Shear Strength of Members without Transverse Reinforcement Based on Development of Critical Shear Crack

The shear strength of beams and one-way slabs has been acknowledged for more than one century as one of the most complex, yet fundamental, topics to be addressed in structural concrete design. The experimental data used to investigate the phenomenon has traditionally been obtained from tests on simply supported beams subjected to point loading and recording only a limited amount of data (applied load and deflection in most cases). Following these experimental evidences, design formulas have been calibrate-din many cases in an empirical manner-by using large amounts of experiments in the form of databases. Also, mechanical models have been proposed as a rational approach to shear design, which can be conceptually very different and relies on different shear-transfer actions. Discussions on the pertinence of these models are mostly based on the general agreement of the failure load to existing datasets more than on a critical review of their basic hypotheses. Within this context, the development of new measuring techniques, such as digital image correlation (DIC), enables a systematic and transparent examination of the shear crack development and of the role of the various potential shear-transfer actions. This allows for a scientific discussion on the correctness of the principles of the design approaches and allows understanding their pertinence and limitations. Based on these possibilities, the main hypotheses of the Critical Shear Crack Theory (CSCT) for shear design are examined in this paper. To that aim, the results of a specific test series on specimens tested under realistic loading conditions are reviewed. The results show that shear strength results from a combination of various shear-transfer actions depending on the development of shear cracks and their associated kinematics. On this basis, the applicability and pertinence of the CSCT main hypotheses are discussed and a complete mechanical approach is formulated. Also, it is shown that the CSCT can be formulated in a simple and consistent manner as closed-form design equations.

Shear in reinforced concrete without transverse reinforcement

Francesco Cavagnis

Since the first applications of structural concrete, the shear behaviour of one-way slabs without transverse reinforcement has been largely investigated. Nevertheless, currently in the scientific community there is no general agreement on the mechanisms of shear failure, on the parameters governing the shear strength and on the predominant shear-transfer actions. Hence, several mechanical models, based on very different hypotheses, and empirical formulations, calibrated on the available experimental results, have been proposed in the last decades. In addition, these experimental results have been traditionally obtained from tests on simply supported beams subjected to point load, whereas in most one-way slabs without transverse reinforcement in practice (foundations, retaining walls, slabs of cut-and-cover tunnels, silos) the boundary and loading conditions are typically different. This thesis has therefore the objective to provide new experimental data on reinforced concrete members without transverse reinforcement tested with different loading and boundary conditions, to increase the understanding on the mechanisms of shear failure and to develop a mechanical model based on the new experimental evidence. In the first part of the thesis, the experimental results of 23 tests on 20 beams without transverse reinforcement subjected to different loading (concentrated or distributed) and boundary conditions (cantilevers, simply supported or continuous beams) are presented. Refined measurement techniques allowed detailed tracking of the development of the crack pattern up to failure. The results show that the location, inclination and kinematics of the critical shear crack play a major role on the shear strength. Moreover, the amount of shear transferred by the various potential shear-transfer actions has been estimated on the basis of the experimental measurements and by using suitable mechanical models for each shear-transfer action. The analyses show that, for slender members, the shear-transfer actions contributing to the shear capacity are the inclination of the compression chord, the residual tensile strength of concrete, the dowelling action and the aggregate interlock, and the latter is the predominant one. For squat members or members in which the critical shear crack develops below the theoretical compression strut, differently, the arching action is predominant. In the second part of the thesis, a mechanical model, consistent with the main ideas of the critical shear crack theory, is presented. The shear force that is transferred through the critical shear crack by the various shear-transfer actions is calculated by integration of simple constitutive laws and a failure criterion is obtained by summing the different contributions. The shear and deformation capacity can thus be calculated by intersection of the failure criterion with a load-deformation relationship. It is shown that the failure criteria obtained by integration of stresses at the crack surface can be approximated by power-law equations. Combining the power-law failure criteria with the load deformation relationship, a closed-form equation has been obtained. The closed-form equation provides almost identical results to the mechanical model and allows for direct design and assessment of existing structures. The accuracy of the mechanical model and the closed-form equation has been checked against a large database, showing a good agreement to the experimental results.

EPFL2017

Influence of aggregate fracture on shear transfer through cracks in reinforced concrete

Juan Sagaseta Albajar

Design methods for shear in reinforced concrete structures typically rely upon shear transfer through cracks, which depends upon the crack opening and sliding displacements and the roughness of the crack surfaces. The effectiveness of shear transfer through aggregate interlock is commonly believed to be reduced if the coarse aggregate fractures at cracks, as is frequently the case in high-strength and lightweight aggregate concretes. This paper describes two sets of push-off tests that were carried out to investigate the effect of aggregate fracture on shear transfer through cracks. Marine dredged gravel was used in one set of specimens and limestone in the other. The cracks typically passed around the gravel aggregate but through the limestone aggregate. The experimental results are compared with the predictions of various existing analytical models including those in design codes MC90, Eurocode 2 and ACI-318. The paper also examines the contribution of aggregate interlock to the shear strength of a parallel set of reinforced concrete beams, tested by the authors, which used the same types of aggregate as the push-off specimens.

Magazine of Concrete Research2011

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.