Abstract
Numerous strategies have been advocated to reduce the potential for plastic shrinkage cracking in concrete through mixture proportioning, curing methods, or the use of fiber reinforcement. The effectiveness of each approach must be adequately quantified to determine whether the additional initial cost of each strategy is justified. The majority of current research to characterize plastic shrinkage cracking in concrete relies on manual crack observation and measurement that is typically only performed at select locations. These manual measurements of crack width may provide only limited information and may be subject to operator bias. This paper describes a systematic methodology that uses a semi-automated image analysis approach to accurately quantify the salient features of the plastic shrinkage crack patterns. A restrained slab specimen with a stress riser was used to amplify the potential for plastic shrinkage cracking. The specimens described in this paper were exposed to an accelerated drying environment for the first 6 hours after casting. At an age of 24 hours, the crack was assessed using a series of thirteen gray level images that were acquired along the crack path. Crack contours were extracted from these images using gray level intensity thresholding to create binary images. Subtracting the binary image from a uniformly spaced grid enabled the crack width to be determined at numerous locations. This image analysis technique permits the crack widths to be assessed rapidly at numerous locations without operator bias. The measured crack widths were statistically analyzed using a modified Weibull distribution function. This enabled the crack width distribution to be fully described using only two or three parameters. It is anticipated that this approach can be used to better quantify the effects of short randomly distributed fiber reinforcement on plastic shrinkage crack formation.
Résumé
De nombreuses stratégies ont été préconisées en vue de réduire le potentiel de fissuration de retrait plastique dans le béton, en agissant sur le dosage, les méthodes de cure, ou en utilisant le renforcement par fibres. L'efficacité de chacune de ces approches doit être quantifiée avec soin, afin de déterminer si le coût supplémentaire induit est justifié. La plupart des recherches actuelles pour caractériser la fissuration de retrait plastique dans le béton reposent sur l'observation et la mesure manuelles des fissures effectuées, typiquement, à des endroits prédéterminés. Ces mesures manuelles fournissent des informations limitées et sont sujettes à une erreur systématique (biais) due à l'opérateur. Cet article décrit une méthodologie systématique où les caractéristiques géométriques essentielles de la fissuration de retrait plastique sont observées par traitement d'images semi-automatique. Un échantillon confiné de plaque a été utilisé avec un coin de contrainte afin d'amplifier le potentiel de fissuration de retrait plastique. Les spécimens décrits dans cet article ont été soumis à un séchage accéléré pendant les six premières heures après moulage. Après 24 heures, la fissuration a été observée sur une série de 13 images en nuances de gris, le long de la fissure. Les contours de fissures ont été extraits de ces images en digitalisant les intensités de gris. Puis, en superposant l'image binaire à une grille uniforme, on a pu déterminer la largeur de fissure à un grand nombre d'endroits. Cette technique d'analyse d'image permet de déterminer la largeur des fissures rapidement à de nombreux endroits et sans le biais de l'opératuer. Les largeurs de fissures mesurées ainsi sont analysées statistiquement au moyen, soit d'une distribution standard de Weibull, soit d'une distribution modifiée de Weibull. Cela permet de décrire la distribution des largeurs de fissures par seulement deux ou trois paramètres. On espère que cette approche pourra être utilisée pour mieux quantifier les effets, sur la formation des fissures de retrait, des renforcement par fibres courtes distribuées de manière aléatoire.
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Editorial Note Prof. W. Jason Weiss is a RILEM Senior Member. Prof. Jan Olek is also a RILEM Senior Member. He participates in the work of RILEM TC 186-ISA: ‘Internal sulfate attack’.
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Qi, C., Weiss, J. & Olek, J. Characterization of plastic shrinkage cracking in fiber reinforced concrete using image analysis and a modified Weibull function. Mat. Struct. 36, 386–395 (2003). https://doi.org/10.1007/BF02481064
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DOI: https://doi.org/10.1007/BF02481064