15 May 2001
Lot Density as a Percent of Laboratory Density
(16) Figure 2-8 shows the control charts for mat density. The first test result obtained is plotted
in figure 2-8a. Note that this measurement falls below the desired range. At this point, it should have
been concluded that the process was out of control; thus, the operation should be stopped until the
cause of the deficiency is identified and corrected.
(17) The second, third, and fourth samples were obtained after corrections were made to the
process and found to be higher but still below the desired range. At this point, the weight of the rubber-
tired roller used in compacting the mat was increased from 20 to 25 metric tons (20 to 25 tons), and the
tire pressure was increased from 480 kPa (70 psi) to 620 kPa (90 psi). After these changes, the density
results were generally within the desired range.
(18) The moving average is determined for the last four samples tested (figure 2-8b). Plotting
the moving average smooths out the plot of individual values and allows trends to be spotted earlier.
e. Significance of changes in mixture properties.
(1) General. As a general rule, the flow and stability values are obtainable quickly and are
reasonably reliable indicators of the consistency of the plant-produced mix. A measurable increase in
flow value generally indicates that either the gradation of the mix has changed sufficiently to require a
revision in the optimum asphalt content for the mix, or too much asphalt is being incorporated in the mix.
A review of the control charts should indicate the problem. Substantial changes in stability or void
content also may serve as an indication of these factors. Mix proportions shall be adjusted whenever
any test property consistently falls outside of the specified tolerances. In the case of batch plants, the
use of faulty scales and the failure of the operator to accurately weigh the required proportions of
materials are common causes for paving-mixture deficiencies. Improper weighing or faulty scales may
be detected readily and corrective measures taken by maintaining a close check of load weights.
Figure 2-9 lists other probable causes of paving-mixture deficiencies.
(2) Extraction tests.
(a) Representative samples of paving mixture should be obtained for extraction tests to
determine the percentage of bitumen in the mix and the gradation of the extracted aggregates.
Extraction tests shall be made according to ASTM D 2172. Sieve analyses of recovered aggregates
shall be determined according to D 5444.
(b) Nuclear gages are currently being used to determine asphalt content in accordance
with ASTM D 4125. After the nuclear gage is calibrated, it can be used to check the asphalt content of a