Ruggedness Study Plan
The objective of this ruggedness study was to evaluate the sensitivity of the proposed test methods to the changes in lev- els of operating and environmental factors. The results of rug- gedness testing were used to eliminate or reduce the sources of variation, which would ensure better precision of test results and establish appropriate ranges of operating parameters for standardizing the test methods.
This ruggedness study was conducted in accordance with ASTM C1067: Standard Practice for Conducting a Rugged- ness or Screening Program for Test Methods for Construction Materials. The experimental plan involved determination of seven sources of variation, each with potential to influence test results. Each variable factor was assigned high and low values, which represented the tolerance extremes of the variable fac- tor allowed in the test procedure. The experimental plan used in this study was based on the seven-factor Plackett-Burman design presented in ASTM C1067.
Depending on the significance of the interaction between variable factors, the ruggedness experiment can be conducted as a full factorial or fractional factorial. Since the test param- eters in the specific gravity measurement were not consid- ered highly interactive, a fractional factorial experiment was considered adequate. Following the ASTM C1067 procedure for a design of fractional factorial, eight determinations (test conditions) were considered sufficient for obtaining valid and objective conclusions with a seven-factor experiment.
The development of a ruggedness study as presented in ASTM C1067 included the following steps:
• Identifying seven variable factors for each test method;
• Determining high and low levels for each factor;
• Setting up 8 combinations of 14 factor levels (7 factors ×
2 levels) with 2 replicates for each combination;
• Identifying at least three laboratories to participate in the
study; and
• Determining three to five materials that cover the range of materials properties to which the test method is applicable.
Following the above steps, plans for ruggedness testing of revised AASHTO T 85 and T 84 were arranged. Details of each plan are presented in the following sections.