggregate in Strain Relieving
Interlayers for streets and highways o:'"fers a disposal
method for waste tires and could alleviate a serious solid
waste problem.
15
REFERENCES
1. McDonald, Charles E. - "Bi tuminous Paving As Related To
Lo.rge Commercial Airports In The Urban Envirorur:ent".
Paper presented Highway Research Board Jan. 1971.
2. U.S. Rubber Reclaiming Co. - "Perma Track All-Weather
Rubber Surfacing" - Advertising Bulletins.
3. Bynum, D. Jr., Evertson, J.F., Fleisher, H. 0., and Ray,
D. R. - "Materials Characterization For A Pavement Stress
Relieving Interface", submitted for publication by
Journal of Naterials.
4. Goode, J.E. and Lufsey, L.A. - "A Hew Graphical Chart for
'Evaluating Aggregate Gradations," ~, 1962, P 176-207.
5. Bynul!!, D. Jr., Glla\'lay, B.~li., and LaGrone, B. D. - "Analog
Tests of Pavement Stress Relieving Interface" submitted
for publication by ASCE.
6. Hiss, J.G., Lambert, J. R., EcCarty, W.1'1., - "Joint Seal
JViaterials" Research Report 68-6, 3ureau of Physical
Research, 1':e'.v York state Depart::::.ent of Tray.sportation.
7. James, L.L., -"Ground Tires Re:'uce Pavement Cracking"
The American City - Feb. 1971
8. James, L.L., - "A NeVI Potential For Slur'7 Seals-Improving
Pavement Performance By utilizing Discarded Automobile
Tires In A Stress Relieving Interface", A paper presented
to International Slurry Seal .P_SSOCi2.tion - ]lebo 1971.
Gallaway & LaGrone
ACKNOWLEDGEMENTS
The authors would like to acknowledge Dr. Douglas
Bynum, Jr. for his dedicated efforts in the investigation
and development of the Strain Relieving Interlayer.
We would also like to acknowledge Mr. Loyd James,
City Engineer of the City of College Station, Texas
for his progressive effort in m~~ing the first field
placement of the Strain Relieving Interlayer.
Finally we would like to thank the U.S. Rubber
Reclaiming Co. for sponsoring the development of the
SRI concept.
Gallaway & LaGrone
TABLE 1
GRA.DATION ADJUSTr1E~~T
Optimum Packing Rubber Aggregate Sand Volume
Accum. Accum. R % Pass Fraction % % Pass Fraction % Fraction % Fraction %
4 - 8 100 26 100 56 26 0
8 - 10 74 6 44. 10 5 1
10 - 16 68 16 34 30 14 2
16 - 20 52 6 4 4 2 4-
20 - 50 46 17 0 0 0 17
50 200 29 13 0 0 0 13
200 - Pan 16 16 0 0 0 16
56 = """"2b"" = 2.15
Mix. No.
35 42 37
35 42 37
35 42 37
* w· 1.
Galla\vay & LaGrone
=
TABLE 2
CONSTITUENTS IN NIXES TESTED
Bitumen
25.0 30.0 35.0
14.6 18.0 21.6
15.6 18.7 21.8
Constituent
Emulsion . Rubber
Percent by Volume
35.2 32.9 . 30.5
Percent by \'leight
23.6 22.6 21.6
Constant *, C
26.0 31.2 36.4
25.2 23.5 21.8
CV where constituent weight, Wi' is in pounds and total volume of mix, V, is in cubic Ieet.
Sand
39.8 37.1 34.5
61.8 59.4 56.8
65.9 61.6 57.2
,GALLAWAY EX LA GRONE
R'EFLECTION CRACKING
OF ASP HAL Teo NCR E TEO VE R LAY
(OVERLAY ON PORTLAND CEMENT TREATED BASE)
FIGURE I
, ',.'> ,( "",;0,:; ,; , ," ,-, >~"
' ..
CRACKING DUE TO EXCES SIVE
DEFLECTION OF PAVEMENT
FIGURE 2
· .
c:i~/~i~l ---_-----_--------"'-'--~~~ j _L-:I> \ r--------------- I
" I ,/
L>A
.. . ........... ·1
PAVEME NT ROCKET MOTOR ANALOG
FI G U R E 3
· GALLAWAY ex LAGRONE
,. i'
HI GHWAY ANALOG USED IN LAB STUDY
80
E-< 60 Z 1'1 u
'" W ll.
~
:> 40 ~
E-< Z W E-< Z 0 u Q 20 -H 0 :>
o ~ ____ ~~ ____ ~~ ____ ~~ ____ ~~~ __ ~ o 50 100 150 200 250
PEESSuhE, p, psi
AIR VOID - FUNCTION OF PRESSURE
Figure 5
'GALLAWAY & LAGRONE
1.00r-------~-------r------_.------~,_------._'------~------~
• ~
H
0.50
~
Z 0.1 o H E-< ..: Eii o .... "'" Cl 0.0
- -- - - --"\7 __ _ -
1/16 2/16 3/16
KEY
STRAIN RATE 0.025 0.025 2.500 2.500
4/16 5/16
INTERFACE THICKNESS, ti
, in.
ANALOG DEFORMATION vs. INFERFACE THICKNESS
~ -'! f I
PAVENENT THICKNES$
1 .00 J' 2.00 1.00 2.00
6/16 7/16
Figure 6
·r-" - -- : -_ -i - ,
" "~ " r:. <
• - I .-~' .. - - - -'. :-- : -?-/--,--,",-_:>,- -
" . ~ I - - '.-
; " ,~- ~- :' . .1
DETERIORATED STREET
CROSEN FOR FIELD TEST
FIGURE 7
GALLAWAY a LAGRONE
VERTICAL 01 SPLACE ME NT OF SLAB
FIGURE 8
\ .
PAVEMENT AFTER BEl N G BROKEN UP
BY DR OP HA M MER
, \ .
. " "
~ . -.:: ,
INS TAL LAT 10 f\l 0 F MOD 1 FIE 0 SRI M I X
FIGURE 10
.GAL LAWAY a LAGR 0 NE .' I.
HIGHWAY AFTER
SEVERAL .CONVENTIONAL OVERLAYS
FIGURE II
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