| Fracture in concrete has been a subject of extensive research for the past thirty years. Although much has been accomplished, concrete still remains the least known of the engineering materials with respect to its fracture behavior. The objective of this research was to study the influence of micro-structural parameters, such as aggregate size, and macroscopic parameters, such as specimen size, on the fracture toughness of concrete. In addition, the effect of notch-depth ratio on the fracture behavior of concrete was studied. Over 300 concrete specimens of various sizes were prepared with various maximum aggregate sizes.;There is a well pronounced trend observed in all groups of specimens tested. Critical energy release rate of concrete in the presence of compressive force increases with crack length, i.e., exhibits a typical R-curve behavior. For fixed specimen size, toughness (energy release rate or fracture energy) increases with an increase in maximum aggregate size. Fracture surface interaction, i.e., interlocking, bridging, etc. is more pronounced in the specimens with larger maximum size aggregates. For specimens with fixed maximum aggregate size, toughness increases with an increase in specimen size. For geometrically similar specimens, i.e., the shape and all dimensionless parameters are the same, the toughness evaluated for larger specimens is noticeably higher than that for smaller specimens. Energy release rates for long-notch specimens were relatively lower than those for short-notch specimens. Fracture energy for short-notch specimen is higher than that for long notch specimen.;Concrete surfaces exhibit fractal characteristics over the range of scales studied. Fractal dimension increases with an increase in maximum aggregate size and specimen size as well. The fractal dimension correlates very well with surface roughness. The rougher the surface, the higher the fractal dimension. A good correlation also exits between the fractal dimension and fracture toughness: the tougher the material, the higher the fractal dimension. A simple linear relation between fractal and fracture energy is recommended.;The uncertainty of energy released rate distribution increases with crack length and specimen size. For large specimens the maximum allowable splitting load is more sensitive to the required reliability level than that for small specimens. Reliability increases with aggregate size for all other conditions being fixed. Based on experimental observation and statistical analysis, size effect and reliability prediction equations are proposed. |
