Comparing Concrete Durability Testing Methods

Absorption Tests

Description

Absorption testing is a popular method of determining the water tightness. A water absorption test, such as BS 1881-122:2011 is a test that measures the amount of water that penetrates into concrete samples when submersed. Low absorption is considered a positive result.

Where it’s Most Useful

The Absorption test method is intended to determine the susceptibility of an unsaturated concrete to the penetration of water, for both the concrete surface and interior concrete.  

Limitations

This test does have some inherent limitations including:

• Short time duration of submersion compared to potential long term service conditions;
• Does not account for any type of reactive process that ties up water; and
• Assumes all weight gain is water and not reactive processes.
• Can be misleading when it comes to the use of admixtures, such as hydrophilic crystalline waterproofing admixtures. Absorption testing measures the amount of water that penetrates into concrete samples when submersed, but fails to take into account 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 concrete is saturated and crystals continue to grow. Therefore, in the case of crystalline admixtures, testing the absorption at later ages such as 56 or 90 days will give more realistic results.

Rapid Chloride Permeability Test

Description

The Rapid Chloride Permeability (RCP) test is the standard test method for electrical indication of concrete’s ability to resist chloride ion penetration. Test methods—such as ASTM 1202, Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration—consists of monitoring the amount of electrical current passed through cylindrical specimens for a duration of six hours. The total charge passed, in coulombs, is found with the results relating to the resistance of the sample specimen to chloride ion penetration.

Where it’s Most Useful

The RCP test has gained wide acceptance as a relatively easy and quick method for determining concrete permeability. Overall, as per ASTM, this test method is suitable for the evaluation of materials and material proportions for design purposes, and for research and development. However, it has incorrectly become one of the tests used to evaluate the durability of concrete. RCP can qualify a mix, but not necessarily disqualify it.

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

Limitations

This test does have various drawbacks that make it unreliable, especially for the comparison of different mix designs or for evaluating concrete durability.

Some of these limitations are:

• It may allow for premature measurements (before a steady state is achieved);
• High voltage applied leads to an increase in temperature to the sample, and can cause physical and chemical changes resulting in unrealistic values;
• Electric current that passes through the sample during the test, indicates the movement of not just chloride ions, but all ions in the pore solution (the sample’s electrical conductivity); therefore, this test may not represent the true permeability (or potential permeability) for concrete that contains supplementary cementitious materials (e.g. fly ash, silica fume, or ground granulated blast-furnace slag) or chemical admixtures (e.g. water reducers, superplasticizers, or corrosion inhibitors);
• There can be misleading results largely due to the chemical composition of the pore solution, rather than from the true permeability;
• Concrete resistivity, rather than permeability, is measured—resistance is calculated as volts divided by current and it has been shown there is a fair correlation between concrete resistivity and concrete permeability, but it is not the intended use of the RCP test; and
• The test has low inherent repeatability and reproducibility.

Water Permeability Test

Description

BS EN 12390-8, Testing hardened concrete: Depth of penetration of water under pressure, and DIN 1048 Part 5, Testing concrete; testing of hardened concrete, water permeability, are equivalent tests which measure the depth of water penetration into concrete samples subjected to 0.5 MPa (72.5 psi) of hydrostatic pressure over a period of three days. Concrete specimens are cast and cured for 28 days. After curing, samples are 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 and cracked in half vertically. The maximum depth of water penetration into the sample is then measured.

Where it’s Most Useful

These tests determine the true permeability of concrete by evaluating the resistance of concrete against the penetration of water under hydrostatic pressure. A permeability test should be considered the dominant test to evaluate the case whereby concrete is subjected to hydrostatic pressure.

When analyzing the results, the smaller the depth of penetration results in a higher resistance to water under hydrostatic pressure. This test method is useful for structures such as basements, tunnels, and water reservoirs because it recreates the pressure conditions these structures are subjected to in a realistic way.

Limitations

In spite of many advantages, like other tests, this test method has some limitations, such as:

• For a dense and low permeable mix, the depth of water penetration is low, making it difficult to compare the mixes that have low permeation (for these cases the pressure or testing age can be modified); and
• Evaluating cases in which the concrete is not under hydrostatic pressure, this test method should be considered as an auxiliary test method and tests such as absorption as the dominate test.