National Academies Press: OpenBook
« Previous: Chapter 2 - Research Approach
Page 20
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 20
Page 21
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 21
Page 22
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 22
Page 23
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 23
Page 24
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 24
Page 25
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 25
Page 26
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 26
Page 27
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 27
Page 28
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 28
Page 29
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 29
Page 30
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 30
Page 31
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 31
Page 32
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 32
Page 33
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 33
Page 34
Suggested Citation:"Chapter 3 - Findings and Applications." 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.
×
Page 34

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

20 Findings of Industry Survey Information regarding strand manufacture and the appro- priate test methods was solicited directly from manufacturers of lubricants used to manufacture PC strand and from PC strand manufacturers. Lubricant Manufacturers Some specific information gained from representatives of the lubricant manufacturing industry included the following: • A wide range of lubricants is used in U.S. strand manu- facturing operations. One supplier suggests at least six of their products are used throughout the country in strand production. • Lubricants are often a blend of multiple soaps, including sodium and calcium stearates, and can include fillers such as lime. • The type (formulation) of lubricant recommended by the lubricant manufacturer is dependent on the type of metal pretreatments and the condition of the metal surface. • Rod or wire can be either mechanically or chemically cleaned (acid pickled). Mechanical descaling can be performed using shot blasting, reverse bending, or wire brushing. Chemical cleaning was considered superior to mechanical descaling by lubricant manufacturers. Subsequent pretreat- ments include borax and lime. • Wire that is only marginally cleaned (retained scale) requires a lubricant with a heavy residue. • Products are formulated with different melting points. Twenty or more different sodium soaps are available, as well as different sources of fatty acid, such as stearic acid. • One-to-three different products can be used in line for manufacturing prestressing strand. • Calcium salts of fatty acids are often used in the first set of dies. Some plants use calcium soaps in three die boxes and sodium stearate in the rest. Other plants use sodium soaps in all die boxes. • High-carbon wire and high-speed lines result in greater heat and require lubricant formulation with a higher soft- ening point and a high fat content. (High fat content equals 60% for calcium-based lubricants and 75% for sodium- based lubricants.) Some formulations use a blend of fats (hydrotallow and stearic acid) to achieve certain melting (softening) points. • In general, calcium soaps melt at a lower temperature than do sodium soaps. • Additives in lubricants can include lime, sodium carbon- ate, sodium phosphate, sodium sulfate, antioxidants, and borax. Strand Manufacturers Some specific information gained from representatives of the strand manufacturing industry included the following: • Induction furnaces are typically used during the stabiliza- tion process after the wires have been stranded. The strand surface temperature is typically measured with in-line in- frared sensors and is in the range of 700-785°F as the strand leaves the furnace. The strand remains at approximately this temperature for 2-10 sec, until the steel is quenched in a water bath. Although there may be some limited washing action as the strand passes through the bath water, which is circulated through filters and a heat exchanger, no other cleaning methods were reported. Additional cleaning meth- ods used by some plants may be proprietary. • Problems in the quality of raw materials used in the strand production process are identified when a significant change in drawing performance, such as reduced die life or an in- crease in energy demand, is observed. It is common for adjustments in the drawing process to be necessary since lubrication performance is highly temperature-dependent C H A P T E R 3 Findings and Applications

and modified behavior may be caused by ambient humidity and temperature differences. Such variations are typically accommodated by modifying line speed. • No QC testing is conducted on lubricants used in strand production. • Until recently, the post-production strand QC testing at strand production facilities generally consisted of periodic relaxation strength and modulus of elasticity testing, with no specific tests performed to verify bond performance. However, in 2007, a 2-year quarterly program of NASPA bond tests (pull-out tests from mortar) was initiated, and initially included the participation of 10 NASPA members supplying strand to the domestic U.S. market. One pro- ducer has since stopped supplying to the domestic market and has dropped out. Currently, this program consists of tests conducted on lengths of strand randomly sampled from the first 500 ft. of a 1500-ft. pack of strand supplied for testing. This testing is performed under the supervision of Bruce Russell of OSU, who has been contracted to per- form this work. Three NASPA bond tests, consisting of six strands each are performed. Only 0.5-in. diameter strands have been included in this effort. • Strand producers demonstrated interest in the chemistry- based surface characterization methods and expressed a willingness to use such methods in their plants, if their efficacy can be proven. Findings of Supplemental Investigations Detailed results of supplemental investigations are presented in Appendix D. The significant findings are summarized here. The supplemental investigations focused on the strand and strand/concrete interface, in an attempt to gain a greater un- derstanding of some factors that may influence bond. Studies of surface roughness and the relationship between surface roughness and lubricant residue were carried out using met- allographic methods, scanning electron microscopy with energy dispersive X-ray spectrometry, and electrochemical impedance spectroscopy (EIS). The effect of lubricant residue on cement hydration was evaluated with petrographic and chemical investigations of the concrete immediately adjacent to the strand. Possible relationships between surface-roughness charac- teristics (together with the concentration of residual lubricant on the strand) and bond strengths were assessed. The relation- ship between surface roughness and bond strength is compli- cated by the presence of residual lubricant, as demonstrated by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) analysis; higher concentrations of residual lubricant appear to be associated with greater surface roughness, with the lubricant occurring predominately within depressions on the surface. Based on metallographic studies, a strong correlation between surface roughness and bond strength was not apparent, but only a very limited number of strand cross-sections were evaluated because of the labor- intensive nature of this technique. However, EIS studies were used to measure the capacitance per nominal unit area, a prop- erty controlled by the surface area of the strand on a micro- scopic scale. This microscopic surface area is linked to the microstructural roughness. A strong correlation between bond strength and a parameter equal to the ratio of the lubricant concentration and this capacitance per nominal unit area was found. Since the lubricant concentration is also based on the nominal surface area, this parameter is a measure of the rela- tive concentration of lubricant residue per actual microscopic surface area. Petrographic studies have shown that there is a difference in cement hydration at the interface of strand compared to the surrounding concrete that appears to be a direct result of the presence of residual lubricant. Cement hydration studies comparing the interfacial features of as-received strand with strand that had been cleaned and stripped of its residual lubri- cant demonstrate that the cement particles at the strand in- terface of the as-received strand appears to be less hydrated than the surrounding cement, while the interface of the cleaned strand is similar to the bulk cement. This reduction in cement hydration at the interface was also seen in studies of the transfer zone of a concrete sample containing strand that exhibited poor bond. The residue on the strand appears to have the ability to affect hydration of the cement immedi- ately adjacent to the strand. These supplemental investigations, although not directly involved in formulating the QC tests, provided a greater un- derstanding of concrete, lubricant, and strand interaction. The presence of residual lubricant on the surface of the strand is a result of the amount of lubricant used, heating proce- dures, washing procedures, and the microscopic surface roughness. Residues of chemical surface treatments and/or lubricants appear to have an impact on cement hydration adjacent to the strand, possibly also reducing the strength of the bond. Findings of Evaluation of Test Methods Detailed results of the testing program are presented in Appendix B. The conclusions of the testing program are summarized here. Mechanical Testing As mentioned, the transfer length test is considered the most realistic measure of bond performance and was used as 21

22 the basis for evaluating the ability of the pull-out test meth- ods to accurately measure bond characteristics. The coeffi- cients of determination calculated based on the regression between these methods and the average bond stress over the transfer length are given in Table 3. The concrete pull-out test results correlated better with bond quality than the other pull-out test methods that were evaluated. Pull-out testing from mortar also showed promise as a means to eval- uate bond, and the existing correlation is deemed sufficient to justify further study. This limited program can not be considered a definitive evaluation of these methods; just three strand sources were evaluated. Nevertheless, this con- clusion that pull-out from concrete is the superior test is contrary to that of other studies of pull-out test methods, including that sponsored by NASPA, which have concluded that the mortar pull-out test is superior at assessing bond performance (Russell and Paulsgrove 1999, Russell 2001, and Russell 2006). This correlation, evaluated during the Screening Round of testing, was not explored further in the Correlation Round. Chemical and Surface Testing Transfer length was not measured on the historic strand and only partial transfer length test results were available for the OSU strand. Consequently, the evaluation of the effec- tiveness of the chemical and surface QC test methods in pre- dicting bond performance was determined by comparing the QC test results against performance measured in pull-out tests. The coefficients of determination for these QC methods are given in Table 4. The P-values for selected methods are given in Table 5. Coefficient of Determination (R2) from Regression with Mechanical Test Test Method QCLevel Concrete Pull-Out (0.1-in. and 1st Slip) Mortar Pull- Out (0.1-in. Slip) As received I 0.04 0.35 After Ca(OH)2 dip I 0.61 0.57 After Ca(OH)2 dip— stearate only† I 0.44 0.84 Contact Angle (°) After ignition I N.A. 0.00 pH I 0.97 0.18 Loss on Ignition I 0.86 0.16 Loss on Alkali Bath I 0.17 0.76* As received I 0.72 0.68 After Ca(OH)2 dip I 0.80 1.00* Change in Corrosion Potential after 6 h After ignition I N.A. 0.00 Surface Roughness, Ra I 0.93 0.16 As received II 0.67 0.09 After Ca(OH)2 dip II 1.00 0.00 Corrosion Rate After ignition II N.A. 0.18 Total II 0.81 0.12 Organic Residue Extraction Total—stearate only† II 0.88 0.63 Warm water II 0.34 0.31 Sodium Total II 0.12 0.02 Warm water II 0.39 0.00 Potassium Total II 0.28 0.14 Warm water II 0.17 0.06 Calcium Total II 0.22 0.07 Warm water II 0.21 0.05 Zinc Total II 0.25 0.21 Warm water II 0.30 0.10 Boron Total II 0.28 0.11 Phosphate Total II N.A. 0.17 Combined Index for B, Ca, & Org. Res. Scaled for combination II 0.78 0.28 R2 values presented in bold are for those methods recommended for use in a QC program. * Test method not included in Correlation Round. Regression based on three sources. †Only those sources identified as containing primarily stearate-based compounds by FTIR analysis are considered. Table 4. Coefficient of determination (R2) from linear regression with concrete and mortar pull-out at 0.1-in. and 1st slip.

At the initiation of this study, the surface and chemical methods were divided into Level I and II QC tests, as based on the required effort and complexity of each test. These correlations are discussed separately here, since the level of correlation required to justify the use of each test method is different for each QC level. As can be seen in Table 4, a num- ber of the surface and chemical test methods that showed good correlation with concrete pull-out test results did not corre- late as well with the mortar pull-out test results. This may be indicative of the inadequacy of the surface and chemical methods, but may also be related to inaccuracies or inconsis- tencies in the pull-out test methods. Level I Quality Control Tests The objective of the Level I QC test methods is to quickly and easily determine if strand properties that have been cor- related with questionable bond are present. The minimum correlation required for these tests to be useful is somewhat lower than for the Level II QC tests. • Contact angle—Contact angle correlated with bond only after the strand sample was subjected to exposure to a saturated calcium hydroxide solution. This correlation is higher for those sources judged to carry only stearate-based lubricants, when performance assessed with mortar pull-out is considered. Nevertheless, the P-values calculated when comparing this test against mortar and concrete pull-out testing are low (0.039 and 0.019, respectively), suggesting that the relationships between both pull-out test methods and this surface test are statistically significant. It is likely that this high correlation after the calcium hydroxide solu- tion exposure occurs because the resulting residues are similar compounds (the stearates having converted mostly to calcium stearate) that influence the surface tension in proportion to their concentration on the strand surface. Greater concentrations of residue make the strand surface more hydrophobic and increase the contact angle. It is rec- ommended that this method be included as part of a future QC program. • Examination under UV light—A limited quantity of fluo- rescing material was observed, and no correlation to bond was found. This method should be abandoned. • Testing pH—The pH test was successful in finding a cor- relation with bond as measured by concrete-based pull- out testing on a limited dataset, but it was unsuccessful at finding a similar correlation based on mortar pull-out test results. It also appeared that this test was only effective for differentiating strands produced with a borax pretreatment. Therefore, this method may only be applicable for strand produced with borax pretreatments. More study is needed before a recommendation regarding the adaptation of this method can be made. • Loss on ignition—A good correlation was found between the weight LOI and bond performance measured in concrete pull-out tests. Further statistical analysis suggests that there is greater than 99% confidence that the relationship between concrete pull-out and this test method is significant. This correlation and significance was not found based on mortar pull-out test results. Nevertheless, this is one of the easiest tests to perform and is recommended for future QC testing, although not alone. Some other measure of bond perfor- mance should be included along with LOI in a QC program. • Loss in alkali bath—Multiple cleaning procedures using sodium hydroxide solutions were attempted, but no cor- relation was observed between the weight loss and bond in concrete. Although a higher correlation was found with the mortar test, this higher correlation is only based on three sources. Interestingly, this is the only method suggested by the Wire Association International manual for measuring surface residues on wire. It is recommended that this test be abandoned. 23 P-Value from Regression with Mechanical Test Test Method QCLevel Concrete Pull-Out (0.1-in. and 1st Slip) Mortar Pull- Out (0.1-in. Slip) After Ca(OH)2 dip I 0.039 0.019 Contact Angle (°) After Ca(OH)2 dip— stearate only† I 0.262 0.029 Loss on Ignition I 0.003 0.285 Change in Corrosion Potential after 6 h As received I 0.356 0.006 Total II 0.002 0.353 Organic Residue Extraction Total—stearate only† II 0.006 0.110 †Only those sources identified as containing primarily stearate-based compounds by FTIR analysis are considered. Table 5. P-value from linear regression with concrete and mortar pull-out at 0.1-in. and 1st slip.

• Change in corrosion potential—The drop in corrosion potential showed a good correlation with bond in both the Screening and Correlation Rounds of evaluation. The P-value (0.006) calculated when comparing this test against mortar pull-out testing suggests that the relation- ship between mortar pull-out and this test method is sta- tistically significant. Testing after exposure to a saturated calcium hydroxide solution showed better correlation than testing in the as-received condition in the Screening Round; however, this higher correlation is only based on results from three sources, and it was judged that this additional conditioning effort is not worthwhile. It is hypothesized that the increased tendency for corrosion measured on poor bonding strand is a consequence of greater surface roughness measured at the microscopic scale. This microscopic roughness occurs at too fine a scale to affect bond through mechanical interlock, but makes the strand more likely to accumulate lubricant residue, which leads to poor bonding behavior. It is rec- ommended that this method, conducted on strands in the as-received condition, be included as part of a future QC program. • Surface roughness—The surface roughness parameter, Ra, correlated well with bond in concrete based on only three sources, but not with bond in mortar. Since an increased roughness was associated with poor bond, it appears that correlation to bond is not a direct effect, but is related to the tendency of the wire surfaces to retain residue. The profilometer used to measure this property is convenient for use in a QC setting, but does not appear to be sensitive enough to measure the roughness at the scale needed, nor does it test a sufficiently large surface of the strand for the test result to be representative of a property that can be tied to bond performance. Therefore, it is recommended this method be abandoned. Level II Quality Control Tests The objective of Level II QC testing is to provide a more conclusive prediction of bond performance than possible with Level I QC tests. These tests require either more advanced methods or more complicated equipment. The minimum cor- relation required for these tests is higher than that required for Level I QC tests. • Corrosion rate—A strong correlation was measured between corrosion rate and pull-out bond stress in concrete. How- ever, the correlation between corrosion rate and pull-out bond stress in mortar was relatively weak. Given this lack of consistent correlation, the uncertainty about the mech- anisms involved in establishing the initial good correlation, and the complexity and equipment-dependent nature of this test, it is not recommended for inclusion in a future QC program. • Organic residue extraction—The concentration of the organic residue correlated well with the bond performance in concrete, but only moderately with bond in mortar. Nevertheless, the P-value calculated when comparing this test against mortar pull-out testing for all samples was less than 0.01. This test is time consuming to perform, but gives the best direct measure of the type and quantity of drawing lubricants left on the strand surface during the manufac- turing process. Of all the methods proposed, this method evaluates the property of the strand tied most obviously to bond quality. The presence of organic lubricants on the surface of the strand can only be expected to reduce bond performance. Therefore, it is recommended that this method be included as part of a future QC program. FTIR spec- troscopy should be performed on the organic residues that result to ensure that residues being evaluated are consistent. This is necessary because the effect of residues with different chemistries is unlikely to be proportionally similar (e.g., a stearate-based lubricant residue will likely effect bond differently than a non-stearate-based lubricant residue). FTIR analyses will also identify contamination of the samples from other organic materials, such as oils, greases, or from release agents. The correlation between mortar pull-out stress and residue concentration was much higher when those sources carrying only stearate-based lubricants were included in the correlation analysis. • Elemental analysis—Atomic absorption and visual light spectroscopy were used to determine the surface concen- tration of various elements. The concentrations of sodium and boron showed signs of a correlation with the mechan- ical properties measured in pull-out tests in both concrete and mortar. The concentrations of zinc, however, did not. The elemental analysis gives some insight into the type of pretreatment and lubricant in use and was useful for the purposes of this study. A combined index calculated from the normalized concentrations of boron, calcium, and organic residue (as explained in Appendix B) showed good correlation to pull-out bond in concrete. This correlation was not found with mortar pull-out results. Given the cost and equipment-dependent nature of the atomic absorption testing, it is not recommended for inclusion in a future QC program. In summary, the following methods are recommended for inclusion in future QC programs: • Weight Loss on Ignition (LOI), • Contact Angle Measurement after Lime Dip, • Change in Corrosion Potential, and • Organic Residue Extraction with FTIR Analysis. 24

Test Methods and Precision Testing The recommended QC methods have been written in AASHTO/ASTM standard method format in Appendix C, where they are titled: 1. Test Method for the Determination of the Surface Tension of Steel Strand by Contact Angle Measurement, 2. Test Method for Weight Loss on Ignition (LOI) of Steel Strand, 3. Test Method for Change in Corrosion Potential of Steel Strand, and 4. Test Method for Identification and Quantification of Residue on Steel Strand by Extraction, Gravimetric, and Spectroscopical Analyses. Testing was conducted to provide the basis for a precision statement accompanying the proposed test methods devel- oped for identifying strand bond performance. The results for the four recommended test methods are given in Table 6. Note that each repeat test included testing of three strand sections. The precision and bias statements to be added to the standard test methods take the form illustrated in the following example: Test Method for Weight Loss on Ignition (LOI) of Strand Single-Operator Precision—The single-operator standard deviation was found to be 0.014 mg/cm2. Therefore, results of two properly conducted tests by the same operator on the same source are not expected to differ from each other by more than 0.041 mg/cm2. (These numbers represent, respectively, the (1s) and (d2s) limits as described in ASTM C670 [ASTM 2003].) Bias—Since there is no accepted reference material suitable for determining the bias in this test method, no statement on bias is made. Development of Thresholds For the recommended surface and chemical test methods to be useful in a QC setting, thresholds for acceptable bond behavior are needed. The usefulness of acceptance/rejection thresholds for the surface and chemical test results is de- pendent on the correlation of these results with minimum acceptable bond strengths established by physical test methods. The validity of thresholds developed in this way is also de- pendent on the validity of the physical test methods (such as pull-out tests) used as the basis for measuring bond perfor- mance. At the direction of the NCHRP supervisory panel, the transfer length testing originally planned for this test program as a basis for developing thresholds for the surface and chem- ical test results was not conducted. Instead, the thresholds for the chemical and surface test methods were based on the acceptance limits for the mortar pull-out tests proposed by Russell and adopted by NASPA. The bond strength thresholds proposed by Russell are stated in terms of the force at 0.1-in. slip measured by the NASPA mortar pull-out test procedure. They are based on a set of development length tests conducted in parallel with the development of the NASPA strand bond test (Russell 2001, Russell 2006). The thresholds were derived using develop- ment length tests on four strand sources, (in what is referred to as the NASPA Round III study [Russell 2001]), and they are defined in terms of acceptance criteria for the average force at 0.1-in. slip from six pull-outs with a lower criterion for any single measurement of the six pull-outs. The Round III report proposed thresholds of 7,300 and 5,500 lbs, for the minimum permissible average and single test result, respectively for 1/2-in. diameter strand (Russell 2001). These minimum thresholds have since been increased to 10,500 and 9,000 lbs, but without additional testing (Russell 2006). For 0.6-in. diameter strand, the suggested thresholds are 12,600 and 10,800 lbs for the minimum permissible average and single test result, respectively (Russell 2006). No threshold has been suggested for other sizes of strand. Despite the somewhat limited scope of the development process used to establish these NASPA test thresholds, the threshold determination effort for the surface and chemical testing conducted in this study was performed assuming that these thresholds were well-defined lower bounds for good bonding behavior. As has been done throughout this study, the thresholds were converted to bond stresses calculated as the force divided by the nominal surface area to support comparisons among all of the tested strands. When converted to a bond stress, the minimum threshold on the average of six tests of 10,500 lbs is equal to 0.313 ksi. This value was used as the basis for the threshold analysis. 25 Test Method Organic Residue Extraction(mg/cm2) Weight Loss on Ignition (LOI) (mg/cm2) Average Contact Angle after Lime Dip (°) Change in Corrosion Potential (V) after 6 h Number of Repeats 6 6 6 5 Average of Results 0.069 0.091 73 -0.334 Standard Deviation of Results 0.013 0.014 4 0.047 Table 6. Precision test results for recommended QC tests.

Thresholds Based on Regression with Single Predictor The initial efforts made to define thresholds for each of these recommended QC methods were based on single-predictor linear regressions and are described below. The results are summarized in Table 7. Weight Loss on Ignition (LOI)—The prediction interval for LOI with a one-sided confidence level of 90% is shown in Figure 4. As can be seen in this figure, the prediction interval does not exceed 0.313 ksi anywhere over the range of test results observed in this study. For that reason, no threshold can be determined. Contact Angle Measurement after Lime Dip—The predic- tion interval for Contact Angle after Lime Dip with a one-sided confidence level of 90% is shown in Figure 5. As can be seen in this figure, this prediction interval exceeds 0.313 ksi when the contact angle is less than 73°. Therefore, based on the data and the NASPA-defined threshold on mortar pull-out stress at 0.1-in. slip, a Contact Angle after Lime Dip of 73° or lower is recommended to give a good (90%) confidence of adequate bond. This test must be run on recently manufactured strand with no surface weathering or rust (i.e., bright strand). Change in Corrosion Potential—The prediction interval for Change in Corrosion Potential with a one-sided confi- dence level of 90% is shown in Figure 6. As can be seen in this figure, this prediction interval exceeds 0.313 ksi when the Change in Corrosion Potential is less negative than −0.175 V. Therefore, based on the data and the NASPA-defined thresh- old on mortar pull-out at 0.1-in. slip stress, a Change in Cor- rosion Potential of −0.175 V or more (i.e., less negative) is recommended to give a good confidence of adequate bond. Organic Residue Extraction—The prediction interval for organic residue extraction with a one-sided confidence level of 90% is shown in Figure 7. As can be seen in this figure, the pre- diction interval does not exceed 0.313 ksi anywhere over the range of test results observed in this study. For that reason, no threshold can be determined. A similar analysis was attempted considering only those sources with organic residue that the FTIR analyses indicated was primarily stearate. This was done to eliminate potentially confounding influences of 26 Predictor Constant ( x 0 ) Coefficient ( ) Coefficient of Determination ( R 2 ) Threshold Corresponding to Mortar Pull-Out Stress of 0.313 ksi Weight Loss on Ignition (m g/cm 2 ) 0.445 -1.403 0.16 Not Possible Contact Angle after Lime Dip (°) 1.393 -0.012 0.57 73 Change in Corrosion Potential after 6 h (V)—as Received 0.766 1.741 0.68 -0.175 Extracted Organic Residue (m g/cm 2 ) 0.453 -1.752 0.12 Not Possible Extracted Organic Residue (m g/cm 2 )—Stearate Only 0.436 -1.943 0.63 Not Possible Table 7. Regression coefficients for single-predictor models. Loss on Ignition (mg/cm2) y = -1.4031x + 0.4454 R2 = 0.1595 -0.100 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 -0.05 0 0.05 0.1 0.15 M or ta r P ul l-O ut 0 .1 -in S lip S tre ss (k si) Loss on Ignition (mg/cm2) Prediction Interval Lower Bound Figure 4. Prediction interval (confidence level  90%) for Loss on Ignition. Threshold not possible.

non-stearate-based lubricants and other surface contaminants. The prediction interval for this stearate residue with a one-sided confidence level of 90% is shown in Figure 8. As can be seen in this figure, the R2 is higher, but the prediction interval still does not exceed 0.313 ksi anywhere over the range of test results ob- served in this study, and no threshold can be determined. Thresholds Based on Regression with Multiple Predictors An attempt also was made to determine if combinations of test results (e.g., a combination of contact angle and organic residue extraction test results) correlated with bond perfor- mance. Although numerous linear combinations were exam- ined, the three combinations that showed the best correlation, based on the adjusted coefficient of determination (R2 adj.), were as follows: • Contact Angle Measurement after Lime Dip & Change in Corrosion Potential, • Contact Angle Measurement after Lime Dip & Organic Residue Extraction (100% stearate only), and • Weight Loss on Ignition (LOI) & Contact Angle Measure- ment after Lime Dip & Change in Corrosion Potential. Note that for multiple-predictor regression, a larger number of variables will increase the R2. Therefore, the adjusted R2 statistic, which accounts for the number of degrees of freedom in the dataset, was calculated as a means to compensate for this potentially misleading effect. The regression coefficients 27 (°) Contact Angle after Lime (°) y = -0.0123x + 1.3929 R2 = 0.5701 -0.100 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 60 70 80 90 100 110 M or ta r P ul l-O ut 0 .1 -in S lip S tre ss (k si) Contact Angle After Lime (°) Prediction Interval Lower Bound Threshold on Contact Angle After Lime (°) Threshold on Mortar Pull Out Figure 5. Prediction interval (confidence level  90%) for Contact Angle after Lime Dip. Change in Corr. Pot. (V) y = 1.7413x + 0.7656 R2 = 0.6818 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 -0.350 -0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000 M or ta r P ul l-O ut 0 .1 -in S lip S tre ss (k si) Change in Corr. Pot. (V) Prediction Interval Lower Bound Threshold on Change in Corr. Pot. (V) Threshold on Mortar Pull Out Figure 6. Prediction interval (confidence level  90%) for Change in Corrosion Potential.

and the R2 adj. for these three models are given in Tables 8 to 10. The R2 adj. values for these combinations were high and equal to 0.73, 0.98, and 0.76, respectively. The regression indicated that the last combination of predictors listed above (Contact Angle Measurement after Lime Dip & Organic Residue Extraction) was a good predictor of bond and was performed based only on those strand sources that the FTIR analysis of the organic residue identi- fied as being stearate only. This limited the number of data points used to develop the regression model to five, but was done as a means of eliminating potentially confounding influ- ences of non-stearate-based lubricants on the results obtained by the contact angle and organic residue extraction measure- ment methods. Given the high level of correlation with the multiple regression approach, this model may be particularly useful in a production setting where the lubricant in use is known. The prediction interval can not be shown in a two- dimensional plot as was done with the single-variable models. This is because multiple combinations of variables can give the same output. For this reason, a separate prediction interval must be calculated for each combination of variables. To give a sense of how these multiple regression models might be used, tables have been prepared showing the predicted pull out, the 28 Extracted Organic Residue (mg/cm2) y = -1.7515x + 0.453 R2 = 0.1241 -0.100 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 M or ta r P ul l-O ut 0 .1 -in S lip S tre ss (k si) Extracted Organic Residue (mg/cm2) Prediction Interval Lower Bound Figure 7. Prediction interval (confidence level  90%) for Organic Residue. Threshold not possible. Extracted Organic Residue (mg/cm2) y = -1.9426x + 0.4355 R2 = 0.6282 0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 0.450 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 M or ta r P ul l-O ut 0 .1 -in S lip S tre ss (k si) Extracted Organic Residue (mg/cm2) Prediction Interval Lower Bound Figure 8. Prediction interval (confidence level  90%) for Organic Residue when FTIR analysis indicates organic residue is primarily stearate. Threshold not possible.

lower bound on the prediction interval, and the comparison of the lower bound and the actual pull-out test result with the specified mortar pull-out threshold of 0.313 ksi, for two of the three multiple regression models. These are shown as Table 11, which was developed for the model based on the combination of Contact Angle Measurement after Lime Dip & Change in Corrosion Potential, and as Table 12, which was developed for the model based on the combination of Contact Angle Mea- surement after Lime Dip & Organic Residue Extraction for those residues determined to be stearate only. Using Table 11 as the example, the first row shows the re- sults of these two individual QC tests obtained for Source 349. Based on the regression model, the predicted mortar pull-out stress at 0.1-in. slip is 0.264 ksi. The lower bound on the pre- diction interval for that combination of the two test results must be calculated specifically using those values and is 0.131 ksi. Since 0.131 ksi is less than the mortar pull-out threshold of 0.313 ksi, this source fails to meet the minimum threshold for the combined Contact Angle Measurement after Lime Dip & Change in Corrosion Potential performance. For Source 548, the lower bound on the prediction interval calculated for the specific combination of test results measured for that source is 0.420 ksi, and this source “passes” since this value exceeds the 0.313 ksi threshold. The last column shows whether that strand would be expected to pass based on the actual pull-out test result. The conclusions reached based on the prediction interval and the actual pull-out test results are consistent for Sources 349 and 548. However, this is not always the case; the evaluation process based on the prediction inter- val is by definition conservative, and some sources will be judged as failing that may not fail in the actual pull-out test. A similar example is shown in Table 10, which was developed for the model based on the combination of Contact Angle after Lime Dip & Organic Residue Extraction (for stearate- based residues). Interpretation of Elemental Analyses Relative to Manufacturing Processes The atomic absorption and colorimetric analyses of wash solutions from strands from various sources indicated varied 29 Predictor Coefficient Constant 1.209 Contact Angle after Lime Dip (°) -0.007 Change in Corrosion Potential after 6 h (V)—as Received 1.233 Adjusted Coefficient of Determination (R2 adj.) 0.73 Table 8. Regression coefficients for model based on Contact Angle Measurement after Lime Dip & Change in Corrosion Potential. Predictor Coefficient Constant 0.864 Contact Angle after Lime Dip (°) -0.006 Extracted Organic Residue (mg/cm2) -1.093 Adjusted Coefficient of Determination (R2 adj.) 0.98 Table 9. Regression coefficients for model based on Contact Angle Measurement after Lime Dip & Organic Residue Extraction (stearate-based residues). Predictor Coefficient Constant 1.203 Weight Loss on Ignition (mg/cm2) -0.846 Contact Angle after Lime Dip (°) -0.006 Change in Corrosion Potential after 6 h (V)—as Received 1.178 Adjusted Coefficient of Determination (R2 adj.) 0.76 Table 10. Regression coefficients for model based on Weight Loss on Ignition (LOI), Contact Angle Measurement after Lime Dip & Change in Corrosion Potential. Mortar Pull-Out 0.1-in Slip Stress (ksi) Strand Source ID Contact Angle after Lime Dip (°) Change in Corrosion Potential (V) Experimentally Determined in Pull-Out Test Value Predicted by Regression for QC Results Lower Bound of Prediction Interval Pass/Fail* Based on Prediction Interval from QC Tests Pass/Fail* Based on Pull-Out Test Result 349 87 -0.289 0.156 0.264 0.131 Fails Fails 548 79 -0.080 0.623 0.576 0.420 Passes Passes 697 68 -0.154 0.606 0.559 0.417 Passes Passes 717 94 -0.241 0.206 0.276 0.136 Fails Fails 478 73 -0.272 0.409 0.38 0.232 Fails Passes 960 76 -0.211 0.409 0.435 0.303 Fails Passes 102 87 -0.266 0.315 0.291 0.161 Fails Passes 103 79 -0.172 0.397 0.463 0.331 Passes Passes 151 98 -0.322 0.273 0.149 0.003 Fails Fails * Threshold for passing is 0.313 ksi. Table 11. Evaluation of prediction interval for model based on Contact Angle Measurement after Lime Dip & Change in Corrosion Potential.

levels of sodium, calcium, potassium, zinc, boron, and phos- phate on the strand samples. The presence and concentration of these elements is largely governed by the specific pre- treatment process and wiredrawing lubricants used in the manufacturing of strand from each specific source, although other sources of some of these elements may also be involved. The pretreatments commonly used in strand production are zinc phosphate and sodium borate, also known as borax. Lubricants typically consist primarily of either sodium stearate, calcium stearate, or some other fatty acid blend. Tables 13 and 14 show each strand source ranked in ascend- ing order of concrete or mortar pull-out bond performance together with the prominent elements removed during the extraction process, and the presumed pretreatment and 30 Mortar Pull-Out 0.1-in Slip Stress (ksi) Strand Source ID Contact Angle after Lime Dip (°) Extracted Organic Residue (mg/cm2) Experimentally Determined in Pull-Out Test Value Predicted by Regression for QC Results Lower Bound of Prediction Interval Pass/Fail* Based on Prediction Interval from QC Tests Pass/Fail* Based on Pull-Out Test Result 717 94 0.117 0.206 0.211 0.176 Fails Fails 478 73 0.033 0.409 0.420 0.388 Passes Passes 960 76 0.035 0.409 0.401 0.371 Passes Passes 102 87 0.069 0.315 0.303 0.274 Fails Passes 151 98 0.037 0.273 0.276 0.240 Fails Fails * Threshold for passing is 0.313 ksi. Table 12. Evaluation of prediction interval for model based on Contact Angle Measurement after Lime Dip & Organic Residue Extraction (100% stearate only). Strand ProminentElements Presumed Pretreatment Presumed Lubricants Concrete Pull- out Bond Stress (ksi) 153 Ca, Zn, P zinc phosphate calcium salt of fatty acid - KSU-H Na, K, B borax Na/K stearate 0.209 SC-F Ca, Zn zinc phosphate calcium stearate 0.223 101 Na, K, B borax Na/K stearate 0.241 KSU-F Na, K, B borax Na/K stearate 0.241 102 Na, Ca, B borax Na/Ca stearate 0.441 SC-H Na, K, Zn zinc phosphate sodium stearate 0.472 151 Na, B borax calcium stearate 0.541 SC-IS Na, Zn zinc phosphate sodium salt of fatty acid 0.682 103 Na, Zn zinc phosphate sodium salt of fatty acid 0.944 Table 13. Compounds likely used in manufacture of each source—historic and recently manufactured strands. Strand ProminentElements Presumed Pretreatment Presumed Lubricants Mortar Pull- Out Bond Stress (ksi) 349 Ca, Zn, P zinc phosphate calcium salt of fatty acid and resin 0.156 717 Na, Ca, Zn, P zinc phosphate Na/Ca stearate 0.206 478 Na, K, Zn, P zinc phosphate Na/K stearate 0.409 960 Na, K, Zn, P zinc phosphate Na/K stearate 0.409 697 Ca, Zn, P zinc phosphate calcium salt of fatty acid and resin 0.606 548 Na, K, Zn, P zinc phosphate Na/K salt of fatty acid 0.623 Table 14. Compounds likely used in manufacture of each source—OSU strands.

lubricants used based on the prominent elements. Knowledge about the types of pretreatments and drawing lubricants can be an important part of the interpretation of the QC test results. These results of the elemental analyses suggest that the wires in the sampled strands were pretreated using one of two methods during the manufacturing process. Those strands that carried high amounts of boron typically carried low amounts of zinc and phosphate. As expected, the lubricants appeared to be either calcium or sodium/potassium stearates, but a number of strands showed evidence of both. This may result from different types of lubricants being used in separate dies in the drawing process. There is a greater amount of sodium, potassium, and calcium than would be expected from the stearate compounds alone (based on the concentration of the organic residues extracted). Therefore, other sources of these elements are contributing to the values measured here. Possible sources include chemicals in the pretreatment processes described above, detergents used for cleaning, or fillers, such as lime, used in some drawing lubricants. Interpretation and Applications Development of Quality Control Program for Strand Bond A number of QC test methods for predicting strand bond performance have been developed and evaluated in this test- ing program. The value of these tests was judged based on the correlation observed between these methods and mechanical testing methods. Three pull-out test procedures, differing mainly by the embedment material, were also examined dur- ing this program. Although pull-out testing from concrete appears to correlate best with transfer length, the most reli- able and realistic measure of bond performance, the Correla- tion Round of this test program had to be based on available mortar pull-out results provided from the NCHRP 12-60 Program. The following four test methods showed the best correla- tion with pull-out bond 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 QC tests have been divided into two categories, de- pending 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. A main objective of this study was to develop test methods that were more easily performed at more frequent intervals than mechanical pull-out tests, which are time consuming, especially for a prestressing strand producer. The three rec- ommended Level I QC tests are all easier to conduct than pull-out tests and, although they require some training and the acquisition of some specialized equipment, could be con- ducted by strand producers or precasters. If a QC lab was set up, it is envisioned that performing all three of these tests on a given sample of strand would require less than four hours of an appropriately trained QC inspector’s time. If more than one sample is tested, the amount of time required per sample would be much less since much of the effort would be duplicative. Although, as discussed further in the next sec- tion, the definition of thresholds on all four of these tests was not straightforward, all of these methods showed a correla- tion to bond performance in concrete, mortar, or both and would have value in a QC program as an indicator of bond quality. Recently, strand manufacturers in the United States have begun conducting pull-out testing from a single, specially prepared spool of 1/2-in. strand on a quarterly basis. Although this is obviously better than no testing, it currently repre- sents a small portion of the strand produced annually by each supplier. Therefore, it is suggested that the recommended Level I QC methods could be conducted by strand producers on a weekly basis for each size of strand produced. As a frame of reference, a requirement of weekly testing is much less onerous than the QC program requirements for at least one other reinforcing steel product—during production of epoxy-coated reinforcing steel at many manufacturing facil- ities, a number of QC tests, such as checks of blast cleaning effectiveness and coating flexibility, are conducted more fre- quently than every four hours of production. It is also not un- common for precasters to test concrete properties (including slump, air content, and strength) more frequently than once per day. Regular QC testing should greatly decrease the likelihood that poor bonding strand would reach the market, and this type of testing would be a valuable supplement to the quarterly testing of only a single size of strand currently being performed. When lots of strand are produced that exhibit suspicious behavior identified by these test methods, this could then prompt additional testing using the Level II organic residue extraction test and mechanical pull-out testing. It is also noted that routine QC analyses of new batches of the drawing lubricants are not routinely conducted. Instead, problems with lubricant are generally only noted while the wiredrawing process is ongoing. Although the development of such a test program was beyond the scope of this research, greater QC as part of the lubricant acquisition process also would add to the confidence in bond quality. 31

Thresholds Thresholds for two of these QC tests (Contact Angle Mea- surement after Lime Dip and Change in Corrosion Potential) have been developed. This was done based on prediction in- tervals for the regression calculated from the available data, a minimum criterion on the mortar pull-out stress adopted by NASPA, and a selected confidence level. The available data, consisting of the mortar pull-out results and QC test re- sults for the included strand sources, were not sufficient to allow threshold determination for the other two methods with the same constraints. The thresholds that were possible were calculated in a conservative manner to ensure adequate bond performance. However, the 90% confidence prediction interval thresholds on the change in corrosion potential and contact angle test would suggest that of the nine samples (two of which came from the source) included in the program, only two and three of the nine samples would be judged to be acceptable based on these test methods, respectively. Although any conservative approach for predicting a response based on an empirically developed relationship should be expected to underestimate that response, this is in contrast to the six of nine that would be judged acceptable based on the pull-out test itself. The inabil- ity to develop thresholds for two QC test methods and the strongly conservative nature of the thresholds that were developed has resulted from the large prediction intervals calculated for these relationships. Regression with multiple predictors also has been performed to determine if results of selected QC methods could be combined to better predict bond. The following three com- binations showed the best correlation, based on the adjusted coefficient of determination (R2 adj.): • Weight LOI & Contact Angle Measurement after Lime Dip & Change in Corrosion Potential, • Contact Angle Measurement after Lime Dip & Change in Corrosion Potential, and • Contact Angle Measurement after Lime Dip & Organic Residue Extraction (for stearate based residues). 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 multiple-predictor regressions can not be determined using the same procedure used for single- predictor regressions. Instead, the lower bound on the pre- diction interval must be calculated for each combination of test results. One of the models (for the combination of Contact Angle Measurement after Lime Dip & Organic Residue Extraction) for stearate-based residues accurately predicted performance. However, the two models that included all nine of the strand samples both predicted that three of these strand samples would be judged to be acceptable, in contrast to the six of nine that would be judged acceptable based on the pull- out test itself. Although these multiple-predictor regression models do appear to be more effective than the individual QC tests, the strongly conservative nature of the conclusions regarding acceptable performance is related to the large pre- diction intervals. There are a number of possible reasons, as follows, that the prediction intervals are not smaller: • The QC test methods are inadequate—It is possible that the QC methods do not measure a property of the strand that is sufficiently strongly linked to bond per- formance. It is also possible that the QC tests measure only a part of what determines bond and that other fac- tors exist that are equally or more important. This may mean that, while an individual QC test result is not suf- ficient to determine strand bond performance by itself, the QC result must be combined with the result from another test. • The QC test methods or the mortar pull-out test method were susceptible to large scatter—All four of the recom- mended test methods produced regression models that predicted average results for the range of QC test results obtained that spanned the mortar pull-out threshold. However, the difference between the predicted average result and the lower bound on the prediction interval is strongly influenced by the scatter about the best-fit line. Recall that the line fit shown on the regression plots is the average result (i.e., half of the pull-out test results will be above this line and half will be below). A large amount of variation in the data used in the regression analysis will lead to a large prediction interval. Therefore, despite the fact that the method may test a property strongly linked to bond, if that property is difficult to measure with good precision, defi- nition of threshold may be difficult. It should also be noted that this large scatter may be the result of significant local variations in the bond properties of the strand. It has been suggested by other researchers that the concentration of lubricant residue is highly variable even within a single spool and that significant differences may exist in strand separated by as little as 20 ft. Although some attempt was made to track the proximity of strand samples used in each of the various test methods in the Screening Round, this was not possible in the Correlation Round, and any such variation has become inseparably combined with variations in the test methods. • The sampled sources were too closely grouped in terms of bond performance—In any regression analysis, greater 32

separation of the ranges of dependent variables (QC test result) and independent variable (pull-out response) will result in greater confidence in the model that is developed. Even for the same scatter on the individual test results, if samples with a wider range of performance were used, a smaller prediction interval would result. • A limited number of data points were available for the re- gression analysis—A greater number of data points would increase the confidence in the regression model estimates’ ability to accurately predict performance. This would re- duce the prediction interval. Future of Quality Control Program and Thresholds The discussion of the QC test methods and QC test program given here has focused on the sampled sources used in this test program. A number of thresholds have been proposed based on the relationships between the QC test and the mortar pull-out test results for this sample set. Although a signifi- cant amount of work and scientific rigor has gone into their development, these thresholds should not be considered absolute and immutable. Because of the issues discussed and the finite nature of this test program, the number of samples included in the re- gression analyses was limited. However, it is suggested that the threshold development process could be an ongoing process. If the recommended QC methods were conducted on the samples selected for inclusion in the quarterly mor- tar pull-out test program currently underway by NASPA plants, that would provide nine additional data points for inclusion in the regression dataset each quarter. These data could be included in future regression analysis and used to refine the existing thresholds or perhaps allow the definition of future thresholds for those methods for which determi- nation was not possible at this time. Even if these sources proved to be all of similar bond quality, the additional data would likely serve to improve confidence in the regression model. The proposed thresholds can be applied to new sources of strand, but this should be done with some caution. Strand produced with a different pretreatment process or with a lubricant with significantly different chemistry may not respond similarly to strand produced with a borax or zinc phosphate pretreatment and largely calcium or sodium stearate lubricants. A main objective behind the inclusion of the FTIR test in the organic residue extraction test method is to confirm that the organic component of the lubricant at least is consistent. It is likely that the existing thresholds will reject a larger number of sources than the mortar pull-out test alone. If the type(s) of pretreatment and lubricants used in the wiredrawing process are known, method-specific correlations and thresholds could be determined. For example, if only a borax pretreatment and stearate-based drawing compounds were used to manufacture the strand in a certain plant, the effects of other wiredrawing compounds on the QC test results would not be present, and it is likely that a more consistent response would be achieved (resulting in higher R2 and smaller prediction intervals). Based on the same predefined mortar pull-out threshold, different QC test thresholds could be de- veloped for that particular manufacturing process. This would require each manufacturer to maintain a record of QC test and pull-out test results from strand they produced. With the computational tool developed as part of this program, regres- sion analysis could be conducted. A QC program developed on this basis will be the most effective use of the recommended QC test methods. Computational Tool As mentioned, a computational tool in the form of a Microsoft Excel-based spreadsheet has been developed. This Excel workbook was designed to predict whether prestressing strand will exhibit adequate bond properties based on results from surface and chemical QC test methods developed as part of this project. The workbook performs this prediction according to the procedures outlined in Appendix B of this report. This is done by calculating the prediction intervals for single- and multiple-predictor regressions and for determining the threshold on the QC test that corresponds to a predefined threshold using the mechanical test method. This tool is de- signed to: (1) develop a regression model to predict mortar pull-out stress from inputted surface and chemical QC test and mechanical (mortar) pull-out test results, (2) establish the lower bound of the prediction interval for the regression model for a desired level of confidence, (3) compare the lower bound of this prediction interval with a predefined threshold for the mortar pull-out stress with the chosen level of confidence, and (4) determine a pass/fail threshold for the QC test, if possible. There are individual worksheets for each of the recommended QC tests and for the recommended combinations of these tests. The user has the ability to modify the following inputs: desired level of confidence (default: 90%), threshold for ac- ceptability (default: 0.313 ksi), and the QC test result for a new strand source. The acceptability of a given source is determined by com- paring the lower bound on the prediction interval to a prede- fined pass/fail threshold for the mortar pull-out test. Based on this comparison, a judgment is made as to whether the source exhibiting the QC test results is expected to exceed that 33

threshold with the chosen confidence level. Indication is then given if the source passes (lower bound on prediction interval above threshold) or fails (lower bound on prediction interval below threshold). For the combined, multiple-predictor re- gressions, the prediction interval is different for each set of input QC results. A universally applicable set of thresholds on the QC test re- sults can not be generated, and the prediction interval cannot be simply plotted. Instead, the prediction interval must be cal- culated separately for each combination of QC test results. This tool accomplishes that calculation task. The multiple-predictor analysis is interpreted in terms of the pass or fail statements. 34

Next: Chapter 4 - Conclusions and Recommendations »
Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!