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