Sandberg offers a range of tests, including Chloride Migration Testing and Chloride Diffusion Testing, to assess the durability and performance of concrete. These tests can be conducted at the design stage before construction or on existing structures.
The presence of chloride ions is a major factor in the corrosion of steel embedded in concrete. Chloride-induced corrosion generally causes pitting off the reinforcing steel, resulting in localised heavy corrosion. This has an adverse effect on the performance and durability of reinforced concrete.
This article summarises the differences between the different chloride tests and their advantages and disadvantages. For more information, contact our in-house expert to learn more and discuss how we can help tailor concrete chloride testing to suit your requirements.
Table of Contents
Why Test for Chlorides
Chloride-induced corrosion in concrete structures is a well-known issue which results in the degradation of the affected concrete structure over time. The corrosion process starts long before visible concrete deterioration is evident and is influenced by many factors, including the concrete’s constituent materials.
It is crucial to be aware of this phenomenon to assess the quality and service life of the concrete structure. Understanding more about “Chloride Diffusion” and “Chloride Migration” testing and the relationship between the two provides valuable information when evaluating concrete mix designs before construction and service life or residual service life of existing structures.
Chloride Diffusion Testing
The movement of chloride ions through a concentration gradient from a high chloride concentration zone to a low chloride concentration zone is known as “Chloride Diffusion”. The chloride diffusion coefficient of hardened concrete can be determined in accordance with ASTM C1556 or BS EN 12390-11.
Both test methods require a representative concrete sample to be obtained from existing structures or test specimens moulded in the laboratory. The sample is separated into two test specimens. The initial chloride-ion content is determined on one of the test specimens and the sides of the other specimen are sealed with an appropriate sealant except the finished surface. This test specimen is preconditioned and then placed in a sodium chloride (NaCl) solution for a specified exposure time (typically 90 days), Figure 1.
The test specimen is removed from the sodium (NaCl) chloride solution once the specified exposure time is complete, and thin layers are ground off in parallel to the exposed surface. The acid-soluble chloride concentration of each layer is determined and the initial chloride-ion content is then used to determine the apparent chloride diffusion coefficient of the concrete.
Chloride Migration Testing
The movement of ions caused by an external electrical field is known as “Chloride Migration”. The chloride migration coefficient of hardened concrete is determined in accordance with NT Build 492 or BS EN 12390-18 (Figure 2).
Even though the two test methods differ in certain ways, the overall process consists of the same setup (Figure 3); however, the cell at the negative terminal is filled with a sodium chloride (NaCl) solution (Figure 4), and the cell at the positive terminal is filled with either a sodium hydroxide (NaOH) or potassium hydroxide (KOH) solution. The specimen (typically 95-100mm in diameter and 50+2mm in length) is subjected to an initial 30 VDC voltage before the related current is measured.
This measurement is used to determine the test voltage and duration (minimum of 6 hrs and up to 168 hrs), with the aim of achieving a chloride penetration depth of between 10 and 30mm. Once the test duration is complete, the specimen is removed and split in half and the amount of chloride ion penetration is determined by spraying the split surface with a 0.1M silver nitrate solution. Where chlorides are present, this solution precipitates as buff white silver nitrate (AgCl) deposit. The non-steady-state migration coefficient (x10-12 m2/s) is computed using the penetration depth (Figure 5), the absolute value of applied voltage, average temperature, and other test factors.
Chloride Content Testing
Sandberg also undertakes chloride content testing. Chloride content testing aims to establish the level of chlorides within the concrete structure, either as an overall level or, by taking incremental samples, as a profile through the depth of the structure in accordance with BS EN 14629—more information about chloride content testing.
Conclusion
Considering the cost of construction today and its impact on sustainability, it is necessary for reinforced concrete structures to be durable in order to meet their designed service life.
The durability and service life of reinforced concrete structures rely on the constituent materials (mix design) and pore structure or permeability of the concrete, which affect the quantity and rate of chloride ions transported through concrete; consequently, the higher the permeability of concrete, the greater the ingress of chloride ions. Therefore, tests that can determine the rate of chloride ingress/penetrability provide valuable information on the long-term performance of the concrete.
The NT Build 492 and BS EN 12390-18 test methods are increasingly used as a more accurate alternative to ASTM C1202 (Rapid Chloride Penetration Test). These test methods produce a similar (not identical) result to that of ASTM C1556 and BS EN 12390-11, with a much shorter turnaround. The chloride migration coefficient result can also be used to evaluate the resistance of hardened concrete to chloride ion penetration.
For more information and rates for chloride diffusion and chloride migration testing, please contact our in-house expert.
References
- ASTM C1556-2003 ‘Determining the Apparent Chloride Diffusion Coefficient of
Cementitious Mixtures by Bulk Diffusion’. - BS EN 12390-11:2015 ‘Determination of the chloride resistance of concrete, unidirectional diffusion’
- NordTest Method, NT Build 492 ‘Chloride Migration Coefficient from Non-Steady-State Migration Experiments’
- BS EN 12390-18:2021 ‘Determination of the chloride migration coefficient’
- ASTM C1202-22 ‘Sndard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration’
- Germann Instruments PR-TDS-02-Prooveit
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