Renovating steel structures in residential buildings
One of the main causes for the renovation of steel structures in residential buildings is the corrosion of steel in reinforced concrete. In more than 80% of the cases in which reinforced concrete requires renovation the root cause has been the corrosion of the reinforcing steel (17)
Product iron corrosion is iron oxide: FexOy:
2 Fe + O2 + 2 H2O + heat → 2 Fe(OH)2
1 vol ca. 2,5 vol
We see that iron oxide takes up twice as much space as normal, metallic iron. It’s therefore easy to understand how concrete can crack while the rebar corrodes. The original rebar takes up a certain volume only in a tight concrete matrix. As the metal rods corrode, the volume they take up increases significantly. The concrete is a fixed substance and is resistant to change meaning that as the iron bars corrode, an internal pressure is built up inside the concrete. After a certain amount of time, as corrosion of the rebar continues, this stress will prove to be too much for the concrete to handle and signs of cracking and fissuring will start to show (17).
In reality however when reinforcing steel corrodes, a whole host of iron oxide compounds form. Not just Fe(OH)2. Unfortunately for us that has as a consequence that the volume this mix of iron oxides takes up within the concrete can be up to 3-12 times as much as the volume the original rebar claims. This is highly dependent on the various variables of the electrochemical process. As mentioned higher this will not only lead to cracking and the formation of fissures, this will also expose, slowly but surely, more bare rebar, ready for corrosive attack (17).
Next we have to discuss “rust” and the consequences it has for the adhesive forces within the concrete. When talking about “rust” I am talking about the layer of iron oxides we see when a metal bar is corroding. We have all handled a rusty bar at some point in our lives and we can all remember the stained color our hands had after we put the bar down again. This stained color is easily transferred to your hands because “rust” has very poor adhesive qualities to its parent metal. Much unlike aluminium oxide which forms a protective passive layer on the surface of aluminium with high adhesive forces. Since it is always the outer layer of the parent metal that gets corroded it is on this outer layer that rust forms. However it is also this outer layer that is in contact with the concrete, in reinforced concrete, providing strengthening for the concrete structure. As easily removable rust forms on the surface of the rebar it means that the surface contact between the metal bar and the concrete becomes less and less, meaning that the steel rebar is not able to adequately strengthen the concrete anymore and thus the whole structure loses its strength (17).
Lastly as I explained above “rust”, or the mix of iron oxides that form on the surface of the reinforcing steel, possesses low adhesive forces with its parent metal and is therefore easily removed. It just “flakes off”. This means that as it flakes off, new metal is exposed underneath the iron oxide layer. As a consequence this newly exposed layer will start to corrode, after which the corrosion layer will flake off and the process repeats itself. This process of corroding, flaking off, corroding means that locally, where this corrosion occurs, the metal will start to thin out as over time more and more material is removed.
However it is very unlikely for steel to corrode once used in reinforced concrete as it is in a passive condition encased in an alkaline environment within the concrete (pH > 13). If steel does corrode, then it happens when the reinforced concrete structure has been exposed to environmental aggressors such as moisture or salts-ions that are using moisture as a vector to access the concrete matrix (such as chloride ions for example) (18).
In general the corrosion of steel is started and maintained by one of two mechanisms. The first mechanism is when the pH of the concrete gets systematically lowered. This can happen either by leaching of basic substances with water, leading to a reduction in pH. The second way in which this can happen is through Carbonation. Carbonation is the reaction of carbon dioxide in the air with calcium hydroxide or hydrated calcium silicate in the concrete. As a consequence this neutralizes the environment in the concrete leading to a reduction in pH. The pH of the environment surrounding the steel structure within concrete would have to fall below 9 for corrosion to start. (17)(18)(19).
Another way corrosion can occur within concrete is through reaction with chlorides and oxygen present in the environment. Especially in a maritime setting chloride ions can be quite abundant due to the salinity of the nearby sea (20).
When repairing reinforced concrete the aim should be to completely remove the damage as possible, while at the same time safeguarding the concrete structure from any further corrosion. This might require to provide additional strengthening or even completely rebuild certain parts of the reinforced structure. Deterioration next to repaired concrete patches are quite common due to “Halo of Anodic Ring”-effect; When the same rebar extends in two distinctly different environments, conditions result in an electrochemical process which may result in corrosion where the old and new parent concrete meet. This dividing line is also called the “bond line” (21).
Firstly the damaged concrete needs to be addressed. This is referred to as surface preparation. Executing good surface preparation requires that minimal damage is done to the remaining, “good” concrete at the bond-line. Abrasive blasting or high pressure water jetting can be used to get rid of any loose concrete as a result of micro-cracking. The surface needs to be clean, free of contaminants and roughened to an appropriate degree for the selected repair material (19).
However there are some things to be taken into consideration. Using a jackhammer to remove concrete might cause micro-cracking which is undesirable. Only the lightest of hammers should be used when removing material in this way. For larger projects hydrodemolition would be a better option. Although micro-cracking is minimized, containment and disposal of the water used could pose a problem. (19)
According to Jay H Paul, EVP Hoffman and Klein (18); The most common repair methods for cement are the following: With the Cast-in-place method concrete the defective concrete is replaced with new concrete. With the Form-and-pour method the damaged or deteriorated concrete is replaced by placing repair mortar in a formed cavity. Troweling is used for shallow and/or limited areas of repair. Dry packing is the hand packing of very dry Portland cement mortar that is stamped or rammed into place. The shotcrete method involves pneumatically placing concrete or mortar through a hose at a high velocity onto a surface. Injection grouting is a common method for filling cracks or open joints. Cementious or chemical materials can be used for this method.
4.1 How to protect the rebar from corroding after repair?
To extend the life of repairs and minimize future deterioration various concrete protection systems can be used in addition to the completed renovation-works. Firstly let's review surface applied protection systems. To start. penetrating sealers are materials which, after application, as the name suggests, penetrates the concrete. Such products can include various types of oils and high molecular weight methacrylates. Next Surface sealers are products with a thickness of less than 0.25 mm that lay on the surface of the concrete. These can be a variety of epoxies or other resins types. High-build coating are materials that are thicker than surface sealers. Generally they have a thickness between 0.25 mm and 0.75 mm. Like surface sealers these are also applied to the surface of the concrete. Membranes are thicker than high-build coatings but no more than 6mm. Urethanes and asphaltic products would all be excellent choices as a membranes. Overlays are the thickest of all surface applied protection systems. With a thickness of over 6 mm these are products that are, in general, bonded to the surface of the concrete. This protection method will prevent any foreign materials being absorbed into the reinforced concrete, subsequently preventing carbonation and preventing chloride ions from the environment from attacking the steel rebar. (21)
Secondly let's review Migratory Corrosion Inhibitors. Migratory Corrosion Inhibitors (or MCI’s for short) are chemical compounds that when added to a liquid or gas provide corrosion protection for the base metal. MCI’s come in a variety of forms and can be for example nitrite compounds or chromates. We will discuss in detail MCI’s that have an amine-functional group. Through the process of chemical adsorption these chemicals are adsorbed to the surface of the reinforcing steel and form a tiny film of just a few micrometers thick. Chemical adsorption is not be confused with absorption. Absorption of a certain chemical means the inclusion of that chemical compound within the host compound. Adsorption is different in the sense that the “to be adsorbed”- compound never enters the chemical structure of the host compound, it merely forms a film on its surface. Chemical adsorption of MCI’s by the reinforcing steel leads to the steel becoming resistant to many aggressive and reducing substances from the environment. At the same time the steel itself is extra protected as MCI’s work as a reducing agent for iron oxides that might form on the surface of the reinforcing steel. Due to the chemical adsorption and formation of the protective layer on the surface of the reinforcing steel “popular” corrosion reactants such as O2 and H2O find it impossible to reach the bare surface of the steel. Herein lies the big difference between amine-based MCI’s and nitrites for example. Nitrites and/or chromates have the tendency to protect the reinforcing steel from corrosion through anodic passivization, meaning that the nitrates and chromates themselves sacrificially oxidize in order to protect the parent metal. MCI’s with an amine-functional group simply provide an impenetrable layer and do not take part in the redox-reactions that form metal oxides that might take place at the surface of the reinforcing steel. It forms a barrier for the reactants therefore preventing the corrosion reaction from taking place. (21)
Lastly Cathodic protection is another way of preventing corrosion. The basic principle would be to make the rebar cathodic, preventing further corrosion of the steel. You can do this by making sure the steel is in contact (electrically) with another metal that becomes the anode, with or without the application of an external power supply. Without an external power supply cathodic protection systems are called “sacrificial systems”. The “less noble” metal used to protect the steel will be corroded away to protect the steel from corrosion. Zinc is commonly used in this principle. “Impressed current systems” are cathodic protection systems using an external power supply. With this system a small amount of current is forced through the rebar. The purpose of this current is to stop the flow of current caused by the corrosion process. A metal that corrodes at a very slow rate, such as platinum, is usually used as an anode in this protection system. (21)
