PSI - Issue 42

Michal Vyhlídal et al. / Procedia Structural Integrity 42 (2022) 1000–1007 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Concrete is a composite, which is formed, in a fresh state, from coarse aggregate, fine aggregate, binder (mostly Portland cement) and water, eventually admixtures. Due to the hydration process concrete obtain its valuable properties – high strength (mainly compressive), durability, fire resistance etc. During the hydration and hardening, the shrinkage and other processes occur and lead to the formation of micro and macro-defects (cracks, pores, etc.). Nevertheless, concrete structures are designed using procedures mentioned in standards – e. g. EN 1992-1-1 (2011) or recommendations which do not consider the existence of defects, such as cracks, pores, inclusions, transition zones, etc. However, these discontinuities can act as potential weak elements, which influences the fracture behavior of concrete structures. In the more complex design of these structures (e. g. complex geometry), it is advisable to apply the principles of fracture mechanics. In this paper, material discontinuity is formed by special steel inclusion placed in the middle of the test specimen. Such a specimen made of fine-grained cement-based composite was tested in three-point bending configuration. The aim of this paper is to identify the influence of the interface between steel inclusion and matrix which is formed by the interfacial transition zone (ITZ). In the paper, results of numerical assessment of crack propagation stages are presented.

Nomenclature d

averaging distance Young’s modulus

E

Young’s modulus of the interfacial transition zone

E ITZ F appl F crit ( , ) GMTS H 1 K Ic, ITZ K Ic, MTX MTX p 1 , ν ij ( , ) ̅ ( ) ̅ ,c ITZ K I K Ic

applied loading force

critical value the applied force

shape functions

generalized maximum tangential stress generalized stress intensity factor stress intensity factor in loading mode I fracture toughness (under pure mode I) interfacial transition zone

fracture toughness (under pure mode I) of the interfacial transition zone

fracture toughness (under pure mode I) of the matrix matrix – fine-grained cement-based composite

stress singularity exponent

polar coordinates Poisson’s ratio

stress tensor component

tangential stress

average tangential stress

critical value of the average tangential stress

2. Theoretical background 2.1. The Interfacial Transition Zone

The existence of the ITZ between aggregate and cement paste was first described in the 1950s by Farran (1956). The properties of the ITZ and its impact on the behavior of cement-based composites have been studied from many points of view since that. The ITZ is a region of about 50 µm in size, and its microstructure is formed mainly by ettringite needles and portlandite plates (Diamond and Huang, 1998). The significant feature of the ITZ is mainly