Permeability tests measure the ease with which liquids, ions and gases can penetrate into the concrete. In situ tests are available for assessing the ease with which water, gas and deleterious matter such as chloride ions can penetrate into the concrete.These methods do not measure permeability directly but produce a permeability index, which is related closely to the method of measurement.
Permeability of concrete is important when dealing with durability of concrete particularly in concrete used for water retaining structures or watertight sub-structures. They are useful to determine the quality of the cover concrete and to assess the durability of a concrete structure.
- BS 1881
- AASHTO T277
- ASTM C1202
Modified Figg permeability test can be used for both air and water. In both the air and water permeability test a hole of 10 mm diameter is drilled 40 mm deep normal to the concrete surface. A plug is inserted into this hole to form an airtight cavity in the concrete. In the air permeability test, the pressure in the cavity is reduced to 55 kPa using a hand operated vacuum pump and the pump is isolated. The time for the air to permeate through the concrete to increase the cavity pressure to 50 kPa is noted and taken as the measure of the air permeability of the concrete. Water permeability is measured at a head of 100 mm with a very fine canula passing through a hypodermic needle to touch the base of the cavity. A two-way connector is used to connect this to a syringe and to a horizontal capillary tube set 100 mm above the base of the cavity. Water is injected through the syringe to replace all the air and after one minute the syringe is isolated with a water meniscus in a suitable position. The time for the meniscus to move 50 mm is taken as a measure of the water permeability of the concrete.
For measuring the chloride ion permeability a DC power supply is used to apply a constant voltage between the copper screen and the steel reinforcement. The total current flowing between the mesh and the reinforcing bar over a period of six hours is then measured. The total electric charge (in coulombs) is computed and can be related to the chloride ion permeability of the concrete.
For the ISAT,
- Tests on oven dried specimens give reasonably consistent results but in other cases results are less reliable. This may prove to be a problem with in situ concrete.
- Particular difficulties have also been encountered with in situ use in achieving a watertight fixing.
- The test has been found to be very sensitive to changes in quality and to correlate with observed weathering behaviour.
For Modified Figg permeability test
- The main difficulty in the modified permeability test is to achieve an air or watertight plug.