Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete (2008) / Chapter Skim
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Appendix B - Evaluation of Mechanical and Chemical Test Methods
Appendix B - Evaluation of Mechanical and Chemical Test Methods
Pages 39-122
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From page 39... ...
39 A P P E N D I X B Evaluation of Mechanical and Chemical Test Methods
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55 Surface and Chemical Test Methods and Results ......................................................................................................................... 69 Contact Angle Measurement.................................................................................................................................................
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The mechanical methods included: • Pull out from concrete, • Pull out from Portland cement mortar, • Pull out from gypsum plaster-based mortar. The surface and chemical methods included: • Contact angle, • Examination under UV light, • pH, • Loss on ignition, • Loss in alkali bath, • Change in corrosion potential, • Corrosion rate, • Surface roughness, • Organic residue extraction/Fourier transform infrared (FTIR)
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From page 42... ...
The surface and chemical test methods were evaluated in the Screening Round based on the results of pull-out tests from concrete, again on strand samples from the same source. However, the evaluation of correlation of test results to bond in the Correlation Round of testing was based on results from a mortar pull-out test program associated with NCHRP Project 12-60 Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete.
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This was intended to test the repeatability of the surface and chemical test methods. Transfer length and mortar pullout test results were provided in tabular form by Russell after the chemical and surface testing had been completed.
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Mechanical Test Methods and Results The original project scope included the development of a performance-based test method for use in evaluating strand bond. The Screening Round of the experimental program included mechanical testing of strand sources using transfer length, pull-out from concrete (large concrete block pull-out test [LBPT]
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pull-out and transfer length test data accompanying strand from OSU.
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Table B-3. Transfer length and beam test data accompanying strand from OSU.
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Res. Water Acid Historic Strand KSU-F 1 2 -- -- -- -- -- 1 -- -- 2 2A 2A KSU-H 2 2 -- -- -- -- -- 2 -- -- 2 2A 2A SC-F 2 2 -- -- -- -- -- - -- -- 3 3A 3A SC-H 2 2 -- -- -- -- -- 2 -- -- 3 3A 3A SC-IS 2 2 -- -- -- -- -- 2 -- -- 3 2A 2A 101 2 2 -- 2 -- -- -- 2 -- -- 3 3AB 3A Recently Manufactured Strand 102 2 2 2 3 1 -- 6 2 1 -- 3 3AB 3A 103 2 2 2 3 1 -- 6 2 1 -- 3 3AB 3A 151 2 2 2 3 1 -- 6 2 1 -- 3 3AB 3A 153 -- -- -- 3 -- 3 12 -- 3 3 3 3 3 OSU Strand Samples 349 3 -- -- 4 -- 3 12 -- 3 3 3 3 3 548 3 -- -- 3 -- 3 12 -- 3 3 3 3 3 697 3 -- -- 3 -- 3 12 -- 3 3 3 3 3 717 3 -- -- 3 -- 3 12 -- 3 3 3 3 3 478 *
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The transfer length measured in this test is defined as the intersection of the strain on this plateau and the linear extension of the slope from the transfer region. Because of the problems experienced with this first set of transfer length specimens, a second set was fabricated and tested (Round 2)
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From page 49... ...
Thus, this larger strand had a slightly lower bond stress at transfer. To account for these issues and enable comparisons of stress transfer behavior between the tested strand sources, the average bond stress over the transfer length was calculated based on the measured transfer length and strand tensile stress.
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The elastic loss was determined based on the strain measured immediately after release in the central region of the test prism over which the strain is approximately constant, assuming no relaxation losses in the strand. Results of Transfer Length Testing Table B-6 and Table B-7 show the average bond stress over transfer length measured for Rounds 1 and 2, respectively.
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2, it would be expected that average bond stress over transfer length is inversely proportional to end slip. This has been confirmed in this study as shown in Figure B-18, which depicts the correlation between bond stress and the inverse of end slip for measurements made using a scale, photos, and a set of calipers.
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Examining the end slip measurements for Source 102 over time (Figure B-17) can reveal the limits of the end slip-average bond stress over transfer length correlation.
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The transfer length and the average bond stress over the transfer length are calculated based on numerous discrete measurements. Multiple methods for measuring end slip were employed in this study; all were difficult to correlate with transfer length bond stress.
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This measurement has proven to be a better long-term indicator of transfer length and is easier to perform than the mechanical strain measurement. Transfer Length Testing for Correlation Round of Evaluation As mentioned, the sampled sources used for the Correlation Round of testing were selected by Bruce Russell of OSU.
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Average Bond Stress over Transfer Length (psi) Strand Sample ID Beam End Initial After Cut 28 Days 6 Months 22 Months S-S 437 423 380 364 328 S-N -- 596 558 477 397 N-S -- 525 486 437 380 102-A-0.5-1A N-N 375 354 336 345 341 S-S 397 436 422 385 364 S-N -- 569 569 524 468 N-S -- 513 476 476 416 102-A-0.5-2A N-N 344 364 369 359 331 S-S 422 436 429 390 357 S-N -- 627 547 485 459 N-S -- 612 485 436 402 103-B-0.5-1A N-N 485 443 436 415 378 S-S 514 475 450 428 428 S-N -- 694 626 571 494 N-S -- 626 546 514 459 103-B-0.5-2A N-N 597 514 475 467 428 S-S 371 341 319 332 323 S-N -- 586 413 355 345 N-S -- 360 341 336 327 151-Z-0.5-1C N-N 332 336 319 315 315 S-S 349 359 354 334 309 S-N -- 399 369 330 302 N-S -- 434 427 419 359 151-Z-0.5-2C N-N 330 309 302 309 258 Bond stress values shaded in gray were measured at central cut or at the end of specimen influenced by release method and were ignored during computation of inter-round averages.
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Average Bond Stress over Transfer Length (psi) Strand Source ID After Cut or Release 28 Days 6 Months 18-22 Months 102 363 323 316 314 103 531 427 397 351 151 375 334 323 286 Round 1 measurements made at released ends (S-S)
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. Coefficient of Variation for Average Bond Stresses over Transfer Length (psi)
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Correlation between average bond stress over the transfer length and the inverse of end slip as measured with three different methods. Figure B-19.
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. Also, the test described herein continues up to maximum load while the NASPA test stops at an end slip of 0.1 in.
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(At a presentation to the PCI Prestressing Steel Committee in April 2004, Don Pellow presented some additional information from Russell, indicating that the load rate achieved at OU was actually closer to 7 kips/min.) The specimen configuration is shown in Figure B-30, and the test frame is shown in Figure B-31.
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Slip (psi) Strand Source ID Concrete Portland Cement Mortar Gypsum Plaster Mortar Historic Strand A KSU-F 241 -- -- KSU-H 209 -- -- SC-F 223 -- -- SC-H 472 -- -- SC-IS 682 -- -- 101 241 -- -- Recently Manufactured Strand 102 441 315 588 103 944 397 621 151 541 273 619 153 142/406B -- -- OSU Strand 349 -- 156 -- 548 623 697 -- 606 -- 717 -- 206 -- 478 *
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, which is the rate specified by the NASPA test, as presented in Figure B-39. The average bond stresses (calculated based on the nominal surface area of the embedded section of the strands)
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. Calcium hydroxide flakes were added to simulate the alkalinity of concrete, and the Hydrocal was combined with sand to limit the heat production generated during the rapid plaster hydration.
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Pull-Out Test Results The tests performed in the Screening Round resulted in six data sets for each of the three recently manufactured strand sources and for each of the three pullout test methods for a total of 54 data sets. In each data set is recorded time, load, ram travel, and end slip.
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The program interpolates the end slips, average, and standard deviation at each increment of load. The average bond stress is plotted against the end slip in the figures shown below with the standard deviation for end slip at that load increment included in a horizontal error bar on each side of each data point.
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This similarity is significant because the stress at 0.1-in end slip was not recorded during the historic concrete pull-out tests conducted on the many historic samples that were included in the screening evaluation of the chemical and surface test methods reported in the next section, Surface and Chemical Test Methods and Results. Therefore, in discussing the correlation between bond performance and the chemical test 67 Figure B-37.
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For comparison with mortar pull-out test results for the Screening Round of testing, the loads at 0.1-in. slip provided from OSU were converted to average bond stresses at 0.1-in.
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Surface and Chemical Test Methods and Results In this section, the experimental study conducted to evaluate the surface and chemical test methods is presented. The test procedure and results are covered separately for each method.
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The ignition process was performed on samples to volatilize organic compounds expected to be present in the drawing lubricants. The contact angle is measured on the projected shadow of a small drop of distilled water that has been applied to the strand surface with a syringe as pictured in Figure B-56.
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Contact angle testing after the calcium hydroxide exposure was also included in the Correlation Round. The ignition process to remove organic residue was only used in the 71 Figure B-47.
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Bond stress versus end slip from large concrete block pull-out test.
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This test method was not included in the Correlation Round. Testing pH Testing of the pH of the surface was attempted with each of the strand sources to see if measurement of the alkalinity of a solution generated by placing drops of water on the residue could be linked to bond.
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74 Comparison of bond stress at 1st obs. slip vs.
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Strand Source ID As-Received After Ca(OH) 2 After Ignition Historic Strand KSU-F 80 106 -- KSU-H 91 103 -- SC-F -- -- -- SC-H 87 80 -- SC-IS 83 79 -- 101 55 94 -- Recently Manufactured Strand 102 60 87 -- 103 71 79 -- 151 107 98 -- 153 -- 75 -- OSU Strand 349 -- 87 11 548 -- 79 15 697 -- 68 8 717 -- 94 12 478 *
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SC-H None SC-IS Speckles in crevices 101 None Recently Manufactured Strand 102 Dull glow in crevices (rust) 103 None 151 None Table B-12.
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Average pH reading Strand Source ID Interstitial Valley Peak Historic Strand KSU-F 9.0 8.0 KSU-H 8.8 7.3 SC-H 7.0 7.0 SC-IS 7.0 7.0 Recently Manufactured Strand 102 9.0 8.2 103 7.0 7.0 151 7.7 7.0
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. Average pH Strand Source ID pH Meter Duotest pH-Fix 7.5-9.5 Recently Manufactured Strand 102 9.36 8.7 9.0 103 8.32 7.4 8.0 151 9.13 8.3 8.4 153 -- 7.3 -- OSU Strand 349 -- 7.2 -- 548 -- 7.1 -- 697 -- 7.0 -- 717 -- 7.5 -- 478 *
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The ignition cleaning regime to remove organic residues was used only in the Correlation Round as a frame of reference representing strand without any residue. Surface Roughness Microscopic examinations of sectioned portions of wire taken from strand have indicated that an observable difference in the surface roughness of the good- and poor-bonding strand sources exists.
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Change in corrosion potential versus time for recently manufactured strand. Change in Corrosion Potential (V)
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The corrosion rate test was conducted on samples in the as-received condition and after calcium hydroxide exposure in the Correlation Round. Organic Residue Extraction The tests for identification and quantification of organic drawing-compound residues were based on solvent extraction procedures, together with gravimetric and FTIR analyses.
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2 After Ignition Historic Strand KSU-F 32.8 -- -- KSU-H 56.1 -- -- SC-F -- -- -- SC-H 87 -- -- SC-IS 83 -- -- 101 55 -- -- Recently Manufactured Strand 102 32.7 52.8 -- 103 9.4 10.2 -- 151 23 44.0 -- 153 -- 2.9 19.3 OSU Strand 349 3.1 3.1 13.3 548 3.9 2.0 12.3 697 76.8 46.4 29.4 717 30.5 12.9 14.3 478 *
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Historic Strand KSU-F stearic acid KSU-H stearic acid SC-F stearic acid SC-H stearic acid, acrylic SC-H stearic acid SC-IS fatty acid, trace styrene 101 stearic acid Recently Manufactured Strand 102 stearic acid 103 fatty acid 151 stearic acid 153 fatty acid OSU Strand 349 fatty acid, resin 478 stearic acid 548 fatty acid 697 fatty acid, resin 717 stearic acid 960 stearic acid * Stearate salts are converted to stearic acid by the organic residue extraction process.
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Evaluation of Mechanical Test Methods Before assessing the correlation of the surface and chemical test methods to bond performance, the correlation between the mechanical test methods was evaluated. This was possible only for the recently manufactured strand, since they were the only available sources that could be obtained in sufficient length to support transfer length testing.
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N.D. 0.096 0.007 0.103 Recently Manufactured Strand 102 (warm)
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From page 86... ...
slip during pull-out testing in concrete, mortar, and Hydrocal. This concrete bond stress is compared to the average bond stress over the transfer length for each strand source in Figure B-68.
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This was probably because the lubricant contained mixed forms of stearate. The contact angle on strand after ignition process also did not correlate well with the bond 87 102 103 151 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 Average Bond Stress over Transfer Length (ksi)
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slip Figure B-70. Hydrocal pull-out bond stress versus bond stress over transfer length (recently manufactured strand)
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Slip (psi) Average Bond Stress over Transfer Length (psi)
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does not guarantee satisfactory bonding performance. However, the pH test results appear to be influenced by the presence of borax pretreatments, which were apparently used on a number of the historic and recently manufactured strands, based on the elemental analysis of the residue extracts.
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The correlation here is less clear, even among the recently manufactured strand, which showed better correlation when compared to concrete pull-out results. However, high weight losses above 0.7 mg/cm2 did appear to be consistent with low bond.
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Loss in Hot Alkali Bath No trends were apparent between the weight loss after soaking the strand in a hot alkali (sodium hydroxide solution) bath and average bond stress over transfer length or pull-out bond stress at first or 0.1-in.
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Similar trends were observed based on the transfer length data. Because of this good correlation, this test was included in the Correlation Round of evaluation.
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A similar strong relationship exists between the extracted organic residue and the average bond stress over the transfer length. Because of this good correlation, this test was included in the Correlation Round of evaluation.
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slip in concrete (historic and recently manufactured strand)
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slip in concrete (historic and recently manufactured strand)
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Correlation between sodium concentration from warm-water wash and bond stress at first or 0.1-in. slip in concrete (historic and recently manufactured strand)
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Figure B-95 shows the total concentration of boron compared to the concrete pull-out bond stress. Low boron contents appear to correlate with higher average bond stress over the transfer length.
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Correlation between potassium concentration from the warm-water wash and bond stress at first or 0.1-in. slip in concrete (historic and recently manufactured strand)
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Correlation between zinc concentration from combined warm-water and acid/chloroform washes and bond stress at first or 0.1-in. slip in concrete (historic and recently manufactured strand)
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This may be explained by suggesting that high concentrations of any of these element or organic residues has the potential to produce poor bond, and that while a poor bonding strand may have a low concentration of one single element, it is likely to be high in at least one other. To investigate this idea, analysis was done to compare bond performance associated with various combinations of these elements.
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From page 102... ...
This combined index was defined as the average of the scaled residue concentrations for certain combinations of elements after the concentrations were scaled from 0 to 1, to account for differences in the magnitude of the measured concentrations. This was done for the total concentrations, that is, the sum of the materials removed using water and acid/chloroform washes in the Screening Round or the combined wash solutions in the Correlation Round, measured with warm water as the first step in the wash procedure.
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The correlation between bond and the methods that fall under each of these classifications were evaluated differently based on the strand sources that were collected for testing and the data quantifying bond performance that were available. The evaluation of correlations between the various pull-out testing methods and the bond were based on bond performance as measured with transfer length tests.
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Source Zinc Potassium Sodium Calcium Boron Organic Residue Combined Index for B, Ca, and Organic Residue Recently Manufactured Strand 102 0.003 0.483 1.000 0.648 0.853 0.594 0.698 103 0.642 0.270 0.583 0.055 0.302 0.098 0.152 151 0.003 0.119 0.417 0.064 1.000 0.316 0.460 OSU Strand 349 0.688 0.067 0.251 1.000 0.104 0.186 0.430 548 1.000 0.697 0.769 0.172 0.083 0.403 0.220 697 0.678 0.045 0.248 0.882 0.114 0.160 0.386 717 0.638 0.034 0.577 0.675 0.177 1.000 0.617 478 0.688 1.000 0.704 0.119 0.146 0.283 0.183 960 0.694 0.944 0.678 0.109 0.177 0.297 0.194 Table B-28. Scaled concentrations and combined index for comparison with mortar pull out.
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Correlation with Bond -- Mechanical Test Methods The concrete pull-out test results correlated better with bond quality than the other pull-out test methods that were evaluated, based on comparisons with transfer length tests conducted on three strand sources. However, pull-out testing from mortar also showed promise as a means to evaluate bond, and the existing correlation is deemed sufficient to justify further study.
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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 B-31.
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From page 107... ...
For the contact angle and organic residue extraction measurement methods, coefficients of determination have been calculated using only data from sources identified in the FTIR analyses as carrying only stearate-based lubricants. This was done to eliminate the potentially confounding influences of non-stearate-based lubricants.
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From page 108... ...
of Steel Strand, • Determination of the Surface Tension of Steel Strand by Contact Angle Measurement, • Change in Corrosion Potential of Steel Strand, and • Organic Residue Extraction with FTIR Analysis. Precision Testing The recommended QC methods have been written in AASHTO/ASTM standard method format in Appendix C, where they are titled: 1.
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From page 109... ...
Therefore, results of two 109 Test Method Strand Source Organic Residue Extraction 102 Contact Angle 102 Change in Corrosion Potential 103 Weight Loss on Ignition 717 Sample Set (three pcs. of strand)
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From page 110... ...
Two tests, namely the Test Method for Change in Corrosion Potential of Strand and the Test Method for Contact Angle Measurement of a Water Droplet on a Strand Surface, were conducted on the same sources of strand that were tested as part of the Screening Round of evaluation in 2004. The results of the Precision testing program obtained in 2007 gave average results that differed from the earlier result by an amount greater than the acceptable single-operator ranges listed in the precision statements above.
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From page 111... ...
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. As has been done throughout this study, the thresholds were converted to bond stresses calculated as the force divided by the nominal surface area (the nominal perimeter of the strand multiplied by the embedment length)
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Fitted line plot for mortar pull-out stress versus the change in corrosion potential as-received. SUMMARY OUTPUT Regression Statistics Multiple R 0.826022976 R Square 0.682313957 Adjusted R Square 0.636930237 Standard Error 0.097193005 Observations 9 ANOVA df SS MS F Significance F Regression 0.1420221 0.142022 15.03433 0.00607324 Residual 7 0.066125 0.009446 Total 8 0.208147 Coefficients Standard Error t Stat P-value Lower 95% Upper 95% Intercept 0.765888672 0.105371 7.268469 0.000167 0.516724919 1.015052 Change in Corr.
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From page 113... ...
Instead, what is needed to interpret and practically apply a change in corrosion potential test result is the computation of a lower bound on the interval that, with 90% confidence, includes the pull-out stress for a strand sample with that change in potential test result. This type of interval is known as a one-sided prediction interval and is a standard part of regression theory and practice.
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From page 114... ...
To help with the calculations below, the vector contains Prediction Interval (x x t n p 0 0 1 0 90 )
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From page 115... ...
4) , the 90% prediction interval for a new strand sample that has a change in corrosion potential as-received of x0 = −0.1 V, would be This means that for a measured change in corrosion potential as-received of −0.100 V, the mortar pull-out stress at 0.1-in.
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From page 116... ...
. βˆi βˆ0 SUMMARY OUTPUT Regression Statistics Multiple R 0.922308764 R Square 0.850653457 Adjusted R Square 0.761045531 Standard Error 0.078849246 Observations 9 ANOVA df SS MS F Significance F Regression 0.1770613 0.05902 9.49306 0.016594131 Residual 0.0310865 0.006217 Total 8 0.208147 Coefficients Standard Error t Stat P-value Lower 95% Upper 95% Intercept 1.202523363 0.243392 4.940688 0.00432 0.576864667 1.828182 Weight Loss on Ignition -0.84478002 0.617114 -1.368919 0.229318 -2.431123313 0.741563 Contact Angle After Lime Dip -0.00632965 0.003492 -1.812644 0.129631 -0.015305971 0.002647 Change in Corr.
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From page 117... ...
Thresholds Based on Regression with Single Predictor The test methods that were recommended for inclusion are: • Weight LOI, • Contact Angle Measurement after Lime Dip, • Change in Corrosion Potential, • Organic Residue Extraction. The efforts made to define thresholds for each of these methods were based on single variable linear regressions and are described individually below.
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From page 118... ...
Prediction interval (confidence level 90%) for Contact Angle after Lime Dip.
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From page 119... ...
, were: • Contact Angle Measurement after Lime Dip & Change in Corrosion Potential, • Contact Angle Measurement after Lime Dip & Organic Residue Extraction (100% stearate only) , • Weight Loss on Ignition (LOI)
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From page 120... ...
The regression that indicated that the last combination of predictors listed above (Contact Angle Measurement after Lime Dip & Organic Residue Extraction) was a good predictor of bond was performed based only on those strand sources that the FTIR analysis of the organic residue identified as being stearate only.
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From page 121... ...
This included limited mechanical testing (pull-out testing from concrete, Portland cement mortar, and 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)
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From page 122... ...
Contact Angle Measurement After Lime & Organic Residue Extraction (100% stearate only)
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