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