asphalt-rubber hot mix research hot mix research findings ... asphalt concret e inc luding aging,...

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  • 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

    -2-

  • 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.

    - 3-

  • 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.

    -4-

  • 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

  • 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.

    - 6-

  • 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

    -7-

  • 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.

    -8-

  • 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 procedu

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