Passive concrete

Silt, sand, concrete chips, shells, and so on, foul many cooling water systems. These siliceous materials produce indirect attack by establishing oxygen concentration cells. Attack is usually general on steel, cast iron, and most copper alloys. Localized attack is almost always confined to strongly passivating metals such as stainless steels and aluminum alloys. 

The passivating action of an aqueous solution within porous concrete can be changed by various factors (see Section 5.3.2). The passive film can be destroyed by penetration of chloride ions to the reinforcing steel if a critical concentration of ions is reached. In damp concrete, local corrosion can occur even in the presence of the alkaline water absorbed in the porous concrete (see Section 2.3.2). The Cl content is limited to 0.4% of the cement mass in steel-concrete structures [6] and to 0.2% in prestressed concrete structures [7]. 

Fig. 19-1 Experimental setup for the cathodic protection of an active steel concrete-passive steel cell.

However, if the interpretation of the potentials measured for regions with a covering as uniform as possible and aeration or moisture is extended to estimate the potential gradients corresponding to the explanation for Fig. 3-24, there follows the possibility of classifying the state of corrosion [52-54]. Furthermore, the sensitivity of the estimate can be raised by anodic polarization according to the explanation given for Fig. 2-7, because the depassivated steel is less polarizable than the passive steel in concrete [43]. 

Calcium hydroxide leached from incompletely cured concrete causes serious corrosion of lead (see Section 9.3). This is because carbon dioxide reacts with the lime solution to form calcium carbonate, which is practically insoluble. Carbonate ions are therefore not available to form a passive film on the surface of the lead . Typically, thick layers of PbO are formed, which may show seasonal rings of litharge (tetragonal PbO) and massicot (orthorhombic PbO) . 

Little information is available about the corrosion of metals in concrete, although it seems likely that all Portland cements, slag cement and high-alumina cement behave similarly Concrete provides an alkaline environment and, under damp conditions, the metals behave generally as would be expected e.g. zinc, aluminium and lead will react, copper is unaffected, while iron is passivated by concrete. 

Zinc will initially react with cement-based materials with the evolution of hydrogen. This reaction can be controlled by the presence of soluble chromate either in the cement (over 70 ppm) or as a chromate passivation treatment to the zinc surface. Zinc can therefore be used to provide additional protection to steel in concrete. It is more effective in cmbonated concrete than in chloride-contaminated concrete. 

Many passive metals suffer pitting attack when aggressive ions (usually chloride) enter the system. It is possible to forestall pitting, or to stop it once started, using cathodic protection. It is not necessary to polarise to the protection potential of the metal a negative shift of 100 mV from the natural corrosion potential in the environment will often be sufficient. This technique has been applied to various stainless steels and to aluminium . The philosophy is not unlike that applied to rebar in concrete. 

The proof of protection is more difficult to establish in this case for two reasons. First, the object is to restore passivity to the rebar and not to render it virtually immune to corrosion. Second, it is difficult to measure the true electrode potential of rebars under these conditions. This is because the cathodic-protection current flowing through the concrete produces a voltage error in the measurements made (see below). For this reason it has been found convenient to use a potential decay technique to assess protection rather than a direct potential measurement. Thus a 100 mV decay of polarisation in 4 h once current has been interrupted has been adopted as the criterion for adequate protection. It will be seen that this proposal does not differ substantially from the decay criterion included in Table 10.3 and recommended by NACE for assessing the full protection of steel in other environments. Of course, in this case the cathodic polarisation is intended to inhibit pit growth and restore passivity, not to establish effective immunity. 

Where installation costs are not prohibitive, the ideal remedy would be a suspended concrete floor above a well ventilated space, with the ground below the ventilated space sealed by concrete incorporating a polymeric radon barrier. This would effect a passive remedy requiring little maintenance and no running costs. 

Precast concrete piles will be used with an allowable compression force of 80 kips (356 kN) and an allowable tension force of 50 kips (222 kN), both with a safely factor of 3 against ultimate capacity. Because battered piles will resist all lateral forces without the need for passive soil pressure, a safety factor of 1.2 may be used. Permissible blast capacities will be adjusted accordingly. 

Steel, aluminum, concrete, and other materials that form part of a process or building frame are subject to structural failure when exposed to fire. Bare metal elements are particularly susceptible to damage. A structural member undergoes any combination of three basic types of stress compression, tension, and shear. The time to failure of the structural member will depend on the amount and type of heat flux (i.e., radiation, convection, or conduction), and the nature of the exposure (one-sided flame impingement, flame immersion, etc.). Cooling effects from suppression systems and effects of passive fire protection will reduce the impact. 

Measures to reduce the impact of fire include active and passive systems. Active systems include automatic sprinkler, water deluge, water mist, gaseous agent, dry chemical, foam, and standpipe handle systems. Passive protection is provided by fire resistive construction, including spray-applied or cementitious fireproofing of steel, concrete/masonry construction, and water-filled steel columns. Chapter 7 provides details on the design of fire protection systems. 

In addition, further oxidation and cathodic reactions lead to the production of oxides and oxyhydroxides of Fe (III), which produces a low-permeability, passive film that slows down the corrosion rate considerably. Where corrosion can continue (by depassivation), the expansion of corrosion products at the cement-steel interface and the subsequent spalling of cover concrete can occur. Many examples of this can be seen in concrete structures. 

The protection of steel reinforcements. Concrete produces a layer of passivity at the steel/concrete interface and any breakdown of this can increase the chance of reinforcement corrosion. In addition, it is important that concrete be maintained in a state of low permeability to minimize the passage of moisture and air to the steel. 

The action of an admixture in relation to attack on reinforcement can be considered either in direct chemical reaction with the steel or, alternatively, a breakdown of the passive layer imparted by concrete which normally prevents corrosion at the cement/steel interface. In this respect, any accelerating water-reducing admixtures containing calcium chloride can be considered hazardous as far as raising susceptibility of steel reinforcement to corrosion is concerned. It is particularly so at calcium chloride contents in the concrete at or above 1.5% by weight of cement as discussed in the section on accelerators. The use of such materials has been controlled by relevant codes of practice where embedded metal is present in the concrete. 

The formation of the passive layer at the concrete/reinforcement interface referred to earlier (Section 1.4) is due to the alkaline nature of the concrete. The alkalinity is due to calcium, sodium and potassium hydroxides which, over a period of time, react with atmospheric carbon dioxide to form carbonates. This reduction in alkalinity in reflected in a diminished protective capacity towards the steel reinforcement. 

The presence of calcium chloride at concentrations greater than about 1.5% by weight of cement can lead to breakdown of the passive layer of Fe203 normally present at the steel/concrete interface, rendering the... 

There is very limited information available on the effect of calcium formate and nitrate but certainly the passive layer at the concrete/steel... 

A corrosion-inhibiting admixture is a chemical compound which, when added in small concentrations to concrete or mortar, effectively checks or retards corrosion. These admixtures can be grouped into three broad classes, anodic, cathodic and mixed, depending on whether they interfere with the corrosion reaction preferentially at the anodic or cathodic sites or whether both are involved [48]. Six types of mechanisms, viz. anodic (oxidizing passivators), anodic (non-oxidizing passivators), cathodic, precipitation... 

Corrosion-inhibitive properties of the compound Na2P03F have been tested by Andrade et al., either by incorporating it in a mortar or as a penetrant[68]. This compound, which is currently available as a proprietary product, is reported to act as an anodic inhibitor, possibly with some cathodic action. The minimum required ratio of phosphate to chloride was suggested as 1 1. The mechanism of action of this admixture is to stabilize the passive layer of iron oxide on the steel and also increase the density of concrete, thus decreasing the permeability... 

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