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Alternative Title:Quality control analysis : part I : asphaltic concrete : Louisiana highway research.
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Abstract:This report deals with the statistical evaluation of results from several hot mix plants to determine the pattern of variability with respect to bituminous hot mix characteristics.
Individual tests results when subjected to frequency distribution indicated normal (Gaussian) distribution. Further analysis showed the overall variability of each characteristic for binder course mixes to be less than that for wearing course mixes. Also, the natural tolerances for bitumen content and aggregate gradation were outside the engineering (job mix) tolerances indicating a need for either a much closer control in plant operation and materials uniformity or a revision in engineering tolerances.
The variability in the case of Marshall stability was considerably different for each plant. Furthermore, lack of uniformity was indicated as evidenced by considerable between-days variation.
If acceptance tolerances are to be written in the specifications, then the number of samples to be tested for a particular characteristic should be specified. For Marshall stability, eight random samples obtained from trucks representing a day's operation should be tested. Furthermore, if the minimum specifications are to be met 100 per cent of the time, then it is minimum specifications be maintained at 30 above the minimum requirement, or 3(190) = 570 lbs. above the absolute minimum specified for the type of mix.
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