Spalling of concrete cover induced by reinforcement

This paper investigates the phenomenon of cover spalling in reinforced concrete induced by bond or by the action of an inner pressure. This research is based on an experimental programme comprising a series of bond tests and a series of tests with inner-pressure on cylindrical openings. The inner-pressure test series (aimed at representing the conditions occurring for instance due to corrosion of reinforcement) was performed with hydraulic inflator devices embedded within concrete openings near to the free concrete surface. By means of detailed surface measurements performed with Digital Image Correlation, the mechanisms triggering spalling failures and the associated resistance are discussed and analysed thoroughly. This series investigates in addition a number of phenomena relevant to spalling failures, such as the influence of the casting position, group and size effects. The observed response, analysed by means of a mechanical analogy, is later used to investigate a series of structural tests performed on pull-out specimens. This analysis highlights the analogies and differences between the two types of tests (subjected to an imposed pressure or to bond stresses). On that basis, a comprehensive approach for treatment of bond-related cases failing by cover spalling is proposed, showing consistent agreement to the experimental evidence.

Interaction between Bond and Deviation Forces in Spalling Failures of Arch-Shaped Members without Transverse Reinforcement

Miguel Fernández Ruiz, Aurelio Muttoni, Sylvain Plumey

This paper investigates the structural behavior of reinforced concrete (RC) arch-shaped members without transverse reinforcement subjected to bending. Such members have typical applications in tunnels, cut-and-cover structures, shells, vaults, ducts, silos, tanks, and off-shore structures. Although such members are mostly subjected to axial forces, bending moments may also develop when the shape of the structure does not perfectly match the ideal funicular shape. In this case, when the intrados reinforcement is in tension, deviation forces developed by the reinforcement increase the splitting stresses originated by bond and can lead to spalling of the reinforcement cover. Such a failure mode is particularly brittle and dangerous, leading to a sudden loss of load-carrying capacity of the structure. In this paper, a series of six tests on 400 mm (15.7 in.) thick arch-shaped beams are presented. They are aimed at investigating spalling failures before and after yielding of the tensile reinforcement. These results, as well as others taken from the literature, were compared to an analytical model accounting for the interaction between bond and deviation forces, showing a good agreement and explaining the various failure modes observed. On that basis, a practical formula for the design of such members is proposed.

ACI Structural Journal2010

Shear Strength of Arch-Shaped Members without Transverse Reinforcement

Stefano Campana, Miguel Fernández Ruiz, Aurelio Muttoni

Arch-shaped members are used in many reinforced concrete structures such as tunnels, bridges, silos, or offshore constructions. They are generally designed to carry significant axial forces, and, in most cases, are not provided with transverse reinforcement. Depending on the applied actions, however, bending moments and shear forces may also develop. This can potentially lead to spalling failures of the concrete cover or to shear failures where the strength may be reduced in comparison with straight members. Such reduction is due to the curvature of the element and to the corresponding deviation forces originated by the compression and tension chords. In spite of the significance of the topic, little research has been performed. In this paper, the authors summarize the results of a comprehensive investigation comprising an experimental part on nine arch-shaped specimens with thickness equal to 400 mm (15.7 in.). On the basis of these results, the basic failure mechanisms and shear-transfer actions are identified. By using the principles of the critical shear crack theory for assessment of the shear strength, a design approach for such members is presented that accounts for the various mechanical and geometrical parameters. The approach provides consistent and accurate predictions when compared with the tests presented in the paper.

ACI Structural Journal2014

Stress Fields for the analysis of the faillure load and cracking state of dapped-end beams (Aplicación de modelos de campos de tensiones para el análisis en servicio y rotura de apoyos a media madera)

Miguel Fernández Ruiz, Jaime Mata Falcon, Aurelio Muttoni

The automatic generation of elastic-plastic stress fields (EPSF) for structural concrete members by means of the finite element method has been demonstrated as a suitable approach for the design and assessment of D regions. This methodology allows accounting for an elastic-plastic behaviour of the materials. For concrete, no tensile strength is considered, and the compression softening hypothesis of Vecchio & Collins is incorporated to take into account the effect of the transversal cracking on the compressive strength of concrete. In this paper, the application of this tool for the analysis of the failure load and cracking conditions at serviceability is investigated over a series of experimental tests on dapped-end beams (DEB). The failure of DEB occurred in most cases by spalling of the concrete cover on top of the suspension reinforcement. The models with the EPSF predicted also consistently this failure mode. Contrary to classical approaches of stress fields, where the tensile strength of the concrete cover is neglected (leading thus to conservative estimates of the strength for members failing by cover spalling), it is shown in this paper that EPSF can be suitably adapted to reproduce this failure mode by introducing a fictitious reinforcement in the cover. This reinforcement is introduced to reproduce cover spalling for a critical opening of the spalling cracks developing. With this assumption, a very good estimate of the experimental ultimate loads is consistently obtained, also for prestressed elements and for steel-fibre reinforced members (for which the contribution of the fibres is introduced in the model as an equivalent plastic tensile strength of the concrete). A methodology for the determination of the crack width in the critical regions of DEB for service load levels by using EPSF is also investigated. It is assumed a rigid-plastic approximation for the bond stresses near the crack. The methodology gives also reasonable and consistent estimates of the crack widths for DEB specimens, particularly for those with diagonal reinforcement, in which the cracking process is more stable.

VI Congreso ACHE2014

Miguel Fernández Ruiz, Jaime Mata Falcon, Aurelio Muttoni

VI Congreso ACHE2014