What is Capillary absorption testing?
Capillary absorption testing is the most common method to assess water transport within concrete and its results are strongly indicative of the concretes resistance to the most common deterioration methods.
Capillary absorption is the primary transport mechanism for water, and the compounds it contains such as carbonates and chlorides, to penetrate through the concrete and create deterioration through processes such as reinforcement corrosion from carbonation or chloride contamination, freeze thaw or External Sulphate Attack. Limiting capillary absorption is vital to the creation of long lasting, durable concrete structures.
How does Capillary absorption testing work?
Capillary pores are tiny, tube-like structures within concrete and on its surface. Capillary absorption testing works based on the concept of sorptivity, i.e., the capacity of these pores to absorb and transmit water, a critical property for durability and corrosion resistance.
A dry concrete sample, sealed on all but one side, is exposed to water and the volume absorbed through capillary suction is recorded over time. The rate this happens at is known as the capillary absorption coefficient and is controlled by factors such as the size, distribution, and connectivity of the pores in the pore structure of the concrete sample tested.
High coefficients indicate a porous network that could allow for easier ingress of harmful substances, thereby reducing the concrete's durability and increasing its vulnerability to degradation mechanisms.
What is Capillary absorption testing of concrete used for?
Deterioration process | Defects | Control of repairs |
Complementary technique which can be | - |
How do I carry out Capillary absorption testing?
BS 1881-122 provides a simple procedure for carrying out capillary absorption testing on a concrete structure.
First, identify areas of the concrete structure that require testing and extract cylindrical core samples with a diameter of 75 mm +/- 3 mm using a diamond-tipped core drill. A minimum of three samples are required.
Wrap the cores and store in sealed polyethylene bags for transport to the testing facility.
Clean the cores to remove debris and cut them to have flat, perpendicular ends.
Dry the cores in an oven until the mass remains constant to remove all moisture then cool to room temperature in a desiccator.
Coat all sides but the exposed surface with epoxy resin or similar materials to water proof them.
Weigh each core to obtain its dry mass.
Immerse the sample in water on top of a rack so the water can contact the exposed surface.
At specified time intervals, remove the cores, blot the wet surface, and weigh to determine the mass of water absorbed at each interval.
Calculate the absorption rate by plotting mass increase over time.
What equipment and expertise are required for Capillary absorption testing?
This is an incredibly simple test that requires no specialised equipment. The typical tools used for this test include:
A precise electronic scale capable of measuring to at least 0.1 gram for weighing the samples.
An oven capable of maintaining a set temperature, usually around 105°C, for drying the samples to constant weight.
A desiccator for cooling the samples to room temperature after drying, without absorbing moisture from the air.
Epoxy resin for coating the samples to prevent water absorption through the sides.
A water bath for immersing the bottom surface of the concrete cores to allow capillary absorption.
Callipers or a ruler for measuring the dimensions of the cores.
Blotting paper or cloth for gently drying the wet surface of the samples after immersion.
What are the advantages of Capillary absorption testing?
Simple and easy test methodology.
Besides the core samples, only a balance (for weighing), an oven (for drying samples), and a water bath are needed, all of which are standard in most testing laboratories.
While understanding concrete properties and the testing procedure is necessary, the test does not demand specialized skills, making it accessible to a broad range of practitioners.
The capillary absorption of concrete is linked to nearly all deterioration mechanisms involving ingress of damaging materials.
Acts as a cost-effective, initial screening method to determine if further, more specialized tests (such as chloride ingress testing) are necessary, potentially saving time and resources.
What are the disadvantages of Capillary absorption testing?
Removing cores is destructive, potentially compromising structural integrity and requiring repair post-testing.
The accuracy of the test can be affected by the conditions under which the core was stored and tested, including temperature and humidity variations.
The process from core extraction to testing requires significant time.
A core may not represent the entire structure's condition, necessitating multiple extractions for comprehensive analysis.
How accurate is Capillary absorption testing?
Capillary absorption testing is a valuable method for evaluating the permeability and overall durability of concrete, offering insights into how water moves through the material under capillary action.
The accuracy of this test is influenced by several critical factors, beginning with the meticulous preparation of samples. Ensuring that samples are correctly cored, uniformly cut, thoroughly dried, and their surfaces (except the one exposed to water) properly sealed, typically with epoxy resin, is paramount. Any inconsistency in these preparatory steps can lead to significant variations in the test results.
The environment in which the test is conducted also plays a crucial role in its accuracy. Temperature and humidity need to be consistent to avoid altering the water movement through the concrete, which can affect absorption rates. Moreover, the precision in executing the test according to established standards, including the depth of water immersion, duration of the test, and timing of weight measurements, is essential for obtaining reliable data.
In essence, capillary absorption testing can be highly accurate for assessing water ingress through capillary action in concrete, provided that the sample preparation, testing environment, procedural execution, and result interpretation are meticulously managed. It serves as an effective tool within a broader suite of assessments aimed at determining concrete durability, but it is most informative when used in conjunction with other testing methods.
What are the limits of Capillary absorption testing?
Capillary absorption testing for concrete has notable limitations. Being a manual test, it introduces variability due to the hands-on nature of sample preparation, execution, and measurement, impacting result consistency. The test yields a capillary absorption coefficient that reflects complex physico-chemical mechanisms, complicating interpretation. This coefficient isn't a direct measure of transport properties but indicates how water, potentially carrying damaging ions like chlorides and sulphates, penetrates the concrete.
Additionally, the test focuses on surface or near-surface permeability, which may not represent the entire structure's behaviour, especially in heterogeneous materials. While it provides insights into water ingress susceptibility, it doesn't directly measure specific degradation processes such as chloride ingress or sulphate attack, necessitating further targeted testing for comprehensive assessment.
In summary, the limitations of capillary absorption testing include its manual nature, the complexity of interpreting the results, and its limited scope in assessing overall structural durability, underscoring the need for complementary analyses.
Ancillary information
Maturity of test: > 10 years
Qualification & interpretation : Specialised lab
Service disruption: Yes
Preliminary works: Yes
Time consumption High (> one day)
Cost Medium
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