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This paper presents the results of compressive strength – fcm and fracture toughness tests of concrete composite modified with the addition of siliceous fly ash (FA). The investigation embraced concrete with 0%, 10%, 20% and 30% FA addition. The parameters of linear and nonlinear fracture mechanics were analysed in this studies, i.e. the critical stress intensity factor – and the critical crack tip opening displacement – . These parameters were determined based on the results obtained from the MTS 810 press and the Digital Image Correlation (DIC) system with ARAMIS software. Concrete with FA addition up to 23% is characterized by regular fracture toughness and above this limit the fracture toughness gets low. On the other hand, the optimum amount of FA addition causing maximum fracture toughness of the material is 17% (Fig. 8). Furthermore, it was observed that there is a qualitative correlation between parameters: fcm, and The values of parameters of fracture mechanics calculated on the basis of data from the ARAMIS software show the behaviour of the structural material exactly at the moment of the initial crack inception. The research results presented show the usefulness of the DIC method in this type of experiments, whereas the article confirmed the high potential of DIC technique in the determination of fracture toughness in concrete composites with FA addition.
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