Detection of Failures around Main Cracks and Their Relation to the Fracture Toughness of Cement Pastes

Fluorescence microscopy examinations revealed the characteristics of crack path and the presence of local failures around the main cracks in hardened cement pastes. Reduction in the water: cement ratio and the addition of silica fume led to a decrease in locally damaged regions around the main cracks. However, the R - curve of the silica fume-free cement paste with the water: cement ratio of 0.55 was much different from that of the silica fume-bearing paste. The silica fume-bearing cement paste exhibited a relatively flat R -curve, whereas reduction in the water: cement ratio in the silica fume-free paste led to a rising R-curve. These results were related to the toughening mechanisms of cement pastes derived from the characteristics of the crack path and the areas of local failures around main cracks that were revealed by fluoresence microscopy. Presumably, these differences in fracture toughness resulted from the nature of hydration products and/or their inherent microstructure.