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New Mexico State Highway and Transportation Department
DETERMINATION OF IGNITION OVEN CALIBRATION FACTORS
1. GENERAL
1.1. Both the asphalt calibration factor and the aggregate calibration factor must be
checked in order to ensure that variations in oven operation and efficiency, possible
differences in weighing accuracy, and the breakdown of aggregate during the ignition
process are properly accounted for. These calibration factors may be affected by oven
setup and efficiency; by the type, source and gradation of the aggregate used; by the type
or grade of asphalt cement used; and by the addition of hydrated lime (or Anhydrite
Material). Therefore, to optimize accuracy, calibration factors must be established for
each individual oven and for each mix design tested.
1.2. The Contractor shall be responsible for obtaining the materials, fabricating the
required sets of calibration specimens for each oven that will be involved in ignition
testing for a given project, and for delivering the calibration specimens and specimen
fabrication information to the appropriate laboratories for testing. This includes all
laboratories used for process control testing, quality assurance testing, and independent
assurance testing. (Note 1)
1.3. Each set of oven calibration specimens shall consist of four (4) ignition specimens
1.4. All oven calibration specimens for a specific Job Mix Formula (JMF) shall be
prepared by, and within the same laboratory facility at the same time.
1.5. All oven calibration specimens sent to the project and project related laboratories
shall be tested within seven (7) calendar days from the date of fabrication. Samples sent
to non-project level laboratories for potential support applications or samples built for
contingency purposes in questions arise with the initial test results will be sent complete
to the District Laboratory Supervisor with the oil blended into the aggregate.
2. SAMPLING
2.1.
Aggregate samples shall be obtained in accordance with AASHTO T 2, as
modified by the Department and described in the Technician Training and Certification
Program (TTCP) Manual, and shall be representative of the material to be used in
producing asphalt mixture. Each virgin aggregate sample shall be washed, dried and
sieved in accordance with AASHTO T11 and T27, as modified by TTCP.
2.2.
RAP, if required, shall be sampled in accordance with T2. All RAP shall be
fractionated so that 100% passes the 1 inch sieve.
2.3.
Retain a sufficient amount of aggregate and RAP (as sampled from the
stockpile) on each screen to build all of the burn-samples required for the HMA mix
being calibrated. Obtain a sufficient amount extra minus 200 material to accommodate
that which is lost during the wash process.
2.4.
Asphalt binder samples shall be obtained in accordance with AASHTO T 40 and
shall be representative of the material to be used in producing asphalt mixture.
2.5.
Hydrated lime or Anhydrite Material samples shall be obtained in accordance
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with AASHTO T 218, as modified by the Department and described in the TTCP
Manual, and shall be representative of the material to be used in producing asphalt
mixture.
3.
SPECIMEN PREPARATION
3.1. Dry the virgin aggregate samples obtained from the project stockpiles to constant
mass at 110 ± 5 °C (230 ± 9 °F). Cool to room temperature of 25 ± 5 °C (77 ± 9 °F).
3.2. The RAP samples may be placed in a refrigerator, at the option of the lab, to a
temperature of 4 oC (40 oF), or greater.
3.3. These samples will be used to prepare all of the ignition specimens for each oven,
as required in Section 3.4. Each aggregate sample shall be of an appropriate size so that,
when the design asphalt binder content is added, it conforms to the requirements of
AASHTO T 308 for the nominal maximum aggregate size of the mix.
3.4. Four (4) oven-dried aggregate samples shall be prepared for each ignition oven to
be calibrated, plus one Job Mix Formula (JMF) gradation sample for the butter batch.
Each of these specimens shall be prepared and documented as follows:
3.5.
The weights required for each material used in the calibration samples shall be
determined as shown in the following example. For each sample, the total weight of
asphalt binder to be used is determined by the following procedure: If RAP is used,
subtract the RAP weight from the total aggregate weight required, and provide the
appropriate amount of additional virgin asphalt to bring the total asphalt up to the design
content.
EXAMPLE:
Design asphalt content = 5.6%
Design RAP Percentage = 25%
Percent Oil in the RAP = 4%
Total weight of aggregate (including RAP) + lime = 2000.0 grams
Total weight of virgin aggregate + lime = 2000 x .75 = 1500.0 grams
Total weight of RAP required = 2000 x 0.25 = 500.0 grams
(To be added proportionately for each sieve from the stockpiled
RAP gradation)
Total weight of oil in RAP = 500 x 0.04 = 20.0 grams
Total weight of Sample = {[2000.0/(100 – 5.6)] x 100} = 2118.6 grams
Total weight of asphalt = 2118.6 – 2000.0 = 118.6 grams
Check #1: (118.6/2118.6) x 100 = 5.6%
Weight of virgin oil required = 118.6 – 20.0 = 98.6 grams
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Check #2: (1500.0 + 500.0 + 20.0 + 98.6) = 2118.6 grams
3.6. The latest version of the Excel Binder Ignition Calibration Excel Spreadsheet
provided by the Department shall be used to build and document all binder ignition
samples for use on NMDOT projects.
3.7. Enter the target gradation of the Job Mix Formula shown on the approved HMA
Mix Design to the nearest 0.1%.
3.8. For mixes containing RAP;
3.8.1. Enter the existing gradation of the in-situ RAP as it exists in the stockpile being
used in the column designated “Raw Rap Gradation”. DO NOT enter the
unbound RAP aggregate gradation in this column;
3.8.2. Enter the unbound, extracted RAP aggregate gradation in the column designated
“Extracted RAP Aggregate Gradation”.
3.9. The target weight of virgin aggregate required on each screen shall be determined
from the target gradation entered from the HMA mix design. If RAP is used, the RAP
contribution will be subtracted from the total on each screen. The actual weight of
virgin aggregate on each screen shall be weighed and recorded to the nearest 0.1 gram.
3.10. The required weights on each screen for the raw RAP will be also be shown. The
actual RAP weight on each screen will be weighed and entered to the nearest 0.1 gram
under the appropriate columns of the spreadsheet.
3.11. If the difference between the heaviest recorded weight and the lightest recorded
weight for all of the samples in a set exceeds two (2.0) grams, individual samples shall
be discarded and replaced with newly fabricated samples for whom the difference is less
than two (2.0) grams.
3.12. The weight of the virgin aggregate and for the raw RAP on each sieve shall be
within +/- 1.0 % of the mix design target weights. If the weights are not within +/- 1.0 %
of the target weights, new calibration samples shall be prepared.
3.13. The actual weight of virgin binder added shall be within 0.1% of the target weight,
and shall be recorded on the Department spreadsheet.
3.14. The target weight of hydrated lime or Anhydrite Material shall be weighed and
recorded to the nearest 0.1 gram on the spreadsheet. The method used to combine the
hydrated lime into the aggregate shall be the same as that used during the mix design
process.
3.15. Prior to mixing any ignition specimens, all bowls, sample pans, and mixing tools
shall be preheated to the laboratory mixing temperature prescribed in the mix design. At
the time mixing of the specimens begins, the temperature of the asphalt binder and
aggregate shall be at the mixing temperature specified by the asphalt mix design ± 5 °C
(9 °F).
3.15.1. Using one of the prepared aggregate samples, mix an initial batch at the design
asphalt binder content for the purpose of "buttering" the mixing bowl. This batch
shall be emptied after mixing and the sides of the bowl and the stirrers shall be
cleaned of mixture residue by scraping with a small limber spatula but shall not be
wiped with a cloth or washed with solvent.
3.16. Using the remaining aggregate samples prepare the ignition specimens at the
design asphalt binder content. The virgin asphalt binder grade and source shall be the
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same as will be used in the asphalt mixture to be tested. The percent asphalt binder
content is based on the weight of total mix.
3.17. For specimens to be delivered to another laboratory, allow the mixture to cool and
then package taking care to avoid losing any material.
3.18. As detailed above, ensure that all of the following information has been recorded
on the Department Spreadsheet, and furnished electronically to the Project Manager, to
the District Laboratory Supervisor and the Contractor for each oven to be calibrated.
a.
b.
c.
d.
e.
f.
g.
h.
i.
The aggregate gradation from the approved mix design;
The total aggregate weight on each screen;
The actual weight of virgin aggregate for each burn specimen prepared;
The gradation of RAP from the approved mix design;
The gradation of the raw RAP, as sampled from the stockpile;
The actual weight of raw RAP from each sieve for each burn sample prepared;
The design asphalt content for the ignition specimens,
The actual weight of virgin asphalt;
The date the specimens were fabricated.
Note 2: Information entered into the Department provided Excel spreadsheet will flag
if any of the calibration samples do not comply with this procedure. The electronic
copy of the completed Excel Spreadsheet verifying compliance with these procedures
(no flags) must be sent to each laboratory that will be testing the designated HMA mix
design, along with the appropriately identified sets of calibration specimens. If any
flags are shown, all of the samples will be rejected and discarded and new samples
will be prepared and submitted by the fabricator.
4. ASPHALT CALIBRATION FACTOR PROCEDURE
4.1. Check to insure that the accuracy of the oven temperature setting has been verified
within the last 4 months.
4.2. Verify and record the as-received cold weight of each calibration specimen. The
spreadsheet will flag if the as-received weight differs from that provided by the
fabricator of the specimens by more than four (4.0) grams. When this happens, the
complete set of specimens shall be discarded and replaced.
Note: If contingency calibration specimens were not prepared from the same
materials at the same time as the rest of the samples, then all calibration samples for
all ovens must be discarded and new samples prepared for each oven to be
calibrated.
4.3. Record the initial weight of each of the burn specimens at a temperature of 149 oC
(300 oF).
4.4. Burn all four (4) specimens. Perform all burns at a temperature of 538 +/- 5 o C
(1000 +/- 9 oF).
4.5. If using a properly documented NCAT Type A oven, the internal scale can be used
to confirm that the final equilibrium weight has been achieved;
4.6. Weigh and record the final weight of each burned specimen at 149 oC (300 oF).
These weights shall then be entered into the Excel Spreadsheet for the appropriate set of
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samples.
4.7. The spreadsheet will subtract the asphalt binder content recorded for each of the
four (4) specimens from the burned asphalt binder content. The asphalt calibration factor
is the average of the four (4) resultant values recorded to the nearest 0.01 percent.
% ACDIFF = %ACMEASURED -%ACDESIGN
Asphalt Correction CF = %ACDIFF1 + %ACDIFF2 + %ACDIFF3 + %ACDIFF4
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Note 3: The ignition oven temperature used for testing hot mix asphalt samples on
project shall be the same as that used for determining the asphalt calibration factor.
4.8.
Normally, the measured asphalt content of the ignition specimens will be greater
than the reported actual asphalt content. The resulting asphalt correction factor is
subtracted from the ignition oven asphalt content determination when performing
subsequent field testing on plant produced mixture. Due to the effects of noncombustible additives to the asphalt, it is possible that the indicated measured asphalt
content could be slightly less than the burned actual asphalt content, and will generate a
negative correction factor. The correction factor determined from this procedure shall be
used, regardless of whether it is positive or negative.
4.9.
After entering all of the results from the calibration burns, submit the completed
Excel Spreadsheet to the District Laboratory Supervisor for incorporation into the
project Binder Ignition Calibration Summary.
4.10. If, in the opinion of the District Laboratory Supervisor, the difference between all
of the asphalt correction factors from each of the ovens being calibrated are not
adequately consistent, new samples shall be prepared and burned. Results from the
original calibrations samples shall be discarded.
4.11. If there are any discrepancies or problems with any set of burn specimens, all
results for the calibration set in question must be discarded and a complete new set
burned. If a contingency set of specimens (fabricated from the same materials and at the
same time as the original burn specimens) is not available, then all specimen results from
all ovens must be discarded and new specimens built for each oven to be calibrated.
5. AGGREGATE CALIBRATION FACTOR PROCEDURE
5.1.
Use the gradation results from the samples that were burned for the Asphalt
Correction Factor determination to establish the Aggregate Correction Factor.
Note 4: Theoretically, only the organic material (asphalt binder) will actually be
lost during the ignition process. The weight of the aggregate on each sieve may
change, but the actual material should not be lost. It will show up on a smaller
sieve, or in the minus 200 material. Therefore it is not necessary to burn an
independent set of samples for this determination.
5.2.
Perform a wash/gradation on each of the burned samples in accordance with
AASHTO T11 and T27, as modified by TTCP.
5.3.
Determine the percent passing for each of the designated sieves, and enter into the
excel spreadsheet.
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5.4.
The spreadsheet will determine the difference between the target percent passing
for each sieve and the average percent passing for the same sieve. It will also check if
the difference for any sieve exceeds:
5.4.1. 5% for any material retained on or above the #8 sieve;
5.4.2. 3% for any material passing the #8 sieve, but retained on or above the
#200 sieve;
5.4.3. 0.5% for material passing the #200 sieve.
5.5.
If the difference on any +200 sieve meets or exceeds the differences shown in
5.5.1 or 5.5.2 above, the correction factors for all of the sieves greater than the #200
sieve will automatically be determined. The correction factor for each sieve shall be
determined by subtracting the average after-burn weight from the target weight for each
sieve. The average difference will be used as the correction factor for that sieve. If no
differences exceed the percentages shown in 5.5.1 or 5.5.2, no correction factor shall be
used on any sieve.
5.6.
If the difference between the target minus 200 sieve and the after-burn minus
#200 sieve exceeds 0.5% determine the correction factor for the minus #200 sieve in the
same manner as described in 5.6, above. If the difference does not exceed 0.5%, then no
correction factor shall be used.
5.7.
If the differences are sufficient to require the use of the aggregate correction
factors for any oven on the project, then the aggregate correction factors determined for
each oven will be used for all ovens on the subject project, even though they may not
have been large enough to require their use for an individual oven. This procedure will
be used for both the plus 200 material and for the minus 200 material.
6. RECALIBRATION
6.1.
The calibration factors shall be verified or new calibration factors shall be
established when any of the following conditions occur:
6.1.1.
6.1.2.
6.1.3.
6.1.4.
6.1.5.
6.1.6.
The source of aggregate is changed,
The source or grade of asphalt binder is changed,
Hydrated lime is added to or deleted from the JMF,
A new JMF is required,
The oven location or venting set-up is changed,
The Project Manager or the District Laboratory Supervisor has reason to
suspect a material change has occurred that may affect the calibration
factors.
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