Chlorine and Sodium Measurements

Concrete exposed to deicer salts, or to a marine environment is subjected to chloride and sodium loading. The ability of concrete to resist the penetration of chlorides and sodium is a primary design consideration in marine or cold environments. The ingress of chlorides into concrete is a major problem due to chloride-induced corrosion of the reinforcing steel and deicer salt scaling [a process by which a thin layer ( 1 mm) of concrete deteriorates from the surface of the concrete]. The penetration of sodium from sea water or deicer salts is generally 

Traditionally, the penetration of chlorides and sodium is measured destructively by grinding layers of concrete and chemically analyzing the powder samples. These data are used to calculate diffusion coefficients for the ions. This procedure is very slow, has low spatial resolution, and is destructive. The measured data are critically important for the development of service life models and therefore a rapid, high-resolution method to monitor the ingress of these ions is desirable. 

The SPRITE class of measurements is required for MRI measurements of the low sensitivity, low concentration and short signal lifetimes of these nuclei. The absolute sensitivity of 35C1 is only 0.4% and 23Na is 9% compared with 3H [36]. 

The short signal lifetimes of these nuclei (bulk T2 around a hundred ps and T) around a few ms) [14] require the use of pure phase encode methods. The low concentration of these nuclei coupled with the low absolute sensitivity requires a measurement technique that is rapid enough to permit numerous signal averages. 

It is important to note that sodium and chlorine are both quadrupolar nuclei and Eq. (3.4.1) is therefore not true for SPI/SPRITE measurement of these species however, Eq. (3.4.2) is correct when centric scan SPRITE methods are employed. 

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