Reinforced concrete is one of the most widespread construction materials in the world, thanks to the high availability of raw materials and excellent mechanical properties. Concrete offers an intrinsic protection to steel reinforcement, preventing corrosion and loss of bearing capacity. Corrosion may anyway occur, during the service life of the structure, mainly as a consequence of the penetration of chlorides and/or carbonation from the environment. The study of corrosive phenomena in concrete over the last few decades has resulted in the development of models predicting the temporal evolution of degradation, which are used during design stage to ensure the durability of the structure and prevent premature failure. However, some issues are still associated to the applicability of these models, such as that they do not consider cracks in concrete, even if these are almost inevitable in real structures, and the reliability in predicting, from short-term analyses, the long-term interactions of concrete with the environment. Moreover, while knowledge on the processes that lead to corrosion initiation are deeper, there are still some open questions and debates on the processes that occur and influence the propagation of corrosion in the long term. In this research, the effects of cracks were studied on different durability-related performances of concrete. Three different cement types were considered, an Ordinary Portland, a Portland-Limestone and a Pozzolanic cement, and two w/c ratios, 0.45 and 0.55. Uncracked and micro-cracked concretes, obtained through a specifically developed loading procedure, were tested. Firstly, results on the effects of cracks on the resistance to chloride penetration will be presented (Chapter 3), considering different concrete types, different exposure times, and different testing procedures, including an accelerated testing procedure, a conventional medium-term test, and simulated tidal exposure conditions. To this part of the research, also long-term testing on uncracked specimens was associated, to evaluate the long-term development of concrete resistance to chloride penetration. Chloride-induced corrosion initiation and propagation were evaluated on reinforced concrete in uncracked and longitudinally microcracked configurations, and results will be presented in Chapter 4. Specimens were reinforced with carbon steel and stainless steel rebar, which can be employed in chloride-bearing environments as additional protection. Corrosion was monitored through electrochemical measurements over two years and a half, and, at the end of exposure, destructive tests were performed to characterize corrosion. The effects of cracks on carbonation penetration resistance were evaluated on uncracked and micro-cracked concretes, subjected to accelerated carbonation at short-term testing. Carbonation-induced corrosion, on the other hand, was evaluated on uncracked and longitudinally micro-cracked concretes, reinforced with carbon steel bars. Part of the specimens were then exposed to natural carbonation under outdoor unsheltered environment for more than one year, to detect corrosion initiation through electrochemical techniques, while the remaining cracked and uncracked specimens were subjected to accelerated carbonation to induce corrosion initiation and monitor corrosion propagation under outdoor unsheltered conditions (results presented in Chapter 5). Finally, in Chapter 6, results on the characterization of long-term natural corrosion propagation will be presented. The specimens analysed for this purpose were designed and cast in 1998 with different cement types, and initially corrosion was induced either by accelerated carbonation or by forcing the penetration of chlorides. They were then left under unsheltered outdoor conditions for more than 20 years, during which corrosion propagated naturally. Corrosion was characterized at multi-scale levels, by a combination of different techniques. Electrochemical measurements were performed, and destructive tests to evaluate chloride content and carbonation depth; subsequently, X-ray computed tomography was performed and image analysis was carried out to visualize the distribution and the extension of corrosion attacks on the rebar, estimate the section loss, the dimension and morphology of corrosive attacks, and the characteristics of the steel-concrete interface. Finally, thin sections were observed through optical microscope, while through the combination of SEM imaging, EDS semiquantitative analyses and Raman spectroscopy it was possible to identify morphological elements and mineralogical compositions of corrosion products.
Il calcestruzzo armato è uno dei materiali da costruzione più diffusi al mondo, grazie all'elevata disponibilità di materie prime e alle eccellenti proprietà meccaniche. Il calcestruzzo offre una protezione intrinseca alle armature in acciaio, prevenendo la corrosione e la perdita di capacità portante. La corrosione può comunque verificarsi, durante la vita utile della struttura, principalmente come conseguenza della penetrazione di cloruri e/o carbonatazione dall'ambiente. Lo studio dei fenomeni corrosivi nel calcestruzzo negli ultimi decenni ha portato allo sviluppo di modelli che prevedono l'evoluzione temporale del degrado, utilizzati in fase di progettazione per garantire la durabilità della struttura e prevenire il collasso. Tuttavia, alcuni problemi sono ancora associati all'applicabilità di questi modelli, come il fatto che non vengono considerati gli effetti delle fessure nel calcestruzzo, anche se queste sono quasi inevitabili nelle strutture reali, e l'affidabilità nel prevedere, da analisi a breve termine, le interazioni a lungo termine del calcestruzzo con l'ambiente. Inoltre, mentre le conoscenze sui processi che portano all'innesco della corrosione sono più approfondite, rimangono ancora aperti alcuni dibattiti sui processi che influenzano la propagazione della corrosione a lungo termine. In questa ricerca sono stati studiati gli effetti delle fessure su diverse prestazioni del calcestruzzo legate alla durabilità. Sono stati presi in considerazione tre diversi tipi di cemento (Portland, Portland al calcare e pozzolanico), e due rapporti acqua/cemento (0.45 e 0.55). Sono stati analizzati calcestruzzi non fessurati e micro-fessurati, ottenuti attraverso una procedura di carico appositamente sviluppata. In primo luogo, vengono presentati i risultati relativi agli effetti delle fessure sulla resistenza alla penetrazione dei cloruri (Capitolo 3), considerando diversi tipi di calcestruzzo, diversi tempi di esposizione e diversi metodi di prova, tra cui una prova accelerata, una prova convenzionale di diffusione a medio termine, ed esposizione a cicli di asciutto/bagnato. A questa parte della ricerca sono state associate anche prove a lungo termine su provini non fessurati, per valutare lo sviluppo a lungo termine della resistenza del calcestruzzo alla penetrazione dei cloruri. L'innesco e la propagazione della corrosione indotta dai cloruri sono stati valutati su calcestruzzo armato in configurazione non fessurata e micro-fessurata in direzione longitudinale all’armatura, e i risultati vengono presentati nel Capitolo 4. I campioni sono stati rinforzati con armature in acciaio al carbonio e in acciaio inossidabile, che possono essere impiegate in ambienti con presenza di cloruri come protezione aggiuntiva. La corrosione è stata monitorata attraverso misure elettrochimiche per due anni e mezzo e, al termine dell'esposizione, sono state eseguite prove distruttive per caratterizzare la corrosione. Gli effetti delle fessure sulla resistenza alla penetrazione della carbonatazione sono stati valutati su calcestruzzi non fessurati e micro-fessurati, sottoposti a carbonatazione accelerata. La corrosione indotta dalla carbonatazione, invece, è stata valutata su calcestruzzi non fessurati e micro-fessurati longitudinalmente, rinforzati con barre di acciaio al carbonio. Una parte dei provini è stata esposta alla carbonatazione naturale in ambiente esterno non riparato per più di un anno, per rilevare l'innesco della corrosione attraverso tecniche elettrochimiche, mentre i restanti provini fessurati e non fessurati sono stati sottoposti a carbonatazione accelerata per indurre l'innesco della corrosione e monitorare la propagazione della corrosione in ambiente esterno non riparato (i risultati sono presentati nel Capitolo 5). Infine, nel Capitolo 6, vengono presentati i risultati relativi alla caratterizzazione della propagazione naturale della corrosione a lungo termine. I provini di calcestruzzo armato erano stati progettati e gettati nel 1998 con diversi tipi di cemento, e inizialmente la corrosione era stata indotta da carbonatazione accelerata o da esposizione ai cloruri. Sono poi stati lasciati in condizioni di esposizione esterne non riparate per più di 20 anni, durante i quali la corrosione ha propagato naturalmente. In questa ricerca, la corrosione è stata caratterizzata su tre diversi livelli, mediante una combinazione di diverse tecniche di indagine. In particolare, sono state eseguite misure elettrochimiche e prove distruttive per valutare il contenuto di cloruri e la profondità di carbonatazione (livello macroscopico). Successivamente, è stata eseguita la tomografia computerizzata a raggi X e l'analisi delle immagini ha permesso di visualizzare la distribuzione e l'estensione degli attacchi corrosivi sulle armature, di stimare la perdita di sezione, la dimensione e la morfologia degli attacchi corrosivi e le caratteristiche dell'interfaccia acciaio-calcestruzzo (livello mesoscopico). Infine, le sezioni sottili sono state osservate al microscopio ottico, mentre attraverso l’acquisizione di immagini tramite SEM, analisi SEM-EDS e tramite spettroscopia Raman, è stato possibile identificare elementi morfologici e composizioni mineralogiche dei prodotti di corrosione (livello microscopico).
Effects of cracks and long-term corrosion propagation on reinforced concrete structures durability
RUSSO, NICOLETTA
2021/2022
Abstract
Reinforced concrete is one of the most widespread construction materials in the world, thanks to the high availability of raw materials and excellent mechanical properties. Concrete offers an intrinsic protection to steel reinforcement, preventing corrosion and loss of bearing capacity. Corrosion may anyway occur, during the service life of the structure, mainly as a consequence of the penetration of chlorides and/or carbonation from the environment. The study of corrosive phenomena in concrete over the last few decades has resulted in the development of models predicting the temporal evolution of degradation, which are used during design stage to ensure the durability of the structure and prevent premature failure. However, some issues are still associated to the applicability of these models, such as that they do not consider cracks in concrete, even if these are almost inevitable in real structures, and the reliability in predicting, from short-term analyses, the long-term interactions of concrete with the environment. Moreover, while knowledge on the processes that lead to corrosion initiation are deeper, there are still some open questions and debates on the processes that occur and influence the propagation of corrosion in the long term. In this research, the effects of cracks were studied on different durability-related performances of concrete. Three different cement types were considered, an Ordinary Portland, a Portland-Limestone and a Pozzolanic cement, and two w/c ratios, 0.45 and 0.55. Uncracked and micro-cracked concretes, obtained through a specifically developed loading procedure, were tested. Firstly, results on the effects of cracks on the resistance to chloride penetration will be presented (Chapter 3), considering different concrete types, different exposure times, and different testing procedures, including an accelerated testing procedure, a conventional medium-term test, and simulated tidal exposure conditions. To this part of the research, also long-term testing on uncracked specimens was associated, to evaluate the long-term development of concrete resistance to chloride penetration. Chloride-induced corrosion initiation and propagation were evaluated on reinforced concrete in uncracked and longitudinally microcracked configurations, and results will be presented in Chapter 4. Specimens were reinforced with carbon steel and stainless steel rebar, which can be employed in chloride-bearing environments as additional protection. Corrosion was monitored through electrochemical measurements over two years and a half, and, at the end of exposure, destructive tests were performed to characterize corrosion. The effects of cracks on carbonation penetration resistance were evaluated on uncracked and micro-cracked concretes, subjected to accelerated carbonation at short-term testing. Carbonation-induced corrosion, on the other hand, was evaluated on uncracked and longitudinally micro-cracked concretes, reinforced with carbon steel bars. Part of the specimens were then exposed to natural carbonation under outdoor unsheltered environment for more than one year, to detect corrosion initiation through electrochemical techniques, while the remaining cracked and uncracked specimens were subjected to accelerated carbonation to induce corrosion initiation and monitor corrosion propagation under outdoor unsheltered conditions (results presented in Chapter 5). Finally, in Chapter 6, results on the characterization of long-term natural corrosion propagation will be presented. The specimens analysed for this purpose were designed and cast in 1998 with different cement types, and initially corrosion was induced either by accelerated carbonation or by forcing the penetration of chlorides. They were then left under unsheltered outdoor conditions for more than 20 years, during which corrosion propagated naturally. Corrosion was characterized at multi-scale levels, by a combination of different techniques. Electrochemical measurements were performed, and destructive tests to evaluate chloride content and carbonation depth; subsequently, X-ray computed tomography was performed and image analysis was carried out to visualize the distribution and the extension of corrosion attacks on the rebar, estimate the section loss, the dimension and morphology of corrosive attacks, and the characteristics of the steel-concrete interface. Finally, thin sections were observed through optical microscope, while through the combination of SEM imaging, EDS semiquantitative analyses and Raman spectroscopy it was possible to identify morphological elements and mineralogical compositions of corrosion products.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/190573