Nicola Gehri

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Gehri

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Nicola

ORCID

0000-0002-0321-7446

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Publications 1 - 10 of 10

Experimental investigation of the shear response of large-scale fibre-reinforced concrete panels
Gehri, Nicola; Mata Falcón, Jaime; Kaufmann, Walter (2023)
Engineering Structures
Although fibre-reinforced concrete has been the subject of extensive research for decades, its mechanical behaviour in shear is still not well understood. Furthermore, hardly any large-scale shear tests on elements with moderate fibre contents have been conducted to date. The present paper addresses this knowledge gap by presenting and discussing the results of an experimental campaign on large-scale fibre-reinforced concrete shear panels. Six panels with dimensions of 2.00 m × 2.00 m × 0.27 m were tested in the Large Universal Shell Element Tester at ETH Zurich by applying homogeneous shear. In four of these experiments, the in-plane shear was combined with homogeneous uniaxial compression by simulating the longitudinal restraint provided by the chords in a girder. Five tests focused on concrete of normal strength with moderate fibre contents and one test studied the behaviour of ultra high-performance fibre-reinforced concrete. The combined application of digital image correlation instrumentation and distributed fibre optical sensing allowed gaining a deeper insight into the structural behaviour of the panels. Overall, the test results showed the high efficiency of the fibres as shear reinforcement in large-scale web elements. However, failure of the panels without any steel reinforcing bars was rather brittle, with only few cracks forming over the element. The provision of a moderate amount of distributed longitudinal bar reinforcement has been found to be highly beneficial to the crack behaviour and thus significantly increase the ductility.
Cracked Membrane Model with Fixed, Interlocked Cracks: Numerical Implementation and Validation
Beck, Alexander; Gehri, Nicola; Mata Falcón, Jaime; et al. (2020)
Journal of Structural Engineering
The cracked membrane model with fixed, interlocked cracks (CMM-F), whose basic concepts were already outlined in this journal more than 20 years ago, is the most general approach for cracked reinforced concrete members subjected to in-plane stresses as long as one set of uniformly spaced cracks is considered and steel and bond stresses are modeled by equivalent, uniformly distributed stresses. However, the CMM-F was not implemented so far, due to the numerical intricacy of the general solution procedure. In this paper, these issues are overcome by determining the variation of steel and concrete stresses and strains between cracks using the tension chord model (TCM), rather than by iteratively integrating over a crack element. After a discussion of the compression field approaches, the TCM and the CMM-F, including the constitutive relationships of concrete and reinforcement and aggregate-interlock models, response predictions obtained from the CMM-F are validated against experimental data. While the agreement is generally good, the predicted stresses at the cracks and the crack kinematics differ significantly between the different aggregate-interlock relationships. These values should be measured in future experiments, using appropriate instrumentation, to validate the aggregate-interlock models.
Exploring the shear behaviour of fibre-reinforced concrete
Gehri, Nicola (2024)
It is widely acknowledged that the addition of fibres can improve the mechanical behaviour of reinforced concrete and, thus, fibres may partially replace conventional reinforcement. Furthermore, the substitution of manually assembled and placed reinforcing bars with an equivalent dosage of fibres directly added to the concrete mix can yield considerable economical and ecological benefits. However, due to the often unfavourable softening behaviour after cracking of the concrete when subjected to tension, in practice, the application of fibre-reinforced concrete is nowadays essentially limited to structurally low-demanding or secondary elements. On the other hand, fibres offer substantial potential as shear reinforcement: various experiments have shown that even a moderate fibre dosage can prevent brittle shear failures in beams without conventional transverse steel reinforcement. Nevertheless, the theoretical understanding of the effectiveness of fibres as shear reinforcement is still limited and tests representative for real structures are scarce. To this end, extensive experimental and theoretical investigations of the shear behaviour of fibre-reinforced concrete were carried out in this thesis. To gain a better understanding of the structural response of fibre-reinforced concrete, the crack behaviour of fibre-reinforced girder web elements was investigated in detail. Comprehensive knowledge of the crack pattern and the crack width and slip is of particular importance in fibre-reinforced concrete, as the stresses transferred across cracks by the fibres are directly related to the crack kinematics. The first part of this thesis addresses the development of an innovative, refined measurement technique for the automated and reliable extraction of the crack pattern and crack kinematics based on digital image correlation measurements. This technique offers detailed information of the crack behaviour, particularly in large-scale experiments with complex crack patterns, where conventional crack measurement methods are insufficient. Moreover, an approach is proposed for the determination of characteristic crack properties in homogeneous large-scale panel experiments directly from digital image correlation measurements, providing highly valuable information for the development and validation of sound mechanical models. The second part of this thesis explores the shear behaviour of fibre-reinforced concrete through a combination of experimental and theoretical investigations. To this end, large-scale shear tests on fibre-reinforced concrete panels that are representative of girder web elements were conducted in the Large Universal Shell Element Tester at ETH Zurich. The tests were instrumented with the refined crack measurement technique developed in the first part of the the-sis, which provided deep insight into the structural behaviour of fibre-reinforced concrete members subjected to shear. The obtained results build the basis for the development of a mechanically sound model for fibre-reinforced concrete subjected to shear and the derivation of practical design recommendations that aim at fostering the use of fibre-reinforced concrete in future structural applications.
Automated Crack Detection and Measurement on large-scale Experiments based on Digital Image Correlation
Gehri, Nicola; Mata Falcón, Jaime; Kaufmann, Walter (2020)
The acquisition and evaluation of the crack behaviour in experiments on quasi-brittle materials, such as concrete, mortar, or masonry is essential for understanding their structural behaviour. This article presents a fully automated procedure to detect cracks and measure crack kinematics in experiments instrumented with digital image correlation (DIC). With appropriate parameters, the procedure allows detecting crack locations with high precision and measuring very accurately crack kinematics even for large-scale experiments with complex crack patterns.
Combined application of distributed fibre optical and digital image correlation measurements to structural concrete experiments
Mata Falcón, Jaime; Häfliger, Severin; Lee, Minu; et al. (2020)
Engineering Structures
The combined application of distributed fibre optical strain measurements on reinforcing bars and digital image correlation (DIC) measurements on the concrete surface has a great potential to increase knowledge in many fields of structural concrete. This paper explores the advantages of these measurement techniques for concrete tests and the key aspects to be considered in order to obtain reliable measurements suitable for quantitative analysis. The uncertainty of DIC analysis is highly dependent on the test conditions and user carefulness, and should be assessed for each test. A procedure to quantify the DIC uncertainty in large scale structural tests is presented, showing that it is highly dependent on the quality of the calibration. Comparative tests on distributed fibre optical strain measurements with different fibre coatings show that polyimide-coated fibres capture properly high strain gradients and, therefore, should be used when instrumenting reinforcing bars in RC specimens. Moreover, the measuring noise was found to be dependent on the absolute strain level. Combined plots of crack kinematics and reinforcement strains, stresses and forces are shown for the results of a series of two concrete panel tests subjected to diagonal tension. Crack locations predicted by both measurements match perfectly in these experiments.
Refined extraction of crack characteristics in large-scale concrete experiments based on digital image correlation
Gehri, Nicola; Mata Falcón, Jaime; Kaufmann, Walter (2022)
Engineering Structures
The accurate extraction of the crack patterns and measurements of crack kinematics are essential for understanding the mechanical behaviour in experiments on structural concrete as well as in the validation and further development of sound mechanical models. This paper presents important refinements of the authors' recently published automatic crack detection and measurement procedure (ACDM) based on surface displacement measurements obtained with digital image correlation (DIC). The proposed refinements are crucial for reliably assessing the crack behaviour in large-scale experiments with complex crack patterns, since the original methods of ACDM may fail or result in biased measurements at locations with closely spaced cracks, crack intersections or cracks with high morphological curvature. The main refinements are (i) a Canny edge-based crack detector, which is applied on the DIC major principal strain field and (ii) enhancements in the crack kinematic measurement to assess the reliability of the results. The latter includes the automatic selection of optimum reference points used in the crack kinematic measurement to increase its reliability and remove uncertain results. The refined ACDM procedure is validated using several large-scale 2.0 × 2.0 m shear panel experiments with highly complex crack patterns. Compared to the original ACDM, significantly thinner cracks can be detected with a much higher reliability of crack locations and crack kinematic measurements, particularly close to crack intersections and at closely spaced cracks. Additionally, two approaches for the statistical consolidation of the large amount of gathered data into characteristic crack properties in large-scale homogeneous concrete element experiments are proposed and compared. The results show that the statistical consolidation of the ACDM data using a 95%-quantile match well with the direct extraction of the best-fit homogeneous crack properties from the full-field DIC displacements. The consolidated data provides highly valuable insight into the mechanical behaviour, especially regarding crack phenomena.
Cracked Membrane Model for Strain-Softening Fiber-Reinforced Concrete
Gehri, Nicola; Mata Falcón, Jaime; Kaufmann, Walter (2025)
Journal of Structural Engineering
This paper presents the extension of the cracked membrane model, developed originally for predicting the behavior of conventionally reinforced concrete members subjected to in-plane loading, to include the effect of fiber reinforcement. This mechanically sound model combines the tension chord model with appropriate compatibility conditions for membrane elements, and thus expresses equilibrium at the cracks and yields explicit information on the crack spacings and kinematics. The model can readily be extended by incorporating well-established constitutive models for the crack-bridging fiber stresses. In its general formulation with fixed cracks, the extended model accounts for the interaction of crack-bridging fiber stresses and aggregate interlock and can capture crack sliding failure mechanisms as observed in experiments on fiber-reinforced concrete members with anisotropic or uniaxial bar reinforcement. The response predictions were validated against the experimental data of all shear panel tests available within the existing literature that contain fiber reinforcement. The model predictions correlate very well with the load-deformation behavior of the panels, including shear strength, corresponding deformation and failure modes, confirming the general applicability of the general model for a wide range of fiber contents and concrete strengths. Additionally, a simplified version of the model considering rotating, aggregate interlock-free cracks was derived, yielding reliable response predictions for members with low amounts of fibers. While the global response is accurately predicted by the general model, experiments with direct and detailed measurements of the crack and kinematics and stresses at the cracks are scarce. Future studies should thus focus on validating the postulated stress transfer mechanism across cracks in membrane elements by relying on more tests with direct and detailed measurements of the kinematics and stresses at the crack.
Automated crack detection and measurement based on digital image correlation
Gehri, Nicola; Mata Falcón, Jaime; Kaufmann, Walter (2020)
Construction and Building Materials
The acquisition and evaluation of the crack behaviour in experiments on quasi-brittle materials, such as concrete, mortar, or masonry is essential for understanding their structural behaviour. This publication presents a fully automated procedure to detect cracks and measure crack kinematics in laboratory experiments instrumented with digital image correlation (DIC). Crack lines are extracted using well-established image processing methods showing excellent agreement with the physical crack pattern. In contrast to most existing crack detectors that rely on pixel intensities of true images, the presented crack detection is based on the DIC principal tensile strain field what allows the extraction of much finer cracks and more reliable crack locations. The crack widths and slips are measured using the DIC displacement field accounting for local rotations of the specimen. Additionally, automated visualisations of the crack kinematic measurements including data smoothing are presented. Several sensitivity analyses evaluating the performance and the uncertainty of the crack detector and the crack kinematic measurements have been conducted. These analyses show that the obtained results depend on the DIC configuration and that the procedure is limited in the case of very closely spaced cracks. With appropriate DIC parameters, the procedure allows detecting crack locations with high precision and measuring crack kinematics very accurately even in large-scale experiments with complex crack patterns.
Discussion on “Explicitly quantifying the effect of bond stress on the failure of reinforced concrete membranes” by Fei Geng, Chao Xu, and Zengping Wen [structural concrete, 2023, DOI: 10.1002/suco.202200561]
Gehri, Nicola; Kaufmann, Walter (2023)
Structural Concrete
The authors of this paper address the relevant topic of the effect of bonds on the behavior of reinforced concrete elements under in-plane loading, focusing on the compressive stresses in the concrete. They claim that in previous models, “the contribution of bond stress to the compression in the cracked concrete struts was usually neglected” and present a model according to which “for pure shear, the contribution of bond stress to the compressive stress in the concrete is at least two times of the average (apparent) compression.” They further argue that these increased compressive stresses explain the compression softening effect of concrete, rather than the common understanding of a reduced compressive strength due to imposed transverse strains. However, the entire paper, and thus all its conclusions, are built on the ill-founded assumption that bond increases the concrete compressive stresses, while in reality, the concrete compressive stresses are reduced by the bond between the cracks. In addition, the authors' model—apart from being limited to specific situations as it requires coupling of the diagonal compressive struts to the intersections of longitudinal and transverse reinforcement—is based on several other mechanically unsound assumptions. The most severe issues will be clarified in the following discussion.
Limit analysis and design recommendations for fiber‐reinforced concrete subjected to shear
Gehri, Nicola; Amin, Ali; Kraus, Michael Anton; et al. (2025)
Structural Concrete
It is widely recognized in both research and practice that fibers have great potential to partially, or completely, substitute conventional transverse reinforcement in webs of girders subjected to moderate shear forces. However, despite extensive research in recent years, there is still no consensus in the research community on the mechanically sound modeling of the shear strength of fiber-reinforced concrete (FRC). Accordingly, current code provisions for the design of FRC to resist shear rely on (semi-)empirical approaches and are thus justifiably restrictive. This article addresses this knowledge gap by proposing the application of the theory of plasticity to the design of FRC webs. Limit analysis methods for conventionally reinforced concrete are extended to FRC by incorporating the residual crack-bridging stress offered by the fibers in a consistent manner. This stress, as well as the limits of applicability of the theory of plasticity, are derived through a sensitivity analysis using the recently developed Cracked Membrane Model for FRC considering fixed, interlocking cracks. Based on these investigations, a straightforward and efficient mechanically based shear design model suitable for the design of FRC webs with and without conventional transverse bar reinforcement is proposed. The model is validated against a large database of panel and beam experiments, providing valuable insights into its reliability.

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