Concrete carbonation progress

The almost neutral environment of carbonated concrete hinders formation of a protective film on the steel and thus the basis for conditions of passivity. The anodic curve therefore shows a progressive increase as shown in Figure 7.6, curve a. On a semilogarithmic scale (E vs log i) over a wide range of current densities the anodic curve is a straight Hne with a slope of between 60 and 120 mV/decade. 

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. 

The reaction of lead with concrete differs from that of aluminium and of zinc in that it is not normally rapid during the early wet stage. It is, however, progressive in damp conditions, and this is said to be due to the fact that the concrete prevents the formation of a protective basic lead carbonate film on the surface of the lead. The packing of lead cables in plaster of Paris is reported to be of doubtful value in preventing corrosion from surrounding concrete. 

Our study applies an adsorption process using chemically activated carbon fibers (CACF) to effect NOx> a prevalent atmospheric pollutant difficult to control. A concrete understanding of adsorption and desorption behavior and surface chemistry on was progressed. 

Fukushima, T. (1991) Predictive Methods on the Progress of Neutralization (Carbonation) of Concrete by Unsteady State Dynamic Analysis Considering the Influence of Tendency of Increase in Concentration of Atmospheric Carbon Dioxide, Proceedings of the 2nd CANMET/ACI International Conference, held in Montreal, Canada, August 4—9, 1991, edited by V.M.Malhortra, Supplementary Papers, American Concrete Institute, pp.-545-564. 

Calcite is the dominant cement in all of the sandstones except the chlorite cement-rich Ran-zano. The absence of volumetrically significant cements other than carbonate precludes a definitive placement of carbonate cementation within a progression of diagenetic events. No petrographic evidence marks any particular group of concretions as temporally distinct from another. IGV provides a crude estimate of burial at the time of cementation, and suggests that most of the concretions formed after considerable compaction. 

From the viewpoint of prediction of service lives, the photochemical deterioration processes of polymers used as paints and finishes are theoretically analyzed based upon unsteady state dynamics. Theoretical results are compared with experimental data under natural and accelerated exposure. Infrared spectra and scanning micrographs show that the deterioration proceeds continuously inwards from the surface, but differently with the exposure conditions. Parabolic (/t ) law was derived approximately for the increase in the depth of the deteriorated layer of polymers with time. Paying attention to the influence of the deterioration of polymeric finishes, the parabolic law involving a constant term was also derived for the progress of carbonation of concrete. These parabolic laws well predict the progress of deterioration and explain the protective function of finishes on reinforced concrete. 

This report deals with dynamic processes of the deterioration of polymers often used as paints and finishes in housing, and also refers to their influence as the reduction in protective performance on the durability of reinforced concrete. The deterioration processes of polymers by the simiiltaneous action of ultraviolet (UV) light and diffusive oxygen is explained theoretically based upon unsteady state dynamics. The parabolic law (/t" law) is derived for a typical path for the progress of the deterioration of polymers inwards from the surface (l), and compared with some experimental data. The same parabolic law involving a constant term was also derived for the carbonation of concrete, which well explains the retardation effects of finishes on the carbonation (2). 

Paying attention to the influence of the deterioration of polymeric finishes, the parabolic law involving a constant term was also derived for the progress of carbonation of concrete. These parabolic laws combined predict the progress of the deterioration under natural weathering and explain well the protective performance of finishing materials on reinforced concrete. 

From Figure 5.7, which shows the progress in time of the carbonation depth for different values of K, it can be seen that the carbonation depth is less than 20 mm (minimum thickness of concrete cover in many existing structures) after 50 y only... 

There is some discussion in the previous chapter as to whether the carbonation front is truly as well defined as the indicator shows it to be (see Figure 3.1(b)) but for most practical considerations it is a reliable technique. Some aggregates can cause problems usually making the colour transition difficult to see. Also very poorly consolidated concrete and concrete underground exposed to dissolved carbonates in the water may not show clearly defined carbonation fronts due to the non-uniform progress of the carbonation front. 

A number of empirical calculations have been used to derive values of A and n based on such variables as exposure conditions (indoors and outdoors, sheltered, unsheltered), 28 day strength and water cement ratio. A wider range of empirically derived equations is given in Table 3.1. These cover different exposure conditions, curing and concrete properties. The easiest solution for a given structure is to take some measurements of carbonation depth, assume n = 1/2 and calculate A. This can be used to predict the rate of progression of the carbonation front. The time taken to reach the steel can then be estimated and the rate of depassivation calculated. 

The chemical corrosion resistance of AAS concrete is veiy high. The hardened paste is completely resistant to sodium sulfate, and has a very high resistance to magnesium chloride and nitrate attack (Tailing and Brandstetr, 1993). AAS concrete also protects the steel reinforcement effectively against corrosion by chloride solutions, mainly because of a very low diffusion rate of chloride ions in the hardened paste (Dhir et al, 1996). However, the carbonation of AAS concrete surfaces by the CO2 of the air progresses faster than in comparable mixes produced with Portland cement. 

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. 

Materials - parking garage repair completed with carbon fiber. Civil Engineering, 69(3), 24,1999. 237a. McConnell V, Composites make progress in reinforcing concrete. Reinforced Plastics, 40-46, Jul/ Aug 1999. 

The corrosion of steel fibres in cement matrix was a subject of concern in the first period of appUcation of SERC, but later it was proved that only short sections 2-3 mm long that stretch out of the concrete surface corrode and disappear in a short time. Cracked specimens maintained during 18 months in four different environments proved that no decrease of strength could be found in the non-cracked concrete and rusty fibres were found only in the immediate surface in the carbonated zone (Nemeeger et al. 2003). The test carried on by Granju and Balouch (2005) have confirmed that even fibres which bridged open cracks of 0.5 mm wide and maintained a year in a marine environment exhibited only minor traces of corrosion that did not progress inside the concrete matrix. 

Problem. An accelerated test method for comparing the carbonation properties of concrete shall be developed according to the following principle a geometrically well-defined concrete specimen is placed in a pressure tank of steel. Carbon dioxide CO2 (g) is added nnder pressnre at 20 °C. The firee gas volume in the tank is 4.00 . The progress of the carbonation is recorded by measiuring the drop of pressnre in the tank. 

As concrete attack progresses, concrete begins to fail (lose its structural integrity) even before gross melting of its constituents occurs. The loss of structural integrity accompanies the release of water and carbon dioxide from the concrete in three phases ... 

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