Prestress Loss of Post-Tensioned Near-Surface-Mounted Carbon Fiber-Reinforced Polymer for Bridge Strengthening

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Title: Prestress Loss of Post-Tensioned Near-Surface-Mounted Carbon Fiber-Reinforced Polymer for Bridge Strengthening

Author(s): Yail J. Kim, Jae-Yoon Kang, Jong-Sup Park, and Woo-Tai Jung

Publication: Structural Journal

Volume: 115

Issue: 5

Appears on pages(s): 1495-1506

Keywords: bridge; carbon fiber-reinforced polymer (CFRP); long-term; loss; near-surface-mounted (NSM); post-tension; retrofit; strengthening

Date: 9/1/2018

Abstract:
This paper presents short-term and long-term prestress losses in post-tensioned near-surface-mounted (NSM) carbon fiber-reinforced polymer (CFRP) composites for strengthening a decrepit bridge. Predictive models are developed to examine the time-dependent behavior of the NSM CFRP, comprising carbon fibers and a resin matrix at macro- and microscales (analytical and closed-form mathematical approaches, respectively). Emphasis is placed on the relaxation of the CFRP, which is a primary component in prestress loss. Parameters influencing the relaxation (characteristic time, and viscous and plastic moduli) are calibrated using the Generalized Reduced Gradient Method. The anchor set of the post-tensioned NSM CFRP is the major attribute for short-term prestress losses, followed by the elastic shortening of the bridge. Regarding long-term losses, the relaxation of the NSM CFRP accounts for 92% of the total time-dependent loss (major relaxation takes place within the first-year period after strengthening), and the creep and shrinkage components are responsible for the remaining portions of 5% and 3%, respectively. The contribution of the fibers to the composite’s relaxation is negligible, whereas that of the matrix is noticeable. According to a parametric study, unlike the characteristic time, the plastic and viscous moduli affect the relaxation responses. Design recommendations are proposed with relaxation losses varying from 3% to 8% of the initial post-tension stress, depending on matrix properties.