Media

Concrete durability testing

To have a sustainable concrete structure, durability is key. Use of inappropriate materials, poor construction practices, curing and mix designs, results in concrete structures often showing serious premature deterioration. This is a global problem that annually costs public and private sectors worldwide billions of dollars.

Measuring the durability of concrete to determine its longevity in service is challenging. It is, however, important in identifying how sustainable structures will ultimately be.

Tests such as water absorption, surface absorption, water permeability, or chloride permeability are commonly specified.
These tests should consider the unique properties of the mix design when used to determine concrete’s durability.

 They also have some limitations and it is necessary to understand and recognise them.

Absorption Tests

Description: Absorption testing is a common method of determining water resistance. Tests such as BS 1881-122:2011 measure the amount of water that penetrates into concrete samples when submersed. Low absorption is a positive result.

Where it’s most useful: The test is intended to determine the susceptibility of unsaturated concrete to water penetration, for both surface and interior concrete.

Limitations:

• Short submersion time compared to long-term service conditions
• Does not account for any reactive process that binds water
• Assumes all weight gain is water and not reactive processes
• Can be misleading when using admixtures such as hydrophilic crystalline waterproofing admixtures. Absorption testing
• measures the amount of water penetrating into concrete samples when submersed, but fails to consider the inherent use of water in the crystallization process, particularly in the early stages of curing (less than 28 days).

The absorption test results will improve over time as the crystals continue to grow. Therefore, for crystalline admixtures, testing the absorption at later ages (56 or 90 days) will give more realistic results.

Rapid Chloride Permeability test

Description: The Rapid Chloride Permeability (RCP) test is an electrical test used to estimate the durability of concrete. Test methods – like ASTM 1202, Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration – monitor the amount of electrical charge passed through cylindrical specimens for six hours. The total charge passed, in coulombs, is related to the specimen’s ability to resist chloride ion penetration. Lower levels of charge passed indicate higher resistance.

Where it’s most useful: The RCP test has gained wide acceptance as a relatively easy and quick method for determining concrete permeability.

Overall, this test method is suitable for evaluating materials and material proportions for design purposes, and for research and development.

However, it has incorrectly become one of the tests used to evaluate concrete durability. RCP can qualify a mix, but not necessarily disqualify it.

The RCP test should not be used to accurately determine the permeability of concrete mixes containing supplementary cementitious materials or chemical admixtures.

Limitations: Various drawbacks make the test unreliable, especially for comparing different mix designs or for evaluating concrete durability.

• It may allow for premature measurements (before a steady state is achieved);
• The high voltage applied increases temperature in the sample, and can cause physical and chemical changes, and unrealistic values;
• Electric current passing through the sample indicates the movement of not just chloride ions, but all ions in the pore solution (the sample’s electrical conductivity); this test may thus not represent true (or potential) permeability of concrete containing supplementary cementitious materials (fly ash, silica fume, or ground granulated blast-furnace slag) or chemical admixtures (water reducers, superplasticisers, corrosion inhibitors);
• Possible misleading results due to the chemical composition of the pore solution, rather than the true permeability;
• Low repeatability and reproducibility.

Water Permeability test

Description: BS EN 12390-8 and DIN 1048 Part 5 are equivalent tests measuring depth of water penetration into concrete samples subjected to 0.5 MPa of hydrostatic pressure over three days.

Concrete specimens are cast and cured for 28 days and then placed in the testing device.

The sample holders are open at both ends, with one end subjected to hydrostatic pressure.

After three days, the samples are removed from the testing apparatus, cracked in half vertically and maximum depth of water penetration measured.

Where it’s most useful: This test determines the true permeability of concrete by evaluating concrete’s resistance to penetration of water under hydrostatic pressure and should be the preferred test if concrete is subjected to hydrostatic pressure.

The smaller the depth of penetration, the greater the resistance to water under hydrostatic pressure. This test is useful for basements, tunnels, and water reservoirs, since it realistically recreates pressure conditions to which these structures are subjected.

Limitations:

• For dense and low-permeability mixes, the depth of water penetration is low, making it difficult to compare mixes that have low permeation (in these cases the pressure or testing age can be modified); and
• For concrete not under hydrostatic pressure, this test should be used in conjunction with absorption testing.

 Conclusion

Environmentally specific mix designs are necessary for every unique condition, as are service-environment-appropriate test
methods.

Absorption tests are best suited to concretes with little or no hydrostatic pressure and permeability tests should be the main test to evaluate concrete subjected to hydrostatic pressure.

The focus of specifications for testing concrete durability should be on determining the concrete’s permeability, resistance to environmental factors, and tendency to crack.

Alireza Biparva, B.Sc., M.A.Sc. Kryton
Research and Development Manager
Email: alireza@kryton.com

Source: Concrete Trends