As the demand for new concrete structures continues to boom, better and more efficient ways to test the quality and durability of concrete are needed. Multiple tests for obtaining and understanding concrete durability by measuring its ability to resist chloride penetration have widely become accepted. This method, commonly known as RCPT (rapid chloride permeability test), 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. This test is especially important for DOT’s when bridges and highways are undergoing construction, as well as for monitoring existing structures to determine and prevent deterioration of concrete and rebar corrosion due to chloride penetration.
Unfortunately, not only is the RCPT approach labor intensive, it is also time-consuming, as the test itself takes 6 hours to complete and 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. Moreover, this test method is often very expensive due to the amount of work required to perform the analysis.
Using Electrical Resistivity to Test the Durability of Concrete
A much faster and less expensive method for determining the quality and durability of concrete is electrical resistivity. 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 to improve this measurement process. This method assesses the ability of concrete to resist a chloride passage by measuring the concrete’s resistance to the flow of electrical current. As a result, it evaluates the quality control and service life prediction of concrete.
Electrical resistivity testing can be used to:
- Ensure quality
- Measure chloride permeability
- Determine the Chloride Diffusion Coefficient
- Detect cracks
- Determine the setting time of fresh concrete
- Measure moisture content
Methods of Measuring Electrical Resistivity of Concrete
Different techniques for measuring electrical resistivity of concrete, include bulk resistivity, surface resistivity, and embedded electrodes for fresh concrete.
Bulk Resistivity: ASTM C1876, AASHTO TP 119 & CSA 23.3-26C. Two electrodes are used to send and read the current flow in a hardened concrete sample using two parallel electrode plates which transmit an alternate current (AC).
Embedded Electrodes in Fresh Concrete: embedded electrodes are used to track changes in the concrete hydration process, enabling the user to measure water content, predict concrete setting time, and detect cracking.
Surface Resistivity: AASTHO T358. Uses the Wenner-Array setup in which 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 concrete’s electrical potential.
The Time-Savings Potential of Surface Resistivity
All of these methods of testing concrete quality and durability are valid. However, the potential of electrical resistivity, in particular; surface resistivity to monitor your concrete, yields huge advantages. This is especially true when compared to current chloride permeability tests.
The RCPT method takes 6 hours for test results to be returned, not to mention the additional time needed to prep concrete samples for testing. But, with surface electrical resistivity testing, according to AASHTO T358: Surface Resistivity Indication of Concrete’s Ability to Resist Chloride Ion Penetration, you can evaluate the quality and permeability of your concrete in seconds. This saves significant time in both labor costs and quality control testing costs for DOTs and testing agencies. States such as Florida and Louisiana have already realized the significant cost savings associated with this testing method and have incorporated it in their specifications.
Giatec’s Surf™ device is the fastest and most accurate technology for the measurement of surface resistivity.
Surf can be used to evaluate;
- Performance-based quality control of concrete
- Estimation of chloride diffusion coefficient of concrete
- Service life design of concrete structures
- Estimation of the remaining life of concrete structures
- Crack detection in concrete elements under load
- Monitor the setting time of fresh concrete
Surf™ has a unique and customized setup, which enables the measurement of electrical resistivity with high accuracy by reducing the duration of the test, minimizing the ambient effects and reducing human error. The patented technology automatically measures resistivity 8 times around the concrete specimen using four channels of 4-probe array which are located at 90° from each other. These measurements are used to estimate the resistance of chloride penetration in the concrete.
Chloride Ion Penetrability
AASHTO T358: Surface Resistivity (kohm-cm)
a = 1.5 in.
ASTM C1202 – Rapid Chloride Penetrability (Coulombs)
4-by-8 in. (100-by-200-mm)
6-by-12 in. (150-by-300-mm)
a = Wenner probe tip spacing
This shows the qualitative relationship between the rapid chloride penetrability test (RCPT), and the surface electrical resistivity of concrete. This device can also be used for durability-based quality control of concrete and for monitoring the service life design of a structure.
Electrical resistivity testing is an important step in the process of assessing durability factors of concrete, such as permeability, diffusivity, and other micro-structure characteristics which can be correlated to other properties, such as; chloride diffusion coefficient, water absorption, and the rate of corrosion of rebar in reinforced structures.
These characteristics provide, not only a quantitative assessment of concrete, but also general guidance on the current durability status of any given concrete element. This technology has the potential to save significant costs associated with testing and labor for DOTs, testing agencies, and contractors.