Ion Transport in Concrete

The principles that apply to aqueous solutions are, basically, also valid for concrete, because the transport of electrical current is due to ion movement in the water-filled pore system (Section 2.1). Positive ions (Na, K, Ca ) migrate in the direction of the current negative ones (OH , SO4 , CT) in the opposite direction. 

Applying the concept of transport numbers (Eq. (12)) to concrete it can be shown that for chloride-free concrete, assuming that the pore solution contains 0.5 mole/1 of NaOH, the transport numbers for OH and Na are 0.8 and 0.2, respectively. For concrete contaminated by chloride salts, assuming that the pore solution contains 0.5 mole/1 of NaOH and 0.5 mole/1 of NaCl, the transport number of OH, Na, and CT are 0.52, 0.20 and 0.28, respectively. In a general sense, these estimated ion transport numbers have been confirmed by experimental works [31, 32). 

Electrical current flow by ion migration in concrete is important for electrochemical rehabilitation techniques such as chloride removal (Chapter 20), but also for (macrocell) corrosion processes (Chapter 8). 

Ion migration of concrete can be tested as a measure of its resistance to chloride penetration using migration tests, of which various versions exist. Some of these tests measure non-steady-state migration, expressed as the depth of penetration of chloride ions into a specimen in an electrical field [34, 35]. Other methods apply an electrical field across a specimen until steady-state flow of chloride ions is detected in the downstream cell [36]. Because of their specific nature, a detailed description of these methods will be omitted. 

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Mclnerney, M.K., Cooper, S.C., Hock, V.F. and Morefield, S.W. (2004). Measurements of Water and Ion Transport in Concrete via Electro-Osmosis. Proc. Corrosion 2004. Paper 04351. NACE. 

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