Investigation of jointed concrete specimens using compressional wave velocity and resistivity

Recent developments in making new devices for geophysical surveys and the low costs of these surveys have made the geophysical methods such as compression sound waves velocity (P) and electrical resistivity (ρ) very common in the estimation of physical and mechanical properties of rocks. Raymer and colleagues proposed two new relationships between porosity and compression waves for low porosities and very high porosities. They used an interpolation method to estimate an average porosity between two limits. Studies have shown a good correlation (usually between 0.7 and 0.8) between the physical properties of rocks and the compression sound wave velocity.
Compression sound wave velocity is a measure of intact rock wave velocity. Therefore, ignoring the crack effects adumbrates the results and generalizes it to rock mass.
In this study, we made some artificial fractures in concrete samples in order to study the effect of discontinuities on the physical properties (P wave and electrical resistivity) in laboratory.
In order to provide concrete samples, both B and C mix designs for modeling the fine and coarse grain were prepared. To simulate the various discontinuities in the samples, materials with different cohesion and negligible density of viscosity were used. Paper and talc were used to simulate high and very low cohesions, respectively and foam (unolit) was used for distant discontinuities. To prepare the standard samples, templates were constructed according to the NQ ISRM Standard. 20 templates (the number of samples, including 10 for each mix design) were considered. After preparing the concrete mix design based on both B and C mix designs, and lubricating the templates, the concrete was poured along with placement of discontinuities and artificial porosity. The natural porosities of the samples were determined according to ISRM (1981).
The Pundit Device was used to test the compression wave velocity according to the standard ASTM D2845 (1978). To measure the electrical resistivity of the samples, the standard ASTM D5334-08 (2001) was used. This test is done in the full saturation. Since the porosity between individual samples for each type (fine or coarse grain) is constant, an increase in the total porosity is the result of an increase in the number of artificial joints and pores. In other words, increasing the joints (discontinuities) and artificial pores, causes an increase in the total porosity in the samples and a reduction in the wave velocity.
Saturated samples followed the empirical model of Raymer and colleagues very well. In case of the specimens with equal artificial pores and joints, the wave velocity in the saturated state was larger than that in the dry state. Changes in the compression wave velocity with an increase in the number of joints are greater in dry samples than in saturated samples. In other words, by increasing the number of the joints, the saturation effect of the fluid filling the pores becomes more obvious. The best fit of the data on the electrical resistivity and total porosity (or an increase in the ratio of the joint volume to the pore volume) is a logarithmic relationship that has a correlation coefficient at least equal to 0.97. Considering that water does not replace with the artificial discontinuities, electrical resistivity increases with an increase in discontinuities.