Fracture toughness parameters to assess crack healing capacity of fiber reinforced concrete under repeated cracking-healing cycles

This paper is the continuation of a research focused on the assessment of the crack-sealing capacity of Steel Fiber Reinforced Concrete (SFRC) with crystalline admixtures subjected to repeated cracking-healing cycles. In the first study, the work was focused on the quantitative evaluation of the crack-sealing performance by means of image analysis. To this purpose, crack sealing effectiveness was evaluated as a function of the presence of crystalline admixtures, maximum crack opening, duration of the healing period, exposure conditions (immersion in water, exposure to open-air exposure or wet/dry cycles), fiber orientation and number of cracking and healing cycles. The outcomes of the self-sealing phenomenon were analyzed defining a crack-sealing index calculated from images taken by means of a digital microscope both at the beginning and at the end of each healing exposure period. In this paper, it has been tried to move a step further, correlating the Sealing Index (crack closure in %) with parameters obtained from fracture toughness tests on specimens subjected to repeated cracking-healing cycles, with the aim of quantifying the retention and/or recovery of mechanical properties along the testing path. This is meant to simulate a real structural service scenario, in which a healed crack may reopen and be allowed enough time to re-heal, this repetition of events being likely to occur several times during the structure service life. To this purpose, equivalent tensile stresses (obtained from absorbed energy per unit fracture surface) were determined from nominal tensile stress vs. crack opening displacement curves obtained from a dedicated testing methodology, namely the Double Edge Wedge Splitting (DEWS) tests, and their evolution along the cracking and healing cycles was assessed. Results showed that, an increase of the Sealing Index, i.e. a more effective sealing of the cracks, also results into a slight increase of SFRC toughness performances as a consequence of both the through-crack matrix continuity reconstitution as well as of a likely improved bond between fibers and matrix. The method proposed in the paper can be further employed to build up a data-base in order to establish, through suitable meta-analysis procedure, sound correlation between parameters representative of crack self-sealing and material performance recovery (self-healing)