Quantification of thermally induced damage and its effect on dynamic fracture toughness of two mortars

- The CT values for two mortars decrease with the increase of temperatures.
- The damage variable is introduced to quantify the thermally induced damage.
- The fracture toughness increases with the loading rate.
- The fracture toughness deceases with the increase of temperature.
- A formula is used to relate thermal damage and rate dependence to fracture toughness.

Concrete structures have a risk of simultaneous exposure to high temperature and blasting or impact load. Influences of thermally induced damage on the dynamic fracture toughness of concretes are thus significant to the safety design of concrete structures. We study the effect of thermally induced damage on the dynamic fracture toughness of two mortars. The notched semi-circular bend (NSCB) specimens were heat-treated at 150 °C, 250 °C, 350 °C, 450 °C and 600 °C, and then tested at room temperature of 25 °C with a split Hopkinson pressure bar (SHPB) apparatus. Before the dynamic fracture tests, X-ray Computer Tomography (CT) scanning was utilized to quantify the thermally induced micro-cracks and chemical changes of two mortars in terms of the CT value. In addition, densities and P-wave velocities of two mortars were also measured. The CT value, density and P-wave velocity decrease with the increase of the heat-treatment temperature, which can be attributed to the evolution of micro-structures due to the undesirable chemical reactions and the increase of micro-cracks. The results of dynamic tests show that the dynamic fracture toughness increases with the loading rates and deceases with the heat-treatment temperatures. The thermally induced damage is a combination of (a) the thermally induced micro-cracks and various chemical changes (which is characterized by the CT value), and (b) the deterioration of the binding property of reaction products. Thus the damage variable is introduced to represent the thermally induced damage for two mortars. A formula for describing the dependence of the dynamic fracture toughness on the loading rate effect and the thermal effect is developed by using the damage variable. The formula can predict the trend of the fracture toughness of two mortars well.