Calibrating a crack self-healing kinetic law for Ultra High Performance Concrete from five years of experimental work in the Horizon 2020 ReSHEALience project

To enhance the long-term durability of Ultra-High Performance Concrete (UHPC) in extremely aggres-sive environments, crystalline admixtures are used to stimulate the self-healing of cracks with different widths, also ensuring the recovery of the mechanical and durability performance of the material. The geometrical characteristics of these cracks play a crucial role in determining the extent of the penetra-tion of harmful substances into the composite cementitious matrix and are also important factors gov-erning the self-healing mechanisms in concrete. Among these geometric characteristics, crack width is of paramount importance and should be given priority in the evaluation of crack self-healing processes. This study, conducted under the umbrella of the ReSHEALience project, funded by the European Com-mission in the framework of the Horizon 2020 Research and Innovation programme (GA760824), used image collection techniques to capture the geometric characteristics of concrete cracks in beams and Double Edge Wedge Splitting (DEWS) test specimens and created a database of crack closures through image processing techniques. A set of plural experimental campaigns which have spanned over five years and have included different environmental exposure conditions, matrix compositions and times has been considered. The results show that the smaller crack width range (0-50 μm) has a higher rate of crack healing in all environments, while the 50-300 μm crack closure is slower. Moreover, complete immersion in a salt water environment resulted in faster closure of cracks with a width range of 0-20 μm in a shorter period of time. Based on the analysis of this database, this paper presents the calibration of a crack closure kinetic model for different crack width ranges (0-20, 20-50, 50-100, 100-300 μm) in different healing environments and along time, in order to provide a basis for further refine crack heal-ing simulation and modelling and further contribute to lay out a durability based design approach for UHPC structural applications.