Resistivity is one of the key controlling factors once corrosion begins. Corrosion requires a flow of ions between anodic and cathodic sites on the reinforcement of concrete; the resistivity controls the rate of this flow of ions and therefore directly controls the rate of concrete corrosion.
What this means is that by measuring concrete resistivity it is possible to determine if a structure that is corroding will deteriorate quickly or if the corrosion rate is slow and can be addressed in a more conservative manner. This can be a very useful test in combination with a Half-Cell potential measurement to locate hot spots for corrosion likelihood.
The Surface Resistivity (SR) test is a much quicker and easier test for estimating concrete permeability. The traditional testing method, Rapid Chloride Penetration was laboratory based and very labour intensive, taking 4 days of preparation and testing including cutting, epoxying, desiccation and testing.
A study initiated in 2002 by the Florida Department of transport has determined a strong correlation between Rapid Chloride Penetration [RCP] measurements and resistivity measurements (see image below). This discovery now means permeability testing can now be done in 10 minutes instead of 10 days using the same cylinders or cores used to determine compressive strength.
The versatility of the method can be seen in these example applications:
- Estimation of the likelihood of corrosion
- Indication of corrosion rate
- Correlation to chloride permeability
- On site assessment of curing efficiency
- Determination of zonal requirements for cathodic protection systems
- Identification of wet and dry areas in a concrete structure
- Indication of variations in the water/cement ratios within a concrete structure
- Identification of areas within a structure most susceptible to chloride penetration
- Correlation to water permeability of rock
Principle of Operation
Operating on the principle of the Wenner probe, the ResiPod is designed as a resistivity meter to measure the electrical resistivity of concrete or rock. A current is applied to the two outer probes, and the potential difference is measured between the two inner probes. The current is carried by ions in the pore liquid. The calculated resistivity depends on the spacing of the probes.
Resistivity ρ= 2πaV/l [kΩcm]
When taking measurements the unit automatically changes its measurement methods to suit the subjects. Normally it will attempt to drive a full 200uA current through the concrete, if this is not possible due to a high resistance it will drive 50 uA and if that is not possible due to a very high resistance the device will instead drive a voltage across the outside probes and measure the induced current to obtain a resistivity estimate. This allows the Resipod to measure much higher resistivity’s than a basic Wenner Probe.
Resipod Models and Probe Spacing
Wider probe spacings provide a more consistent reading when measuring on an inhomogeneous material like concrete. However, if the spacing is too wide, there is more danger of the measurement being affected by the reinforcement steel. The industry standard 50 mm probe spacing has long been seen as a good compromise.
The 38mm model is designed specifically to comply with the AASHTO standard (under development) for “Surface Resistivity Indication of Concrete’s Ability to Resist Chloride Ion Penetration”.
The contact extension cable kit can also be used to attain variable spacing for the contacts. Spacing from 20-85mm can be achieved using the kit, which is bought as an optional accessory.
| Range|| 0–1000 kΩcm (probe spacing)|
| Resolution (nominal current 200μA) || ±0.2 kΩcm or ±1% (whichever is greater)|
| Resolution (nominal current 50μA) || ±2 kΩcm or ±5% (whichever is greater)|
| Resolution (nominal current <50μA) || ±2 kΩcm or ±5% (whichever is greater)|
| Frequency|| 40 Hz|
| Memory Non volatile|| 500 measured values|
| Power Supply|| >50 hours autonomy|
| Charger connection|| USB type B, (5V, 100mA)|
| Dimensions|| 197 x 53 x 69.7 mm|
| Weight|| 318 g|
| Operating temperature|| 0° to 50°C|
| Storage temperature|| -10° to 70°C|