Shear Behaviour of Reinforced Concrete Beams without and with Small Amounts of Shear Reinforcement

Frederik Autrup

Research output: ThesisPh.D. thesis

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Abstract

The assessment of the load-bearing capacity of existing concrete structures is, in many cases, performed on the basis of design models. In these design models, the minimum shear reinforcement ratio, ρw,min, and the maximum spacing of the shear reinforcement, sl,max, have been introduced to prevent a brittle shear failure. However, many existing concrete structures do not comply with these requirements. In such cases, it is not possible to account for the shear reinforcement when assessing the shear capacity. The development of an accurate shear model that accounts for even small amounts of shear reinforcement is, therefore, a need to suitably evaluate the level of safety and avoid unnecessary strengthening of existing concrete structures and the associated environmental impact.
When assessing the load-bearing capacity of RC bridges, the bridge deck is typically simplified to act as a beam. For reinforced concrete beams without and with small amounts of shear reinforcement, it is well-known that the shear failure is characterised by the development of a critical shear crack. After the development of the critical shear crack, the shear capacity is governed by the sum of shear forces carried by each of the potential shear-transfer actions, such as aggregate interlock, the inclination of the compression chord, residual tensile stresses, dowel action and the shear reinforcement. The development of an accurate shear model should therefore account for the shear contribution from each of these shear-transfer actions. This thesis investigates these shear-transfer actions of reinforced concrete beams without and with small amounts of shear reinforcement.
A literature review reveals that only limited experimental investigations have been performed on beams with small amounts of shear reinforcement. Therefore, two large experimental campaigns Shear test series I and Shear test series II are performed as part of the PhD study. Both experimental campaigns are designed to investigate the influence of small amounts of shear reinforcement on the shear behaviour and shear capacity of RC beams. Shear test series I shows that the shear capacity of beams with a shear reinforcement ratio below the minimum requirements according to the current design standards is similar to identical beams without shear reinforcement. However, Shear test series II also shows that the shear capacity increases as the spacing and diameter of the shear reinforcement are decreased for beams with a constant shear reinforcement ratio.
The thesis shows that the available aggregate interlock models in the literature, in general, overestimate the stresses when compared to Mixed-Mode tests in the post-peak regime (shear slip larger than 0.2 mm). The Mixed-Mode tests imitate most accurately the crack kinematics of the critical shear crack in RC beams. Measurements of the crack kinematics of beams from the experimental campaign Shear test series I show that the crack development is more severe for beams with small amounts of shear reinforcement than for beams without shear reinforcement. By applying a well-known aggregate interlock model from the literature to the measured crack kinematics, the more severe crack development shows to result in an overestimation of the shear contribution from aggregate interlock. A new expression for the aggregate interlock stresses is therefore proposed. Comparing this expression with the shear and Mixed-Mode tests shows a good agreement.
The thesis also shows that the dowel tests performed in the literature to evaluate the shear force carried by dowel action of the tensile reinforcement in RC beams without shear reinforcement are conducted without subjecting the reinforcement to a tensile force. Dowel tests of members with a realistic activation of the tensile reinforcement are therefore performed during the PhD study. The tests show that the level of tension in the reinforcement significantly influences the relation between the dowel force and the dowel displacement. The tests also show that the relation between the dowel force and the dowel displacement is initially linear elastic, followed by a non-linear plastic regime. Such models are currently missing in the literature. Therefore, a linear elastic and a modified rigid plastic dowel model is proposed to evaluate the shear force carried by dowel action. A comparison of the proposed models with tests shows a reasonably good agreement.
The thesis furthermore shows that only a few models are available in the literature for estimating the shear force carried by dowel action of beams with shear reinforcement, and they are purely empirical. The linear elastic dowel model for beams without shear reinforcement is therefore extended to cover beams with shear reinforcement.
 Furthermore, this thesis estimates the shear force carried by each potential sheartransfer action based on the proposed models and the DIC measurements of 16 shear tests from the experimental campaign Shear test series I. The shear tests include beams with a shear reinforcement ratio ranging between 0.04-0.17 %, beams with an effective height varied between 330-660 mm and identical beams without shear reinforcement. Comparing the estimated shear force and the actual shear force shows a good agreement in both the pre-peak and the post-peak regimes. Finally, it is shown that the governing shear-transfer action changes as a function of the shear reinforcement ratio. For beams without or with a low shear reinforcement ratio, the shear force is primarily carried by the brittle concrete contributions, i.e. aggregate interlock and residual tensile stresses. For beams with a larger shear reinforcement ratio, the shear force is primarily carried by the more ductile contributions, i.e. dowel action and activation of the shear reinforcement.
Original languageEnglish
Awarding Institution
  • University of Southern Denmark
Supervisors/Advisors
  • Jørgensen, Henrik Brøner, Principal supervisor
  • Hoang, Linh Cao, Supervisor
Publisher
DOIs
Publication statusPublished - 22. Dec 2022

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Print copy of the thesis is restricted to reference use in the Library. 

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