Concrete degradation in infrastructures
By Louis-Samuel Bolduc Eng., M.Sc., Frédéric Gagnon Eng., M.Sc.,
Durability of concrete infrastructures is a growing concern, particularly in cold regions. Maintenance and repair require large amounts of the public funds since most of these structures have reached the end of their service life. For example, in a 2006 study, the Residential and Civil Construction Alliance of Ontario estimated that the cost to rehabilitate the public infrastructure of the province would be 19 billion dollars. In January 2009, an investment of 16,2 billion dollars was promised by the Provincial Government of Quebec to improve the condition of roadways, and to continue the work undertaken for the repair of the bridges in Quebec. Furthermore, the Federal Government launched in 2006 an investment of 33 billion dollars in infrastructures, with a portion that will be used to repair various concrete structures.
This article is the first in a series of two on the mechanisms governing concrete degradation. The emphasis of this first is on the deterioration of public infrastructures (bridges, marine structures, parking structures, etc.), and on the three durability issues that are generally associated with them: the reinforcing steel corrosion, the effects of freeze-thaw cycles, both phenomenon being the result of our climate and use of deicing agents, and alkali-aggregate reaction, which is the result of the use of certain types of crushed stones in the concrete mixes.
Reinforcing Steel Corrosion
The most expensive degradation mechanism in reinforced concrete structures is the corrosion of steel reinforcing bars. Corrosion is the deterioration of iron through a reaction of steel, water and oxygen, to form an iron oxide (rust). The reaction products occupy a volume greater than the original steel, causing stresses on the material surrounding the reinforcing bars. The concrete layer covering the bars (also called the concrete cover) acts as a barrier and protects the steel against aggressive agents. Therefore, the stresses generated by the steel corrosion lead to the spalling of the concrete cover, which in turn expose unprotected steel to the aggressive environment. This greatly reduces the service life of the structure, since there is no more protection over the reinforcing bars (see figure 1).
Why is there corrosion of the reinforcing steel? In sound concrete, the pH of the solution found in the material porosity is high (pH?13), and this environment creates a protective layer at the surface of the steel. This protection is called the "passive film". This layer is destroyed when aggressive agents penetrate into the concrete porosity and reach the reinforcing bar. The main mechanisms that attack the protective layer are the carbonation (decrease of pH because of CO2 ingress) and the penetration of chloride ions (from deicing salts or seawater). Therefore, when the passive film is weakened, corrosion can be initiated.
To limit the problems associated with corrosion, it is necessary to improve the barrier that protects the reinforcing bars. It can be done by using good quality concrete (lowering the penetration of aggressive agents) and by increasing the thickness of the concrete cover. Also, it is now possible to use reinforcing systems that do not corrode (stainless steel, composite materials), or to apply alternative solutions, such as electrified grids, sacrificial cathodic protection, migrating corrosion inhibitors, etc.
Effects of Freeze-Thaw Cycles
It is a known fact that infrastructures can be deteriorated by frost action, but how does it work? How does the frost action affect the integrity of a material like concrete? The answer is simple: water found in the concrete expands when it freezes (volume increase of approximately 9%). Why is it a problem with concrete structures? The problem comes from the fact that water is confined in the concrete porosity, and that it does not have enough room to expand freely. This phenomenon brings stresses in the cementitious matrix and generates microcracking. Upon thawing, water goes back to its liquid form and fills the microcracks. At the next cycle, the microcracks are enlarged by the freezing water, and so on. After several cycles, the state of microcracking is so advanced that the strength of the concrete is significantly reduced.
To decrease the unwanted effects of the frost action, a chemical called air-entraining admixture is added to concrete mixtures. This product is similar to liquid soap. It modifies the surface tension of water to generate microscopic air bubbles within the bulk of the concrete. These bubbles provide space where the pressure resulting from the freezing of the water can be dissipated during the freezing process. To offer an efficient protection against the frost action, the numerous bubbles must be uniformly distributed and closely spaced, so the pressure can be easily released from the concrete porosity. In cold regions, specifications are usually very strict regarding the distribution of these air bubbles.
The third concrete pathology, which is largely spread over Eastern Canada, is caused by what is known as alkali-aggregate reaction (AAR). AAR is a chemical reaction involving the solution in the concrete porosity and certain types of aggregates. The product of this reaction is an expansive gel that emerges around the aggregates. This swelling generates internal pressure, which leads to cracking of the concrete (see figure 2).
In Eastern Canada, AAR is a major issue because several quarries contain reactive aggregates. Even if it does not necessarily lead to structural problems, AAR can contribute to accelerate existing problems, and the remedial cost can be extremely high. For example, the rehabilitation cost of the Beauharnois hydroelectric power plant (dam affected by AAR) was of approximately 1,5 billion dollars!
Different chapters of the Canadian standard CSA-A23.2 explain how to identify and prevent AAR. However, there are almost no approved and recognized remedial methods. A global research effort is currently underway to develop new repair procedures.
Concrete can be subjected to different degradation mechanisms. In the case of public infrastructures exposed to harsh environments, the three main pathologies affecting the integrity of these structures are steel corrosion, the frost action and the alkali-aggregate reaction. Very effective methods are now available to control these degradation mechanisms, but poor design and/or deficient construction can quickly bring infrastructures to a dangerous state of disrepair.
There are other degradation mechanisms known as "chemical attacks". These can affect concrete constructions such as foundations, silos, sewers, etc. Even if they are not as known as those presented in this article, chemical attacks cause headaches to owners, engineers and insurers. These attacks will be discussed in the next article.