Carbonation-induced Corrosion

C. M. Chun, J. D. Mumford and T. A. Ramanarayanan, Carbon-induced Corrosion of Nickel Anode,... 

L. J. Parrot, Design for avoiding damage due to carbonation-induced corrosion , Proc. of Canmet/ACI Int. Conf. on Durability of Concrete,... 

The time for initiation of carbonation-induced corrosion is the time required for the carbonation front to reach a depth equal to the thickness of the concrete cover. It depends on all the factors mentioned above (that influence the carbonation rate) and on the thickness of the concrete cover. If the evolution of carbonation in the course of time and the thickness of the concrete cover are known, the initiation time can be evaluated. It should, however, be taken into consideration that the carbonation front may not be uniform across the concrete surface. 

The lower limits of relative humidity near which the corrosion rate becomes negligible depend on the characteristics of the concrete, on the amount of chlorides in the concrete and the type of salt they originate from. In any case, this limit is at much lower relative humidity than that which makes carbonation-induced corrosion negligible. In the presence of high chloride contents, above all with hygroscopic salts admixed like calcium or magnesium chloride, even for relative humidities of 40-50%, the corrosion rate can be up to 2 pm/y. 

For carbonation-induced corrosion, the service life (ti) is expressed as the sum of the initiation (h) and propagation (tp) periods up to the threshold at which deterioration becomes unacceptable = h + tp (Figure 4.1). The initiation time (h) may be calculated as a function of the properties of concrete, in particular the coefficient K of carbonation, the environment and the thickness of the concrete cover x), for example with models by Tuutti, Bakker, or Parrott (Chapter 5). The propagation time (tp) can be estimated if the corrosion rate is known, once the maximum acceptable penetration of corrosion has been fixed. A maximum penetration for corrosion attack that is often accepted in reinforced (but not prestressed) concrete elements is 100 tm. 

Carbonation of concrete can be modelled relatively simple with good accuracy (Chapter 5). The following model is a simplified representation of the DuraCrete model for carbonation-induced corrosion initiation [21], Initiation of corrosion by carbonation may be set as a hmit state. The design equation g is then given by... 

Various researchers have tested the effect of coatings (and other surface treatments) on active reinforcement corrosion. Due to the extreme variation of materials (both coatings and concrete) and test methods and conditions, it is beyond the present scope to summarise that work. The overall impression is that coatings do not effectively reduce the rate of chloride-induced corrosion, see for example [13]. Subject to many practical influences, it is possible that coatings reduce the rate of carbonation-induced corrosion. 

Carbonation-induced corrosion. If carbonation has been identified as the cause of corrosion of steel reinforcement and other deterioration processes can be neglected, the evaluation of the depth of concrete to be removed can be carried out as shown in Figure 19.1. The present conchtion of the structure has to be expressed by means of ... 

Many factors influence the ability of reinforced concrete to resist carbonation induced corrosion. As the carbonation rate is a function of thickness, good cover is essential to resist carbonation. As the process is one of neutralizing the alkalinity of the concrete, good reserves of alkali are needed, that is, a high cement content. The diffusion process is made easier if the concrete has an open pore structure. On the macroscopic scale this means that there should be good compaction. On a microscopic scale well cured concrete has small pores and lower connectivity of pores to the CO2 has a harder job moving through the concrete. Microsilica and other additives can block pores or reduce pores sizes. 

Figure 6.6 (a) Roof structures before treatment showing obtrusive anticarbonation coatings and exposed reinforcement due to chloride and carbonation-induced corrosion, (b) Roof structures after repair and coating where the visual effect of patch repairs are minimized by application of Keim Concretal Lazure coating. 

One of the major issues facing any consultant or owner of a structure suffering from chloride or carbonation induced corrosion is what form of repair to undertake. As we have seen from the previous sections there are coatings, sealants, membranes and enclosures, specialized patch repair materials, options for total or partial replacement, impressed current and galvanic cathodic protection, electrochemical chloride removal, realkalization, electro-osmosis and corrosion inhibitors. These can be applied to structures suffering different degrees of corrosion due to chloride attack or carbonation or a combination of these two. Each treatment will have implications for the future maintenance requirements, time to next major intervention and ultimate service life of the structure. 

Figure 8.3 Flow diagram for selecting treatment for carbonation induced corrosion.

Parrott, L.J. (1994b). Carbonation-Induced Corrosion, Paper presented at the Institute of Concrete Technology Meeting, Reading, 8 Novembei Geological Society, London. 

Most work on linear polarization probes has been done in chloride corrosion condition. Ho vever, the only methods of assessing carbonated concrete are destructive drilling or coring for carbonation depth measurement and trying to interpret half cell potentials which is difficult (Section 4.7.2). Linear polarization is therefore very useful in asse.ssing carbonated structures, particularly as half cell potentials are so difficult to interpret for carbonation induced corrosion. 

As described in detail in the RILEM 124 SRC recommendation (RILEM, 1994), the basic repair methods for carbonation induced corrosion consist in repassivation of the steel by a mortar layer or by local repair. The procedure for patch repair is described in the published guidelines (e.g. Bentur et al., 1997). Limitation of the concrete moisture content is another repair principle, its aim is dry out the concrete, e.g. when additional facade elements are mounted to reduce energy consumption. When carbonated concrete is not completely removed in the vicinity of the reinforcement, corrosion occurs after repair when sufficient moisture is present (Schiessl and Breit, 1996). 

The pore solution is effectively neutralized by this reaction. Carbonation damage usually appears as a well-defined front parallel to the outside surface. Behind the front, where all the calcium hydroxide has reacted, the pH is reduced to around 8, whereas ahead of the front, the pH remains above 12.6. When the carbonation front reaches the reinforcement, the passive film is no longer stable, and active corrosion is initiated. Figure 1.14 shows that active corrosion is possible at the reduced pH level. Damage to the concrete from carbonation-induced corrosion is manifested in the form of surface spalling, resulting from the buildup of voluminous corrosion products at the concrete-rebar interface (Fig. 1.15). 

Carbonation-induced corrosion. Carbon dioxide present in the atmosphere can reduce the pore solution pH significantly by reacting with calcium hydroxide (and other hydroxides) to produce insoluble carbonate in the concrete as follows ... 

Chun C M, Mumford J D, Ramanarayanan T A, Carbon-induced corrosion of nickel anode , J. Electrochem. Soc., 2000 147 3680-3686... 

---