AASHTO T358 is the standard method of test for surface resistivity indication of concrete’s ability to resist chloride ion penetration. It measures how strongly a saturated concrete specimen resists the flow of electrical current, using a four-pin Wenner probe placed on the curved surface of a cylinder, and reports the result in kΩ·cm. Higher resistivity means a denser pore structure and a lower risk of chloride ingress reaching the reinforcement. The test takes seconds per specimen and is non-destructive, which is why many state DOTs accept it in place of the six-hour ASTM C1202 rapid chloride permeability test for durability-based quality control.
As the demand for new concrete structures continues to increase, contractors need better and more efficient ways to test concrete quality and durability. For this purpose, several techniques have been developed and studied. One of those is Rapid Chloride Permeability Test (RCPT), which measures the concrete’s ability to resist chloride penetration and is standardized by ASTM C1202. Another is the electrical resistivity of concrete, which measures the concrete’s resistance to the flow of electrical current and is standardized by AASHTO T358, ASTM C1876, AASHTO TP 119 & CSA A23.2-26C. Additionally, they are useful for monitoring existing structures to determine and prevent deterioration of concrete and rebar corrosion due to chloride penetration.
Unfortunately, apart from being labour-intensive, the RCPT approach is also time-consuming. 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 then letting the specimens soak with water for another 18 hours prior to the test. When this is completed, there are even more steps to follow. What is more, this test method is often very expensive due to the amount of work required to perform the analysis.
Using Electrical Resistivity to Test Concrete Quality and Durability
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 concrete quality
- Measure chloride permeability
- Determine the Chloride Diffusion Coefficient
- Detect cracks
- Determine the setting time of fresh concrete
- Measure moisture content
Learn more about the science behind how electrical resistivity evaluates the quality of concrete.
Methods of Measuring Electrical Resistivity of Concrete
Here are some different techniques for measuring the electrical resistivity of concrete including bulk resistivity, surface resistivity, and embedded electrodes for fresh concrete.
Bulk Resistivity: ASTM C1876, AASHTO TP 119 (a provisional method for bulk resistivity) & CSA A23.2-26C. 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). Bulk electrical conductivity of hardened concrete is covered separately by ASTM C1760.
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 the concrete setting time, and detect cracking.
Surface Resistivity: AASHTO 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 these methods of testing concrete quality and durability are valid. However, the potential of surface resistivity to monitor 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 and extra time to prep concrete samples for testing. However, with surface electrical resistivity testing and 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 labour 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 into their specifications.
An Overview of AASHTO T358
AASHTO T358 at a glance
| Item | AASHTO T358 |
|---|---|
| Full title | Standard Method of Test for Surface Resistivity Indication of Concrete’s Ability to Resist Chloride Ion Penetration |
| What is measured | Electrical resistivity of the concrete, read at the specimen surface |
| Probe configuration | Four-pin Wenner array, pin spacing a = 1.5 in. (38 mm) |
| Specimens | Cylinders, typically 4 in. by 8 in. (100 mm by 200 mm) or 6 in. by 12 in. (150 mm by 300 mm), tested saturated |
| Readings per specimen | Two sets of four readings, taken at 0°, 90°, 180° and 270° around the cylinder |
| Result | Surface resistivity in kΩ·cm, classified into chloride ion penetrability bands |
| Test duration | Seconds per specimen, with no cutting or vacuum saturation (specimens are tested saturated) |
Two points determine whether a result is valid. Specimen size changes the number. A 6 in. by 12 in. cylinder returns a lower reading than a 4 in. by 8 in. cylinder of the same concrete, which is why the classification table below has separate columns for each. Comparing a 4 by 8 result against a 6 by 12 threshold will misclassify the concrete. Moisture condition changes the number too. Resistivity is a measurement of ionic movement through the pore solution, so a specimen that is not properly saturated will read high and make poor concrete look good.
How the AASHTO T358 test is performed
- Prepare a minimum of three specimens, either cores or standard cylinders, at 4 in. by 8 in. (100 mm by 200 mm) or 6 in. by 12 in. (150 mm by 300 mm).
- Condition the specimens so they are saturated at the time of testing. Resistivity is measured through the pore solution, so moisture state governs the result.
- Set up the four-pin Wenner probe array with 1.5 in. (38 mm) pin spacing. Alternating current is applied through the two outer pins and the potential difference is measured between the two inner pins.
- Take two sets of four readings around the circumference of each cylinder, at 0°, 90°, 180° and 270°, to average out local variation in the specimen.
- Calculate surface resistivity from the applied current, the measured potential, and the geometry, and report the result in kΩ·cm.
- Classify the result against the penetrability bands for the specimen size tested.
What’s the Latest Technology to Measure Concrete Quality through Surface Resistivity?
Giatec’s Surf™ is a laboratory surface resistivity meter built around the AASHTO T358 method.
Surf™ can be used to evaluate:
- Performance-based quality control of concrete
- Estimation of the 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 a 4-probe array which are located at 90° from each other. These measurements are used to determine electrical surface resistivity, AASHTO T358, which can be correlated to ASTM C1202, the resistance of chlorine penetration in the concrete.
AASHTO T358 vs. ASTM C1202: Classification Table
| 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) | ||
| High | <12 | <9.5 | > 4000 |
| Moderate | 12-21 | 9.5-16.5 | 2000-4000 |
| Low | 21-37 | 16.5-29 | 1000-2000 |
| Very Low | 37-254 | 29-199 | 100-1000 |
| Negligible | >254 | >199 | <100 |
a = Wenner probe tip spacing
The ASTM C1202 column is shown for reference, not as an equivalence. Surface resistivity and rapid chloride permeability are correlated for a given concrete, but they measure different physical responses, and a resistivity result is not a substitute for a coulomb value where a specification calls for one by name.
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 the concrete quality 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 labour for DOTs, testing agencies, and contractors.
Frequently Asked Questions
What is AASHTO T358?
AASHTO T358 is the standard method of test for surface resistivity indication of concrete’s ability to resist chloride ion penetration. A four-pin Wenner probe is placed against the surface of a saturated concrete cylinder, alternating current is passed through the outer pins, and the potential difference across the inner pins is measured. The result, in kΩ·cm, indicates how readily chloride ions could move through that concrete.
What is the difference between AASHTO T358 and ASTM C1202?
They answer the same durability question by different routes. ASTM C1202, the rapid chloride permeability test, measures the total charge passed through a prepared specimen over six hours and reports coulombs. AASHTO T358 measures electrical resistivity at the specimen surface in seconds and reports kΩ·cm. C1202 requires cutting, vacuum conditioning, and an 18-hour soak before the six-hour test; T358 requires none of that and does not damage the specimen, so the same cylinder can be measured again later.
Does specimen size affect an AASHTO T358 result?
Yes, and this is the most common source of error. A 6 in. by 12 in. cylinder gives a lower resistivity reading than a 4 in. by 8 in. cylinder of identical concrete, because the geometry relative to the probe spacing differs. AASHTO T358 provides separate penetrability classification ranges for each specimen size. Always classify against the column for the size you actually tested.
Does temperature or moisture affect surface resistivity?
Both do. Resistivity measures how easily ions move through the concrete pore solution, so a drier specimen reads higher and a warmer specimen reads lower. This is why AASHTO T358 specifies the moisture condition of the specimen at test, and why field resistivity readings taken on a structure need their surface condition and temperature recorded alongside the value.
Can surface resistivity be used on an existing structure, or only on cylinders?
AASHTO T358 as written is a laboratory method for cylindrical specimens, and its classification table is tied to that geometry. Resistivity can be measured on an in-service structure, and it is a useful durability indicator there, but the numbers should not be read against the T358 cylinder classification bands. For in-place assessment of reinforced concrete, resistivity is most useful alongside half-cell potential and corrosion rate rather than on its own.
**Editor’s Note: This post was updated for accuracy and comprehensiveness in October 2026.
