Corrosion in concrete. Civil works construction technology. Civil engineering department 5th year. Linnea Sofie Johansson IST

Transcription

1 Corrosion in concrete Civil works construction technology Civil engineering department 5th year Linnea Sofie Johansson IST Elin Magdalena Fryklund IST Bojan Fajdiga IST Antonio Barhouche IST Maxime De Almeida IST /11/2017

2 TABLE OF CONTENTS 1 What is the carbonation problem? What is carbonization of the concrete and how is process going Factors affecting Concrete Carbonation... 4 Water Cement Ratio ( w/c) Carbonation depth... 5 Assessing the depth of the carbonated layer How can we design a concrete structure for 100 years without corrosion? Use of adequate type of concrete with different ratio water/cement Latex modified concrete (with polymer) Grouts for bonded post-tensioned concrete Corrosion inhibitors Types of inhibitors Major commercially available corrosion inhibitors: Epoxy-coated reinforcing steel Performance of epoxy-coated steel bars Corrosion-resistance reinforcing bars Examples of new coating materials How can we stop the problem in an existing structure?

3 1 WHAT IS THE CARBONATION PROBLEM? This Digest discusses the carbonation of normal dense concrete, which results from the reaction of atmospheric carbon dioxide gas with hydrated cement compounds. It relates particularly to the assessment of the risk of corrosion to embedded steel. The Digest describes the carbonation process and how the depth of carbonation can be measured; it outlines the various factors influencing the depth and rate of carbonation. 1.1 WHAT IS CARBONIZATION OF THE CONCRETE AND HOW IS PROCESS GOING The term Carbonation of concrete means the chemical reaction between carbon dioxide in the air and hydration products of the cement. Carbon dioxide (CO 2) is a minor component of the atmosphere: about 0.03% by-volume. It can react with and decompose the hydration products of the cement in concrete to form various carbonate minerals. The process includes: 1. Diffusion of CO 2 in the gaseous phase into the concrete pores, 2. Its dissolution in the aqueous film of these pores, 3. The dissolution of solid Ca(OH) 2 in the water of the pores, 4. The diffusion of dissolved Ca(OH) 2 in pore water and its reaction with the dissolved CO 2, result of several steps through which the calcium carbonate is formed may simply be described by the following reaction which is assumed to be irreversible: Ca2+(aq) + 2(OH )(aq) + CO2(aq) = CaCO3(s) + H2O or we can write like this: Ca(OH) 2 +CO 2 = CaCO 3 + H2O Figure 1: Formula for Carbonation of concrete This process, always start from the exposed concrete surface. The outer layer of concrete in which appreciable reaction with carbon dioxide has taken place is called the carbonated layer. This cancer of reinforced concrete is a serious problem because the construction itself is vulnerable to decay. 3

4 Figure 2: Carbonated Layer (appreciable reaction with CO2) Carbonation of concrete is associated with the corrosion of steel reinforcement and with shrinkage. However, it also increases both the compressive and tensile strength of concrete, so not all of its effects on concrete are bad. Figure 3: Examples of Carbonation in concrete (Corrosion of Steel Reinforcement) 1.2 FACTORS AFFECTING CONCRETE CARBONATION The rate of carbonation depends on porosity (for CO 2 to Diffuse) & moisture content of the concrete (for dissolution of solid Ca(OH) 2). The diffusivity of CO 2 depends upon the pore system of hardened concrete and the exposure condition. The pore system of concrete depends upon the type and the content of cement, water/cement ratio, and the degree of hydration. The main factors affecting concrete carbonation are: 1. Pore system of Hardened Concrete which in turn depends upon w/c ratio, type of binder, and degree of hydration, 2. Relative humidity (for dissolution of Ca(OH)2), 3. The concentration of CO 2. Water Cement Ratio ( w/c) The carbonization rate in concrete is directly dependent on the ratio of water and cement (v / c) to concrete, the higher the ratio, the higher the depth of carbonization in the concrete. For concrete of basic quality, the expected carbonization rate is very low. For example, for concrete with a ratio of water and cement of 0.45 and a range of reinforcement to the edge of concrete of 25mm, it will take more than 100 years for carbonization to come into concrete, which is in the immediate vicinity of 4

5 steel. Carbonization of concrete or mortar is a bigger problem in Europe - which has led to the application of electrochemical realkalinisation of concrete - than in the United States. Figure 4: Water/Cement Ratio (w/c) 1.3 CARBONATION DEPTH Cement paste has a ph of about 12-13, which provides a protective layer (passive coating) to the steel reinforcement against corrosion. Loss of passivity occurs at about ph 11. Carbonation of the concrete, caused by carbon dioxide in the atmosphere, has the effect of reducing the ph. Figure 5: ph value of concrete before carbonation and after A low ph, reacted layer of concrete forms at the surface and penetrates inward to a depth x proportional to the square root of the exposure time t. x = Kc t 1/2, where Kc is called the carbonation coefficient 5

6 Figure 6: Evolution of accelerated carbonation depth in time For example, if the carbonation depth is 1mm in a one-year-old concrete, it will be about 3mm after 9 years, 5mm after 25 years and 10mm after 100 years. When x is equal to the rebar cover depth, steel depassivation takes place and corrosion begins. Assessing the depth of the carbonated layer There are several ways of assessing the depth of the carbonated layer. The affected depth from the concrete surface can be readily shown by the use of phenolphthalein indicator solution. This is available from chemical suppliers. Phenolphthalein is a white or pale yellow crystalline material. Carbonation depth is assessed using a solution of phenolphthalein indicator that appears pink in contact with alkaline concrete with ph values in excess of 9 and colourless at lower levels of ph. The test is most commonly carried out by spraying the indicator on freshly exposed surfaces of concrete broken from the structure or on split cores. Alternatively, the powder from drill holes can be sprayed or allowed to fall on indicator-impregnated paper. The test is covered by BS EN 14630, Products and systems for the protection and repair of concrete structures. Test methods. Determination of carbonation depth in hardened concrete by the phenolphthalein method. Figure 7: Measuring carbonation with Phenolphthalein indicator 6

7 Figure 8: Difference between Carbonated and Non-Carbonated piece On this pictures we can see that the indicator has not changed colour near the top and bottom surfaces, suggesting that these near-surface regions are carbonated to a depth of at least 4 mm from the top surface and 6 mm from the lower surface. Where the indicator has turned purple - the centre of the slab - the ph of the concrete pore fluid remains high (above 8.6, probably nearer 10). Whether the cement paste here is completely Noncarbonated is unclear, despite the strong purple indicator colour; a more complete assessment would require microscopic examination. Indicator was not applied to the concrete at the right of this image and so the concrete here retains its original colour. 7

8 2 HOW CAN WE DESIGN A CONCRETE STRUCTURE FOR 100 YEARS WITHOUT CORROSION? Corrosion cannot be avoided and will appear soon or later, but several solutions exist to slow the phenomenon. Thanks to those possibilities, it is possible to increase the lifetime of structures considerably, although the cost increases also. 2.1 USE OF ADEQUATE TYPE OF CONCRETE WITH DIFFERENT RATIO WATER/CEMENT Reducing the corrosion of steel in concrete is possible first just by modifying the water/cement ratio and using an appropriated type of cement. Those are several possibilities, using a conventional concrete: - The grain sizes influences the water absorption of the concrete. By using smaller grain, it is possible to increase the density of the concrete and to make it impermeable. - Decreasing the water/cement ratio permits to decrease penetration of chlorides in concrete. In Belgium, in 2007, the CTSC (Construction Scientific and Technical Center) have conducted research about that type of influence 1. The figure below shows that for the same cement, with the same dosage of cement, a different water/cement ratio influences the carbonation coefficient. The carbonation coefficient indicates the concrete resistance of the carbonation phenomenon. Higher is this coefficient, faster is the carbonation and lower is the phenomenon resistance. It depends on the depth of carbonation D (in mm) which can be measured with a phenolphthalein solution, a ph indicator, and on the age of the concrete t (in years). D = k c / t Figure 9: Inlfuence of the w/c ratio on the carbonation coefficient of concretes made with cement CEM I 42,5, dosage 300 kg/m3 The results show that higher is the w/c ratio, more important is the carbonation of the concrete (and consequently the corrosion of steel). - The puzzuolanic additives reduce the content of Ca(OH)2, and so reduce the concrete resistance of carbonation. It s why those additives have to be used in limited quantities. Indeed, the CSTC noticed that with a constant dosage of cement and a constant w/c ratio, cements with fly ash (CEM II and CEM V) and with blast furnace slag (CEM III) have a higher carbonation coefficient than Portland cement. So those type of cements are more vulnerable 1 Corrosion des armatures induite par la carbonatation du béton : comment s en prémunir? Dossiers du CSTC, N 3, Cahier n [18/10/2017] 8

9 to carbonation. Although silica fume permits to have a denser structure, and therefore less permeable. Figure 10: Influence of the type of cement on the carbonation of concrete (same ratio w/c=0,525 and dosage 300kg/m3) - In figure 2, it is also possible to see that a satisfactory concrete curing (concrete protection during the grip and hardening phases) permits to reduce the apparition of the carbonation phenomenon. - It is also important to notice that the dosage in cement, with an equivalent w/c ratio and type of cement, doesn t have a significant impact on the carbonation of concrete as it has be demonstrated in the CSTC researches. Figure 11: Influence of cement dosage on the carbonation of concrete - Having a satisfactory cover of steel by the concrete is also an important rule to reduce steel corrosion in concrete. All of those parameters can reduce the carbonation of the concrete and thus corrosion of steel. Although, it is important to maintain the desired properties of concrete that should not be affected by the changes in the composition of the concrete, particularly in terms of resistance. 9

10 2.2 LATEX MODIFIED CONCRETE (WITH POLYMER) 2 The latex modified concrete (LMC) is a concrete composed by polymers. Compared to a conventional concrete, the LMC has a lower water/cement ratio and includes 15% of polymers, usually Styrene- Butadiene Rubber (SBR). Several experimental programs proved that this type of concrete has a better resistance to carbonation and corrosion of steel in concrete. The test used is called Accelerated corrosion cell. The accelerated corrosion cell proved to be a good and simple test to assess the durability of concretes especially with respect to chloride ion penetration. Figure 12: Schematic diagram of the accelerated corrosion cell The results are presented in figures X. The variable used to describe the corrosion phenomenon is the Corrosion Current, which corresponds to the speed of the steel corrosion chemical reaction. 2 OKBA S.H, EL-Died AS, REDA M.M. Evaluation of the corrosion resistance of Latex modified concrete (LMC). Cement and Concrete Research, vol. 27, No 6. 5, pp , 1997 [18/10/2017] 10

11 The LMC proved to be superior in its corrosion resistance compared to conventional concrete 3, which recommends its use in structures exposed to severe aggressive environments. Figure 13: Corrosion current for a conventional concrete and a Latex modified concrete at different test ages To understand why, the composition of LMC was also studied. The LMC is characterized by a dense microstructure compared to that of the conventional concrete. Also, great reduction was observed in the size of the crystals formed throughout the polymer cement matrix, such as calcium hydroxide crystals, indicating a more dense microstructure than that of the conventional concrete. The LMC was observed to have a pore size which is much smaller than that of the conventional concrete. Most of the pores were found to be filled with the polymer latex. 2.3 GROUTS FOR BONDED POST-TENSIONED CONCRETE For the post-tensioned concrete, grouts are used to provide a non-corrosive environment and corrosion protection to the steel prestressing tendons. Like concrete, grout consists of Portland cement, water and admixtures. And like concrete, the permeability of grouts can be reduced by lowering the water-cement ratio, adding silica fume, or latex modifiers. Although, grout viscosity during grouting should be high enough to be effectively and adequately pumped, completely filling the post-tensioning duct. And reducing the water-cement ratio, or others methods to reduce the grout permeability also have the consequence to increase the density and the viscosity of the grout. It is why, in order to reduce the phenomenon of corrosion of the steel prestressing tendons, the solutions detailed before can be applied by using a high-range water reducer. Indeed, it can maintain adequate fluidity at reduced water-cement ratios in grouts with silica fume, and polymer reducers. The length of open time is controlled by the superplasticizer dosage rate. Styrene-butadiene latex modifiers, when used in conjunction with superplasticizers, produce fluid grouts with prolonged open times. 2.4 CORROSION INHIBITORS In order to protect the reinforced and prestressed concrete structures, chemical inhibitors 4 can be added to Portland cement concrete mixes during batching (usually in very small concentrations). This 3 SHAKER F.A, EL-Died AS, REDA M.M. Durability of styrene-butadiene latex modified concrete (LMC). Cement and Concrete Research, vol. 27, No 5, pp , 1997 [18/10/2017] 4 MG RICHARDSON, Corrosion inhibitors for steel in concrete: State-of-the-art Report, [04/11/17] 11

12 corrosion-protection system works by delaying the onset of steel corrosion. This technique presents a long-term durable solution to prevent the corrosion of steel in concrete structures. Inhibitors are often used in combination with low-permeability concrete. They increase the threshold chloride concentration needed to initiate corrosion. They can be used to protect uncoated highstrength steel strands used in prestressed concrete structure, and also in cement grouts for filling the ducts of post-tensioned cables. There are three major concerns regarding the use of corrosion inhibitors: - Long-term stability and performance of inhibitor. - The effect of the inhibitor on corrosion propagation after corrosion initiation. - The effect of the inhibitor on the concrete s physical properties over the service life of the structure. In fact, inhibitors may have an effect on the corrosion process after corrosion initiation; some of them will affect the chloride transport in the concrete structure, and can reduce the rate of chloride ion migration and therefore delay the corrosion. The use of these inhibitors should not affect the concrete properties; structural, permeability, etc. This technique should not increase the amount of any concrete cracking. Types of inhibitors Corrosion inhibitors are either organic or inorganic. They are classified based on the protection mechanism. Organic inhibitors consist primarily of amines and esters. They form a protective film on the surface of steel reinforcing bars and/or delay the arrival of chloride ions. Inorganic inhibitors protect the steel reinforcing bars through oxidation-reduction reactions at the steel surface. - Active type of inhibitor (anodic): Facilitates the formation of an oxide film on the surface of the steel reinforcement bars. - Passive type of inhibitor: Reduces the rate of chloride ion migration. Major commercially available corrosion inhibitors: - DCI (Dares Corrosion Inhibitor) and DCI-S - DCI (Calcium nitrite) is an inorganic inhibitor. It is an anodic type of inhibitor (active) and functions by passivating the anode. Chloride ions accelerate corrosion through the formation of Fe ++ ions. Nitrite ions inhibit corrosion through the formation of passive iron oxide (Fe 2O 3). - Rheocrete 222 and Rheocrete 222+ Rheocrete is an organic inhibitor. It protects the steel bars by forming a corrosion-resistant organic film on the steel surface, and by coating the pores of the concrete matrix and slowing the migration of chloride ions. - Armatec 2000, Ferrogard 901 and MCI 2000 Protection of the steel bars is made by forming a continuous monomolecular film on the steel surface, and covering both anodic and cathodic sites. - Catexol 1000 CI Catexol is a water-based formulation of amine derivatives. It protects by forming a protective barrier, which stabilizes the passive iron oxide layer. 2.5 EPOXY-COATED REINFORCING STEEL The corrosion of steel reinforcement bars is mainly caused by the chlorides. Epoxy-coated reinforcing steel (ECR) can prevent the deterioration by corrosion and extend the life cycle of the structure. The 12

13 epoxy coating is a barrier system intended to prevent moisture and chlorides from reaching the surface of the reinforcing steel. Epoxy coatings sometimes referred as powders are solid dry powders. Theses powders are electrostatically sprayed over cleaned and preheated steel reinforcing bars. The physical properties of the epoxy powders do not change with change of temperature. Performance of epoxy-coated steel bars 5 - The epoxy-coated reinforcing steel doesn t influence the physical properties of the concrete. It is not linked to cracking or any mechanical characteristics. - Corrosion in the epoxy-coated steel is 81 %, less compared to normal reinforcing steel. - Once the concrete is cracked, the ECR (epoxy-coated reinforcing steel) allows more corrosion, and does not perform as well as the normal steel. - The use of ECR has to be combined with adequate good-quality concrete cover, adequate inspection and finishing, in order to improve the corrosion protection. Figure 14: Epoxy-coated steel bars 2.6 CORROSION-RESISTANCE REINFORCING BARS Before choosing the type of reinforcing steel bars in the concrete structures, it is important to consider the various severe environments (during transportation, storage and installation) in which the bars will be exposed. During storage, reinforced bars can be exposed to condensation, rain, chemical attacks and seawater. After the bars are embedded in concrete, they are exposed to a high ph and moist environment. Therefore, the coatings of the bars need to be stable in all these environments and should not be deteriorate. Coating defects are a major factor in coating deterioration. Gaps called holidays in the coating can allow the chloride ion to access directly to the steel surface, and also reduce the overall electrical resistance. This will affect the durability of the coating, and the reinforcing bars. The use of highly corrosion-resistant reinforcing bars can provide an additional layer of corrosion protection for reinforced concrete structures. Even though this is much more expensive than the regular coatings (epoxy coating for example), this corrosion-resistant materials may be very costeffective. It increases the life cycle of the bars, regarding permeable concrete, concrete cracking, and harsh service environments; especially that the repair of corrosion-induced deterioration is costly and hard to do. 5 National Precast Concrete Association, Working with epoxy-coated rebars [04/11/17] 13

14 A 75- to 100-year design life can be achieved by extending the corrosion initiation period and reducing corrosions rates. The use of a less sensitive coating material to depassivation can extend the corrosion initiation period. New materials are being tested and started to be used for the coating of the reinforcing bars; including organic and inorganic coatings, ceramic, and metallic-claddings. Examples of new coating materials - Hot-dipped galvanized bars 6 - Bars coated with zinc using Delot process - Ceramic-clad bars - Copper-clad bars - Galvan (aluminum and zinc) coated bars. Figure 15: Hot dipped galvanized steel bars 6 Zinc protects, Hot dipped galvanized reinforcing Steel A concrete investement, [04/11/17]. 14

15 3 HOW CAN WE STOP THE PROBLEM IN AN EXISTING STRUCTURE? The problem with carbonation in already existing concrete structure can be handle in different ways depending how deep the carbonisation has gone into the concrete. If it has gone so deep that it has reached the steel reinforcement there will be a need for repairing the structure by replacing the damaged concrete with new or at least cement based material. If it has not gone that far there are some ways to stop it for going further with for example surface treatments. One way to measure the depth of the carbonisation is with phenolphthalein indicator that will change colour depending on the ph-value of the concrete. The test is performed on freshly exposed surfaces or from drilled cores 7. For structures that has been exposed to carbonation for a short period of time where the lowered phvalue haven t penetrated so deep into the concrete yet, surface treatments and or polymer modified cementitious mortars are a good way of stopping it from going further. Surface treatments works as an effect barrier to CO 2 diffusion. They are classified by a European standard pren which divides the different treatments in to three groups Hydrophobic impregnation, Impregnation and Coatings 8. Hydrophobic impregnation is consisting of alkyl alkoxysilanes and is regularly mentioned as a water repellent agent which will extend the service life for the concrete structure. This is done by protecting the steel reinforcement from chlorides and by changing the content of the moist inside the structure. The treatment makes the surface layer become hydrophobic which means that the water can t enter but at the same time it stills allowed water to pass through by vaporizing. This means that rain and chloride ingress will not be able to penetrate through the surface layer 9. Figure 16: Hydrophobic impregnation 10 Impregnation creates a thin layer that is discontinuous and moderately fills the capillaries 11. Impregnation that contains corrosion inhibitors will decrease the corrosion rate but will still not change the concentration of any corrosive agent. The impregnation is applied by spraying or putting it in a gel. It is important that the inhibitor goes through the concrete to the reinforcement 12. Coatings will create a continuous layer on the concrete surface. Coating with acrylic paint is the protection against penetration of water and CO 2. Acrylic paint also a good resistance against the sunlight and weathering. If the concrete structure is exposed to an environment, where it is in water

16 or nearly never dry, coating with cement is to prefer. If the environment instead is chemical aggressive, plastic-based coating can be used which also makes the mechanical properties of the surface better 13. Figure 17: Coating 14 These different types of treatments can be done alone or combined if they are not sufficient enough alone. Except for surface treatment, the use of cementitious mortar or rendering concrete is the most efficient way to protect the concrete. The cementitious mortar or rendering concrete will be a layer that absorbs CO 2 but this can create an aesthetic problem since it demands a certain thickness to be effective and protect the concrete structure. 15 A different kind of treatment against carbonation of concrete is realkalisation which is a method where applied DC voltage between the reinforcement in the concrete and a steel mesh on the surface of the concrete will drive alkali ions, from an applied fibre pulp with high alkali level, into the concrete. The ph value in the concrete will then rise and the protective oxide layer around the steel reinforcement will return 16. If the carbonisation has gone too deep into the structure so that there is no chance to stop the carbonation in the concrete the damaged concrete needs to be removed and the corroded steel needs to be cleaned. After the cleaning process the renovation method can be decided. Carbonated concrete that is intact and do not protect any reinforcement structure does not necessarily need to be removed, but if the carbonated concrete is next to reinforcement that do not already have corrosion damage the carbonated concrete needs to be removed. If there is reinforcement that lies close to or on the surface of the structure it should be removed if it is not necessary for the structure, otherwise it should be knocked in the structure or covered by at least 20 cm of new concrete. The kind of reparation mortar that is used depends on the current concrete structure

17 Figure 18: Reparation with mortar There are different methods of applying of new concrete on the damaged area. It could be done by hand, where it is very important to first make sure that the surface of the application is first moisturized. When the mortar is applied by hand it is important that the area first is elutriated. When this has dried the reparation mortar is applied. Another method that can be used is applying new concrete with a casting mold. This method is often used on horizontal surfaces. In this case smooth concrete surfaces need to be rugged in order to improve the adhesion. Another way to apply new concrete on a surface is with shotcrete. There are different types of shotcrete, with and without fibre reinforcement, adhesion and salt resistant attributes. This method is often used when applying new concrete in walls and ceiling of tunnels. This type of concrete needs to be protected from dehydration after the application by watering the product and covering it with plastics 17. Even if the carbonation process is slow, sometimes it is not detected until the corrosion of the reinforcement has gone too far. In those cases, it is not possible to only clean the reinforcement since it has lost its capacity 18. In this case, the reinforcement either needs to be exchanged or strengthened by putting up another piece of reinforecement next to the corroded parts of the existing one /05o3_Bil1_10årsDrift_och_underhållsplan_2008_2017.pdf

18 Figure 19: Reinforcement of the corroded parts 18