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