Strength of cracked concrete: Shear behaviour of straight and arch-shaped members

Jens-Christian Kragh-Poulsen

Research output: Book/ReportPh.D. thesisResearch

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Arch-shaped and straight concrete members are commonly used in civil and marine structures, e.g. caissons, silos, tunnels, etc. However, shear capacity formulas in standards and guidelines for non-shear reinforced concrete are based on empirical formulas, calibrated on straight concrete members. These formulas are frequently used for arch-shaped concrete members despite the fact that they are not necessarily applicable to such structures. Neither are they necessarily applicable to large (thick) structures. At present, scanty experimental data are available on strength and behaviour of reinforced arch-shaped concrete members without shear reinforcement subject to shear and bending. The aim of the present thesis is to improve our general knowledge of the strength of cracked concrete in such members.
Part I of the thesis is an introduction to the general theoretical and physical background of the thesis. Here concrete is reviewed as a material, showing how complex it is, and a reason is given for modelling the carrying capacity of a structure relatively simply. An introduction is also given to the theory of plasticity, the basic theory used in this thesis. An example is given of using the crack growth theory on two reinforced concrete beams without shear reinforcement, and the background for the crack growth theory is reviewed. This example indicates that application of linear-elastic fracture mechanics to the analysis of reinforced concrete members may give reliable results for predicting crack propagation. It also shows that to predict the load-carrying capacity linear-elastic fracture mechanics does not suffices. To account for the transmission of shear stress in cracks another approach must be used.
Part II deals with straight concrete beams and plates without shear reinforcement. Recently it has been shown that the effectiveness factors used in the crack sliding theory are not well suited for predicting the shear strength of beams with large depths. Therefore, formulas for the effective concrete compression strength and the effective tensile strength are re-formulated by changing the size effect factor and adding a term to take into account the maximum aggregate size. The theory is also improved to predict better the shear strength of deep beams and beams with short shear span by including the effect of the transverse stress. Experimental data from slender beams and deep/short beams with rectangular cross sections are used for validation of the crack sliding theory. The outcome of this analysis is that the new theory in general is substantially better at predicting shear strength than the old one.
Part III deals with curved concrete beams and plates without shear reinforcement. Here shear strength and spalling strength of curved beams and plates are treated. Little research on the topic is available and no design codes take curvature into account. This may lead to uneconomic or unsafe design solutions. The objective of this study is to develop an understanding of the shear behaviour of such members. It presents the results of a test series of 12 arch-shaped and 3 straight concrete beams without shear reinforcement, all tested in a three-point bending test. The radius of the arch-shaped members is varied both in direction and magnitude. The maximum aggregate size and compressive strength are the main varying material properties. The experimental results together with a test series from literature are analysed with respect to the influence of curvature on shear capacity and crack development. Digital image correlation is used to analyse crack development with high accuracy, a method not yet seen in the literature on arch-shaped structures investigated for shear.
Finally, a theory of the spalling strength of concrete members with curved reinforcement bars based on the plasticity theory is developed. This theory is compared with existing experimental work and shows good agreement.
Original languageEnglish
PublisherTechnical University of Denmark, Department of Civil Engineering
Number of pages206
ISBN (Print)87-7877-545-0
Publication statusPublished - 2020

Bibliographical note

PhD thesis R-444


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