An analysis of shear fracture toughness KIIc and microstructure in concretes containing fly-ash

• Estimation of the shear fracture toughness KIIc in concretes containing 0%, 10%, 20% and 30% volume content of FA additive.
• Determination of two regions of the fracture toughness depending on the amount of FA additive and maximum KIIcmax.
• The microstructural observations of type of phases in the interfacial transition zone between paste and coarse aggregates.
• The SEM analysis of microcracks in the interfacial transition zone between paste and coarse aggregates.

The present study was focused on estimation of the fracture toughness in Mode II (shear) and internal microstructure of concretes containing 0%, 10%, 20% and 30% volume content additive of the Fly ash (FA). The addition of the FA changes the shear fracture toughness KIIc. The regular value of the shear fracture toughness is included within the additive range from 0% to 23%. The volume content of the FA, corresponding to i.e. maximum increase of KIIc is equal to 17%. If the FA content in higher than 23%, the composite becomes weaker as for response against to shear cracking.The most compact microstructure with well developed fibrous C-S-H phase were characterized by a concrete containing 20% FA additive. The biggest micro-cracks (from 2.5 to 3.0 μm) in interfacial transition zone (ITZ) between paste and coarse aggregates were observed in concretes containing 30% FA, while the smallest one (to 700 nm) in concretes with 20% FA additive.