AN EXPERIMENTAL METHODOLOGY TO ASSESS EFFECTS OF HEALING ON FREEZE-THAW DAMAGED ULTRA HIGH-PERFORMANCE CONCRETE

This paper presents the experimental investigation of the self-healing capacity and ability to maintain the structural performance of a Ultra High-Performance (Fiber Reinforced) Concrete (UHPC/UHPFRC), with a crystalline admixture to stimulate the healing, after freeze-thaw cycles. To the aforesaid purpose ultrasonic pulse velocity tests, four-point flexural tests (before and after freeze-thaw, and after self-healing), and crack closure quantification have been performed. 20 mm thin beams were pre-cracked up to a cumulative crack width of 0.3 mm by means of four-point flexural test and subjected to freeze and thaw cycles between -20° C to 38° C for 17 days, each cycle lasting for 20 hrs. The flexural tests showed that freeze-thaw did not deteriorate the specimens' flexural strength. However, freeze-thaw caused some damage which was noticeable in the ultrasonic test. After the freeze and thaw cycles specimens were immersed in water for self-healing. The self-healing progress was measured periodically after 1, 2, 3, and 6 months of healing through ultrasonic test and microscopy image processing. The results showed that the freeze-thaw damages were healed throughout the specimens, and that previously undergone damage didn't affect neither the stimulated autogenous healing capacity of the investigated material nor its mechanical performance. This can be likely attributed to both closure of the cracks, which were almost fully healed within 3 months, and likely also to improved bond strength between the fibers and concrete matrix, due to the deposition of the healing products along the interface

Fatigue behavior and effect of stimulated autogenous self-healing in Ultra High-Performance Concrete

This paper investigates the mechanical response under fatigue cycles in Ultra-High-Performance Con-crete (UHPC) under four-point flexural loading, focusing on the effect of damage recovery triggered by stimulated autogenous self-healing. Thin beam specimens were pre-cracked up to 0.25 mm residual crack opening displacement under monotonic loading and then subjected to cyclic loading with a fre-quency of 5.5 Hz and a load amplitude equal to 10-80% of the load corresponding to the load at residual pre-crack width. Cyclic loading was applied for 700,000 cycles or up to the attainment of 1 mm total crack opening displacement at maximum load, whichever was reached first. Specimens were then healed underwater, and the fatigue tests were repeated to failure after the scheduled healing period of 1, 3, or 6 months. Self-healing performance was assessed via ultrasonic pulse velocity test and micro-scopic image analysis. Furthermore, the effects of self-healing in fatigue-crack growth rate, stiffness degradation, and critical crack opening were identified together with the benefits brought in as residual fatigue life recovery. The three-month healed specimens showed up to twenty times reduction in the rate of crack opening displacement as compared to the same specimen before the healing period