Minu Lee

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Lee

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Minu

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0000-0002-6489-8310

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Publications 1 - 10 of 14

New structural engineering possibilities of digital fabrication towards leaner concrete structures
Mata Falcón, Jaime; Lee, Minu; Gebhard, Lukas; et al. (2022)
The design possibilities of concrete structures are typically tightly constrained by the high costs and material waste of formworks and reinforcement for producing non-conventional shapes. Digital fabrication technologies aim at producing structures with increased degrees of geometrical freedom at little extra cost and avoiding material waste. This might allow (i) revisiting abandoned structural typologies (e.g. shells, folded roofs, ribbed or mushroom slabs) and (ii) exploring new concepts towards leaner construction. In this lunch talk, researchers from the Group of Concrete Structures and Bridge design at ETH Zurich will present recent structural engineering developments within the domain of Digital Fabrication with Concrete, making use of technologies such as 3D concrete printing, knitted textile reinforcement, robotic reinforcement assemblies and 3D formwork printing.
Direction-dependent behaviour and size effect of steel fibre reinforced concrete based on the double punch test
Karrer, Simon; Markic, Tomislav; Lee, Minu; et al. (2022)
Materials and Structures
The post-cracking behaviour of steel fibre reinforced concrete (SFRC) is typically determined following an inverse analysis of flexural prism tests. Although these tests have significant practical merit, it has been argued that the anisotropy of the material due to the dispersion and orientation of the fibres cannot be accounted for in these tests. Multidirectional double punch tests on cubes have been proposed to overcome these issues. These tests are also well-suited to study size effects. However, no generally accepted inverse analysis method for these tests presently exists. This paper presents a simple and mechanically consistent inverse analysis procedure to estimate the residual post-cracking strength of SFRC from the results of double punch tests conducted on cubes. To explore the potential and limitations of this methodology, an experimental investigation was conducted on 30 double punch tests on cubes of varying sizes, varying fibre dosage and loading direction with respect to the concrete casting direction. The results demonstrate that the approach provides useful comparative information on the anisotropy of the material, however further investigation on the input parameters is required to prove its reliability in quantifying the residual tensile stress offered by the fibres.
Shear strength of concrete beams using stay-in-place flexible formworks with integrated transverse textile reinforcement
Lee, Minu; Mata Falcón, Jaime; Kaufmann, Walter (2022)
Engineering Structures
This study investigates the fabrication and the structural performance of concrete beams using weft-knitted tubular fabrics as stay-in-place formworks with integrated textile reinforcement. The use of non-corrosive high-strength textile materials for flexible formworks offers great potential for efficient construction processes and a bespoke design of material-efficient concrete structures. To this end, an experimental campaign consisting of ten three-point bending tests on beams with rectangular cross-sections made with flexible stay-in-place formworks was conducted, where aramid rovings were integrated within the textile as transverse reinforcement to withstand the shear forces while the longitudinal reinforcement consisted of conventional deformed steel bars. The use of digital image correlation measurements and distributed fibre optical sensing allowed the refined analysis of the deformations, including the strains in the textile reinforcement and the estimation of the crack kinematics, which were used to assess various contributions from the reinforcement and the concrete to the shear strength in the governing crack. The amount of textile reinforcement proved to be a decisive parameter in increasing the shear strength, although the full tensile capacity of the rovings could not be exploited due to the lack of ductility in the material behaviour. The thorough consideration of the shear transfer mechanisms revealed a strong dependence of the concrete contribution, specifically aggregate interlock and the formation of a direct strut, on the crack patterns. The combination of the brittle aramid rovings as shear reinforcement and conventional steel reinforcing bars in the tension chord provided a large deformation capacity if the shear reinforcement was able to sustain the load until a bending failure was reached.
Analysis of the tension chord in the flexural response of concrete elements: Methodology and application to weft-knitted textile reinforcement
Lee, Minu; Mata Falcón, Jaime; Kaufmann, Walter (2022)
Engineering Structures
The use of high-strength fibrous materials offers great potential to serve as the main reinforcement in the flexural response of lightweight concrete structures. Specifically, weft-knitted textile reinforcement allows creating complex geometries (i.e. doubly curved or folded) and introducing straight inlays as well as spatial features such as ribs within the fabric to enhance the bond conditions between the reinforcement and the concrete. While the evaluation of uniaxial tension tests to characterise the mechanical behaviour is usually straightforward, the vast majority of bending tests of textile reinforced concrete elements in the literature are limited to load–displacement data, which does not give direct information on the mechanical behaviour and makes it difficult to compare specimens with different geometries or reinforcement contents. This study presents a methodology based on digital image correlation measurements to directly assess the response in the tension chord of reinforced concrete beams in four-point bending. This methodology was applied and validated in an experimental campaign consisting of 16 bending tests with weft-knitted textile reinforcement. Various fibre materials, coating types and specimen geometries were tested, and the results were compared to corresponding uniaxial tension tests with the same type of reinforcement, generally showing a good agreement in the load-deformation behaviour and the crack kinematics. However, the textile reinforcement could not be utilised to the tensile strength obtained in uniaxial tension since all bending beams failed prematurely either in a combined bending-shear type failure (for most of the cement paste-coated textiles) or due to the delamination of the textile within a shear crack (for all epoxy-coated textiles). The Tension Chord Model with an assumed constant bond-slip relationship and an equivalent reinforcement ratio in bending yielded a good prediction of the stress–strain relationship, the tension stiffening effect, the crack widths and spacing.
Influence of short glass fibres and spatial features on the mechanical behaviour of weft-knitted textile reinforced concrete elements in bending
Lee, Minu; Mata Falcón, Jaime; Kaufmann, Walter (2022)
Construction and Building Materials
Weft-knitted textiles made from high-strength fibrous materials offer great potential for use as a flexible stay-in-place formwork and reinforcement system since they allow creating complex geometries (i.e. doubly curved or folded) and introducing spatial features such as ribs within the fabric. However, the closed surface of the textile and its placement at the bottom edge of the concrete element present major challenges regarding bond, which may lead to premature failure due to delamination of the reinforcement initiated by a substantial opening of a governing crack. This study investigates the influence of short integral glass fibres and spatial bond ribs on the mechanical behaviour of weft-knitted textile reinforced concrete elements subjected to bending and their potential to increase the shear resistance. To this end, an experimental campaign consisting of 14 four-point-bending tests was conducted, where the specimens were examined regarding their load-deformation behaviour, crack kinematics and failure modes. The contribution of the short glass fibres to the load-bearing mechanism was estimated with Pfyl’s fibre engagement model, based on the material characterisation results from prism tests on fibre reinforced concrete members. The Tension Chord Model was used to predict the stress-strain relationship of the reinforcement and the crack widths in the constant moment zone, which yielded a good correlation between the predictions and the experiments. The short fibres mostly contributed to the flexural response in the serviceability limit state, but only slightly increased the shear resistance due to their low fibre effectiveness. The introduction of bond shear connectors was shown to be essential to prevent the premature delamination of the reinforcement. The use of tall connectors allowed doubling the bearing capacity with respect to the flat reinforcement without any spatial features, effectively bridging the governing crack and reaching the tensile capacity of the longitudinal textile reinforcement.
Structural safety of RC bridge piers under train impact
Lee, Minu (2016)
Load-deformation behaviour of weft-knitted textile reinforced concrete in uniaxial tension
Lee, Minu; Mata Falcón, Jaime; Kaufmann, Walter (2021)
Materials and Structures
Weft-knitted textiles offer many advantages over conventional woven fabrics since they allow the fabrication of doubly curved geometries without the need of stitching multiple patches together. This study investigated the use of high-strength continuous fibres as knitted textile reinforcement, focusing on various knitting patterns, fibre materials, coating types and spatial features to enhance the bond conditions between concrete and reinforcement. The bond is of particular interest since the contact surface of knitted textiles is fundamentally different due to their closed surface, compared to commercially available textile reinforcement, which is normally formed as orthogonally woven grids of rovings. An experimental campaign consisting of 28 textile-concrete composites was conducted, where digital image correlation-based measurements were used to assess the load-deformation behaviour and to analyse the crack kinematics. The results showed a beneficial post-cracking behaviour for epoxy coated configurations with straight inlays. The comparison of these configurations with conventional textile reinforcement generally showed a similar behaviour, but with higher utilisation compared to the filament strength. The Tension Chord Model, which assumes a constant bond stress-slip relationship, was adapted for the specific geometry of the knitted reinforcement, and it was used for the estimation of bond stresses and a post-diction of the experimental results, generally showing a good agreement.
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.
Concrete structures with stay-in-place flexible formworks and integrated textile reinforcement
Lee, Minu (2023)
The reduction of the concrete volume used in the construction sector – triggered by the urgent demands on sustainability – has become one of the critical drivers for developing new composite materials and structural typologies. This thesis presents a novel approach for stay-in-place fexible formworks with integrated textile reinforcement based on the KnitCrete technology, reducing the environmental footprint of concrete structures through structurally informed geometries and slender dimensions, enabled by the use of weft-knitted fabrics and non-corrosive high-strength fbrous materials. The frst part of the thesis explores the possibilities arising from knitted textiles – including the feasibility of creating doubly curved geometries and introducing continuous rovings and spatial ribs – and revisits conventional reinforcement types and their suitability for complex geometries, proposing two directions for lean reinforcement strategies: (i) use of high-strength fbrous materials for the manufacturing of weft-knitted textiles and (ii) guiding conventional reinforcement (i.e. deformed steel bars or post-tensioning tendons) with integrated features within the fexible formworks. The second part of the thesis aims at characterising the mechanical behaviour of weft-knitted textile reinforced concrete regarding the (i) strength, (ii) stifness, (iii) bond, and (iv) deformation capacity. To this end, several experimental campaigns are conducted to prove the feasibility of the manufacturing procedure and examine the structural response under various loading conditions, including uniaxial tension, bending, and shear. The investigations focus on various knitting patterns, textile materials, coating types, and the infuence of shear connectors to enhance the bond conditions and the addition of short fbres to the concrete to improve the post-cracking behaviour. Furthermore, the combination of brittle and ductile reinforcement materials and the optimisation of the geometry by means of thinwalled cross-sections are studied. The load-deformation and failure behaviour is evaluated using refned measuring techniques, i.e. digital image correlation and distributed fbre optical sensing, which allow assessing the mean strains in the reinforcement and the crack kinematics. Analytical methods following the Tension Chord Model, which considers the stress transfer between the reinforcement and the concrete based on mechanically consistent assumptions, allow the adaptation to the specifc geometry of the weft-knitted textile reinforcement, the back-calculation of bond shear stresses, and the consideration of the short fbres in the concrete. The resulting predictions are validated with the experimental results. The load-bearing capacity of concrete beams with integrated transverse textile reinforcement is analysed regarding various contributions from the concrete and the high-strength rovings to the shear transfer across the governing crack and modelled using numerical simulations based on the Compatible Stress Field Method. The third part of the thesis discusses potential structural applications based on the fndings on the mechanical behaviour obtained from the experimental investigations. The implications of using brittle reinforcement materials are addressed, and various means to implement ductility in the global structural response are discussed, eventually proposing a classifcation framework for safe and reliable design principles to achieve an adequate post-cracking behaviour. The thesis concludes with a case study examining the structural behaviour of doubly curved concrete shells using non-linear fnite element analyses, highlighting the potential of the developed reinforcement approach for structures with complex geometries.
Advances in Structural Applications of Digital Fabrication With Concrete
Mata Falcón, Jaime; Gebhard, Lukas; Lee, Minu; et al. (2023)
Open Conference Proceedings ~ Visions and Strategies for Reinforcing Additively Manufactured Constructions 2023
The construction industry needs to reduce its large environmental footprint drastically. Building with less material is one of the main levers for reducing this negative impact. This material reduction can be achieved with structurally efficient geometries requiring a higher degree of complexity than typically applied in conventional construction practices. Digital fabrication with concrete has been proposed as one of the solutions to facilitate the fabrication of efficient structures. Over the last few years, extensive research has been conducted within the National Centre of Competence in Research (NCCR) Digital Fabrication at ETH Zurich to investigate digital fabrication with concrete for structural applications. Various digital technologies were investigated, including 3D concrete printing, digital casting, Mesh Mould, printed polymer formworks and knitted formworks. This contribution highlights the main findings of these investigations with a particular focus on the development of reinforcement strategies, as these strategies are an essential step to ensure compliance with existing design guidelines and ease of mass-market adaptation. Promising future research areas are identified based on the assessment of the technology readiness and sustainability potential of the investigated approaches.

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