Experimental tests on existing RC beams strengthened in flexure and retrofitted for shear by C-FRP in presence of negative moments

Davide Lavorato,Gabriele Fiorentino,Alessandro Vittorio Bergami,Camillo Nuti,Alessandra Fiore,Silvia Santini

doi:10.1007/s40091-018-0193-1

Abstract

The shear strength of reinforced concrete beams extracted from existing buildings often reveals insufficient transversal steel reinforcement, mainly due to design or construction defects or increased design load requirements. FRP wrapping is one of the best solutions to improve beam shear strength as the retrofitting intervention is fast and the cost is modest. Design codes provide clear indication about the retrofitting design of simply supported beams, while the case of a beam with negative moments at the end is not considered, although this is in the case of a beam in a framed structure. One of the main uncertainties lies in the effectiveness of the FRP U sheet anchorage behavior in the area of negative bending moments with cracked concrete. This may limit the shear strength of the retrofitted beam. In this study, two beams extracted from an existing building constructed in the 1930s in Rome and retrofitted by carbon fiber-reinforced polymer (C-FRP) U strips placed at beam ends, where also negative bending moments were present, and have been evaluated with experimental tests at the laboratory of the Department of Architecture of Roma Tre University. Beam steel and concrete characteristics were evaluated by means of different tests. The experimental results are discussed considering the final results in terms of maximum shear resistance in the presence of negative bending moments. Load deflections at different points along the beam, shear-C-FRP deformation along the reinforcement strips and the damage state for different load levels, are presented. The importance of avoiding possible fragile mechanisms in the sections retrofitted with FRP is clearly shown.

Highlights

Recent earthquakes which hit Italy Between August and October 2016 (Fiorentino et al 2017) demonstrated the high vulnerability of the Italian building stock
The experimental results about the elastic and failure tests described in §4 are discussed in terms of: (1) behavior of the non-retrofitted beams during the elastic tests and (2) behavior of the retrofitted beams during the failure tests
Two beams (TM1 and TM2) extracted from an existing reinforced concrete (RC) building located in Rome and dated around 1930 have been retrofitted in bending by adding an RC slab and reinforced in shear with carbon fiber-reinforced polymer (C-FRP) strips, using the same provisions adopted in the real building

Summary

Introduction

Recent earthquakes which hit Italy Between August and October 2016 (Fiorentino et al 2017) demonstrated the high vulnerability of the Italian building stock. This vulnerability is evident for masonry and for reinforced concrete (RC). Many existing RC beams have insufficient shear reinforcement, due to design or construction defects or increased design load requirements. The beam retrofitting with carbon fiber-reinforced polymer (C-FRP) sheets or strips is a valid solution to increase the shear strength. International and National Guidelines and Codes (CNR-DT 200 R1/2013 2013; Fib 2001; Italian Council of Public Works 2009) give design prescriptions for shear retrofitting of beams by C-FRP which are essentially based on exhaustive experimental data for supported beams only

Methods

Results

Conclusion