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If we were to make a concrete using cement paste and zero porosity aggregate, with no interfacial transition zones present, the ionic diffusivity and fluid permeability of the concrete would rigorously have to be lower than the corresponding values for the cement paste, as predicted by analytical bounds like the Hashin bounds [49]. This is because the aggregates, assumed to be fully dense, would have transport coefficients of zero, so that the mixture must have lower bulk properties, which can only decrease as more of the second phase (aggregate) is added. However, having a third phase, the interfacial transition zones, can modify this picture.
The study of the transport properties of concrete using composite theory then requires, as experimental input, simultaneous measurement of the cement paste host and concrete transport properties while the aggregate volume fraction is systematically varied. Unfortunately, there are not many experimental results available [50,51]. Results from recent work, however, demonstrate that the effective transport properties of concrete can increase greatly as more aggregate is added past a critical amount [52]. There are also data showing that concrete can have up to 100 times the water permeability of the cement paste from which it is made [2]. Experimental results for the ionic diffusivity show effects beyond that of simply adding insulating, non-porous aggregate to a porous phase [50,51]. Elastic modulus data also clearly show the effect of a third phase, the interfacial transition zone phase [53,54,55]. The only possible microstructural explanation of all this behavior, besides that of an unreasonable amount of microcracking, is the effect of transport of fluid or ions through the interfacial transition zones. It is already known that the interfacial transition zone regions contain pores larger than those in the bulk paste. However, if the interfacial transition zones do not percolate, their effect on transport will be fairly small, as any transport path through the concrete would have to go through the bulk cement paste. Transport properties would then be dominated by the bulk cement paste transport properties.
Before exploring the interfacial transition zone percolation problem further, it is important to note that real concrete always contains air voids, especially in air-entrained concrete, where air voids are introduced deliberately to help the porous material resist freeze-thaw damage [56,57]. The volume fraction of air voids can be as great as 5-10%, so they are an important constituent of concrete [56,57]. As long as they remain filled with air, they can be treated in a transport model simply as more zero-property aggregate, with a known size distribution [56,57], since the presence of air voids also causes interfacial transition zone regions to develop [58].