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Expert Task Group on Asphalt Mixtures & Construction; Asphalt Binders; and Models Technology April 2001 Attachment J Standard Practice for The Evaluation of Different Superpave Gyratory Compactors (SGC's) Used in the Design and the Field Management of Superpave Mixtures AASHTO Designation: TP XX-01(1) (DRAFT - April 10, 2001)

1. Scope

1.1 This method covers the procedure for evaluation of different SGC's used in the design and the field management of Superpave mixtures. SGC's shall satisfy AASHTO PP35 and shall be operated according to AASHTO TP4. Evaluation of SGC's should include the SGC used for mix design evaluated with the SGC used for production quality control (QC) and the SGC used for production quality acceptance (QA). The evaluation will assist in the identification of within procedure differences that may impact the field management of asphalt mixes. If differences are attributed to mechanical differences in SGC's, a supplemental offset procedure is provided.

1.2 This practice may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of the user of this procedure to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use.

2 Referenced Documents

2.1 AASHTO Standards (Note: Update to reflect current AASHTO designations)

MP1Specification for Performance-Graded Asphalt Binder
MP2Specification for Superpave Volumetric Mix Design
TP4Method for Preparing and Determining the Density of Hot Mix Asphalt (HMA) Specimens by Means of the SHRP Gyratory Compactor
PP28Practice for Designing SUPERPAVETM of Hot Mix Asphalt (HMA)
T166Bulk Specific Gravity of Compacted Bituminous Mixtures (Method A)
T209Maximum Specific Gravity of Bituminous Paving Mixtures
PP35Practice for Evaluation of Superpave Gyratory Compactors (SGCs)

3. Summary of Method

3.1 This method is intended to provide a uniform process to assist in the identification of within procedure differences that may impact the field management of asphalt mixes.

3.1.1 If differences are attributed to mechanical differences in SGC's, a supplemental offset procedure is provided.

3.2 The design, QC, and/or QA SGC's shall be evaluated. All SGC's shall satisfy AASHTO PP35 and shall be operated according to AASHTO TP4.

3.3 Laboratory prepared or production mix shall be utilized in the evaluation.

3.4 Documentation of within procedure differences and assessment of compacted specimens shall be utilized in the evaluation.

3.5 The evaluation shall be conducted in two phases. The initial evaluation shall use multiple operators and existing within procedure handling practices. The second phase, if required, shall utilize a single operator and consistent, within procedure handling practices. One laboratory shall be used in the determination of the compacted bulk specific gravities of the gyratory specimens.

4. Significance of Use

4.1 SGC's fabricated according to AASHTO TP4 and satisfying AASHTO PP35 create cylindrical specimens from loose, hot mix asphalt (HMA) through a gyratory (kneading) effort. AASHTO PP35 ensures that different manufacturer models of SGC's are statistically comparable based upon a single laboratory, single operator evaluation. However, within procedure differences may impact the comparability of SGC's. In addition, variability within the manufacturing process may result in mechanical differences in SGC performance.

5. Responsibilities Specific to the Standard

The laboratories used in the evaluation are identified in the following sections.

5.1 Mix Design Laboratory - Based upon the contract document, the Superpave mix design can be conducted by either the specifying agency, contractor, or private consultant.

5.2 Field Quality Control Laboratory - QC testing is performed by the contractor to ensure the quality of the production process. QC results are not used in the acceptance of production mixes.

5.3 Field Quality Acceptance Laboratory - QA testing is required by the specifying agency for the acceptance of production mixes. Based upon the contract document, QA testing can be conducted by either the specifying agency, contractor, or private consultant

5.4 Independent Quality Assurance Laboratory (IQAL) - Based upon the contract document, a IQAL may be employed to arbitrate differences in contractor and specifying agency results. Typically the IQAL is a private consultant or a different specifying agency laboratory.

5.5 The actual operator responsible for each laboratory shall be used in the initial phase of the evaluation.

6. Procedure: Phase I - Initial Evaluation

6.1 The specifying agency shall identify the laboratories to be included in the evaluation.

6.2 Prior to compaction of any specimens, each operator shall verify the SGC calibration according to AASHTO TP4.

6.3 Evaluation mix shall conform to one of the following sections.

6.3.1 Laboratory Prepared Mix: A mix similar to the anticipated production mix should be used. The mix should use the same asphalt binder anticipated for production. One laboratory shall prepare sufficient mix for four (4) SGC specimens per each evaluation SGC compacted to 115 mm in height and 150 mm in diameter.

6.3.1.1 The mixing temperature shall be according to AASHTO PP28.

6.3.1.2 The laboratory prepared mix shall be short-term aged according to AASHTO PP2. The alternative 2-hour procedure should be used if it was employed in the original mix design.

6.3.2 Plant Produced Mix: A mix similar to the anticipated production mix should be used. The mix should use the same asphalt binder anticipated for production. One sample shall be taken from a production haul vehicle, according to AASHTO 168, of sufficient size to fabricate four (4) SGC specimens per each evaluation SGC compacted to 115 mm in height and 150 mm in diameter.

6.3.2.1 The production mixing temperature shall be recorded and compared to the recommended mix temperature specified in AASHTO PP28.

6.3.2.2 No short-term aging is required of plant produced mix.

6.3.2.3 Logistics of the SGC's may result in the cooling of mix samples prior to compaction. The cooling and reheating of asphalt mixes can affect the measured volumetrics. Reheating of mix is not part of mix design, as specified in AASHTO PP28. The laboratory prepared or production mix shall be split and provided to each laboratory. Any differences in handling shall be recorded. If possible an observer, from either the specifying agency or contractor, should assist in recording any within procedure differences.

6.3.2.4 The mix shall be brought up to the required compaction temperature specified in AASHTO PP28. This can be performed in either the SGC mold or in a pan. The actual method employed by each laboratory and operator shall be recorded. In addition, the make and model of the oven used shall be recorded.

6.3.2.5 Four (4) SGC specimens shall be compacted in each SGC according to AASHTO TP4 to the design number of gyrations (Ndes) anticipated for the production mix. Each individual operator shall perform the compaction and bulk specific gravity (Gmb) of the specimens.

7. Reporting

7.1 Phase 1: Initial Evaluation - The results of each laboratory shall be compiled as follows in Table 1.

7.2 The sample average (symbol for sample average, x with a line over it) and sample standard deviation (s) shall be calculated based on the four (4) compacted specimens' bulk specific gravity and SGC height data according to equations 7.3 and 7.4.

7.3 Sample average,

Equation Sample Average = (sum of x sub i from i = 1 to n) divided by n

Where n is the number of specimens (4). Report the average to three (3) significant digits.

7.4 Sample standard deviation,

Equation: sample standard deviation = ((sum of (sample average minus x sub i) squared from i = 1 to n) divided by (n -1)) raised to the 0.5 power

Report the standard deviation to four (4) significant digits.

7.5 The sample standard deviations for the various compaction levels for each laboratory should be within the typical values indicated in Table 2.

Table 2. Typical Standard Deviations for Bulk Specific Gravities of Specimens Compacted in the SGC.
SGC Compaction LevelTypical Standard Deviations
Nini0.008
Ndes0.006

Note 1: Values for tables 2 & 3 are based on four (4) specimens.

Note 2: Values are based upon an analysis of 20 production mixes from HMA plants located throughout the United States. Data was conducted as part of FHWA Demonstration Project No. 90, "Superpave Asphalt Mix Design & Field Management." Ninety-eight percent (98%) of the production data analyzed is within the above typical standard deviations, + 2s.

7.6 The absolute difference of the averages () between of any two laboratories should be within the typical values indicated in Table 3.


Table 3. Typical Absolute Difference of the Averages between Two Laboratories
SGC Compaction LevelTypical Absolute Differences
Nini0.022
Ndes0.015

Table 1. SGC Evaluation Form
Laboratory 
Operator's Name 
SGC Make / ModelMake:Model:Meets PP35 Y(_) N(_)
Oven Make / ModelMake:Model:
Mix Designation 
Mix Type
(_) Lab or (_) Production
 
Short-Term Aging(_) 4-hours       (_) 2-hours       (_) None
Mixing Temperature, °CSpecified (range):Actual:
Compaction Temp., °CSpecified:Actual:
Heating Method(_) In Mold       (_) In Pan

Comments:

 

 


SGC CompactionBulk Specific Gravity
ABCDAve. (x with a line over it)St.Dv. (s)
Nini = ____

Ndes= ____

      
Recorded Observations:

 

 

 

 


8. Procedure: Phase II - Single Operator Evaluation

8.1 If one or more of the SGC's evaluated is not providing results within the typical ranges as indicated in Tables 2 and 3, the second phase of the evaluation should be employed.

8.2 A meeting between the laboratory operators will be held to identify and discuss within procedure differences - see section 9 for discussion topics. The specifying agency should establish a uniform procedure to address the identified differences. This should be based on the consensus of the group.

8.3 Repeat section 6 with the following exceptions:

8.4 A single operator shall be used throughout the evaluation.

8.5 The laboratory prepared mix or production mix shall be mixed or sampled, split, and allowed to cool to ambient temperature for a minimum of 12 hours. The split samples shall be uniformly handled and reheated to the specified compaction temperature.

8.6 Table 1 shall be used to summarize the evaluation data.

8.7 A statistical evaluation shall be conducted using the typical values provided in Tables 2 and 3.

8.8 If one or more of the SGC's evaluated still does not provide results within the typical ranges, the single operator should verify the calibration of the SGC's or contact manufacturer for service.

9. Discussion Topics

9.1 Within procedure differences that may impact the field management of asphalt mixes. The following provides topics for discussion in identifying potential differences. This is by no means a complete list.

9.1.1 SGC Calibration - Most versions of the SGC manufactured today allow the use of both 100 mm and 150 mm molds. During the calibration process it is not always apparent which setting is in use. Verify each compactor is set to 150 mm per AASHTO TP4. A setting of 100 mm will result in a lower then specified consolidation pressure, which will in turn result in lower densities.

9.1.2 Sample Segregation - If a compactor is not providing results within the typical standard deviations, a comparison of each specimen's gradation should be performed. Extraction of the aggregate can be accomplished through either solvent or ignition method. If segregation has occurred, a review of splitting techniques should occur. In addition, the technician may be employing a 4-inch funnel, typically used for Marshall and Hveem, to charge the molds. This can also cause within specimen segregation.

9.1.3 Method of Heating - The method of heating specimens may affect the densities. Heating in the pan may age the mix more than heating in the mold. This can result in lower densities. Also, the method of transfer from the pan to the mold may result in cooling of the mix, which in turn can result in lower densities.

9.1.4 Use of One Mold - The use of one mold may affect the compaction effort. If the mold is not reheated between specimens, the mold may cool which can result in lower densities.

9.1.5 Rodding of the Sample - Operators familiar with Marshall may rod the specimen prior to compaction. This is not part of AASHTO TP 4, and should not be performed.

9.1.6 Forced-Draft Oven - The oven set temperature should not be significantly above the compaction temperature. Over heated molds and mix may result in excessive aging of the mix. Which may result in lower densities. Also, the level of forced-draft-ness may be discussed.

9.1.7 Reheating - Logistics may require some of the compacted specimens to be reheated. Reheated may result in higher absorption of the asphalt binder. The lower effective asphalt content can result in lower densities.

9.1.8 Different Mix - The evaluators may wish to include a different mix in the evaluation, ex. a coarse versus a fine mix.

9.2 SGC Calibration - Since the SGC is a relatively new piece of equipment, there may be unforeseen issues with calibration. If after the second phase of evaluation a given SGC is still not comparing to the other unit, the manufacturer should be consulted to verify the calibration.

1 Recommended by the TRB Mixture/Aggregate ETG in April of 2001.

 
Updated: 04/07/2011
 

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