Structures Exposed to the Atmosphere

The most frequent type of macrocell in reinforced-concrete structures exposed to the atmosphere is that established between more superficial rebars that have been depassivated by carbonation or chloride penetration, and internal passive rebars. Another example may be walls where chloride penetrates from one side and oxygen penetrates from the other side, which may occur in hoUow structures Hke tunnels and offshore platform legs or with ground retaining walls. 

For reinforced-concrete structures buried in soil or immersed in water, cathodic areas may be due to noble metals present in the environment and electrically connected with the steel embedded in the concrete. For example, this is the case with copper grounding systems. 

It should be observed that, because of the current flowing from the anodic area towards the cathodic areas, theoretically some Fe ions migrate away from the corroding site and thus they do not precipitate locally as expansive oxides. This could mean that corrosion products due to macroceUs may have less expansive effect than corrosion products due to microceUs. 

Depassivation of rebars due to carbonation or chloride penetration often does not extend to the whole surface of the reinforcement but, for instance, it is limited to the outer layer of rebars, or to parts where the concrete cover has a lower thickness 

Luca Bertolini, Bernhard Elsener, Pietro Pedeferri, Rob P. Polder 

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Values of K found for real structures exposed to the atmosphere but protected from rain, vary from 2 to 15 mm/y. Indicatively 2 < fC < 6 for concrete of low porosity (that is well compacted and cured, with low w/c) whose cement content is above 350 kg/m 6 < fC < 9 for concrete of medium porosity K > 9 for highly porous concrete with cement content below 250 kg/m ... 

On structures exposed to the atmosphere, the corrosion rate can vary from several tens of fim/y to localised values of 1 mm/y as the relative humidity rises from 70 to 95 % and the chloride content increases from 1 % by mass of cement to higher values. These high corrosion rates have been observed in particular on heavily chloride containing structures such as bridge decks, retaining walls and pillars in the Swiss Alps. 

For a given potential of the steel, the highest content of chlorides compatible with conditions of passivity is the critical chloride content (or chloride threshold) at that potential. As already discussed in Chapter 6, for structures exposed to the atmosphere (whose reinforcement operates at a potential around 0 V SCE) the critical level is usually considered to be in the range of 0.4% to 1 % of the cement content. For structures immersed in water (whose reinforcement operates instead at a much lower potential, around —400 to —500 mV SCE) or for reinforcement that is cathodically polarized for any reason, the chloride threshold is much higher. 

The action of macrocells in structures buried in the soil or immersed in water is different from that of structures exposed to the atmosphere two circumstances promote macrocell effects while another reduces them. First, concrete is wetter than in aerated structures and its resistivity is lower, particularly in structures immersed in seawater. This reduces the ohmic drop in the concrete and increases the size of the effective cathodic area in relation to the anodic one. Secondly, the soil or the seawater around the concrete is an electrolyte of low resistivity, and the macro-cell current can also flow outside the concrete. This further reduces the ohmic resistance between the anodic area and passive reinforcement. Thirdly, there is, however, a mitigating aspect. Oxygen can only diffuse with great difficulty through wet concrete and thus it hardly reaches the surface of the embedded steel. Depletion of oxygen at the surface of the rebar that is observed in this case makes initiation of corrosion very difficult, and, even when corrosion initiates, the driving voltage for the macrocell is very low. 

The size of the surfaces acting as anode and cathode (and thus r, which is their ratio) also depend on the resistivity of the concrete and the geometry of the system. For reinforced - concrete structures exposed to the atmosphere, usually concrete has a high resistivity and thus only those areas near the site of active corrosion act as cathode (e. g. r can approach unity). In the case of structures immersed in seawater or buried, the concrete is wet and has a low resistivity and, moreover, the sea or the soil also act as electrolytes. Therefore, even areas very far from the anodic areas can fimction as cathodes, so that ratio r can be very small. 

In most structures exposed to the atmosphere, the informative recommendations on cement content and iv/c together with the minimum thickness of the concrete cover required by Eurocode 2, wiU provide a service Hfe of at least 50 y. Therefore, by simply following these standards it would be possible to eHminate the vast majority of forms of deterioration, including corrosion, which are found today and that are connected to incorrect design, material composition or construction practice. 

Table 16.3 Interpretative criteria for measurement of electrical resistivity of concrete structures exposed to the atmosphere for OPC concrete [16]...

C. Andrade, ). Sarria, C. Alonso, Statistical study on simultaneous monitoring of rebar corrosion rate and internal RH in concrete structures exposed to the atmosphere , in Corrosion of Reinforcement in Concrete Construction, C. L. Page,... 

Cathodic protection of reinforced-concrete structures exposed to the atmosphere was apphed for the first time to bridge decks contaminated by de-icing salts by R. F. StratfuU in California in 1973 [1,2]. In the years following, design and protection criteria were elaborated, as well as power supply and monitoring systems completely different from those used for cathodic protection of buried steel structures or structures operating in seawater. Above all, it was proved that cathodic protection was a rehable repair technique even in the presence of high chloride contents, where traditional systems of rehabilitation are inefficient or very costly. 

Initial current densities in the range of 5-15 mA/m of reinforcing steel surface area are generally needed for protecting reinforced concrete structures exposed to the atmosphere. Much lower current densities are required under conditions that reduce the access of oxygen towards the surface of the steel, such as in water-saturated concrete. For elements operating under water, current densities typically in the range 0.2 to 2 mA/m are sufficient... 

The scope of application of CP is enormous and continuously increasing. It is possible to protect vessels and ships, docks, berths, pipelines, deep wells, tanks, chemical apparatus, underground and underwater municipal and industrial infrastructure, reinforced concrete structures exposed to the atmosphere, as well as underground parts, tunnels, and other metal equipments using cathodic protection. Apart from reduction of general corrosion, cathodic protection reduces SCC, pitting corrosion, corrosion fatigue, and erosion-corrosion of metallic materials. 

For structures exposed to the atmosphere the design should allow easy drainage with ample supply of air. Alternatively, the opposite hinder air transport to cavities by complete sealing. 

Sometimes, critical situations in structures exposed to the atmosphere occurs to bolted joints. These are very often the seat of crevices, anodic with respect to the surrounding areas, and, if unprotected, they can be sevaely corroded. If the outer surface is carefully painted, the situation is generally not dramatic, as the nonconducting paint limits the extent of the cathodic area and, hence, the damage to the anodic area. 

Upon contact with the atmosphere, copper forms a brown-black patina that becomes blue-green over the years. The changing color is explained as follows. Initially the oxides CU2O and CuO arc formed. Then, in the presence of pollutants, the oxides slowly transform into greenish hydroxy-sulfates Cu(0H)x(S04)y, hydroxy-carbonates Cu(0H)x(C03)y or hydroxy-chlorides Cu(OH)xCly. Normally the patina has little or no influence on the rate of corrosion, which remains constant during the time of exposure. Uncoated copper structures exposed to the atmosphere, such as for roofing, corrode only very slowly and last many years. More critical than the loss of copper due to atmospheric corrosion is a possible environmental pollution by copper ions formed by corrosion reactions and carried away by rainwater. 

Facilities and structures exposed to the atmosphere are subject to corrosion primarily from the chemical contaminants of the local atmosphere. For example, a galvanized metal roof wiU last 30 to 50 yr in a rural environment but only 5 to 10 yr in a heavily polluted, industrial environment. If structures and facilities are near the ocean, the impact of corrosion due primarily to the chlorides in the atmosphere will be significant if no protective measures are taken. For atmospheric corrosion to occur there must be a metallic substrate, moisture, and a chemical agent. 

Cathodic protection is by far the most versatile method of corrosion control, since it is applicable to any electrically continuous structure within a suitable electrolyte. Inasmuch as the steel embedded in concrete, and not the concrete itself, requires the protection from metallic corrosion, damp concrete serves as a suitable electrolyte, and even structures exposed to the atmosphere, such as bridge decks, can be protected cathodically. 

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