ReSHEALience (Rethinking coastal defence and green-energy Service infrastructures through enHancEd-durAbiLity high-performance cement-based materials) is a research project funded/supported by the European Commission coordinated by Politecnico di Milano which includes the present thesis. The main purpose of the project is to study the long-term behavior of structures built in high-performance cement-based materials, also known as UHPFRC (Ultra High Performance Concrete), when exposed to highly aggressive environments. This document focuses on one of the six pilot projects, the study of a Mix Design of fiber reinforced concrete used in order to build tanks containing the cooling water used in the towers of geothermic systems. Such study has been developed by analyzing 42 tests, 20 of which are thin beam and 22 deep beam. The choice of these elements was based on the information the team needed to collect. The thin and deep beam were therefore exposed to different kind of tests. The monotonic flexural and tensile tests were carried out to characterize the mechanicals of the material, whereas the precracking tests aimed at evaluating its self-healing skills. Sustainability plays a crucial role in several fields. For instance, in the area of materials technology this idea is strictly connected to the one of durability, which can be defined as the preservation of the physical and mechanical properties and of the structures in time. The improvement of such characteristic in a specific material clearly implies a prolongation of the life cycle (non sono certa si dica così) of the structures built with said material. It also entails a reduction in the maintenance, which could be expensive both from an energetic and from an economic perspective. Durability must not be seen as a plus, it is instead the aim to reach and to control. It is essential to make solid assessments regarding this idea and to adopt a holistic approach. Concrete is subject to several deterioration mechanisms, mainly caused by the opening of cracks which allow aggressive species to penetrate inside the material, causing a damage. The self-healing of the cracks, which happens thanks to a reaction of delayed hydration and calcium carbonate precipitation, is an efficient and innovative solution to this kind of problem. Specific parameters and indices – such as index of crack healing, damage, thoughness and stress recovery- have been introduced in the present work in order to calculate numerically the phenomenon in question. As noted in the indirect tensile tests, the addition of fibers to the mix – 1,54% in its volume- makes the mechanical behavior of the material strain hardening. This situation occurs when fiber is considered in parallel direction to the stress. With regard to the self-healing of the material, it is important to highlight that after a month of exposure to geothermic water a partial healing took place.
ReSHEALience (Rethinking coastal defence and green-energy Service infrastructures through enHancEd-durAbiLity high-performance cement-based materials) è il progetto di ricerca finanziato dalla Commissione Europea e coordinato dal Politecnico di Milano in cui si inserisce il presente elaborato. Tale progetto si pone come principale obiettivo lo studio del comportamento a lungo termine di strutture realizzate attraverso l’utilizzo di materiali cementizi ad elevate prestazioni noti con l’acronimo di UHPFRC (Ultra High Performance Concrete) esposte in ambienti particolarmente aggressivi. Il focus di questo documento è uno dei sei progetti pilota, ovvero lo studio di un Mix Design di calcestruzzo fibrorinforzato per la realizzazione di vasche contenenti l’acqua di raffreddamento delle torri di impianti geotermici. (1) Tale studio è stato sviluppato analizzando 42 provini, di cui 20 travi sottili, e 22 travetti (2) selezionati in funzione delle informazioni da ottenere, sottoposti in seguito a diversi tipi di prova. Le prove monotone a flessione e a trazione indiretta sono state effettuate per eseguire la caratterizzazione meccanica, quelle di prefessurazione e di rifessurazione per valutare la capacità di riparazione autogena. Il tema della sostenibilità sta acquisendo un’importanza sempre crescente. Nell’ambito della tecnologia dei materiali tale concetto è indissolubilmente legato a quello di durabilità, definita come la conservazione delle caratteristiche fisiche e meccaniche dei materiali e delle strutture nel tempo (3). È evidente come al migliorare di tale caratteristica in un materiale corrispondano un allungamento del ciclo di vita delle strutture realizzate con tali materiali e la diminuzione degli interventi di manutenzione necessari, che risultano dispendiosi in termini energetici ed economici. La durabilità non deve esser vista come un ‘bonus’ ma l’obiettivo da controllare; risulta altresì fondamentale effettuare solide valutazioni relative a tale concetto, adottando un approccio olistico. (1) Il calcestruzzo è soggetto a numerosi meccanismi di degrado causati soprattutto dall’apertura di fessure che consentono alle specie aggressive di penetrare all’interno del materiale danneggiandolo. La capacità di riparazione autogena delle fessure, dovuta a reazione di idratazione ritardata e precipitazione del carbonato di calcio, è un’efficace ed innovativa soluzione a questo problema. Per valutare numericamente tale fenomeno sono stati introdotti, nello sviluppo dell’elaborato, parametri ed indici ad hoc quali l’indice di richiusura percentuale delle fessure, gli indici di recupero di resistenza, danno e tenacità. Come osservato dalle prove a trazione indiretta, se valutato in direzione parallela allo sforzo, l’aggiunta di fibre all’impasto (1,54% in volume) rende il comportamento meccanico del materiale di tipo incrudente. Per quanto riguarda la capacità di riparazione autogena del materiale si è notato che, a seguito di un mese di esposizione in acqua geotermica, vi è innanzitutto una richiusura parziale delle fessure. Inoltre si è ottenuto un recupero parziale delle proprietà meccaniche.
Caratterizzazione meccanica e capacita di riparazione autogena di compositi cementizi fibrorinforzati ad elevate prestazioni (UHPFRC)
PASSONI, STEFANO;PAPA, LORENZO
2017/2018
Abstract
ReSHEALience (Rethinking coastal defence and green-energy Service infrastructures through enHancEd-durAbiLity high-performance cement-based materials) is a research project funded/supported by the European Commission coordinated by Politecnico di Milano which includes the present thesis. The main purpose of the project is to study the long-term behavior of structures built in high-performance cement-based materials, also known as UHPFRC (Ultra High Performance Concrete), when exposed to highly aggressive environments. This document focuses on one of the six pilot projects, the study of a Mix Design of fiber reinforced concrete used in order to build tanks containing the cooling water used in the towers of geothermic systems. Such study has been developed by analyzing 42 tests, 20 of which are thin beam and 22 deep beam. The choice of these elements was based on the information the team needed to collect. The thin and deep beam were therefore exposed to different kind of tests. The monotonic flexural and tensile tests were carried out to characterize the mechanicals of the material, whereas the precracking tests aimed at evaluating its self-healing skills. Sustainability plays a crucial role in several fields. For instance, in the area of materials technology this idea is strictly connected to the one of durability, which can be defined as the preservation of the physical and mechanical properties and of the structures in time. The improvement of such characteristic in a specific material clearly implies a prolongation of the life cycle (non sono certa si dica così) of the structures built with said material. It also entails a reduction in the maintenance, which could be expensive both from an energetic and from an economic perspective. Durability must not be seen as a plus, it is instead the aim to reach and to control. It is essential to make solid assessments regarding this idea and to adopt a holistic approach. Concrete is subject to several deterioration mechanisms, mainly caused by the opening of cracks which allow aggressive species to penetrate inside the material, causing a damage. The self-healing of the cracks, which happens thanks to a reaction of delayed hydration and calcium carbonate precipitation, is an efficient and innovative solution to this kind of problem. Specific parameters and indices – such as index of crack healing, damage, thoughness and stress recovery- have been introduced in the present work in order to calculate numerically the phenomenon in question. As noted in the indirect tensile tests, the addition of fibers to the mix – 1,54% in its volume- makes the mechanical behavior of the material strain hardening. This situation occurs when fiber is considered in parallel direction to the stress. With regard to the self-healing of the material, it is important to highlight that after a month of exposure to geothermic water a partial healing took place.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/146969