CC1 Material Characterization - Flexible

A considerable number of measurements of the physical properties of the NAPTF test pavements were made at all stages of construction and after the construction was completed. The measurements were made for three purposes: construction QC, construction acceptance, and material characterization. Tests were conducted on the subgrade materials, base, subbase and surface layers.

Properties of all of the materials used in each of the test item component layers were measured before, during, and after construction and stored in a database available for download or direct access on the FAA Airport Pavement Technology website. The database is an historical record of all of the testing conducted on the pavement materials and contains information about material properties of the component layers from quality control (QC), acceptance and material characterization tests. The QC tests were conducted primarily to ensure uniformity of quality and compliance with design parameters.

Subgrade Testing

CBR – California Bearing Ratio

A material known as County Sand and Stone Clay (CS&SC) was used for the low strength subgrade with the target CBR of 4. A material known as DuPont Clay was used for the medium strength subgrade with the target CBR of 8. For the high strength subgrade, the locally available sand was used with a target CBR of 20. Designers were forced with the problem of not merely obtaining a minimum subgrade strength for each type of material but also of controlling the CBR for each material within a relatively small range. This was achieved by controlling the water content of the material. 

Initial CBR and Soil Classification for Each Subgrade

Prior to the initiation of full scale testing, about 6 months after the initial construction, test pits were opened to a depth of 4 ft. to 5 ft. below the surface of the flexible pavements on stabilized base. CBR and other tests were performed at several depth of the subgrade. The CBR results are summarized in the following table.

Test Pit CBR Results (McQueen 2000)

Resilient Modulus

After the completion of the subgrade, resilient modulus tests (ASTM D1587) were conducted on low and medium strength subgrades using extracted Shelby thin-wall tube samples.  The subgrade resilient modulus values varied from approximately 2,600 to 7,500 psi (14 to 52 MPa) depending on confining pressure and deviator stress for low strength soils and between 5,000 to 12,500 psi (34 to 86 MPa) for medium strength soils.

P-154 Subbase and P-209 Base Material

The base and subbase courses were composed of granular materials constructed on the finished, prepared subgrade in accordance with specifications detailed in Advisory Circular 150/5370-10A for Item P-209 and Item P-154 respectively. The P-209 base material used at the NAPTF were partial blend milestone materials and Maryland materials. The P-154 subbase material was a uniformly graded, manufactured argillite screenings product called Grey Quarry Blend Fines. Table 5 and Table 6 summarize the specified gradation requirements for P-154 and P-209 and the gradation used at the NAPTF. For both P-154 and P-209, the material passing the No. 40 sieve was required to have a liquid limit and plasticity index of maximum 25 and 6, respectively. For P-154, the liquid limit and plasticity index were 16 and 3.1, respectively, therefore the requirements were met.

P-154 Gradation Data and Requirements for Argillite Screenings Subbase

P-209 Gradation Data and Requirements for Crushed Aggregate Base

QC testing and inspection was conducted to ensure uniformity and quality of the subbase material. The QC plan for the subbase P-154 material consisted of gradations on bulk samples from the compacted lifts, moisture and density measurements on the compacted lifts, thickness measurements, grade and surface condition. The compacted subbase thicknesses were measured using rod and level survey equipment at intervals of 10 ft. in each direction.  The dry densities and moisture contents for P-209 base and P-154 subbase aggregates were determined using modified Proctor test (AASHTO T180).

Resilient modulus tests were conducted following the SHRP Protocol P-46 testing procedure.

Hot Mix Asphalt Materials

Hot Mix Asphalt (HMA) in CC1 consisted of mineral aggregate and bituminous material mixed in a central mixing plant and placed on a prepared course. The mixed material was spread, finished and compacted in accordance with specifications detailed in Advisory Circular 150/5370-10A, Item P-401. The same material was used for HMA surface as stabilized base layers. QC testing of the P-401 material during production was required to ensure that the final product met P-401 specifications. During production, aggregate gradation, binder content, mix temperature and mixture properties were monitored. QC tests for the P-401 mix included aggregate gradation, mat and joint density, asphalt content, air voids and stability. The specification for Hot Mix Asphalt (HMA) required tight control of plant produced and field placed material.

The Marshall Test properties of the P-401 mixtures from the truck were measured during production using the Asphalt Institute MS-2 method. The field densities of P-401 cores extracted from NAPTF flexible test items were measured using ASTM D 2726 procedure. Resilient modulus tests (ASTM D 4123) and fatigue tests (AASHTO TP 8-94) were conducted at the University of Illinois Advanced Transportation Engineering Laboratory (ATREL) facility. A total of seven cores were extracted from the NAPTF flexible test items for resilient modulus testing. The average laboratory measured P-401 AC resilient modulus at 77°F (25°C) was 426 ksi (2940 MPa).

The primary acceptance test results for the HMA are summarized as follows:

• Average asphalt content = 5.4%
• Average compaction = 98% of 75 blow Marshall density
• Average plant air voids = 3.6%

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