Abstract
•Mesoscopic length scale parameters in un-reinforced concrete are investigated experimentally.•The experimental results are post-processed according to the Theory of Critical Distances.•Aggregates acts as barriers slowing down/affecting the crack propagation process.•The critical distance approaches the average distance between notch tip line and first aggregates.
The Theory of Critical Distances (TCD) groups together a number of approaches that make use of specific length scale parameters to model and estimate the strength of cracked/notched engineering materials. In the TCD framework, the critical distance is assumed to be an intrinsic property and this length is somehow related to the material micro-/meso-/macro-structural features. In recent years, a number of comprehensive theoretical/experimental investigations have proven that the TCD is successful also in assessing the static/dynamic Mode I/Mixed-Mode I-II strength of unreinforced concrete containing geometrical features of all kinds. However, the scientific community has not yet agreed on a commonly accepted answer to the most obvious fundamental research question, i.e. “what is the physical meaning of the TCD critical distance?” In order to answer this question, a number of experimental results were generated by testing specimens of unreinforced concrete under static and dynamic Mode I bending. According to what is recommended by RILEM for the determination of fracture parameters of plain concrete, this comprehensive experimental work involved not only plain samples, but also specimens containing crack-like saw-cut notches. The specimens being tested were manufactured by using different bespoke mixes so that the meso-structural features of the concrete materials being tested could be controlled and then modelled in a very accurate way. The results from this systematic experimental/theoretical study led to the conclusion that the TCD critical length approaches the average distance between the crack-like saw-cut notch tip line and the first aggregates, with these aggregates acting as barriers slowing down/affecting the crack propagation process.