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Average carbonation depths based on measurements taken from each of the four faces of every specimen are presented in Table 2 values presented are after 7, 14 and 28 days of exposure to carbonation chamber conditions. Figure 2 shows a mix design nomogram built for 21, 28, and 42 days after casting the specimens. When mixture proportions are known, mix design nomograms are a useful tool to estimate the carbonation depth of any concrete structure, since the parameters employed fall well within the range used for concrete fabrication.
The ability to predict carbonation depth based on a variety of different mixtures depends on the possibility of testing a variety of available cement types, including those which contain slags, pozzolans, metakaolin, silica fume, calcite or fly ash.
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Diffusion equations can be used to estimate the carbonation depth as a function of time, using the well-know model:. To exemplify a complementary application of the nomogram used here it is possible to conduct a simplified transformation between accelerated and natural CO 2 exposure. Assuming that the humidity level in the accelerated carbonation chamber can be extrapolated to a natural environment, it is possible to consider the product of time by gas CO 2 concentration as a constant, as shown in eq.
In this case, t would be of seven yr and eight mo. Using this transformation, the predicted carbonation depth after 50 years of service life is shown in Table 3.
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Another application of the mix design nomogram is the development of concrete structures with specific properties. Carbonation data from several concrete families exposed not only to accelerated, but also to natural environmental conditions, are desirable in order to achieve a more accurate and effective application of mix design nomograms.
As expected, the behavior of the former group indicated that cement content improves carbonation resistance if the slump is constant. Previous studies have proposed that cement content can affect carbonation depths, assuming that can occur changes in alkaline reserve. This is true only when the slump concrete workability is held constant.
Table 3 shows how concrete properties can be estimated from the mixture design nomogram. Although the data recorded in this study cannot be used to estimate carbonation rates for concrete made with other cement types or under different environment conditions, the method employed here to construct nomograms may represent a useful tool to understand the behavior of new concretes made with different materials. Therefore, special caution must be taken in research studies in order to ensure that mixture proportion planning can, in fact, help to generate reliable results which allow generalizations.
First, it is important to underline that results reported based on the nomograms used pertain only to the materials and conditions tested here. If the methodology we used is not followed or environmental conditions vary with respect to this study, our predictions may change or even become invalid.
A novel method to predict concrete carbonation
For this reason, results obtained under different conditions should be used to design novel nomograms which are specific for the methodology and materials used e. Accelerated carbonation tests can be used to evaluate and predict the performance of a concrete mixture exposed to natural aggressive conditions when natural relative humidity RH is similar to that present in experimental conditions. New nomograms applicable to specimens constructed with a different concrete composition, as well as under different conditions can be fabricated if additional tests are performed.
By using all the information available from such nomogram, it is possible to determine mixture proportions which comply with predefined performance criteria. In the present study, special attention was given to the relationship between the design of mixture proportions and degree of carbonation in concrete structures.
This approach can be applied to other properties of concrete such as permeability, resistivity, elasticity modulus, among others. The authors are indebted to Fernanda Pereira for her valuable assistance with laboratory tests.
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Research significance Because carbonation of reinforced concrete can lead to corrosion of reinforcement steel bars, it is highly relevant that the concrete technologist specifies the appropriate concrete mixture proportion in order to minimize carbonation rates. For a given workability, the equation which best fits the tendency predicted by this law, generalized by Powers 12 , is: where fc is compressive strength MPa , while k 1 and k 2 are constants that depend on the materials used.