COMPETENCIAS GENERALES DE BACHILLERATO

To maintain the passivity of the steel reinforcing bar in concrete, a high pH environment is required. Neutralisation of the pore solution in concrete will change the alkaline condition in the pore solution to aggressive conditions (Page, 1998). Since concrete is a porous material, some aggressive gases may diffuse such as CO2 or any other acidic gases, and then concrete loses its alkalinity and hence carbonation occurs. The carbonation process is usually a very slow process. The carbon dioxide is present in the air with a concentration of about 0.03 % (Gu et al., 2001). When concrete structures are exposed to the air for long time, the alkalinity of the concrete will be depleted, as illustrated by the following equations;

[2.7]

[2.8]

[2.9]

[2.10]

- - [2.11]

Practically, carbon dioxide from the air will penetrate through concrete cover, react with alkaline hydroxides in the concrete and form carbonate compounds. When more carbon dioxide diffuses into the concrete, more acid will be formed resulting in more depletion of the calcium hydroxide according to equation [2.10] and as illustrated in Figure 2-7. According to equation [2.11], calcium-silicate-hydrate (C-S-H) will react with carbon dioxide to form calcium carbonate and hydrated silica gel. Removal of the alkali hydroxides which are dissolved in the pore solution will lead to a significant reduction of alkalinity in the concrete, to a value of less than 10, and consequently, to an unstable passive film on the steel leading to steel corrosion (Page, 2007).

Figure ‎2-7 Schematic representation of alkalie neutralization in hydrated cement and the formation of a "carbonation front" after (Currie and Robery, 1994)

Once the concrete is carbonated, the condition of the electrolyte becomes more aggressive and as a result enhances corrosion of the reinforcement. The corrosion rates in the carbonated concrete were found to be governed mainly by the electrolytic material conductivity and they increase as the relative humidity (pore saturation) increases (Alonso et al., 1988). Chlorides (0.4 – 1.0 % by mass of cement) also influence the corrosion rates in carbonated mortar (Glass et al., 1991). High levels of industrial pollutants with the presence of chloride ions can accelerate corrosion of the reinforcement. For instance, in the Arabian Gulf region, many concrete structures deteriorated within a short period of time after the end of construction. The investigation carried out found that the main cause of the deterioration was corrosion of the reinforcing steel. The primary reason for corrosion of the reinforcement was due to a combination of the presence of chloride ions and the industrial pollutants (Kumar, 1998). Furthermore, it was found that, the corrosion rate of the reinforcing steel in concrete

contaminated with a combination of chloride ions and carbonation could be 5 to 20 times higher than the corrosion rate in concrete contaminated with chloride only (Yokota et al., 2005).

This enhancement in the corrosion rates of steel bar embedded in carbonated concrete could be due partly to a release of bound chloride ions in the concrete pore solution. Experimentally, the release of aggressive substances such as chloride and sulphate ions in the pore solution was confirmed by Anstice et al., (2005). In their study cement pastes were exposed for 30 min each day to various concentrations of CO2; 100 %, 5 % (plus 21 % oxygen and 74 % nitrogen) and air. It was found that chloride and sulphate ions in the extracted pore solution were released and the concentration of these substances increased as the degree of carbonation increased as shown in Figure 2-8. The release of chloride ions was attributed to the decomposition of bound chloride from calcium chloro-aluminate hydrates according to the following equation;

[2.12]

Figure ‎2-8 Concentration of chloride and sulphate ions found in the pore solution extracted from cement paste speciemns exposed to various levels of carbon dioxide after

(Anstice et al., 2005)

Inadequate compaction, poor curing and failure to achieve the minimum specified concrete cover to the steel reinforcing bar are the most common problems that enhance carbonation- induced corrosion of reinforcement (Page, 1998). Three main parameters have been found to have an effect on the carbonation rate. These are the permeability of concrete, the total alkali content of the hydration products and the level of moisture in the concrete. Good quality concrete will result in low permeability and hence the rate of penetration of CO2 is reduced. Carbonation rate is a function of relative humidity (RH). With low relative humidity, the penetration of the carbon dioxide is high. In practice, the pores are partly filled with water that allows the penetration of CO2. At the same time, the reaction rate of Ca(OH)2 with CO2 is also high. Therefore, in the relative humidity range of 50 % - 75 %, the carbonation rate of concrete is most rapid, as shown in Figure 2-9, and thus, carbonation reaches the reinforcing steel faster (Richardson, 2002). This is verified in a study performed by Houst and Wittmann

0 5 10 15 20 25 0.03 5 100 Con cen tr at ion , mM o l/ l CO₂ Concentration, % Free Chloride Sulphate

(2002) where the depth of carbonation in mortar exposed to outdoors, one face was exposed directly to rain and the other face was sheltered, was found higher in the sheltered face than that in the exposed face because pores in the exposed face were blocked by rainwater. The carbonation depth in both faces of mortars made with different water to cement ratios and exposed to outdoors is illustrated in Figure 2-10.

Figure ‎2-10 Carbonation profiles for mortars made of various w/c ratios: (A) 0.6, (B) 0.7, (C) 0.8 and (D) 0.9 and exposed to outdoors for 40.5 months after (Houst and

Wittmann, 2002)

Furthermore, wet/dry cycling may enhance the carbonation process. When concrete is dry, carbon dioxide gas penetrates into concrete pores. Then, during wet cycles, the carbon dioxide dissolves in the water and leads to carbonation problems. Semi-dry and wet cycles are the most significant factor in carbonation induced corrosion (Hansson et al., 2007).

The rate of carbonation front penetration reduces with time. This could be attributed to three factors. First, carbon dioxide has to go further through the concrete. Secondly, continuous hydration of the concrete leads to more impermeable concrete. Finally, the carbonation may lead to precipitation of the carbonate in the existing pores and also, the carbonation reaction

releases water that may result in a wetter concrete internally than near surface and further hydration and hence more impermeable concrete (Hansson et al., 2007).

The carbonation of concrete is not like chloride–induced corrosion, general corrosion occurs rather than localized corrosion. The corrosion products will generate stresses in the concrete cover causing cracking before rust staining appears on surface of the concrete

2.1.4.2.1 Cover thickness and quality of concrete

The thickness and quality of concrete cover are very important factors in control of carbonation-induced corrosion. The carbonation rate increases when a poor quality of concrete is used. This concrete quality will lead to a structure with open pores. The diffusion of carbon dioxide will then be rapid since these open pores are well connected together. The most vulnerable area to carbonation attack is the corner of the structure. This is because a great surface area is exposed and hence, carbon dioxide can diffuse from both sides (Broomfield, 2007).

A good quality control is required and it is very important to ensure the required concrete cover depth is achieved. This is because a reduction in the expected time required for carbonation to reach the steel reinforcing bar of approximately 36 % when 20 % of the minimum specified cover is not achieved (Page, 2007). In practice, the latter is true and a survey carried out by Clark et al., (1997) showed that the 5 mm tolerance that should have been applied to nominal cover according to former standards i.e. BS 8110-Part 1 (1985) was commonly not achieved indicating poor quality control. The recommended allowance in

design for cover deviation has therefore been raised to 10 mm according to BS EN 1992-1-1 (2004) standard.

2.1.4.2.2 Depth of carbonation

For carbonation of concrete exposed to a constant atmosphere the rate of movement of the carbonation front can be modelled using Fick‘s first law of diffusion. From this, the carbonation depth (x) can be shown to have a parabolic relationship with the exposure time (t);

- _[2.13]

This equation was derived by Kropp (1995) where,

D is the constant diffusion coefficient and depends on porosity, pore size distribution, tortuosity and pore continuity, saturation degree, relative humidity and temperature;

C1-C2 is the concentration difference of CO2 between the external surface (C1) and the carbonation front (C2) and depends on the concentration of CO2 in the atmosphere;

a is the alkaline component and depends on the cement content and content of CaO in the binder;

t is exposure time;

K is carbonation coefficient.

The carbonation depth is simply estimated by spraying phenolphthalein (RILEM Committee, 1988). The colour will change from pink to colourless, around a pH value of 10, indicating

carbonated concrete in a freshly exposed concrete. It is important to know that, the carbonation coefficient (K) depends on the above parameters, D, (C1-C2) and a. In fact these parameters are related directly to the quality of concrete and to the condition of environment (Page and Treadaway, 1982). This form of relationship