Review of a Statistical Specification for Pugmill Mixed Material

About the report:

Since the spring of 1964, the Virginia Highway Research Council has been developing and implementing statistical specifications for highway operations. One of these specifications is used for the acceptance of pugmill mixed materials. The purpose of this study was to review the data obtained under the present statistical specification for pugmill mixed material, and to recommend any revisions thought to be needed in the tolerances. It was found that: 1. The variability as measured by standard deviations of materials produced under the statistical specification is more than, and probably less than, the variability values the specification was based on. 2. Generally, producers are unable to attain an average percentage passing a given sieve exactly equal to the job mix specified. Thus, the ability to remain within tolerances on means is a function of variability plus the ability to "hit" the job mix. In fact, in this study a large percentage of material out of tolerance was usually the result of a large miss from the specified job mix. 3. Considering the average variabilities and average miss from the job mix, the current statistical tolerances for means are such that for the average producer no more than 2.5% of material will fall outside the tolerances. 4. The tolerances on means are somewhat inconsistent in that those on the smaller sieve sizes are more restrictive relative to average conditions.

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