Elastic Recovery Testing in Oklahoma

By Ken Hobson, Bituminous Branch Manager, Oklahoma DOT khobson@odot.org


The Oklahoma Department of Transportation Material's Division has performed elastic recovery tests on asphalt cements with polymers since 1992. The initial in-house procedure was OHD L-42 (Method of Test for Elastic Recovery by Means of the Ductilometer). It used AASHTO's (American Association of State Highway and Transportation Officials) T 51, Standard Test Method for Ductility of Bituminous Materials, as part of the procedure.

One important difference was the shape of the middle mold sections. The square middle mold sections as described in ASTM's ( American Society for Testing and Materials) D 5976 are used. The standard hourglass middle sections, as used in ductility tests, were found to be unacceptable for this procedure. This test was done on the RTFO (Rolling Thin Film Oven), residue (AASHTO T 240) at 25¡C.

In 1998, OKDOT (Oklahoma Department of Transportation) adopted the PG (Performance Graded) binder specifications. Recently, the elastic recovery test was added to specifications for PG binders PG70-28 and PG76-28. To distinguish the additional testing required these binders are now identified as PG70-28 OK and PG76-28 OK respectively.

PG binders that follow the "rule of 90" generally have been modified. The "rule of 90" is generally taken to mean that if one algebraically adds the numbers in the PG designation, and that number is over 90, then that binder may have been modified, e.g., PG70-28 is 70 + | -28 | = 98. As the algebraic difference increases above 90 there is a greater tendency for increased percentages of additives. The elastic recovery test is one way to verify elasticity.

These procedural notes do not address all of the safety problems, if any, associated with their use. It is the responsibility of the user to establish appropriate safety and health practices and determine the application of regulatory limitations prior to performing these tasks.

OKDOT's OHD L-42 Procedure is shown below.

Method of Test For Elastic Recovery by Means of the Ductilometer

    1. Polymerized Asphalt Cement 25¡C
    2. Polymerized Asphalt Emulsion Residue Less than 200 Penetration 10¡C
    3. Polymerized Asphalt Emulsion Residue Greater than 200 Penetration 4¡C
    4. Asphalt Cement 25¡C
  2. PROCEDURE. Condition the ductilometer and samples to be tested at the temperature prescribed for that material. Prepare the apparatus and test specimen in accordance with AASHTO T-51, "Ductility of Bituminous Materials" except parts a and a' of the ductility mold shall be modified to produce a specimen with straight sides. Condition the specimen in the testing machine at the specified test temperature for 85 - 95 minutes. Test the specimens in accordance with AASHTO T-51 with the following modifications. The rate of pull shall be 5 cm/minute unless otherwise stated. After the 20 cm elongation has been reached, stop the ductilometer and hold the sample in its elongated position. After 5 minutes, clip the sample approximately in half by means of scissors or other suitable cutting devices. Let the sample remain in the ductilometer in an undisturbed condition for one (1) hour. At the end of this time period, retract the half sample specimen until the two broken ends "just" touch. At this point, note the elongation in centimeters.
  3. CALCULATIONS. Calculate the percent recovery by the following formula:
% Recovery = (20 Ð x) / 20 * 100

Where: x = Observed elongation after rejoining of sample, centimeters.

The ASTM D 5976 standard specification is similar but in the equation above use 10cm as the elongation length and replace 20 with 10 in the equation above.

Listed below are step-by-step instructions.

  1. Heat the sample for no more than 2 hours and not more than 110¡C above the softening point (AASHTO T 53).
  2. Obtain sufficient bitumen material from the residue as obtained from the RTFO (AASHTO T 240) test.
  3. Use a mold assembly with dimensions as shown in Fig. 1 of ASTM D5976. Brass molds are normally used and a flat brass plate. Place disposable paper under mold to facilitate easy cleanup.
  4. Assemble the mold on a brass plate. Coat the surface of the plate and interior parts of the middle sections. Thinly coat the top parts of the middle sections and overlap the joints. The coating material is a mixture of talc and glycerin. Other coating materials are mixtures of glycerin and dextrin, talc, or kaolin (china clay). This keeps the sample from sticking to the sidepieces and bottom of the plate. It also acts as weak glue to hold the assembly together.
  5. Stir the sample to a homogonous consistency and pour into a small container. 50 ml Tri-Pour beaker (Polypropylene, disposable) and 6 oz. tins have been found suitable.
  6. Quickly pour a thin stream into the mold from the small container. Pour in a back-and-forth motion until more than level full.
  7. Cool at room temperature for 30 to 40 minutes.
  8. Place assembly in water bath for 30 minutes at 25¡C (0.1¡C).
  9. Remove assembly from water bath and trim level with a hot straight edged putty knife or spatula (preferably with a 2 inch wide blade). Support one end of the assembly with a roll of tissue paper in one hand while striking off the excess material level with the top of the mold. Wipe excess bitumen material from putty knife on tissue paper and dispose. Use sufficient thickness of tissue paper to ensure safety from burns.
  10. Place mold assembly back into water bath for 85 to 95 minutes.
  11. While submerged in water remove the sidepieces and slide briquet off plate and attach ends to studs, pins or clips on the ductilometer.
  12. Immediately start the ductilometer motor to pull at a rate of 5 cm/min 5.0%.
  13. After 10 cm 0.25 cm elongation, stop the machine. Hold at this position for 5 minutes.
  14. After the 5 minutes, clip the sample with scissors (any type of conventional scissors capable of cutting polymer-modified asphalt at the test temperature) near the middle.
  15. After one hour, retract the half sample until the two cut ends "just" touch.
  16. Record the reading for elongation in centimeters.
  17. Report the % Recovery.
    % Recovery = (10 Ð x) / 10 * 100

    Where: x = Observed elongation after rejoining of sample, centimeters.

    Alternatively, one can quickly obtain the % Recovery by "Reverse Numbering".
    e.g. Scale of 0 to 10 centimeters and a reading of 3.1 cm. The % Recovery would be 69%.
    The higher the reading of elongation the lower percent recovery is obtained.
  18. Soak mold pieces in mineral spirits under a ventilated hood. Wipe dry with a cloth or paper towel. Rinse with acetone and let air dry.



Mold assembly

With talc and glycerin

Overfill molds

Place in water bath

Heat tool to trim

Trim level

Attach to ductilometer


Pulled to 10 cm



"Just" touch ends together

Lab technician shown is Larry Boston. OKDOT materials division is an AASHTO accredited lab. For more information on related topics: http://www.okladot.state.ok.us - Oklahoma Transportation or http://www.telepath.com/khobson/odot/liqlab/liqlab.html - Liquid Asphalt Lab photos, Superpave files, etc.

The University of Texas at Austin  •  UT's Cockrell School of Engineering