Electrical Resistivity of Concrete
Durability performance of existing infrastructures in North America is a huge concern. For example, according to the 2013 ASCE report, almost 11% of bridge structures in the US are structurally deficient. In addition, the lack of reliable quality control tools and procedures can affect the durability of future construction.
Currently, there is a huge gap between the state-of-art knowledge in concrete research, and the current industry practice. While the cutting edge research has been quite active in developing more reliable and accurate techniques, the tendency in the industry is toward old-fashion test methods. Recent advances in concrete testing technology is hardly accepted by the industry. We here at Giatec are trying to bridge this gap by developing smart testing techniques for quality control and condition assessment of concrete.
A very good example of these old-fashion practices in quality control of concrete is the common use of the compressive strength testing as the only method for quality control purposes. Compressive strength in concrete is not necessarily an indicative test for durability of concrete. The only other method employed in some major projects is the use of the Rapid Chloride Permeability Test, informally known as an RCP test.
The RCP test is a widely accepted approach for durability assessment of concrete material. In this test, the electrical charge passed through concrete is used as an indication of the concrete's ability to resist chloride ion penetration. The test should be carried out on 28-day saturated samples. It takes lots of preparation, and the test takes a period of at least 6 hours to finish. The test preparation and setup is labor intensive, and time consuming.So what is this "Durability thing" that we are speaking all about today? Concrete is a porous material, and the durability of concrete depends largely on the properties of its microstructure such as the pore network, size, and interconnections. A finer pore network with fewer connectivity, leads to lower permeability. Porous microstructure with larger degree of interconnections, on the other hand, results in higher permeability and reduced durability in general. Now, Electrical resistivity measurement can be used to evaluate the durability of concrete material.
In order to get into the electrical resistivity topic in concrete testing, first lets begin with the electrical properties of concrete. The electrical properties of concrete, largely depends on the moisture content of concrete. Depending on the moisture content, concrete may exhibit conductive or insulated characteristics. For example, a concrete sample might exhibit high electrical resistance when it is dry, but the same concrete can have much lower resistance if it is saturated. Moreover, concrete has capacitive properties, which means it can hold electrical charge. This makes it difficult to measure the resistance of concrete.
Lets start with the general concepts of electrical resistivity. When concrete is subject to an electric field, the ions start to move in the pores of concrete. The ability of concrete to withstand the transfer of ions is what we call real resistance of concrete. In this context, resistivity measurement can be used to assess the size and extent of the interconnectivity between pores. In addition, there is a capacitance property of concrete that tends to store the electrical charge. In measuring the electrical resistivity, we should always consider this effect in our calculations.
In order to find a practical approach to test concrete material, different test procedures have been proposed and put into test. The electrical response of concrete is normally studied by electrical circuit modeling. This means that concrete acts as an electrical circuit, with real resistance, and capacitance behavior. Then, the response of such a system is normally determined with the aid of an equivalent circuit model. These models represent the electrical properties of concrete as well as the electrode-concrete interface.
Based on the proposed models, different measurement techniques have been developed including two-point uniaxial and four-point also known as Wenner probe techniques. The two point technique delivers what is usually called bulk electrical resistivity. The reason is that the current passes through the bulk of concrete and the resistance is somewhat an average of the entire concrete bulk between the two electrodes. The four point technique, or the surface electrical resistivity of concrete makes it possible to measure the electrical resistivity from the surface of concrete. This can be important for the devices used in field inspection. It is more sensitive to the local properties of concrete and variation of moisture on the concrete surface.
The inherent electrical resistivity of concrete is affected by the following four parameters:
1- Connectivity of pores
2- Porosity and conductivity of pore solution
3- Moisture content
5- The geometry of the specimen
6- Electrical signal frequency
However, if the effect of these factors is appropriately taken into account, any resistivity measurement technique should deliver the same resistivity value.
Now, lets talk a little bit more about these parameters. The porosity of concrete can significantly affect the electrical resistivity measurements. The more porous the concrete is, the more space for ionic movement. In a big picture, these pores can make way for chemical agents to penetrate into concrete. The other issue with the pores is. to what extent are these pore networks connected to each other? We should also consider the electrical properties of the pore solution. The conductivity of pore solution can affect the total electrical resistivity of concrete.
The other influential parameter is the temperature. In general, the increase in the temperature decreases the electrical resistivity of concrete. This can be explained by the ARRENIUS equation. In simple terms, an increase in the temperature will increase the rate of ionic movements. The more ionic movement means more electrical conductivity, and lower resistivity values.
Most electrical resistivity test methods employ Alternate Current. If we test the resistivity of concrete using AC current, the response may vary depending on the frequency range that is used. When testing on low frequency, the concrete-electrode interface becomes important. The effect of concrete bulk is well determined using higher frequency range.
It is worth mentioning that the variation of concrete moisture content can effect the electrical resistivity of concrete. Increasing the moisture content will result in a decrease in the electrical resistivity. This is an interesting relationship, because one can track the changes in the moisture content of concrete by measuring its electrical resistivity.
Finally, lets talk about the effect of concrete sample geometry. In theory, resistivity is a property of concrete material, and the shape of the sample does not affect it. In doing so, however, one should take into account the confined geometry effect of the measured values. Certain mathematical formulations can be used to determine the geometry factor to get the correct resistivity value for any sample size or shape. One approach is to fill the same geometry with a liquid that has a known electrical resistivity, and extract the geometry factor from resistance and resistivity.
The electrical resistivity test techniques can be used in several areas. The test method has been successfully employed in the following areas seen on this slide, it helps analyze
1- Quality Control
2- Chloride Permeability
3- The Chloride Diffusion Coefficient
4- Crack Detection
5- Setting Time of Fresh Concrete
6- Moisture Content Measurement
Speaking of the applications of the electrical resistivity measuring technique, one can consider the resistivity technique as a suitable replacement for more traditional tests such as the previously discussed: RCP. This table shows a proposed relationship between the RCP test results, and the electrical resistivity of concrete. Similar research has shown a reliable relationship between the results of these tests. Considering the time and the cost of RCP test, electrical resistivity techniques can be a powerful alternatives.
In theory, we expect a linear relationship between the RCP test results, and the conductivity of concrete. If we combine and simplify the famous Ohm Law and equations relating electrical charge and the current, we can deliver this linear relationship for the RCP test. However, in practice, a nonlinear relationship is observed.
This can be explained by the generation of heat during the RCP test which changes the test condition, and the change in the pore solution properties.
It is proven that there is a strong relationship between the electrical resistivity of concrete and the Diffusion Coefficient of concrete. The Diffusion coefficient of concrete is an important factor in the service life design of new structures, as well as for the maintenance and rehabilitation of existing structures. There is a linear relationship with electrical resistivity of concrete in SSD condition and the diffusion coefficient. This concludes, that the resistivity can be used as a reliable technique to obtain the diffusion coefficient of concrete required in service life estimation of structures.
So to conclude todayo's presentation , it can be said with confidence that the electrical resistivity measurement techniques can be used in durability performance assessment of fresh and existing concrete. If the effect of influential parameters are correctly taken into account, the electrical resistivity can deliver reliable results. The test is proven to have a consistent relationship with conventional test procedures such as an RCP test and Diffusion coefficient measurement.
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Thank you for your time and that concludes this presentation.