AD01'tS USE OF CRUMB RUBBER IN ASPHALT PAVEMENTS

DOUGLAS A. FORSTJfE,P.E. , 'oc';:,;,- _

AND GEORGE:B. WAY~ P.E.

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ARJ[ZONA DEPARTMENT o.F TRANSPORTATION MATERIALS SECTION

WASHTO

PHOENIX~AruzONA MARCJH: Jl7,J.994

HISTORY

ADOT'S USE OF CRUMB RUBBER IN ASPHAL T PAVEMENTS

Charles MacDonald, as the City of Phoenix Materials Engineer, began to experiment with mixing crumb rubber from ground tires with asphalt in the early 1960's. He, along with other associates, patented what is currently described as the MacDonald Process or Wet Process for Making Asphalt Rubber.

The Arizona Department of Transportation (ADOT) monitored the development of Asphalt Rubbei· and placed a Band Aid Asphalt Rubber (AR) Treatment in 1964. In 1968, technology advanced enough that AR could be placed with a distributor truck by using a diluent (kerosene). From 1968 - 1972, ADOT placed six projects with AR in a seal coat type application. In 1972, the first field experiment was constructed using AR as a seal coat or Stress Absorbing Membrane (SAM), as well as an interlayer under a hot mix surfacing. The interlayer application is typically referred to as a Stress Absorbing Membrane Interlayer (SAM!). Both the SAM and SAM! applications showed great promise in reducing reflective cracking (1). From 1974 until 1989, approximately 700 miles of State highways were built using a SAM or SAM! application of AR. In addition to this, 42 research reports were published, greatly increasing the state-of-the­knowledge concerning AR .• '

In addition to reducing reflective cracking it was noted early on that AR is a waterproofing membrane. Several projects were built to control sub grade moisture in order to control expansive (swelling) clays or to reduce structural pavement sections. This application proved to be very successful (2).

CURRENT PRACTICE

In 1989, Larry Scofield documented in a research report, liThe History, Development, and Performance of Asphalt Rubber at ADOT" (3). In that report tho· following conclusion is stated, "asphalt rubber has successfully been used as an encapsulating membnme to control pavement distortion due to expansive soils and to reduce reflection cracking in overlays on both rigid and flexible pavements. During the twenty years of asphalt rubber use, ADOT has evolved from using slurry applied asphalt rubber chip seals to utilizing reacted asphalt rubber as a binder in open and dense graded asphalt concrete." He noted that AR could be used as a binder for Hot Mix Asphaltic Concrete (HMAC). 'Concurrent with this conclusion, it became evident that AR as a binder could provide a HMAC mix suitable for addressing cracked pavements.

In 1988, an open graded asphalt rubber asphaltic concrete friction course (AR-ACFC) was placed on several miles of Interstate 19, south of Tucson. This AR-ACFC mix, containing 10.0 percent asphalt rubber as the binder, was placed on top of a plain jointed concrete -pavement. Since 1988 no cracks have reflected through. Following that project, several other projl'cts have

USE OF CRUMB RUBBER IN ASPHALT PAVEMENTS -

PAGE 2

been built with asphalt rubber as the binder. These projects were built with the expressed purpose of controlling reflective cracks with a very thin layer of very elastic material. To date, all projects have performed as expected. As a further extension of this work, a structural overlay was designed and built in 1990 on Interstate 40 near Flagstaff, using AR as the asphaltic concrete binder and the ACFC binder. This project also contained numerous Strategic Highway Research Program (SHRP) test sections as well as ADOT test sections. The purpose of the project was to overlay a severely cracked and failed concrete pavement. As of the most recent reviews of this project in May and June of 1993 the asphalt rubber sections built as the top portion (overlay top 2" AR-AC, 112" AR-ACFC) have the least percentage of reflective cracks.

Cost comparisons would indicate that asphalt rubber can be from two to four times as expensive as asphalt. After incorporation into the mix, the finished project (A..'t-AC or AR­ACFC) is generally from 25% to 75% more expensive than AC and 80% to 160% more expensive than an ACFC. These higher costs need to be examined in light of actual usage. On the 1-19 project, only an AR-ACFC was placed for the same cost as a 1.5" AC overlay. Such a thin dense AC mix surely would have cracked after the first winter, whereas after six winters, the AR-ACFC has not cracked. Similarly, the 2" AR-AC on the 1-40 Flagstaff project was placed for the same cost as 3" of AC. It should be noted that a 4" AC overlay showed the highest percentage of reflective cracking, whereas the sections with a 2" AC plus 2" AR-AC overlay have shown the least percentage of reflective cracking after three winters. In terms of field performance asphalt rubber appears to be a cost mective material when used as a relatively thin overlay (2 inches or less) or ACFC.

With regard to the price of asphalt rubber, Table I shows bid prices since 1985. In 1985, one Arizona company became the owner of all asphalt rubber patent rights, which had previously been owned by two Arizona companies. As can be seen, since 1985 asphalt rubber prices have been going down, except for 1990 when prices came down substantially. The 1990 price reduction was probably due to substantially greater usage. ADOT monitors the price of all the products it buys and has used asphalt rubber only when its usage appeared to be well suited to the problem and cost effective. In 1992, the patents on AR began to exptre. Since then, the AR price has continued to drop with increased competition.

REASONS FOR USE

In general' ADOT is using asphalt rubber mixes to reduce reflection cracking, improve durability of surface courses and in urban areas to reduce noise. By using asphalt rubber as a binder the film thicknessjs increased to a value of 19 - 36 microns. The grade of asphalt is an AC-I0, in contrast to typical grades of AC-20 and AC-30 in the mountains and AC-30 or AC-40 in the deserts. The 20 percent rubber particles change the asphalt rubber temperature susceptibility such that at high temperatures the mixture is much more viscous than the neat asphalt. Typically the asphalt rubber mixes are 0.5" - 1.0" thick when open graded and 1.0" - 2.0" thick when gap graded. For Arizona's climate and materials, asphalt rubber appears to provide an excellent wearing course.

REFERENCES

1. Way, G. B., "Prevention of Reflective Cracking Minnetonka-East", Report Number 1979GWI, Arizona Department of Transportation, August 1979.

2. Forstie, D., Walsh H. and Way, G. B., "Membrane Technique for Control of Expansive Clays", Transportation Research Record Number 705, 1979.

3. Scofield, L. A., "The History, Development, and Performance of Asphalt Rubber at ADOT", Report Number AZ-SP-8902, Arizona Department of Transportation, December 1989.

DD

YEAR

1985

1986

1987

1988

1989

1990

1991

1992*

1993

1994

TABLE I

ASPHALT RUBBER BID PRICES AND USAGE

NUMBER TOTAL OF PROJECT TONSOFAR

3 903

3 164

1 275

" 1 390

6 3,541

6 5,612

6 3,256

9 9,723

18 16,087

, 2 2,606

• Patents expired in 1992

AVERAGE LOW BID $/TON

505

454

481

405

443

408

448

350

325

263

0.5

. I, 0 -fl'-'--...

ARACFC AC ARAC ACFC

"

".i ,

35

30 I

~ 25 I I I-

- I I ~ CJl 20

~ I I m;i~:r I I -I

8 15~ I r;"""1 ta i.;:t'i.f.:E ~~~f~1 ,~ ~~fi,I"

I I ~i!'OT,(\;:! ~~~.,: 10

5

.' AC

AVE. BID PRICES (W/O BINDER & ADMIX)

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il\l~f).nM;~~l.t1 LIIIIIIIIIIIIIIII ~"I~~t""''fl ~?&~ ~ Riit~ IIII11IIII11II11IIU

In .11111111111111~ I!' "" f~- :f'~ ;1. .. lk~'

!: •• !_~IIIIIIIIIIIIW

ARAC

;

• •

III r~x~\~~ril ~

~~1~&~1 ;~~~~ • 1,,,,,,,·,1'1 ~J{&~~s'~

ACFC

, I

I;Jli~C'jl' F ;,1 ~]i,;;f,jr- ,< I I Kill 1990

~~~~.,,, ~;:~ ~',' - - -~-- '11Lft1m1 ..... 1 .... , • .[. ,,-, .", I I [] 1991

~l~'~1 . ~"-"K'.~~ 1II111111111111111U I I 0 1992

.:!* 'tol i' IF~I*I ~-i' ,,',.' is! j;:J ,j/ 1I11111111111111m I III 1 993

1!llI1994 ~,,, 'W~~J .~='! ~t b~~'~·. _111111111111111111 1

ARACFC

AVERAGE BID PRICE (WITHOUT BINDER AND ADMIXTURE)

1990

1991

'1992

1993

1994

1990

1991

1992

1993

1994

DOLLARS PER TON

AC ARAC ACFC ARACFC

20.57

20.00

14.60

..

25.09

24.41

28.56

19.91

23.33

19.64

22.40

18.82

TOTAL COST

DOLLARS PER SQ YD PER INCH

2.92

1.52 3.02 1.41 ,,-

1.43 ..,. 3.20 1.61 "

1.15 2.34 1,48

2.68

18.20

37.34

20.04

22.36

19.96

2.99

4.51

2.74

.2.68

2.48 ,

,-