Concrete permeability reinforcement protection

The neutralization of concrete leads to reduction of the corrosion protective function of concrete against reinforcing steel, and has an important influence on the durability of reinforced concrete structures. The neutralization is influenced by various factors (concentration of CO2 gas, type of concrete, water-to-cement ratio(W/C), water content, type of finishes and their thickness and permeability, temperature and humidity conditions, etc.). From the physicochemical point of view, this process can be considered to be the diffusion of CO2 inwards into concrete from the surface, accompanied by the conversion of Ca(0H)2 into CaC03. In this context, -unsteady state dynamics has been done for the progress of neutralization of concrete in order to rationally understand the process and the influence of finishes on the process (, . ... 

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. 

Exterior-use paints have a pigment volume concentration of 30-55%. They may be formulated with all of the aforementioned dispersions. Properties are adapted as appropriate to the substrates, climatic conditions, market circumstances, etc. The most important properties of exterior-use paints are their water and water vapor permeability (moisture balance), chalking resistance, and adhesion to the substrate. Special requirements apply in certain areas of application (e.g., COj protection on reinforced concrete, fungal and algal resistance in moist surroundings, crack bridging on cracked substrates, and alkali resistance on new mineral substrates). 

In steel-reinforced concrete stractures made with calcium alununate cement and with a sufficiently low water/cement ratio, the reinforcement is sufficiently protected from corrosion. However, in mixes made with too much water, corrosion of the steel may take place, especially after conversion of the hardened paste has occurred, as the cement paste becomes too porous and too permeable for oxygen of the air. Carbonation of the paste, which progresses especially easily in porous mixes, enhances the corrosion process even further, as the pH of the pore solution drops from its original 10-12 to lower values, making the steel susceptible to corrosion. 

There are several methods that can be used to control corrosion of steel reinforcements in concrete. First, the design of the structure should provide for drainage of salt-containing waters away from the reinforced concrete. Second, concrete of adequate thickness, high quality, and low permeability should be specified to protect the reinforcements from the environment. Third, chloride content of the concrete mix should be kept to a minimum. For further protection, the steel reinforcements can be epoxy-coated. In many parts of North America, steel reinforcements used in bridge decks are now epoxy-coated as a standard construction procedure. Cathodic protection is also being used, both with impressed current anodes and with sacrificial anodes [61]. (See Chapter 13.)... 

The concrete is said to carbonate by this process. In permeable, poor-quality concrete, the concrete cover over the embedded reinforcement can sometimes carbonate in the course of a few years. The carbonated concrete is almost neutral when its pH range is reduced to ca. 8 thus, the pH-provided protection of the reinforcement is removed. Therefore, deep carbonation may result in reinforcement corrosion, and spalling and discolouration of the concrete over the corroded reinforcement may occmr. 

In reinforced concrete members exposed to chlorides and subjected to intermittent wetting, the degree of protection against corrosion is determined primarily by the depth of cover to the reinforcing steel and the permeability of the concrete. 

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