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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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9Overview The work plan developed to achieve these objectives was divided into the following three phases. The initial phase involved gathering information about strand manufacturing and potential test methods from the prestressed concrete, strand manufacturing, and wiredrawing lubricant industries, as well as from the available literature. Based on this information, a number of chemical and surface test methods and performance-based (i.e., mechanical) test methods that were considered to have potential for use in a quality control program were proposed for evaluation. In the second phase of this work, the proposed surface and chemical test methods were conducted on a limited number of available sources of strand with variable bond properties: (1) to evaluate the ability of these methods to predict bond performance and (2) to assess their suitability for routine quality control operations. In addition, performance-based tests were conducted on some of the same strand sources. Those test methods that showed good correlation with bond performance were selected for further study, while those that did not were abandoned. A parallel set of investiga- tions, termed supplemental investigations, was conducted to learn more about the relationship between bond and residual lubricants. In the third and final phase, the promising surface and chemical test methods were performed on a different group of strand sources to validate their correlation with bond performance. At the direction of the supervisory panel, bond performance was quantified by another researcher using the pre-existing performance-based test procedure. Testing was also conducted to support the development of a precision statement. Finally, statistical analysis was performed to iden- tify minimum acceptance thresholds for the surface and chemical test methods that would predict adequate bond performance as defined by the pre-existing performance-based test procedure. Industry Survey Letters and accompanying questionnaires were sent out to obtain additional information about strand production. One letter was sent to strand producers and the other was sent to drawing lubricant manufacturers. No one returned the ques- tionnaire. Instead, the research team received a few phone calls, primarily from strand producers who expressed concern that this research would interfere with their business. Several lubricant producers were cooperative when con- tacted by phone. They shared some information about their products and their use in strand production. To facilitate discussion, a meeting was held with members of the project team, NASPA, and Bruce Russell, co-principal investigator of NCHRP Project 12-60: Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete, and a consultant to NASPA. This meeting was held at WJE’s Northbrook, IL, headquarters on 7 January 2004. The objective of this meeting was to present the strand pro- ducers with an opportunity to give feedback on the proposed work plan for this project, which had been provided in advance for their review, and to provide the project team the opportunity to gain some insight into industry practice and the applicability of the proposed testing methods. In addition, it was hoped that this meeting would lay the groundwork for future interactions regarding this project. Quality Control Program Development The purpose of a QC program is to assess, by routine mon- itoring and testing, whether a particular level of quality is maintained during production. In this context of strand bond, the desired QC program would evaluate the surface condition of strand so that steps can be taken in a timely manner, if needed, to ensure that the bond between strand and concrete products is reliable and structurally adequate. C H A P T E R 2 Research Approach

In the past, the quality of bond has been evaluated using mechanical methods, such as pull-out or transfer length tests. However, such methods are expensive, time consuming, and conducted infrequently; currently, routine pull-out tests are conducted quarterly by strand manufacturers. A main goal of this project was to develop fast, accurate, reproducible, simple to conduct, and inexpensive QC test methods for detecting and measuring the level of deleterious residues on strand that could be performed frequently (i.e., weekly). A number of methods were proposed involving testing sur- face and chemical properties of the strand that could be linked to strand bond. All tests were intended as part of a routine testing program that could be conducted by strand manufac- turers, precasters, or other interested parties. Quality Control Program Overview The individual tests that were proposed required a varied range of time, expertise, and equipment. Therefore, it was envisioned that they would be applied as part of a two-tiered QC program, in which the value of the test is proportionate to the test complexity, with the following distinct components: • Level I QC component (QC-I) and • Level II QC component (QC-II). The Level I QC component consists of relatively quick, simple, and inexpensive tests that can be conducted by strand manufacturing personnel. These tests would be performed on a daily basis. Each test would take less than one-half hour to perform. The Level II QC component consists of tests that require more in-depth training and advanced equipment, and could be performed by testing laboratories on behalf of strand man- ufacturers. These tests would be performed at longer intervals, with changes in processes, or as dictated by the Level I QC test results. Evaluation of Proposed Surface and Chemical Test Methods Several chemical and surface tests were proposed to predict poor bonding characteristics for strand. The purpose of the experimental program conducted in this research program was to determine if these proposed tests would be applicable for use in a QC program. To do this, several rounds of exper- imentation were needed. Screening Testing The first round of experiments consisted of “screening” ex- periments. The objective for the screening experimentation was to eliminate those tests that were not helpful for predict- ing bond performance. Thus, the first step of the analysis in this round of testing was to estimate the correlation between each surface or chemical test and bond performance, and the second step was to identify those methods where some degree of correlation was indicated. For each source of strand, bond performance was measured in terms of pull-out stresses, transfer lengths, or both. Accordingly, for the purpose of this analysis, the bond performance was treated as the independ- ent variable. The best experimental design for estimating a correlation is to place the design points as far apart as pos- sible in terms of the independent variable. Thus, the optimal statistical design is to run each test on strands that show a range of bonding performance. For the screening experi- ments high, medium, and low bonding sources were desired. However, efforts to obtain a very low bonding strand were not successful. Although reports of low bonding strand inci- dents continue to surface in the precast concrete industry, “unused” samples of such strand remained elusive. There- fore, the screening tests on new strands were run on what are essentially high and intermediate bond strands. Correlation Testing The second round of experiments was performed for confirmation and calibration purposes. This round involved running additional tests using those methods that showed promise in the screening experiments. These selected tests were conducted on five new strand sources. This complete dataset was then used to assess the correlation between the QC tests and bond performance, and to determine if the tests were able to accurately identify good and bad strand. It was also used as a basis for discussing pass/fail criteria for accept- able bond performance. Precision Testing A third round of testing was conducted to determine the precision (i.e., repeatability) of those methods, showing good correlation with bond strength. This was used to develop pre- cision statements included in the published test methods. Basis for Evaluation—Transfer Length and Pull-Out Tests Transfer length is the most reliable and realistic measure of bond performance. During the screening testing, the eval- uation of correlations between the pull-out tests and bond performance were based on performance as measured with transfer length tests conducted on the same sources of strand. Pull-out testing was conducted as part of the screening studies using three materials as the test matrix: a concrete, 10

a Portland cement mortar, and a gypsum plaster mortar. Based on comparisons with transfer length tests conducted in this study and described in Appendix B, the concrete pull-out test showed the best correlation with bond quality. 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 pro- gram associated with NCHRP Project 12-60. The principle in- vestigator from that project supplied the strand samples for this portion of the study. No pull-out testing was conducted in the Correlation Round of the experimental program. Strand Samples To assess the effectiveness of the mechanical and surface chemistry-based testing procedures, it was essential that samples representing the range of possible performance be evaluated. Since neither precasters nor strand suppliers were enthusiastic about associating themselves with poor-bonding strand, obtaining samples of strand from the lower end of the performance spectrum was difficult. The strand sources included in testing for this program are listed in Table 1. This table also includes a result from con- crete pull-out tests or mortar pull-out tests (the bond stress at the observed first slip or after 0.1-in. slip at the non-loaded end of the strand). Each pull-out stress is the average of the pull-out stresses from at least six individual pieces of strand. The bond stresses are calculated from the measured loads based on the actual surface area and the embedment length of the strand. These strand sources fall into three groupings: historic, re- cently manufactured, and OSU (Oklahoma State University) strand. Historic Strand—This study initially identified samples of strand for testing from prior unpublished tests conducted at Kansas State University (KSU) by Bob Peterman and at StressCon Corporation, Inc. by Don Logan that cover a wide range of pull-out behavior. These are referred to as “historic” strand and were manufactured between 1997 and 2004. Figure 2 is a plot of first slip bond stress or bond stress at 0.1-in. end slip versus maximum bond stress from the data available from historic concrete pull-out tests. When suggested minimum pull-out loads for acceptable bonding performance (suggested by Logan, based on a limited number of flexural beam tests conducted in the mid-1990s and his engineering judgment) are converted to bond stresses, they are 425 and 955 psi for the first slip and maximum stresses, respectively. These thresholds have been reproduced in Figure 2. Recently Manufactured Strand—Figure 2 also shows the concrete pull-out performance of recently manufactured sam- ples identified during this project. These recently manufactured 11 Strand Geometry Mortar Pull-Out Testing Concrete Pull-Out Testing (LBPT) Strand Source ID Size (in.) Measured Diameter (in.) Pitch (in.) Lay (Handed- ness) Location Date 0.1-in. Slip Stress (psi) Location Date 0.1-in. Slip Stress (psi) Historic Strand KSU-F 1/2 Special 0.524 7 5/8 Left -- -- -- KSU Mar 2004 241 KSU-H 1/2 Special 0.523 7 1/2 Left -- -- -- KSU Mar 2004 209 SC-F 1/2 0.503 8 Left -- -- -- SC May 1997 223 SC-H 1/2 Special 0.530 7 1/4 Left -- -- -- SC Nov 2002 472 SC-IS 1/2 0.501 7 Left -- -- -- SC Mar 2003 682 101 6/10 0.601 8 1/2 Left -- -- -- SC Oct 2004 241 Recently Manufactured Strand 102 1/2 0.501 7 1/2 Left KSU Jun 2005 315 KSU Jun 2005 441 103 1/2 0.503 8 Left KSU Jun 2005 397 KSU Jun 2005 944 151 1/2 Special 0.517 7 1/2 Left KSU Jun 2005 273 KSU Jun 2005 541 153 6/10 0.588 9 Right -- -- -- KSU/SC Jun / Aug 2006 142/406 OSU Strand 349 1/2 0.505 8 3/4 Left OSU Jun 2004 156 -- -- -- 548 1/2 0.500 7 5/8 Left OSU Jan-Feb 2004 623 -- -- -- 697 1/2 0.503 7 1/4 Left OSU May 2004 606 -- -- -- 717 1/2 0.500 8 Left OSU Feb 2004 206 -- -- -- 478 * 1/2 0.499 7 5/8 Left OSU May-June 2004 409 -- -- -- 960 * 1/2 0.500 7 1/2 Left OSU May-June 2004 409 -- -- -- * Samples designated 478 and 960 were from same source. KSU = Kansas State University; OSU = Oklahoma State University; SC = StressCon Corporation, Inc. Table 1. Strand sources.

samples were obtained in large quantities for the purpose of this research and were used in the screening experiments. Notice that the greatest variation in the recently manufac- tured strand is not in terms of maximum stress but in terms of stress at 0.1-in. slip. The recently manufactured strand sources (102, 103, and 151) were selected because initial testing indicated that they represented a range of first-slip pull-out performance. None of these strands had significantly low maximum load pull-out performance. Source 103 is the strand used by StressCon Corporation, Inc. in their ordinary production of precast/prestressed concrete and has a proven record of good bond from pull-out tests, flexure beam tests, transfer length tests, and end slips observed in hollow core precast/prestressed concrete members. The bond stress at 0.1-in. slip of Source 102, measured in concrete pull-out testing performed as part of this project, is slightly above Logan’s 425 psi threshold. Of the 31 results from historic and other sources presented on this plot, 13 are to the left of Source 102. Only one of these (D-5/96) was available in enough quantity to enable additional testing, and the condition of this strand was variable. OSU Strand—The sample sources used for the Correlation Round of testing were selected by Bruce Russell of Oklahoma State University (OSU). These sources of strand had been tested in work performed by Russell for NCHRP Project 12-60: Transfer, Development, and Splice Length for Strand/ Reinforcement in High-Strength Concrete, the Oklahoma Department of Transportation, and the NASPA (also known as the Committee of the American Wire Products Association [AWPA]). Two of the six strand sources provided by Russell were actually the same strand source, a fact that was not known by the research team before the testing was com- pleted. This was intended to test the repeatability of the surface and chemical test methods. Complete mortar pull- out and some transfer length test results were provided in tabular form by Russell after the chemical and surface testing had been completed. Transfer Length Testing The transfer length test is not proposed as a QC test method, but was conducted as a basis for evaluating the pro- posed QC test methods, since the transfer length quantifies bond performance under the most realistic conditions. Transfer length is defined as the distance over which the effective prestressing force is transferred to the concrete ele- ment. In other words, this is the distance from the end of the strand where no stress is applied to the concrete to the point where the maximum amount of stress has been transferred into the concrete. The test for transfer length involves casting a prism of concrete around a stressed strand or strands and then measuring the strain profile along the length of the prism after the stress is released. The transfer length is defined as the length over which the measured strain in the prism increases from zero at the ends of the prism to the edge of the strain plateau region in the middle of the prism. The end slip (i.e., the distance that the end of the strand moves relative to its original position) is proportional to transfer length and also was measured. The strain profiles of the transfer length prisms were monitored over time, starting with the initial reading immediately after release; additional measurements were taken at 28 days, 6 months, and 18-22 months. 12 A BC D E F G H J S1 S2 S3 S4 S5 hollowcore H-11/02 U -11/02 F-5/97 IT-11/02 TW-5/96 D-5/96 NC-05/01 IS-3/03 101-A-0.6 102-A-0.5 103-B-0.5102-A-0.5 103-B-0.5 151-Z-0.5 200 400 600 800 1000 1200 200 300 400 500 600 700 800 900 1000 Bond Stress at 1st Slip or 0.1 in (psi) M ax S tre ss (p si) Historical results at first slip NCHRP tests at 0.1 in slip Max Threshold 1st Slip Threshold Figure 2. Correlation between maximum stress and first observed or 0.1-in. slip stress for historic and recently manufactured strand.

The data obtained during the transfer testing are not re- ported as a length, but instead as an average bond stress over the transfer length. This was done to enable comparisons of stress transfer behavior between the tested strand sources, since this approach eliminates complications from strands of varying sizes and varying initial stress conditions. The aver- age bond stress over the transfer length, Ut, is calculated as (Eq. 1) where fse is the effective prestress after transfer, Aps is the cross- sectional area of the strand, Cp is the circumferential perimeter of the strand (4/3 π db) and Lt is the transfer length. Average bond stress is thus dependent both on effective prestress as well as transfer length for a given strand geometry. For the purpose of this calculation, fse was taken as the difference be- tween the stresses in the strand before release and the elastic losses only. 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 con- stant, assuming no relaxation losses in the strand. Proposed Quality Control Test Methods The test methods that were proposed and conducted as part of the screening and correlation test program are summarized in Tables 2 and 3. These tables also list the QC levels for these tests, if applicable. These test methods consist of (1) surface and chemical testing, (2) pull-out testing, and (3) transfer length testing. Since insufficient lengths of strand were available for pull-out and transfer length testing during the Screening Round from the historic sources, these tests were conducted only on the recently manufactured strand. The surface and chemical testing program has been conducted on both the historic strand samples and on the recently manufactured samples. Surface and Chemical Testing The surface and chemical test methods that were attempted are described briefly. More complete descriptions are given in Appendix B. Contact Angle Measurement The contact angle is a measure of surface tension (wet- ability). It was anticipated that the presence of drawing lubri- cants would affect this property. The contact angle is measured on the projected shadow of a small droplet of distilled water applied to the strand surface. Measurements were taken with the strand: (1) in an as-received condition, (2) after immersing the strand sample in a saturated calcium hydroxide [Ca(OH)2] U f C L t se ps p t = A solution, and (3) after an ignition process. The calcium hy- droxide exposure (also called a lime dip) will convert sodium soaps (e.g., sodium stearates) to insoluble calcium salts. For example, water-soluble sodium stearate (a soap or wetting agent) is converted to a film of insoluble calcium stearate (a wax-like, water repellent that increases the surface energy of the strand). This conversion reaction was chosen to simu- late the reaction of concrete with surface residues of soaps and is intended to produce a condition where the effect of similar calcium stearate compounds on the contact angle are com- pared, even if the original residue did not result from a calcium stearate-based lubricant. The ignition process was performed on samples to volatilize organic compounds expected to be present in the drawing lubricants. Examination under Ultraviolet Light Certain lubricant additives (e.g., hydrocarbon oils, fluo- rescein additives, and some inorganic deposits) will fluoresce under ultraviolet (UV) radiation. An examination under UV light was conducted using a range of light sources, the most promising of which was a 366-nm wavelength. Testing pH Testing of the pH of the surface was attempted with each of the strand sources to see if alkalinity of a solution generated by placing drops of water on the residue could be linked to bond. Testing of the pH of the surface was conducted using indicator papers, indicator solutions, and a pH meter. Loss on Ignition The weight loss on ignition (LOI) represents the weight of compounds that can be volatilized or burned off the strand surface at high temperature. This property was measured with the expectation that the weight lost would consist mainly of the organic component of residues, such as drawing lubricants. Loss in Hot Alkali Bath The weight loss after hot alkali bath (LAB) represents the weight of compounds that can be washed off the strand sur- face in a hot sodium hydroxide solution. As with the LOI test, this property was measured with the expectation that the weight lost would consist mainly of drawing lubricants. Change in Corrosion Potential Past studies of the corrosion resistance of prestressing strand in concrete have suggested that strand with a coating of residue does not corrode as readily as a clean strand. To assess the 13

Test Method Condition/Type of Test QC Level Property Measured Objective As received After Ca(OH)2 dipContact Angle Measurement After ignition I Surface energy of strand Detect presence of materials that reduce water surface tension (Na-based soaps) or increase steel surface energy (Ca-based salts) Examination under UV Light - I Presence of fluorescing materials Identify lubricant additives such as hydrocarbon oils, some inorganic deposits, or possibly fluorescing-based tracers that may fluoresce under UV light Universal indicator Indicator solutions pH meter pH Testing High-res. indicator I pH of surface Detect presence of pretreatment lubricant residues containing alkaline salts or alkalies Weight Loss on Ignition (LOI) - I Weight of material burned off strand Determine amount of material that can be oxidized on the strand surface at 415°C, expected to be largely organic Method 1 Weight Loss in Alkali Bath Method 2 I Weight washed off strand Determine amount of material that can be washed off the strand surface after soak in a NaOH solution As received After Ca(OH)2 dipChange in Corrosion Potential After ignition I Average change of potential Assess the potential for corrosion by comparing the corrosion potential to a reference cell monitored versus time Surface Roughness - I Roughness parameters Ra, Rz, Pc Quantify surface profile As received After Ca(OH)2 dipCorrosion Rate After ignition II Corrosion current Determine the shift in potential of a metal sample from a stable corrosion potential due to an external current Warm water/acid- chloroform wash Organic Residue Extraction Hot water/acid- chloroform wash II Weight of extracted organic residue Determine amount of individual components of strand manufacturing lubricants from a warm/hot water wash procedure then an acid/solvent-wash procedure Sodium Calcium Potassium Boron Zinc Atomic Absorption (AA) Spectroscopy Phosphate II Concentrations of inorganic components of extraction residue Quantify inorganic elements (sodium, calcium, potassium, zinc, and boron) in residue Pull Out from Large Concrete Block - II Maximum bond stress and stress at 0.1 in. displacement (or first slip) Mechanically measure stresses required to break bond with concrete Pull Out from Portland Cement Mortar - II Maximum bond stress and stress at 0.1 in. displacement (or first slip) Mechanically measure stresses required to break bond with mortar Pull Out from Hydrocal-Based Mortar - II Maximum bond stress and stress at 0.1 in. displacement (or first slip) Mechanically measure stresses required to break bond with Hydrocal-based mortar Transfer Length - Analytical Length over which the prestress is transferred to a concrete beam Directly measure bond performance in prestressed concrete beam Table 2. Test methods conducted during screening and correlation testing programs.

potential for corrosion, the strand samples were placed in a solution of deionized water, and the corrosion potential measured with a reference cell (saturated calomel reference electrode), was monitored versus time. This corrosion poten- tial is determined by the amount of ferrous ions in solution surrounding the sample, and a greater drop in this potential is indicative of a greater tendency to corrode. Measurements were taken with the strand in an as-received condition, after immersing the strand sample in a saturated Ca(OH)2 solution, and after an ignition process. 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. Based on images captured using a scanning elec- tron microscope, the depth of the roughened surface features is typically 3 µm (0.0001 in.) or less. Trials with a portable profilometer suitable for a QC setting were conducted to de- termine whether these physical measurements could accurately represent the surface roughness and to investigate the corre- lation with bond performance. This system works by measuring the deflection of a diamond probe, with a 2-µm tip radius, as it is dragged 2 mm across the surface of the sample. Corrosion Rate To further explore the interaction between strand bond and corrosion, the instantaneous rate of corrosion of samples of strand in a salt solution was measured with a polarization resistance technique. The polarization resistance technique measures the corrosion current, which quantifies the rate at which the electrochemical corrosion reaction is occurring. This is a much faster test than the test for change in corrosion potential, but requires specialized equipment (a potentiostat). Measurements were taken with the strand in an as-received condition, after immersing the strand sample in a saturated Ca(OH)2 solution and after an ignition process. Organic Residue Extraction The tests for identification and quantification of organic drawing-compound residues were based on solvent extrac- tion procedures, together with gravimetric and Fourier trans- form infrared spectroscopical (FTIR) analyses. Essentially, the amount of material extracted from a defined length of strand was determined by weighing the extraction residue on an analytical balance. The material in the extraction residue was then identified by FTIR analysis of the residue. The FTIR spectrum obtained is like a fingerprint of the material. The extraction procedure used is a modification of a pro- cedure found in ASTM C114 for organic materials in cement. Multiple extractions were used to differentiate between various forms of drawing-compound residue. The strand was first washed with warm or hot water to remove water-soluble materials, such as sodium stearate. Then, the strand was ex- posed to hydrochloric acid and chloroform to extract water- insoluble residues such as calcium stearate and stearic acid. At the conclusion of the Screening Round, it was observed that the water temperature had little effect in most cases, but that the residue concentrations measured with the warm water method seemed to generally correlate better with bond tests. Therefore, only a warm-water wash was used in the Correla- tion Round. In addition, to minimize the effort spent on per- forming the time-consuming chloroform organic extraction, the wash solutions from the warm water and acid-chloroform washes were combined, and a single separation was performed. Therefore, only one FTIR scan and residue weight determi- nation was made per piece of strand in this round of testing. However, this is considered essentially equivalent to the com- bination of sequential warm water and acid-chloroform washes performed in the Screening Round. Since quantifying the water-soluble materials was still of interest, because it might provide insight into possible cleaning methods, separate warm water washes were performed on additional pieces of strand. This wash solution was acidified and saved for elemental analysis. Atomic Absorption and Colorimetric Analysis To identify the chemical composition of residual inorganic components of pretreatment chemicals and drawing com- pounds, chemical analyses of the acidified water extract and acid/solvent extract solutions, which had been obtained dur- ing the organic residue extraction procedure and had been separated from the chloroform, were performed. Either zinc phosphate or borax (sodium borate) is often applied to the wire before the drawing process begins to help drawing lubricants stick to the surface of the rod stock. Most common drawing lubricants are expected to include stearate salts, particularly sodium and calcium stearates. The elemental concentrations 15 Test Method QC Level Coefficient of Determination (R2) from Regression with Average Bond Stress over Transfer Length Concrete Pull Out II 0.98 Mortar Pull Out II 0.85 Hyrdocal Mortar Pull Out II 0.36 Table 3. Coefficient of determination (R2) from linear regression with average bond stress over transfer length.

of sodium, potassium, calcium, and zinc were determined by atomic absorption spectroscopy. The solutions were also scanned for detectable quantities of aluminum during the Screening Round. Colorimetric analyses of the wash solutions for boron and phosphate ( as total phosphate) were per- formed using visible light spectroscopy. Pull-Out Testing The original project scope included the development of a performance-based test method for use in evaluating strand bond. As a result, in the initial phases of this study, efforts were made to develop a procedure for quantifying bond using a pull-out test conducted on untensioned strand embedded in some material. Two types of pull-out tests have been commonly used to evaluate strand bond. The first method involves pulling untensioned strand out of a block of concrete. The second method involves pulling untensioned strand out of a steel cylinder filled with mortar. In its current form, the concrete pull-out test resembles a method developed by Moustafa (1974). The method was pri- marily developed to judge the capacity of strand to be used as lifting loops to handle product during shipping and erection. The test developed by Moustafa was modified by Logan (1997) to judge the bond quality of strand in pretensioned applications. Further developments of the method have oc- curred and are the basis for the testing reported herein. In its current form, the mortar pull-out test method resem- bles a method originally developed for the Post Tensioning Institute in 1994 (Hyett et al. 1994, Post-Tensioning Institute 1996). The method was primarily developed to judge the bond quality of prestressing strand used in rock anchors. The method became the basis of ASTM A981-97 (2002) Standard Test Method for Evaluating Bond Strength for 15.2 mm (0.6 in.) Di- ameter Prestressing Steel Strand, Grade 270, Uncoated, Used in Prestressed Ground Anchors (ASTM 2002). Later, this method was modified by Russell and Paulsgrove (1999) for NASPA and became known as the NASPA test. The NASPA test has been modified slightly by this research project to make it less sensitive to the test apparatus. One of the goals of this project was to try to eliminate vari- ables by using standardized and universally available embed- ment media referred to in the NCHRP Project 10-62 Request for Proposal (RFP) as a “surrogate homogeneous material.” Accordingly, a third type of pull-out test was attempted: pulling untensioned strand out of a steel pipe filled with a modified gypsum plaster (Hydrocal). The three types of pull-out tests were performed on the three sources of strand at KSU in March and May of 2005. Each of these test procedures and their results are described briefly below. More complete descriptions are given in Appendix B. In each of these methods, the load applied to pull out the strand and the movement (or slip) of the non-loaded (free) PO43− end of the strand were monitored throughout testing. To allow comparison of data among strand of different sizes, the bond stress has been calculated from the measured loads based on the nominal surface area (equal to 4/3 π db l, where db is the nominal strand diameter and l is the embedment length) of the embedded section of the strands. Two characterizations of performance are determined during strand bond pull-out tests. The first characterizes the early part of the bond stress- slip relationship, while the second is based on the maximum stress measured throughout the test. In concrete pull-out tests performed on historic strand, the early performance was char- acterized in terms of the stress at which movement is first vi- sually observed at the loaded end of the strand, called the stress at “first observed slip” or “first slip.” For the tests conducted as part of this experimental program, the stress selected to characterize the early part of the bond stress-slip relationship is the bond stress at 0.1-in. slip, measured at the non-loaded end of the strand. This 0.1-in. slip criterion was adopted to give a more precisely defined location on the stress-slip curve. Large Concrete Block Pull-Out Test The large concrete block pull-out test (LBPT) involves pulling six untensioned strands bonded over 18 in. from a large (2 ft × 2 ft × 2 ft 8 in.) block of concrete. This concrete was produced from a conventional mix design used by a pre- caster, and is produced with a coarse aggregate with Mohs hardness greater than 6.0. The strength of the concrete at the time of the test is 3500 to 5900 psi. The test is conducted in load-rate control with a load rate of 20 kips per minute. The bond stress at 0.1-in. slip and the average maximum bond stress for each strand tested are reported and averaged. In addition, for the concrete pull-out tests conducted as part of this study, the load at which slip at the loaded end was first observed visually also was recorded. This “observed first slip” was determined because it relates back to historic pullout data recorded by Logan (1997) and others. Mortar Pull-Out Test In the mortar pull-out test, each prestressing strand is em- bedded in 5-in. diameter by 18-in. long steel cylinders filled with mortar (Portland cement, sand, and water). The top 2 in. of the embedded portion of the strand is debonded, leaving 16 in. of strand in contact with the mortar. Six cylinders are tested for each source of strand. The mortar is produced with a Type III cement-to-sand ratio equal to 2:1 by weight, and the required mortar strength at time of the test is 3500 to 5000 psi. The bond stress at 0.1-in. slip and the average maximum bond stress for each strand tested are reported and averaged. The test was conducted in load-rate control with a load rate of 5 kips per minute, which was reportedly similar to the rates 16

achieved during testing conducted by previous researchers, such as in the NASPA-funded work, which was performed under ram displacement-rate control. The mortar pull-out test was conducted with load-rate control instead of displacement- rate control because load rate, which may influence the results of the physical tests on structural materials, is independent of the stiffness of the testing frame. It is desirable that the test method be universally applicable, and so should not be influ- enced by the load frame. Hydrocal Pull-Out Test A range of Hydrocal-based mixtures was evaluated as possible surrogate materials for use in a pull-out test. The final mixture used in this testing contained Hydrocal White (a material similar to plaster of Paris made by United States Gypsum), Ottawa graded sand (ASTM C778), calcium hy- droxide flakes, USG Retarder for Lime-Based Plasters, and water. This formulation of Hydrocal is almost pure plaster of Paris and was chosen because it is produced at only one manufacturing facility from consistent raw materials. Also, like cement, it is a calcium compound (plaster of Paris is hemihydrated calcium sulfate). Calcium hydroxide flakes were added to simulate the alkalinity of concrete, and the Hydrocal was combined with sand and plaster retarder to limit the heat production generated during the rapid plaster hydration. In this test, each prestressing strand is embedded 12 in. in a 3-in. diameter steel cylinder filled with the Hydrocal mortar (gypsum/lime plaster, sand, and water). Six cylinders are tested for each source of strand. The required mortar strength at time of the test is 3000 to 4000 psi. The test is conducted similarly to the mortar pull-out test. The bond stress at 0.1-in. slip and the average maximum bond stress for each strand tested are reported and averaged. Interpretation of Historic and Recent Concrete Pull-Out Test Results The “first observable slip” and “0.1-in. slip” are measured in different ways, yet are considered to be close in value to one another. This was verified during the current test pro- gram, which showed that the first observed slip at the live end of the strand occurred at generally the same time at which 0.1-in. of end slip was measured in the pull-out test. This similarity is significant because the stress at 0.1-in. end slip was not determined during the historic concrete pull-out tests conducted on the historic samples that were included in the screening testing program. However, because of the similarity, when evaluating the correlation between bond performance and the screening test results, both the “first observable slip” and “0.1-in. slip” characterizations of bond are used together. Mortar Pull-Out Testing for Correlation Round of Evaluation As mentioned, the sample sources used for the Correlation Round of testing were selected by Bruce Russell of OSU. Mortar pull-out data were provided for each of the sources. The reported NASPA pull-out forces represent the average load at 0.1-in. slip for multiple (5 to 12) specimens, all tested on the same day with the same batch of mortar. Per the pro- tocol outlined by Chandran (2006), the mortar pull-out force was measured on strands embedded in 5-in. diameter by 18-in. long cylinders (with 16 in. of strand in direct contact with mortar). These mortar pull-out tests were conducted under displacement-rate control, with an additional criterion for load rate. 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. slip. Statistical Evaluation of Results A large experimental program was conducted to support the evaluation of the various proposed test methods for strand bond that were intended for use as part of a QC program. These were classified as performance-based (i.e., mechanical) tests and surface and chemical tests. The correlation between bond and the methods that fall under each of these classifica- tions was 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. The correlations between the surface and chemi- cal test methods were evaluated based on the results of pull-out tests from concrete in the Screening Round and based on the results of pull-out tests from mortar in the Correlation Round. Statistical analyses have been performed with two objectives: (1) to determine and quantify the relationship between the chemical and surface test results and bond performance, and (2) to allow the determination of acceptance thresholds for the chemical and surface test results that can predict, with a given level of confidence, that adequate bond performance can be achieved. The first objective was achieved based on standard linear regression techniques, while the second objec- tive requires the determination of prediction intervals. A more extensive discussion of both of these analysis methods is given in Appendix B. Regression To provide a quantitative measure of the goodness-of-fit to aid in the evaluation of these methods, a linear regression has been performed, and the coefficient of determination (R2) was 17

determined for the relationship between each proposed test method and the bond quality measure. No physical basis for a linear relationship between these measures of bond is known; however, the linear relationship was assumed as the simplest model relating the parameters. The R2 is a measure of the ad- equacy of a regression model (i.e., it describes the amount of variability in the data explained by the regression model). The closer the R2 is to 100%, the more completely the model de- scribes the relationship between the test method results and the basis for evaluation. To further evaluate the validity of these methods, the signif- icance of the linear models developed based on these data was evaluated by the calculation of P-values for the coefficients (slope) from the linear models. The coefficient from the linear model is judged to be significant when there is a sufficiently high confidence that it is not equal to zero. If this is the case, the re- lationship represented by the model is statistically significant and the results of the surface tests are meaningful in the predic- tion of the pull-out test result. A 95% confidence level is com- monly used to evaluate significance. The level of confidence of significance on the coefficient is given by (1 − P-value) × 100%, so a P-value < 0.05 implies that the confidence interval does not include zero with higher than 95% confidence. For the contact angle and organic residue extraction test 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 elim- inate the potentially confounding influences of non-stearate- based lubricants. Analyzing these data in this manner has a practical motivation, since such models could be useful in a production setting where the lubricant in use is known to be stearate-based only. In addition to the regression with a single predictor, re- gression analyses were also performed based on selected com- binations of the surface and chemical test results to see if the pull-out performance could be better predicted using more than one predictor variable. When regression was performed with multiple predictors, the R2 adjusted was calculated and used to interpret how well the model fits the data. The R2 ad- justed is the most appropriate measure of goodness-of-fit for multiple-predictor regression. Since adding predictors makes the model more flexible and thus better able to fit the data, the R2 adjusted measure includes a penalty for additional predictors in the model. Prediction Intervals The models generated by the regression analysis allow for the prediction of the pull-out stress based on results obtained with the surface and chemical QC test methods. However, the prediction formulas give the average estimated pull-out stress, but do not account for variation that is bound to occur in the QC test results or uncertainty in the regression model. Instead, what is needed to interpret and practically apply a given QC test result is the computation of a lower bound on the interval that, with a given confidence, includes the pull-out stress for a strand sample with that QC test result. This type of interval is known as a one-sided prediction interval and is a standard part of regression theory and practice. The prediction interval concept is a necessary part of the development of acceptance/rejection thresholds for the rec- ommended QC test methods, since, to conservatively ensure that a specified pull-out bond stress is achieved, the threshold on the QC test must be chosen as the value where the prediction interval lower bound is equal to the pull-out stress threshold. The regression model gives an estimate of the average pull-out stress if the pull-out test was actually conducted repeatedly on the same source of strand. For a given measurement of the predictor, half of the actual pull-out test results would be ex- pected to fall above this average and half would fall below. The distribution of individual pull-out observations about that average pull-out stress is the basis for the prediction in- terval, which is calculated based on the variability in the data used for the regression. This concept is demonstrated graphically in Figure 3, which shows the prediction interval lower bound plotted along with the regression line, and data for the mortar pull-out plotted versus the change in corrosion potential. If a specified thresh- old on mortar pull-out is defined as 0.313 ksi, the threshold on the corrosion potential is the value where the pull-out threshold and the curve delineating the lower bound of the prediction interval intersect, shown by the red lines in the plot. In this case, the threshold would be approximately −0.175 V. If a multiple-predictor regression model is used for predic- tion of the pull-out stress, the prediction interval is still needed. Determining the prediction interval for models based on mul- tiple predictors is possible; however, it is more complicated and can not be shown graphically. The predicted pull-out stress is not uniquely determined by a single combination of predic- tors, but can be found based on numerous combinations of those predictors. However, the prediction interval for the pull-out stress will be different depending on the specific com- bination of predictors used. That means that when multiple re- gression is used to improve the predictive ability of the model, a single threshold can not be defined. Instead, for a specific set of predictors, a new prediction interval must be calculated based on the set of data used to develop the regression model. The lower bound of the newly calculated prediction interval must then be compared with the specified pull-out threshold. Computational Tool To facilitate the implementation of the prediction interval concept, a computational tool in the form of a Microsoft 18

Excel worksheet was developed that demonstrates the calcu- lation of prediction intervals for single- and multiple-predictor regressions. This tool employs the calculation outlined in Appendix B. Selection of Confidence Level For the threshold determinations performed based on the data collected in this study, the confidence level was taken as 90%. This means that for a given surface and chemical test result, 10% of the pull-out results would be expected to fall below that prediction interval. This confidence level is lower than the 95% confidence interval that is most commonly used as the basis for probabilistic design in structural engineering analysis. Using a confidence level as high as 95% will result in very conservative thresholds for the surface and chemical tests, so a 90% confidence level was used instead. Precision A final round of testing using the QC methods that corre- lated well with bond performance was conducted to provide the basis for a precision statement to be included in the test methods. To determine the precision (i.e., the repeatability) of the methods, the selected tests were repeated up to six times on samples of strand obtained from the same source. This testing was conducted on a single source identified as a middle-range performer in that particular test during the correlation testing. The results of this testing were developed according to ASTM practice and are reported based on the standard deviations measured among the test results. A deter- mination of bias in the testing methods is not possible at this time since a known reference sample can not be selected in a universally acceptable manner. Supplemental Investigations of Strand Bond In addition to testing performed to evaluate the proposed QC testing methods, supplemental investigations were con- ducted to provide insight into the causes of poor bond. These investigations aided in the development and interpretation of the results of the QC tests. The supplemental investigations included studies of: (1) surface roughness and distribution of lubricant residue, (2) the concrete/strand interface, and (3) local variation of strand diameter. The studies of surface roughness included metallographic studies of strand from poor and good bonding sources, scanning electron micro- scopical studies of the strand surface, and study of surface roughness by electrochemical impedance spectroscopy. Inves- tigation of the concrete/strand interface included a study of the cement hydration at the strand interface, and a petrographic and chemical investigation of the strand/concrete interface in transfer length prisms in which poor bond was observed. 19 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 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 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) Specified Pull- Out Stress Threshold Change in Corr Pot. where pull-out threshold intersects prediction interval Figure 3. Threshold determination using the prediction interval.

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

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