The fast-paced environment of the construction industry results in a constant search for better and more efficient ways to evaluate the quality of concrete. Despite this increase in state-of-the-art knowledge regarding concrete research, current industry practices still rely on compressive strength testing to measure concrete quality. Results of a compressive test alone do not represent the quality of the concrete, or its inherent durability. When strength is emphasized as the only quality specification, contractors concentrate their efforts on delivering a material that is compliant with strength requirements, which ultimately does not represent the durability of the concrete.
As a result, multiple tests for obtaining and understanding durability by measuring concrete’s ability to resist chloride penetration have widely become accepted. This method commonly known as RCP measures chloride penetration on a range from high to negligible and is standardized by ASTM C1202: Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration. However, not only is this approach labor intensive, it is also time-consuming, as the test itself takes 6 hours to complete and it requires extensive sample preparation. This includes cutting and conditioning the samples into a desiccator for 3 hours and letting the specimens soak with water for 18 hours prior to the test, and even more steps. In addition, this test method is often very expensive due to the amount of work required to perform the analysis.
A much faster and less expensive method for determining the quality and durability of concrete is electrical resistivity. This method assesses the ability of concrete to resist a chloride passage or flow of electrical current. As a result, it evaluates the quality control and service life prediction of the concrete. The reason behind this is twofold.
- Firstly, when concrete is subject to an electric field, the current starts to move through the pores of the material. The ability to withstand this current transfer indicates the resistivity.
- Secondly, the resistivity demonstrates the permeability of fluids throughout the concrete and therefore, indicates how durable the concrete is, especially as chloride penetration can cause corrosion of reinforcing steel.
Like strength, electrical resistivity is a property of any given concrete. Its main advantage is the ease of measurement and testing speed. Electrical resistivity has been used to measure the quality of concrete since the early 1910s, but in recent years state of the art devices have been developed improving this measurement process.
How Can Resistivity Help the Construction Industry?
The durability of concrete is largely related to its microstructure, such as its porous network and interconnections. For instance, a concrete with low porosity and interconnections has lower conductivity and thus lower permeability. In this sense, the idea behind electrical resistivity is to quantify the conductive properties of the concrete, making a direct comparison with the durability of the material.
Furthermore, electrical resistivity can be used for several applications, both in the lab and field. For instance, one can track changes in the mix design, such as alterations in cement or additions, by comparing resistivity values from lab specimens with field samples. Furthermore, resistivity results can be correlated to other characteristics, such as; chloride diffusion coefficient, water absorption, and corrosion rate of rebar in reinforced structures. Those characteristics provide, not only a quantitative assessment of concrete, but also general guidance on the current durability status of any given concrete element.
There are different ways to measure the electrical resistivity of concrete, the most common are; bulk resistivity, surface resistivity, and embedded electrodes for fresh concrete. Giatec has built devices to measure the electrical resistivity of concrete based on these techniques. They are;
- RCON: Laboraory Device for Testing Bulk Electrical Resistivity
- SURF: Laboratory Device for Testing the Surface Electrical Resistivity of Concrete
- SmartBox: Wireless Concrete Sensor for Resistivity and Temperature Monitoring
Bulk resistivity uses a two point technique, meaning that two electrodes are used to send and read the current flow in a concrete sample. In this method, a hardened concrete specimen is placed between two parallel electrode plates which transmit an alternate current (AC). The resistivity is calculated as an average of the entire material between the plates in a matter of seconds.
Governed by AASHTO TP 119: Standard Method of Test for Electrical Resistivity of a Concrete Cylinder Tested in a Uniaxial Resistance Test, the release of CSA A23.2-26C: Bulk electrical resistivity of concrete, and ASTM C1876-19: Standard Test Method for Bulk Electrical Resistivity or Bulk Conductivity of Concrete has brought even more relevancy to measuring bulk electrical resistivity of concrete.
As there is a correlation between bulk resistivity and the RCP test (refer to the table below), one can perform resistivity measurements to gather the durability status of the concrete. Since this test is completely non-destructive, it is also possible to perform a compressive strength test and obtain the compressive strength of the same sample.
Generally, both the surface and the bulk electrical resistivity will deliver the same results as long as factors are taken into consideration. Surface is guided by AASHTO 358-19: Surface Resistivity Indication of Concrete’s Ability to Resist Chloride Ion Penetration. However, due to its high acceptance, there are current studies focusing on the standardization of this technique.
The method uses the Wenner-Array setup, where four electrodes are positioned in a straight line and equally spaced. The two outer electrodes send the current into the concrete, while the two inner electrodes measure the electrical potential. This approach allows the assessment of concrete samples and facilitates the evaluation process on job site conditions as local properties can be measured in a simple manner and requires almost no preparation.
Embedded Electrodes: Fresh Concrete
For electrical resistivity of fresh concrete, embedded electrodes can be used to track changes in the concrete hydration process. When concrete hardens, there is an intrinsic reduction of moisture due to the hydration process, therefore the results of the electric resistivity test will increase with the hydration process. In addition, the results using this method enables the user to obtain parameters such as water content, and allows the ability to predict setting time and detect cracking.
Due to its high versatility and utility, more studies are being developed in this field and, currently, there have been efforts put in place to standardize this technique in the coming years.
Factors that Influence Electrical Resistivity Measurements
Although electrical resistivity tests have the advantage of being easy and rapid to perform, there are key factors that can influence the results.
Electrical resistivity will continuously play an important role in the quality assessment of concrete. In fact, due to advantages, such as; ease of use, fast results, and good correlation with durability properties, there are current studies to standardize electrical resistivity and it’s direct correlation in assessing the durability of concrete.