Permeability of concrete is important when dealing with durability of concrete particularly in concrete used for water retaining structures or watertight sub-structures.Structures exposed to harsh environmental conditions also require low porosity as well as permeability. Such adverse elements can result in degradation of reinforced concrete, for example, corrosion of steel leading to an increase in the volume of the steel, cracking and eventual spalling of the concrete. 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.
PROCEDURE
A comprehensive review of the wide range of test methods is given in the Concrete Society Technical Report No. 31. Two of the most widely established methods are the initial surface absorption test (ISAT) and the modified Figg air permeability test. The former measures the ease of water penetration into the surface layer of the concrete while the latter can be used to determine the rate of water as well as air penetration into the surface layer of the concrete which is also called the covercrete. Another newly developed technique uses modification of the laboratory test to determine chloride ion permeability. All the site tests emphasize the measurement of permeability of the outer layer of concrete as this layer is viewed as most important for the durability of concrete.
EQUIPMENTS
1. Initial surface absorption test
Details of the ISAT is given in BS 1881:Part 5 which measures the surface water absorption. In this method, a cup with a minimum surface area of 5000 mm2 is sealed to the concrete surface and filled with water. The rate at which water is absorbed into the concrete under a pressure head of 200 mm is measured by movement along a capillary tube attached to the cup. When water comes into contact with dry concrete it is absorbed by capillary action initially at a high rate but at a decreasing rate as the water filled length of the capillary increases. This is the basis of initial surface absorption, which is defined as the rate of water flow into concrete per unit area at a stated interval from the start of test at a constant applied head at room temperature.
2. Modified Figg permeability Test
The modified Figg permeability test can be used to determine the air or wate permeability of the surface layer of the concrete. 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 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.
3. In situ rapid chloride ion permeability test
This method was originally designed for laboratory application but has been modified for in situ use. The procedure for the laboratory test is given in AASHTO T277 and ASTM C1202. The technique is based on the principle that charged ions, such as chloride (Cl- ), will accelerate in an electric field towards the pole of opposite charge. The ions will reach terminal velocity when the frictional resistance of the surrounding media reaches equilibrium with the accelerating force. This is the basis of “electrophoresis”, which is utilized in many chemical and biological studies.
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.
APPLICATIONS
The methods described do not measure permeability directly but produce a ‘permeability index’, which is related closely to the method of measurement. In general, the test method used should be selected as appropriate for the permeation mechanism relevant to the performance requirements of the concrete being studied. Various permeation mechanisms exist depending on the permeation medium, which include absorption and capillary effects, pressure differential permeability and ionic and gas diffusion.
Most of these methods measure the permeability or porosity of the surface layer of concrete and not the intrinsic permeability of the core of the concrete. The covercrete has been known to significantly affect the concrete durability since deterioration such as carbonation and leaching starts from the concrete surface. This layer thus provides the first defense against any degradation.
RANGE AND LIMITATIONS
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. The main application is as a quality control test for precast units but application to durability assessment of in situ concrete is growing.
