Search

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., Construction and Building Materials, 256, ISSN:0950-0618

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., Journal of Structural Engineering, 146 (2), ISSN:0733-9445, ISSN:1943-541X

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., Engineering Structures, 251, ISSN:0141-0296