The degradation of concrete structures under the effect of chemical attacks has become a major concern for civil engineers. The most common examples of chemical attacks are the corrosion of reinforcement bars as a result of chloride exposure, external sulfate attacks, carbonation and alcali-aggregate reactions. For the corrosion case, the cost of repair of the concrete structures exposed to this problem is estimated at 20 billion dollars in the U.S. only.
All of these examples of premature concrete structure degradations have their origin in ionic exchanges between the material and its environment. The corrosion of reinforcement bars is caused mainly by the penetration of external chloride in the concrete porous network. The presence of chloride depassivates the steel, which initiates corrosion. The expansive corrosion product, when in sufficient amount, will cause the concrete to crack and eventually fall apart. External sulfate attack is based on the same principle. In that case, it is the penetration of sulfate ions in the concrete that is at the origin of the degradation. When the proper chemical conditions are met, the sulfates ions react with the solid phases of the material to form gypsum and ettringite. If the amount of these two products is important, severe cracking can occur, again leading to a degradation of the structure.
The reverse situation is also possible. Ions originally present in the concrete porous network as a result of cement hydration can be leached out of the material to the external environment. For example, a structure in contact with pure water will lose some of its hydroxyl ions, which causes a reduction of pH in the material. This drop of pH will lead to the dissolution of portlandite (calcium hydroxide) and the decalcification of C—S—H, the binding phase of the material. Consequently, the structure is bound to lose its mechanical resistance.
A good knowledge of ionic transport mechanisms in cement-based materials and the implementation of the related physical laws in a computer model would be necessary to evaluate properly the service-life of concrete structures. Such a model could be used to plan reparation schedules for structures, based on their estimated remaining service-life. It could also help to choose the proper concrete mixture for a given structure, given the environment to which it is exposed.
Despite this need, few significant developments related to the ionic transport in cement-based materials have been published in recent years. Most models used to predict the service-life of concrete structures are based on Fick's law, which simplifies the problem too much to yield reliable predictions.
Fick's law accounts for the transport of ions as a result of diffusion. The transport of particles by diffusion is the result of their thermal agitation which causes random collisions that eventually disperses the particles from high concentration regions to weak concentrations regions. When the particles are electrically charged like ions, their charge influences the transport by diffusion through the electrical coupling between the ions and the chemical activity of the solution. As a consequence, the movement of one ionic species is influenced by all the other species and the use of Fick's law is inappropriate.
The electrical coupling and the chemical activity effect emphasizes the multiionic aspect of ionic transport. Most of the time, the chloride ion is under scrutiny, as it is an important factor in the rebar corrosion phenomenon. However, few models consider the other ionic species involved in the transport process. They use Fick's law to model the transport of ions by diffusion. This approach is at the core of most ionic transport models in cement-based materials.
This is the case for example in the models published by Gospodinov et al. in “Diffusion of sulfate ions into cement stone regarding simultaneous chemical reactions and resulting effects” published in “Cement and Concrete Research”, vol. 29, p.1591-1596 in 1999, by Hansen and Saouma, in “Numerical simulation of reinforced concrete deterioration—part 1: chloride diffusion”, published in “ACI Materials Journal”, vol. 96, no. 2, p. 173-180, 1999, by Martin-Pérez in “Service-life modeling of R. C. highway structures exposed to chlorides”, in a Ph.D. thesis, University of Toronto, 1998, by Nagesh and Bhattacharjee in “Modeling of chloride diffusion in concrete and determination of diffusion coefficients”, published in ACI Materials Journal, vol. 95, no. 2, p. 113-120, 1998, by Saetta et al. in “Analysis of chloride diffusion into partially saturated concrete”, published in “ACI Materials” Journal, vol. 90, no. 5, p. 441-451, 1993, and Swaddiwudhipong et al. in, “Chloride ingress in partially and fully saturated concretes”, published in “Concrete Science and Engineering”, vol. 2, p. 17-31, 2000.
There are however an increasing number of papers where the electrical coupling is taken into account. This is the case for the work of Masi et al. in “Simulation of chloride penetration in cement-based materials”, published in “Cement and Concrete Research”, vol. 27, no. 10, p. 1591-1601, 1997 and Truc et al. in, “Numerical simulation of multi-species diffusion”, published in, “Materials and Structures”, vol. 33, p. 566-573, 2000. It should be emphasized that there are very few models accounting for the chemical activity effects. Li and Page in, “Modeling of electrochemical chloride extraction from concrete: influence of ionic activity coefficients”, published in “Computational Materials Science”, vol. 9, p. 303-308, 1998 include it in their model, as well as the electrical coupling. However, their model is limited to ionic transport in saturated cement-based systems exposed to an electrical current. It is not relevant to predict the service-life of concrete structures.
Ions can also move under the effect of fluid displacement in the porous network of the material. This phenomenon is called advection. This fluid displacement occurs as a result of capillary forces arising in unsaturated materials. A structure becomes unsaturated following the various wetting/drying cycles to which it is exposed during its service-life. In the models cited previously, only those of Martin-Pérez, Nagesh and Bhattacharjee, Saetta et al., and Swaddiwudhipong et al. consider capillary forces in unsaturated cement-based materials as a transport mechanism.
Both diffusion and advection drag the ions through the porous network of concrete. The ions may then undergo some chemical reactions with the hydrated cement paste. For example, the penetration of sulfate (SO42−) ions in cement-based materials may lead to the formation of ettringite and gypsum, while the penetration of chloride is at the basis of the formation of chloroaluminates. Studies performed on simple cement systems clearly showed that these chemical reactions are multiionic. For instance, in addition to SO42−, The formation of ettringite also involves Ca2+, OH− and Al(OH)4− ions. The last three ions are involved in the formation of chloroaluminates. Furthermore, the formation of ettringite, gypsum, and chloroaluminates is influenced by the presence of the alkali ions Na+ and K+.
Some studies have indicated that surface binding phenomena (also called physical adsorption) can have a significant influence on ionic transport mechanisms. This appears to be particularly the case for chloride penetration problems.
All of the previously cited models take into account interactions between ions in solution and the hydrated cement paste. They all use, without any exception, a simplified interaction model based on the concentration of only one ionic species. It is called the interaction isotherm, which consists in an experimental curve relating the amount of a given ion bound to the solid phase as a function of its concentration in solution. In most cases, the isotherm is determined for the chloride ions. This method is used in the model based on one ion as well as in the multiionic ones. Even if it allows to model the interactions involving chloride or sulfate, in most cases it does not allow to take into account other chemical reactions occurring simultaneously like the dissolution of portlandite or the decalcification of the C—S—H. The use of a simple chemical model thus limits the possibility of considering some reactions that are bound to have an important effect on the ionic transport in the material. Furthermore, it blends into one unique curve the chemical reactions, where products are formed or dissolved, with the surface interactions, where ions are adsorbed onto the surface.
Therefore, since it is essential to be able to determine the behavior of hydrated cement systems and concrete structures, there is a need for a method of calculating such a service-life accurately.