Data 12-mar-2015
Titolo della tesi Self-healing capacity of cementitious composites
Abstract in italiano Capacità di Auto-riparazione dei Compositi Cementizi
Abstract in inglese The self- healing capacity of cementitious composites employed for either new or repairing applications opens challenging perspectives for the use of a construction materials intrinsically able to recover its pristine durability levels, thus their guaranteeing a longer service life of the designed applications and a performance less sensitive to environmental induced degradation. One possibility of achieving the aforementioned self-healing capacity stands in the use of additives featuring a “delayed crystalline” activity. These additives are able, when in contact with water or atmosphere humidity, to form chemical compounds which are able to reseal the cracks thus guaranteeing the recovery of a pristine level of mechanical performance. In order to approach the investigation, besides conventional concrete (with and without the aforementioned admixtures) the characterization of the self-healing capacity of High Performance Fiber Reinforced Cementitious Composites (HPFRCCs) with steel fibers and combination with the natural ones was also studied, i.e. their capacity to completely or partially re-seal cracks, as a function of the material composition, maximum crack opening and exposure conditions. This also implies a new structure concept and a wider worthiness of the sustainability of engineering applications which can be achieved thanks to the use of high performance cement based materials, which encompasses and overcomes the traditional one related to the use of by-products in mix-compositions, which can be effectively pursued also in this case In order to quantify this self-healing ability and its effects on the recovery of mechanical properties a methodology has been developed and will be presented in this dissertation. It consisted in pre-cracking up to different crack opening levels (a three point bending scheme with COD measurement was employed) prismatic beam specimens, made with both concrete, with or without the aforementioned additive. Whereas, for HPFRCCs, through this dissertation topic has been investigated including the effect of different flow-induced alignment of fibers, triggered through tailored casting, which can result into a material exhibiting either a strain hardening or softening behavior, whether stressed parallel or perpendicularly to the fibers. In all cases, after pre-cracking specimens were initially pre-cracked, according to a 4-point bending scheme, and up to different values of crack openings. Specimens were then submitted to different exposure conditions (natural winter or summer environment, underwater, exposure to humid or dry air, wet-and-dry cycles, accelerated winter or summer representative temperature and humidity cycles) for different exposure times. After scheduled exposure duration the specimens were tested up to failure according to the same scheme employed for pre-cracking and results, in terms of load-crack opening curves were compared to those obtained from virgin specimens before any “treatment”. Non-destructive tests (UPV), performed before and after the precracking as well as after the environmental conditioning, together with dedicated microscopic investigation completed the experimental program. The significant amount of garnered experimental results also allowed suitable self-healing indices to be defined and quantified, as from the measured recovery of mechanical properties, including load bearing capacity, ductility and stiffness; which is a much needed approach in order to consistently consider the self-healing phenomenon into a durability based design.
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