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Expert Task Group on Asphalt Mixtures & Construction; Asphalt Binders; and Models Technology August 2001 Attachment F - Recommendations for Changes to AASHTO T-209

The Expert Task Group (ETG) for Superpave Mixtures and Aggregates is recommending changes to be made to AASHTO T-209, "Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures". These changes, with accompanying rationale, are detailed below.

Change: Eliminate provision for manual agitation of the specimen (Section 9.4); require continuous mechanical agitation. The suggested change is shown below.

Existing Section 9.4

9.4 Remove air trapped in the sample by applying gradually increased vacuum until the residual pressure manometer reads 3.7 ± 0.3 kPa. Maintain this residual pressure for 15±2 min. Agitate the container and contents during the vacuum period either continuously by a mechanical device, or manually by vigorous shaking at intervals of about 2 minutes. Glass vessels should be shaken on a resilient surface such as a rubber or plastic mat, and not on a hard surface, so as to avoid excessive impact while under vacuum.

Proposed Section 9.4

9.4 Place the container with the sample and water on a mechanical agitation device and anchor it to the surface of the device. Remove air trapped in the sample by applying gradually increased vacuum until the residual pressure manometer reads 3.7±0.3 kPa. Maintain this residual pressure for 15±2 min. Agitate the container and its contents using the mechanical device during the vacuum period. Glass vessels should be shaken on a resilient surface such as a rubber or plastic mat, and not on a hard surface, so as to avoid excessive impact while under vacuum.


Rationale: Data from AMRL proficiency samples #31 and #32 were analyzed to determine the relative variability of test results obtained using manual and mechanical agitation. Table 1 shows the results of the analysis.

Table 1. Statistical Results for AMRL Proficiency Samples #31 and #32
(AASHTO T209)
Property Sample I.D.
Sample #31 Sample #32 Combined (#31 & #32)
Agitation Method Manual Mechanical Manual Mechanical Manual Mechanical
No. of Samples 234 156 234 156 468 312
Mean Gmm 2.509 2.508 2.502 2.503 2.506 2.505
Between Lab
Std. Deviation
0.0270 0.0119 0.0202 0.0123 0.0241 0.0123

An analysis of variance (ANOVA), or "F" test, showed that the differences in mean Gmm values obtained using the two agitation methods are not statistically significant. However, a two-sample "F" test for sample variance showed that the difference in the variance (or standard deviation) obtained from data representing each agitation method is statistically significant at a 0.05 level of significance (α = 0.05). Based on these findings, it is recommended that manual agitation be eliminated to improve testing precision in AASHTO T209.

Change: Change the minimum sample size table in Section 7.2 to conform to standard Superpave mixture size terminology; reduce entries to streamline table. A suggested format is shown below.

Existing Table (Section 7.2
Size of Largest Particle of Aggregate in Mixture, mm (in.) Minimum Sample Size, g
50.0 (2) 6000
37.5 (1 1/2) 4000
25.0 (1) 2500
19.0 (3/4) 2000
12.5 (1/2) 1500
9.5 (3/8) 1000
4.75 (No. 4) 500
Proposed Table (Section 7.2)
Nominal Maximum Aggregate Size, mm Minimum Sample Size, g
37.5 or greater 5000
19 to 25 2500
12.5 or smaller 1500

Rationale: The left column heading in the existing table is ambiguous and not consistent with standard Superpave terminology for mixture designation. The change would simplify and standardize the terminology and streamline the table by reducing the number of minimum size entries. A suggested format is shown below

 
Updated: 04/07/2011
 

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