Issue 78

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

concretes made with granite aggregate, the intrinsic mechanical characteristics of the aggregate itself played a more significant role in the failure mechanism.

Splitting tensile strength of concrete – mean value [MPa]

Increase in strength of the splitting tensile strength [%]

Flexural strength of cement matrix [MPa]

Increase in strength of the cement matrix [%]

Aggregate crushing – mean value [%]

Aggregate type

Series

H16N L16N H16G L16G

8.79 6.86 8.79 6.86

3.47 2.67 3.81

+28.1

Gravel

8.2

+30.0

+28.1

Granite

15.8

+13.4

3.36 Table 6: Laboratory results of the concrete’s splitting tensile strength in respect of the cement mortar strength and the aggregate crushing value. Comparative analysis with other experimental results A review of the professional literature has shown that both geometric and mechanical characteristics of aggregates significantly affect the splitting tensile strength of concrete. In the research performed by Piasta et al. [37], the crushing strength of different types of aggregate (basalt, granite, gravel, dolomite, granodiorite and quartzite) with Dmax = 16 mm was examined. The gravel aggregate exhibited the lowest crushing value (aggregate crushing value ACV ≈ 9%). The highest crushing value was obtained for the granite aggregate (aggregate crushing value ACV ≈ 15%). Comparing the results obtained in [37] for concretes with gravel and granite aggregates (D max = 16 mm, w/c = 0.55), an increase in the crushing value by 67.7% corresponded to a reduction in splitting tensile strength of 8.5%. Although the ACV results reported in [37] are consistent with those obtained in the present study, an opposite relationship between crushing resistance and splitting tensile strength was observed In the own laboratory investigation, when comparing the results obtained for concretes with gravel and granite aggregate for D max = 16 mm and w/c = 0.55, with the decrease in the aggregate crushing ACV of 48.1%, concrete tensile strength decreased by 20.5%. This difference in results may suggest the influence of other aggregate characteristics, not only crushing strength, on the splitting tensile strength. It may also indicate the importance of other components of the concrete structure, such as, for example, the bond between aggregate grains and cement mortar known as the interfacial transition zone (ITZ). The importance of the interfacial transition zone (ITZ) has also been highlighted by Hansen et al. [38]. The authors investigated the influence of coarse aggregate characteristics on the mechanical behaviour of concrete by comparing concretes containing rounded and crushed gravel aggregates. Similar to the present study, their research was based on evaluating how selected aggregate characteristics affect the tensile behaviour of concrete. Hansen et al. demonstrated that aggregate characteristics, particularly particle shape, significantly influenced the relationship between compressive and splitting tensile strength. The authors attributed these differences to the quality of the aggregate–paste bond and the characteristics of the interfacial transition zone, indicating that the tensile behaviour of concrete is governed not only by the intrinsic properties of the coarse aggregate but also by the effectiveness of stress transfer across the ITZ. These observations are consistent with the interpretation proposed in the present study, where the differences observed between concretes containing rounded gravel and crushed aggregate are also discussed in terms of crack propagation mechanisms and the aggregate–matrix interaction. The role of interfacial transition zone Both the aggregate type and the properties of the cement mortar influence concrete’s fracture behaviour. However, the interfacial transition zone is another key factor responsible for its overall mechanical response. Individually, the hardened cement mortar and the coarse aggregate tend to exhibit linear, brittle elastic behaviour. On the other hand, concrete cannot be described by the linear elastic law since it is a quasi-brittle material. This distinction is mainly associated with the formation and propagation of microcracks, which are predominantly localised within the ITZ. As a result, the interfacial transition zone is generally regarded as the weakest region in the microstructure of normal-strength concrete. A comparison of stress–strain relationships for individual concrete phases is presented in [39] (see Fig. 8). The formation of the ITZ is primarily attributed to several interrelated mechanisms, including inadequate particle packing, water absorption by aggregates, chemical reactions, and the development of interfacial bonds. The aggregate type is of importance due to mechanical interlocking. Crushed aggregate, such as granite, usually has a more irregular morphology compared to gravel one, which promotes stronger adhesion between aggregate particles and the hardened cement paste. Another important factor is the mineral composition of the aggregate as well as the use of mineral admixtures, both of

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