Issue 41

H. Šimonová et alii, Frattura ed Integrità Strutturale, 41 (2017) 211-219; DOI: 10.3221/IGF-ESIS.41.29 212 I NTRODUCTION ement-based composites belong to traditional and broadly used building materials [1]. Despite their long-term using, the investigation of damage of elements made of such materials under static loading is still developing. Concrete, as representative of such composites, shows nonlinear, more precisely, quasi-brittle behaviour – the ability to carry load continues even after the deviation from the linear branch of load–displacement diagram until the peak point and then the decrease of loading force follows until the failure, so called tensile softening. On the other hand, the individual components of primarily considered two-stage composite, cement paste and aggregate, show very often brittle elastic behaviour. This difference in cement-based composites behaviour is caused by development of multiple microcracking predominantly in the so called interfacial transition zone (ITZ) [2] and others toughening mechanisms. The aim of this paper is quantification of basic mechanical fracture parameters of selected fine-grained cement-based composites – static modulus of elasticity, fracture toughness and fracture energy and investigation of effect of ITZ between cement matrix (MTX) and aggregate (AGG) on stress distribution in cement based composite with crack terminating at the ITZ–MTX interface. I NTERFACIAL TRANSITION ZONE (ITZ) he existence of interfacial transition zone (originally „aureole de transition“) between aggregate and cement paste was introduced in the fifties of last century by Farrran [3]. Properties of ITZ and its impact on behaviour of cement-based composites have been studied numerically and experimentally from many points of view since that. Number of publications concerned with mechanical properties of individual components of cement-based composites are connected with homogenization techniques, such as e. g. Mori-Tanaka scheme [4] or generalized self-consistent scheme [5] used in this paper to estimation of ITZ’s elasticity modulus. Although investigation of concrete fracture is connected with the recognition that for description of its structural behaviour are necessary other independent material parameters, such as fracture toughness [6, 7] and not only compressive/tensile strength, only a few publications about ITZ are concerned with this fact, e.g. [8, 9]. Fracture mechanics based on analytical-numerical approaches, mainly connected with finite element method (FEM), is widely used to simulate structural response of materials with internal defects (microcracks, voids, pores) which lead to initiation, propagation of cracks and consequent fracture. The problem with crack–interface interaction of two elastic materials is long-term investigated by team gradually created by prof. Knésl from Institute of Physics of Materials of the Academy of Sciences of the Czech Republic, v. v. i. [10, 11]. E XPERIMENTAL PART Materials and specimens wo fine-grained cement-based composites mixtures with various water to cement ( w / c ) ratios and amount of plasticizer have been prepared for purpose of this study. Mixtures have been prepared on the basis of the standard ČSN EN 1961 [12]. Portland cement type 42.5 R as a binder and quartz sand with the maximum nominal grain size of 2 mm standardize according to ČSN EN 196-1 [12] for the fine aggregate were used. The w/c ratio was different for both mixtures. The second mixture’s w / c ratio was reduced by addition of super-plasticizer SVC 4035 in amount of 1 % by cement mass. Mixtures were prepared by a mixing device with controllable mixing speed. The basic information about the composition and properties of the fresh composites are given in Tab. 1. The properties of the fresh composites were determined in accordance with ČSN EN 1015-3 [13] and ČSN EN 1015-6 [14]. The three specimens of 1000 mm in length and with 60 × 100 mm in cross-section were made from each mixture and primarily used for recording the length changes. After the stabilization of shrinkage values, approximately after 90 days, the beam specimens with nominal dimensions 40 × 40 × 160 mm were cut from these specimens and subsequently used for three-point bending fracture tests. This procedure was chosen primarily because of exclusion of specimen boundary effect. The specimens for microscopy measurements were prepared from former mentioned specimens. C T T