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•The presented procedure records the full crack behaviour in large-scale experiments.•DIC results of the specimen’s surface provide highly accurate crack measurements.•Complex crack ...patters are automatically extracted using 2D image processing methods.•Several sensitivity analyses help in understanding the measurement uncertainty.•Crack measurements are represented with automated data visualisations tools.
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.
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•Refined methods for the accurate acquisition of crack measurements are proposed.•Crack locations and crack kinematics are automatically extracted from DIC results.•The procedure ...captures the complete crack behaviour in large-scale concrete tests.•The statistical consolidation to characteristic crack properties is proposed.•The procedure is implemented in a user-friendly open-source tool.
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.
•3D concrete printing and structural testing of nine reinforced beams.•Aligned interlayer fibres and steel cables as interlayer shear reinforcement.•Full-field digital image correlation and precise ...analysis of crack kinematics.•Development of a mechanical model for interlayer shear reinforcement in 3D printed beams.
3D concrete printing (3DCP) offers many new possibilities. This technology could increase the productivity of the construction industry and reduce its environmental impact by producing optimised structures more efficiently. Despite significant developments in materials science, little effort has been put in developing reinforcement strategies compatible with 3DCP and on the characterisation of their structural behaviour. Consequently, 3DCD still lacks compliance with structural integrity requirements. This study presents an experimental investigation consisting of nine four-point bending tests on extrusion 3DCP beams reinforced with various types of reinforcement. As interlayer shear reinforcement, aligned end-hook fibres (0.3 and 0.6%) or steel cables (0.1%) placed between the layers of printed concrete were used. As longitudinal reinforcement, unbonded post-tensioning and conventional bonded passive reinforcement were explored. The crack patterns and their associated kinematics were tracked using digital image correlation. The results show that the post-tensioned beams failed in a brittle manner due to the crushing of concrete in bending, with deformations localised in a few bending cracks. In the beams with conventional bonded longitudinal reinforcement, both bending as well as shear cracks were generated, and the brittle failure of the interlayer shear reinforcement limited the ultimate load. Estimations based on the measured crack kinematics show that the interlayer shear reinforcement carried most of the applied shear force. Based on these results, a simple mechanical model is developed to understand the mechanical behaviour and to pre-design the required amount of interlayer shear reinforcement.
Concrete tensile stresses influence the cracking behaviour and the stiffness of reinforced concrete (RC) members. Most design codes account for this tension stiffening effect using an effective ...reinforcement ratio. Although this ratio has a significant influence on the design of RC structures, its quantification is controversial in many cases, and typically relies on empirical geometry‐based expressions. One main reason for this knowledge gap is that the area of concrete in tension can only be verified indirectly, for example, through crack widths and spacings and using a suitable mechanical model. This indirect validation is subject to considerable uncertainty as it depends on parameters that scatter (e.g., bond stresses and the concrete tensile strength), and further assumptions relating internal stresses to the applied loads are required. This article outlines how refined measurements of the reinforcing steel strains and the concrete deformations in the compression zone, combining distributed fibre optic sensing (DFOS) and digital image correlation (DIC), can be used in order to obtain a more reliable quantification of the parameters affecting tension stiffening and hence, the effective reinforcement ratio. Selected models are validated against experimental data of an RC beam tested under four‐point bending, underlining the potential of DFOS and DIC as valuable tools for a better understanding of RC structures and the enhancement of mechanical models.
Digital fabrication technologies utilizing concrete (DFC) have recently enabled form freedom for the production of a variety of concrete-made objects having mainly architectural and aesthetic ...functions. Structural elements or civil/building structures made by DFC demonstrate a high engineering potential, mainly for tailoring the final shape while optimising the structural/functional performance, material use, overall costs, and architectural effectiveness. However, the design of structurally efficient DFC constructions or components is often faced with a lack of a common structural engineering approach that can adapt to specific DFC particularities.
In this paper, we provide a systematic overview of a number of DFC structural projects developed thus far. A comprehensive discussion about structural engineering details is provided, addressing the related fundamental structural issues and envisioning opportunities and challenges toward achieving the full potential of DFC.
•Reinforced concrete ties were successfully instrumented with fibre optical sensing.•The novel instrumentation gives unmatched insights on the structural behaviour.•Local and average bond stresses ...were studied.•The influence of load cycles, relative rib area and bar diameter on bond is studied.•The influence of typically neglected design parameters on bond was shown.•Meaningful insights on the behaviour of embedded reinforcing bars were gained.
The interaction of reinforcing steel and concrete is decisive for the structural behaviour of reinforced concrete (RC) in the serviceability and ultimate limit state. In the past decade, distributed fibre optical sensing (DFOS) and digital image correlation (DIC) have emerged as valuable tools to investigate and comprehend the mechanics of this interaction in more detail. This paper presents and discusses the results of an experimental campaign comprising 21 RC ties subjected to monotonic and cyclic uniaxial tension, where these refined instrumentation methods were utilised to study the influence of the reinforcing bar diameter, rib geometry and mechanical properties on the structural behaviour. Local strains were measured along the reinforcing bars with DFOS, from which the distributions of steel and bond stresses, as well as slip were derived. The crack pattern and kinematics on the surface were determined through DIC. The results allowed studying the influence of the deformation level, cyclic loading and the rib geometry on splitting and the magnitude of local and average bond stresses. The results indicate that the bond and crack behaviour depend on parameters typically neglected in design standards, such as the relative rib area, elastic steel stiffness, and bar diameter. Moreover, the observed average bond stresses in the elastic range were lower than proposed by design standards for serviceability verifications. These observations allow identifying weaknesses and inconsistencies in common bond models, serving as a starting point for improvements in modelling.
Most of the concrete volume in multistorey buildings is cast in solid slabs, which are frequently flat slabs supported on columns. By using two‐way spanning ribbed slabs, concrete consumption could ...be significantly reduced. However, due to the high costs associated with formwork, such a complex rib configuration is rarely used nowadays. With the advent of technologies for automated formwork fabrication, the material‐saving potential inherent in this structural system could again be exploited. This paper investigates the feasibility of material‐efficient ribbed concrete slabs on a building scale using conventional concrete and steel reinforcing bars cast inside a three‐dimensional‐printed plastic‐based formwork. To that end, the code‐compliant design of ribbed slabs is first discussed, followed by the introduction of a concept for an automated design‐to‐production workflow. The sustainability of this slab system is compared to a solution using conventional formwork in a case study consisting of a multibay office building with slabs spanning 8 m in both directions, revealing that ribbed slabs use 40% less concrete than solid slabs. Several representative structural elements of the case study (ribs, slab‐column transition) were produced at full‐scale and tested until failure to investigate the feasibility of production and structural performance. Three T‐beams with various rib shapes (straight, kinked with diaphragms, curved) were tested in a three‐point bending configuration, showing a ductile behavior with longitudinal reinforcement yielding and indicating the relevance of torsional effects in curved ribs. Punching tests on two slab‐column connections (ribbed, solid) revealed that the optimized ribbed slab could prevent brittle punching failures and achieve an ultimate load 105% higher than the solid reference slab. All specimens' load‐bearing behavior could be predicted using established design formulas, showing the feasibility of producing code‐compliant ribbed slabs with the applied technology.
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•An experimental campaign of large-scale fibre-reinforced panels is presented.•Six 2.00m×2.00m panels were subjected to homogeneous in-plane loading.•A combination of DIC and DFOS ...allowed a deep insight into the structural behaviour.•The ductility of the panels without any steel reinforcing bars was rather low.•A moderate amount of bar reinforcement significantly increased ductility.
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.
•A new stay-in-place flexible formwork with integrated textile reinforcement was developed and experimentally examined.•The integration of high-strength aramid rovings considerably increased the ...shear strength of the tested concrete beams.•Various shear transfer mechanisms were investigated based on refined measurements using DIC and DFOS.•High-strength textiles and longitudinal steel bars provided large deformation capacity when bending failure was reached.
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.
•A stress field is proposed for modelling bond in axisymmetric RC ties.•Bond is determined based on the concrete bearing capacity in tension and conpression.•Interface compatibility is used to ...account for the effect of steel strains on bond.•The results match well with test results obtained using distributed fibre optical sensing.•New insights on the behaviour of embedded reinforcing bars are gained.
The interaction of the reinforcing bars and the surrounding concrete plays an important role in structural concrete as it is decisive for the reinforcing bar anchorage and the load-deformation behaviour, including stiffness and deformation capacity. Therefore, understanding this interaction – commonly known as bond – is highly relevant to a safe and efficient structural concrete design. Bond depends on many parameters, which affect the local reinforcing bar-concrete interface as well as the global load transfer between the reinforcing bars and the surrounding concrete. The latter governs the formation of transverse and longitudinal cracks and the activation of transverse confinement, which are as decisive for bond as the local interface characteristics. Nonetheless, most bond models and code provisions focus on the local interface and merely account for the global configuration by means of empirically calibrated factors accounting, e.g. for good or bad bond conditions in confined or unconfined situations, respectively.
This study investigates the potential of stress fields for modelling bond based on the lower-bound theorem of plasticity theory. Stress fields facilitate the investigation of the local interface and the global load transfer consistently in one model, providing valuable insight into the flow of forces. For simplicity, the fundamental case of a concrete tie reinforced with one deformed steel bar is studied in this paper, superimposing axisymmetric discontinuous stress fields originating at each reinforcing bar rib. Each stress field consists of a triaxial nodal zone adjacent to a rib and a conical compression field that spreads the bond force radially, and is equilibrated along its outer perimeter by axial tensile stresses and confining hoops. The model relies on geometric parameters and basic material properties. The validation against experiments on reinforced concrete ties shows a good correlation between the predicted and observed bond strength and the crack spacing. The predictions can be improved by assigning stiffnesses to the stress field components and requiring interface compatibility, i.e. ensuring contact between the concrete and the reinforcing bar at every rib. Even when assuming a simplified, linearly elastic constitutive behaviour for concrete, the consideration of interface compatibility allows for reproducing the steel strain dependency of the local and average bond stresses and the occurrence and impact of splitting cracks.
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