Reinforced concrete (RC) has long been considered an intrinsically durable material. Only since the ‘80, when faced with obvious cases of deterioration, safety risks and increasingly high maintenance costs, they have begun to understand the importance of coping with its deterioration. It has therefore begun to study the phenomena that cause its degradation and the techniques of prevention, construction and management of the structures. There are numerous factors that can determine a progressive deterioration of the structure and can affect the concrete and the reinforcement. Corrosion of reinforcement induced by chloride is one of the main durability problems affecting RC structures. Different models have been developed to evaluate the durability in the design phase, as regards the prevention of corrosion in environments contaminated by chlorides, but refer mainly to non-cracked concrete. The cracks, however, are often present in RC structures and accelerate the penetration of chlorides with the consequent reduction in the useful life of the structure. This study, conducted at the Department of Chemistry, Materials and Chemical Engineering "Giulio Natta" of the Politecnico di Milano, deals with the effect of cracks on the durability of reinforced concrete in an environment contaminated by chlorides. First, the resistance to chloride penetration was evaluated in different types of cracked and non-cracked concrete, made with Portland cement, Portland limestone and pozzolanic concrete, using two distinct water / cement ratios and different curing times. Two chloride exposure tests were carried out on prismatic samples of 120x90x50 mm size: an accelerated Rapid Chloride Migration test (RCM) and an immersion test for 32 and 90 days. Secondly, the time of priming of corrosion was studied on samples reinforced with carbon and stainless steel, made with Portland cement with limestone. The samples were made and cracked at the ITC-CNR of San Giuliano Milanese. The load-induced micro-cracks were obtained with a specifically developed technique and characterized by a surface opening of between 7 μm and 70 μm and a penetration depth ranging from 1 mm to 35 mm. In the first chapter, after an introduction to the main causes of degradation of reinforced concrete, the experimental techniques present in the literature for cracking concrete and for exposure to chlorides are illustrated and compared. The description of the materials used, of the cracking methods and of the exposure tests, performed during this thesis work, are reported in the second chapter. Chapter 3 reports the mechanical characterization of the concrete and the characterization of the cracks. Next, the data collected during the entire period of experimental tests and their elaboration are reported, in order to evaluate and quantify the effect of the cracks on chloride penetration resistance, in chapter 4, and on the triggering time, in the chapter 5.
Il calcestruzzo armato (CA) è stato a lungo considerato un materiale intrinsecamente durevole. Solo a partire dagli anni Ottanta, dinanzi ad evidenti casi di degrado, a rischi per la sicurezza ed a costi di manutenzione sempre più alti, si è iniziato a comprendere l’importanza di far fronte al suo deterioramento. Si è cominciato quindi a studiare i fenomeni che ne causano il degrado e le tecniche di prevenzione, costruzione e gestione delle strutture. Sono numerosi i fattori che possono determinare un progressivo deterioramento della struttura e possono interessare il calcestruzzo e o le armature. La corrosione dell’armatura di rinforzo del calcestruzzo armato indotta dai cloruri è uno dei principali problemi di durabilità che colpiscono le strutture in CA. Sono stati sviluppati diversi modelli per valutare la durabilità in fase di progetto, per quanto riguarda la prevenzione della corrosione in ambienti contaminati da cloruri, e si riferiscono principalmente al calcestruzzo non fessurato. Le fessure, tuttavia, sono spesso presenti nelle strutture in CA e accelerano la penetrazione dei cloruri con la conseguente riduzione della vita utile della struttura. Questo studio, condotto presso il Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta” del Politecnico di Milano, tratta l’effetto delle fessure sulla durabilità del calcestruzzo armato in ambiente contaminato da cloruri. In primo luogo, la resistenza alla penetrazione dei cloruri è stata valutata in diversi tipi di calcestruzzo fessurato e non, realizzati con cemento Portland, Portland al calcare e pozzolanico, utilizzando due distinti rapporti acqua/cemento e diversi tempi di stagionatura. Sono state condotte, su campioni prismatici di dimensione 120x90x50 mm, due prove di esposizione ai cloruri: una prova accelerata di migrazione in cella inclinata ed una di immersione per 32 e 90 giorni. In secondo luogo, è stato studiato il tempo di innesco della corrosione su campioni armati con acciaio al carbonio e inossidabile, confezionati con cemento portland al calcare. I campioni sono stati confezionati e fessurati all’ITC-CNR di San Giuliano Milanese. Le micro-fessure, indotte da carico, sono state ottenute con una tecnica specificamente sviluppata e caratterizzate da un’apertura superficiale compresa tra i 7 μm e i 70 μm e da una profondità di penetrazione che varia da 1 mm a 35 mm. Nel primo capitolo, dopo un’introduzione alle principali cause di degrado del calcestruzzo armato, vengono illustrate e confrontate le tecniche sperimentali, presenti in letteratura, per la fessurazione del calcestruzzo e per l’esposizione ai cloruri. La descrizione dei materiali utilizzati, delle metodologie di fessurazione e delle prove di esposizione, eseguite nel corso di questo lavoro di tesi, sono riportate nel secondo capitolo. Il capitolo 3 riporta la caratterizzazione meccanica dei calcestruzzi e la caratterizzazione delle fessure. A seguire, sono riportati i dati raccolti durante l’intero periodo di prove sperimentali e la loro elaborazione, al fine di valutare e quantificare l’effetto delle fessure sulla resistenza alla penetrazione dei cloruri, nel capitolo 4, e sul tempo di innesco, nel capitolo 5.
Effetto delle fessure sulla durabilità di calcestruzzi armati in ambiente contaminato da cloruri
POZZATO, EDOARDO
2018/2019
Abstract
Reinforced concrete (RC) has long been considered an intrinsically durable material. Only since the ‘80, when faced with obvious cases of deterioration, safety risks and increasingly high maintenance costs, they have begun to understand the importance of coping with its deterioration. It has therefore begun to study the phenomena that cause its degradation and the techniques of prevention, construction and management of the structures. There are numerous factors that can determine a progressive deterioration of the structure and can affect the concrete and the reinforcement. Corrosion of reinforcement induced by chloride is one of the main durability problems affecting RC structures. Different models have been developed to evaluate the durability in the design phase, as regards the prevention of corrosion in environments contaminated by chlorides, but refer mainly to non-cracked concrete. The cracks, however, are often present in RC structures and accelerate the penetration of chlorides with the consequent reduction in the useful life of the structure. This study, conducted at the Department of Chemistry, Materials and Chemical Engineering "Giulio Natta" of the Politecnico di Milano, deals with the effect of cracks on the durability of reinforced concrete in an environment contaminated by chlorides. First, the resistance to chloride penetration was evaluated in different types of cracked and non-cracked concrete, made with Portland cement, Portland limestone and pozzolanic concrete, using two distinct water / cement ratios and different curing times. Two chloride exposure tests were carried out on prismatic samples of 120x90x50 mm size: an accelerated Rapid Chloride Migration test (RCM) and an immersion test for 32 and 90 days. Secondly, the time of priming of corrosion was studied on samples reinforced with carbon and stainless steel, made with Portland cement with limestone. The samples were made and cracked at the ITC-CNR of San Giuliano Milanese. The load-induced micro-cracks were obtained with a specifically developed technique and characterized by a surface opening of between 7 μm and 70 μm and a penetration depth ranging from 1 mm to 35 mm. In the first chapter, after an introduction to the main causes of degradation of reinforced concrete, the experimental techniques present in the literature for cracking concrete and for exposure to chlorides are illustrated and compared. The description of the materials used, of the cracking methods and of the exposure tests, performed during this thesis work, are reported in the second chapter. Chapter 3 reports the mechanical characterization of the concrete and the characterization of the cracks. Next, the data collected during the entire period of experimental tests and their elaboration are reported, in order to evaluate and quantify the effect of the cracks on chloride penetration resistance, in chapter 4, and on the triggering time, in the chapter 5.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/150818