Carbonation of Concrete

1. Classification of deterioration of concrete

In general concrete deterioration can be classified by physical, chemical and reinforcement corrosion. Among them chemical deterioration can be also classified by sulfate, acid, sea water, alkali-aggregate reaction, leaching, and carbonation or neutralization. Figure.1 shows the classification of concrete deterioration. This study will focus more on carbonation of concrete.

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Figure.1 Classification of concrete deterioration

2. Carbonic acid attack phenomenon

As well known, the process of neutralization in concrete of RC structures is that cement paste hydrated by reaction with cement and water is composed of cement gel, capillary pores between gels, and a large amount of calcium hydroxide and due to those of porous properties, cement paste is easily exposed to carbon dioxide in the atmosphere, and carbon dioxide penetrates from surface into inner voids of concrete, and eventually the cement paste is neutralized despite of large concentrations of Na+, K+, and Oh-. The chemical equation of neutralization is shown in Eq. (1). Of course, the higher alkalinity in concrete, the more vulnerable to being permeated because of the increase of solubility, and also in the winter case 2H2O generated in the process of neutralization of cement paste causes frequently frost action that makes cracking and spalling.

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Eq. 1

On the other hand, in order to prevent H2O from penetrating into inner concrete, as following the Eq(2).


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Eq. 2


Besides, a tendon surface in RC structures is mixed with the anode and cathode field together so that if water exists around it, as following Eq(3) and(4) the anode and cathode reaction occurs and then as following Eq(5) mono-ferrous hydroxide is turned into bi-ferrous hydroxide.


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Eq. 3, 4, 5
Neutralization extended over a long term ultimately decreases pH value of surface of a reinforcing bar less than 11.5pH, and the low pH value makes ambient chlorine ions density ratio (Cl-/OH-) increase up to over 0.3 and leads to corroding the tendon. Certainly, corrosion of a reinforcing bar is directly related to deterioration of bearing strength of structures, attributing to falling off covering concrete and suffering a deficit in cross section.In order to rehabilitate deteriorated structures, traditional methods are for now used such as filling high alkali mortar after chipping the deteriorated part off. These methods are in general related with a long period of mending time, noise, vibration, dust scattering, expenditure in excess.


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Figure.2 Procedure of concrete carbonation
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Figure.3 Procedure of rebar corrosion


Source: Denise Dal Molin in P.K. Mehta and P.J.M Monteiro, concrete

3. Test method of concrete carbonation

To measure carbonated depth of concrete phenolphthalein indicator solution is used. The phenolphthalein solution is made of 1% of phenolphthalein powder and 99% of ethyl alcohol. Phenolphthalein is insoluble in water and is usually dissolved in alcohols for use in experiments. The acid-base indication abilities of phenolphthalein also make it useful for testing for signs of carbonation reactions in concrete. Concrete has naturally high pH due to the calcium hydroxide formed when Portland cement reacts with water. The pH of the ionic water solution present in the pores of fresh concrete may be over 14. Normal carbonation of concrete occurs as the cement hydration products in concrete react with carbon dioxide in the atmosphere, and can reduce the pH to 8½ to 9, although that reaction usually is restricted to a thin layer at the surface. When a 1% phenolphthalein solution is applied to normal concrete it will turn bright pink. If the concrete has undergone carbonation, no color change will be observed.


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Figure. 4 Example of use of phenolphthalein indicator solution


3.1 Acceleration test

For specimen preparation of the acceleration test, intended specimen sizes according to materials’ types should be determined. And molds as designated sizes are prepared and then places the fresh concrete or mortar into the molds. The specimens will be cured for 28 days as the standard test pieces. After the standard curing, the specimens should be sealed with silicon on the 5 sides as seen in Figure.5 (left) below and after one day curing for sealing the specimens are placed in carbonation acceleration equipment as seen in Figure below. Note that the unsealed side should be perpendicularly placed to table plate. Finally the carbonated depth will be usually measured after 1, 2, 4, 6, and 8 weeks by using phenolphthalein indicator solution. Figure 6 shows the diagram and picture of the carbonation acceleration equipment. And Figure 7 shows precedure of measurement of carbonated depth.
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Figure. 5 Specimens for acceleration test


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Figure. 6 Diagram and picture of acceleration test equipment

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Figure. 7 Procedure of measurement of carbonated depth


3-2 Direct (field) test


One method of testing a structure for carbonation is to drill a fresh hole in the surface to collect core sample and split off the sample and then treat the cut surface with phenolphthalein indicator solution. Figure. 8 shows the procedure of direct test.

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Figure. 8 Procedure of measurement of direct test

4. Methods to prevent carbonation and to recover alkalinity from carbonated concrete

4. 1 Methods to prevent concrete carbonation


In order to prevent carbonation of concrete several methods are used. First of all, increasing cement contents is used. In other words, low water-cement ratio can play a significant role in slowing speed carbonation concrete. Figure. 9 shows one test result on carbonation depth according to water-cement ratio.

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Figure. 9 test result on carbonation depth according to water-cement ratio


Also, we can prevent carbonation by applying certain protectors on concrete surface such as paints and tiles and so on. This is because the protectors hinder from permeating carbon dioxide. In addition by adding admixtures to prevent carbonation in fresh concrete we can prevent carbonation of concrete. This method is to make organization of concrete more tightly. Besides the methods mentioned above we can prevent concrete carbonation by securing enough concrete covering depth and increasing relative humidity.

4. 2 Methods to recover carbonated concrete

And in order to recover alkalinity from carbonated concrete, we can use electronic and chemical re-alkalization method. This theoretical method uses the principle that alkali moves toward inner concrete by using electric field effect. First of all, alkali solution with electrode net as seen in Figure. 10 should be applied on concrete surface with the electrolyte. The electrode net should be connected with rebar in concrete by direct current. If electric current flows between the electrode net as anode and the inner rebar as cathode, various electronic and chemical processes occur in concrete and then by reaction of ambient rebar or movement of external substances it is possible to realkalize the carbonated concrete. Figure. 10 shows the principle of the method.


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Figure. 10 The principle of electronic and chemical re-alkalization method



However, even though it is possible to realkalize carbonated concrete using the electronic and chemical method, in realistic it is difficult to apply in real fields because of difficulty of construction and cost. Thus, it is necessary to develop more simple method to recovery carbonated concrete. Based on theory of the electronic and chemical re-alkalization we can apply to make development of alkali recovery agents in chemical. The principle for this basic agent can be found in that as described in Eq(2) above, soluble alkali in concrete is independently able to push alkaline ions outward a surface reversely and spread out according to neutralization and in that as the following Eq(6) increasing density of alkali ions for LiOH is able to restrain alkali-reactivity. In this principle, with rapid diffusion speed and insoluble calcium salts of H+ and OH-, the creation of calcium hydroxide and consumption of SO42- in making reaction of ettringite play a significant role in making reaction of ettringite.

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Figure. 11 The principle to develop alkali recovery agent


5. Case studies of recovering alkalinity from carbonated concrete

5-1 Lap test


Hexahedron bars of cement mortar, 40×40×160 mm, were made of a general portland cement and washed sea sand with 0.6 of water-cement ratio and 1:3 of cement-sand rate. The samples were demolded after 24 hours and then cured at 25℃ under water for 28 days. After curing the samples were sealed with silicon on the 5 sides and then placed in carbonation acceleration equipment under the conditions of 20±3℃, 55% of relative humidity, and 10±2% of carbon dioxide concentration. Measurement of carbonated depth was implemented at 1, 2, and 4 weeks. Figure. 12 shows the results of carbonated depth. As shown in Figure. 12 at 4 weeks the carbonated depth was 16.5mm.



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Figure. 12 test result of carbonated depth of mortar for lap test by acceleration


The carbonated samples by acceleration were applied with the alkali recovery agents to evaluate its performance. The alkali recovery agent was applied twice successively with 0.15kg/㎡ on the mortar surface and then measured the recovered depth of alkalinity 4 weeks later.
Figure. 13 shows the result of recovered depth. As seen in Figure. 13, 4 weeks later carbonated concrete area was almost recovered. Thus it is possible to say that the alkali recovery agent is effective to recover alkalinity from carbonated mortar.

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Figure. 13 Result of recovered depth of mortar

5-2 Field test


In order for the field test a certain structure built before 60 years was designated to apply this test. As seen in Figure. 14, the alkali recovery agent was applied twice successively with 0.15kg/㎡on the concrete surface and collected core sample and measured carbonated depth after 4 weeks. Figure x shows the results of the test. As seen in Figure. 15, carbonated area was considerably recovered outward surface. This indicates that the alkali recovery agent was also effective in the real field of concrete.

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Figure. 14 Application of the alkali recovery agent on bottom of slap

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Figure. 15 Result of recovered depth of the structure
Source:
1. www.wikipedia.org/
2. Concrete, microstructure, properties, and materials, Third Edition. P.K. Mehta and Paulo J.M Monteiro.

Work created by Dahee