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13 The data obtained during the transfer testing are not re- solution, and (3) after an ignition process. The calcium hy- ported as a length, but instead as an average bond stress over droxide exposure (also called a lime dip) will convert sodium the transfer length. This was done to enable comparisons of soaps (e.g., sodium stearates) to insoluble calcium salts. For stress transfer behavior between the tested strand sources, example, water-soluble sodium stearate (a soap or wetting since this approach eliminates complications from strands of agent) is converted to a film of insoluble calcium stearate varying sizes and varying initial stress conditions. The aver- (a wax-like, water repellent that increases the surface energy age bond stress over the transfer length, Ut, is calculated as of the strand). This conversion reaction was chosen to simu- late the reaction of concrete with surface residues of soaps and f se Aps is intended to produce a condition where the effect of similar Ut = (Eq. 1) C p Lt calcium stearate compounds on the contact angle are com- pared, even if the original residue did not result from a calcium where fse is the effective prestress after transfer, Aps is the cross- stearate-based lubricant. The ignition process was performed sectional area of the strand, Cp is the circumferential perimeter on samples to volatilize organic compounds expected to be of the strand (4/3 db) and Lt is the transfer length. Average present in the drawing lubricants. bond stress is thus dependent both on effective prestress as well as transfer length for a given strand geometry. For the Examination under Ultraviolet Light purpose of this calculation, fse was taken as the difference be- tween the stresses in the strand before release and the elastic Certain lubricant additives (e.g., hydrocarbon oils, fluo- losses only. The elastic loss was determined based on the rescein additives, and some inorganic deposits) will fluoresce strain measured immediately after release in the central region under ultraviolet (UV) radiation. An examination under UV of the test prism over which the strain is approximately con- light was conducted using a range of light sources, the most stant, assuming no relaxation losses in the strand. promising of which was a 366-nm wavelength. Proposed Quality Control Test Methods Testing pH The test methods that were proposed and conducted as part Testing of the pH of the surface was attempted with each of the screening and correlation test program are summarized of the strand sources to see if alkalinity of a solution generated in Tables 2 and 3. These tables also list the QC levels for these by placing drops of water on the residue could be linked to tests, if applicable. These test methods consist of (1) surface and bond. Testing of the pH of the surface was conducted using chemical testing, (2) pull-out testing, and (3) transfer length indicator papers, indicator solutions, and a pH meter. testing. Since insufficient lengths of strand were available for pull-out and transfer length testing during the Screening Round Loss on Ignition from the historic sources, these tests were conducted only on the recently manufactured strand. The surface and chemical The weight loss on ignition (LOI) represents the weight of testing program has been conducted on both the historic compounds that can be volatilized or burned off the strand strand samples and on the recently manufactured samples. surface at high temperature. This property was measured with the expectation that the weight lost would consist mainly of the Surface and Chemical Testing organic component of residues, such as drawing lubricants. The surface and chemical test methods that were attempted Loss in Hot Alkali Bath are described briefly. More complete descriptions are given in Appendix B. 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, Contact Angle Measurement this property was measured with the expectation that the The contact angle is a measure of surface tension (wet- weight lost would consist mainly of drawing lubricants. ability). It was anticipated that the presence of drawing lubri- cants would affect this property. The contact angle is measured Change in Corrosion Potential on the projected shadow of a small droplet of distilled water applied to the strand surface. Measurements were taken with Past studies of the corrosion resistance of prestressing strand the strand: (1) in an as-received condition, (2) after immersing in concrete have suggested that strand with a coating of residue the strand sample in a saturated calcium hydroxide [Ca(OH)2] does not corrode as readily as a clean strand. To assess the
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Table 2. Test methods conducted during screening and correlation testing programs. Condition/Type QC Test Method Property Measured Objective of Test Level As received Detect presence of materials that reduce water surface tension Contact Angle Measurement After Ca(OH)2 dip I Surface energy of strand (Na-based soaps) or increase steel surface energy (Ca-based salts) After ignition Identify lubricant additives such as hydrocarbon oils, some Examination under UV Light - I Presence of fluorescing materials inorganic deposits, or possibly fluorescing-based tracers that may fluoresce under UV light Universal indicator Indicator solutions Detect presence of pretreatment lubricant residues containing pH Testing I pH of surface pH meter alkaline salts or alkalies High-res. indicator Weight of material burned off Determine amount of material that can be oxidized on the Weight Loss on Ignition (LOI) - I strand strand surface at 415°C, expected to be largely organic Method 1 Determine amount of material that can be washed off the strand Weight Loss in Alkali Bath I Weight washed off strand Method 2 surface after soak in a NaOH solution As received After Ca(OH)2 dip Assess the potential for corrosion by comparing the corrosion Change in Corrosion Potential I Average change of potential potential to a reference cell monitored versus time After ignition Surface Roughness - I Roughness parameters Ra, Rz, Pc Quantify surface profile As received Determine the shift in potential of a metal sample from a stable Corrosion Rate After Ca(OH)2 dip II Corrosion current corrosion potential due to an external current After ignition Warm water/acid- Determine amount of individual components of strand chloroform wash Weight of extracted organic Organic Residue Extraction II manufacturing lubricants from a warm/hot water wash Hot water/acid- residue procedure then an acid/solvent-wash procedure chloroform wash Sodium Calcium Atomic Absorption (AA) Potassium Concentrations of inorganic Quantify inorganic elements (sodium, calcium, potassium, zinc, II Spectroscopy Boron components of extraction residue and boron) in residue Zinc Phosphate Maximum bond stress and stress at Mechanically measure stresses required to break bond with Pull Out from Large Concrete Block - II 0.1 in. displacement (or first slip) concrete Pull Out from Portland Cement Maximum bond stress and stress at Mechanically measure stresses required to break bond with - II Mortar 0.1 in. displacement (or first slip) mortar Pull Out from Hydrocal-Based Maximum bond stress and stress at Mechanically measure stresses required to break bond with - II Mortar 0.1 in. displacement (or first slip) Hydrocal-based mortar Length over which the prestress is Directly measure bond performance in prestressed concrete Transfer Length - Analytical transferred to a concrete beam beam
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15 Table 3. Coefficient of determination (R2) Organic Residue Extraction from linear regression with average bond stress over transfer length. The tests for identification and quantification of organic drawing-compound residues were based on solvent extrac- Coefficient of tion procedures, together with gravimetric and Fourier trans- Determination (R2) from Regression form infrared spectroscopical (FTIR) analyses. Essentially, Test Method QC Level with Average Bond the amount of material extracted from a defined length of Stress over Transfer Length strand was determined by weighing the extraction residue on Concrete Pull Out II 0.98 an analytical balance. The material in the extraction residue Mortar Pull Out II 0.85 was then identified by FTIR analysis of the residue. The FTIR Hyrdocal Mortar Pull Out II 0.36 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. potential for corrosion, the strand samples were placed in Multiple extractions were used to differentiate between various a solution of deionized water, and the corrosion potential forms of drawing-compound residue. The strand was first measured with a reference cell (saturated calomel reference washed with warm or hot water to remove water-soluble electrode), was monitored versus time. This corrosion poten- materials, such as sodium stearate. Then, the strand was ex- tial is determined by the amount of ferrous ions in solution posed to hydrochloric acid and chloroform to extract water- surrounding the sample, and a greater drop in this potential insoluble residues such as calcium stearate and stearic acid. is indicative of a greater tendency to corrode. Measurements At the conclusion of the Screening Round, it was observed were taken with the strand in an as-received condition, after that the water temperature had little effect in most cases, but immersing the strand sample in a saturated Ca(OH)2 solution, that the residue concentrations measured with the warm water and after an ignition process. method seemed to generally correlate better with bond tests. Therefore, only a warm-water wash was used in the Correla- Surface Roughness tion Round. In addition, to minimize the effort spent on per- forming the time-consuming chloroform organic extraction, Microscopic examinations of sectioned portions of wire the wash solutions from the warm water and acid-chloroform taken from strand have indicated that an observable difference washes were combined, and a single separation was performed. in the surface roughness of the good- and poor-bonding strand Therefore, only one FTIR scan and residue weight determi- sources exists. Based on images captured using a scanning elec- nation was made per piece of strand in this round of testing. tron microscope, the depth of the roughened surface features However, this is considered essentially equivalent to the com- is typically 3 µm (0.0001 in.) or less. Trials with a portable bination of sequential warm water and acid-chloroform washes profilometer suitable for a QC setting were conducted to de- performed in the Screening Round. Since quantifying the termine whether these physical measurements could accurately water-soluble materials was still of interest, because it might represent the surface roughness and to investigate the corre- provide insight into possible cleaning methods, separate warm lation with bond performance. water washes were performed on additional pieces of strand. This system works by measuring the deflection of a diamond This wash solution was acidified and saved for elemental probe, with a 2-µm tip radius, as it is dragged 2 mm across the analysis. surface of the sample. Corrosion Rate Atomic Absorption and Colorimetric Analysis To further explore the interaction between strand bond To identify the chemical composition of residual inorganic and corrosion, the instantaneous rate of corrosion of samples components of pretreatment chemicals and drawing com- of strand in a salt solution was measured with a polarization pounds, chemical analyses of the acidified water extract and resistance technique. The polarization resistance technique acid/solvent extract solutions, which had been obtained dur- measures the corrosion current, which quantifies the rate at ing the organic residue extraction procedure and had been which the electrochemical corrosion reaction is occurring. separated from the chloroform, were performed. Either zinc This is a much faster test than the test for change in corrosion phosphate or borax (sodium borate) is often applied to the wire potential, but requires specialized equipment (a potentiostat). before the drawing process begins to help drawing lubricants Measurements were taken with the strand in an as-received stick to the surface of the rod stock. Most common drawing condition, after immersing the strand sample in a saturated lubricants are expected to include stearate salts, particularly Ca(OH)2 solution and after an ignition process. sodium and calcium stearates. The elemental concentrations
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16 of sodium, potassium, calcium, and zinc were determined by end of the strand were monitored throughout testing. To allow atomic absorption spectroscopy. The solutions were also comparison of data among strand of different sizes, the bond scanned for detectable quantities of aluminum during the stress has been calculated from the measured loads based on Screening Round. Colorimetric analyses of the wash solutions the nominal surface area (equal to 4/3 db l, where db is the - for boron and phosphate ( PO3 4 as total phosphate) were per- nominal strand diameter and l is the embedment length) of formed using visible light spectroscopy. 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- Pull-Out Testing slip relationship, while the second is based on the maximum The original project scope included the development of a stress measured throughout the test. In concrete pull-out tests performance-based test method for use in evaluating strand performed on historic strand, the early performance was char- bond. As a result, in the initial phases of this study, efforts acterized in terms of the stress at which movement is first vi- were made to develop a procedure for quantifying bond using sually observed at the loaded end of the strand, called the stress a pull-out test conducted on untensioned strand embedded in at "first observed slip" or "first slip." For the tests conducted some material. Two types of pull-out tests have been commonly as part of this experimental program, the stress selected to used to evaluate strand bond. The first method involves characterize the early part of the bond stress-slip relationship pulling untensioned strand out of a block of concrete. The is the bond stress at 0.1-in. slip, measured at the non-loaded second method involves pulling untensioned strand out of a end of the strand. This 0.1-in. slip criterion was adopted to steel cylinder filled with mortar. give a more precisely defined location on the stress-slip curve. In its current form, the concrete pull-out test resembles a method developed by Moustafa (1974). The method was pri- Large Concrete Block Pull-Out Test marily developed to judge the capacity of strand to be used as lifting loops to handle product during shipping and erection. The large concrete block pull-out test (LBPT) involves The test developed by Moustafa was modified by Logan pulling six untensioned strands bonded over 18 in. from a (1997) to judge the bond quality of strand in pretensioned large (2 ft × 2 ft × 2 ft 8 in.) block of concrete. This concrete applications. Further developments of the method have oc- was produced from a conventional mix design used by a pre- curred and are the basis for the testing reported herein. caster, and is produced with a coarse aggregate with Mohs In its current form, the mortar pull-out test method resem- hardness greater than 6.0. The strength of the concrete at the bles a method originally developed for the Post Tensioning time of the test is 3500 to 5900 psi. The test is conducted in Institute in 1994 (Hyett et al. 1994, Post-Tensioning Institute load-rate control with a load rate of 20 kips per minute. The 1996). The method was primarily developed to judge the bond bond stress at 0.1-in. slip and the average maximum bond stress quality of prestressing strand used in rock anchors. The method for each strand tested are reported and averaged. In addition, became the basis of ASTM A981-97 (2002) Standard Test for the concrete pull-out tests conducted as part of this study, Method for Evaluating Bond Strength for 15.2 mm (0.6 in.) Di- the load at which slip at the loaded end was first observed ameter Prestressing Steel Strand, Grade 270, Uncoated, Used in visually also was recorded. This "observed first slip" was Prestressed Ground Anchors (ASTM 2002). Later, this method determined because it relates back to historic pullout data was modified by Russell and Paulsgrove (1999) for NASPA recorded by Logan (1997) and others. and became known as the NASPA test. The NASPA test has been modified slightly by this research project to make it less Mortar Pull-Out Test sensitive to the test apparatus. One of the goals of this project was to try to eliminate vari- In the mortar pull-out test, each prestressing strand is em- ables by using standardized and universally available embed- bedded in 5-in. diameter by 18-in. long steel cylinders filled ment media referred to in the NCHRP Project 10-62 Request with mortar (Portland cement, sand, and water). The top 2 in. for Proposal (RFP) as a "surrogate homogeneous material." of the embedded portion of the strand is debonded, leaving Accordingly, a third type of pull-out test was attempted: 16 in. of strand in contact with the mortar. Six cylinders are pulling untensioned strand out of a steel pipe filled with a tested for each source of strand. The mortar is produced with modified gypsum plaster (Hydrocal). a Type III cement-to-sand ratio equal to 2:1 by weight, and The three types of pull-out tests were performed on the three the required mortar strength at time of the test is 3500 to sources of strand at KSU in March and May of 2005. Each of 5000 psi. The bond stress at 0.1-in. slip and the average these test procedures and their results are described briefly maximum bond stress for each strand tested are reported below. More complete descriptions are given in Appendix B. and averaged. In each of these methods, the load applied to pull out the The test was conducted in load-rate control with a load rate strand and the movement (or slip) of the non-loaded (free) of 5 kips per minute, which was reportedly similar to the rates