Tests conducted in the early 1980s on uncoated and epoxy-coated steel strands found that the measured transfer and development 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 the FHWA and others that the AASHTO equation was not conservative enough for modern strands with larger diameter and higher ultimate tensile strength. As a result, an increase of the development length by 60 percent over the length computed using the AASHTO equation was mandated.
Numerous test programs conducted by industry, academia, and state agencies in response to this mandate, including a major study by the FHWA, provided data to reevaluate the transfer and development length performance of modern strands. Beam test results were summarized by FHWA, which resulted in the formulation of new 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 used throughout the United States. The presence of surface residues arising from manufacturing processes, rust and other foreign materials as well as from the use of corrosion inhibitors suggested a possible source for the wide scatter that was observed in the test data; however, the conclusions were not definitive.
Strand surface condition has a major impact on transfer and development length, yet it has not been evaluated systematically to date. Strand surface condition needs to be quantified, and acceptance test criteria need to be developed to control its impact. This issue has been unresolved for nearly 20 years, resulting in wide variations in the design of beam-ends and pile-cap embedments. Research into strand surface conditions is needed to produce more consistent, economical, and safe designs nationally; the potential for cost savings and national implementation are high.
The objectives of this study were to (1) identify the common types of uncoated steel strand residues, their impact on bond characteristics and strand performance, and recommended methods for their reduction; (2) develop a quality control and assurance method for assessing the level of deleterious residues and related test specification requirements and thresholds for strand acceptance; (3) develop a performance-based test procedure and a minimum specification requirement for strand acceptance based on bond behavior; and (4) determine if the use of surface treatments applied to strands has a significant impact on their bond characteristics.
Product Availability: The project final report is available as NCHRP Report 621, Acceptance Tests for Surface Characteristics of Steel Strands in Pre-Stressed Concrete.
Unpublished appendices A, D, and E are available here.