Since the 1980s, the importance of studying the durability of reinforced concrete (RC) structures has been recognised, as a consequence of numerous cases of deterioration that have led to safety risks and high maintenance and restoration costs. Different causes can promote the progressive damage of a reinforced concrete structure, including the action of the environment, which can be physical (e.g. the action of temperature), biological, mechanical (e.g. the action of the loads applied on the structure) or chemical (presence of aggressive substances in the environment). The experience showed that the most common cause of deterioration of RC structures is the corrosion of the reinforcement, which can be induced by the presence of chlorides present in the environment. In order to design a structure to withstand the actions of the environment for a pre-defined service life, several models have been developed that study the evolution in time of the degradation phenomena. Most of these are based on the assumption that concrete is uncracked, that is a situation that rarely occur in practice, since cracks are often present in RC structures. In recent years, various studies have been carried out to evaluate the effects of cracks on service life, i.e. on corrosion initiation and propagation time. Although the studies generally agree that the presence of cracks can promote the penetration of aggressive substances, such as chlorides. These studies were carried out using different cracking methods that cause cracks significantly different, concretes with different compositions and different tests methods to assess the resistance to chloride penetration and the effects of crack on corrosion and propagation time. These differences therefore make it difficult to draw a general conclusions. This study, conducted at Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta” of Politecnico di Milano, focuses on the effects of cracks on the durability of RC structures in chloride-contaminated environments. Prismatic samples, with dimensions of 120x90x50 mm, and cubic samples, 100 mm side dimension, were made with different types of cement (Portland, Portland-limestone and Pozzolanic) and different water/cement ratios (0.45 and 0.55). Unreinforced and reinforced prismatic specimens (with carbon steel and 304L stainless steel bars) were made. Part of the prismatic specimens were subjected to a specific cracking procedure, to obtain load-induced micro-cracks, 10÷70 µm wide and 8÷47 mm deep. Specimens were cast and cracked at ITC-CNR in San Giuliano Milanese. The cubic samples were used to assess the compressive strength of the concrete after different curing times. The uncracked prismatic samples, at the end of curing time, were exposed to chlorides by immersion for different exposure times (32, 90 and in some cases 180 days). The resistance to chloride penetration was evaluated using two different methods, colorimetric analysies and potentiometric titration analysis. The reinforced samples were exposed to a solution containing chlorides and monitored over time by electrochemical measurements to assess the effects of cracks on corrosion. In this thesis, in the first chapter, after a general overview of the main causes of RC structures deterioration, part of the literature concerning the effects of cracks on durability is analysed, reporting the main cracking procedures and exposure tests to chlorides, and the main results obtained. The description of the materials used, the methodology followed for the cracking of the samples as well as the tests carried out, is exposed in the second chapter. The third chapter reports the mechanical characterization of the concrete and the cracks obtained, considering crack width and depth as geometrical parameters. Finally, in chapter 4, the results obtained from the experimental tests are reported and discussed, evaluating in particular the effects of the cracks on chloride penetration resistance of concrete and corrosion of steel reinforcement.
A partire dagli anni Ottanta, si è compresa l’importanza di studiare la durabilità delle strutture in calcestruzzo armato (c.a.), a seguito di numerosi casi di degrado che hanno comportato rischi per la sicurezza, nonché costi elevati per manutenzione e restauro delle opere. Diversi sono i fattori che causano un progressivo danneggiamento di una struttura in c.a.; fra le cause troviamo l’azione dell’ambiente che può essere di natura fisica (ad esempio l’azione della temperatura), biologica o meccanica (ad esempio l’azione dei carichi applicati sulla struttura) o chimica (presenza di sostanze aggressive nell’ambiente). L’esperienza mostra che la causa più ricorrente di degrado delle opere in c.a. è rappresentata dalla corrosione delle armature, che può essere indotta dalla presenza di cloruri nell’ambiente. Al fine di progettare una struttura perché resista per una ben definita vita di servizio alle azioni dell’ambiente, sono stati sviluppati diversi modelli che studiano l’evoluzione del degrado nel tempo. La maggior parte di questi parte dall’ipotesi che il calcestruzzo sia privo di fessure, una condizione che accade tuttavia raramente nella pratica. Negli ultimi anni sono stati realizzati diversi studi per valutare l’effetto delle fessure sulla vita di servizio, ovvero sul tempo di innesco e propagazione della corrosione. Sebbene in generale gli studi concordino sul fatto che la presenza di fessure possa favorire la penetrazione di sostanze aggressive, come i cloruri, a causa della notevole. Tuttavia, tali ricerche sono state svolte utilizzando metodologie di fessurazione diverse che hanno portato a fessure anche notevolmente differenti, calcestruzzi con differente composizione e prove diverse per valutare la resistenza alla penetrazione dei cloruri e gli effetti su tempo di innesco e propagazione. Queste diversità rendono quindi difficile trarre conclusioni univoche riguardo gli effetti delle fessure, e dei relativi parametri geometrici, sulla durabilità. Questo studio, condotto presso il Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta” del Politecnico di Milano, è incentrato sull’effetto delle fessure sulla durabilità delle strutture in c.a. in ambiente contaminato da cloruri. Sono stati realizzati campioni prismatici di dimensioni 120x90x50 mm e campioni cubici di lato 100 mm, confezionati con calcestruzzo avente differente tipo di cemento (Portland, Portland-calcare e Pozzolanico) e differenti rapporti acqua/cemento (0,45 e 0,55). Sono stati confezionati sia provini prismatici non armati, che armati con barre in acciaio al carbonio e in acciaio inossidabile 304L. Parte dei campioni prismatici sono stati sottoposti a fessurazione ricorrendo ad una particolare tecnica, che ha consentito di ottenere micro-fessure indotte da carico. Il confezionamento e la fessurazione dei provini sono stati realizzati presso l’ITC-CNR di San Giuliano Milanese. I campioni cubici sono stati utilizzati per valutare la resistenza alla compressione dei calcestruzzi dopo diversi tempi di stagionatura, mentre i campioni prismatici non armati sono stati, al termine della stagionatura, esposti ai cloruri ricorrendo ad una prova di immersione, per diversi periodi di tempo (32, 90 e in alcuni casi 180 giorni). La resistenza alla penetrazione dei cloruri è stata valutata con due differenti metodi, analisi colorimetrica e analisi per titolazione potenziometrica. I campioni armati sono stati esposti ad una soluzione contenente cloruri e monitorati nel tempo attraverso misure elettrochimiche, per valutare l’effetto delle fessure sull’innesco e la propagazione della corrosione. In questa tesi, nel primo capitolo, dopo un inquadramento generale sulle principali cause di degrado delle strutture in c.a., viene analizzata parte della letteratura riguardante l’effetto delle fessure sulla durabilità, riportando le principali tecniche di fessurazione e di esposizione ai cloruri, ed i relativi risultati ottenuti. La descrizione dei materiali utilizzati, la metodologia seguita per la fessurazione dei campioni nonché per lo svolgimento delle prove condotte, viene trattata all’interno del secondo capitolo. Nel terzo capitolo viene riportata la caratterizzazione meccanica dei calcestruzzi e delle fessure ottenute, considerando come parametri geometrici l’apertura superficiale e la profondità della fessura. Infine, nel capitolo 4, vengono riportati e discussi i risultati ottenuti dalle prove sperimentali, valutando, in particolar modo, l’effetto della fessura sulla penetrazione dei cloruri e sulla corrosione delle armature.
Studio della durabilità di calcestruzzi armati in ambiente contaminato da cloruri e in relazione alla presenza di fessure
FILIPPI, ANDREA
2019/2020
Abstract
Since the 1980s, the importance of studying the durability of reinforced concrete (RC) structures has been recognised, as a consequence of numerous cases of deterioration that have led to safety risks and high maintenance and restoration costs. Different causes can promote the progressive damage of a reinforced concrete structure, including the action of the environment, which can be physical (e.g. the action of temperature), biological, mechanical (e.g. the action of the loads applied on the structure) or chemical (presence of aggressive substances in the environment). The experience showed that the most common cause of deterioration of RC structures is the corrosion of the reinforcement, which can be induced by the presence of chlorides present in the environment. In order to design a structure to withstand the actions of the environment for a pre-defined service life, several models have been developed that study the evolution in time of the degradation phenomena. Most of these are based on the assumption that concrete is uncracked, that is a situation that rarely occur in practice, since cracks are often present in RC structures. In recent years, various studies have been carried out to evaluate the effects of cracks on service life, i.e. on corrosion initiation and propagation time. Although the studies generally agree that the presence of cracks can promote the penetration of aggressive substances, such as chlorides. These studies were carried out using different cracking methods that cause cracks significantly different, concretes with different compositions and different tests methods to assess the resistance to chloride penetration and the effects of crack on corrosion and propagation time. These differences therefore make it difficult to draw a general conclusions. This study, conducted at Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta” of Politecnico di Milano, focuses on the effects of cracks on the durability of RC structures in chloride-contaminated environments. Prismatic samples, with dimensions of 120x90x50 mm, and cubic samples, 100 mm side dimension, were made with different types of cement (Portland, Portland-limestone and Pozzolanic) and different water/cement ratios (0.45 and 0.55). Unreinforced and reinforced prismatic specimens (with carbon steel and 304L stainless steel bars) were made. Part of the prismatic specimens were subjected to a specific cracking procedure, to obtain load-induced micro-cracks, 10÷70 µm wide and 8÷47 mm deep. Specimens were cast and cracked at ITC-CNR in San Giuliano Milanese. The cubic samples were used to assess the compressive strength of the concrete after different curing times. The uncracked prismatic samples, at the end of curing time, were exposed to chlorides by immersion for different exposure times (32, 90 and in some cases 180 days). The resistance to chloride penetration was evaluated using two different methods, colorimetric analysies and potentiometric titration analysis. The reinforced samples were exposed to a solution containing chlorides and monitored over time by electrochemical measurements to assess the effects of cracks on corrosion. In this thesis, in the first chapter, after a general overview of the main causes of RC structures deterioration, part of the literature concerning the effects of cracks on durability is analysed, reporting the main cracking procedures and exposure tests to chlorides, and the main results obtained. The description of the materials used, the methodology followed for the cracking of the samples as well as the tests carried out, is exposed in the second chapter. The third chapter reports the mechanical characterization of the concrete and the cracks obtained, considering crack width and depth as geometrical parameters. Finally, in chapter 4, the results obtained from the experimental tests are reported and discussed, evaluating in particular the effects of the cracks on chloride penetration resistance of concrete and corrosion of steel reinforcement.| File | Dimensione | Formato | |
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STUDIO DELLA DURABILITA’ DI CALCESTRUZZI ARMATI IN AMBIENTE CONTAMINATO DA CLORURI E IN RELAZIONE ALLA PRESENZA DI FESSURE.pdf
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https://hdl.handle.net/10589/166425