Fracture properties of alkali-activated slag and ordinary Portland cement concrete and mortar

Three-point bending (TPB) tests were conducted on notched beams to compare the fracture properties of alkali-activated slag (AAS) concrete and ordinary Portland cement (OPC) concrete at three different compressive strength levels (30, 50 and 70 MPa). Parallel comparisons were also conducted between AAS mortar (AASM) and OPC mortar (OPCM). Load vs. mid-span deflection (P-δ) curves, load vs. crack mouth opening displacement (P-CMOD) curves and load vs. crack tip opening displacement (P-CTOD) curves of the tested beams were obtained. The fracture energy GF and the characteristic length (lch) of the AAS and OPC concrete and mortar were calculated and analyzed. It was found that the GF of AAS concrete was always higher than that of OPC concrete given the same compressive strength, due to their denser and stronger interfacial transition zones (ITZs). At a compressive strength of 30 MPa, the GF of AASM was also larger than its OPC counterparts. However, the GF of AASM became lower than that of OPCM when the compressive strengths were 50 and 70 MPa, as more initial micro-cracks were formed in the AASM matrix with strength increase. In addition, the lch values of AAS concrete and mortar were all smaller than those of OPC, implying that the formers were more brittle given the same compressive strengths. The elastic modulus of AAS concrete and mortar were found to be always lower than those of OPC. Micro-structural observations were carried out to explain the above phenomena.