Permeation capacity as measured by water vapor transport

Measurements of water vapor transport through concrete has been mentioned briefly in the introduction of the concept of permeation capacity, and the peculiarity inherent in the transport of water vapor through partly dry specimens was illustrated in Fig. 2.8. An ASTM standard test method for water vapor transmission of materials (ASTM E96-00)62 specifies a method to measure the rate of water vapor transmission across a unit thickness of material as driven by vapor pressure differences maintained between the two surfaces of the material. The method is not specifically designed for concrete, and does not consider the degree of saturation of the specimen itself; this is obviously a controlling factor with partly dry concrete. A somewhat similar but more flexible ISO specification63 was designed for building materials in general. It provides for water vapor transport measurements under several different boundary conditions, and it further specifies that the specimen be conditioned to 50%RH.

 

Using a variant of one of the ISO methods, Nilsson64 has measured long-term water vapor diffusion coefficients for very mature concretes, but only between boundary conditions of 65% RH and 100% RH. Moisture diffusion coefficients showed the expected dependency on w:c ratio. As expected, moisture diffusion coefficients were found to be substantially lower for concretes with silica fume, and were especially low for concretes containing both silica fume and fly ash. Somewhat similar studies were carried out by Jooss and Reinhardt,65 but in this case the study was focused on effects of increasing temperatures. Higher temperatures were found to increase diffusion coefficients substantially. In their analyses, Jooss and Reinhardt65 formally recognized the separate contributions to the observed diffusion coefficients by diffusion through the unfilled pores and by liquid transport through the filled `choke points’ such as was illustrated in Fig. 2.8, but were unable to break down the overall flow into the separate components. This is a fundamental problem that apparently remains to be solved.