Influence of cracking and rough surface properties on the transfer of forces in cracked concrete

Aggregate interlocking is acknowledged as one of the most significant actions transferring shear forces in cracked concrete structures and has been investigated for several decades. Despite the many experimental programmes and previous efforts to develop models based on mechanical approaches, a number of instrumental issues of the phenomenon are still not fully understood. For example, most researches have focused on the capacity to transfer forces through a given crack surface. However, the development of secondary cracks developing from the initial crack due to stress concentrations has traditionally been disregarded, despite the fact that these secondary cracks are governing in many cases for the overall strength. Also, other important aspects have not been comprehensively investigated, such as the contribution of the residual strength of concrete both in tension and shear during crack development. In this paper, the results of an experimental programme aimed at the fundamental understanding of the transfer of forces in cracked concrete is presented. This programme comprises detailed measurements of the surface roughness after failure. On that basis, a model considering both the crack surface properties and those of the concrete material is presented, accounting also for the potential development of secondary cracking. The model estimates the transferred forces by considering surface patches in contact and the contribution of the residual strength of the fracture process zone. The results of the model are compared to the test results showing consistent agreement both in terms of failure mode and the capacity to transfer forces as a function of crack opening and sliding.

Assessment of Shear Strength for Existing Bridges with Low Amounts of Shear Reinforcement

Miguel Fernández Ruiz, Aurelio Muttoni, Michael Markus Rupf

Design of concrete girder bridges has significantly evolved during the last decades. This has been particularly relevant with respect to shear design, motivated by changes in actions and design models. As a consequence, assessing the shear strength of existing bridges leads in many cases to unsatisfactory safety levels. Furthermore, many existing bridges do not comply with current code regulations with respect to minimum amounts of shear reinforcement. This situation can lead to expensive retrofitting and strengthening of a significant number of existing bridges. The assessment of the shear strength of these bridges has thus become a significant task for structural engineers. Design codes are not always appropriate for assessing the strength of existing bridges. They propose safe models providing sufficient accuracy for design of a wide number of structures. However, these models do not account for some particularities of prestresses bridges and neglect a number of shear-transfer actions that can be relevant for their strength. This is typically the case of shear carried by the inclination of the compression chord, the increase of the stress in the tendons or the effective strength of concrete in the web of cracked prestressed girders. Accounting for more realistic approaches for these parameters may significantly increase the estimated strength of a structure with respect to code provisions, avoiding in many cases unnecessary retrofitting or strengthening. In this paper, the results of a tests campaign carried out at Ecole Polytechnique Fédérale de Lausanne on 10 prestressed concrete girders (10 meters long, 0.78 m high) are outlined. The specimens were provided with very low amounts of shear reinforcement and some of them present defective stirrup anchorage to simulate realistic conditions of existing structures. The experimental results are discussed with reference to the stress field method where the role of the different shear-transfer actions is investigated and compared to current code provisions.

fib Symposium Tel-Aviv 20132013

Fatigue Behaviour of Steel Reinforcement Bars at Very High Number of Cycles

Marina Rocha Pinto Portela Nunes

A large portion of reinforced concrete (RC) bridges in the western world were built in the second half of the last century. RC bridge deck slabs are nowadays often subjected to increased traffic loading and volume than originally designed for and thus, steel reinforcement bars (rebars) are more susceptible to fatigue damage. Fatigue life of rebars can be largely affected by the crack initiation phase characterised by the growth of short cracks. The approaches available for fatigue damage evaluation of rebars fail to predict the crack initiation phase. Microstructural barriers control the short crack behaviour which can be significantly different from the stable long crack growth described by Paris' law. The stochastic nature of the fatigue life comes mainly from the scatter of these short cracks. Research on this domain is attractive since it can help to understand more accurately the fatigue behaviour of rebars. A better understanding can result in more accurate fatigue damage evaluation of RC elements. The aim of this thesis is to predict the scatter and fatigue behaviour of hot rolled (HR), cold worked (CW) as well as quenched and self-tempered (QST) rebars incorporating the crack initiation phase. The research commences with an experimental investigation on the fatigue strength of QST rebars under high and very high cycle fatigue (HCF-VHCF) at constant amplitude (R= 0.1). A non-destructive inspection technique was applied for surface crack detection based on the frequency change monitored during the tests. Surface and near surface macro residual stresses on QST rebars were determined by X-ray diffraction and Cut Compliance techniques. Surface imperfections and roughness were identified with Scanning Electron Microscopy mainly near the ribs. A parametric study of the rebar geometry, using 3D Finite Element Models, allowed to determine the influence of rib inclination and rebar diameter on the stress concentration factors. A short crack growth model was developed to study the scatter resulting from the interaction between short crack and microstructural barriers. The model includes dispersion of grain orientation ratio, grain size variation and different phases (ferrite-pearlite and martensite). This model was then model to include the surface roughness effects and long crack propagation. The stress concentration factor was considered as a constant parameter. The model predicted the fatigue behaviour of HR-CW and QST rebars.

EPFL2014

The mechanics of shear failures in RC slabs based on refined measurements

Raffaele Cantone

Reinforced concrete planar members, as slabs and shells, are structural elements commonly used in the construction technique, which are typically designed without the arrangement of shear reinforcement. Despite the fact that this solution allows for fast and economic construction, the absence of shear reinforcement can give rise to the potential localization of strains within a critical shear crack and eventually to the shear failure of the member. In the case of redundant systems, most research on the mechanics of shear failures has been devoted to the strength of the member, neglecting in many cases the development of shear deformations due to inclined cracking as well as the redistributions of internal forces, which are instrumental for the analysis of the response of these members. The contribution to the state-of-the-art includes a series of theoretical works explaining the observed responses for a series of experimental programmes. These experimental campaigns comprise tests in tension, shear tests in one- and two-way slabs as well as punching tests. For their instrumentation, in addition to classical measurement devices, Fibre-Optic Measurements and Digital Image Correlation were intensively used.This thesis starts by revisiting the basis of the interaction between reinforcement and concrete. A series of bond tests show the stress concentrations occurring near the ribs and its complex transfer of forces with the surrounding concrete. In addition, tests on beams failing in shear show a complex interaction between bond stresses and kinking on the reinforcement due to the development of dowel action. These phenomena are normally neglected for concrete design due to the ductile nature of reinforcement, but may be relevant for fatigue and negative tension-stiffening effects.An important step in the knowledge is performed on the understanding of the shear response with respect to the characterization of the deformations in concrete members. Based on a series of test results, a complete description of the deformation field (including shear strains) is presented. On that basis, a rational model is proposed, consistent with the mechanical model of the Critical Shear Crack Theory. This model allows for a precise description of the response and also to describe the through-thickness distribution of the shear deformation.A general frame for modelling of reinforced concrete slabs is thus presented accounting for the redistribution of internal forces during propagation of the shear crack. This approach is used to investigate a testing programme performed on three wide slabs, analysing in a scientific manner the influence of the width of the member on the shear resistance. The detailed experimental data allow to capture the crack propagation and internal forces redistributions. Clear conclusions and answers are obtained, showing the influence of the shape of the failure surface and of its propagation on the load-carrying capacity.The research ends with a final investigation on the dowelling action of compression reinforcement, with an application to slabs failing in punching. Based on a large testing programme including eleven axisymmetric punching tests, an analytical approach is developed to estimate the contribution of the dowel action on the load-carrying capacity. This approach is formulated within the frame of the Critical Shear Crack Theory, and is incorporated in a consistent and efficient manner for design purposes.

EPFL2021