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Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete (2008)

Chapter: Chapter 4 - Conclusions and Recommendations

« Previous: Chapter 3 - Findings and Applications
Page 35
Suggested Citation:"Chapter 4 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Page 35
Page 36
Suggested Citation:"Chapter 4 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Page 36

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

Regular QC testing should decrease the likelihood that poor bonding strand would reach the market. Lots of strand exhibit- ing unacceptable behavior identified by these test methods should then be tested further using the Level II organic residue extraction test and mechanical pull-out testing. The determination of thresholds for two of the individual QC tests (Contact Angle Measurement after Lime Dip and Change in Corrosion Potential of Strand) was possible based on the relationships between the QC test and the mortar pull- out test results for this sample set; however, these thresholds are conservative. The available data were not sufficient to allow threshold determination for the other two individual methods with the same constraints. The threshold determination process is governed by the prediction intervals, which are de- termined by the uncertainty in the regression results. Sources of uncertainty, which ideally would be minimized, include inability of the test methods to predict bond, scatter in both the QC and mortar pull-out test results, close grouping of sources in terms of bond performance, and a limited number of data points for the regression analysis. Although a significant amount of work and scientific rigor has gone into the development of the thresholds, they should not be considered absolute. Additional data could possibly be used to reduce the uncertainty alluded to above and may allow a reduction in the thresholds. Specifically, if the QC tests were conducted on the samples included in the quarterly pull-out testing program currently being conducted by NASPA, this information would be valuable to further refine the regression relationships. Another possible means for implementing these test meth- ods is the development of process-specific regression models and thresholds. The dataset for this study included strand sources manufactured with a number of different pretreatment and lubricant processes. Limiting the data included in the re- gression analysis to a single production process, such as might be done at an individual strand manufacturing facility, would likely significantly improve the correlation of the QC test methods, since the QC test results would be influenced mainly by variations in concentration of a specific lubricant and pre- treatment and not the simultaneous variations of a variety of lubricant and pretreatment chemistries and concentrations. A better correlation would also allow the development of less restrictive thresholds. Future Work Although a variety of researchers has made conclusions about the most appropriate mechanical test methods for evaluating strand bond, the limited results of this research program suggested that concrete pull-out test results corre- lated better with transfer length test results than with mortar pull-out test results. Although the mortar pull-out test demon- strated good correlation, it is not clear that this is the best test for measuring the strand bond quality. Further work is needed to resolve the questions related to performance-based (i.e., mechanical) tests of strand bond that were left unanswered by the elimination of further development of mechanical tests from this research program. It is recommended that a multi- laboratory test program be conducted following the concepts outlined by ASTM E1169-07 Standard Practice for Conduct- ing Ruggedness Tests (ASTM 2007) and conducted at qualified laboratories that are not already involved in the historically contentious discussions regarding strand bond issues. Although a small task if an effective test method can be de- fined, further work is also needed to ascertain the extent and significance of local variations in the strand performance within a single spool. During the program outlined in this document, the relationship between test results and proxim- ity on the strand could not be tracked. Nevertheless, such a variation has direct impact on the evaluation of any test method, since such variation would become combined with variations related to the test method itself. As stated, additional work is needed to refine the thresholds for bond acceptability that will be used to establish alert (pass/fail) thresholds for the QC test results. The incorporation of additional data into the regression analysis would improve the confidence in the validity and usefulness of the QC test methods and may also allow less restrictive thresholds to be defined. The threshold on the mortar pull-out test result adopted by NASPA is based on development length testing of four strand sources. For the sake of the QC threshold determination con- ducted during this program, it has been assumed that this threshold is a well-defined absolute. However, although addi- tional work to refine this threshold would require a significant effort, such an effort would be valuable and would provide greater confidence in the performance of strand. 36

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TRB's National Cooperative Highway Research Program (NCHRP) Report 621: Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete explores tests to identify and measure residues on the surface of steel pre-stressing strands and to establish thresholds for residue types found to affect the strength of the strand's bond to concrete.

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