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Title: Modelling the Bond Fatigue Behavior of Concrete Beams Strengthened with Prestressed NSM CFRP Rods

Author(s): Noran Wahab, Timothy Topper, and Khaled A. Soudki

Publication: Symposium Paper

Volume: 322


Appears on pages(s): 13.1-13.16

Keywords: bond, debonding, prestressed, fatigue load, NSM, shear stress, rod

DOI: 10.14359/51706964

Date: 6/18/2018

Near surface mounted (NSM) is a promising strengthening technique provided that the full bond between the strengthening material and the existing structure develops. Wahab et al. (2011 and 2012) tested reinforced concrete beams to asses the bond strength between the prestressed NSM carbon fibre reinforced polymers (CFRP) rods and the concrete. The beams were tested in four points bending under different fatigue load levels. Failure was by slipping between the CFRP rod and the epoxy that started at the support and traveled to the loading point as the number of cycles was increased. Modeling of the failure of the beams is presented in this paper. When the CFRP rod is prestressed, the shear stress is largest at the free end and decreases along the transfer length. The prestressing shear stresses are taken equal to the cracking shear stress under monotonic load for non-prestressed beams. During loading, following flexural cracking, debonding occurs at the loading point and progresses towards the supports as the load is cycled. The debonding is modeled as a crack growing at the interface between the CFRP rod and the epoxy. A bond stress versus slip model is proposed. It consists of an ascending branch representing the strains and stresses ahead of the crack tip and a descending branch representing the region behind the crack tip. The driving force for the crack is the shear stresses at the interface between the CFRP rod and the epoxy. The overlapping of the two shear stress distributions increases the shear stress near the end of the beam, which becomes the critical shear stress that causes failure. The failure shear stress at the end is higher than the prestressing shear stress and is estimated to be 25.5 MPa (3.698 ksi) for the spirally wound rods and 30.4 MPa (4.409 ksi) for the sand coated rods. The model also predicts the number of cycles and the forces in the CFRP rod at all locations in the shear span at the onset of failure with reasonable accuracy.