Complex conductivity of rammed earth

Abstract

Searching for recyclable materials of construction, in the objective of building sobriety and resilience, is a major issue of our current societies. Mudbricks of compacted rammed earth represent an ancient construction material with many advantages associated with its availability, cost of production, potential reuse, and with a very low carbon footprint. Moisture content affects the mechanical resistance of such materials, which could become mechanically weak above a critical value. Therefore, non-intrusive characterization techniques able to image the water content distribution of these materials is highly in demand. We apply a recently developed theory of complex electrical conductivity (alias induced polarization) to characterize core samples of rammed earth materials in the laboratory. Complex conductivity describes both the ability of a porous material to conduct an electrical current (characterized by the in-phase conductivity) and its ability to store reversibly electrical charges (characterized by two interconnected properties namely the quadrature conductivity and the normalized chargeability). Samples of rammed earth and clayey soils with different pore water salinities, saturations, and compaction states are measured with the complex conductivity method in the frequency range 100 mHz-45 kHz. The in-phase and quadrature conductivities of the complex conductivity of rammed earth are connected to the water content offering therefore a new non-intrusive tomographic technique to study the water content distribution in walls made of rammed earth. The data are all consistent with the so-called dynamic Stern layer model of complex conductivity for clayey materials. This new approach provides a general method to image the change in the water content of walls made of rammed earth, a task that electrical conductivity imaging cannot perform as a stand-alone technique.

Moisture–Conductivity Calibration for Electrical Imaging of Horticultural Substrate

“…The results in this study indicate that the magnitude of complex conductivity and the in-phase conductivity follow good power-law relations with the VWC, and these results are consistent with those in the previous works [11][12][13][14][15]. Moreover, the results in the previous studies also show a relatively regular relationship [12,15] or good power-law function [11,13,14] that the quadrature conductivity increases with the increase in saturation (or VWC), but this work does not show good results that are consistent with those in the previous studies, probably due to the following reasons. On the one hand, in this study, although a set of compressed substrates with nearly equal dry densities could be offered, the presence and different volumes of spherical substrate blocks produced by the continuous mixing process led to the relatively large differences in pore structure, while in the previous works, the sample desaturation using an air injection device [12,15] or the related sample preparation [11,13,14] caused relatively small differences in the pore structure.…”

“…As the water content increases, the pore water connectivity is improved, and thus, the conductivity increases [17,19,20]. The results in this study indicate that the magnitude of complex conductivity and the in-phase conductivity follow good power-law relations with the VWC, and these results are consistent with those in the previous works [11][12][13][14][15]. Moreover, the results in the previous studies also show a relatively regular relationship [12,15] or good power-law function [11,13,14] that the quadrature conductivity increases with the increase in saturation (or VWC), but this work does not show good results that are consistent with those in the previous studies, probably due to the following reasons.…”

“…Moreover, the results in the previous studies also show a relatively regular relationship [12,15] or good power-law function [11,13,14] that the quadrature conductivity increases with the increase in saturation (or VWC), but this work does not show good results that are consistent with those in the previous studies, probably due to the following reasons. On the one hand, in this study, although a set of compressed substrates with nearly equal dry densities could be offered, the presence and different volumes of spherical substrate blocks produced by the continuous mixing process led to the relatively large differences in pore structure, while in the previous works, the sample desaturation using an air injection device [12,15] or the related sample preparation [11,13,14] caused relatively small differences in the pore structure. On the other hand, the removal of electrode polarization through the equivalent circuit modelling method can result in the removal of a part of the material polarization.…”

“…In most related studies of root zones, ERT (generally at a frequency of less than 1 kHz) is widely applied, and calibrations between direct current (DC) (or low frequency) conductivity and moisture are performed, while in those using EIT (usually at a higher frequency), links between alternating current (AC) (or complex) conductivity and moisture are required [1,2,11]. At present, there are some published works focusing on the complex conductivity (or spectral induced polarization (SIP)) measurements of soils in the frequency range below several tens of kHz.…”

“…At present, there are some published works focusing on the complex conductivity (or spectral induced polarization (SIP)) measurements of soils in the frequency range below several tens of kHz. In these studies, the effect of the water saturation, which can be related to the VWC through the porosity, on the complex conductivity were investigated, and it was shown that the magnitude of complex conductivity, the in-phase conductivity, and even the quadrature conductivity follow power-law relations with the saturation [11][12][13][14][15]. Additionally, in some other works focusing on soil moisture measurements, the related electrical measurements were performed in wider frequency ranges of up to several MHz, and despite the presence of the effects of electrode polarization, the related results show good regular relations at different frequencies between the measured resistivity (or impedance, conductivity) and the water content [16][17][18].…”