Spall | EPFL Graph Search

Spall are fragments of a material that are broken off a larger solid body. It can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ball bearing). Spalling and spallation both describe the process of surface failure in which spall is shed. The terms spall, spalling, and spallation have been adopted by particle physicists; in neutron scattering instruments, neutrons are generated by bombarding a uranium (or other) target with a stream of atoms. The neutrons that are ejected from the target are known as "spall". Mechanical spalling Mechanical spalling occurs at high-stress contact points, for example, in a ball bearing. Spalling occurs in preference to brinelling, where the maximal shear stress occurs not at the surface, but just below, shearing the spall off. One of the simplest forms of mechanical spalling is plate impact, in which two waves of compression are re

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

On the compressive and bond strength of reinforced concrete as structural properties

Francesco Rafael Alberto Moccia

Traditionally, the concrete strength is measured on cubes or cylinders having normalized dimensions, suitable vibration and curing conditions and their strength is assessed in laboratory under fast loading rates. However, the in-situ strength of structural elements can considerably differ from that of a small and homogenous control specimen due to a number of issues. Notably, phenomena taking place during the consolidation process of fresh concrete may affect the compressive resistance of tall members as well as the bond strength of reinforcing bars located in top layers. During concrete consolidation, water migrates in the direction of the free surface while concrete settles downwards, phenomena referred as concrete bleeding and plastic settlement respectively. Under these circumstances, a decrease in the concrete properties near the upper surface is observed as well as a development of cracks and voids surrounding horizontal reinforcing bars, causing potential disturbances on the compressive stresses and affecting the mechanical engagement between bars and concrete. In addition, the response of structural concrete may differ from that of material samples due to non-uniform stress states, the material brittleness, the cracking induced by imposed strains, the rheological response of concrete and the presence of embedded disturbances. As a result, the strength measured in material samples needs to be corrected with strength reduction factors to ensure suitable structural analysis. In this thesis, an in-depth investigation is performed on the different phenomena affecting the compressive and bond strength of structural members. These aspects are assessed by means of several testing programmes instrumented with refined measurements techniques such as tomography and Digital Image Correlation. An extensive experimental programme comprising 76 column and prism tests was carried out to evaluate the influence of casting position, loading direction and bar disturbances on the compressive resistance of structural elements. The detailed measurements performed at the fresh and hardened state resulted in the proposal of consistent design rules accounting for the investigated phenomena. Focus was also given on the influence of material brittleness and the implications of internal stress redistributions on the structural response of reinforced concrete columns and compression zones of members in bending. The pertinence of the investigations were validated based on more than 400 column tests collected from the literature. The implications of casting conditions on pull-out and spalling failures were also assessed by means of 137 pull-out tests on reinforcing bars presenting variable diameter, concrete cover, casting height and embedded length. The investigations resulted in the proposal for a physically-consistent approach, evaluating the pull-out resistance as function of casting conditions and reinforcement characteristics. Finally, the phenomenon of cover spalling was examined with respect to the action of a radial inner pressure, as originated by bond engagement or associated to the volumetric expansion of corroded reinforcement. The mechanisms inducing spalling were analysed by means of a comprehensive experimental programme comprising 56 specimens instrumented with Digital Image Correlation. A mechanical model is eventually proposed to evaluate bond-related cases of cover spalling.

EPFL2021

Casting position effects on bond performance of reinforcement bars

Miguel Fernández Ruiz, Francesco Rafael Alberto Moccia, Aurelio Muttoni

The phenomena associated with the consolidation of fresh concrete (bleeding and plastic settlement) are commonly considered significant for the bond performance of reinforcement. However, rules to take care of such influence for design are not consistent amongst design recommendations and may lead to notable differences. With this respect, two failure modes generally govern the bond failure, namely the spalling of the concrete cover (also called splitting failure) and the pull-out of the reinforcement. In this paper, a detailed investigation is presented on the influence of bleeding and plastic settlement on both failures modes, in an effort to understand their conceptual differences and to clarify how shall consistent design recommendations be formulated. Such investigation is based on a comprehensive experimental programme, comprising 137 pull-out tests on specimens with different casting conditions, embedment lengths, loading arrangements and concrete covers. On the basis of the test results, the phenomenological differences between pull-out and spalling failures are clarified, as well as the main influencing phenomena (particularly the potential presence of cracks and voids under the reinforcement and the mechanical properties of concrete). On this basis, a physically-consistent approach is presented to consider the casting conditions on the bond performance and failure modes.

ERNST & SOHN2021

On the compressive and bond strength of reinforced concrete as structural properties

Francesco Rafael Alberto Moccia

EPFL2021

Miguel Fernández Ruiz, Jaime Mata Falcon, Aurelio Muttoni

VI Congreso ACHE2014