Title: Bond of prestressing strand to concrete
Date Published: January - February 2021
Page Numbers: 28 - 48
Authors: Andrew Osborn, Mike Lanier, and Neil Hawkins
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The transfer of prestressing force from prestressed strand to concrete over a predictable length is essential for the reliable performance of prestressed concrete. Although no problems with strand bond were reported in the early days of precast, prestressed concrete construction in the United States, several documented problems have occurred in the past 35 years or so, indicating the need for the quality control and quality assurance program recommended in this article.
Over the past two decades, a direct pullout test method that reasonably predicts the bonding properties of the strand used in precast concrete products has been developed. This test method was developed to qualify the strand as having bond properties consistent with design expectations. Certain concrete formulations may affect bond quality, and these should be evaluated separately on a case-by-case basis.
In July 2020, the PCI Technical Activities Council and Research and Development Council approved a new “Recommended Practice to Assess and Control Strand/Concrete Bonding Properties of ASTM A416 Prestressing Strand,” which was subsequently published in November 2020. It establishes minimum test pullout values that can be used to define transfer and development lengths for prestressing strand in concrete. This article summarizes background information on strand bond and documents the many years of research that led to the final published recommended practice.