Concrete carbon dioxide effects

Carbon dioxide is, of course, fundamentally important to plants because of photosynthesis. Most plant cell cultures are heterotrophic, non-photosynthetic and use a chemical energy source. It is reasonable to suspect, however, that some of the control mechanisms for the photosynthetic dark reactions would be regulated by C02 concentration. This could affect both cell growth and, indirectly, production of useful compounds. More concretely, C02 is known to promote synthesis of ethylene [38] on the other hand, C02 concentrations of 5-10% inhibit many ethylene effects [53]. 

Effects of Carbon Dioxide. Concrete is known to be affected by the take-up of CO2 from ambient air, i.e. carbonation (32,34-37). 

Woods (32) states "The reaction between atmospheric carbon dioxide and dense hardened concrete is very slow, and even after a considerable number of years, may affect only a thin layer nearest the exposed surfaces. A principal product of the reaction is calcium carbonate, the presence of which may enhance the early resistance of concrete to attack by some chemicals in solution, such as sulfates. In practice, however, any beneficial effect that may exist appears to be of relatively small moment." The harmful effect of carbonation arises when the carbonated layer created on the surface of reinforced concrete over the years reaches the steel reinforcement. The alkaline protective layer is then considerably less alkaline, and the steel bars may start to rust. 

In moist environments, carbon dioxide present in the air forms an add aqueous solution that can react with the hydrated cement paste and tends to neutralize the alkalinity of concrete (this process is known as carbonation). Also other acid gases present in the atmosphere, such as SO2, can neutralize the concrete s alkalinity, but their effect is normally limited to the surface of concrete. 

The beneficial effects of this type of repair are durable if the external layer offers an effective barrier to penetration of carbon dioxide. The thickness and permeability of this layer must therefore be sufficient to prevent its carbonation during the complete design life of the repair. In fact, the original concrete underlying this layer cannot be counted on to resist carbonation. 

The effects of filling and sealing with polymers in latex-modified mortar and concrete are reflected in the reduced transmission of such gases as air, carbon dioxide (CO2), oxygen (O2), and water vapor, as well as increased water impermeability. The carbonatlon resistance of the latex-modified mortar and concrete is remarkably improved with an increase in... 

Phase 1 is the chemical corrosion phase, where no bacteria are involved. In this phase, due to the combined effect of hydrogen sulfide and atmospheric carbon dioxide, concrete s pH is highly reduced to less than 10. In phase 2, the first stage of microbial succession starts, where a certain species of SOB neutrophilic sulphur-oxidizing bacteria (NSOM) further reduces pH, resulting in the... 

The pH of the electrolyte does not only have an effect on the passivation potential, but also on the passivation current density, because both the metal dissolution kinetics and the solubility of hydroxides depend on pH. Figure 6.16 shows that the passivation current density of iron becomes smaller at higher pH. This has been explained by a lowering of the solubility of ferrous hydroxide, which precipitates at the surface. Since both the passivation potential and the passivation current density decrease with increasing pH, spontaneous passivation of iron becomes possible in basic, aerated media. This explains why steel reinforcements in concrete (pH >13) resist corrosion well as long as chemical reactions with carbon dioxide from air (carbonation of concrete) do not modify the alkalinity. 

Concrete may deteriorate if adequate precautions are not exercised to protect it from adverse effects that could result from exposure to natural or artificial conditions. Several physical, chemical, and electrochemical processes are known to induce cracking of concrete. Concrete can have durability problems as a consequence of its exposure to seawater, sulfates, chlorides, freeze-thaw action, carbon dioxide, etc., or when it is attacked by artificially induced processes such as exposure to acids and salts in chemical plants or to fire. In recent years, a new type of durability problem was encountered that involved use of steam cured concrete products. The distress was caused by the formation of delayed ettringite. If the raw materials in concrete are not carefully controlled, there may be an eventual failure of concrete elements, eg., the presence of excess alkali in concrete that promotes alkali-aggregate expansion reaction, harmful impurities in the aggregates, or the presence of excess amounts of dead-burnt MgO. Thermal techniques in combination with others have been employed with success to examine the raw materials as well as the failed concrete. The knowledge gained from such work has been applied to produce more durable concrete. 

Several variables affect the rate of carbonation. In general, low-permeability concrete is more resistant. Carbonation tends to proceed most rapidly at relative humidity levels between 50 and 75 percent. At lower humidity levels, carbon dioxide can penetrate into the concrete relatively rapidly, but little calcium hydroxide is available in the dissolved state for reaction with it. At higher humidity levels, the water-filled pore structure is a more effective barrier to the ingress of carbon dioxide. Clearly, environmental cycles of alternate dry and wet conditions will be associated with rapid carbonation damage. 

A high moisture content will also substantially reduce the rate of diffusion of carbon dioxide and, hence, the rate of carbon-ation of the concrete. An important effect of the moisture content of concrete is its effect on the electrical resistivity of the concrete. Progressive drying of initially water-saturated concrete results in the electrical resistivity increasing, and steel corrosion would be negligible even in the presence of chloride ions, oxygen and moisture. 

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