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2.4.5 Precision of AASHTO T304 dations other than Method A were also highly correlated
to the Method A results and maintained the same rank-
AASHTO T304 reports a single-operator standard devia- ing of fine aggregates. The results from AASHTO T304
tion (Std) of 0.13% voids and a multilaboratory standard devi- Method C are affected by the fine aggregate gradation
ation of 0.33% voids (71). This means that two properly con- and are not recommended for comparing particle shape
ducted tests should not differ (D2S) by more than 0.37% and and texture.
0.93% voids, respectively, for a single operator and between · The current Superpave consensus aggregate properties
two different labs. AASHTO T304 testing is included as part do not address the angularity of the material that pass
of the AASHTO Materials Reference Laboratory (AMRL) the No. 4 sieve but are retained on the No. 8 sieve. It is
proficiency samples testing program. The precision results doubtful that the current AASHTO T304 apparatus
for the four latest proficiency samples are shown in Table 9. could accommodate material of this size fraction.
The average uncompacted voids contents for the samples · Numerous test procedures are available to assess fine
tested in Table 9 ranged from 42.7% to 44.7%. The data in
aggregate texture and angularity. Several of the imaging
Table 9 indicates that AASHTO T304 is more variable in
techniques and the CAR test appear to be promising.
practice than reported in the test method. The Southeast
Researchers using the DST (ASTM D3080) have indi-
Asphalt User/Producer Group conducted a round-robin for
cated that it is difficult to obtain consistent results; how-
AASHTO T304 Methods A, B, and C. The study included
ever, to date, the majority of the work to correlate fine
seven aggregate sources from the southeastern United States:
two natural sands, two granite sources, two limestone sources, aggregate shape and texture to performance has been
and standard graded sand. The standard graded sand had completed using AASHTO T304 Method A.
been previously used to establish the precision statement · The results of studies relating the uncompacted voids
for AASHTO T304. Sixteen laboratories participated in the content from AASHTO T304 Method A to performance
study, although not all of the data were returned for all of the are mixed. Generally, studies indicated a trend between
samples. The results indicated that Method C was more vari- uncompacted voids content and improved rutting per-
able than Methods A and B, which had similar variability. For formance, but in some cases the trend was weak. Subtle
Method A, the single operator standard deviation was 0.57% differences in uncompacted voids content can be over-
voids and the multilaboratory standard deviation was 0.75% whelmed by the effect of the coarse aggregate or other
voids, which correspond to D2S limits of 1.61 and 2.12, HMA mixture properties. Several studies supported the
respectively (72). The variability of the bulk dry specific grav- 45% uncompacted voids criteria for high traffic, but
ity measurements (72) used in the calculations to determine several also indicated performance was unclear between
the uncompacted void content significantly increases the test 43% and 45% (or higher) uncompacted voids. There
variability. The AMRL results and Southeast Asphalt User/ is clear evidence that good-performing mixes can be
Producer Group Study indicate that the AASHTO T304 pre- designed with uncompacted voids contents between 43%
cision statement may need to be revised. and 45%, but evaluation of these mixes using a rutting
performance test is recommended.
· Higher uncompacted void contents generally resulted in
2.4.6 Summary of Findings on Fine Aggregate
Texture and Angularity higher VMA and lower densities at Ninitial.
· The variability of AASHTO T304 method A appears to
The findings on fine aggregate texture and angularity are be larger than reported in the test method. Much of this
as follows: variability appears to be related to variability in the fine
aggregate specific gravity measurements used to calcu-
· The results of AASHTO T304 Methods A and B are late the uncompacted voids. Ongoing research to improve
highly correlated, with Method B producing larger fine aggregate specific gravity measurements may also
uncompacted void contents. Tests using alternative gra- benefit AASHTO T304.
TABLE 9 AMRL AASHTO T304 proficiency sample results (71)
Sample Number Multilaboratory Precision Single Operator
Numbers of Labs First Sample Second Sample Precision
Std. D2S Std. D2s Std. D2s
119 120 136 0.937 2.651 1.012 2.863 0.358 1.103
123 124 183 1.129 3.194 1.149 3.250 0.406 1.147
127 128 211 1.291 3.651 1.349 3.815 0.377 1.066
131 132 242 0.917 2.594 0.858 2.428 0.381 1.077
