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  • Cross and Pokhrel 1

    EVALUATION OF VACUUM DRYING FOR DETERMINATION OF BULK SPECIFIC GRAVITY

    OF HMA SAMPLES

    6784 Equivalent Words

    by:

    Stephen A. Cross, Ph.D., P.E. Professor

    School of Civil and Environmental Engineering Oklahoma State University Stillwater, Oklahoma 74078

    Tel: (405) 744-7200 Fax: (405) 744-7554

    e-mail: steve.cross@okstate.edu

    and

    Gyanendra Pokrel Graduate Research Assistant

    School of Civil and Environmental Engineering Oklahoma State University Stillwater, Oklahoma 74078

    Tel: (405) 744-7200 Fax: (405) 744-7554

    e-mail: gyanendra120@hotmail.com

    Paper submitted for presentation and publication at the 88th Annual Meeting of the Transportation Research Board, January 11-15, 2009, Washington, D.C.

    TRB 2009 Annual Meeting CD-ROM Paper revised from original submittal.

  • Cross and Pokhrel 2

    EVALUATION OF VACUUM DRYING FOR DETERMINATION OF

    BULK SPECIFIC GRAVITY OF HMA SAMPLES

    ABSTRACT

    Stephen A. Cross and Gyanendra Pokhrel

    Text 3840 words Abstract 194 words 2 Figures 500 words

    9 tables 2250 words Total 6784 words

    Determination of bulk specific gravity (Gmb) of bituminous paving mixtures is an important part of the Superpave mix design system and construction quality control/ quality assurance programs. Two problems commonly associated with AASHTO T 166 are the time it takes to dry test specimens and potential damage to the sample caused by drying at elevated temperatures. A vacuum drying technique has been developed to overcome these shortcomings. Field core samples and laboratory compacted samples of dense graded HMA and SMA were obtained for testing. The objectives of the study were to determine if dry mass and resultant Gmb of laboratory compacted and pavement core samples determined using vacuum drying procedures produces statistically similar results to AASHTO T 166 procedures. A second objective was to compare the dry mass obtained from AASHTO T 166 Method A to that of Method C. Comparisons of the means were made using ANOVA techniques along with paired t-testing. The practical significance of any statistically different results was also evaluated by using the precision statement of AASHTO T 166. Results indicated that vacuum drying is an acceptable procedure for determining dry mass and bulk specific gravity of HMA samples.

    TRB 2009 Annual Meeting CD-ROM Paper revised from original submittal.

  • Cross and Pokhrel 3

    EVALUATION OF VACUUM DRYING FOR DETERMINATION OF BULK SPECIFIC GRAVITY OF HMA SAMPLES

    INTRODUCTION Determination of bulk specific gravity (Gmb) of bituminous paving mixtures is an important part of the Superpave mix design system and construction quality control/ quality assurance programs. Two problems commonly associated with AASHTO T 166 are the time it takes to dry test specimens and potential damage to the sample caused by drying at elevated temperatures. According to InstroTek (1), the CoreDryTM apparatus (figure 1) was introduced to overcome these problems. The CoreDryTM system uses high vacuum in conjunction with a thermoelectric cold trap to draw moisture out of a specimen, evaporate the moisture, and subsequently condense the moisture in a separate chamber. As such, the specimen remains at or near room temperature, which helps to retain the HMA characteristics that can be altered due to prolonged exposure to heat and oxidation potential present in forced-draft ovens. The CoreDry apparatus is also reported to provide a constant mass in less time than traditional oven-drying techniques (1).

    FIGURE 1 CoreDryTM vacuum drying device. The CoreDryTM is already accepted by ASTM and the test procedure is available as ASTM D 7227-06 Rapid Drying of Compacted Asphalt Specimens Using Vacuum Drying Apparatus (2). However, there was little peer reviewed literature found verifying that dry mass and resultant Gmb determined from CoreDryTM is the same as that determined using AASHTO T 166 procedures.

    TRB 2009 Annual Meeting CD-ROM Paper revised from original submittal.

  • Cross and Pokhrel 4

    Hall (3) investigated the ability of the CoreDryTM vacuum drying system to provide consistent and accurate estimates of constant mass for compacted HMA specimens. Hall concluded that the CoreDryTM vacuum drying system consistently provides a reasonable estimate of constant mass for specimens with degrees of saturation ranging up to fully saturated. Retzer (4) evaluated how well CoreDryTM dry mass compared to conventional oven drying procedures using 100 mm and 150 mm diameter field cut cores. Cores were first tested in CoreDryTM and then oven dried at 110 5C (230 9oF) (AASHTO T166, Method C) to a constant mass. Initial results indicated that the average difference in density was 0.17 % and never exceeded 0.60 %. Similarly, the difference in bulk specific gravity determined from the two dry masses averaged 0.004 and never exceeded 0.016. Retzer (4) reported possible bias in his results due to testing order. To overcome bias, 12 more samples with a 90 second vacuum saturation were tested. This time, comparisons of specimen weight before saturation, after saturation, after drying with CoreDryTM and after oven drying were made. The average efficiency of CoreDryTM in removing water from vacuum saturated samples was found to be 86.9% whereas the average efficiency was found to be 91.2 % for oven drying. The authors final conclusions were that CoreDryTM can be accepted as an alternative to oven drying methods of AASHTO T 166. OBJECTIVES The objectives of this study were to determine if dry mass and resultant Gmb of laboratory compacted and pavement core samples determined using the CoreDryTM apparatus produces statistically similar results to AASHTO T 166 procedures. This was accomplished by comparing dry mass and Gmb of laboratory compacted and field core samples using the CoreDryTM apparatus to the dry mass and Gmb obtained by Methods A and C of AASHTO T 166. For laboratory molded samples, one extra dry mass measurement was obtained and used in the analysis, initial dry mass after cooling to room temperature. A second objective was to compare the dry mass obtained from AASHTO T 166 Method A to that of Method C. Comparisons of the means were made using ANOVA techniques along with paired t-testing. The practical significance of any statistically different results was also evaluated by using the precision statement of AASHTO T 166 (5). MATERIALS Laboratory Compacted Samples For laboratory compacted samples, three mixes were used; an ODOT S-3 mix, an ODOT S-4 mix and an ODOT Stone Matrix Asphalt (SMA) mix. ODOT S-3 and S-4 mixes are fine graded mixes with nominal maximum aggregate size of 19.0 mm (3/4 inches) and 12.5 mm (1/2 inches), respectively. The S-3 and S-4 mixture were made with PG 64-22 asphalt. For the laboratory compacted S-3 and S-4 mixes, two different sample heights, 95 5 mm and 115 5 mm were prepared. The 95 5 mm and 115 5 mm high samples were compacted at 7 0.5% and 4 0.5% VTM, respectively. Eight to nine samples were made for each mix at each height, for a total of 33 samples. Sample heights and void contents were selected to match typical laboratory compacted samples for mix design, moisture damage testing (AASHTO T 283) and rut testing.

    TRB 2009 Annual Meeting CD-ROM Paper revised from original submittal.

  • Cross and Pokhrel 5

    Twenty-five field produced SMA samples were obtained from a contractors laboratory for an Interstate paving project. The sample heights ranged from 116 122 mm. Void contents varied.

    Field Cores Field cores of HMA were provided from various projects by a commercial testing laboratory and two contractor labs. There were two different sizes of cores provided, 100 mm (4 inches) and 150 mm (6 inches) nominal diameters, respectively. The cores provided by the commercial testing laboratory were from old projects that were slated for disposal. Therefore, not all of these cores could be identified by project or mix. TEST PLAN Test Procedures CoreDry All CoreDry testing was performed in accordance with the manufacturers recommendations (1) and the procedures of ASTM D 7227 (2). The Gmb was determined using dry mass determined from CoreDry and the appropriate volume determined from either AASHTO T 166 or AASHTO TP 69 (ASTM D 6752). AASHTO T 166 The Gmb of each sample was determined in accordance with AASHTO T 166-05 Method A and Method C (5). There are two Gmb procedures under Method A and one under Method C. The procedures are based on how dry mass of the sample is determined. Method B uses a volumeter and does not appear to be that popular among DOTs. AASHTO T 166 is not recommended for samples where the water absorption exceeds 2 percent, by volume. Method A Note 1 of Method A (5) defines constant mass as the mass at which further drying at 52 3C (125 5F) does not alter the mass by more than 0.05 %. Samples partially saturated with water are dried overnight at 52 3C (125 5F) and then weighed at two-hour drying intervals until the mass loss is less than 0.05 %. Dry mass determined in this manner was used with the submerged mass and saturated surface dry mass (SSD) to determine Gmb and was designated as Method A dry mass or Gmb. Note 1 of Method A (5) states that recently molded lab samples, which have not been exposed to moisture, do not require drying. Note 2 of Method A (5) states that the sequence of testing may be changed to expedite testing. For lab molded samples, the dry mass is usually recorded first. The dry mass and Gmb determined in this manner was designated Initial. Method C Method C of AASHTO T 166 (5) outlines procedures for determining dry mass and Gmb of laboratory compacted and field core samples which h