ASPHALT-RUBBER HOT MIX

RESEARCH FINDINGS

RUSSELL SCHNORMEIER

TECHNICAL DIRECTOR ARPG

11'ltroduction

As phalt-Rubber ( A-R ) has been used in 4 5 s tates and

four contine nts for 20 years in the following types o f

application:

SAM- Stress Absorbing Membra n e (Seal Coats)

SAMI- Stress Absorbing Membrane Interlayer

As phalt-Rubber Hot Mix

Design mixtures of crumb rubbe r from waste tires and

asphalt cement were developed by C.H. McDonald along with

indus try , demonstrations (FHWA), re sear c h by Universities

a nd agencies both here in the United States and abroad.

As more and mo re projects were placed, it become obv i ous

Asphalt-Rubber did n ot fall within kno wn properties of

s tandard asphalt . Observations we re made by Engineers a nd

Scientist indicating property values far better than the

standards.

When As phalt-Rubber hot mixes are test.ed using t h e

same procedures as for convent ional asphalt cement ( AC ).

Some engineers have shied a way from the A-R mi x es because

of the difference from conventio nal test values.

In 1987,the Asphalt-Rubber Producer Group (ARPG)

commenced a program to determine criteria of property

values for Asphalt Rubber using kn own standar d test

methods.

A r esear c h contract was established with the

University of Nevada-Reno, t he Univer sity of Ari zon a a nd

the U.S. Army Corps o f Engineer, Waterways EXpe rime nt

Station (WES) through its Construction Productivity

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Kdvancement Research (CPAR) . Also, ARPG member

engineer ing person al and ,their testing facilities we r e

included along with Sout hwest Testing Labo r atories

Houston, Texas,and Wester n Technology Laboratories,

Phoen i x , Ar izona .

Aggregates and asphalt ceme nts selected for this

r esearch have been o r a r e to be included in the Str a t egic

Hi ghwa y Research Pr oB r am (SHRP)

Goal s and Objec tives

The purpos e of this researc h is to:

1. Develop mix design procedures using

a. Mars hall Method b. Hveem Method c . Gyrotory Compaction Method

2. Develop design properties s uch as air voids, flow s tab ility and water sens itivity. Evaluate paveme nt desgins using dense and open g raded Asphalt - Rubber mixes .

3. Set phys i c al properties us ing a specific des ign. Establis h property parameters for Asphalt - Rubber mixtures using current tes t methods.

4. Establish standards for test methods.

5. Compare physical properties and performance values with s tandard asphalt in dense graded asphalt concret e inc luding aging, fatigue, flexibility, a nd duc tility f o r high and low t e mperatures.

6 . Establish p ractical and theor etical equivalenc y values f o r Aspha l t-Rubber hot mix , both as an overlay a nd for new construction.

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Design Approach for the Research

A pre-agreed resea r ch flow chart wi 't h goals and

timing is shown in Figure 1.

1. As goals were met and new information became ava i lable,

strategy changes were made. The total program is

monitored by an advisory committ ee selected from two

states agencies, two uni versit i es and two private

companies, along with t he Asphalt-Rubber P r oducers Group.

Where Are We Now ?

Binder contents have been determined for the dense

graded hot mix, with and without rubber. Selections were

based on Marshall and Hveem test methods by the advisory

committee_

A design binder content using the gyrotory compaction

method with Asphalt-Rubber could not be established.

Modifications of the gyrotory test procedure are being

investigated.

Binders were tested before and after aging. Aging

methods used were Rolling Thin Film Oven, Weatherometer

through 160 cycles , ASTM Method D-1754. Ductility, ASTM

D-11 3, tests were performed, however the rubber particles

in the mix did cause problems.

Job Mix Formul a

It was the decision of the research team to use a

single aggregate and asphalt source. This aggregate and

asphalt sou r ce would be one of the select SHRP materials

i nclude d in the Strategic Highway Research Program.

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Aggregate i s a 100% crushed granite from

Watsonville, Cal if o r nia, meeting gradation requirements of

Cal Trans 1/2-inch hot mix, Nevada type 2 hot mix and

ASTH D 3315 1/2-inch dense. The selected g radation is as

follow a nd in Figure 2.

Aggr egate Gradat ion Sieve Size % Passing

3/4 1/2 3/8 .. .8 016 <30 '50 UI OO # 200

100 98 85 58 40 28 20 14

9 5

The asphalt cement is from Witco Oildal e,

Cali f ornia, refinery. Grades of asphalt ceme nt used in the

research are AR-8000, AC-20 and AC-5.

Crumb rubber was supplied by Baker Rubber, Inc. o f

South Bend, Indiana. It is a mbient gr ound rubber having

a rubbe r hydrocarbon content of appr o ximately 45-percent

and a specific g ravity between 1.1 and 1.2 with par -ticle

size as follows:

Sieve Size 010 #16 #30 .4 0 '50 080 HOO #200

% Passing 100

99.8 78.0 48.8 26.6 9.2 6 . 6 0 . 2

Thi s r ubbe r is a stan da r d for the ind ustry when using

dense grade d mixtu r es.

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The mix design for rubber content was set by the

research committee at 16-17-percent by weight of the

asphalt cement based upon labor at ory test re sults.

Binder Contents

Binder contents for conventional asphalt cement and

Asphal t-Rubber dense graded mixtures selected, were based

on the Marshall and Hveern Tests. Air voids were set

between two-and- five-percent with stabi lity and flow

established using industry and current practices.

The Marshall Method (ASTM D-1559) was modified in the

testing procedure for A- R hot mix as follow:

1. Material placed in a 275-degree oven for one-hour after mixing and before compacting to insure mixture was at the appropriate temperature for compaction.

2. Compacted samples were allowed to remain in the mold overnight before extruding.

The Hveem method (ASTM D- 1560 and 1561) was modified

in the testing procedure for A-R Hot Mix as follow:

1. Samples were mixed and placed in a 300-degree oven for three hours.

2 . Samples were extruded immediately after a leveling load was applied. Samples were not cooled down prior to extrusion.

Binder contents were selected using both test methods

within the criteria established and they produced the same

binder contents. The conclusion was, either Marshall o r

Hveern method can be used for designing A- R Mixes.

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General Observations

The halfway point of this research has revealed

Asphalt -Rubbe r has significantly modified the physical

properties of hot asphalt mixtures when compared, to

conventional mixes. Modifications include inc reased high

temperature modulus. viscosity, elasticity and less age

hardening. Figures 3 through 7 show the general range of

mix properties of Asphalt-Rubber mixes and standard

asphalt. As phalt-Rubber mixes establishes a property

range different from known standar ds .

The graphs show the physical properties of Asphalt­

Rubber mixes compared to standard AC. Standard marshall

stabilities accepted in the industry are in the r anges

shown in Figure 3. Usually, stability ranges from 1,800 to

3,500 pounds. Asphalt-Rubber on the other hand, provides

stability values of 1 , 000 to 1,500.

Unit weights are the same except, Asphalt - Rubber mix

has two-percent higher binder content as shown in Figure

4. Void contents are inc reas ed with higher A-R binder

contents. Standard asphalt mixes range from three-to­

five-percent voids and Asphalt-Rubber can be a s low a~

two-percent with as high as three-percent more binder as

shown in Figure 5.

Marshal l flows are dramatically increased in a

Asphalt-Rubber hot mix as shown in Figure 6. The

relatio nship between stability and flow gives an

indication of flexibility of the mix. Current flow

requirements are 8 to 16, whereas Asphalt-Rubber fl ow

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values are 18 to 25, stablilities are l ower .

Flows and stability indicate a very flexible elastic

material to prevent cracking.

Hveem stabilities plotted reveal higher binder

contents give lower stabilities (Figure 7). Acceptable

ranges for standard hot mixes are 35 to 45 whereas the

higher Asphalt-Rubber content offer s lesser stabilities.

However due to the flexibility and elasticity of A- R

mixes, the lower range of stability maybe acceptable.

The next phase of this r esearch is to compare binder

properties and mix properties via fatigue, creep, and

stress-strain analysis. This seement will also include a

mix design procedure for binder contents for open graded

Asphal t-Rubber hot mix. The interesting aspect of this

research is the binder contents . When using standard

asphalt cement the binder content is limited to runoff of

AC. The binder content can not exceed the r unoff factor

for practical placement.

Asphalt-Rubber however has allowed the binder

content to increase by 50% or more. A higher binder

content in open graded mix offer s thicker films and

greater durability. At the same time the r ubber inclusion

offers age hardening resistance.

The purpose of this part of the research i s to

develop a mix design method to determine optimum binder

content needed for an open graded hot mix seal to obtain

maximum durability.

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Ultimate Goals of this Research

Research will provide the parameters of properties

required for Asphalt-Rubber using accepted test methods.

It will allow any qualified engineer, technician and/or

laboratory to test and develop a hot Asphalt-Rubber mix.

Design procedures and property parameters will be

published and available to the public.

Also, acceptable design procedures for open eraded

Asphalt - Rubber hot mix will be established.

The last phase of this research is to establish

equivalency factors for pavement thicknesses. Test data

and demonstration projects over the past 20 years will be

used to obtain equivalency values.

Predicted equivalency values indicate that one -inch

of Asphalt -Rubber hot mix is equivalent to one-and-one-

half to two inches of standard neat asphalt concrete.

Based on original AASHTO road test values and layer

equivalency, we can surmise and project conservative

coefficient values as follows:

EquivCllent Thickness Coefficient

Asphalt-Rubber Hot Mix Asphalt Concrete Hot Mix Stabil ized Base Asphalt Cement Treated Base Quality Aggregate Base

1 inch .1-1 /2 inch

3 inch 5 inch 6 inch

When comparing standard asphalt concrete with

Asphalt-Rubber concrete it is possible to reduce the

.60

.40

.20

.12

.10

pavement thickness by up to 33-percent based OIl these

equivalent values. -9-

Research has also shown Asphalt-Rubber can tolerate

25-to-50-percent greater deflections under traffic. This

indicates a lesser thickness for overlays can be applied

with a minimum of one-inch.

Asphalt-Rubber thickness can be reduced whe n compared

to standard asphalt. Sections can be reduced because of

its flexibility at low temperatures and its stiffness at high

temperatures. Also, Asphalt -Rubber has been shown to be

impermeable and has sealed the subgrade structure to

increase its load carrying capacity.

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ARPG CPAR Research Gradation

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