EFFECT OF AUTOGENOUS SELF-HEALING ON HIGH TEMPERATURE EXPOSED ULTRA HIGH-PERFORMANCE CONCRETE

Mechanical properties of Ultra High-Performance Concrete (UHPC) degrade when exposed to elevated temperatures, even more than ordinary concretes due to its dense microstructure. Concerning, in particular, the special application of nuclear power plants, in which UHPC can find a promising use, concrete can be subjected to moderately high temperature (usually lower than 400 °C) along the working life, this making of interest the study on the influence and persistence of UHPC's innate self-healing capabilities over the thermal degradation. In this context, the paper focuses on an experimental study of UHPC recovery ability by autogenous self-healing after being exposed to high temperatures. The UHPC specimens have been made with hybrid fibers, that is, polypropylene and steel fibers, and have been pre-cracked up to a cumulative crack width of 0.3 mm under 4-point flexural test. The pre-cracked specimens have been exposed to a temperature of 200 °C or 400 °C, with a heating rate of 1 °C / minute from room temperature and kept at the target temperature for two hours, with a following slow cooling at a rate of <1 °C / minute. The specimens have been kept in the lab environment for 24 hours after reaching room temperature. Then they have been tested for residual flexural capacity or allowed to self-heal under water immersion for six months. The damage and healing evolution have been monitored periodically using ultra-sonic pulse velocity survey and digital microscope inspection. In spite of the thermal degradation, during the healing period UHPC showed a significant recovery in terms of strength assessed by ultrasonic pulse velocity tests.