PSI - Issue 13

Oldřích Sucharda et al. / Procedia Structural Integrity 13 (2018) 1533 – 1538 Sucharda O., Lehner P., Kone č ný P., Ponikiewski T./ Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Extensive research on the reliability of concrete structures is very important today. The ratio between input costs, performance and service life must be considered. Also the development of tools for predicting the durability of concrete structures that help develop durable concrete and construction systems is useful. Increased focus on higher durability helps maintain the required level of safety and maintenance for a longer period of time, which saves the cost of premature repair and reconstruction (Ghosh et al., 2011; Lehner et al., 2018). 1.1. Steel fiber reinforced concrete Self-compacting concrete (Paja̧k and Ponikiewski, 2013) allows the simplification of concrete processing technology and the production of complex cross-sections and structural shapes. The difference compared to standard concrete is mainly in mixture composition. The proportion of cement and small aggregate is increasing. Also, the amount of plasticizer is significantly greater. The basic properties and advantages of self-compacting concrete are mentioned in (Goodier, 2003). Other options for improving of the mechanical properties of concrete is the use of steel fibres (Abrishambaf et al., 2015). The selection of quantity and type of fibres depends mainly on the purpose of use. Due to this there are also a number of special tests for SFRC, which are harmonized in recommendations and national standards (162-TDF, 2002; UNE 14651:2005+A1, 2008). A great emphasis on the correct description of the material properties is important. (Katzer and Domski, 2012). Important information includes knowledge of stress-strain dependence, tensile strength and compressive strength, and fracture mechanical properties. Especially in cases of advanced computational analyses involving physical non linearity, a more complex procedure for using a fractional mechanism is a prerequisite. (Goodier, 2003; Lehner et al., 2018; Paja̧k and Ponikiewski, 2013; Sucharda et al., 2017). 1.2. Nonlinear analysis and fracture mechanics Design and non-linear analysis of structures taking into account the actual behavior of SFR concrete is typically based on concrete constitutive models. Basic approaches and recommendations can be found in Model Code 2010 (FIB, 2013). A number of approaches can be found in theoretical and experimental research where the main part includes correct selection of fracture parameters (Köksal et al., 2013), also tensile strength (Abrishambaf et al., 2015) and several another material properties (Hoover and Bažant, 2013). The typical possibilities of use of constitutive concrete models in the numerical modelling of concrete element tests and their comparison is presented in (Sucharda et al., 2017). Correct description of fatigue properties and crack formation in the composite materials can be combined with modelling of structures within several available models (Ghosh et al., 2017; Konečný et al., 2015). There are also a number of design approaches or recommendations specifically for SFRC. Especially in case of advanced design using computer programs, it is necessary to know exact mechanical properties. However, this information is not often available from standardized tests. For example, in the tensile strength of fibre concrete testing is very demanding and sensitive (Abrishambaf et al., 2015). There is also a large scattering of measured values that leads to computational difficulty and inaccuracies. Characteristics of concrete in compression are, as opposed to tension, often available. Description of the dependence between the cracking strains/crack opening and the tensile stress is most frequently expressed by the graphs in Fig. 1 (Cervenka and Cervenka, 2002). Typically for concrete the Exponential Crack Opening Law is used (see Fig. 1a). For SFRC the Fracture Energy can be used (see Fig. 1b). In this approach, softening coefficients can be considered, which takes into account the real behaviour of SFR concrete. It can be seen that the dependence of strength on crack opening is different. Particularly for SFR, the variability of softening correction coefficients is large, and depends on the choice of concrete fibres and concrete. It is also appropriate to know the dependence of the working diagram on the degree of reinforcement. This can then be useful and appropriate for new design situations and a better understanding of the behaviour and modelling of wire-reinforced concrete.