The durability of concrete structures depends on the permeability of reinforcement cover by concrete. It is this thin layer of concrete over reinforcement on which life of a structure depends.
The permeability tester for concrete cover is a non-destructive instrument for the determination of air permeability of cover concrete.
The permeability of concrete cover depends on the condition of concreting at site such as segregation and bleeding, finishing and curing, the formulation of micro-cracks, etc.
The composition and properties of the cover concrete may differ very considerably from those of the good quality of cover concrete. In addition, the concrete test specimens used for quality controls can never represent the quality and properties of the cover concrete since they are produced and stored in a completely different manner.
Durability of concrete structure under aggressive environmental influences depends essentially on the quality of a relatively thin surface layer (20 – 50 mm). This layer is intended to protect the reinforcement from corrosion which may occur as a result of carbonation or due to ingress of chlorides or other chemical effects. The influence mentioned is enhanced by damage due to frost/thaw or frost/thaw/salt.
There is no generally accepted method to characterize the pore structure of concrete and to relate it to its durability. However, several investigations have indicated that concrete permeability both with respect to air and to water is an excellent measure for the resistance of concrete against the ingress of aggressive media in the gaseous or in the liquid state and thus is a measure of the potential durability of a particular concrete.
There is at present no generally accepted method for a rapid determination of concrete permeability and of limiting values for the permeability of concrete exposed to different environmental conditions.
The Permeability Tester permits a rapid and non-destructive measurement of the quality of the cover concrete with respect its durability. The general arrangement of the permeability tester is shown in fig below:
Fig: General arrangement of Concrete Permeability Tester
Principle of Concrete Permeability Tester:
The rate at which the air from the concrete cover may extracted, is a measure of permeability of concrete. This method can be used to assess the resistance of concrete to carbonation, penetration of aggressive ions and quality of grout in post tensioned ducts.
Description of Concrete Permeability Tester:
The technical details of the instrument are given below:
(1) Display Unit
- Non volatile memory for upto 200 measured objects
- Display on 128 X 128 graphic LCD
- RS 232 C interface
- Integrated software for printout of measured objects and transmission to PC
- Operation with 6 batteries LR6 1.5 v for about 60 hrs. or commercial power unit 9 VDC/0.2 A.
- Temperature range –100C to +600C
- Carrying case 320 / 285 / 105 mm, total weight 2.1 Kg
(2) Control Unit and vacuum cell
- The volume of inner chamber and hose and the cross sectional area of the inner chamber are terms in the formula for calculating kT and L. They must therefore not be changed.
- Vacuum connection –small flange 16 KF
- Carrying case 520 / 370 / 125 mm , total weight 6.3 kg
(3) Resistance probe WENNER –PROCEQ
- Electrode spacing 50 mm
- The instrument is operated with a commercial vacuum pump.
- Technical data as per DIN 28400
- Suction capacity :1.5 m3/h
- Final total pressure :approx 10 bar
- Suction side connection : small flange 10 KF / 16KF
- High water vapour toleration
Methodology of Concrete Permeability Tester:
It operates under vacuum and can be used at the site and also in the laboratory. The essential features of the method of measurement are a two chamber vacuum cell and a pressure regulator which ensures an air flow at right angles to the surface and into the inner chamber.
Dry surface without cracks should be selected for test. It should be insured that inner chambers should not be located above the reinforcement bar.
Pressure loss is calibrated from time to time and after a large change in temperature and pressure. 3 to 6 measurements of electrical resistance of the concrete and its mean value is taken for the measurement of coefficient of permeability.
This permits the calculation of the permeability coefficient kT on the basis of theoretical model. In case of dry concrete, the results are in good agreement with laboratory methods, such as oxygen permeability, capillary suction, chloride penetration and others. The quality class of the cover concrete is determined from kT using a table as shown below.
Table – Quality class of cover concrete
The humidity, a main influence on the permeability, is compensated by additionally measuring the electrical resistance ? of the concrete. With kT and ? the quality class is obtained from a monogram shown in fig below:
Limitations of Concrete Permeability Test
- The determination of kT and p should not be carried out on wet surfaces (the moisture entering the unit could damage the membrane in the pressure regulator).
- The most accurate values are obtained for dry concrete ( p measurement is superfluous).
- In order to obtained an exact idea of the quality of the cover concrete of a structure or of a finished component, several measurements must always be carried out .
- The quality classification of cover concrete from table and the monogram from figure related to young concrete i.e. concrete age about 1-3 months. Some measurements on concrete a few years old have shown that the classification in Table and the monogram cannot be directly applied.
- The moisture content of the concrete has a major effect on the gas permeability. The correction of this effect by the measurement of the electrical resistance generally leads to satisfactory results in the case of young concrete. For old concrete, further investigations must be carried out.
- The investigations were performed using a vacuum pump with a suction capacity of 1.5 m3/h and a motor power of 0.13 kW, this pump makes it possible to achieve a vacuum of a few mbar. Pumps of lower power do not reach the same vacuum and it is therefore advisable to use only pumps of similar power.
- There may be three further reasons why the desired vacuum (10-50 mbar ) is not reached.
The concrete cover is too permeable (normal function of the unit). The concrete surface is too uneven: the rubber seals can compensate only a certain degree of unevenness (abnormal function). The unit has a leak (abnormal function).