Title: Stepped Reinforced Concrete Beams Retrofitted with Carbon Fiber-Reinforced Polymer Sheets and Ultra-High- Performance Concrete

Author(s): Yail J. Kim and Aliasghar Hassani

Publication: Structural Journal

Volume: 120

Issue: 2

Appears on pages(s): 91-104

Keywords: carbon fiber-reinforced polymer (CFRP); rehabilitation; retrofit; stepped beams; strengthening

DOI: 10.14359/51734827

Date: 3/1/2023

Abstract:
This paper presents the effectiveness of structural retrofit for reinforced concrete beams containing variable steps. A strengthening system, consisting of externally bonded carbon fiber-reinforced polymer (CFRP) sheets and ultra-high-performance concrete (UHPC) infills, is proposed to improve the flexural response of the stepped beams alongside mitigated stress concentrations at the reentrant corner. Incorporating a step size ranging from 20 to 60% of the effective depth, unstrengthened (CONT) and strengthened groups are tested with and without anchorage: plain CFRP (CF), U-wraps (CFU), and U-wraps plus mechanical fasteners (CFUM). The bond between UHPC and a concrete substrate gradually grows up to 2.5 MPa (362 psi) at 28 days, accompanied by the improved certainty of adhesion. The load-carrying capacity of the strengthened beams increases by more than 23.1% relative to their unstrengthened counterparts; however, the efficacy of the retrofit system decreases with the incremented step size, which alters the failure characteristics and energy dissipation of those beams. According to analytical investigations, the asymmetric stress distribution of CFRP resulting from the geometric discrepancy is responsible for the progression of damage in the direction of the reduced section. The implications of concrete strength are insignificant for the development of interfacial stresses between the CFRP and UHPC, while the elastic modulus and application thickness of the CFRP and bonding agent dominate the magnitude of the stresses. Implementable recommendations are proposed for practice.