Proposed NCHRP 20-7 Project
AASHTO Subcommittee on Materials
I.
TITLE
Improvements to AASHTO T 209
II.
BACKGROUND/NEEDS STATEMENT
AASTHO T 209 Theoretical Maximum Specific Gravity and Density of Hot Mix Asphalt is the basis for
HMAC compaction specifications, which indicate a percentage of the maximum specific gravity obtained
with this test procedure. The test procedure also includes supplemental procedures for porous aggregates.
The supplemental procedure (Dry Back) accounts for the water absorption of the aggregate during testing.
Theoretically, the Dry Back procedure provides a more accurate value with which to measure compaction
in the field. AASHTO currently recommends using the Dry Back Procedure when the water absorption of
the individual aggregate is greater than 1.5%. It has been found that this value requires the Dry Back
procedure to be run on mixes where the difference between the two procedures is insignificant in relation
to the effect on compaction. In addition the Dry Back procedure includes steps that are very subjective in
determining when the mix has reached the saturated surface dry condition.
There is also interest to include the use of asphalt pavement core samples to the AASHTO T 209 standard
for forensic investigations and as an alternate sample type. AASHTO T 209 currently does not address
the testing of pavement core samples. There are two primary concerns when performing AASHTO T 209
on asphalt pavement core samples: 1) meeting the minimum sample size required by AASHTO T 209,
and 2) effect on test results and the supplemental procedure due to the cut/exposed aggregate faces from
the drilling operation. Asphalt pavement core samples are typically sized 6-inch in diameter by the depth
of the compacted asphalt pavement layer. Smaller nominal maximum aggregate size mixtures compacted
in thin layer depths (e.g., 9.5 mm NMAS compacted to 1.5 inch depth) may result in sample sizes slightly
to moderately smaller than the minimum sample size specified in AASHTO T 209.
III.
LITERATURE SEARCH SUMMARY
The current body of research related to AASHTO T 209 does not relate to the modified “dry back”
procedure. The literature search using TRIS included the following results:
AASHTO T 209: Effect of Agitation Equipment Type on Theoretical Maximum Specific Gravity Values;
NCHRP Research Results Digest, Issue 369, February 2012.
Evaluation of the supplemental procedure of the maximum specific gravity test for bituminous paving
mixtures; Stuart, K.D., FHWA, June 1990.
IV.
RESEARCH OBJECTIVE
The purpose of this research would be to first determine what is the appropriate trigger measure such as
asphalt or water absorption and identify the appropriate trigger value and determine if there are
improvements that can be made to the Dry Back procedure to make it less subjective and produce more
repeatable and reliable test results. Secondly, the purpose is to explore methods that may allow for the
prediction or modeling of the Dry Back procedure to reduce the frequency that the Dry Back procedure
needs to be completed. Finally, the purpose is to include appropriate revisions throughout the standard to
allow use of this test method to test pavement core samples for theoretical maximum specific gravity.
The results of the research will be used to modify the existing AASHTO T 209 test procedure.
V.
WORK TASKS
Tasks anticipated in the project include the following:
 Perform literature review including recent research revolving around AASHTO T 209 and the
impacts high absorptive aggregates may have on laboratory testing and pavement performance.
Include any information on theoretical maximum specific gravity testing on pavement core
samples. Include other agency test procedures for modifications to the AASHTO T 209 standard
and interview these agency’s material experts on the purpose of the modification.
 Perform laboratory investigation of significance of the water absorption of the aggregate of the
dry back procedure on both field mixture samples and pavement core samples developed from the
literature review. Include investigation on aggregate sources from various locations with different
absorption and geological characteristics.
 Perform laboratory investigation on pavement core samples where the samples have a slightly to
moderately smaller sample size than the minimum sample size specified in AASHTO T 209 and
compare to pavement core samples of same mixture where the minimum sample size is met.
 Analyze data to determine the correct trigger value for the Dry Back procedure and improved
methods to ensure proper drying back of the field mixture sample and pavement core sample.
Also determine if modifications can be made to the procedure to reduce the time of the drying
process and/or model the procedure to reduce the frequency that the testing needs to be
completed. Analyze data to determine significance of differences in test results when potentially
slightly to moderately smaller sample sizes from pavement core samples are used in the test.
 Prepare and provide revisions to AASHTO T 209 in tracking changes format to include
appropriate revisions to the supplemental procedure (dry back) and to include appropriate
revisions to include the testing of pavement core samples.
VI.
ESTIMATE OF PROBLEM FUNDING AND RESEARCH PERIOD
It is estimated that the research will take 2 years to complete and will require approximately $80,000.
Summary: $10,000 - Literature Search
$55,000 - Material Testing
$25,000 - Analysis and Report
$10,000 - Research Administration
VII.
URGENCY, PAYOFF POTENTIAL, AND IMPLEMENTATION
Benefits of this research related to the dry back procedure include getting quicker results in the field with
which to measure density and make decisions on mix quality. Setting appropriate trigger values would
reduce the need to run this procedure, which takes significantly longer than the normal test, on many
aggregates, thus reducing testing costs for both the agency and contractors and provide results to the
density technician in a more timely fashion for in-field density monitoring. Additionally, the agency
needs to produce a more reliable and repeatable measure for compaction, yielding better confidence in our
density test results.
Benefits of this work related to including the use of pavement core samples in AASHTO T 209 would
provide a standard test procedure for agencies to perform forensic analysis of pavements or to optionally
perform the maximum theoretical specific gravity test on pavement core field samples with confidence.
The investigation would provide changes to asphalt paving specifications and to the testing requirements
involved in mix design and construction of hot mix asphalt, specifically AASHTO T 209.
VIII.
PERSONS DEVELOPING THE PROBLEM
Cole F. Mullis, P.E.
Pavement Services Engineer
Oregon Department of Transportation
800 Airport Rd. SE
Salem, OR 97301
(503)986-3115
Cole.F.MULLIS@odot.state.or.us
IX.
PROBLEM MONITOR
X.
DATE AND SUBMITTED BY
Subcommittee on Materials
Tech Section 2c
Timothy L. Ramirez, P.E.
Engineer of Tests
Pennsylvania Department of Transportation
81 Lab Lane
Harrisburg, PA 17110-2543
(717) 783-6602
tramirez@pa.gov