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Suggested Citation:"Chapter 1 - Introduction." 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 1 - Introduction." 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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Page 4
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
×
Page 5
Page 6
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
×
Page 6
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
×
Page 7
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Suggested Citation:"Chapter 1 - Introduction." 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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3The transfer of prestressing force from prestressed con- crete (PC) strand to concrete over a predictable length is essential for the reliable performance of prestressed concrete. It also is essential that lubricants be used in the wiredrawing process to manufacture PC strand so that the process is cost effective and does not damage the wire. However, residual films of lubricant and other contaminants remaining on the strand surface after manufacture are known to be highly effective in preventing the cementitious bond developed between the concrete and steel. Residual films on wire can be difficult to remove since some residual films, including those resulting from calcium stearate-based lubricants, are water insoluble. The residual film that persists on strand is influenced by many factors, including the condition of the raw rod stock, the pretreatment and lubrication materials and procedures, and the production system, particularly the die condition and line speed. Therefore, to produce strand that reliably bonds with concrete in prestressed elements, the manufacturing process must be carefully controlled, and the appropriate surface treatments must be selected throughout the wire drawing and stranding processes. Finally, a set of testing pro- cedures to be used as part of a routine quality control (QC) program is needed to rapidly assess factors that are known to affect bond properties. Background to Strand Bond Uncertainty Tests conducted in the early 1980s on uncoated and epoxy- coated steel strands found that the measured transfer and de- velopment lengths of the uncoated strands were in excess of lengths computed using the equation found in the AASHTO standard specifications. Publication of these results led to concerns by FHWA and others that the AASHTO equations for transfer and develop- ment lengths were not conservative enough for modern strands with larger diameters and higher ultimate tensile strengths. As a result, an increase of the development length by 60% over the length computed using the AASHTO equa- tion was mandated in 1988 (U.S. DOT 1988). Numerous test programs conducted by industry, acade- mia, and state agencies in response to this mandate, includ- ing a major study by FHWA (Lane 1998), provided data to reevaluate the transfer and development length perfor- mance of modern strands. Beam test data summarized by FHWA in 1998 led to the formulation of new proposed equations for both transfer and development lengths for uncoated strands. Following completion of these tests, the industry became aware of variations in the surface condition of strands pro- duced in the United States. The presence of surface residues arising from varying manufacturing processes suggested a possible source for the wide scatter that was observed in the test data; however, the conclusions were not definitive. The North American prestressing strand producers created an organization called the North American Strand Producers Association (NASPA) to study the problem and recommend solutions to its members. It was primarily concluded that calcium stearate, a non-water soluble metallic soap used as a lubricant in the initial wiredrawing process, was not being consistently or adequately removed during the stress relieving and strand rinsing operations. Therefore, manufacturers were advised to use an alternative lubricating soap, namely sodium stearate, which is water soluble. Other changes, not shared with those outside the strand production industry, may also have been instituted. In spite of the efforts made by NASPA to date, there are still occasional incidents where strand bond problems occur. From the late 1980s to the present, the research team has in- vestigated and is aware of other investigations involving cases of strand slippage problems. Accordingly, rapid QC tests that could be performed frequently were judged to be needed to assess the acceptability of strand surfaces meant to be bonded C H A P T E R 1 Introduction

to concrete. It was the goal of this research project to develop such methods. A more complete description of the history of strand bond- related research is given in Appendix A. Manufacture and Surface Condition of Prestressing Strand The production of prestressing strand, as observed at one manufacturing facility, includes the following basic compo- nents (Figure 1): • Coils of nominal 1/2-in. AISI C1080 steel rod stock are cleaned and pretreated. This can be achieved using a variety of methods; in one example, the raw steel coils are cleaned by pickling (dipped in acid, then rinsed with water) and then phosphate treated (coils are submerged in a zinc phos- phate solution, then rinsed in water and dried). • Rods from several coils are butt welded end to end, and then fed into the wiredrawing machine. • The wire drawing machine consists of a series of eight successively smaller dies that draw down the rod stock to wire with a diameter of about one-third the strand diame- ter. For 1/2-in. diameter strand, the center, or “king” wire, has a diameter of 0.174 in., approximately 5% greater than that of the six outer wires. Integral with each die is a box containing wiredrawing lubricant that the wire passes through before entering the die. (This arrange- ment allows for different lubricants to be used with dif- ferent dies, and it is not uncommon to use a different lubricant for the first [“ripper”] die than for subsequent dies.) Both the die and the capstan, which pulls the wire through the die, are water cooled since the performance of the lubricant and properties of the wire are very sensitive to temperature. At the end of the machine, the individual wire is spooled. • The spools of wire are installed in a skip strander. In this machine, the six outer wires are helically wound around the king wire to form the seven-wire strand. • The strand is drawn under tension through an induction furnace. This stage imparts the stress relieving and low- relaxation properties to the strand. The plant visited has a box installed at the end of the induction furnace that con- tains equipment to wash and cool the strand. • A venting hood is situated between the furnace and the final cooling bath to draw off any vapors created by treat- ment in the induction furnace. • The strand is cooled in a spray chamber with recirculated water. This process may remove some additional wire- drawing lubricants if they are sufficiently water soluble. • The strand is spooled, packaged, warehoused, and shipped to customers. Pretreatment and Lubrication The character and quantity of the residual film on the prestressing strand is governed by the pretreatment of the rod, lubricants used during manufacture, and postdrawing processes including stress relieving. The purpose of the pre- treatment, which typically is conducted on the spooled rod stock, is to provide a foundation for the drawing lubricants. The drawing lubricants are applied to minimize friction, which dictates the amount of energy required for drawing, and to prolong the life of the dies. The cleaning and pretreatment performed on the rod stock are critical and influence all remaining steps in the produc- tion process. This is because the surface quality of the result- ant steel that must be subsequently drawn through the dies governs the lubricant selection and effectiveness. For strand production, the most common coating applied to rod stock during the pretreatment process is zinc phosphate, which serves as a carrier for the lubricants applied during the wiredrawing process. The phosphating process often con- sists of some of the following steps: (1) mechanical cleaning, (2) pickling—cleaning in acid, (3) rinsing—using neutralizing lime solution or water to remove all chloride if hydrochlo- ric acid solution (HCl) is used for pickling, (4) activating prerinse—a dip in a solution or suspension of titanium phos- phate to produce a thinner zinc phosphate layer, and (5) zinc phosphating in a solution that may also contain orthophos- phoric acids and sodium nitrate (Wire Industry 1992, Liberti 1994). The cleaning (i.e., descaling) of the wire may be done me- chanically or chemically. Mechanical methods of descaling include reverse bending, belt sanding, or shot blasting. Chemical descaling usually involves a soak in an acid solution and is typically more effective than mechanical methods. As a result, acid baths are most common in strand plants, since high carbon steel is difficult to clean using other methods. Zinc phosphating deposits a thin layer of zinc phosphate crystals on clean wire to act as substrate for dry lubricating soaps and to provide a barrier to metal-to-metal contact. The best results are achieved with a dense layer of fine crystals. The phosphating process is influenced by temperature and acidity of the phosphate bath, since these factors influence the phosphate solubilities. A longer immersion time will produce a heavier coating (Liberti 1994). It has been observed that zinc phosphate/lubricant coated wires have better corrosion per- formance suggesting that a residual film may be left on the wire produced with such a pretreatment (Rutledge 1974). Phosphate coatings are themselves difficult to remove (Wire Industry 1992). Borax and lime may also be used for pretreatment, either alone or in combination with zinc phosphate. A fundamental difference between phosphate coatings and other coatings 4

5I II Figure 1. Schematic of strand manufacturing process. III IV

such as borax is that the phosphate reacts with the steel surface to provide the foundation for lubricant, but the borax does not. Borax is more likely to be removed during processing than phosphate coatings but is less effective at aiding lubrication (Hajare 1998). Therefore, although a non-phosphate-based process is used in at least one North American manufac- turer’s plant for pollution control reasons, that approach is not common. The most common combination of pretreatments is to follow the zinc phosphate treatment with a bath in hot borax solution prior to drying. The alkalinity of borax serves to neutralize acid from the pickling process not removed by washing. However, borax has some disadvantages including its highly hygroscopic nature (i.e., it absorbs water) (Wire In- dustry 1992). It should also be noted that both zinc and borax can retard Portland cement hydration. Following the pretreatment processes, lubricant is applied to the wire at each die during the drawing process. Dry lubricants are used exclusively by strand manufacturers in the United States in the wiredrawing process for strand. The lubricity agent in such lubricants is typically a chemical compound of a metallic element (calcium, sodium, aluminum, potassium, barium, or combinations of these) plus a fatty acid (such as stearic acid). The dry lubricants may also contain borates. It is commonly felt that calcium-based dry lubricants provide the lubricating properties needed for wiredrawing more cheaply and effectively than any other material. Powdered wiredrawing lubricants are usually classified by their solubility in water. Insoluble lubricants are usually cal- cium based (e.g., calcium stearates; partially soluble lubricants are usually mixtures of sodium stearates and calcium stearates; and soluble lubricants are typically sodium stearates). Within each classification, additives are used to modify the proper- ties of the lubricant to a considerable extent. Thickeners or fillers are usually unreactive, fine powders blended into the lubricant base to increase its viscosity, or resistance to flow under pressure at a given temperature. Although lime (possibly limestone dust in quantities of 30% to 70%) is the most pop- ular thickener additive in general wiredrawing lubricants, the choice of thickener depends on the application and the end use of the wire; for example, coatings that must be easily cleaned should contain soda ash, borax, or other soluble material. Calcium and sodium sulfate compounds are also potential fillers. Extreme pressure additives are used in dry lubricants to reduce friction and increase die life. Molybdenum disulfide is the most popular of such agents; however, it is relatively expensive and may leave a very slippery and difficult- to-clean surface on finished wire. Graphite, sulfur, chlorine, and phosphates are also possible additives (Gzesh and Colvin 1999). One of the key properties that determines which lubricants are most suitable in a given operation is the softening point (related to melting point) since this governs how the lubricant is applied to the wire and how quickly it is removed during drawing. If the lubricant is too soft at operating temperatures, it may come off before the drawing is complete. If too hard, the lubricant film is not applied uniformly and scratching or feathering (flaking) will result (Gzesh and Colvin 1999). The temperatures generated at the strand surface are determined by the plant configuration, including the drawing speed, die geometry, and area reduction in each die. As a result, the de- sirable lubricant properties vary from plant to plant and even from die to die. The softening point is determined by the alkali and fatty acids (such as stearic acid or tallow acids) on which the lubricant is based. The viscosity of the lubricant, which affects the thickness of the coating applied to the steel, is determined by the fat content and filler materials. To min- imize the amount of residual film, the ideal lubricant system would be one that, at the plant die operating temperatures, provides a coating of just sufficient thickness to facilitate drawing, but which would be nearly all removed from the wire by the dies. As previously stated, the two most common lubricants are sodium and calcium stearate-based materials. These lubricants are compounds made from sodium hydroxide or calcium hydroxide and a fatty acid (stearic acid) in combination with additives to impart special properties to the lubricant. These materials are soaps (Ivory soap, for instance, is 99% sodium stearate) and are supplied in dry form. Calcium stearate typ- ically has a lower softening point than sodium stearate (Gzesh and Colvin 1999). Calcium stearate may have been more appealing to strand manufacturers at one time because its lower softening point permits calcium stearate to produce a more effective coat- ing on the wire early in the drawing sequence, when the rate of draw is slower, and the wire is at lower temperatures. In addition, it is typically cheaper than sodium stearate. As a result, it is not uncommon for calcium stearate to be used for the first one to three drafts (dies) and then for sodium stearate to be used for the remainder (Wire Industry 1991). The drawback in the use of calcium stearate-based lubri- cants is that they are more difficult to remove from the drawn wire, since they are water insoluble. In fact, calcium stearate lubricants may be chosen in certain applications (such as nails or coat hangers) because they leave a residue film that makes certain subsequent wire processing proce- dures easier (Platt 1991). On the other hand, sodium soaps or lubricants are “generally used when subsequent operations demand wire that may be readily cleaned” (Wire Association 1965, p. 285). The strategy of using calcium stearate in only the first die(s) does not necessarily limit the residual film of the final product. This is because 80% of the lubricant needed is applied in the ripper box (the first die). Subsequent applications of lubricant retard loss, but typically do not significantly add 6

to the residual lubricant (Gzesh and Colvin 1999, Wire Association 1965). The effectiveness of lubricants is also influenced by the pretreatment selected for use during cleaning. For example, insoluble lubricants (calcium and aluminum) are most com- patible with both borax and zinc phosphate coatings. Soluble lubricants (sodium and potassium) react with borax pretreat- ments to such a degree that the film is weaker and adequate lubrication may not be provided (Dove et al. 1990). When the strand bond problems first began to surface in the United States in the early 1990s, one of the causes was thought to be the use of calcium stearates. As a result, North American strand producers reportedly stopped using calcium stearates, at least in the second and subsequent dies. How- ever, it has been reported that European strand producers still use them. Residual Film Residual films are always present after wire drawing (Wire Association 1965). Prior to about 20 years ago, residual films and possibly other organic residues on prestressing strand that may have been detrimental to bond with the concrete were burned off during the stress-relieving operation (Preston and Sollenberger 1967). However, as noted in a 1982 article (Quick 1982, p.104-105), the replacement of open flame fur- naces with far more efficient induction furnaces greatly im- proved line speed, but residues were no longer being burned off during stress relieving operations. Although the newer in- duction coils were effective in heating the strand and altering the physical characteristics of the steel, the short duration heating does not burn-off surface contaminate like convec- tion heating had done. “Contaminants, such as the efficient wiredrawing lubricant calcium stearate, which do not sublime at stress-relieving temperatures in an induction furnace and are insoluble in water, are of particular concern . . . Induction heating only promoted surface flow of this contaminant, re- sulting in a glazed surface appearance which tended to seal other surface contaminants (i.e., zinc phosphate).” In addition, convection heating, unlike induction heating, is a combustion- based process and that “may have aided in oxidizing impurities on strand surfaces” (Rose and Russell 1997, p. 57). The link between residual films and poor bond was veri- fied when scanning electron microscopy with energy disper- sive spectroscopy (SEM/EDS) analyses conducted on strand tested in structural bond tests confirmed the presence of “copious amounts of surface process chemical . . . on the outer wires of uncleaned strand which failed bond develop- ment tests” (Quick 1982, p. 107). The link between lower amounts of lubricant and increased bond strength has also been demonstrated recently by others (Maehata and Ioka 2006). Additional evidence of an increase in residual film on pre- stressing strand was found in the early 1980s, when bright prestressing strand exhibited approximately six times the chloride ion corrosion threshold of black reinforcing bar in an FHWA sponsored study (Pfeifer 1986). It is believed that the unexpected corrosion protection was due to the presence of residual rod treatments and wiredrawing lubricants, namely zinc phosphate and calcium stearate, on the strand as manufactured. The corrosion performance of strand that had been subsequently “ultrasonically cleaned” by the manufac- turer was indistinguishable from that of the “as manufac- tured” strand, suggesting that the drawing lubricants were not removed by the cleaning. Similar corrosion behavior was noted in 1984 by another researcher, who reportedly cleaned the strand with xylene prior to testing (Stark 1984). Quantifying the amount of residual film present on pre- stressing strand is typically performed through gravimetric methods that consist of weighing a segment of strand before and after stripping with sodium hydroxide (Wire Association 1965) or some solvent. In several investigations of suspected strand bond problems, the research team has employed a method involving a solvent (acid/chloroform) extraction of residue from the strand surface and the cement paste in con- tact with the strand. The removal of residual films on drawn wire is not a trivial process. The cleaning mechanisms applicable to wire reviewed in a recent article included: detergency (displacement of soil by active agents with greater affinity for the substrate surface), mechanical removal (external physical action), chemical re- action (conversion of soil from an insoluble form to a soluble form), and dissolution (soil dissolved with solvent cleaner) (Colvin and Carlone 1998). For strand production, a post- drawing cleaning operation employing the first three mecha- nisms listed above is not typically performed because of the high line speeds involved. However, individual wires may be dipped in water before stranding or the stand may be rinsed with water to cool and clean the strand, if soluble lubricants have been used. When insoluable lubricants are present, cleaning is more difficult. In tests of cleaning solutions, a sodium hydroxide solution was able to remove all but 10 of 300 to 400 mg/ft2 of an insoluble stearate lubricant residue originally on the tested wire. However, cleaning effectiveness is enhanced by in- creased solution temperature and higher rinse volumes and temperatures, all of which require additional effort to produce (Colvin and Carlone 1998). Multi-pass immersions in sodium hydroxide solutions have been used in the production of other wire products where surface cleanliness is critical, such as aluminum-clad steel wire, but these methods are not ideal because of the hazardous nature of the caustic solutions and the large volume of waste solution that is generated (Chow 2001). As alternatives, in-line methods incorporating neutral 7

salt water electrolysis and high-frequency focused ultrasound have been proposed (Quick 1982, Chow 2001). However, because of cost, these methods have not found widespread acceptance in strand production. Research Objectives The original objectives of this study were to: (1) identify the common types of strand residues, determine their impact on bond characteristics and strand performance, and recom- mend methods for their reduction; (2) develop quality control and assurance methods for assessing the level of deleterious residues and recommend thresholds for strand acceptance; and (3) develop a performance-based test procedure and a minimum specification requirement for strand acceptance based on bond behavior. At the direction of the supervisory panel, this third objective was modified during the execution of this project to include the adoption of a pre-existing performance-based test procedure. Organization of Document This final report summarizes the findings of this study and discusses their potential application. The following appendices have been written to provide more background and additional details of this work. These include Appendix A - Review of Strand Bond Literature Appendix B - Evaluation of Mechanical and Chemical Test Methods Appendix C - Specifications for Standard Surface Test Methods Appendix D - Supplemental Investigations of Strand Bond Appendix E - Bibliography of Strand Bond 8

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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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