Concrete degradation - Part 2: chemical attacks
By Louis-Samuel Bolduc Eng., M.Sc., Frédéric Gagnon Eng., M.Sc.,
Concrete is the most used artificial material around the world. The annual production is estimated at 1 cubic meter per person! However, this material is very sensitive to its environment. The previous paper on this subject focused on the main mechanisms known to affect the integrity of the concrete in public infrastructures (bridges, sidewalks, dams, etc.), which are the corrosion of the reinforcing steel, frost action and the alkali-aggregate reaction. These three mechanisms affect particularly public infrastructures since these constructions are directly exposed to harsh environments (humidity, deicing salts, etc.).
However, there is another menace that can lead to concrete deterioration: chemical attacks. These pathologies are susceptible to affect every type of concrete structure, and they are caused by materials in contact with concrete, or directly from the ingredients used in the mixture. This paper presents the main chemical attacks, which are: sulfate attacks, acid attacks and bacterial attacks.
As announced by its name, this type of degradation is associated with the presence of a chemical called sulfates (SO42-). The "external" sulfates are present in seawater or in sulfate contaminated soils. When ground-water dissolves sulfates contained in the soil, it can cause expensive deterioration to the basements of homes.
One notable example of external sulfate attack is the case of South California (soil highly contaminated with sulfates), where hundreds of legal actions were taken in the early 1990's by owners who saw their concrete foundations completely destroyed by the action of sulfates. These lawsuits lead to billions of dollars in legal fees between insurance companies, attorneys and consultants.
Another similar degradation related to this chemical is called the "internal sulfate attacks" because the sulfates are present directly within the concrete mixture. The most common example is the conta mination of aggregates by gypsum (source of sulfate). An important example of internal sulfate attack is the case of the Vallée de la Maurienne, in France, where 15,000 cubic meters of concrete were produced in 2004 with aggregates contaminated by gypsum. In the following years, hundreds of claims were made for concrete that was crumbling (figure 1). Damages are estimated at several million Euros.
How sulfates lead to deterioration? They chemically react with phases already present in the cementitious matrix to create expansive products (internal expansion). There are two main expansion products that are well documented: gypsum (expansion of 120%) and ettringite (expansion of 280%). The resulting swelling leads to cracking, and ultimately to the overall concrete degradation.
The solution contained in concrete porosity is very alkaline (pH?13, compared to lemon juice which has a pH?2). In this condition, the solid minerals in the cementitious matrix are stable, and they perform properly. However, when concrete is exposed to an acid (pH<7), the solid minerals become unstable, which leads to the dissolution and the decom position of the material.
For example, stagnant water is very aggressive because its pH can reach 4,4. This is a potential problem for concrete pipes. Another example is concrete exposed to acid rains, which are typically composed of sulfuric and nitric acids (pH between 4,0 and 4,5). In agricultural areas, concrete is frequently exposed to organic acids from silage and manure. In all these cases, disorders range from a simple aesthetic deterioration to the end of the structure's service life!
Another potential pathology of concrete structures is called bacterial attacks. Certain bacteria, generally present in organic materials, transform or produce corrosive by-products. An anaerobic bacteria called desulfovibrio desulfuricans transforms sulfuric bearing compounds present in the concrete mixture in hydrogen sulfides (H2S). This is a very toxic chemical that has a foul smell (rotten eggs). When hydrogen sulfide is exposed to oxygen, the bacteria thiobacillus concretivorus transforms H2S into sulfuric acid, which can be extremely detrimental for concrete structures since the material is then subjected to both acid and sulfate attacks! This bacterial action is usually observed in agricultural silos and in sewer systems. The case of silos is particularly problematic because structural collapses are frequent, and they cause damage to the surrounding environment (figure 2).
Concrete is a widely used construction material, but it is not well understood. As presented in this series of paper, several mechanisms can cause the concrete degradation. In the case of chemical attacks, the most widespread deterioration mechanisms are sulfate attacks (internal or external), which were the subject of numerous lawsuits. Acid attacks dissolve solid minerals in the cementitious matrix. This can lead to potentially serious problems. Finally, bacterial attacks are particularly observed in sewer systems and in agricultural silos.
Nowadays, concrete material science is relatively advanced and the standards that govern concrete construction are well established. However, with the impressive number of ongoing construction projects, faults are common and the states of the art are not always followed. Moreover, several actors are involved in concrete construction projects (quarries, concrete producers, general contractors, laboratories, etc.).
Therefore, it is critical to understand the different pathologies that might affect the concrete material before proposing a diagnostic for the deterioration of the material or the structural failure.