Evaluating percent within limits (PWL) specifications volume 1 : HMA specifications.
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2006-05-01
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Edition:Final report.
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Abstract:The Oklahoma Department of Transportation (ODOT) has let four paving jobs for construction where
percent within limits (PWL) specifications were employed. The PWL specifications are intended to be
used as part of the Quality Assessment program to determine the statistical probability of conformance to
specified material properties and construction details, and to base the pay factor (PF) off the probability of
conformance to the specifications. There is a need to evaluate the performance of the PWL specifications
and assess the suitability of the PWL specifications for future jobs.
Two hot mix asphalt (HMA) projects were let for construction using the proposed HMA PWL
specifications. In addition to the normal quality control, acceptance and assurance sampling and testing,
the ODOT Materials Division performed more extensive sampling and testing on randomly selected lots
from each pavement which were designated “super lots.”
The data obtained were to analyzed 1) to determine whether the PWL specifications are working as
intended, 2) to recommend changes to the PWL specification if necessary, including possible changes to
the limits and ranges contained within the specifications, and 3) to recommend whether ODOT should
continue to pursue PWL specifications as matter of policy in their overall QA/QC programs.
The examination of the draft HMA PWL specification and analysis of the super lot data demonstrated that
the specification concept is sound and will require very little additional work to develop fully. The F and
t-test procedure for process verification is recommended for use.
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