Influence of cracks and spacers on chloride penetration and reinforcement corrosion in concrete

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chloride penetration and reinforcement corrosion in concrete

Øyvind Strømme

Civil and Environmental Engineering Supervisor: Mette Rica Geiker, KT

Co-supervisor: Tobias Alexander Danner, IBM Karla Hornbostel, Statens Vegvesen

Department of Structural Engineering Submission date: June 2017

Norwegian University of Science and Technology

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NTNU- Norwegian University of Science and Technology

MASTER THESIS 2017

SUBJECT AREA:

Concrete Technology

DATE:

11.06.2017

NO. OF PAGES:

11 + 54 + 51

TITLE:

Influence of cracks and spacers on chloride penetration and reinforcement corrosion in concrete

Effekten av riss og avstandsholdere på kloridinntrengning og armeringskorrosjon i betong

BY:

Øyvind Strømme

RESPONSIBLE TEACHER: Mette Rica Geiker

SUPERVISOR(S): Mette Rica Geiker, Tobias Alexander Danner (NTNU) and Karla Hornbostel (NPRA) CARRIED OUT AT: Department of Structural Engineering

SUMMARY:

This thesis documents the influence of cracks and spacers on chloride penetration and reinforcement corrosion in concrete. The study was conducted in connection with the Norwegian Public Road Authorities’ (NPRA) E39 project, WP 7.1.1, wherein NTNU is collecting data regarding the extent of reinforcement corrosion in the vicinity of cracks in field exposed concrete structures. The project was partially funded by the NPRA.

The study was performed on the middle parts of four concrete columns that NTNU had access to through NPRA. The concrete columns were located at marine field station established by DNV GL in 1983. At least until the end of 1992, the columns were dynamically loaded to simulate wave action. The columns were taken out of the water in 2016. The parts of the columns investigated were exposed in the tidal zone for more than 30 years.

Electrochemical potential measurements were performed on the parts of the columns at NTNU in the autumn 2016, and were repeated for this study. Chloride penetration was examined by the use of silver nitrate and µXRF-scans on cores retrieved from the columns. Observations within the individual cores are documented, and comparisons are made between uncracked cores, cracked cores and cores containing spacers. Pieces of rebars were later excavated from the cores, and the extent of corrosion was visually assessed.

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Institutt for konstruksjonsteknikk

Fakultet for ingeniørvitenskap og teknologi

NTNU - Norges teknisk- naturvitenskapelige universitet

MASTEROPPGAVE 2017

FAGOMRÅDE:

Betongteknologi

DATO:

11.06.2017

ANTALL SIDER:

11 + 54 + 51

TITLE:

Effekten av riss og avstandsholdere på kloridinntrengning og armeringskorrosjon i betong

Influence of cracks and spacers on chloride penetration and reinforcement corrosion in concrete

BY:

Øyvind Strømme

FAGLÆRER: Mette Rica Geiker

VEILEDER(E): Mette Rica Geiker, Tobias Alexander Danner (NTNU) og Karla Hornbostel (Statens vegvesen)

UTFØRT VED: Institutt for konstruksjonsteknikk SAMMENDRAG:

Denne oppgaven tar for seg effekten av riss og avstandsholdere på kloridinntrengning og armeringskorrosjon i betong. Studien ble utført i tilknytning til Statens vegvesens E39-prosjekt, WP 7.1.1, der NTNU er med på å samle data om omfanget av armeringskorrosjon i nærheten av riss i felteksponerte betongkonstruksjoner. Oppgaven ble delvis finansiert av Statens vegvesen.

Undersøkelsene i denne studien ble utført på midtre del avfire betongsøyler som NTNU hadde tilgang til gjennom Statens vegvesen. Søylene hadde vært plassert ved en marin feltstasjon etablert av DNV GL i 1983. I alle fall inntil slutten av 1992, ble søylene dynamisk belastet for å simulere bølgevirkninger.

Søylene ble tatt opp av vannet i 2016. Søyledelene som undersøkes i denne oppgaven var eksponert i tidevannssonen i over 30 år.

Målinger av elektrokjemisk potensial ble utført på søyledelene ved NTNU høsten 2016, og ble gjentatt under denne oppgaven. Kloridinnhold ble undersøkt ved bruk av sølvnitrat og µXRF-skanninger på prøvestykker som ble boret ut av søylene. I oppgaven dokumenters observasjoner innad i de enkelte prøvestykkene, og prøvestykkene uten riss, med riss og med avstandsholdere sammenlignes. Deler av armeringsstålet ble senere gravd ut fra prøvestykkene, og omfanget av korrosjon ble undersøkt visuelt.

TILGJENGELIGHET ÅPEN

ÅPEN

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Preface

This master thesis is the finalization of my studies at the master’s programme Civil and

Environmental Engineering at the Norwegian University of Technology and Science (NTNU). The thesis was written at the Department of Structural Engineering, during a period of 20 weeks, from January 12^th, 2017 to June 11^th, 2017.

I would like to thank my supervisor Mette Geiker, Karla Hornbostel and Tobias Danner for all help and encouragement. All three were always ready to provide professional insight and support whenever needed, even outside regular working hours. Tobias Danner also deserves thanks for the many hours we spent together working in the laboratory, and for all the µXRF-scans he made of the samples examined in my thesis.

I would also like to thank Ove Loraas, Gøran Loraas and Steinar Seehuus at the NTNU concrete laboratory. Without them, the rather extensive laboratory work would not have been possible.

Excavation of the full reinforcement cage was undertaken by Skaget Betongsaging AS.

Finally, I would like to thank the Norwegian Public Road Authorities for funding of the project.

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Abstract

Chloride induced reinforcement corrosion is a major durability problem for reinforced concrete structures. When the chloride content at the depth of the reinforcement reaches a threshold level, the steel is depassivated and corrosion may initiate. This thesis studies how chloride penetration and reinforcement corrosion are effected by the presence cracks and spacers in field exposed concrete.

As shown by several studies, cracks may facilitate the initiation of corrosion, by providing easy access into the concrete for chlorides and other aggressive agents. However, to what extent cracks affect corrosion propagation, and how crack characteristics, such as depth, width and frequency, are related to the process, have not been agreed upon. Compared to cracks, the literature is limited with regards to spacer, although recent studies suggest that they may have significant impact on chloride penetration and reinforcement corrosion.

The investigations in this thesis were performed on the middle parts of four concrete columns that NTNU had access to through NPRA and DNV GL. Two of the columns were made of normal density concrete, while the other two were different kinds of lightweight aggregate concrete (Lytag and Liapor). At least until the end of 1992, the columns were dynamically loaded to simulate wave action.

The columns were taken out of the water in 2016. The parts of the columns investigated in this thesis were exposed in the tidal zone for more than 30 years.

Electrochemical potential measurements were performed on the specimens at NTNU in the autumn 2016, and were repeated for this study. Chloride penetration was examined by the use of silver nitrate and µXRF-scans on samples retrieved from the specimens. Patterns observed within the individual samples are documented, and comparisons are made between uncracked samples, cracked samples at samples containing spacers. Pieces of rebars were later excavated from the samples, and the extent of corrosion was visually assessed.

The results did not show any clear influence of cracks on chloride penetration. Within the individual cracked samples, a possible trend towards increased penetration depth close to the cracks was observed, but compared to uncracked samples, the cracked samples did not have any deeper penetration. Cracks were however indicated to affect corrosion, as the only case of corrosion pits were observed in steel from the sample with the deepest (200 mm) and widest (1 mm, at the surface and close to the reinforcement) crack.

Samples with spacers were observed to have increased chloride penetration along the

concrete/spacer-interface. Samples with spacers were also indicated to have deeper penetration than most other samples from their respective columns, although this effect was clearer in the samples made of lightweight aggregate (Lytag), than in those made of normal density concrete.

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Symbols, abbreviations and definitions

Abbreviation Explanation

NDC Normal density concrete LWAC Lightweight aggregate concrete CSE Copper - copper sulfate electrode

Symbol Definition Elemental

mass %XRF

Chloride content calculated by the µXRF-apparatus, based on the total mass of certain elements measured at each point. The elements used in these calculations are Cl, Si, Al, S, Ag, Ca, Ti, Fe, Mn, Mg, Na, K and P.

mass-%cement

Chloride content in percentage by weight of cement.

Pd0.5% Penetration depth defined as the depth at which the chloride content does not exceed 0.5 elemental mass %XRF.

Pd0.15% Penetration depth defined as the depth at which the chloride content does not exceed 0.15 elemental mass %XRF.

PdAgNO3

Penetration depth defined as the depth at which the color change boundary appears when spraying a concrete specimen with silver nitrate. The chloride content at this boundary is assumed to be 0.15 mass-%cement free chlorides, corresponding to 0.36-0.8 mass-%cement total chlorides.

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1 Introduction

Steel reinforcement is a necessity for most modern concrete structures. The material qualities of steel counteract the concrete’s low tensile strength and ductility, enabling the construction of a wide range of structures. However, reinforced concrete structures do not last forever. As the steel is prone to corrosion, early degradation is a problem for these structures. Structures that are exposed to harsh environments, like marine structures, are especially at risk. Chlorides from the sea water penetrate from the surface and in to the reinforcement bars, where at a certain concentration corrosion is initiated. (Alzyoud 2015)

Both risk of corrosion and penetration of chlorides are expected to be influenced by the presence of imperfections in the concrete. Such imperfections include cracks and microstructural weaknesses around spacers. Both of these imperfections are expected to increase the permeability of the concrete, and thus increase the penetration of chlorides ((Wang et al. 1997) and (Alzyoud et al.

2016)). Exactly how chloride penetration and corrosion is affected by this is however not quite clear.

Cracks have not yet been characterized in a way that enables confident prediction of their influence (Otieno et al. 2010) and the influence of spacers has been a subject of a rather limited amount of studies (Alzyoud et al. 2016).

The purpose of this project is to document and discuss the influence of cracks and spacers on reinforcement corrosion and chloride penetration. Samples retrieved from a marine field station after 30 years of exposure to the tidal zone are used for this study. The extent of rebar corrosion was visually assessed and the corrosion potential was investigated by electrochemical potential

measurements. Chloride ingress was investigated by the use silver nitrate (AgNO3) and µXRF-analysis.

1.1 Objectives

This project had the following objectives:

1) Document and discuss the influence of cracks and spacers on chloride penetration 2) Document and discuss the influence of cracks and spacers on reinforcement corrosion

1.2 Research method and limitations

This project uses four samples exposed to the tidal zone of a marine field station. They were for a time dynamically loaded to simulate wave actions, and their environmental and loading conditions can be assumed to be like those of other offshore structures. The results are applicable to concrete with similar composition as these samples. Similarly, the results are viable for the particular crack characteristics observed in this study. All spacers studied were made of plastic, and as such the results can not be applied for spacers of different materials.

This study uses two definitions of chloride penetration depth; Pd0.5% and Pd0.15%.These are the depths at which the chloride content does not exceed 0.5 elemental mass %XRF and 0.15 elemental mass %XRF

respectively. The use of other penetration depths could yield different results. The unit elemental mass %XRF is based on total mass of certain elements measured by the µXRF-apparatus. This study has not tried to convert these values into other units, such as mass-%cement. Values measured in

elemental mass %XRF should only be compared to other values with the same unit, and not to any outside values.

A literature review is presented early in the thesis. Thereafter follows a description of the columns, the sampling process and of the investigations that were conducted. The results of the study are then presented, with the majority of the results being found in the appendices. For readability, some theoretical background and in-depth comments on the results are saved for the following discussion.

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2 Theoretical background

2.1 Chloride penetration and corrosion

2.1.1 Chloride penetration

Marine structures, such as those studied in this project, are heavily exposed chlorides, in the form of salt in the surrounding sea water. Chloride contaminated structures are expected to have a high chloride content near the surface of the structure, with decreasing contents deeper into the concrete (Bertolini et al. 2004). As external chlorides are transported through the porous system of the

concrete, penetration is highly dependent on the concrete porosity and permeability. Studies have shown that splash and tidal zones typically experience the most severe penetration (Cement Concrete & Aggregates Australia 2009).

For structural elements that are completely and permanently saturated, the chlorides will penetrate the concrete by pure diffusion. In most structures, however, the penetration will be helped by other transport mechanisms as well. For parts exposed to the tidal zone, like the samples retrieved for this project, chlorides penetrate by means of both diffusion and capillary suction (Cement Concrete &

Aggregates Australia 2009). Even so, chloride profiles are often, with good approximation, modelled using an equation based on Fick’s second law of diffusion (Bertolini et al. 2004), as presented below:

𝐶_𝑥= 𝐶_𝑠∗ (1 − erf ( 𝑥

2 ∗ √𝐷𝑎𝑝𝑝∗ 𝑡))

In this equation, Cx is the chloride content at the depth x in the concrete, Cs is chloride content at the surface and Dapp is the apparent diffusion coefficient.

The latter two parameters are determined by fitting experimental data to the equation. The surface content is dependent on internal parameters, such as cement content and curing, and external conditions, such as placement and environment. The apparent diffusion coefficient varies with concrete composition, compaction and curing, as well as with exposure conditions and time of exposure. Figure 1 features four different chloride profiles found using the function above with constant Cs and varying Dapp.

Figure 1: Chloride profiles for different values of Dapp (Bertolini et al. 2004).

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Figure 2 is an example of experimental data fitted to a typical chloride profile as found by Fick’s second law of diffusion. As seen from the figure, Fick’s law assumes that the highest chloride content occurs at the surface, whereas in reality, the maxima may occur a few millimeters into the concrete.

Figure 2: Experimental data fitted to a model chloride profile (Song et al. 2008).

The penetration rate of chlorides in a marine structure is affected by the exposure conditions. These are usually divided into the submerged, tidal, splash and atmospheric zone. Research has typically found higher values for Cs and Dapp in the tidal and splash zones (Costa and Appleton 1999, Cement Concrete & Aggregates Australia 2009), suggesting that these zones are most severely attacked by chlorides. However, other studies have chloride ingress to be somewhat more severe in the submerged zone (Oh and Jang 2007, Boubitsas et al. 2014).

As chloride penetration is influenced by concrete permeability, it is assumed that cracks and other imperfections should influence the penetration. If cracks lead to increased chloride ingress, the risk of reinforcement corrosion could also increase.

2.1.2 Reinforcement corrosion

Reinforcement corrosion is one of the leading causes of deterioration of reinforced concrete structures (Ahmad 2003). When a steel bar in concrete corrodes, there will be a reduction of the reinforcement area. At the same time, the formation of rust will cause a volume expansion which leads to tensile stresses in the concrete (Hanjari 2010). Both effects can have severe consequences for structural capacity. They may lead to a redistribution of stiffness and forces, and cause a general decrease in the structure’s load bearing capacity. Studies of what causes and affects corrosion is therefore necessary, in order to be able to prevent or control these damaging effects.

Due to the high pH (around 13-14) of the concrete pore water, reinforcement bars are normally protected by a passive film, preventing corrosion (Bertolini et al. 2004). However, this passive layer may under certain circumstances be broken; either by carbonation or by chloride penetration. In the process of carbonation, CO2 from the external environment dissolves into the pore solution in the concrete, causing a drop in pH to about 9, at which the passive film is no longer stable (Markeset and Myrdal 2008). Chloride penetration, meanwhile, may lead to localized breaking of the film as it reacts with the chlorides. With the passive layer broken, a reinforcement corrosion cell is formed, with the steel bar as a mixed electrode, containing both an anode (area with passive layer broken) and a cathode (area with passive layer intact) (Ahmad 2003). The corrosion process is thus initiated, as illustrated in Figure 3.

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Figure 3: Illustration of corrosion process (Ahmad 2003).

Reinforcement corrosion is often divided into the initiation phase and the propagation phase. The initiation phase is the period of time until the reinforcement becomes depassivated. During this phase, aggressive agents (CO2 and/or chlorides) penetrate from the surface, and when the concrete at the depth of the reinforcement is either carbonated or contains a critical amount of chlorides, the passive layer breaks. This is the start of the propagation phase, which is the period of time when the corrosion process is occurring (Ying and Vrouwenvelder 2007). According to Tuutti’s model of the service life of a structure, the propagation phase, and the structure’s service life, ends when the corrosion has reached a maximum acceptable limit (Bertolini et al. 2004). This is illustrated in Figure 4.

Figure 4: Tuutti’s model of service life regarding reinforcement corrosion in a concrete structure, according to Bertolini et al.

(2004).

The duration of both phases depends on various complex parameters (Bertolini et al. 2004). The length of the initiation phase mainly depends on how fast aggressive substances penetrate to the steel, as well as what concentrations are necessary to start the corrosion process (Tuutti 1982). The threshold level for corrosion may be affected by concrete quality, cover depth, exposure conditions and the presence of cracks (Angst 2016). While the length of the initiation time is usually found to be affected by the presence of cracks, the influence of cracks on propagation time is still heavily

debated (Concrete Society 2015).

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2.2 Influence of cracks and spacers on chloride penetration and reinforcement corrosion

2.2.1 Influence of cracks

Cracks in concrete structures is a common occurrence, and certain types of cracks are in most cases inevitable (Wang et al. 2015). There are various mechanisms that lead to cracking, among others thermal stresses, shrinkage and mechanical loads (Concrete Society 2015). Although most cracks do not directly pose a threat to the structural capacity of a structure, various studies have linked cracks to an increased occurrence of reinforcement corrosion.

The study of the effects of cracks is complicated, as their influence is a function of several complex factors, such as width, depth, orientation and frequency (Otieno et al. 2010). Additionally, concrete quality, cover depth and exposure conditions may heavily influence results (Costa and Appleton 1999, Concrete Society 2015). There seems to be a consensus that cracks facilitate the initiation of corrosion, as they provide easy access into the concrete for aggressive agents, mainly chlorides and carbon dioxide. However, no definite relationship between crack width and corrosion initiation has been found, as seen in the Figure 5. Furthermore, to which extent cracks influence corrosion propagation has not been agreed upon.

Figure 5: Various results on critical cracks width regarding corrosion (Angst 2016)

The decreased initiation time seen in Figure 5, suggests that cracks increase penetration of chlorides.

Research has found both that increased crack width (Audenaert et al. 2009a) and increased crack depth (Nguyen and Vu 2015) can increase penetration depth significantly. Increased crack widths has also been linked to generally increased chloride content due to increased chloride diffusion

coefficient. (Shaou-feng et al. 2011).

As cracks may provide easy access into the concrete for aggressive agents, such as chlorides and carbon dioxide, cracks could be expected to facilitate the initiation of corrosion. Although studies support this, predicting time to corrosion based on crack characterization has proven difficult (Otieno et al. 2010). For example, while the initiation time has generally been seen to decrease with

increasing crack widths, studies show varying results regarding what the critical crack width could be, as seen in Figure 5.

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As mentioned, there has not been reached any agreement on how cracks affect propagation of corrosion, or exactly how crack characteristics such as depth, width, orientation and frequency influence the process. Otieno et al. (2010) concluded from their study in 2010 that cracks increased rate of corrosion and that wider cracks lead to increased corrosion rate. On the other hand,

according to Concrete Society (2015), Schiessl and Raupach concluded that the impact of crack widths on amount of corrosion was rather low. It is suggested that while larger crack widths may be linked to a larger amount of corrosion, this is not because corrosion rate itself has increased. Rather, as initiation time presumably decrease as crack widths increase, the corrosion process initiates earlier where crack widths are large, leading to a larger build-up of corrosion products.

Whether a crack is live (varying with time) or dormant (not varying with time) is important regarding its effect on chloride penetration. Under certain circumstances, dormant cracks may experience self- healing, preventing increased chloride ingress (Concrete Society 2015). There are various ways according to which self-healing may occur. Concrete Society (2015) describes the two main mechanisms as precipitation of calcium carbonate and/or ongoing cement hydration resulting in hydration products. Another mechanism of self-healing can be due to the presence of MgSO4 in seawater. The formation of brucite (Mg(OH)2) and ettringite has been reported to form in cracks in concrete exposed to marine environment (Mohammed et al. 2003). The occurrence of self-healing can thus be evidenced by observing calcium- and/or magnesium-based reaction products in and around the cracks.

While the influence of cracks is still heavily debated, it can generally be assumed that the risk of reinforcement corrosion increases wherever cracks are present.

2.2.2 Influence of spacers

Spacers are unavoidable components of reinforced concrete structures. They are necessary to keep the reinforcement bars in the correct position within the formwork, and to prevent movement as the concrete is cast. While spacers are small and seemingly inconsequential, studies suggest that they may have an impact on both chloride penetration and corrosion (Alzyoud 2015, Alzyoud et al. 2016).

Despite spacers being either impermeable or have low porosity, Alzyoud et al. found that the presence of spacers, in the study at hand, always increased overall transport (Alzyoud et al. 2016).

The results showed a disturbed microstructure around the spacers, creating an effect similar to that of the interfacial transition zone (ITZ) between paste and aggregate. The higher porosity of such an interface likely allowed for increased transport, resulting in increased chloride penetration. These findings are supported by the earlier study also made by Alzyoud (Alzyoud 2015). Plastic spacers tended to overall perform worse than spacers made of cement or steel. Probable explanations for this are stated to be weak bond between plastic and concrete, greater mismatch in material properties related to shrinkage and thermal expansion, as well as plastic spacers generally having a much larger surface area than cementitious and steel spacers.

The aforementioned study suggests that the effects of the concrete/spacer-interface are likely to be much larger than that of the ITZ between cement and aggregate, despite both having similar

microstructure characteristics (Alzyoud et al. 2016). The concrete/spacer-interface has a larger, continuous area that forms a link between the reinforcement and the concrete surface. The ITZ is in contrast typically discontinuous. As spacers are quite numerous in many structures, their combined effect could be significant, and due to the increased transport of aggressive agents, they could be assumed to facilitate corrosion. As few studies have been made on the effect of spacers on corrosion, further investigations are needed.

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2.3 Investigative methods

2.3.1 Potential measurements

One way of getting an indication of corrosion activity, is by performing measurements of

electrochemical potential. The potential, or rather the difference in potential between the cathode and the anode, can be said to be a driving force of the corrosion process (Markeset and Myrdal 2008).

The potential of the steel is measured against an electrode on the surface with known standard potential. Generally, a lower corrosion potential means a higher risk of corrosion. Using a copper - copper sulfate electrode (CSE) directly on the steel surface, passive steel has a potential value between +50 and -200 mV CSE, while steel suffering from chloride induced corrosion has a potential in the range of -400 to -700 mV CSE (Bertolini et al. 2004). As it is normally not possible to measure the potential directly on the steel surface, the readings are affected by the concrete cover. Figure 6 shows an example of a potential field, and how values decrease with increased distance from the steel.

Figure 6: Potential field (Bertolini et al. 2004).

As mentioned above, electrochemical potential of the reinforcement steel is measured by comparing to the known standard potential E0 of a reference electrode. This means that what is measured is the difference in potential between the steel and the reference electrode. Both the measured values themselves and the gradients between adjacent points are used to interpret the measurements. In general, low values and large gradients are considered to indicate corrosion activity.

The conventional way of performing these measurements is by establishing electrical contact between the rebar and the reference electrode, and using a multimeter or computing devise with voltmeter functionality to measure the difference. The reference potential can for instance be a saturated calomel electrode or a copper-copper sulfate electrode and is usually in the form of a rod or of a wheel that can be rolled over the concrete surface (Elsener et al. 2003). Alternately, the potential mapping may be performed without connection to the reinforcement using two external electrodes on the concrete surface (Reichling 2007). The setup for both methods are illustrated in Figure 7.

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Figure 7: Schematic illustration of the setup of (a) conventional and (b) gradient based potential mapping.

It should be noted that this is not a suitable method of estimating the rate of corrosion (Bertolini, Elsener et al. 2004). Consequently, potential measurements can only be used to get an indication on the probability of whether or not corrosion is occurring. According to Gu and Beaudoin (1998) the ASTM has set guidelines as seen in Figure 8 for limits regarding the probability of corrosion related to a copper-copper sulfate electrode (CSE). The exposure conditions are however not specified, and thus direct comparisons to the ASTM-values can not be made. Various parameters, such as oxygen concentration and carbonation might affect potential readings without necessarily affecting the risk of corrosion (Gu and Beaudoin 1998). However, the general concept that risk of corrosion increases as potential values decrease, is typically true.

Figure 8: ASTM’s guideline for probability of corrosion, according to Gu and Beaudoin (1998). Exposure conditions are not specified.

2.3.2 Silver nitrate

One way of indicating the extent of chloride penetration is by using colorimetric analysis. By spraying a concrete sample with a substance that reacts with chlorides, a penetration front can be determined by observing a color change due to the reaction. Studies of different chemical indicators have shown that solutions of silver nitrate, AgNO3, gives a clearer color change boundary compared to solutions based on lead nitrate and thallium nitrate (Otsuki et al. 1993). When silver nitrate is sprayed onto a chloride contaminated concrete surface, the silver will react either with available chlorides, or with hydroxides in the concrete wherever chlorides are not present. Reaction with chlorides creates the whitely colored silver chloride, whereas silver bonding with hydroxides creates a brown color (Stanish et al. 1997). An example photo of a concrete sample sprayed with AgNO3 can be seen in Figure 9.

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Figure 9: Chloride contaminated concrete sample sprayed with AgNO3. Areas with brownish color are areas with no or low chloride concentration (Michel et al. 2013).

It must be noted that this method does not give a complete picture of chloride penetration. Even brownly colored areas might contain some chlorides, and thus the color change appears at the boundary of the critical chloride concentration needed for silver chloride to form (He et al. 2012).

The chloride concentration at the color change boundary may vary with parameters such as the concrete composition and environmental conditions (Baroghel-Bouny et al. 2007). Various studies have found free chlorides threshold value of around 0.15 % by mass of cement (mass-%cement) when using a 0.1 mol/l AgNO3 (Otsuki et al. 1993). According to Baroghel-Bouny et al. (2007), this

corresponds to a total chloride content between 0.36 and 0.8 mass-%cement.

Colorimetric analysis by use of AgNO3 is a low cost, simple and rapid test method (Baroghel-Bouny et al. 2007). Indications on chloride diffusion and penetration can be found almost immediately after retrieving a test sample. However, the method is not optimal at showing concentration differences at different parts of the contaminated samples. While AgNO3 can be used to see a difference in chloride penetration in, for example, cracked and uncracked samples, the method is not applicable to study the concentration in or around the cracks.

2.3.3 µXRF

Micro X-Ray fluorescence (µXRF) is a chemical imaging technique that can be used to investigate, among other things, chloride penetration in concrete. The principle behind this technique is to locate and quantify the different elements in a sample, based on each element’s distinct energy emission when hit by X-rays. When an atom is hit by a high-energy X-ray, an electron is ejected from the inner orbital, i.e. the inner orbital is ionized. An electron from a higher orbital will then rapidly fill the gap left in the inner orbital. As this happens, energy is emitted in the form of an X-ray photon (Bruker 2015). This photon is called the atom’s characteristic radiation. Its energy level is distinct for each and every element (Banica 2009), although an element can have different characteristic radiations depending on which outer orbital is involved. As seen in the schematic illustration in Figure 10, the main orbital transitions are called Kα, Kβ and Lα.

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Figure 10: Emergence of X-ray fluorescence radiation (Bruker 2015).

Performing an µXRF-scan will give an overview of the full elemental composition of the sample. The method is thus useful for more than just determining chloride penetration. It could, for instance, be used for studying self-healing, which might be indicated by a congestion of calcium or magnesium in and around cracks (Mohammed et al. 2003, Concrete Society 2015). As the X-rays interact with the elements at an atomic level, this method can not be used to directly say anything about molecules or chemical bonds in the material (Bruker 2015). One consequence of this, is that an observance of iron can not outright be taken as an indication of corrosion, as iron based compounds are typically a part of the cement and the aggregate as well.

Compared to other methods, image analysis gives more in-depth information on the chloride ingress.

It is well suited for studies on inhomogeneities, as detailed analysis can be made on specific areas of interest. The µXRF-method might be preferable to the more common electron based imaging

techniques, as X-rays penetrate deeper into the material than electrons. X-rays can penetrate up to 1 mm, while electrons typically penetrate a few microns (Moradillo et al. 2016). Because of this, µXRF- scans will give satisfactory results even on surfaces that are not completely polished.

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3 Experimental

3.1 Materials

The samples used in this project come from a DNV field station established in 1983 (Andersen and Espelid 1993). Eight columns were in 1983/1984 installed in a marine environment and dynamically loaded in a three-point bending rig for several years. The columns were taken out of the water in 2016 and cut into three parts. The lower part had been permanently submerged, the middle part had been exposed to the tidal zone and the upper part had been exposed to atmospheric conditions. This project is concerned with the middle part of four of the DNV-columns. The mean water level is not known at the moment.

The sample columns were numbered 2, 4, 6 and 8. All of these had nominal strength class C60.

Column 2 and 6 were made using normal density concrete (NDC), while 4 and 8 were made using different lightweight aggregates (LWAC), respectively Lytag and Liapor (Andersen and Espelid 1993).

The compositions of the different concretes are shown in Table 1. Table 2 summarizes strength properties and loading conditions up until the end of 1992. It is not known per now whether loading continued after this point in time.

Table 1: Compositions of the concretes used for the columns, after Andersen and Espelid (1993).

Normal density concrete (NDC)

Liapor LWA- concrete

(LWAC)

Lytag LWA- concrete

(LWAC)

w/(c+s) 0.38 0.40 0.33

Cement (kg) 420 385 600

Sand (kg) 1870 650 400

Liapor 8 (kg) 715

Lytag (kg) 684

Total water (l) 160 161 199

Water reducing admixture (l) 5 10 10

Air entraining agent (kg) 0.3

Silica (kg) 15

Wet density (kg/l) 2.39 1.94 1.97

Air content (%) 2.80 6.20 4.90

Table 2: Summary of strength properties and loading conditions, after Andersen and Espelid (1993).

Column number 2 4 6 8

Description NDC LWAC NDC LWAC

Date of installation 06/83 10/84 06/83 10/84

Load level 9 6 9 6

Alternation of load until

end of 1992 39 mill 33 mill 39 mill 33 mill

Nominal strength C60 C60 C60 C60

Reinforcement steel KS40 KS40 KS40 KS40

Maximum reinforcement

stress (MPa) 230 165 230 165

Maximum concrete stress

(MPa) 13 7 13 7

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The samples had a length of 1.2 m and a diameter of 0.6 m, as shown in Figure 11. At some point prior to this project, four cores with a diameter of 110 mm had already been drilled from column 6.

These holes after these cores had not been filled afterwards.

Figure 11: Dimensions of the columns.

The columns were reinforced both with longitudinal bars (D = 16 mm) and stirrups (D = 8 mm). The longitudinal reinforcement consisted of three electrically disconnected meshes, with six bars in each mesh. The bars within each mesh were electrically connected to each other by the stirrups. This setup is shown in Figure 12. The outer, middle and inner mesh had a concrete cover of respectively 50 mm, 100 mm and 150 mm. The bars in the inner and outer mesh were continuous through the whole length of the original column (5m), whereas the middle mesh had been divided according to the three exposure zones.

Figure 12: Schematics of the reinforcement (Andersen and Espelid 1993).

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For an as of yet unknown amount of time, parts of the reinforcement in the columns were

cathodically protected by sacrificial anodes, while at the DNV field station. Available documentation does not clarify what parts of the steel were protected, but it can be assumed to concern the middle mesh, as figures from Andersen and Espelid (1993) show that the protected reinforcement bars are divided according to the exposure zones. This is seen in Figure 13. It is not stated by DNV for how long cathodic protection was applied, or if it was applied for an equally long time for each column.

Figure 13: The columns were cathodically protected by sacrificial anodes (Andersen and Espelid 1993).

3.2 Sampling

3.2.1 Set-up in lab

The autumn of 2016, the samples arrived at NTNU and were placed in a laboratory. They were stored at room temperature (around 20 °C) until January 2017, when cores were drilled out.

In order to have reference points for the experimental tests, marks were made every 45° along the periphery of the top flat surface. This is illustrated in Figure 14.

Figure 14: Markings every 45°.

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14 3.2.2 Drilling of cores

A total of eighteen cores were drilled and taken out from the samples. The locations on the columns from where the cores were taken, were mainly chosen based on the visual inspection, although the potential values were also considered. With the purpose of studying the effects of inhomogeneities, it was preferable to have specimens both with and without cracks and spacers. A list of the cores and their properties is presented in section 4.3. The cores were labeled 2A, 2B, etc., referring to the column number. The location from which each core was taken is seen in section 4.1, where the results of the visual mappings are presented.

As mentioned earlier, three cores had been drilled out from column 6 at some point prior to this project. During drilling in the present study, core 6B connected to one of these preexisting drilling holes. An illustration of this is seen in Figure 15.

Figure 15: sample 6B connects to a preexisting drill hole.

After drilling, the samples were wrapped in two layers of plastic, to avoid drying out. They were then placed in a cold storage room, and stored at 5 °C.

3.2.3 Splitting and cutting

After the cores had been drilled out, most of them were cut in half, using a water-cooled saw. As the cracks appeared to be perpendicular to the longitudinal reinforcement, the cores with visible cracks were cut along the longitudinal bars. Cores without cracks were cut perpendicular to the longitudinal reinforcement. This was done for practical reasons, to minimize the amount of steel that had to be cut through. The orientations of the cuts are illustrated in Figure 16. After cutting, each half (hereby denoted sample) was labeled u, d, l or r (up, down, left or right) referring to the orientation each sample had at the field station. One sample from each core, and a few others, were used for examination with silver nitrate, while the other samples were used for µXRF-scans.

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There were concerns about chlorides getting washed out, due to the saw and the cores being continuously cooled by water during cutting. The plan was therefore to dry split some of the half core samples, in order to have surfaces that had not been exposed to water. This was done with some samples from column 8, but it proved difficult because all the reinforcement, stirrups and cables prevented a clear split surface. As the µXRF-scans required a smooth surface, it was decided that the rest of the cores should only be cut.

3.2.4 Retrieval of reinforcement

From 4^th-6^th of April, the reinforcement in the samples examined with silver nitrate was retrieved.

This excludes samples 2Au and 2Fu, which were sent to profile grinding (the results of which are not covered by this study). The concrete samples were crushed without the use of water, and all steel bars where thus dry after the excavation. Both main reinforcement and stirrups from the outer, middle and inner mesh were marked 1, 2 and 3, respectively. All reinforcement parts were then stored in a desiccator to avoid corrosion due to moisture in the air. The rebars closest to the surface retrieved from the three cracked samples and the sample containing a spacer from column 2, were later acid cleaned by Tobias Danner. To clean the rebars, reinforcement and stirrups were immersed in a chemical solution (HCl:H2O = 1:1 and 3g/l methenamine (corrosion inhibitor)) and placed in an ultrasonic bath for 3 minutes. As not enough of the right acid solution was available at NTNU, the rest of the rebars could not be cleaned during the time scope of this thesis.

From 26^th-27^th of April, the full outer reinforcement mesh was retrieved from each column. This was done by and at Skaget Betongsaging AS. No water was used for the excavation, and all steel bars were thus dry. Both stirrups and main reinforcement were tightly wrapped in plastic foil and put in a plastic bag. They were then stored at room temperature in the concrete laboratory at NTNU. They have not yet been acid cleaned, and the extent of corrosion has yet to be examined. As these rebars are still awaiting a full visual assessment, they will not be commented or discussed in the following sections.

Figure 16: The red line shows the direction in which the (a) cracked cores and (b) uncracked cores were cut.

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3.3 Methods of investigation

3.3.1 Visual Inspection of the concrete samples

After the samples arrived at NTNU, visual inspections were made by Karla Hornbostel and Tobias Danner. Cracks, spacers and preexisting drill holes were drawn into a schematic as shown Figure 17.

Figure 17: Schematic of unfolded column (stirrups not included).

Figure 17 shows a schematic of an unfolded column. The orientations of the samples had to be considered when performing the visual inspection. The figures in the results-section are drawn with all the samples having the same orientation as they had at the field station.

3.3.2 Potential measurements

Potential measurements were made by Karla Hornbostel shortly after the samples arrived at NTNU in the autumn 2016. The reference electrode was a copper-copper sulfate electrode (CSE). The

measurements were made on the outer reinforcement mesh with 50 mm between each measuring point. As the entire mesh was electrically connected, it was suitable to use the conventional method of measurement, with contact to the reinforcement. The results in section 4 are related to the same template as the results from the visual inspection Figure 17.

The potential measurements were repeated for this study in January 2017. As the values were similar to those found by Hornbostel, full mappings were not repeated for three of the columns. Only column 8 was fully remapped.

3.3.3 Spraying with silver nitrate

After cutting, excess water on the cut surfaces was wiped off, and the samples listed in Appendix B was sprayed with a 0.1 mol/l AgNO3 solution. The specimens were then left to dry for about 15 to 45 minutes, until the color changes were clearly visible. As recommended by NT Build 492 (Nordtest 1999), penetration depth was then measured at seven points on the samples where the boundary was somewhat uniform. On specimens where the boundary was more variable, fourteen

measurements were made. This was first done by hand with a ruler, but was redone digitally after the color change boundary had been marked. Immediately after the measurements were made, the samples were repacked in plastic and placed in the cold storage room at 5 °C.

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17 3.3.4 µXRF-scan

Thirteen samples were scanned using a M4 Tornado µ-XRF apparatus from Bruker. The scans were performed by Tobias Danner at NTNU. Relevant to this thesis, the samples were examined in two different ways by µXRF; elemental mapping and line scans. For these analyses, X-ray beams were focused to a spot size of 25µm. Elemental mappings scan an area of a flat surface to find the elemental distribution in a sample. Elemental mappings were performed with a 60 µm distance between each measurement point. In an area of 100 x 110 mm, 3000 points were measured. The measurement speed was 1 ms/pixel. The results of chloride mappings of the samples are presented in section 3.4 as intensity maps, where the chloride content at each point is given as percentage of the maximum content found.

For line scans, the µXRF apparatus make scans along a set straight line and finds the content of different elements at a set number of points. Line scans were taken with a 600 µm distance between measurement points in most samples. For each scan 10 cycles were measured with a speed of 25 ms/pixel. The line scans presented in section 3.4 shows the chloride content in elemental mass %XRF1

That is, in percentage by the total mass of certain elements registered by the apparatus. The elements considered in these calculations were Cl, Si, Al, S, Ag, Ca, Ti, Fe, Mn, Mg, Na, K and P. The lines went from the surface and into a depth of 80-150 mm. The number of line scans made in each sample, the number of points scanned at each line, the length of the lines and the distance between each data point are shown in Table 3.

1The µ-XRF instrument at Department of Structural Engineering, NTNU was obtained in November 2016 through funding from NTNU, and the instrument is still being tested (June 2017).

The µ-XRF instrument detects counts per second/eV (number of fluoresced X-rays in a certain energy window).

To determine concentration, a calibration curve is to be established using a set of reference standards.

With µ-XRF it is not possible to detect elements with an atomic number lower than 11 (Na). That means that elements like e.g. O and H are not detected.

Measurements appear normalised to the mass of the following elements: Ca, Si, Al, Fe, Na, K, Mg, Cl, S, P, Ti, Mn, in the volume measured.

The suggested unit to use is [elemental mass %xrf].

Tobias Danner, 7^th June 2017

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Table 3: Information on the line scans made using the M4 Tornado µXRF-apparatus.

Sample

Number of lines

Number of points in each

line

Depth of lines (mm)

Distance between

points (µm) Comments

2Ad 5 250 150 600

2Bl 5 250 150 600

2Cr 6 250 150 600

2Dl 5 250 150 600

2Er 5 200 120 600

2Fd 5 250 150 600

4Bd 4 150 90 600

4Cu 5 250 140 560

4 4 250 100/150 400/600 One line had a depth

of 150 mm

4Dr 5 150 80 530

6Bu 3 250 150 600

6Cd 5 250 150 600

8Cu 4 150 110 730

For both the elemental mappings and the line scans, a current of 600 µA and a voltage of 50 kV was used. The chamber in which the samples were scanned had a pressure of 20 mbar.

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4 Results

Results from each of the investigations conducted are presented and commented below. Some more in-depth comments may be provided in section 5.

4.1 Visual observations

Results from the visual inspection of the columns are presented in Figure 18Figure 21. Figure 22 explains the symbols used. The mappings are based on the template shown in section 3.3.1, with the reinforcement bars moved slightly to match their actual placement. The figures contain spacers, cracks and earlier drill holes, as well as markings of where the samples used in this project were taken from.

Figure 18: Visual mapping of column 2. See Figure 22 for explanations of symbols, and Figure 27 for a registration of observations. The hatched area indicates where the column was attached to the dynamic loading system.

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Figure 22: Explanation of symbols used in the visual mappings

4.2 Potential measurement

The following figures show potential mappings of the columns. Mappings on column 2, 4 and 6 were done by Hornbostel in the autumn of 2016, while column 8 was remapped in January 2017.

Figure 23: Potential map of column 2.

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The lower half of column 2 has significantly lower potentials than the top half. There are not any particularly low potentials measured close to cracks and spacers. The point measured at core 2C seems to have among the highest values of the column.

Column 4 has high potential values throughout. There does not seem to be any changes in potentials near the crack or the spacer.

The lower half of column 2 have significantly lower potentials than the top half. There are not any particularly low potentials measured close to cracks and spacers. Values taken close to the preexisting drill holes do not seem substantially different from neighboring points.

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Column 8 has high potential values throughout. There does not seem to be any changes in potentials near the crack or the spacer.

4.3 Drilling of cores

Figure 27 shows the results of the drilling of the cores. The figure shows whether the cores were retrieved in one piece, and whether they contain main reinforcement, stirrups, cables, spacers and cracks. Observed crack widths are also noted.

Figure 27: Observations made after drilling of the cores.

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4.4 Spraying with silver nitrate

The colorization on the sample surfaces was not as clear as what is shown in Figure 9. It was however possible to determine a boundary between different colored areas in most samples. Figures of all samples that were sprayed, with boundaries drawn wherever visible, are collected in Appendix A, with the exception of sample 2Fd, which was unfortunately not photographed. he color change boundaries are indicated with blue lines. Table 4 and Table 5 show the average penetration depths and standard deviations for all samples where the color change boundary was visible throughout the whole sample. A complete table of measurements can be seen in appendix B.

Table 4: Average penetration depths PdAgNO3 and standard deviations from the NDC-samples sprayed with silver nitrate.

Sample 2Au 2Br 2Cl 2Dr 2Fu 6Au 6Cu 6Dd

Average penetration

depth PdAgNO3 (mm) 102 109 128 110 122 129 49 97

Standard deviation

(mm) 5 8 8 8 19 2 16 4

Table 5: Average penetration depths PdAgNO3 and standard deviations from the LWAC-samples sprayed with silver nitrate.

Sample 4Au 4Bu 4Cd 4Dr 8Bd 8Bu 8Cd 8Dd

Average penetration

depth PdAgNO3 (mm) 40 42 40 51 68 81 66 80

Standard deviation

(mm) 3 2 2 7 2 10 4 0

4.5 µXRF-scans

The µXRF-scans are presented in Appendix C, the results are described and commented in this section. The data-report for each sample in Appendix C contains three figures; two intensity maps and line scan results. Explanations of these will be provided below, followed by descriptions of what is observed in each sample. The figures from Appendix B1 are used as examples.

Intensity maps

Figure 28: Intensity maps from sample 2Ad as seen in Appendix C1.

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Figure 28 is a copy of figures C1 and C2 from Appendix C1, which show an intensity map of the chloride content on the surface of sample 2Ad. C1.1(a) is the map as provided by the µXRF-scanner, while in C1.1(b), relevant markings have been added. The longitudinal reinforcement and stirrups are shown with gray, whereas cracks are traced with white. The white numbered lines show where the line scans where undertaken. Wherever necessary for clarification, specific points are marked and named Point A, Point B, etc. The intensities are normalized for each specific sample, meaning that the intensity map of one sample can not be compared with that of another. The colors associated with each intensity is shown at the right in both figures.

The intensity maps are oriented to match the orientation the samples had in situ as part of the columns. For uncracked samples, such as sample 2Ad shown in the figure, this means that the upper part of the figure always corresponds to the left part of the sample. For the cracked samples and samples with spacers, which were cut along the main reinforcement, the upper half of the figure corresponds to the upper half of the sample.

Table 6-Table 9 summarizes the observations made about the intensity map, divided into uncracked NDC-samples, uncracked LWAC-samples, cracked samples and samples with spacers.

Table 6: Summary of observations made on the intensity maps of the uncracked NDC-samples.

Samples Comment

Uncracked samples (NDC)

2Ad, 2Fd

Mappings at the edges are somewhat skewed due to the curvature of the

surface.

Somewhat variable penetration front.

6Bu

Chloride intrusion both from the surface and from the right edge.

Left edge has rather homogenous penetration.

Large chloride accummulation around the first longitudinal reinforcement.

6Cd

Rather uniform penetration in to a depth of about 60 mm. After this, the

intensities around lines 2 and 3 seem to increase somewhat.

2Fd, 6Cd Small chloride accumulations around the first longitudinal reinforcement.

Table 7: Summary of observations made on the intensity maps of the uncracked LWAC-samples.

Samples Comments

Uncracked samples (LWAC)

4Bd, 4Cu, 8Cu

Mappings at the edges are somewhat skewed due to the curvature of the surface.

4Bd, 4Cu, 8Cu Relatively uniform penetration.

4Bd, 4 Cu There seems to be a steep decrease in intensity from 20 to 30 mm.

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Table 8: Summary of observations made on the intensity maps of the cracked samples.

Samples:

Cracked samples

2Bl Intensities near the surface are somewhat higher at the upper half than the lower half.

2Dl

Intensities near the surface are somewhat higher at the lower half than the upper half.

Intensities seem to be lower where the crack is.

Chloride congestions around first and second stirrup.

2Bl, 2Er Chloride congestions around first and first longitudinal reinforcement.

2Bl, 2Er No obvious changes in intensity near the crack.

Table 9: Summary of observations made on the intensity maps of the samples with spacers.

Samples: Comments

Samples with spacer

2Cr

Some chlorides seem to have penetrated beyond the spacer

There is an accumulation of chlorides between the sample surface, and the spacer 4Dl Increasing intensities towards the spacer.

4Dl, 4Dr

Penetration seems to stop at the spacer, but goes deeper around the upper and lower edges of the vertical part of the spacer (points A and B).

4Dr Penetration is somewhat deeper than in 4Dl.

4Dr, 2Cr Points with high intensities are found rather deep along the edges of the spacer.

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27 Line scans

Figure 29: Line scans from sample 2Ad as seen in Appendix C1.

Figure 29 is a copy of figure C3 from Appendix C1, which gives the results of the line scans from sample 2Ad. Between three and six such scans were performed on each sample. The x-axes show the depth in millimeters from the sample surface. The y-axes show the amount of chlorides in elemental mass %XRF, as defined previously. Due to the aggregate not containing any chlorides, the curves are not smooth like a typical chloride profile, and contain numerous maxima and minima.

The orange and grey straight lines in the figure indicate the threshold limit used for determining penetration depth. As stated introductorily, two different definitions of penetration depths are used:

• Pd0.5%: the depth at which the chloride content does not exceed 0.5 elemental mass %XRF

(grey line)

• Pd0.15%: the depth at which the chloride content does not exceed 0.15 elemental mass %XRF

(orange line)

Table 10Table 12 summarize observations made about the line scans, divided into uncracked samples, cracked samples and samples with spacers. Table 10 lists average penetration depth for both definitions, cover depth and chloride content at cover depth. Cover depth is the depth at which the first steel bar, either main reinforcement or stirrup, is located. The cover depths are taken from

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the intensity scans, but is tabulated here with the other values, where they are more relevant.

Chloride content at cover depth is the content observed in a line passing through the reinforcement, or the average of two lines passing close to the reinforcement. For samples with cracks and spacers, the penetration is assumed to be inhomogeneous, and only maximum values are tabulated.

Table 10: Summary of observations made on the line scans of the uncracked samples.

Samples

Average penetration

depth Pd0.5% (mm)

Average penetration depth Pd0.15%

(mm)

Cover depth (mm)

Chloride content at cover depth (mass-%)

Comments

2Ad 49 82 51 0.51 All curves have forms resembling typical chloride profiles.

2Fd 77 113 55 0.75 Relatively large variation in penetration depths.

4B 31 38 50 0

Penetration depths around 25-35 mm (Pd0.5% ) and 30-40 mm (Pd0.15%) for all lines.

Line 1 has a peak at 5 mm, although this is not seen on the intensity map.

4Cu 31 40 50 0 Penetration depths around 25-35 mm (Pd0.5% ) and 35-45 mm (Pd0.15%) for all lines.

6Bu - 57 -

All line scans are made in the leftmost 25 mm.

Average value would not be representable.

- No lines go through the reinforcement.

Content at around 120-140 mm seem to be as larger or larger than the content at 60-80 mm.

6Cd 24 50 52 0.31 Chloride content decreases toward 50-60 mm in lines 2 and 3, but increases again from 75-95 mm.

8Cu 61 78 60 0.5 All lines seem to more or less fit a typical chloride profile.

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Table 11: Summary of observations made on the line scans of the cracked samples.

Sample

Maximum penetration

depth Pd0.5% (mm)

Maximum penetration

depth Pd0.15% (mm)

Cover depth (mm)

Chloride content at cover depth

(elemental mass %XRF)

Comments

2Bl 45 75 50 0.25

Somewhat greater penetration closer to the crack for Pd0.15%. This is not observed for Pd0.5%.

Line 3 has tall peaks at 115 mm and 130 mm

2Dl 50 95 50 0.4

2Er 65 90 50 0.51

Tall peaks at 55 mm in line 2 and at 65 mm in line 3, which correlates with the

accumulations seen on the intensity map.

Table 12: Summary of observations made on the line scans of the samples with spacers.

Sample

Maximum penetration

depth Pd0.5% (mm)

Maximum penetration

depth Pd0.15%

(mm)

Cover depth (mm)

Chloride content at cover depth

(elemental mass %XRF)

Comments

2Cr 70 120 51 0.6 Chlorides are observed beyond the depth of

the spacer.

4Dl 50 50 55 0 Chlorides seem to be blocked by the spacer in

sample 2 and 3.

4Dr 65 65 55 2 Unusually high chloride content at cover depth.

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4.6 Visual inspection of reinforcement

Corrosion products were found on most reinforcement surfaces that were exposed to air after the samples were cut. The reinforcement bars closer to the surface typically had darker and wetter corrosion products. An example is shown in Figure 30.

Figure 30: Corrosion products in sample 2Cl are darker and wetter at the first stirrup from the surface. The blue line indicates the color change boundary due to spraying with AgNO3.

Upon excavation of the samples, the stirrups and main reinforcement bars were retrieved. Figures of the steel bars are also seen in Appendix A. The white color seen on some of the steel bars is not due to corrosion, but due to the concrete not being completely chiseled off.

Pitting corrosion has so far been observed in a single piece of steel. Stirrup 1 in sample 2Dr had dark red spots in areas that had not been air exposed, as seen in Figure 31. After excavation, the stirrup was subsequently cleaned in acid to remove the corrosion products. The pits found where the

corrosion products had been, as seen in Figure 32, confirms that pitting corrosion had occurred in the stirrup.

Figure 31: Corrosion product seen in stirrup 1 from sample 2Dr. The white spots are leftovers from the concrete.

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Figure 32: Corrosion pits seen in stirrup 1 from sample 2Dr, as observed after acid cleaning of the rebar.

Some corrosion products of interest were also observed in the reinforcement bars listed in Table 13.

The bars will need to be cleaned in acid in order to investigate whether or not pitting corrosion has occurred. As not enough of the right acid solution was available at NTNU, this could not be done within the time scope of writing this thesis.

Table 13: Observations made on the reinforcement bars after excavating.

Sample Bar Comment

6Bu Main reinforcement 3

The bar was exposed to air after the sample broke during drilling. However, due to the neighboring preexisting drill hole, the bar had only a few millimeters of cover depth. The bar may thus have had increased risk of corrosion.

8Au

Main reinforcement 1 A small amount of corrosion products are observed. Neither bar was exposed to air after cutting and splitting.

Stirrup 1

Influence of cracks and spacers on chloride penetration and reinforcement corrosion in concrete

chloride penetration and reinforcement corrosion in concrete

Øyvind Strømme

MASTER THESIS 2017

MASTEROPPGAVE 2017

Preface

Abstract

Table of contents

Symbols, abbreviations and definitions

1 Introduction

1.1 Objectives

1.2 Research method and limitations

2 Theoretical background

2.1 Chloride penetration and corrosion

2.2 Influence of cracks and spacers on chloride penetration and reinforcement corrosion

2.3 Investigative methods

3 Experimental

3.1 Materials

3.2 Sampling

3.3 Methods of investigation

4 Results

4.1 Visual observations

4.2 Potential measurement

4.3 Drilling of cores

4.4 Spraying with silver nitrate

4.5 µXRF-scans

4.6 Visual inspection of reinforcement