Carbonation in concrete structures

Elin Fryklund, Sofie Johansson, Antonio Barhouche, Maxime De Almeida, Bojan Fajdiga

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What is carbonation? The term “Carbonation” of concrete means the chemical reaction between carbon dioxide in the air and hydration products of the cement. The process includes: •  Diffusion of CO2 in the gaseous phase into the concrete pores.

•  Dissolution of solid Ca(OH)2 in the water of the pores.

•  The diffusion of dissolved Ca(OH)2 in pore water and its reaction with the dissolved CO2. Final result of several steps through which the calcium carbonate is formed may simply be described by the following reaction which is assumed to be irreversible:

Ca2+(aq) + 2(OH¯)(aq) + CO2(aq) CaCO3(s) + H2O

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•  The carbonization rate in concrete is directly dependent on the ratio of water and cement (w/c) to concrete

•  For concrete of basic quality, the expected carbonization rate is very low •  For concrete with a ratio of w/c of 0.45 and a range of reinforcement to the edge of concrete of

25 mm

What is carbonation?

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Carbonation of concrete •  Carbonation of concrete is associated with the corrosion of steel reinforcement and with

shrinkage.

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Depth of carbonation

•  The carbonation depth is approximately proportional to the square root of time. For

example, if the carbonation depth is 1 mm in a one-year-old concrete, it will be about 3 mm after 9 years, 5 mm after 25 years and 10 mm after 100 years.

•  The depth can be measured by the use of phenolphthalein indicator solution.

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How can we design a concrete structure for 100 years without corrosion?

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The use of a lower ratio Water/Cement and the choice of an adequate type of cement

Conventional With fly ash

With fly ash With blast furgnace slag

W/C = 0,525 W/C = 0,55 W/C = 0,60

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Latex modified concrete (LMC)

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GROUTS FOR BONDED POST-TENSIONED CONCRETE

No plasticizers

With 0,3% plasticizer 9

Corrosion inhibitors �  Used in combination with low-permeability concrete

�  Works by delaying the onset of steel corrosion

�  Three major concerns: �  Long term stability and performance of inhibitor �  Corrosion propagation after concrete cracking

�  Effect on concrete physical properties

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Epoxy-coated reinforced concrete (ECR)

�  Solid dry powders

�  Electrostatically sprayed over preheated steel

�  Performance: �  Doesn’t influence the mechanical

properties of structure

�  Reduce corrosion up to 80 %

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Corrosion-resistant reinforcing bars

�  Provides additional layer of coating.

�  Materials �  organic, inorganic, ceramic,

copper, zinc, etc.

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Hot-dipped galvanized steel

Bars coated with zinc

How to stop the process in existing structures

�  Depends on the depth of carbonation �  If it’s not that deep, different types of surface

treatments are applicable

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Surface treatments �  Hydrophobic impregnation �  Impregnation �  Coatings

�  Cementitious mortar or

rendering concrete

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Discontinuous layer

Continuous layer

If the carbonation has reached too deep

�  Remove damaged concrete �  Clean the reinforcement and

surface �  Apply new concrete

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Corroded reinforcement �  The carbonation has gone through the reinforcement �  The reinforcement is exposed to corrosion

�  Replace/ strengthen the reinforcement

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Thank you !

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