Modelling the D.C. Electrical Conductivity of Mortar
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ABSTRACT The interfacial zone separating cement paste and aggregate in mortar and concrete is believed to influence many of the properties of these composites. This paper presents a theoretical framework for quantitatively understanding the influence of the interfacial zone on the overall electrical conductivity of mortar, based on realistic random aggregate geometries. These same ideas may also be used to approximately predict the fluid permeability of mortar.
INTRODUCTION The D.C. electrical conductivity of mortar and concrete is an important measure of ionic diffusivity [1], via the Nernst-Einstein relation [2]. Diffusivity is of interest in connection with a range of issues related to durability, such as sulfate attack and chloride ion-induced corrosion Much recent work has been done on understanding how the of steel reinforcing bars [3]. microstructure of cement paste determines its electrical conductivity [4-9]. However, relatively little work has been done on how the conductivity of concrete depends on quantities like the number and arrangement of aggregate particles, and on the cement paste:aggregate interfacial zone [6,10-12]. In this paper, we are concerned with the (approximately 10 pm-1000 1m) length scale that adequately describes a typical mortar [13,14]. Within this framework, mortar (and concrete) can be viewed as a three-phase composite [15-17]: bulk cement paste, aggregate, and interfacial zone cement paste [see Figure 1], where all three phases can be thought of as uniform continuum materials. In such a three-phase composite model, the volume fraction assigned to the interfacial zone phase depends on what thickness is taken to define the boundary between the interfacial zone and the bulk cement paste. For values of the interfacial zone thickness around 20 lsm, the interfacial zone cement paste occupies 20-30 % of the total cement paste volume, and therefore 10-15% of the total mortar volume [14]. Since the interfacial zone cement paste occupies a significant volume fraction, the different microstructure [18,19] and therefore different physical properties of this phase will certainly have an influence on the overall behavior of the mortar/concrete composite [20,21], especially since recent modelling and mercury injection experimental work have shown that the interfacial zone cement paste phase forms a continuous percolating channel [14,22]. RANDOM THEREE-PHASE MODEL FOR MORTAR More details of the following model are given in Ref. [23]. For the purposes of electrical conduction, the aggregate grains are simply inert obstacles to the flow of current. The basic 429 Mat. Res. Soc. Symp. Proc. Vol. 370 ©1995 Materials Research Society
model is then defined by two parameters: 1) the structure of the interfacial layer, and 2) the electrical contrast between this layer and the bulk cement paste. In this paper we replace the variable conductivity (because of varying porosity) interfacial zone region with a shell of fixed width and constant conductivity, a,,, for the sake of simplicity. All the other cem
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