Control of Asphalt Mixtures



This IM describes the Quality Control/Quality Assurance (QC/QA) procedures for monitoring and controlling plant-produced asphalt concrete mixtures on Quality Management of Asphalt (QMA) projects.



Standard Specification 2303 Flexible Pavement

IM 204   Inspection of Construction Project Sampling & Testing

IM 205A     Securing Samples

IM 208   Materials Laboratory Qualification Program

IM 213   Technical Training and Certification Program

IM 216   Guidelines for Validating Test Results

IM 301   Aggregate Sampling & Minimum Size of Samples for Sieve Analysis

IM 302   Sieve Analysis of Aggregates

IM 319   Moisture Sensitivity Testing of Asphalt Mixtures

IM 320   Method of Sampling Compacted Asphalt Mixtures

IM 321   Method of Test for Compacted Density of Hot Mix Asphalt (HMA)(Displacement)

IM 322   Sampling Uncompacted Hot Mix Asphalt

IM 323   Method of Sampling Asphaltic Materials

IM 325   Compacting Asphalt Concrete by the Marshall Method

IM 325G    Method of Test for Determining the Density of Hot Mix Asphalt (HMA) Using the Superpave Gyratory Compactor (SGC)

IM 336   Reducing Aggregate Field Samples to Test Samples

IM 337   Method to Determine Thickness of Completed Courses of Base, Subbase & Hot Mix Asphalt

IM 338   Method of Test to Determine Asphalt Binder Content & Gradation of Hot Mix Asphalt (HMA) by the Ignition Method

IM 350   Method of Test for Determining the Maximum Specific Gravity of Hot Mix Asphalt (HMA) Mixtures

IM 357   Hot Mix Asphalt (HMA) Mix Sample for Test Specimens

IM 501   Equations and Example Calculations

IM 510   Method of Design of Hot Mix Asphalt Mixes




Appendix A contains an outline of the responsibilities required for all parties. Refer to IM 213 Appendix C for individual certification requirements.




Sample and test according to Section 2303. Only the information obtained from random samples as directed and witnessed by the Engineer and validated by comparison to one or more of the paired samples tested by the Contracting Authority will be used for specification compliance. Additional samples of aggregate and uncompacted asphalt mixture may be taken by the contractor to provide better quality control. The results of testing done on additional samples will be for informational purposes only and do not need to be reported.


All testing done by the Contractor that is used as part of the acceptance decision shall be performed in qualified labs by certified technicians. Gyratory compactors not utilized in the independent assurance testing (IM 208 Appendix C) will not be allowed on QMA projects without permission from the District Materials Engineer (DME).


Retain samples taken for acceptance purposes until the contractor’s results have been validated.



The specific ton or truckload to begin sampling will be determined by the Engineer using the spreadsheet ( The total estimated daily production is divided into equal sublots based on the number of samples determined from Table 2303.03-4.



Estimated production = 4,501 tons

Number of Samples = 5

Approximate sublot size = 4501/5 = 900 tons


When production of a bid item is expected to exceed three production days (small quantities excluded) and conditions/resources reasonably allow, test samples immediately “hot-to-hot” (without allowing the sample to cool) for at least one day at the beginning of production to aid in any future investigation of non-correlation that may arise throughout production.


Calibrate the Rice pycnometer at the beginning of a project and anytime that a correlation problem occurs.


B.  Compacted Asphalt Mixture

1.  For class I compaction, the width subjected to the random sampling shall coincide with the width eligible for PWL incentive/disincentive. This width shall be the nominal width of the travelled lane unless otherwise determined by the Engineer. Take samples from no less than 1 foot from the unconfined edge of a given pass of the placing equipment, except when the width of a single pass of the paver exceeds the width eligible for random sampling by more than 1 foot (i.e. For a 14-foot paving width on a 12-foot wide lane, a core location could randomly fall exactly 12 feet from centerline, assuming a two lane roadway. The outside 2 feet would then be deducted from the field voids lot quantity).


2.   The Engineer will provide inspection staff to direct and witness the sampling and perform Gmb measurement during a time agreed between the Engineer and the Contractor. The Engineer should make every effort to meet the Contractor’s schedule.


3.   The Engineer will transport the cores in accordance with IM 320, or secure the cores for transport by the contractor. The Engineer and Contractor will determine that cores are not damaged. The Engineer will decide if a core is damaged prior to testing.



Sample and test according to IM 204. For DSR verification tests performed at the District laboratory, if the G*/sinδ falls below 1.0, obtain a quart sample for full analysis and test all remaining 4 oz. samples until the area of noncompliance is isolated.

The Engineer may price adjust the asphalt binder for the following quality characteristics

·        G*/sinδ (un-aged)

·        Percent Recovery

·        M-value



A.  Defined

Validation is defined as the ability of two labs to achieve similar (statistically equivalent) test values on split or paired samples.


B. Aggregate Gradation Correction Factor

When comparing the cold-feed gradation to the ignition oven extracted gradation, a correction factor to adjust the extracted gradation must be determined in accordance with IM 501. Validation of the cold-feed gradation will be determined by comparing the cold-feed gradation and the corrected extracted gradation as shown on the comparison report for Cold-Feed & Ignition Oven in IM 216 Appendix A. The correction factors will be established by comparing an Agency cold-feed sample to an Agency ignition oven extracted sample. The Engineer may witness and secure a split cold-feed sample according to IM 205 Appendix A for validation in lieu of an ignition oven sample, in which case a correction factor is not needed.


C. Validation Requirements

1.   When any of the following events occur, validation has not been achieved or maintained:

a.        The difference between test results on each of two consecutive split/paired samples exceeds the IM 216 tolerance.

b.        The difference between test results on any two of three consecutive split/paired samples exceeds the IM 216 tolerance.

2.   Consecutive samples may be either validation samples tested sequentially with another lab or mix specific samples when other mixes are being tested for validation between the two labs. It may be necessary to examine validation of test results on consecutive samples of the same mix if more than one mix is being tested between the two labs. Validation problems sometimes only occur during testing of specific mix samples.

3.   When validation for a particular test has not been achieved, all results for that day are considered invalid for that test.

4.   To achieve or reestablish validation, a minimum of two consecutive test results must meet IM 216 tolerances.

5.   When noncomplying material has been removed, the test results corresponding with the material will remain in the validation decision.



A.   Investigation

When validation is not achieved or maintained, the DME will act as appropriate to resolve split/paired test result differences by choosing among the strategies below. The DME shall report the results of the investigation to the Contractor upon its conclusion. The DME may consider results from the Independent Assurance Program in the investigation. When non-validation of test results cannot be explained by an assignable cause as determined by the DME, the Engineer’s results will be used for acceptance.

1.     Retest the same sample

2.     The District labs will test additional verification samples.

3.     The DME will review the sampling and testing procedures of both labs

4.     The DME will immediately test samples sent in by the Contractor without allowing cool down and reheating (hot-to-hot testing).

5.     Both labs will test samples using comparable reheat periods.

6.     The DME will establish a correction factor based on the reheat evaluation outlined in Appendix B.

7.     Both labs will test a sample that was taken and split by the Engineer.

8.     Both labs and a third laboratory designated by the Contracting Authority will test a sample split three ways. The 3rd lab for state projects will normally be the Central Materials Lab.

9.     The DME will establish a correction factor for the Contractor’s gyratory compactor based on the procedure described in Appendix C. The correction factor for Gmb should not exceed 0.030.

10.  Verify both labs are compacting to the number of gyrations specified in the contract documents.


B.   Quality Assurance Protocol

1.   Resolution decisions by the Iowa DOT Central Materials Laboratory will be final.

2.   During the period of production when validation cannot be achieved, the Engineer’s test results will be used for acceptance of the lot. The use of the Engineer’s test values for acceptance will be retroactive to the time when the first sample exceeded the validation tolerance. Similarly, when validation is regained, the use of the Contractor’s test results for acceptance is retroactive to the first test used to reestablish validation.

a.   Over the period which validation cannot be achieved for aggregate gradation, the Engineer’s test results will be used for the entire gradation and applied to any calculations involving the gradation for the entire lot.

b.   If validation cannot be achieved between the ignition oven extracted gradation and the Contractor’s cold-feed gradation, the Agency will run cold-feed gradations for validation in place of the ignition oven.

c.   Over the period which validation cannot be achieved on uncompacted asphalt mixture tests for Gmm or Gmb, the Engineer’s test results will be used as follows:

                                          i.        For lots under the PWL acceptance plan,

The Engineer’s results and any other valid contractor’s results for the lot will be used in the calculations for field voids and lab voids.

ii.   For all other lots, the Engineer’s results will be used for any calculations involving that particular test value.

iii.   Use a maximum lot pay factor of 1.000 for lab voids and field voids when the Engineer’s results are used for any portion of the lot.



Production tolerances are listed in the Section 2303.


Investigate variations between two consecutive test results in Gmb or Gmm of more than 0.030 promptly since these tests reflect significant changes in binder content, aggregate properties and/or gradation. In some cases variations may be attributed to segregation, thoroughness of mixing, sampling procedure, and changes in aggregate production.


If the test results in a series of split/paired samples (minimum of 3 samples) are not variable and random (results are consistently higher or results are consistently lower) and the difference between each split/paired test result is greater than half of the IM 216 tolerance, the DME may establish a correction factor for the Contractor’s gyratory compactor based on the procedure described in Appendix C. The correction factor for Gmb should not exceed 0.030.



For each production sample of loose asphalt mixture the Contractor will determine, report, and plot Gmb, Gmm and Pa. Binder content measurement by an approved method will be determined, reported, and plotted daily. Gradation will be determined, reported and plotted daily. Make the inter lab correlation reports available.


Test results are to be recorded and plotted in the computer programs provided by the Iowa DOT ( The computer programs act as a tool for documenting project data and applying the specification. The specification and IMs will always govern when errors are encountered in the software. Microsoft Excel 2007 (or newer) is required (or equivalent spreadsheet software capable of reading and writing *.xlsm and *.xlsx file types). The recommended minimum system requirements include a 2.3 Ghz processor or higher with at least 2 GB of physical memory and a wireless network adapter with internet access. Copies of the electronic spreadsheet file containing the completed Daily HMA Plant Report shall be provided to the DME and the Engineer within 4 hours of beginning operations on the next working day. The Engineer may extend this time on days when longitudinal joint cores are sampled and tested. Alternatively in these cases, the Engineer may accept partially completed reports until results are available. Use electronic mail (or DocExpress®) as the method of delivery unless otherwise approved by the Engineer. Copies of computer files containing the project information shall be furnished to the Engineer on a CD or portable memory device upon project completion. An additional copy of the files shall be furnished to the DME on a CD or portable memory device.


Keep the charts current and available showing both individual sample results and moving average values for both lab voids and absolute deviation from target. Base moving average values on four consecutive sample results.



At no additional cost to the Contracting Authority, the Engineer may approve the substitution of any mix design which meets or exceeds the requirements of the original mix. Mixture substitutions shall be gyrated to the same level as the original mix requirements. Binder substitutions have an equal or better low temperature PG grade and MSCR designation.


Original Mix

ST Intermediate with a PG 58-28S


Requested Substitution

HT L-2 Surface with a PG 58-28H

The request would be approved provided the HT Surface mix is gyrated to the same level as a ST Intermediate with lab voids within the target range. The binder substitution would be approved since it meets or exceeds the low and high temperature grade and has an equal or better letter designation. The aggregate quality of a HT Surface also meets or exceeds that of a ST Intermediate. The Engineer may approve an alternate maximum aggregate size.


A polymer modified binder may be substituted into the JMF provided the original PG grade and temperature spread is met or exceeded. In this case, verify the JMF target air voids are met at the design binder content. If the original JMF required moisture susceptibility testing and has consistently demonstrated acceptable SIP values in the field, the original anti-strip agent (if needed) and dosage rate may be used in lieu of IM 319 re-evaluation. Plant produced mix will still be tested for moisture susceptibility.



The Contractor is responsible for making changes, as necessary, to achieve target values specified on the JMF. These changes can include adjusting the proportions of aggregate and asphalt binder necessary to meet the JMF. If a change in the target gradation is desired, obtain approval of a new JMF from the DME. Changes in the target gradation cannot be set outside of the control points. The Contractor may change the target binder content to maintain the required mixture characteristics, provided the appropriate documentation and reporting is performed. The Contractor may change binder sources provided the Engineer receives written notification (or e-mail) prior to the substitution. Report all changes in proportions on the Daily HMA Plant Report.


The addition of new materials to the JMF may be approved by the Engineer without evaluating mix volumetrics in the laboratory if the materials are produced from geologically comparable sources, do not constitute more than 15 % of the total aggregate, meet quality requirements, and produce mixes that meet design criteria. When aggregates are introduced from sources that are not geologically comparable or otherwise differ significantly, complete laboratory mix design testing and approval is required.


When a stockpile of recycled asphalt materials (RAM) constitutes less than 15% of the JMF, it may be substituted by another source of equivalent classification and quality (Classified or Unclassified) to finish the project. In this case, update the JMF by entering the new RAM binder content, specific gravity, gradation, and absorption into SHADES. Verify the volumetrics remain compliant with the specifications by testing a lab compacted sample.


Moving averages and the gyratory compaction slope assist in identifying potential problems before they arise. Watch the trends in the moving averages (approaching a specification limit) and the slope of the compaction curve. The slope of the compaction curve of plant-produced material shall be monitored and variations in excess of ± 0.40 of the mixture design gyratory compaction curve slope may indicate potential problems with uniformity of the mixture.