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68 Current standard test methods for determining the spe- cific gravity and water absorption of coarse and fine aggre- gates are AASHTO T 85 (or ASTM C127) and AASHTO T 84 (or ASTM C128), respectively. There are also several modified and new test methods for determining these properties; these range from simple modifications to procedures for deter- mining the SSD state in the standard test methods to new test methods with more complex, costly devices. These test meth- ods were reviewed, and comparisons were provided in terms of precision, ruggedness of equipment, ease of use, soaking and testing time, equipment cost, and potential problems or problematic materials. Based on the review results, 10 test methods were selected for further evaluation in this study. The selected test methods were evaluated through a labo- ratory testing program. The laboratory program was originally planned in two experiments, but it was later expanded into five experiments. Experiment 1 was a preliminary evaluation to compare results and variability of the selected test methods. Experiment 2 was conducted to further evaluate the test methods selected at the conclusion of Experiment 1 with a broader range of aggregate materials. At the conclusion of Experiment 1, six test methods were selected for further evaluation in Experiment 2. The other test methods were not further evaluated in this study because of additional costs of the CoreLok equipment and bags, the repeatability of SSDetect not being improved by removing the P200 fraction, and the poor precision of the Phunque method for testing a complete gradation. Table 6-1 provides a comparison of the six test methods evaluated in Experiment 2. Of the two test methods evaluated for use with coarse aggregate, AASHTO T 85 provided better precision. For the four methods for testing fine aggregate, the SSDetect and Modified AASHTO T 84 methods provided a more reasonable balance of accuracy and precision than the other test methods. Based on the findings of Experiment 2, it was recommended that AASHTO T 85 be used for measuring specific gravity and absorption of coarse aggregate and the Modified AASHTO T 84 and SSDetect methods be used for measuring specific gravity and absorption of fine aggregate. Among the three test methods, SSDetect is the only one that can be completed within one working day. Ruggedness and round robin studies for SSDetect were completed under a previous study. However, the AASHTO T 85 and Modified AASHTO T 84 (removal of P200) can take up to three work- ing days to complete, which hinders the use of these tests for quality assurance (QA) purposes where results are desired as rapidly as possible. In addition, since the P200 portion of fine aggregate is not tested in the Modified AASHTO T 84, it may be assumed to have the same specific gravity as the other fine aggregate portion, or it can be tested according to ASTM C110 or AASHTO T 133 (with ethyl alcohol). After reviewing the results of Experiments 1 and 2, Experiments 3, 4, and 5 were added to the laboratory testing program to answer specific questions related to the test pro- cedures previously evaluated. Experiment 3 was conducted to evaluate modifications relative to the drying and soaking methods in AASHTO T 85 and T 84 to reduce the testing time. Experiment 4 was conducted to determine the effect of P200 on AASHTO T 84 test results. The purpose of Experiment 5 was to investigate the time-zero reading for Phunque methods. In Experiment 3, alternative methods for drying and soak- ing aggregate to reduce the testing time for AASHTO T 85 and T 84 were evaluated. Instead of using the standard oven- drying method, the aggregate sample can be dried using a vacuum-drying method or tested in its in-situ moisture condition. The vacuum-drying method using the CoreDry device was promising, but it was not able to completely dry highly absorptive materials such as BF slag coarse aggregate and recycled concrete. The results of Experiment 4 showed that testing the aggregate in its in-situ moisture condition was a better alternative to the standard oven-drying method. The Gsb and Gssd results for the coarse and fine aggregates determined using the 5- and 10-minute vacuum-soaking method (similar to the one described in AASHTO T 209) were C H A P T E R 6 Conclusions and Recommendations
69 ID Test Method Comments on Accuracy Equipment Ruggedness Ease of Use Time Eqmt. Cost Other Comments Total Operator I. Test Methods for Coarse Aggregate 1 AASHTO T 85 and ASTM C127 Yields more accurate measurements of absorption, Gsb, and Gssd Good Manual 3 days 30 min. $100 ~ $600 More accurate, repeatable, and reproducible than the Phunque method 2 AASHTO TP 77 (Phunque Method) Yields less accurate measurements of absorption, Gsb, and Gssd Fragile in current design Manual 2 days 2 hrs $500 Questionable accuracy for measuring absorption, Gsb, and Gssd, especially for absorptive aggregate II. Test Methods for Fine Aggregate 1 AASHTO T 84 and ASTM C128 Yields less accurate measurements of absorption, Gsb, and Gssd for materials with high P200 content Good Manual 3 days 1.5 hrs $100 ~ $300 Accuracy, repeatability, and reproducibility are more affected by P200 content and less affected by absorption 2 Modified AASHTO T84 (Removal of P200) Yields more accurate measurements of absorption, Gsb, and Gssd Good Manual 3 days 1.5 hrs $100 ~ $300 Repeatability and reproducibility are affected by absorption; may require another test for P200 material 3 ASTM D 7172 (SSDetect) Yields more accurate measurements of absorption, Gsb, and Gssd Good Auto 1 day 1 hr $7,000 Repeatability and reproducibility are affected by absorption; higher equipment cost, faster results 4 AASHTO TP 77 (Phunque Method) Yields less accurate measurements of absorption, Gsb, and Gssd Fragile in current design Manual 2 days 2 hrs $500 Low equipment cost, somewhat fragile flasks, questionable accuracy for measuring absorption, Gsb, and Gssd, especially for absorptive aggregate Table 6-1. Comparison of test methods for determining specific gravity and absorption of aggregate. not statistically different from those measured according to the standard 15-hour soaking method. To reduce the testing time for QA purposes, soaking aggregate samples for 10 min- utes should be considered as an alternative to the standard 15-hour soaking method. The results of Experiment 4 showed the significant error caused by the presence of non-clay fines and clays in P200. Therefore, the fine aggregate portions retained on and passing the No. 200 sieve should be tested separately when the sand equivalent (AASHTO T 176) value of the fine aggregate is less than 75. The portion retained on the No. 200 sieve can be tested according to AASHTO T 84. The P200 can be tested in accordance with ASTM C110, Section 21, ASTM D 854, or AASHTO T 133 (with ethyl alcohol). It is noted that the sug- gested sand equivalent value (less than 75) was determined based on limited data collected in this project, so it should be verified in the future. In addition, only the sand equivalent test was conducted in this research to determine the presence of clay-like materials in the P200. The hydrometer analysis (AASHTO T 88) and the methylene blue test (AASHTO T 330) also may be used if thresholds similar to the one for the sand equivalent test are developed for these tests in the future. Based on the results of the laboratory evaluation program, proposed changes to AASHTO T 85 and T 84 were incorpo- rated into the test procedures. The ruggedness study was then conducted to evaluate the sensitivity of AASHTO T 85 and T 84 to the changes in levels of operating and environmental factors. For AASHTO T 85, the following changes are proposed (see Appendix A): ⢠The aggregate size (either retained on the 4.75-mm (No. 4) sieve or retained on the 2.36-mm [No. 8] sieve) was found to be significant. Thus, in Section 7.2, testing should be conducted on the same aggregate size in the agencyâs and contractorâs laboratories. ⢠The temperature of the aggregate (either at approximately 110°C or at approximately 50°C) at the time of being sub- mersed in the water was found to be a significant factor. Hence, in Section 8.1, an oven-dried sample should be allowed to cool to a temperature (approximately 50°C) that is comfortable to handle before being submerged in water at room temperature. ⢠The method of drying the soaked aggregate sample (using either a dry or wet cloth) also was found to be significant. Therefore, in Section 8.3, the agencyâs and contractorâs laboratories should use the same method to dry the surface of soaked aggregate particles to an SSD condition. ⢠The water bath temperature was found to be significant. Thus, the water bath temperature specified in Section 8.3 should be changed from 23.0 ± 1.7°C (73.4 ± 3°F) to 23.0 ± 1°C (73.4 ± 1.8°F) to improve the test variability. The tolerance of ± 1°C is recommended because it can be rea- sonably controlled in a laboratory.
70 AASHTO T 84 was found to be sensitive to several test param- eters. The following changes are proposed for AASHTO T 84 (see Appendix B): ⢠The P200 portion was found to be the most significant factor and should be tested separately. This suggests that in Section 7.1, only the portion of fine aggregate retained on the No. 200 sieve be tested according to AASHTO T 84. ⢠The next significant factor was found to be the soaking time; thus, in Section 7.1.1, the soak time should be changed (1) from 10 ± 1 minutes to 10 ± 0.5 minutes for vacuum soaking and (2) from 15 to 16 hours instead of from 15 to 19 hours for hydrostatic soaking. These ranges were proposed because they can be reasonably controlled in the laboratory. ⢠The variability of the AASHTO T 84 test results also could be improved by using consistent methods for conducting the cone test (dropping the tamper 25 times at once or in four sets), eliminating air bubbles (mechanical or manual agitation), and determining the final mass of an oven-dried aggregate sample as soon as it is safe to do so. In addition to the statistical significance analyzed above, the practical significance of the aggregate specific gravity test results also was determined for concrete and asphalt mix design methods. The practical ranges of the within-lab precision estimates for Gsb determinations are reasonable for concrete. However, for asphalt, the Gsb test result has a very large impact on VMA, one of the more critical criteria for asphalt mix design and acceptance. To reduce the impact on VMA to a tolerable level, the acceptable range of differences between within-lab replicates alternative methods would have to be less than 0.010, which is about one-third of what is currently practically attainable. In summary, AASHTO T 85, SSDetect (ASTM D 7172), and Modified AASHTO T 84 were found to provide a better bal- ance of accuracy and precision than the other test methods evaluated in this study. Several changes to AASHTO T 85 and T 84 were proposed to reduce testing time and improve test precision. Although the precision estimates for Gsb are reason- able for concrete proportioning, they are not practically attain- able to reduce the impact on VMA required for asphalt mix design to an acceptable level.
