Reinforced Concrete Structures Subjected to Imposed Deformations: A Study of Cracking due to Shrinkage in Slab Foundations for Residential Houses

Detta är en Uppsats för yrkesexamina på avancerad nivå från Lunds universitet/Avdelningen för Konstruktionsteknik; Lunds universitet/Institutionen för bygg- och miljöteknologi

Sammanfattning: The need for limiting crack widths in reinforced concrete structures is due to durability, tightness towards gaseous and liquid substances, and for aesthetic reasons. Although concrete cracks at relatively low tensile stresses and can not be avoided, strategies for crack control can be adopted to meet the requirements. For certain structures, tensile stresses and consequently cracking may arise due to imposed deformations. That is when a restrained structure is exposed to temperature variations or shrinkage strains. Eurocode 2 provides guidance for crack control, but the formulas are adopted by assuming stabilized cracking, which may be a too conservative assumption for restrained structures. The study presented in this master thesis aims to investigate the crack behavior of reinforced concrete structures exposed to imposed deformations, where the focus is on smaller slab foundations for residential houses. The crack behaviour was analysed with nonlinear finite element analysis in Atena 2D. First, real experimental tie-rod tests were modelled with finite elements and analysed with di↵erent fracture energy models, bond models and mesh sizes. It was shown that nonlinear finite element analysis can predict cracks well for the intended purpose. The obtained results and observations were the foundation for a further study of cracking in slab foundations. For that purpose, the drying was simulated with respect to modern concrete properties, and a shrinkage profile could be obtained. A parametric study was performed by varying following parameters: sti↵ness of the sub-base, friction, slab length, slab height, bar diameter, reinforcement ratio, concrete class, type of strain distribution. The obtained results were compared with analytical methods in Eurocode 2 and a method from Chalmers University by Engstro ̈m. The parametric study showed that the number of cracks and the crack widths can be controlled by varying di↵erent parameters. The length of the slab and the sti↵ness in the sub-base were crucial parameters for the crack growth, but are, however, parame- ters that for a designer are di cult to control. Crack widths could on the other hand be limited by increasing the bar diameter or the reinforcement amount. Eurocode 2 guidelines for minimum reinforcement may be to conservative for smaller concrete slabs. For slabs in which the reinforcement ratio did not fulfil the requirement of minimum reinforcement, the crack widths obtained from the finite element analysis still turned out to be acceptable to some extent. The overall conclusion is that new guidelines for these types of structures is necessary.

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