Issue 41

H. Šimonová et alii, Frattura ed Integrità Strutturale, 41 (2017) 211-219; DOI: 10.3221/IGF-ESIS.41.29 213 Composite ID Components and properties Units 04042016 09052016 Quartz sand [kg] 45.9 45.9 Cement I 42.5 R [kg] 15.3 15.3 Super-plasticizer SVC 4035 % by cement mass ‒ 1.0 Water to cement ratio [ ‒ ] 0.5 0.35 Workability [mm] 140 135 Bulk density [kg/m 3 ] 2200 2280 Table 1 : Composition and properties of fresh composites. Fracture tests The fracture tests in three-point bending were carried out using a Heckert FP 10/1 testing machine with measuring range of 0 − 2000 N. The beam specimens were provided by initial central edge notch with approximately depth 1/3 of specimen depth situated in the middle of span length; span length was 120 mm. The displacement increment loading was performed, which allowed to record load versus displacement diagrams ( L–δ diagrams) during the tests. The L–δ diagrams were used for the determination of elasticity modulus from the first (almost linear) part of the diagram, and for the calculation of effective fracture toughness using the effective crack extension method [15] and specific fracture energy using work-of-fracture method [16]. Because of stability loss during loading, it was not possible to reconstruct the descending part of L–δ diagrams. Therefore, the work of fracture W F * value is determined as area under L–δ diagrams before stability loss occurred. For details about determination above mentioned mechanical fracture parameters see [17]. The results of performed fracture tests evaluation are introduced in Tab. 2 in form mean values and standard deviations ordinarily from six specimens. The monitored mechanical fracture parameters were following: modulus of elasticity E , effective fracture toughness K Ice and specific fracture energy G F * (determined using mentioned work of fracture W F * value). Composite ID Parameter Units 04042016 09052016 Modulus of elasticity E [GPa] 32.1±1.6 34.2±2.6 Fracture toughness K Ice [MPa·m 1/2 ] 0.759±0.054 1.093±0.067 Fracture energy G F * [J·m –2 ] 10.76±1.78 24.37±2.89 Table 2 : Selected fracture tests results of 04042016 and 09052016 composites. Microstructure of tested specimen’s material Microscopy measurements for quantitative description of microstructure of ITZ were carried out using scanning electron microscopy (SEM) MIRA3 TESCAN. The projection of specimen’s surface was performed using secondary electrons (SE) or backscattered electrons (BSE). The selected micrographs caused by detection of SE with accelerating voltage of electrons 20 kV are introduced bellow. Microstructure of fracture surfaces at the aggregate–cement paste interface for both tested specimens are introduced in Fig. 1. SEM image of fracture surface of the specimen with greater w / c ratio (on the left) shows less compact microstructure in compare with the other one. It is possible to identify the elemental minerals forming interface, namely ettringite, portlandit and C-S-H gel. On the contrary, there wasn't found any ettringite on the right SEM image. It can be caused by random selection of fracture surface part. The other advantage of SEM is a possibility of length measurements. It has to be mentioned that measurements are made on a two dimensional sections through a three dimensional microstructure. Nevertheless, the distances, used here for the primary information about size and shape of the ITZ, are uncorrected distances measured on 2D sections.