Fracture Mechanical Behavior of Aggregate–Cement Matrix Interfaces

Cracks in concrete propagate preferentially along cement-aggregate interfaces. Therefore, these interfaces act like weak links. The fracture mechanical behavior of the interfaces can be described quantitatively by means of the specific fracture energy G F, which is obtained from the load-displacement curve of the wedge-splitting test. The G F values of the selected sandstone-matrix and limestone-matrix composites varied between 6 and 18 N/m, although the specific fracture energy of the matrix was about 80 N/m; this means that the resistance against crack propagation of interfaces between aggregate surfaces and matrix is only 1/6th of the cement-matrix resistance. This demonstrates the different behavior of the interfaces compared to the matrix. Testing composite specimens of different dimensions showed that there is only a small size effect in the fracture surface area range of 22.5–85 cm 2 (ligament length approximately 4.8–8.4 cm). In the range of 22.5–45 cm 2 (ligament length approximately 4.8 cm), an augmentation of the fracture surface area leads to an increase of the G F value of the cement matrix. Above 45 cm 2 (ligament length approximately 4.8 cm), G F remains almost constant.