Using digital image correlation to evaluate fracture toughness and crack propagation in the mode I testing of concretes involving fly ash and synthetic nano-SiO₂

Abstract This paper presents the experimental results and deep discussion on the simultaneous effect of fly ash (FA) and synthetic nano-SiO 2 on the main strength parameters and fracture toughness expressed by critical stress intensity factor, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Ic</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">S</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> of a new concrete based on ternary blended cements (TC). Four series of concretes were made, one of which was the control concrete and the remaining three were TC. The effect of constant content of 5% nano-SiO 2 and various FA contents such, i.e.: 0, 15 and 25% wt. as partial replacement of cement were studied. During the studies the DIC technique was applied to determine the deformation of the concrete beams in the propagation area of the modelled initial crack. Based on the studies it was found that the addition of 5% nano-SiO 2 without FA increases the strength and fracture toughness of concrete by approx. 20%. On the other hand supplementing the composition of the binder with 5% nano-SiO 2 in combination with the 15% FA additive causes an increase in all mechanical parameters by another approx. 20%. However, an increase in the FA content in the concrete mix by another 10% causes a smaller increase in the all analysed factors, i.e. by approx. 10% compared to composite with the addition of nano-SiO 2 modifier only. In addition, based on the studies using DIC technique it was observed that in concrete including only nano-SiO 2 the crack paths were almost perfectly rectilinear in shape, with a significant width of opening. However, in concrete containing 5% nano-SiO 2 + 15% FA the crack paths were curvilinear with characteristic additional microcracks in the vicinity of the main crack, whereas in concrete with the addition of 5% nano-SiO 2 + 25% FA the crack paths were very strongly curved and had pronounced branching and numerous additional microcracks in the vicinity of the main crack. From an application point of view, concretes involving FA and nano-SiO 2 can be used in the execution of specific types of concrete and reinforced concrete structures that require a material with reduced brittleness and at the same time increased fracture toughness.