The present thesis is the result of a thorough experimental investigation on the self-healing capacity of a peculiar mix composition of High Performance Fibre Reinforced Cementitious Composites (HPFRCC), containing nanoadditions of alumina nanofibers (ANF) (0.25% by volume). With the purpose of drawing a comprehensive and accurate analysis of the effect of the crack healing, it has been resorted to an approach that correlates the amount of crack closure with the recovery of mechanical properties. Beam specimens (100 mm wide x 500 mm long x 25 mm thick), pre-cracked in 4-point bending up to a value of crack opening (COD) equal to 0.27 mm, were submitted in two different exposure conditions: geothermal water immersion and exposure in a chamber at constant temperature T = 20 °C and relative humidity RH = 95%. After the scheduled exposure times, ranging from one month to six months, specimens were re-cracked up to the same value of COD, according to the ilk test set up employed for pre-cracking. The first part of the investigations, focused on the capability of the material to completely or partially re-seal the cracks, which is a function of its average crack width and exposure conditions. In this regard, the value of crack closure was obtained via an image analysis processing of the crack observed, by an optical microscopy before and after the healing exposure. This method only enables the evaluation of the healed crack surfaces. Moreover, outcomes of the self-healing phenomenon in the inner parts of the cracks, if any, were analysed in terms of recovery of stiffness, strength and ductility. As a matter of fact, the thesis also aimed at evaluate the autogenous self-healing, both through visual image analysis of the healed cracks and through a permeability test. Disk specimens, pre-cracked in splitting test up to a value of crack opening equal to 0.5 μm, were plunged into geothermal water immersion. After the same scheduled exposure times, specimens have been subjected to a pressure caused by the head of a column of water (500 mm height and 85 mm diameter). Finally, mechanical characterization completed the experimental program. Tensile characterization on the mixes has been performed by different techniques, mainly based on 4-point bending test and Double Edge Wedge Splitting (DEWS). This analysis considers different flow-induced alignments of fibres, which can result into either strain-hardening or softening behaviour, depending respectively on whether the material is stressed parallel or perpendicularly to the fibres. The obtained result of this experimental campaign were compared to those arising from previous studies concerning various HPFRCC mix compositions. In those cases, there is either no presence of nanoadditions, or it is possible to find a presence of different nanoadditions in form of cellulose nanocrystals and cellulose nanofibrils.
La presente tesi è il risultato di un'approfondita indagine sperimentale sulla capacità di auto-guarigione di un mix di High Performance Fibre Reinforced Cementitious Composites (HPFRCC), con aggiunta di nanofibre di allumina (ANF) (0,25% in volume). Al fine di tracciare un'analisi completa e accurata dell'effetto della guarigione della fessura, si è analizzata la relazione tra la percentuale di chiusura della fessura ed il recupero delle proprietà meccaniche. I provini della trave (100 mm di larghezza x 500 mm di lunghezza x 25 mm di spessore), pre-fessurati mediante una prova a flessione su quattro punti fino ad un valore d’ampiezza della fessura (COD) pari a 0,27 mm, sono stati posti in due diverse condizioni di esposizione: immersione in acqua geotermica ed esposizione in una camera a temperatura costante T = 20 °C e umidità relativa RH = 95%. Dopo i tempi di esposizione programmati, che vanno da un mese a sei mesi, i provini sono stati sottoposti nuovamente alla fessurazione fino a raggiungere lo stesso valore di COD, secondo il test impiegato per la pre-fessurazione. La prima parte dello studio, si è concentrata sulla capacità del materiale di richiudere le fessure completamente o parzialmente, in funzione dell’ampiezza media delle fessure e delle condizioni di esposizione. A questo proposito, il valore della chiusura della fessura è stato ottenuto attraverso l'analisi della fessura, osservata con un microscopio ottico prima e dopo la guarigione. Questo metodo consente la valutazione della guarigione superficiale della fessura. Inoltre, i risultati del fenomeno di autoguarigione nelle parti interne delle fessure, se presenti, sono stati analizzati in termini di recupero di rigidità, resistenza e duttilità. La tesi ha, anche, valutato l'autoguarigione autogena, sia attraverso l'analisi visiva dell'immagine delle fessure guarite, sia attraverso un test di permeabilità. Provini a disco, pre-fessurati con una prova di splitting fino ad un valore di ampiezza della fessura pari a 0,5 μm, sono stati immersi in acqua geotermica. Dopo i tempi di esposizione programmati, i provini sono stati sottoposti ad una pressione causata da un battente geodetico (colonna d'acqua di 500 mm di altezza e 85 mm di diametro). Infine, la valutazione del recupero meccanico ha completato la campagna sperimentale. Le proprietà meccaniche delle miscele sono state analizzate con diverse tecniche, basate principalmente sulla prova a flessione su quattro punti e sul Double Edge Wedge Splitting (DEWS). Questa analisi considera i diversi allineamenti delle fibre indotti dalla direzione del getto, che possono indurre un comportamento di hardening o softening, a seconda che il materiale sia sollecitato parallelamente o perpendicolarmente alle fibre. I risultati ottenuti sono stati confrontati con quelli conseguiti da studi precedenti riguardanti varie composizioni di mix HPFRCC. Questi ultimi, senza aggiunta di nanoallumina oppure con aggiunta di nanocristalli di cellulosa e nanofibre di cellulosa.
Capacità di autoriparazione di compositi cementizi con nanofibre di allumina
D'AMBROSIO, LEONARDO MARIA
2019/2020
Abstract
The present thesis is the result of a thorough experimental investigation on the self-healing capacity of a peculiar mix composition of High Performance Fibre Reinforced Cementitious Composites (HPFRCC), containing nanoadditions of alumina nanofibers (ANF) (0.25% by volume). With the purpose of drawing a comprehensive and accurate analysis of the effect of the crack healing, it has been resorted to an approach that correlates the amount of crack closure with the recovery of mechanical properties. Beam specimens (100 mm wide x 500 mm long x 25 mm thick), pre-cracked in 4-point bending up to a value of crack opening (COD) equal to 0.27 mm, were submitted in two different exposure conditions: geothermal water immersion and exposure in a chamber at constant temperature T = 20 °C and relative humidity RH = 95%. After the scheduled exposure times, ranging from one month to six months, specimens were re-cracked up to the same value of COD, according to the ilk test set up employed for pre-cracking. The first part of the investigations, focused on the capability of the material to completely or partially re-seal the cracks, which is a function of its average crack width and exposure conditions. In this regard, the value of crack closure was obtained via an image analysis processing of the crack observed, by an optical microscopy before and after the healing exposure. This method only enables the evaluation of the healed crack surfaces. Moreover, outcomes of the self-healing phenomenon in the inner parts of the cracks, if any, were analysed in terms of recovery of stiffness, strength and ductility. As a matter of fact, the thesis also aimed at evaluate the autogenous self-healing, both through visual image analysis of the healed cracks and through a permeability test. Disk specimens, pre-cracked in splitting test up to a value of crack opening equal to 0.5 μm, were plunged into geothermal water immersion. After the same scheduled exposure times, specimens have been subjected to a pressure caused by the head of a column of water (500 mm height and 85 mm diameter). Finally, mechanical characterization completed the experimental program. Tensile characterization on the mixes has been performed by different techniques, mainly based on 4-point bending test and Double Edge Wedge Splitting (DEWS). This analysis considers different flow-induced alignments of fibres, which can result into either strain-hardening or softening behaviour, depending respectively on whether the material is stressed parallel or perpendicularly to the fibres. The obtained result of this experimental campaign were compared to those arising from previous studies concerning various HPFRCC mix compositions. In those cases, there is either no presence of nanoadditions, or it is possible to find a presence of different nanoadditions in form of cellulose nanocrystals and cellulose nanofibrils.| File | Dimensione | Formato | |
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Descrizione: Capacità di Autoriparazione di Composti Cementizi con Nanofibre di Allumina
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https://hdl.handle.net/10589/152977