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22 the basis for evaluating the ability of the pull-out test meth- performance (Russell and Paulsgrove 1999, Russell 2001, ods to accurately measure bond characteristics. The coeffi- and Russell 2006). This correlation, evaluated during the cients of determination calculated based on the regression Screening Round of testing, was not explored further in the between these methods and the average bond stress over the Correlation Round. transfer length are given in Table 3. The concrete pull-out test results correlated better with bond quality than the Chemical and Surface Testing other pull-out test methods that were evaluated. Pull-out testing from mortar also showed promise as a means to eval- Transfer length was not measured on the historic strand uate bond, and the existing correlation is deemed sufficient and only partial transfer length test results were available for to justify further study. This limited program can not be the OSU strand. Consequently, the evaluation of the effec- considered a definitive evaluation of these methods; just tiveness of the chemical and surface QC test methods in pre- three strand sources were evaluated. Nevertheless, this con- dicting bond performance was determined by comparing clusion that pull-out from concrete is the superior test is the QC test results against performance measured in pull-out contrary to that of other studies of pull-out test methods, tests. The coefficients of determination for these QC methods including that sponsored by NASPA, which have concluded are given in Table 4. The P-values for selected methods are that the mortar pull-out test is superior at assessing bond given in Table 5. Table 4. Coefficient of determination (R2) from linear regression with concrete and mortar pull-out at 0.1-in. and 1st slip. Coefficient of Determination (R2) from Regression with Mechanical Test QC Test Method Mortar Pull- Level Concrete Pull-Out Out (0.1-in. (0.1-in. and 1st Slip) Slip) As received I 0.04 0.35 After Ca(OH)2 dip I 0.61 0.57 Contact Angle (°) After Ca(OH)2 dip-- I 0.44 0.84 stearate only 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 Change in Corrosion Potential after 6 h After Ca(OH)2 dip I 0.80 1.00* After ignition I N.A. 0.00 Surface Roughness, Ra I 0.93 0.16 As received II 0.67 0.09 Corrosion Rate After Ca(OH)2 dip II 1.00 0.00 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.
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23 Table 5. P-value from linear regression with concrete and mortar pull-out at 0.1-in. and 1st slip. P-Value from Regression with Mechanical Test QC Test Method Mortar Pull- Level Concrete Pull-Out Out (0.1-in. (0.1-in. and 1st Slip) Slip) After Ca(OH)2 dip I 0.039 0.019 Contact Angle (°) After Ca(OH)2 dip-- I 0.262 0.029 stearate only 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. At the initiation of this study, the surface and chemical Greater concentrations of residue make the strand surface methods were divided into Level I and II QC tests, as based more hydrophobic and increase the contact angle. It is rec- on the required effort and complexity of each test. These ommended that this method be included as part of a future correlations are discussed separately here, since the level of QC program. correlation required to justify the use of each test method is · Examination under UV light--A limited quantity of fluo- different for each QC level. As can be seen in Table 4, a num- rescing material was observed, and no correlation to bond ber of the surface and chemical test methods that showed good was found. This method should be abandoned. correlation with concrete pull-out test results did not corre- · Testing pH--The pH test was successful in finding a cor- late as well with the mortar pull-out test results. This may be relation with bond as measured by concrete-based pull- indicative of the inadequacy of the surface and chemical out testing on a limited dataset, but it was unsuccessful at methods, but may also be related to inaccuracies or inconsis- finding a similar correlation based on mortar pull-out test tencies in the pull-out test methods. 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 Level I Quality Control Tests produced with borax pretreatments. More study is needed The objective of the Level I QC test methods is to quickly before a recommendation regarding the adaptation of this and easily determine if strand properties that have been cor- method can be made. related with questionable bond are present. The minimum · Loss on ignition--A good correlation was found between the correlation required for these tests to be useful is somewhat weight LOI and bond performance measured in concrete lower than for the Level II QC tests. pull-out tests. Further statistical analysis suggests that there is greater than 99% confidence that the relationship between · Contact angle--Contact angle correlated with bond only concrete pull-out and this test method is significant. This after the strand sample was subjected to exposure to a correlation and significance was not found based on mortar saturated calcium hydroxide solution. This correlation is pull-out test results. Nevertheless, this is one of the easiest higher for those sources judged to carry only stearate-based tests to perform and is recommended for future QC testing, lubricants, when performance assessed with mortar pull-out although not alone. Some other measure of bond perfor- is considered. Nevertheless, the P-values calculated when mance should be included along with LOI in a QC program. comparing this test against mortar and concrete pull-out · Loss in alkali bath--Multiple cleaning procedures using testing are low (0.039 and 0.019, respectively), suggesting sodium hydroxide solutions were attempted, but no cor- that the relationships between both pull-out test methods relation was observed between the weight loss and bond in and this surface test are statistically significant. It is likely concrete. Although a higher correlation was found with the that this high correlation after the calcium hydroxide solu- mortar test, this higher correlation is only based on three tion exposure occurs because the resulting residues are sources. Interestingly, this is the only method suggested by similar compounds (the stearates having converted mostly the Wire Association International manual for measuring to calcium stearate) that influence the surface tension in surface residues on wire. It is recommended that this test proportion to their concentration on the strand surface. be abandoned.
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24 · Change in corrosion potential--The drop in corrosion this test, it is not recommended for inclusion in a future potential showed a good correlation with bond in both QC program. the Screening and Correlation Rounds of evaluation. The · Organic residue extraction--The concentration of the P-value (0.006) calculated when comparing this test organic residue correlated well with the bond performance against mortar pull-out testing suggests that the relation- in concrete, but only moderately with bond in mortar. ship between mortar pull-out and this test method is sta- Nevertheless, the P-value calculated when comparing this tistically significant. Testing after exposure to a saturated test against mortar pull-out testing for all samples was less calcium hydroxide solution showed better correlation than 0.01. This test is time consuming to perform, but gives than testing in the as-received condition in the Screening the best direct measure of the type and quantity of drawing Round; however, this higher correlation is only based on lubricants left on the strand surface during the manufac- results from three sources, and it was judged that this turing process. Of all the methods proposed, this method additional conditioning effort is not worthwhile. It is evaluates the property of the strand tied most obviously hypothesized that the increased tendency for corrosion to bond quality. The presence of organic lubricants on the measured on poor bonding strand is a consequence of surface of the strand can only be expected to reduce bond greater surface roughness measured at the microscopic performance. Therefore, it is recommended that this method scale. This microscopic roughness occurs at too fine a be included as part of a future QC program. FTIR spec- scale to affect bond through mechanical interlock, but troscopy should be performed on the organic residues makes the strand more likely to accumulate lubricant that result to ensure that residues being evaluated are residue, which leads to poor bonding behavior. It is rec- consistent. This is necessary because the effect of residues ommended that this method, conducted on strands in with different chemistries is unlikely to be proportionally the as-received condition, be included as part of a future similar (e.g., a stearate-based lubricant residue will likely QC program. effect bond differently than a non-stearate-based lubricant · Surface roughness--The surface roughness parameter, Ra, residue). FTIR analyses will also identify contamination of correlated well with bond in concrete based on only three the samples from other organic materials, such as oils, sources, but not with bond in mortar. Since an increased greases, or from release agents. The correlation between roughness was associated with poor bond, it appears that mortar pull-out stress and residue concentration was much correlation to bond is not a direct effect, but is related to higher when those sources carrying only stearate-based the tendency of the wire surfaces to retain residue. The lubricants were included in the correlation analysis. profilometer used to measure this property is convenient · Elemental analysis--Atomic absorption and visual light for use in a QC setting, but does not appear to be sensitive spectroscopy were used to determine the surface concen- enough to measure the roughness at the scale needed, nor tration of various elements. The concentrations of sodium does it test a sufficiently large surface of the strand for the and boron showed signs of a correlation with the mechan- test result to be representative of a property that can be tied ical properties measured in pull-out tests in both concrete to bond performance. Therefore, it is recommended this and mortar. The concentrations of zinc, however, did not. method be abandoned. 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 Level II Quality Control Tests the normalized concentrations of boron, calcium, and The objective of Level II QC testing is to provide a more organic residue (as explained in Appendix B) showed good conclusive prediction of bond performance than possible correlation to pull-out bond in concrete. This correlation with Level I QC tests. These tests require either more advanced was not found with mortar pull-out results. Given the cost methods or more complicated equipment. The minimum cor- and equipment-dependent nature of the atomic absorption relation required for these tests is higher than that required testing, it is not recommended for inclusion in a future QC for Level I QC tests. program. · Corrosion rate--A strong correlation was measured between In summary, the following methods are recommended for corrosion rate and pull-out bond stress in concrete. How- inclusion in future QC programs: ever, the correlation between corrosion rate and pull-out bond stress in mortar was relatively weak. Given this lack · Weight Loss on Ignition (LOI), of consistent correlation, the uncertainty about the mech- · Contact Angle Measurement after Lime Dip, anisms involved in establishing the initial good correlation, · Change in Corrosion Potential, and and the complexity and equipment-dependent nature of · Organic Residue Extraction with FTIR Analysis.