Influence of rock inclusion composition on the fracture response of cement-based composite specimens

ScienceDirect Available online at www.sciencedirect.com Procedia Structural Integrity 33 (2021) 966–981 2452-3216 © 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the scientific committee of the IGF ExCo 10.1016/j.prostr.2021.10.107 10.1016/j.prostr.2021.10.107 2452-3216 © 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( https://creativecommons.org/licenses/by-nc-nd/4.0 ) Peer-review under responsibility of the scientific committee of the IGF ExCo Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2019) 000–000 www.elsevier.com/locate/procedia 2452-3216 © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo IGF26 – 26th International Conference on Fracture and Structural Integrity Influence of rock inclusion composition on the fracture response of cement-based composite specimens Michal Vyhlídala*, Iva Rozsypalováa, Hana Šimonováa, Barbara Kucharczykováa, Leona Vavrob, Martin Vavrob, Jiří Němečekc, Pavla Rovnaníkováa, Zbyněk Keršnera aBrno University of Technology, Faculty of Civil Engineering, Veveří 331/95, 602 00 Brno, Czech Republic bCzech Academy of Sciences, Institute of Geonics, Studentská 1768, 708 00 Ostrava-Poruba, Czech Republic cCzech Technical University in Prague, Faculty of Civil Engineering, Department of Mechanics, Thákurova 7, 166 29 Prague, Czech Republic Abstract This paper concerns the results of research into the influence of the composition of rock inclusions on the fracture response of cement-based composite specimens. Specially designed specimens of the nominal dimensions 40 × 40 × 160 mm with inclusions in the shape of prisms with nominal dimensions of 8 × 8 × 40 mm were provided with an initial central edge notch with a depth of 12 mm. These specimens, which were made of fine-grained cement-based composite with different types of rock inclusion – amphibolite, basalt, granite, and marble – were tested in the three-point bending configuration. Fracture surfaces were examined via scanning electron microscopy and local response in the vicinity of rock inclusions was characterized via the nanoindentation technique. The aim of this paper is to analyse the influence of the chemical/petrographic composition of rock inclusions on the effective mechanical fracture parameters of cement-based composites, as well as on the microstructural mechanical parameters of the interfacial transition zone. The results of this research indicate the significant dependence of the effective fracture parameters on the petrographic and related chemical composition of the rock inclusions. © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo Keywords: Cement-based composite; Force–displacement diagram; Fracture test; Inclusion; Mechanical fracture parameters; Rocks. * Corresponding author. Tel.: +420-541147362. E-mail address: [email protected] Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2019) 000–000 www.elsevier.com/locate/procedia 2452-3216 © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo IGF26 – 26th International Conference on Fracture and Structural Integrity Influence of rock inclusion composition on the fracture response of cement-based composite specimens Michal Vyhlídala*, Iva Rozsypalováa, Hana Šimonováa, Barbara Kucharczykováa, Leona Vavrob, Martin Vavrob, Jiří Němečekc, Pavla Rovnaníkováa, Zbyněk Keršnera aBrno University of Technology, Faculty of Civil Engineering, Veveří 331/95, 602 00 Brno, Czech Republic bCzech Academy of Sciences, Institute of Geonics, Studentská 1768, 708 00 Ostrava-Poruba, Czech Republic cCzech Technical University in Prague, Faculty of Civil Engineering, Department of Mechanics, Thákurova 7, 166 29 Prague, Czech Republic Abstract This paper concerns the results of research into the influence of the composition of rock inclusions on the fracture response of cement-based composite specimens. Specially designed specimens of the nominal dimensions 40 × 40 × 160 mm with inclusions in the shape of prisms with nominal dimensions of 8 × 8 × 40 mm were provided with an initial central edge notch with a depth of 12 mm. These specimens, which were made of fine-grained cement-based composite with different types of rock inclusion – amphibolite, basalt, granite, and marble – were tested in the three-point bending configuration. Fracture surfaces were examined via scanning electron microscopy and local response in the vicinity of rock inclusions was characterized via the nanoindentation technique. The aim of this paper is to analyse the influence of the chemical/petrographic composition of rock inclusions on the effective mechanical fracture parameters of cement-based composites, as well as on the microstructural mechanical parameters of the interfacial transition zone. The results of this research indicate the significant dependence of the effective fracture parameters on the petrographic and related chemical composition of the rock inclusions. © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo Keywords: Cement-based composite; Force–displacement diagram; Fracture test; Inclusion; Mechanical fracture parameters; Rocks. * Corresponding author. Tel.: +420-541147362. E-mail address: [email protected] 2 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 1. Introduction Cement-based composites, with concrete being the main representative of such composites, are widely used building materials (Aïtcin, 1998), (Neville, 2011). Concrete structures such as highway bridges, tunnels, dams, etc. are important parts of the infrastructure which should serve for many generations after their construction. In many cases, such structures show nonlinear, or more precisely, quasi-brittle behaviour – the ability to carry load continues even after the deviation from the linear branch of the force–displacement diagram until the peak point, after which a decrease in loading force follows until failure occurs, which is a phenomenon known as tensile softening (Karihaloo, 1995). The reason for this behaviour is, apart from strong heterogeneity, the existence of internal defects (pores, cracks, transition zones, etc.) or material discontinuities (e.g. inclusions), which work as obstacles to or promoters of crack propagation. Nevertheless, these discontinuities, which form stress concentrators that serve as potential weak elements in composites, are not given any consideration at all in the standards, e.g. EN 1992-1-1 (2004). In this paper, material discontinuities are formed by rock inclusions placed in the middle of the test specimens. These specimens made of fine-grained cement-based composite with different types of rock inclusion – amphibolite, basalt, granite, and marble – were tested in the three-point bending configuration. The rock inclusions were made using a saw with a diamond blade. After the tests, the fracture surfaces were examined via scanning electron microscopy (SEM) and local response in the vicinity of the rock inclusions was characterized via the nanoindentation technique (Zacharda et al., 2018). Assuming that the test specimens were manufactured, compacted and tested in the same way and that the inclusions’ surfaces had the same roughness, the only way to explain the deviation in overall fracture response should be, according to Randl (2013) and the fib Model Code for Concrete Structures 2010 (2013), chemical adhesion. The aim of this paper is to identify the influence of mineralogical composition of rock inclusions on the overall fracture response of the above-described cement-based composite specimens. 2. Theoretical background 2.1. Interface shear transfer The bond mechanism is the interaction between reinforcement and concrete. The components of bond resistance are a combination of different mechanisms – chemical adhesion, friction, mechanical interlocking and the dowel action of reinforcement crossing the interface (Randl, 2013). The dowel action of reinforcement crossing the interface is the result of the lateral displacement of the upper and lower reinforcement ends due to shear slip along the interface. The transfer of shear forces along the interface is thus provided by the bending, shear and axial stresses in reinforcement bars caused by this lateral displacement (Randl, 2013). The mechanical interlocking resistance is the result of the forces acting perpendicular to the ribs of reinforcement. This resistance takes place in the case of excessive and irregular roughness when keying and undercutting effects occur. This effect will only take place if the aggregates/ribs protrude sufficiently from the surface (Randl, 2013). The frictional resistance is the result of the compression forces perpendicular to the interface and also depends on the degree of interface roughness. In the fib Model Code for Concrete Structures 2010 (2013), there is a recommendation for the values of the coefficient of friction µ for a constant confining stress  c depending on whether the interface is smooth, rough or very rough. Several parameters are also described for the classification of the concrete surface roughness, such as mean roughness Ra and the mean peak-to-valley height Rz. Although adhesive shear resistance is in the range of lower units of MPa for concrete grades ≤ C50/60, adhesive bonding can significantly affect overall shear resistance (Randl, 2013). Adhesive resistance is a result of chemical and physical bonding due to Van der Waals forces. For this effect to occur, the related slip at failure must be very small, otherwise the effect will vanish. Adhesive resistance strongly depends on the real surface of the contact area, and the quality, composition and properties (e.g. porosity) of concrete (Randl, 2013). It is connected to the formation of the interfacial transition zone – see the next section – at the aggregate/matrix or reinforcement/matrix interface, which is regarded as the weakest element of cement-based composites. Michal Vyhlídal et al. / Procedia Structural Integrity 33 (2021) 966–981 967 Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2019) 000–000 www.elsevier.com/locate/procedia 2452-3216 © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo IGF26 – 26th International Conference on Fracture and Structural Integrity Influence of rock inclusion composition on the fracture response of cement-based composite specimens Michal Vyhlídala*, Iva Rozsypalováa, Hana Šimonováa, Barbara Kucharczykováa, Leona Vavrob, Martin Vavrob, Jiří Němečekc, Pavla Rovnaníkováa, Zbyněk Keršnera aBrno University of Technology, Faculty of Civil Engineering, Veveří 331/95, 602 00 Brno, Czech Republic bCzech Academy of Sciences, Institute of Geonics, Studentská 1768, 708 00 Ostrava-Poruba, Czech Republic cCzech Technical University in Prague, Faculty of Civil Engineering, Department of Mechanics, Thákurova 7, 166 29 Prague, Czech Republic Abstract This paper concerns the results of research into the influence of the composition of rock inclusions on the fracture response of cement-based composite specimens. Specially designed specimens of the nominal dimensions 40 × 40 × 160 mm with inclusions in the shape of prisms with nominal dimensions of 8 × 8 × 40 mm were provided with an initial central edge notch with a depth of 12 mm. These specimens, which were made of fine-grained cement-based composite with different types of rock inclusion – amphibolite, basalt, granite, and marble – were tested in the three-point bending configuration. Fracture surfaces were examined via scanning electron microscopy and local response in the vicinity of rock inclusions was characterized via the nanoindentation technique. The aim of this paper is to analyse the influence of the chemical/petrographic composition of rock inclusions on the effective mechanical fracture parameters of cement-based composites, as well as on the microstructural mechanical parameters of the interfacial transition zone. The results of this research indicate the significant dependence of the effective fracture parameters on the petrographic and related chemical composition of the rock inclusions. © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo Keywords: Cement-based composite; Force–displacement diagram; Fracture test; Inclusion; Mechanical fracture parameters; Rocks. * Corresponding author. Tel.: +420-541147362. E-mail address: [email protected] Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2019) 000–000 www.elsevier.com/locate/procedia 2452-3216 © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo IGF26 – 26th International Conference on Fracture and Structural Integrity Influence of rock inclusion composition on the fracture response of cement-based composite specimens Michal Vyhlídala*, Iva Rozsypalováa, Hana Šimonováa, Barbara Kucharczykováa, Leona Vavrob, Martin Vavrob, Jiří Němečekc, Pavla Rovnaníkováa, Zbyněk Keršnera aBrno University of Technology, Faculty of Civil Engineering, Veveří 331/95, 602 00 Brno, Czech Republic bCzech Academy of Sciences, Institute of Geonics, Studentská 1768, 708 00 Ostrava-Poruba, Czech Republic cCzech Technical University in Prague, Faculty of Civil Engineering, Department of Mechanics, Thákurova 7, 166 29 Prague, Czech Republic Abstract This paper concerns the results of research into the influence of the composition of rock inclusions on the fracture response of cement-based composite specimens. Specially designed specimens of the nominal dimensions 40 × 40 × 160 mm with inclusions in the shape of prisms with nominal dimensions of 8 × 8 × 40 mm were provided with an initial central edge notch with a depth of 12 mm. These specimens, which were made of fine-grained cement-based composite with different types of rock inclusion – amphibolite, basalt, granite, and marble – were tested in the three-point bending configuration. Fracture surfaces were examined via scanning electron microscopy and local response in the vicinity of rock inclusions was characterized via the nanoindentation technique. The aim of this paper is to analyse the influence of the chemical/petrographic composition of rock inclusions on the effective mechanical fracture parameters of cement-based composites, as well as on the microstructural mechanical parameters of the interfacial transition zone. The results of this research indicate the significant dependence of the effective fracture parameters on the petrographic and related chemical composition of the rock inclusions. © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo Keywords: Cement-based composite; Force–displacement diagram; Fracture test; Inclusion; Mechanical fracture parameters; Rocks. * Corresponding author. Tel.: +420-541147362. E-mail address: [email protected] 2 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 1. Introduction Cement-based composites, with concrete being the main representative of such composites, are widely used building materials (Aïtcin, 1998), (Neville, 2011). Concrete structures such as highway bridges, tunnels, dams, etc. are important parts of the infrastructure which should serve for many generations after their construction. In many cases, such structures show nonlinear, or more precisely, quasi-brittle behaviour – the ability to carry load continues even after the deviation from the linear branch of the force–displacement diagram until the peak point, after which a decrease in loading force follows until failure occurs, which is a phenomenon known as tensile softening (Karihaloo, 1995). The reason for this behaviour is, apart from strong heterogeneity, the existence of internal defects (pores, cracks, transition zones, etc.) or material discontinuities (e.g. inclusions), which work as obstacles to or promoters of crack propagation. Nevertheless, these discontinuities, which form stress concentrators that serve as potential weak elements in composites, are not given any consideration at all in the standards, e.g. EN 1992-1-1 (2004). In this paper, material discontinuities are formed by rock inclusions placed in the middle of the test specimens. These specimens made of fine-grained cement-based composite with different types of rock inclusion – amphibolite, basalt, granite, and marble – were tested in the three-point bending configuration. The rock inclusions were made using a saw with a diamond blade. After the tests, the fracture surfaces were examined via scanning electron microscopy (SEM) and local response in the vicinity of the rock inclusions was characterized via the nanoindentation technique (Zacharda et al., 2018). Assuming that the test specimens were manufactured, compacted and tested in the same way and that the inclusions’ surfaces had the same roughness, the only way to explain the deviation in overall fracture response should be, according to Randl (2013) and the fib Model Code for Concrete Structures 2010 (2013), chemical adhesion. The aim of this paper is to identify the influence of mineralogical composition of rock inclusions on the overall fracture response of the above-described cement-based composite specimens. 2. Theoretical background 2.1. Interface shear transfer The bond mechanism is the interaction between reinforcement and concrete. The components of bond resistance are a combination of different mechanisms – chemical adhesion, friction, mechanical interlocking and the dowel action of reinforcement crossing the interface (Randl, 2013). The dowel action of reinforcement crossing the interface is the result of the lateral displacement of the upper and lower reinforcement ends due to shear slip along the interface. The transfer of shear forces along the interface is thus provided by the bending, shear and axial stresses in reinforcement bars caused by this lateral displacement (Randl, 2013). The mechanical interlocking resistance is the result of the forces acting perpendicular to the ribs of reinforcement. This resistance takes place in the case of excessive and irregular roughness when keying and undercutting effects occur. This effect will only take place if the aggregates/ribs protrude sufficiently from the surface (Randl, 2013). The frictional resistance is the result of the compression forces perpendicular to the interface and also depends on the degree of interface roughness. In the fib Model Code for Concrete Structures 2010 (2013), there is a recommendation for the values of the coefficient of friction µ for a constant confining stress  c depending on whether the interface is smooth, rough or very rough. Several parameters are also described for the classification of the concrete surface roughness, such as mean roughness Ra and the mean peak-to-valley height Rz. Although adhesive shear resistance is in the range of lower units of MPa for concrete grades ≤ C50/60, adhesive bonding can significantly affect overall shear resistance (Randl, 2013). Adhesive resistance is a result of chemical and physical bonding due to Van der Waals forces. For this effect to occur, the related slip at failure must be very small, otherwise the effect will vanish. Adhesive resistance strongly depends on the real surface of the contact area, and the quality, composition and properties (e.g. porosity) of concrete (Randl, 2013). It is connected to the formation of the interfacial transition zone – see the next section – at the aggregate/matrix or reinforcement/matrix interface, which is regarded as the weakest element of cement-based composites. 968 Michal Vyhlídal et al. / Procedia Structural Integrity 33 (2021) 966–981 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 3 2.2. The Interfacial Transition Zone The existence of the Interfacial Transition Zone (ITZ) between aggregate and cement paste was first described in the 1950s by Farran (1956). The ITZ is a region of about 50 µm in size, when using ordinary Portland cement (Scrivener et al., 2004). It should be noted that the ITZ is not a separate region, but the region of transition and it is difficult to determine the exact boundaries (De Rooij et al., 1998). On the surface of the aggregate grain, there is a thin coating of 1 µm in thickness, called “duplex film”, which consist of a calcium hydroxide (CH) layer and a thin layer of short fibers of calcium-silica-hydrate (C-S-H) gel (Barnes et al., 1978). The remaining microstructure of the region is formed mainly by ettringite needles and portlandite plates, while the amount of unhydrated cement grains is reduced (Diamond et al., 1986). The ITZ’s significant feature is mainly its higher porosity compared to the bulk matrix (Scrivener et al., 2004). The local increase in porosity is in a good agreement with the lower values displayed by the mechanical fracture parameters of the ITZ, see e.g. Zacharda et al. (2018). These lower values are inevitably connected with the bond resistance. 3. Experimental part 3.1. Rocks Four basic types of rocks were selected for the preparation of rock inclusions. Specifically, these were: (i) amphibolite from the former Rožná I uranium mine, (ii) olivine basalt from the Bílčice quarry, (iii) biotite granite from the Černá Voda-Nový lom quarry, and (iv) marble from the Horní Lipová-Mramorový vrch quarry (Fig. 1). These rocks were chosen deliberately, as they essentially represent the main raw materials used in the production of crushed aggregates in the Czech Republic. More than 200 deposits of crushed stone are currently quarried on the territory of the Czech Republic (Starý et al., 2020), of which about 23 % are granite deposits, approx. 12 % basalt deposits, around 7 % amphibolite deposits and approx. 2 % marbles. In terms of the total production of crushed aggregates in the Czech Republic, basaltic volcanites account for about 25 % and acidic plutonites such as granites about 20 % of the currently produced aggregates (Starý et al., opus cit.). Fig. 1. Rock specimens used for inclusion preparation after fracture tests (in order from left to right): amphibolite, basalt, granite and marble. Dark grey to black, coarse-grained amphibolite from the Rožná I mine (approximately 40 km NW from Brno) is mostly formed from amphibole (approx. 60–70 % of the rock volume), which varies in composition from tschermakite to magnesiohornblende (Bukovská et al., 2019). Other rock components consist of plagioclase, the basicity of which corresponds to andesine up to labradorite (approx. 20–30 vol. %) and rarely occurring quartz (up to 10 vol. %). Typical accessory minerals are represented by titanite, zircon and opaque phases, probably pyrite. The rock exhibits plane parallel structure and granonematoblastic texture. 4 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 Olivine basalt from the Bílčice quarry (approx. 40 km NE from Olomouc) typically exhibits massive to vesicular structure and porphyritic texture, with pilotaxitic texture of the rock matrix. Phenocrysts are predominantly formed by olivine (approx. 15–20 %), while other rock forming minerals are represented by pyroxene (in particular augite, approx. 37–45 %), calcium-rich plagioclase of labradorite composition (about 20–30 %) and magnetite (up to 20 %). The proportion of amorphous phase (basaltic glass) is up to 3 %. In addition to aggregate production, this basalt was also used in the past for mineral wool manufacturing (Slivka and Vavro, 1996). The “light Silesian granite” from the Černá Voda-Nový lom quarry (approx. 10 km N from the city of Jeseník) is petrographically represented by light grey to grey, medium-grained biotite granite, which typically features holocrystalline, equigranular, hypautomorphic to panxenomorphic granitic texture and massive structure. Its mineral composition is relatively simple, with felsic rock components formed by quartz (approx. 30 %), K-feldspars (approx. 40 %) and plagioclase (approx. 25 %), while biotite (approx. 5 %) is the basic mafic mineral. Accessory minerals include zircon, titanite, apatite, magnetite and rare allanite (Malíková et al., 2019). The marble quarried at the Horní Lipová-Mramorový vrch deposit (approx. 7 km W from the city of Jeseník) is a well-known building and decorative stone material often referred to as “dark Lipová marble”. The rock typically has a light grey to dark grey colour, often with well-visible banding. It is almost entirely (often more than 90 %) composed of calcite, other minerals such as graphite, quartz, muscovite, and pyrite rarely occur, some of them even only as accessories. The rock exhibits massive to plane parallel structure and granoblastic texture. The chemical composition of the rocks used for inclusion preparation was determined semiquantitatively using a XEPOS X-ray fluorescence (XRF) energy dispersive spectrometer (Spectro Analytical Instruments GmbH, Germany). The milled rock sample was mixed with wax and a tablet was moulded and then analysed in a protective atmosphere (He). The results of determining the chemical compositions of the rocks are shown in Table 1. Table 1. Chemical composition of rock inclusions determined using XRF spectrometry [%]. Inclusion SiO2 TiO2 Al2O3 Fe2O3* MnO MgO CaO Na2O K2O P2O5 LOI  Amphibolite 44.80 0.88 15.25 12.08 0.22 9.56 12.47 1.57 1.05 0.05 1.57 99.50 Basalt 42.21 2.66 13.36 13.72 0.22 8.26 12.90 3.80 0.76 0.97 0.56 99.42 Granite 71.60 0.29 13.70 2.68 0.04 0.41 1.95 3.45 5.02 0.11 0.43 99.68 Marble 2.31 0.12 1.12 0.72 0.01 0.70 53.10 0.19 0.21 0.06 41.31 99.85 Explanations: * = iron in the form of Fe2O3, LOI = loss-on-ignition; the samples were burned in a muffle furnace for 3 hours at 1100ºC 3.2. Physicomechanical properties and fracture tests of rocks The tested rocks were acquired in the form of blocks of irregular shape and with a side length of about 0.3–0.4 m. Cylindrical samples measuring 48 mm in diameter were subsequently drilled from the blocks under laboratory conditions. The ends of the drill cores were finally cut perpendicularly to their length, so that the L:D ratio (length-todiameter ratio or slenderness ratio) of the prepared test specimens was about 2:0.7. Basic physical and mechanical characteristics were tested on dry specimens according to standard procedures represented by relevant European standards and suggested testing methods of the International Society for Rock Mechanics. The mechanical properties of the studied rocks were determined by computer-controlled mechanical presses: the FPZ 100 (VEB TIW Rauenstein Thüringer, Germany) and the ZWICK 1494 (Zwick/Roell, Germany). In order to determine the fracture toughness and other important mechanical fracture properties of the input rocks, the three-point bending test was performed. For this test, long cylindrical specimens with a chevron (V-shaped) notch perpendicular to the specimen axis were used. A clip-on gauge type of extensometer was attached at the mouth of each chevron notch, allowing the relative crack face opening (CMOD – crack mouth opening displacement) to be measured. Cylindrical test specimens of 48 mm in diameter and about 190 mm in length were drilled from the rock blocks. A diamond blade was used to cut the chevron notches with an internal angle of 90° and a thickness of 1.5 mm perpendicular to the core body axis and positioned in the centre of each sample. After the chevron notches had been cut, the test specimens were dried to a constant weight. Fracture toughness was calculated from measured force F vs. CMOD diagrams obtained from a three-point bending test which was carried out at room temperature on an FPZ 100 Michal Vyhlídal et al. / Procedia Structural Integrity 33 (2021) 966–981 969 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 3 2.2. The Interfacial Transition Zone The existence of the Interfacial Transition Zone (ITZ) between aggregate and cement paste was first described in the 1950s by Farran (1956). The ITZ is a region of about 50 µm in size, when using ordinary Portland cement (Scrivener et al., 2004). It should be noted that the ITZ is not a separate region, but the region of transition and it is difficult to determine the exact boundaries (De Rooij et al., 1998). On the surface of the aggregate grain, there is a thin coating of 1 µm in thickness, called “duplex film”, which consist of a calcium hydroxide (CH) layer and a thin layer of short fibers of calcium-silica-hydrate (C-S-H) gel (Barnes et al., 1978). The remaining microstructure of the region is formed mainly by ettringite needles and portlandite plates, while the amount of unhydrated cement grains is reduced (Diamond et al., 1986). The ITZ’s significant feature is mainly its higher porosity compared to the bulk matrix (Scrivener et al., 2004). The local increase in porosity is in a good agreement with the lower values displayed by the mechanical fracture parameters of the ITZ, see e.g. Zacharda et al. (2018). These lower values are inevitably connected with the bond resistance. 3. Experimental part 3.1. Rocks Four basic types of rocks were selected for the preparation of rock inclusions. Specifically, these were: (i) amphibolite from the former Rožná I uranium mine, (ii) olivine basalt from the Bílčice quarry, (iii) biotite granite from the Černá Voda-Nový lom quarry, and (iv) marble from the Horní Lipová-Mramorový vrch quarry (Fig. 1). These rocks were chosen deliberately, as they essentially represent the main raw materials used in the production of crushed aggregates in the Czech Republic. More than 200 deposits of crushed stone are currently quarried on the territory of the Czech Republic (Starý et al., 2020), of which about 23 % are granite deposits, approx. 12 % basalt deposits, around 7 % amphibolite deposits and approx. 2 % marbles. In terms of the total production of crushed aggregates in the Czech Republic, basaltic volcanites account for about 25 % and acidic plutonites such as granites about 20 % of the currently produced aggregates (Starý et al., opus cit.). Fig. 1. Rock specimens used for inclusion preparation after fracture tests (in order from left to right): amphibolite, basalt, granite and marble. Dark grey to black, coarse-grained amphibolite from the Rožná I mine (approximately 40 km NW from Brno) is mostly formed from amphibole (approx. 60–70 % of the rock volume), which varies in composition from tschermakite to magnesiohornblende (Bukovská et al., 2019). Other rock components consist of plagioclase, the basicity of which corresponds to andesine up to labradorite (approx. 20–30 vol. %) and rarely occurring quartz (up to 10 vol. %). Typical accessory minerals are represented by titanite, zircon and opaque phases, probably pyrite. The rock exhibits plane parallel structure and granonematoblastic texture. 4 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 Olivine basalt from the Bílčice quarry (approx. 40 km NE from Olomouc) typically exhibits massive to vesicular structure and porphyritic texture, with pilotaxitic texture of the rock matrix. Phenocrysts are predominantly formed by olivine (approx. 15–20 %), while other rock forming minerals are represented by pyroxene (in particular augite, approx. 37–45 %), calcium-rich plagioclase of labradorite composition (about 20–30 %) and magnetite (up to 20 %). The proportion of amorphous phase (basaltic glass) is up to 3 %. In addition to aggregate production, this basalt was also used in the past for mineral wool manufacturing (Slivka and Vavro, 1996). The “light Silesian granite” from the Černá Voda-Nový lom quarry (approx. 10 km N from the city of Jeseník) is petrographically represented by light grey to grey, medium-grained biotite granite, which typically features holocrystalline, equigranular, hypautomorphic to panxenomorphic granitic texture and massive structure. Its mineral composition is relatively simple, with felsic rock components formed by quartz (approx. 30 %), K-feldspars (approx. 40 %) and plagioclase (approx. 25 %), while biotite (approx. 5 %) is the basic mafic mineral. Accessory minerals include zircon, titanite, apatite, magnetite and rare allanite (Malíková et al., 2019). The marble quarried at the Horní Lipová-Mramorový vrch deposit (approx. 7 km W from the city of Jeseník) is a well-known building and decorative stone material often referred to as “dark Lipová marble”. The rock typically has a light grey to dark grey colour, often with well-visible banding. It is almost entirely (often more than 90 %) composed of calcite, other minerals such as graphite, quartz, muscovite, and pyrite rarely occur, some of them even only as accessories. The rock exhibits massive to plane parallel structure and granoblastic texture. The chemical composition of the rocks used for inclusion preparation was determined semiquantitatively using a XEPOS X-ray fluorescence (XRF) energy dispersive spectrometer (Spectro Analytical Instruments GmbH, Germany). The milled rock sample was mixed with wax and a tablet was moulded and then analysed in a protective atmosphere (He). The results of determining the chemical compositions of the rocks are shown in Table 1. Table 1. Chemical composition of rock inclusions determined using XRF spectrometry [%]. Inclusion SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 * MnO MgO CaO Na 2 O K 2 O P 2 O 5 LOI  Amphibolite 44.80 0.88 15.25 12.08 0.22 9.56 12.47 1.57 1.05 0.05 1.57 99.50 Basalt 42.21 2.66 13.36 13.72 0.22 8.26 12.90 3.80 0.76 0.97 0.56 99.42 Granite 71.60 0.29 13.70 2.68 0.04 0.41 1.95 3.45 5.02 0.11 0.43 99.68 Marble 2.31 0.12 1.12 0.72 0.01 0.70 53.10 0.19 0.21 0.06 41.31 99.85 Explanations: * = iron in the form of Fe2O3, LOI = loss-on-ignition; the samples were burned in a muffle furnace for 3 hours at 1100ºC 3.2. Physicomechanical properties and fracture tests of rocks The tested rocks were acquired in the form of blocks of irregular shape and with a side length of about 0.3–0.4 m. Cylindrical samples measuring 48 mm in diameter were subsequently drilled from the blocks under laboratory conditions. The ends of the drill cores were finally cut perpendicularly to their length, so that the L:D ratio (length-todiameter ratio or slenderness ratio) of the prepared test specimens was about 2:0.7. Basic physical and mechanical characteristics were tested on dry specimens according to standard procedures represented by relevant European standards and suggested testing methods of the International Society for Rock Mechanics. The mechanical properties of the studied rocks were determined by computer-controlled mechanical presses: the FPZ 100 (VEB TIW Rauenstein Thüringer, Germany) and the ZWICK 1494 (Zwick/Roell, Germany). In order to determine the fracture toughness and other important mechanical fracture properties of the input rocks, the three-point bending test was performed. For this test, long cylindrical specimens with a chevron (V-shaped) notch perpendicular to the specimen axis were used. A clip-on gauge type of extensometer was attached at the mouth of each chevron notch, allowing the relative crack face opening (CMOD – crack mouth opening displacement) to be measured. Cylindrical test specimens of 48 mm in diameter and about 190 mm in length were drilled from the rock blocks. A diamond blade was used to cut the chevron notches with an internal angle of 90° and a thickness of 1.5 mm perpendicular to the core body axis and positioned in the centre of each sample. After the chevron notches had been cut, the test specimens were dried to a constant weight. Fracture toughness was calculated from measured force F vs. CMOD diagrams obtained from a three-point bending test which was carried out at room temperature on an FPZ 100 970 Michal Vyhlídal et al. / Procedia Structural Integrity 33 (2021) 966–981 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 5 power press with displacement control at a constant loading rate of 0.1 mm∙min–1. For more details about the methods employed in this test, see Vavro and Souček (2013) or Vavro et al. (2019). The inclusions were made using a saw with a diamond blade to cut them from all above-mentioned rock types – amphibolite, basalt, granite, and marble. 3.3. Matrix material The matrix of the test specimens was prepared from a fine-grained cement-based composite. The fresh mixture consisted of CEM I 42.5 R Portland cement (Mokrá cement plant, Czech Republic), ČSN EN 196-1 (2005) standard quartz sand with a maximum grain size of 2 mm, and water in the ratio 1:3:0.35 (cement:sand:water). To ensure workability, a polycarboxylate-based high-range water-reducing admixture (Sika SVC 4035) was added in an amount of 1 % by cement mass. The properties of the fresh composites were determined in accordance with ČSN EN 1015-3 (2000) and ČSN EN 1015-6 (1999). Workability of mixture was 140 mm and bulk density was established as 2.280 g/cm3. For more details, see Vyhlídal et al. (2019). 3.4. Specimens The specimens with nominal dimensions of 40 × 40 × 160 mm containing an internal inclusion with nominal dimensions of 8 × 8 × 40 mm placed in the middle of the span above the initial notch were manufactured for the fracture tests, see Fig. 2. The only difference between the test sets was the type of rock inclusion – amphibolite, basalt, granite, or marble. Each test set contained three test specimens. Fig. 2. Specimen geometry and fracture test configuration (Vyhlídal et al., 2019). Three-part polyethylene (PE) moulds were used to produce the test specimens. The rock inclusions were fixed in position in each part of the moulds before they were filled, see Fig. 3. The mixture was prepared under laboratory conditions using a hand-held paddle mixer. After pouring and compaction of the fresh mixture, the moulds were covered with a thin PE foil and stored under stable laboratory conditions with a temperature of (22 ± 2) °C for 3 days. After demoulding, the test specimens were cured in a water bath until they were tested. The initial notch was made just before the fracture tests using a saw with a diamond blade, the notch depth being approximately 1/3 of specimen depth. 6 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 Fig. 3. Moulds defining the final shape of the specimens with fixed rock inclusions. 3.5. Fracture tests of test specimens To determine the influence of the ITZ on the fracture behaviour of fine-grained cement-based composite, fracture tests were conducted on the aforementioned specially designed specimens via three-point bending. The experiments were conducted using a very stiff LabTest 6-1000 multi-purpose mechanical testing machine (LaborTech Ltd., Czech Republic) with a load range of 0−1000 kN. The fracture tests were conducted under monotonic loading conditions with a constant displacement increment of 0.02 mm∙min–1. The load span was 120 mm. During the experiment, besides the force (F), vertical mid-span displacement (d) and CMOD were continuously recorded. In order to measure CMOD, a strain gauge was fixed between steel blades, which were placed in close proximity to the notch. The mid-span displacement was measured using inductive sensors. As a result, both F−d and F−CMOD diagrams were obtained. An illustration of the three-point bending fracture test configuration is shown in Fig. 4. Fig. 4. Three-point bending fracture test – LabTest testing machine (on the left); detail of measuring equipment (on the right). Michal Vyhlídal et al. / Procedia Structural Integrity 33 (2021) 966–981 971 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 5 power press with displacement control at a constant loading rate of 0.1 mm∙min–1. For more details about the methods employed in this test, see Vavro and Souček (2013) or Vavro et al. (2019). The inclusions were made using a saw with a diamond blade to cut them from all above-mentioned rock types – amphibolite, basalt, granite, and marble. 3.3. Matrix material The matrix of the test specimens was prepared from a fine-grained cement-based composite. The fresh mixture consisted of CEM I 42.5 R Portland cement (Mokrá cement plant, Czech Republic), ČSN EN 196-1 (2005) standard quartz sand with a maximum grain size of 2 mm, and water in the ratio 1:3:0.35 (cement:sand:water). To ensure workability, a polycarboxylate-based high-range water-reducing admixture (Sika SVC 4035) was added in an amount of 1 % by cement mass. The properties of the fresh composites were determined in accordance with ČSN EN 1015-3 (2000) and ČSN EN 1015-6 (1999). Workability of mixture was 140 mm and bulk density was established as 2.280 g/cm3. For more details, see Vyhlídal et al. (2019). 3.4. Specimens The specimens with nominal dimensions of 40 × 40 × 160 mm containing an internal inclusion with nominal dimensions of 8 × 8 × 40 mm placed in the middle of the span above the initial notch were manufactured for the fracture tests, see Fig. 2. The only difference between the test sets was the type of rock inclusion – amphibolite, basalt, granite, or marble. Each test set contained three test specimens. Fig. 2. Specimen geometry and fracture test configuration (Vyhlídal et al., 2019). Three-part polyethylene (PE) moulds were used to produce the test specimens. The rock inclusions were fixed in position in each part of the moulds before they were filled, see Fig. 3. The mixture was prepared under laboratory conditions using a hand-held paddle mixer. After pouring and compaction of the fresh mixture, the moulds were covered with a thin PE foil and stored under stable laboratory conditions with a temperature of (22 ± 2) °C for 3 days. After demoulding, the test specimens were cured in a water bath until they were tested. The initial notch was made just before the fracture tests using a saw with a diamond blade, the notch depth being approximately 1/3 of specimen depth. 6 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 Fig. 3. Moulds defining the final shape of the specimens with fixed rock inclusions. 3.5. Fracture tests of test specimens To determine the influence of the ITZ on the fracture behaviour of fine-grained cement-based composite, fracture tests were conducted on the aforementioned specially designed specimens via three-point bending. The experiments were conducted using a very stiff LabTest 6-1000 multi-purpose mechanical testing machine (LaborTech Ltd., Czech Republic) with a load range of 0−1000 kN. The fracture tests were conducted under monotonic loading conditions with a constant displacement increment of 0.02 mm∙min–1. The load span was 120 mm. During the experiment, besides the force (F), vertical mid-span displacement (d) and CMOD were continuously recorded. In order to measure CMOD, a strain gauge was fixed between steel blades, which were placed in close proximity to the notch. The mid-span displacement was measured using inductive sensors. As a result, both F−d and F−CMOD diagrams were obtained. An illustration of the three-point bending fracture test configuration is shown in Fig. 4. Fig. 4. Three-point bending fracture test – LabTest testing machine (on the left); detail of measuring equipment (on the right). 972 Michal Vyhlídal et al. / Procedia Structural Integrity 33 (2021) 966–981 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 7 3.6. Scanning electron microscopy measurements After the fracture tests, the resulting fracture surfaces were examined via scanning electron microscopy at the AdMaS science centre, which is part of The Faculty of Civil Engineering at Brno University of Technology, and at the Institute of Theoretical and Applied Mechanics (ITAM) of the Czech Academy of Sciences. A TESCAN MIRA3 XMU scanning electron microscope with an environmental probe with 3D imaging was used. In this paper, micrographs created by the detection of secondary electrons (SE) are presented, while micrographs created by the detection of backscattered electrons (BSE) are omitted. SE are created by inelastic scattering of the beam electrons, while BSE are created by elastic scattering and are in fact primary electrons returning after a Coulomb interaction. SE thus provide information about topography, while BSE, in contrast, provides information about composition depending on atomic number Z (Goldstein et al., 2018). 3.7. Nanoindentation measurements Nanoindentation was applied in the vicinity of each inclusion (ITZ zone) to reveal any changes in micromechanical response (Němeček et al., 2013). A Hysitron TriboLab TI-700 nanohardness tester equipped with a Berkovich diamond tip was used. A load-controlled test with the trapezoidal loading function (linear loading for 1 s, holding for 20 s and 1 s unloading) to a maximum force of 2 mN was prescribed for each indent. A rectangular matrix of about 100−200 indents was positioned partly to the inclusion, ITZ and bulk material, see Fig. 5. The matrix contained several rows with an inter-indent separation of 2−3 µm. Fig. 5. Matrix of indents. Young's modulus E and hardness H were estimated by Oliver and Pharr’s (1992) theory (assuming Poisson's ratio ν = 0.2) as 𝐻𝐻 = 𝐹𝐹 𝐴𝐴𝑐𝑐 (1) 𝐸𝐸 = 1 1−ν2 𝑆𝑆√π 2𝛽𝛽√𝐴𝐴𝑐𝑐 (2) where F is the maximum indentation force, S and Ac are the contact stiffness and area, respectively, and β is the tip correction factor. During the holding period, time-dependent deformation is characterized with the creep indentation parameter, the CIT, as 8 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 100 1 12 ),,( 21  − =h hh CIT ttP (3) which is defined as a relative change between indentation depths h1 encountered at time t1 and h2 at time t2, respectively (i. e. the CIT depends on the contact force F and the time of holding period). Creep was also described with the creep compliance function assuming step loading as: 𝐽𝐽(𝑡𝑡) = 2ℎ2(𝑡𝑡) 𝜋𝜋(1−𝑣𝑣2)𝐹𝐹 𝑡𝑡𝑡𝑡𝑡𝑡𝛼𝛼 (4) where h(t) is the depth of the indent at time t, F is the loading force and α is the angle between the surface and edge of the tip (for a Berkovich diamond tip α = 19.7°). Although the assumption of step loading is not perfectly fulfilled, Eq. 4 gives a good estimate for the J(t). 4. Results In this section, the results of fracture tests, nanoindentation measurements and SEM measurements are presented. 4.1. Physico-mechanical properties and fracture tests of rocks Before fracture testing, some fundamental physical and mechanical rock properties were determined since these were assumed to influence the fracture mechanical behaviour of the studied rocks. Specifically, bulk density  , ultrasonic wave velocity vP, water absorption capacity under atmospheric pressure watm, total porosity φ, and uniaxial compressive strength  c were determined on cylindrical specimens with an L:D ratio of 2 (48 mm in diameter, 96 mm high). Tensile splitting strength  t, determined by the Brazilian test, was measured on disc-like specimens with an L:D ratio of 0.7 (48 mm in diameter, 34 mm thick). Obtained results which represent the average value calculated from at least five individual measurements are shown in Table 2. Table 2. Physical and mechanical properties of rocks. Inclusion material  [kg∙m-3] vP [km∙s-1] watm [%] φ [%]  c [MPa]  t [MPa] Amphibolite 2990 6.68 0.13 0.81 193 13.5 Basalt 2970 5.49 1.16 3.44 232 12.3 Granite 2620 4.80 0.31 1.50 185 7.5 Marble 2710 4.92 0.17 0.69 107 8.9 As stated in Chapter 3.2., for the purpose of estimating fracture behaviour, the chevron bend (CB) test was performed and the mode I fracture toughness and other important mechanical fracture properties of the selected rocks were evaluated (see Table 3). Here, Eagg is the bending Young's modulus, νagg represents Poisson's ratio, KIc, agg is the mode I stress intensity factor (fracture toughness), GIc, agg is the mode I critical strain energy release rate, and GF, agg represents fracture energy. Table 3. Mechanical fracture properties of rocks. Inclusion material Eagg [GPa] νagg [–] KIc, agg [MPa∙m1/2] GIc, agg [J∙m–2] GF, agg [J∙m–2] Amphibolite 143.0 0.16 3.370 79.60 448.0 Basalt 87.8 0.15 2.250 57.40 339.0 Granite 59.6 0.18 1.260 26.70 189.4 Marble 108.1 0.20 1.850 31.60 249.2 Michal Vyhlídal et al. / Procedia Structural Integrity 33 (2021) 966–981 973 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 7 3.6. Scanning electron microscopy measurements After the fracture tests, the resulting fracture surfaces were examined via scanning electron microscopy at the AdMaS science centre, which is part of The Faculty of Civil Engineering at Brno University of Technology, and at the Institute of Theoretical and Applied Mechanics (ITAM) of the Czech Academy of Sciences. A TESCAN MIRA3 XMU scanning electron microscope with an environmental probe with 3D imaging was used. In this paper, micrographs created by the detection of secondary electrons (SE) are presented, while micrographs created by the detection of backscattered electrons (BSE) are omitted. SE are created by inelastic scattering of the beam electrons, while BSE are created by elastic scattering and are in fact primary electrons returning after a Coulomb interaction. SE thus provide information about topography, while BSE, in contrast, provides information about composition depending on atomic number Z (Goldstein et al., 2018). 3.7. Nanoindentation measurements Nanoindentation was applied in the vicinity of each inclusion (ITZ zone) to reveal any changes in micromechanical response (Němeček et al., 2013). A Hysitron TriboLab TI-700 nanohardness tester equipped with a Berkovich diamond tip was used. A load-controlled test with the trapezoidal loading function (linear loading for 1 s, holding for 20 s and 1 s unloading) to a maximum force of 2 mN was prescribed for each indent. A rectangular matrix of about 100−200 indents was positioned partly to the inclusion, ITZ and bulk material, see Fig. 5. The matrix contained several rows with an inter-indent separation of 2−3 µm. Fig. 5. Matrix of indents. Young's modulus E and hardness H were estimated by Oliver and Pharr’s (1992) theory (assuming Poisson's ratio ν = 0.2) as 𝐻𝐻 = 𝐹𝐹 𝐴𝐴𝑐𝑐 (1) 𝐸𝐸 = 1 1−ν2 𝑆𝑆√π 2𝛽𝛽√𝐴𝐴𝑐𝑐 (2) where F is the maximum indentation force, S and Ac are the contact stiffness and area, respectively, and β is the tip correction factor. During the holding period, time-dependent deformation is characterized with the creep indentation parameter, the CIT, as 8 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 100 1 12 ),,( 21  − =h hh CIT ttP (3) which is defined as a relative change between indentation depths h1 encountered at time t1 and h2 at time t2, respectively (i. e. the CIT depends on the contact force F and the time of holding period). Creep was also described with the creep compliance function assuming step loading as: 𝐽𝐽(𝑡𝑡) = 2ℎ2(𝑡𝑡) 𝜋𝜋(1−𝑣𝑣2)𝐹𝐹 𝑡𝑡𝑡𝑡𝑡𝑡𝛼𝛼 (4) where h(t) is the depth of the indent at time t, F is the loading force and α is the angle between the surface and edge of the tip (for a Berkovich diamond tip α = 19.7°). Although the assumption of step loading is not perfectly fulfilled, Eq. 4 gives a good estimate for the J(t). 4. Results In this section, the results of fracture tests, nanoindentation measurements and SEM measurements are presented. 4.1. Physico-mechanical properties and fracture tests of rocks Before fracture testing, some fundamental physical and mechanical rock properties were determined since these were assumed to influence the fracture mechanical behaviour of the studied rocks. Specifically, bulk density  , ultrasonic wave velocity vP, water absorption capacity under atmospheric pressure watm, total porosity φ, and uniaxial compressive strength  c were determined on cylindrical specimens with an L:D ratio of 2 (48 mm in diameter, 96 mm high). Tensile splitting strength  t, determined by the Brazilian test, was measured on disc-like specimens with an L:D ratio of 0.7 (48 mm in diameter, 34 mm thick). Obtained results which represent the average value calculated from at least five individual measurements are shown in Table 2. Table 2. Physical and mechanical properties of rocks. Inclusion material  [kg∙m-3] v P [km∙s-1] w atm [%] φ [%]  c [MPa]  t [MPa] Amphibolite 2990 6.68 0.13 0.81 193 13.5 Basalt 2970 5.49 1.16 3.44 232 12.3 Granite 2620 4.80 0.31 1.50 185 7.5 Marble 2710 4.92 0.17 0.69 107 8.9 As stated in Chapter 3.2., for the purpose of estimating fracture behaviour, the chevron bend (CB) test was performed and the mode I fracture toughness and other important mechanical fracture properties of the selected rocks were evaluated (see Table 3). Here, Eagg is the bending Young's modulus, νagg represents Poisson's ratio, KIc, agg is the mode I stress intensity factor (fracture toughness), GIc, agg is the mode I critical strain energy release rate, and GF, agg represents fracture energy. Table 3. Mechanical fracture properties of rocks. Inclusion material E agg [GPa] ν agg [–] K Ic, agg [MPa∙m1/2] G Ic, agg [J∙m–2] G F, agg [J∙m–2] Amphibolite 143.0 0.16 3.370 79.60 448.0 Basalt 87.8 0.15 2.250 57.40 339.0 Granite 59.6 0.18 1.260 26.70 189.4 Marble 108.1 0.20 1.850 31.60 249.2 974 Michal Vyhlídal et al. / Procedia Structural Integrity 33 (2021) 966–981 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 9 4.2. Fracture tests of specimens During the fracture tests, all possible crack propagation directions were observed, see Fig. 6. The crack propagation paths labelled a) and b) in Fig. 6 were observed only for the one specimen with a marble inclusion, while for the rest of the specimens with a marble inclusion, as well as for those with basalt inclusions, crack propagation path c) was observed. In the case of specimens with amphibolite or granite inclusion, the d) crack propagation path occurred. It is evident, that the crack propagation paths in the case a) and b) are caused by low hardness (3 on Mohs scale) and especially by perfect cleavage of calcite as a dominant rock-forming mineral of marble. Crack propagation path c) indicates a high degree of cohesion between rock inclusion and cement matrix and in the case of basalt it is probably due to the vesicular texture, i. e. the presence of pores on the surface of the inclusion. The pores increase the real surface of the contact area between cement matrix and aggregate inclusion which probably contributes to adhesive resistance improvement. Case d), found for amphibolite and granite, is then unfortunately probably due to the method of preparation of inclusions. Because of being sawn using a diamond blade, the inclusions have flat and smooth surfaces, which causes them to have lower cohesion with the cement matrix than there probably should be. Therefore, in the event of a future continuation of these experiments, it will be appropriate to consider another method of preparing the inclusions, for example by means of water jet cutting. Fig. 6. Illustration of crack propagation paths. Specimens after fracture testing can be seen with their crack propagation paths in Fig. 7. Please note that the specimens are labelled with the first three initial letters of the material from which the inclusions are made (e.g. AMP for amphibolite, BAS for basalt etc.). The specimens in the right side of Fig. 7 are reference specimens which were made only from fine-grained cement-based material (matrix) for the determination of the mechanical fracture properties of the matrix. Fig. 7. Specimens after fracture testing (left), and selected details with inclusions: amphibolite, basalt, granite, marble. 10 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 The measured F–d diagrams were used to estimate values for the maximal force Fmax, Young’s modulus of elasticity E, specific fracture energy GF, fracture toughness KIc and effective fracture toughness KIc,e. Young’s modulus of elasticity E was estimated from the first, almost linear part of these diagrams – see Karihaloo (1995). Specific fracture energy GF was calculated using the work-of-fracture method. It represents the energy necessary for the creation of a unit area of a crack (RILEM, 1985). Fracture toughness KIc was estimated from Fmax according to Karihaloo (1995). It represents a linear elastic brittle material's resistance to crack propagation. In contrast, the effective fracture toughness KIc,e was determined based on the Effective Crack Model (Karihaloo, 1995), in which the difference between the initial tangent stiffness and the secant stiffness of the specimen at peak load Fmax is considered. The determined mechanical fracture parameters can be seen in Table 4. Table 4. Mechanical fracture parameters (Vyhlídal et al., 2019). Inclusion material Fmax [kN] E [GPa] GF [J∙m–2] KIc [MPa∙m1/2] KIc,e [MPa∙m1/2] Amphibolite 0.53 37.7 30.5 0.295 0.40 Basalt 0.79 42.1 42.0 0.443 0.74 Granite 0.83 46.5 42.4 0.462 0.67 Marble 0.83 39.8 57.7 0.462 0.97 4.3. Nanoindentation measurements The results for E, H, CIT and J(t) were evaluated with regard to their dependence on distance from an inclusion. E, H exhibit a gradual increase with distance, defining a weaker ITZ around the rock inclusion in the region of 0–20 m. The region is characterized by a lower modulus and a lower hardness compared to the bulk for all specimens, as already detected in (Zacharda et al., 2018, see Table 5). Slightly lower E and H values among the specimens can be found for specimens with amphibolite inclusions. The CIT parameter in the ITZ around inclusions is always higher due to the higher creep encountered in this zone. The highest amount of creep and the highest CIT and J(t) are exhibited by the specimens with amphibolite inclusions, especially in the ITZ of these specimens. Microstructurally, the ITZ can be described as having a higher porosity around the aggregate (Scrivener et al., 2004). Consistently, the evolution of Young’s modulus and hardness has a negative correlation with porosity, while CIT and the amount of creep scales with porosity. To quantify the influence of micromechanical parameters measured by nanoindentation, the mean hardness (H50) and average creep compliance J50(t) values were calculated over an ITZ region of 50 µm, while the mean Young’s modulus values were calculated over ITZ regions of 20 µm (Emic,20) and 50 µm (Emic,50) due to the higher values of porosity in the first 20 µm of the ITZ; see (Bourdette et al., 1995) or (Scrivener et al., 1987). Table 5. Results of nanoindentation measurements (Zacharda et al., 2018). Inclusion material Emic,20 [GPa] Emic,50 [GPa] H50 [GPa] J50(t) [GPa–1] Amphibolite 23.2 25.8 0.75 0.188 Basalt 32.8 36.1 1.32 0.053 Granite 34.2 37.9 2.12 0.045 Marble 34.4 34.5 1.33 0.063 4.4. Microstructure of the ITZ In the following four figures, the micrographs display the hardened cement that adhered to the inclusions after the mechanical tests; the microstructures of the specimens with amphibolite, basalt and granite are very similar in Michal Vyhlídal et al. / Procedia Structural Integrity 33 (2021) 966–981 981 Vyhlídal et al./ Structural Integrity Procedia 00 (2019) 000–000 15 References Aïtcin, P.-C., 1998. High performance concrete. 1st edition. E & FN Spon. ISBN 9780367865986. Akoglu, H., 2018. User's guide to correlation coefficients. Turkish Journal of Emergency Medicine. 18(3), 91−93. Barnes, B.; Diamond, S.; Dolch, W., 1978. The contact zone between portland cement paste and glass “aggregate” surfaces. Cement and Concrete Research, 8 (2), 233 – 243. 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