Issue 78

M. S ł owik et alii, Fracture and Structural Integrity, 78 (2026) 393-408; DOI: 10.3221/IGF-ESIS.78.23

d) Figure 9: Examples of crack propagation and view of failure sections in tested specimens: L16N series (a), H16N series (b), L16G series (c), and H16G series (d). The comparison of specimens made of concrete with gravel and granite aggregates after failure revealed that the failure crack paths differed. In case of specimens made of gravel-based concrete, the crack propagated more often through the cement matrix and the ITZ - the weak region around the aggregate grains (see Fig. 10 a). When observing the failure crack in specimens made of concrete with granite aggregate, it appeared that the failure crack followed a less tortuous path and that it often ran through the aggregate grains (see Fig. 10b). In the case of concretes in series L16N and H16N (with gravel aggregate), noticeable differences in main crack paths were observed depending on the cement matrix strength. For the L16N series with a weaker matrix (f ct,cm = 6.86 MPa), failure cracks predominantly followed the ITZ, resulting in debonding of the aggregate grains, a characteristic behaviour associated with a weaker bond at the aggregate–matrix interface. In contrast, the H16N series with a stronger matrix (f ct,cm = 8,79 MPa) displayed a more mixed character of failure. While some grains still debonded similarly to the L16N series, others, particularly the larger ones, were fractured. This indicates a stronger aggregate–cement matrix bond, resulting in a partially altered failure mechanism. For the granite aggregate series (L16G and H16G), no visible differences in failure behaviour were observed between specimens with a weaker and stronger cement matrix. In both cases, the majority of cracks propagated through the aggregate grains, reflecting a strong mechanical bond and interlocking effect between the angular, crushed granite and the surrounding cement matrix. The failure mode was dominated by grain fracture rather than ITZ debonding, regardless of the hardened cement mortar strength. The examples of the two dominant crack propagation mechanisms observed in the investigated concretes are presented in Fig. 10.

Figure 10: Representative examples of the post-failure observations obtained through the optical microscopy (Keyence VHX 7000) illustrating the dominant crack propagation mechanisms: (a) crack propagation along the interfacial transition zone (ITZ); (b) crack propagation through an aggregate particle. The observed crack propagation patterns provide a mechanical explanation for the different influence of cement matrix strength on the splitting tensile behaviour of the investigated concretes. In the gravel-based concretes, increasing the cement matrix strength enhanced stress transfer across the aggregate–matrix interface, resulting in a more efficient utilisation of the aggregate capacity and a significant increase in splitting tensile strength. In contrast, although a stronger cement matrix also improved the aggregate–matrix interaction in the granite-based concretes, the lower crushing resistance of the granite particles limited the beneficial effect of matrix strengthening. Consequently, the tensile behaviour of the investigated concretes was governed by the combined influence of cement matrix strength, the efficiency of stress transfer through the ITZ and the mechanical characteristics of the coarse aggregate, rather than by any of these factors acting independently.

405