Primary crack spacing model of reinforced concrete elements

Regimantas Ramanauskas

Doctoral dissertation

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The intention of the present study is to develop a unified approach for serviceability analysis reinforced concrete tensile and flexural elements, with a focus on mean crack spacing. The current research is mainly quantitative in nature, with the development of the strain compliance approach based on collected data of 170 tensile and 96 flexural specimens. Furthermore, statistical analysis and individual physical parameter impact on the crack spacing and accuracy in general are provided. The key feature of the proposed strain compliance concept is the merging of two distinct cracking analysis methods, referred to as the stress transfer and the mean deformation approaches. Compatibility is ensured by equating the mean reinforcement strains, as estimated by the referred techniques individually. Thus, the lack of knowledge on the spatial strain distribution in the mean deformation approach is addressed by the stress transfer technique, which contains such knowledge.

The technique has been derived for tensile elements with the inclusion of a reference element notion, that is defined by reference values of bar diameter and reinforcement ratio. Moreover, the mean crack spacing must also be known. Consequently, the bond stresses can be evaluated for this reference case and the predictions can then be extended to alternative configurations of ratios of reinforcement and bar diameters.

The concept has been extended with modifications to the assumed strain profile for flexural elements. The notion of the reference element has been eliminated with bond stresses accounted for directly from selected models, such as design codes. A central zone concept is introduced, which governs the averaged behaviour of the reinforcement strains within the middle between consecutive primary cracks. A constitutive length model was derived. In-depth comparisons with experimental data and parametric investigations were carried out.

With the rise of machine learning in the field of civil engineering, it is imperative that research stays ahead of the trend to be able to analyse the implications. A multipurpose study was carried out, resulting in the development of an artificial neural network for estimating the spacing between cracks with very good generalisation abilities, good adequacy in terms of accuracy and consistency. Incidentally, the gathered experimental data was validated for robustness and the general features of the strain compliance method were found to be in good agreement with the neural network predictions and the experimental results.

The research concludes with the validation of the strain compliance technique as a more adequate approach in terms of scatter and accuracy than the present design codes. Moreover, the concept has been shown to be mechanically sound.

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154 p.
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