Corrosion, Repair and Maintenance of Structures Essay
Chapter 1.0 Introduction
In many places around the world, there are structures that have existed for a long time and either they are still being used or they are simply left there standing with no use at all to the public. For that reason, these structures that are no longer being used must be rehabilitated in order for them to be re-used in some way. When pertaining to structures, to rehabilitate means ‘to restore, to repair, to rescue from a state of corrosion’.
The idea of rehabilitation came after the World War II when old buildings were starting to show damage through time or damage from human intervention. The easiest way to fix them was to restore them. That process today is very widely used and is a very familiar process, which applies to almost every structure on earth—from structures that need to be repaired to structures that need to have a completely different look or structures that need to expand. It can also be applied on structures that need to be strengthened in order to prolong its life for another set of years.
Rehabilitation of structures, as a process, can save time, money and work. That is because there is no need for the demolition of an already existing structure and building again from scratch. Simply put, rehabilitation means that some changes or improvements need to be made. This process also saves space since there is no need for a new area in order to build a new structure.
In this paper, I am going to divide this process into three stages. The first stage is the corrosion, where I am going to explain all the factors that lead to corrosion of structures and how it affects them. The second stage is the part of repairing a structure after it has undergone corrosion. In this stage, I will mention ideas and ways that have been developed through the years which can be applied to existing structures that have been affected by corrosion. The third and final stage that will be discussed in this paper are the ways on how to maintain a structure and how to prevent further deterioration after repair in order to minimise its cost efficiency.
Chapter 2.0 Corrosion Factors
Corrosion is caused by a lot of factors and it often results to the failure of a structure. All these factors have to be studied in order to prevent structure failures. Corrosion, most of the time, is caused by extreme weather conditions but it is not the only factor that causes corrosion and it is also not the main cause of structure failures. By analysing corrosion and explaining how it happens, we can separate it in different areas.
Factors of Corrosion
Corrosion depends on many factors which we will focus on such as material, environment, stress, temperature and time factors. These factors are dependent and interconnected with each other.
The material factor has to do with the materials that are used for the structure. Materials also has a connection with the environment where the structure is put up (i.e. weather, temperature, moisture, etc.), which we will discuss later in this report. The material factor is important as it all starts from the selection and planning before being able to build the structure. The selection process is divided into three stages.
The first one is listing the requirements taking into consideration the strength and other properties of the material. The second stage is choosing the most convenient material focusing on its technical properties and evaluating them. The last stage is based on the economical factor, which means that after going through the first two stages, we came up with a variety of materials and the only thing left to do is to choose the cheapest and most suitable material for the structure. The final process has to do with the fact that the structure is for repair and that redesigning the whole structure is not easy which means that more attention should be given on delivery time and completion.
Environment is a crucial factor in corrosion of materials. This is caused by a lot of factors and the main factor that needs to be considered is that the materials have a tendency to expand or shrink depending on the changes in the temperature (Table 2 below shows some data on expansion of different materials) and also in areas where the weather changes from rainy to dry or vice versa in a short span of time. Moreover, water as a factor greatly affects a structure because it can penetrate in the material and cause it to crack. If the material is steel, its exposure to water will cause it to rust and the rust can spread and also affect other materials.
Another factor to be considered is moisture that can be caused by rain, condensation, leakage from pipes or drains and rising damp. For the underwater structures and especially for those in salty waters (i.e. sea) more problems arise because salt can accelerate the phases of corrosion, and due to that, extra care has to be provided in choosing the materials. Wind also has to be considered. In some areas, winds can prove to be very strong. And strong wind coupled with the effects of some other factors can cause extreme deterioration also because it can bring dust and dirt, which can penetrate materials causing discolouration and corrosion.
Finally, pollution is another factor that contributes to the environmental corrosion of a structure. As we know, atmospheric pollution can be transferred to the ground in the form of acid rain. This acid rain can react with the materials that were used for the structure and can cause its rapid deterioration. Regular cleaning of the structure would be helpful in preventing any unwanted deterioration.
Stress connects and interacts mostly with the environmental and the temperature factors. Changes in temperature followed by expansion and contraction causes cracks on structure which can result in structure failure in a worst case scenario. In some situations, the structure fails without showing external deformation making it hard to avoid. Even though cracks cannot be seen by the human eye, there are multiple cracks attacking the materials under the surfaces of a structure. These cracks in microstructures can form intergranular or transgranular morphology. These kinds of cracks can cause a big problem because they are difficult to detect. The photo on the right is shows an intergranular SCC (Stress Corrosion Cracking) following the grain boundaries. Problems like this can cause an unpredictable failure of the structure resulting in loss of materials.
Changes in local temperature can affect the stability of a structure in a major way because of the expansion and contraction that the structure is being exposed to. Usually materials tend to contract during low temperatures and expand during high temperatures which can cause small but dangerous cracks that can further lead to the failure of the structure. In order to prevent such a disaster from happening, the most common recourse is to select proper materials that can adapt to the environment where the structure will be erected. The temperature factor, of course, depends on the environment factor and the stress factor that we talked about earlier in the report.
Time in corrosion is a very big issue. Time is the factor that produces all the problems and is the one that makes all the others affect the structure. Without time, corrosion would not happen. Of course, time is not the only factor that causes corrosion as we have already discussed. The ones that have a clear connection with the time factor is the environment, temperature and stress factor.
From all that were mentioned, one final but very important factor must be taken into consideration. Corrosion does not have the same effects on all materials; different type of material will have a different kind of reaction with the various factors.
Types of Corrosion Affecting Structures
First of all, let us consider that we have a reinforced concrete structure.
The mild steel that is used for this structure to reinforce the concrete creates a layer on its surface in order to keep it in a passive state. This layer is caused by the alkalinity of the concrete. Therefore, the corrosion that will affect the structure will be caused by the steel that is reinforcing the concrete, and there are three types of such based on the steel corrosion. These are Chloride Contamination, the Patch Accelerated Corrosion and Carbonation.
The presence of chloride ions in the atmosphere, which are formed usually where structures are exposed to de-icing salts (used to maintain safe transportation during winter time) or to a marine environment, can cause the destruction of the protective oxide layer of the reinforcing steel and lead to its corrosion.
Patch Accelerated Corrosion
This type of corrosion is either known to the public as the “Ring Anode Corrosion” or the “Halo Effect”. This phenomenon is often detected on concrete restoration projects where there is presence of concrete spalling on previous patch repairs. It is most commonly found on the exterior of the structure and is caused by its exposure to freeze and thaw cycles.
This process is very similar to Chloride Contamination. A state called carbonation, it is caused by the reaction of carbon dioxide within calcium hydroxide in the presence of water. It can cause the loss of alkalinity in concrete. This loss of alkalinity that happens to the surrounding concrete of the steel can cause the destruction of the oxide layer that is protecting the reinforcing steel. This kind of corrosion is a big threat to old buildings.
Areas where Corrosion Applies
Corrosion can be found in different kinds of structures. One of them is the bridges where there can be corrosion on the reinforced concrete caused by the presence of de-icing salts or by their exposure to marine environments. Parking garages are also exposed to corrosion, and this can be caused again by the presence of de-icing salts transferred there by the parked cars. Of course, buildings are exposed to corrosion too, and it most likely applies on spandrel beams, columns and balconies.
The types of corrosion usually found on these structures are the types of corrosion that were discussed earlier, namely, chloride contamination and carbonation, which can also be caused by exposure to sea spay usually to buildings close to the sea, and also through contact with atmospheric carbon dioxide. Another area where corrosion can be applied are the marine structures, like concrete piers and wharfs since they are exposed to chloride contaminated salt water and airborne chlorides along with high temperatures and high humidity.
Deterioration of Concrete in General
Concrete, like other materials deteriorate due to internal and external forces causing concrete failure. Other causes are water infiltration, carbonation, corrosion of reinforcing steel, shrinkage, drying, thermal contraction and poor placement practices. From that, water can cause a big problem to concrete since it can penetrate and freeze during low temperatures. This can apply extreme pressure to concrete and make it weak and finally cause it to fail. Also, water carries chemicals like acids, sulphates or chlorides that can attack concrete and corrode the reinforcing steel inside it and expand the corrosion from inside to outside.
Another cause is carbonation, where water from rain can be combined with carbon dioxide in air and after some time infiltrate concrete and react with the calcium hydroxide and create calcium carbonate which can decrease the concrete’s pH level and attack the surrounding layer of the reinforced steel. This can cause corrosion of the steel since it is no longer protected and this as we said before can affect concrete as well because of the presence of moisture.
Deterioration of Structures and Service Life
The deterioration of structures is a process that comes in stages and from that, we can point out the starting stages of the actual corrosion which is the concrete cracking and the excessive deflection. This can result in a reinforced concrete failure due to loss of structural strength.
Service Life of a Structure
Each structure has its own service life. This service life is said to be the time period that when ended will need human intervention such as conducting repairs, strengthening, maintenance or rehabilitation. By knowing the service life of a structure, we can determine the time when such actions will be required. The service life of a structure is separated into four stages.
The first of the four phases is the time period from completion of building the structure to its corrosion initiation. The second phase starts after the corrosion initiation and until corrosion induced cracking. In the third phase, we have the time period from the concrete cracking up to the excessive deflection of the reinforced concrete members. As a fourth and final phase, it is the time from the loss of serviceability to the final collapse of the structure, which means that the structure arrived to a point where it had lost its flexural and shear strength.
By determining the corrosion stage of the structure, its service life can be made known and as result, the phase in which the structure is currently experiencing at the moment can be identified. Furthermore, the time period of each of the four phases of the service life of a structure can be determined once a performance-based assessment criterion is established. The period of each phase can be calculated with the help of the following formula:
S (t) = Structural Response (Load Effect)
L (t) = Acceptable Limit for Structural Response (Structural Resistance)
By knowing the time periods of each of the four phases of the service life of a structure, a probable failure can be determined by using the following formula:
pf (t) = Probability of an event
Deterioration Stages of a Structure
Let us go back through to the stages of deterioration of a structure and discuss it. First we have the concrete cracking which comes at the end of the second phase of the service life of a structure and is after the initiation of reinforcement corrosion. So, after the initiation of corrosion, some rust is produced on the reinforcement. That covers more space than the initiation and as a result, some pressure propagates on the surrounding concrete which by itself cannot take the pressure since it exceeds its tensile capacity and starts cracking.
Furthermore, after the concrete cracking and at the end of phase three of the service life of a structure, the excessive deflection comes which is restrained within an acceptable limit. After the end of the fourth phase, we have the loss of strength of the structure which can lead to a structure failure. In that stage the corrosion on the reinforcement reduces the strength of the RC structure and results to a break at the critical cross section of a structural member.
Deterioration of Strength
There are two different ways of determining the strength deterioration of an RC structure, namely, the destructive load test and the non-destructive measurement of corrosion current density. Both of those methods can be used in the laboratory in order to measure the strength deterioration of a structure, but in practice and on the field, it can only be measured based on the reduction of the cross sectional area of the reinforcement bars.
Deterioration of Stiffness
Finding the stiffness deterioration of RC structural members is very important for the deterioration of the structure as a whole system where the load redistribution and the failure mode both depend on the stiffness of its members.
Rate of Deterioration
The rate of strength and stiffness deterioration is different since the stiffness is deteriorating much more severely than the strength. This is based on the fact that stiffness is measuring mechanical properties related to geometry and the cross sections of a structure way more than strength does. Factors that are related to corrosion and can affect the geometry of a structure are concrete cracking, delamination and debonding. After all those factors, and once the corrosion actively propagates, it results to a severe increase of deflection. Then again, deterioration of strength is not that affected from those factors which can only damage tensile sections of RC members.