hot asphalt-rubber seal coats to cure cracking … asphalt-rubber seal coats to cure cracking...

HOT ASPHALT-RUBBER SEAL COATS TO CURE CRACKING STREETS

by

James A. Stokely Director of Street ~laintenance City of Phoenix 324 west Washington Street Phoenix, Arizona 85003

Introduction

Charles H. McDonald Engineering Supervisor City of Phoenix Materials Testing Section 841 East Jefferson Street Phoenix, Arizona 85034

This article is a sequel to an article by Mr. McDonald which

appeared in Public Works Magazine for July, 1966, entitled "New

patching Material Corrects Pavement Failures". The 1966 article

presented an innovative approach to the problem of fatigue or

"alligator" type cracking. An asphalt-rubber composition was

used to provide the cementing agent for chip seal coats or resur-

facing of asphalt pavements over elastic subgrades or bases.

The new concept employed ground tread rubber off of old tires

to obtain a reaction product between hot asphalt and rubber . •

It also called for much higher percentages of rubber than were

previously used. In numerous small scale applications the con-

cept produced a highly elastic membrane tolerating high pavement

deflections without cracking. The current article tells about

the practical application of the process on a large scale.

Hot Asphalt-Rubber Seal Coats to Cure Cracking Streets

The Article

4-Page. 2

The City of Phoenix, like other cities experiencing rapid

growth, has many miles of older streets which were not designed

and built to handle today's traffic loads and volume. While

our forward-looking program of full improvement of major streets

is making progress with limited funds, our Street Naintenance

Department faces the task of holding some of these old streets

together for many years.

In too many cases we are seeing surface distress in the

form of fatigue or "alligator" cracking and we know it is caused

by elastic, verticle movement under traffic of some member of

the street's substructure. Heavy overlays, adequate to control

the cracking (three to four inches in thickness), are costly

and tend to impair drainage and other benefits of the original

deSign and construction of the curb and gutter. Thus, it

becomes a matter of constant and expensive patching or complete

reconstruction of large sections of pavement and base.

Constant patching of major streets is becoming more and

more impractical and, in some cases, almost impossible during the

daylight hours. As traffic becomes heavier over the years, the

problem will worsen. In some areas it has even been necessary

to schedule all maintenance as night work, which always means

more cost, less efficiency, and poor workmanship.


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p1ge 3

As for the other alternative of reconstruction, the money

is simply not there in most cases. Such reconstruction of a

major street in our city will often cost $24,000 per city block.

With the asphalt-rubber treatment we believe that we have the

solution to both of the above dilemmas at a cost of approximately

$2,400 per city block and with far less disruption of traffic.

The treatment described here is by no means a "cure all".

Sound judgment is needed in using it where it will do the most

good. For example, it is not intended or recommended for areas

where plastic flow of materials has occurred or will occur.

After extensive experimenting since the winter of 1963-64,

and some moderate sized full scale projects to check the research,

the City let to bid a large contract in the summer of 1971 which

included some 208,000 square yards of major streets and airport

apron work. This work was accomplished at a contract cost of

approximately 51 to 52 cents per square yard. Additional con-

struction costs were assumed by the Street Maintenance Department

of the City in the form of preparatory work and cleanup work.

As in all new processes, it took a long time to get the

"bugs" out of the process for practical application. The

greatest problem was the difficulty or unreliability of getting

a uniform spread of the membrane out of an asphalt distributor

because of the high viscosity of its composition. This prevented

what we call "good workmanship". It was finally solved by first


obtaining the reaction between the hot asphalt and the rubber

and then diluting the material with kerosene to the point where

it could be applied with an asphalt distributor. This is not

as easy as it sounds. It was necessary for the contractor to

install mixing devices into his distributors to properly blend

the asphalt, rubber, and kerosene. Without this good workman-

ship factor all previous experiments were considered failures

due to lack of uniformity.

The process is begun by heating the asphalt to a tempera

ture of between 3500 F and 4000 F. Ground tire tread rubber,

#16 to #25 mesh, produced by Atlos Rubber Company of Los Angeles,

is the second ingredient.. This is added to hot asphalt in the

proportion of 75 percent asphalt (120-150 penetration grade)

and 25 percent rubber. A reaction between the hot asphalt

and the rubber results causing a gelling effect to occur within

30 minutes to one hour after the first introduction of the

rubber. The addition of the rubber cools the mixture below

the boiling point of kerosene, and from 5-1/2 percent to 7-1/2

percent kerosene is added to reduce the viscosity sufficiently

for spraying.

We might note here that the rubber only partially dissolves

in the composition so that the particles themselves serve as

units of elastic interference to propagation of cracking from

below. Should a crack begin to propagate through the membrane,


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it encounters an elastic rubber particle and is stopped, where-

upon it would start at another location and be stopped, and so

on. This principle of introduction of ground rubber has been

employed in the manufacture of plastics for a number of years

in reducing their tendency to shatter.

The 1971 City of Phoenix contract called for the Street

Maintenance Department to clean the street and apply a light

tack coat of diluted asphalt emulsion SS-lh prior to the appli-

cation of the asphalt-rubber. This work usually preceded

the contractor's operation by one or two days. The contractor

then applied the hot, kerosene diluted, asphalt-rubber. This

was followed with an application of approximately 38 pounds

per square yard of hot 3/8" nominal sized chips, precoated

with .30 to .50 percent of asphalt and then rolled with a

minimum of three coverages by three pneumatic rollers carrying

5,000 pounds on each wheel and 100 pounds per square inch in

the tires in order to insure optimum initial embedment of the

cover aggregate.

We have found that the precoating of the cover aggregate

in any chip seal work is of great importance to eliminate the

dust problem. Creation of dust pollution would be serious

in the city from an ecological standpoint and could not be

tolerated from a public relations standpoint.


After the rolling has been completed, traffic follows

right behind the operation with very little interference. We

averaged approximately 70,000 square yards of production

placed per seven-hour day (work time restricted because of

traffic control) .

Excess chips are s\vept up the following night after

application by the Street l'1aintenance Department, and as soon

as the streets are cleaned the Traffic Engineering Department

moves in to restore the traffic delineation stripes.

Here, we will take up some of the more specific recom-

mendations for placing this material that have not been

previously touched on. For the airport apron area we used

the following grading with the idea in mind that the nominal

chip size should be small to minimize any possible damage to

jet engines from loose chips.

However, provision should be made for a flush coat of

.05 to 0.1 of a gallon per square yard of emulsified asphalt

(SS-lh) to be used wherever chip embedment is 50 percent or

less or to tie down any chips that may otherwise come loose.

The grading for the 1/4" nominal sized chip was as follows:

1/4" Nominal Chip (Airport Work Only)

Sieve Size

3/8 " 1/4" #8 #200

Percent passing

100 80-100

0-5 0-2


The grading for the chip used on the street work was as

follows:

Sieve Size

1/2" 3/8" 1/4" #8 #200

3/8" Nominal Chip

Percent Passing

100 70-100

0-10 0-5 0-2

o The chips are heated to a temperature between 250 F and

o 325 F and precoated at the time of heating in the hot plant

with a small amount of asphalt to control dust, as previously

stated.

Hot asphalt-rubber should not be applied when the ambient

temperature is below 700 F or when the pavement temperature is o

below 85 F. The distributor bar widths should not exceed 10'

or maldistribution is apt to occur.

The application rate of the hot asphalt-rubber should be

approximately 0.5 gallons per square yard for most conditions.

This can be increased up to 1.0 gallons per square yard for

severely cracked situations, although it is usually more

economical to place a less costly crack-sealing or leveling

material, such as slurry seal or a thin overlay prior to plac-

ing a lighter application of asphalt-rubber. At the applica-

tion rate of 0.5 gallons per square yard we have found that

it is desirable to use the 3/8" nominal sized chip at an

application rate of 35 to 40 pounds per square yard in order

l.. Hot Asphalt-Rubber Seal Coats P~e 8 to Cure cracking Streets

, to prevent excessive embedment. A smaller chip will be completely

embedded under heavy traffic but will serve under light traffic.

It should be noted that the grading is intended to provide

a "one sized chip" for more uniform embedment and less chip

loss due to either under-embedment, or "drowning" of chips.

Public Ii'orks magazine reported on the ecological and

recycling aspects of the use of rubber in asphalt in an

article in their Novemeber, 1971 issue entitled "l'lasted Solids

(Solid waste) and Pavement Design", by Douglas Bynum, Jr.

He states that over 100 million tires a year are thrown away

and that this constitutes approximately one million tons of

solid waste of an originally expensive product that should be

recycled as a valuable resource, particularly when it is con-

sidered that new rubber costs approximately 50 cents a pound.

Mr. Bynum goes on to state,"Asphalt binder costs about $0.015

per pound and increased binder costs are usually avoided.

However, an economic analysis shows that a 20 percent costs

saving could be realized with a $0.29 per pound binder, if the

pavement life were increased by a factor of 4." The solution

to both the pavement deterioration problem and the old tire

disposal problem is the recycling of these old used rubber

tires and thereby "kill two birds with one stone".

In conclusion, our previous experience indicates that we

may expect many years (possibly ten) of essentially maintenance-

free service from this application. Whenever it becomes apparent


, that additional maintenance is required, another application

of hot asphalt-rubber will satisfy requirements without con-

cern for reducing curb height or disturbing the drainage.

The attached photographs show some of the details of the

application (before and after pictures) and photographs of

test areas and their adjacent controls after a number of years

of service.

PHOTO NO. 1

This photo shows how the curb height on a portion of the project has been reduced by repeated overlays in the past in an effort to control fatigue cracking.

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PHOTO NO. 2

Photo of typical pavement condition on the project taken prior to placement of asphalt-rubber treatment for future reference.

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PHOTO NO. 3

Another photo of typical pavement condition prior to asphalt-rubber treatment. Hundreds of these "before" photos have been taken for future reference.

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PHOTO NO: 4

Production train for Placing aSPhalt-rubber. Note Paper cutOff being Placed at transverse jOint.

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PHOTO NO. 5

Spreading hot asphalt-rubber.

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PHOTO NO. 6

Spreading 3/8" nominal sized chips with selfpropelled chip spreader.

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PHOTO NO. 7

chips into freshly spread asphalt-The operator of the third roller can be

the left of the second operator.

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PHOTO NO. 8

East end of asphalt-rubber treatment in foreground on i'Tashington Street at 24th Street looking east.

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Completed asphalt-rubber treatment before striping on l'lashington Street at 24th street looking west.

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PHOTO NO. 10

Asphalt-rubber treated west 1,000 feet + of jet runway, Sky Harbor International Airport, Phoenix, Arizona. Treated June, 1968.

This area was severely "fatigue" cracked in the characteristic "alligator" pattern. Note lac), of reflection cracking and "rubber burns" by landing commercial jets that have failed to dislodge the asphalt-rubber treatment.

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PHOTO NO. 11

Taxiway "e" west end, Sky Harbor International Airport, Phoenix, Arizona.

Asphalt-rubber placed in 1968 (upper part of photo) . Note how it is controlling crack reflection on this heavy conunercial jet taxiway. No reflection cracking has developed at the time of this writing and none is anticipated for many years. Test installations under heavy traffic conditions going back over four years indicate that the useful life of this treatment is unknown at this time.

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PHOTO NO. 12

Small asphalt-rubber, "Band-aid" type, test panel installation-on Transcontinental Highway US-66 at an elevation of 7,000 feet--still intact after two hard winters ..

The test installation was later destroyed by reconstruction.

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PHOTO NO. 13

James A. Stokely. one of the co-authors of the article.

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PHOTO NO. 14

Charles H. McDonald, one of the co-authors of the article.

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II PLACED in 1969, asphalt~rubber shows no sign of re flective crocking, although the adjacent surface is badly crocked. Asphalt-rubber seal coats will reduce Joss of curb depth, an inevitable effect of repeated overlaying.

This article is a sequel to an article by Mr. McDonald which appeared in PUBLIC WORKS for July, 1966, elltitled "New Patching Material Corrects Pavement Failures." The 1966 article presented an innovative approach to the problem of fatigue or ·"alligator" type cracking. An asphalt-rubber composition was used to provide the cementing agent for chip seal coats or resurfacing of asphalt pavements over elastic subgrades or bases. The new concept employed ground tread rubber from old tires to obtain a reaction product between hot asphalt and rubber. It also called for much higTzer percentages of rubber that were previoIlsly used. In numerOllS small scale applications the concept produced a highly elastic membrane tolerating high pavement defiections without cracking. The current article tells about the practical appUcation of the process on a large scale.

JAMES A. STOKELY Director of Street Maintenance,

and

CHARLES H. McDONALD Engineering Supervisor,

Materials Testing Section. Phoenix. Arizona

THE City of Phoenix, like other cities experiencing rnpid growth,

has many miles of older streets which were not designed and built to handle today'. traffic loads and volume. While our forward-looking program of full improvement of major streets is making progress with limited funds, our Street Maintenance Department faces the task of holding some of these old streets together for many years.

In too many cases we are seeing surface distress in the form of fa .. tigue or Ualligator" cracking and we

PUBLIC WORKS for July, 1972

Funcllonal Products & Systems Department, Midland, MIchigan 48640

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in the manufacture of plastics for a number of years in reducing their tendency to shatter.

Precoated Chips Used The 1971 City of Phoenix contract

called for the Street Maintenance Department to clean the street and apply a light tack coat of diluted asphalt emulsion 88-1h prior to the application of the asphalt-rubber. This work usually preceded the contractor's operation by one or two days. The contractor then applied the hot, kerosene diluted, asphaltrubber. This was followed with an application of approximately 38 pounds per square yard of hot % inch nominal sized chips, precoated with 0.3 to 0.5 percent of asphalt, and rolled with a minimum of three coverages by three pneumatic rollers carrying 5,000 pounds on each wheel and 100 pounds per square inch in the tires in order to insure optimum initial embedment of the cover aggregate.

We have found that the precoating of the cover aggregate in any chip seal work is of great importance to eliminate the dust problem. Creation of dust pollution would be serious in the city from an ecological standpoint and could not be tolerated from a public relations standpoint.

After the rolling has been completed, traffic follows right behind the operation with very little interference. We averaged approximately 70,000 square yards of production placed per seven-hour day (work time restricted because of traffic control) .

Excess chips are swept up the following night after application by the Street Maintenance Department, and as soon as the streets are cleaned the Traffic Engineering De-

partInent moves in to restore the traffic delineation stripes.

Here, we will take up some of the more specific recommendations for placing this material fuat have not been previously touched on. For the airport apron area we used the following grading with the idea in mind that the nominal chip size should be small to minimize any possible damage to jet engines from loose chips.

However, provision should be made for a flush coat of .05 to 0.1 gallon per square yard of emulsified asphalt (SS-lh) to be used wherever chip embedment is 50 percent or less or to tie down any chips that may otherwise come loose. The grading for the % inch nominal sized chip was a$ follows:

Sieve Size 3/a" tA" #B

#200

Percent Passing 100

BO·100 0-5 0·2

The grading for the % inch nominal chip used on the street work was as follows:

Sieve Size 1/2 " 3/S " %." #B

#200

Percent Passing 100

70·100 0·10 0·5 0·2

The chips are heated to a temperature between 250'F and 325'F and precoated at fue time of heating in the hot plant with a small amount of asphalt to control dust, as previously stated.

Some Restraints Hot asphalt-rubber should not be

applied when the ambient temperature is below 70°F or when the

1/1 AFTER chips are spread on hot asphalt·rubber mix, they are set in place by at least three passes by three pneumatic rollers. Precoated chips prevent dust problem.

pavement tempe~ure. is_below 85'F. The distrib'il.tor'·· bar widths should not exceed 10 feet or maldistribution is apt to occur.

The application rate of the hot asphalt-rubber should be approximately 0.5 gallon per square yard for most conditions. This can be increased up to 1.0 gallon per square yard for severely cracked situations, although it is usually more economical to place a less costly crack-sealing or leveling material, such as slurry seal or a thin overlay prior to placing a lighter application of asphalt-rubber. At the application rate of 0.5 gallon per square yard we have found that it is desirable to use the % inch nominal sized chip at an application rate of 35 to 40 pounds per square yard in order to prevent excessive embedment. A smaller chip will be completely embedded under heavy traffic but will serve under light traffic. It should be noted thst the grading is intended to provide a "one sized chip" for more uniform embedment and less chip loss due to either under-embedmen~ or "drowning" of chips.

PunLIC WORKS magazine reported on the ecological and recycling aspects of the use of rubber in asphalt in an article in their November, 1971 issue entitled "Wasted Solids (Solid Waste) and Pavement Design", by Douglas Bynum, Jr. He states that over 100 million tires a year are thrown away and that this constitu tes approximately one million tons of solid waste of an originally expensive product that should be recycled as a valuable resource, particularly when it is considered that new rubber costs approximately 50 cents a pound. Dr. Bynum goes on to state, "Asphalt binder costs about ljiO.015 per pound and increased binder costs are usually avoided. However, an economic analysis shows that a 20 percent costs saving could be realized with a $0.29 per pound binder, if the pavement life were increased by a factor of 4." The solution to both the pavement deterioration problem and the old tire disposal problem is the recycling of these old used rubber tires and thereby "kill two birds with one stone."

In conclusion, our previous experience indicates that We may expect many years (possibly ten) of essentially maintenance free service from this application. Whenever it becomes apparent that additional maintenance is required, another application of hot asphalt-rubber will satisfy requirements without concern for reducing curb height or disturbing the drainage. 000

PlTBLIC WORKS for July, 1972

know it is caused by elastic, vertical movement under traffic of some member of the street's substructure. Heavy overlays, adequate to control the cracking (three to four inches in thickness), are costly and tend to impair drainage and other benefits of the original design and construction of the curb and gutter. Thus, it becomes a matter of constant and expensive patching Qr complete reconstruction of large sections of pavement and base.

Constant patching of major streets is becoming more and more impractical and, in some cases, almost impossible during the daylight hours. As traffic becomes heavier over the years, the problem will worsen. In some areas it has even been necessary to schedule all maintenance as night work, which always means more cost, less efficiency and poor workmanship,

As for the other alternative of reconstruction, the money is simply not there in most cases. Such reconstruction of a major street in our city will often cost $24,000 per city block. With the asphalt-rubber treatment we believe that we have the solution to both of the above dilemmas at a cost of approximately $2,400 per city block and with far less disruption of traffic.

The treatment described here is by no means a "cure all." Sound judgment is needed in using it where it will do the most good. For example, it is not intended or recommended for areas where plastic flow of materials has occurred or will occur.

After extensive experimenting since the winter of 1963-64, and some moderate sized full scale projects to check the research, the City let to bid a large contract in the summer of 1971 which included some 208,000 square yards of major streets and airport apron work. This work was accomplished at a contract cost of approximately 51 to 52 cents per square yard. Additional construction costs were assumed by the Street Maintenance Department in the form of preparatory work and cleanup work.

Some Problems As in all new processes, it took a

long time to get the "bugs" out of the process for practical application. The greatest problem was the difficulty or unreliability of getting a uniform . spread of the membrane out of an asphalt distributor because of the high viscosity of its composition. This prevented what we call "good workmanship.)! It was finally solved by first obtaining the reaction

prmT .rr W()RK~ fn'l' .111111 1972

between the hot asphalt and the rubber and then diluting the material with kerosene to the point where it could be applied with an asphalt distributor. This is not as easy as it sounds. It was necessary for the contractor to install mixing devices into his distributors to properly blend the asphalt, rubber, and kerosene. Without this good workmanship factor all previous experiments were considered failures due to lack of uniformity.

The process is begun by heating the asphalt to a temperature of between 350'F and 400'F. Ground tire tread rubber, #16 to #25 mesh, produced by Atlos Rubber Company of Los Angeles, is the second ingredient. This is added to hot asphalt in the proportion of 75 percent asphalt (120-150 penetration grade) and 25 percent rubber. A reaction between

the hot asphalt and JJ* rubbe, results causing a gellin! effect to occur within 30 minutes to one hour after the first introduction or the rubber. The addition of the rubber cools the mixture below the boiling point of kerosene, and from 5lh percent to 7% percent kerosene is added to reduce the viscosity sufficiently for spraying.

We might note here that the rubber only partially dissolves in the composition so that the particles themselves serve as units of elastic interference to propagation of cracking from below. Should a crack begin to propagate through the membrane, it encounters an elastic rubber particle and is stopped, whereupon it would start at another location and be stopped, and so on. This principle of introduction of ground rubber has been employed

JiIJ HOT kerosene~diluted asphalt~rubber mix is applied under pressure to existing surface. Then, precooted % inch stone chips are spread at rate of 38 pounds/square .yard. Daily production averaged 70,000 square yards.

47

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