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35 CHAPTER 4 Conclusions and Recommendations The predictable transfer of prestressing force from strand conduct the tests: Level I (QC-I) and Level II (QC-II) tests. to concrete is essential for the reliable performance of pre- The QC level is shown in the bulleted list above. stressed concrete. Residual films of lubricant and other con- Regression with multiple predictors has also been performed taminants remaining on the strand surface after manufacture to see if results of these methods could be combined to better have been shown to reduce the bond between the concrete and predict bond. The three combinations that showed the best steel. A set of QC procedures has been developed for use by correlation, based on the adjusted coefficient of determina- strand manufactures or their customers as part of a routine tion (R2 adj.), were QC program to enable rapid detection of potential bond prob- lems related to strand residues. · Weight Loss on Ignition (LOI) & Contact Angle Mea- An experimental program was conducted to evaluate a surement after Lime Dip & Change in Corrosion Potential number of test methods proposed for this purpose. This (R2 adj. = 0.76), included limited mechanical testing (pull-out testing from · Contact Angle Measurement after Lime Dip & Change in concrete, from Portland cement mortar, and from gypsum Corrosion Potential (R2 adj. = 0.73), and plaster-based mortar) and extensive surface and chemical · Contact Angle Measurement after Lime Dip & Organic testing (contact angle, examination under UV light, pH, LOI, Residue Extraction (when organic residue is primarily loss in alkali bath, change in corrosion potential, corrosion rate, stearate, R2 adj. = 0.98). surface roughness, organic residue extraction/FTIR analysis, and elemental analysis). These tests, as well as transfer length The adjusted coefficients of determination for each of these tests, have been conducted on a range of strand sources to combinations were higher than the coefficients of determi- establish correlations between the proposed QC tests methods nation for the single-predictor regression models. and bond quality. Thresholds for two of these individual QC tests and all of Although pull-out testing from concrete appears to corre- the combinations have been developed based on prediction late best with transfer length, the most reliable and realistic intervals for the regression calculated from the available data measure of bond performance, the Correlation Round of this and a minimum criterion on the mortar pull-out stress test test program was based on available mortar pull-out results adopted by NASPA. Thresholds for multiple-predictor regres- provided by Russell of OSU from the NCHRP 12-60 Program. sions are not determined using the same procedure used for The following four test methods showed the best correlation single-predictor regressions. Instead, the lower bound on the with bond in concrete, mortar or both, and are recommended prediction interval must be calculated for each combina- for inclusion in future QC programs: tion of test results. A computational tool in the form of a Microsoft Excel spreadsheet has been developed for this pur- · Weight LOI (QC-I), pose, and is called the NCHRP No. 10-62 Prediction Interval · Contact Angle Measurement after Lime Dip (QC-I), Calculation.xls. · Change in Corrosion Potential (QC-I), and It is suggested that the three recommended Level I QC · Organic Residue Extraction with FTIR Analysis (QC-II). tests be adopted as part of a routine QC program for strand producers. To supplement the quarterly mortar pull-out The quality control tests have been divided into two cate- testing program currently underway, this test should be gories, depending on the complexity and time required to conducted on a weekly basis for each size of strand produced.