What Is Permeability Of Concrete?
Permeability is the ability of a given sample of concrete to permit liquids or gases to pass through it. It is the measurement of the amount of water, air, and other such substances that can enter the concrete through its pores. Concrete contains some pores in its structure. These pores are the route for substances to enter the concrete element.
One of the chief reasons for the deterioration of concrete due to corrosion of steel present inside concrete is permeability. When the reinforcement steel meets with water or moisture, corrosion of steel takes place. The rust formed due to corrosion of steel becomes approximately 5 to 6 times the volume of steel-oxide layer. Due to this, development of cracks on concrete begins, and also the concrete starts spalling.
To avoid corrosion in reinforcement and spalling of concrete, the concrete made, should be impermeable. In some cases, the concrete slabs are made permeable purposely. This is to avoid the surface run-off of water. Permeable concrete is used for making pavements of parking areas, areas with light traffic, residential streets, walkways, etc.
To know the permeability of concrete, various permeability tests are performed on concrete. Let’s explore the permeability test in detail below.
Permeability Test Of Concrete
Permeability tests are important especially for Reinforced Cement Concrete (RCC) as the reinforcement bars present inside the concrete element are prone to corrosion when in contact with water. Due to corrosion, the volume of concrete expands and leads to cracking of the concrete element.
So it is necessary to check the permeability of concrete. To check the permeability of concrete, there are various methods. Some of them are most commonly used. They are as follows:
- Rapid Chloride Permeability Test. (RCPT)
- Water permeability test by pressure.
Rapid Chloride Permeability Test (RCPT)
The rapid chloride permeability test is a test to determine the concrete’s ability to resist the penetration of chloride ions in it. Chloride ions are passed through the sample of concrete electrically.
The readings are taken as the current passes through the concrete sample. On average 2 to 3 samples are taken from each batch of concrete and the average reading is taken as the final value.
Apparatus Required
This test is conducted using equipment called rapid chloride permeability test equipment. One of the two reservoirs, present in the equipment, consists of 3% of NaCl solution, and the other one consists of 0.3m NaOH solution respectively. The sample of concrete to be used for testing shall be of 50mm and 100mm thickness.
Procedure Of The Test
To conduct the test, concrete samples of 50mm and 100mm thickness are cast. The concrete sample is placed in between the two reservoirs. Once the concrete sample is placed between the reservoirs, an electric current of 60V is applied and passed through the concrete block for 6 hours. The amount of current passed through the concrete block is measured & can be seen on the LCD screen on the equipment, which is connected to the cell.
Usually, 2 to 3 samples of concrete are taken from a single batch of concrete and the permeability is measured. The average reading of 2 to 3 samples is taken as a final result. This is done to achieve more accuracy of test results.
The formula of Rapid Chloride Permeability Test
The formula of RCPT test is as follows:
Q= 900 × ( I0 + 2×I30 + 2×I60 + 2×I120…….. + I360 )
Where,
Q = Current flowing through one cell (measured in columns)
I0 = Initial current reading immediately after the voltage is applied (measured in amperes) It = Current reading at t minus after the voltage is applied (measured in amperes)
Table For Interpretation Of Test Result
| Charge (coulombs) | Chloride permeability |
| >4000 | High permeable concrete |
| 2000-4000 | Moderate |
| 1000-2000 | Low |
| 100-1000 | Very low |
| <100 | Negligible |
Water Permeability Test By Pressure
Water permeability test, also known as water penetration test, is generally performed to measure the permeability of concrete, having high permeability.
In this test, water with pressure is continuously passed through a disc-shaped concrete sample for a fixed time interval. The amount of water passing through the concrete disc determines the permeability of concrete.
Apparatus Required
- A disc-shaped concrete sample having a thickness of more than 20mm.
- A Permeameter.
- Jar for water collection.
Procedure Of The Test
The concrete sample is placed in the Permeameter. Then the permeamtere is filled up with water. After that, the water is given some pressure and is let to pass through the concrete disc. The water passing through the concrete disc is collected in a water collection jar.
The formula of water permeability test:
k = QL /t Ah
Where,
k = Coefficient of permeability
Q = Discharge of weather into collection unit t= Elapsed time in secs.
L = Top length of concrete specimen
A= Top area of concrete specimen
h = applied pressure head in ‘m’
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