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19 0.700 Mortar Pull-Out 0.1-in Slip Stress (ksi) 0.600 y = 1.7413x + 0.7656 2 R = 0.6818 0.500 Specified Pull- Out Stress 0.400 Threshold 0.300 Change in Corr. Pot. (V) 0.200 Prediction Interval Lower Bound Threshold on Change in Corr. Pot. (V) 0.100 0.000 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 Change in Corr. Pot. (V) Change in Corr Pot. where pull-out threshold intersects prediction interval Figure 3. Threshold determination using the prediction interval. Excel worksheet was developed that demonstrates the calcu- a middle-range performer in that particular test during the lation of prediction intervals for single- and multiple-predictor correlation testing. The results of this testing were developed regressions. This tool employs the calculation outlined in according to ASTM practice and are reported based on the Appendix B. standard deviations measured among the test results. A deter- mination of bias in the testing methods is not possible at this time since a known reference sample can not be selected in a Selection of Confidence Level universally acceptable manner. For the threshold determinations performed based on the data collected in this study, the confidence level was taken Supplemental Investigations as 90%. This means that for a given surface and chemical test of Strand Bond result, 10% of the pull-out results would be expected to fall below that prediction interval. This confidence level is lower In addition to testing performed to evaluate the proposed than the 95% confidence interval that is most commonly used QC testing methods, supplemental investigations were con- as the basis for probabilistic design in structural engineering ducted to provide insight into the causes of poor bond. These analysis. Using a confidence level as high as 95% will result in investigations aided in the development and interpretation of very conservative thresholds for the surface and chemical tests, the results of the QC tests. The supplemental investigations so a 90% confidence level was used instead. included studies of: (1) surface roughness and distribution of lubricant residue, (2) the concrete/strand interface, and (3) local variation of strand diameter. The studies of surface Precision roughness included metallographic studies of strand from A final round of testing using the QC methods that corre- poor and good bonding sources, scanning electron micro- lated well with bond performance was conducted to provide scopical studies of the strand surface, and study of surface the basis for a precision statement to be included in the test roughness by electrochemical impedance spectroscopy. Inves- methods. To determine the precision (i.e., the repeatability) tigation of the concrete/strand interface included a study of the of the methods, the selected tests were repeated up to six cement hydration at the strand interface, and a petrographic times on samples of strand obtained from the same source. and chemical investigation of the strand/concrete interface in This testing was conducted on a single source identified as transfer length prisms in which poor bond was observed.