Permeability of Concrete | Permeability tests on concrete

Hello friends, in today's post we will learn about the Permeability of concrete. Before we start learning about the Permeability tests, we should know about what is porosity ?

The Permeability of Concrete :

Porosity :

To know about the Porosity, we understand it by simple concept. For eg. Every material either it may be sand, soil, cement, etc. has pores, which contains voids in it. 

Porosity is the property of the material measured concerning the amount of void space as a percentage of total volume. 

Porosity is expressed in percentage. It varies from 0% to 100%.

Permeability of Concrete:-

Concrete is a composite material which is made from the materials such as Cement, Sand & coarse aggregate. 

Every material having few % pores or voids in it. Aggregates having a more void ranging from 1 mm-10 mm which cement paste fills these. 

Even cement has voids which ranging from 1-10 micron. Due to this interconnected as well as  the continuous link to fill one void by other material concrete is prone to permeate fluid or gases into it.

or

It may be also define in other words, such as the Presence of voids in concrete makes permeable which in turn allows water or gas to flow into it. 

The permeability of concrete is the ability of concrete to resist the water flow or any other substance into it when the external force is applied.

Why the Permeability of Concrete is Important?

Durability of concrete is most important as well as the complex property of concrete. If concrete is permeable, Deleterious materials like water, CO2, SO2 & Cl which permeates through the pores of the concrete and reacts with the reinforcement which results in rust, it also increases the volume of the reinforcement.

As a result, it cause damages to the structure. Prior understanding of the extent, but also the rate of permeation helps to design structure better.

For example of a dam is made of concrete. In monsoon season, the rate of flow of water is increased. When the water pressure is applied from one side of the dam, it resists the flow of water to the other side due to the impermeable nature of concrete. 

If the concrete is permeable as well as it allows water into it, the reinforcement present in concrete undergoes to corrosion and forms rust which ultimately leads to surface cracking and collapse.

Factors affecting the Concrete Permeability :

1. Water-Cement ratio:
When Excessive water is added to the concrete mix, it increase the workability of concrete. 

This additional mixing of water, more than requirement increases the porosity in concrete also causes to reduce the durability of concrete. 

To resist the entry of water into the concrete 0.4 water-cement ratio is adopted. Experiments proved that taking a water-cement ratio of 0.4 makes concrete impermeable.

2. Improper compaction of concrete:
Due to Improper compaction of the concrete, there are major problem for porosity in concrete. 

Therefore, the Concrete should be properly compacted using hand compaction method or machine compaction methods. 

Poorly compacted concrete results in the formation of honeycomb which ultimately makes steel to corrode and forms surface cracks.

3. Improper Curing :
Concrete should be adequately cured by considering the atmospheric weather. Improper curing in concrete leads to the formation of cracks and it increases the permeability of concrete.

Two Different purposes to check the Permeability of concrete:-

1. Measuring the Permeability of concrete as a quality control parameter :
To determine and study the property of the material that we are going to use. Suppose we need a zero permeable concrete for construction. 

After testing, we found that there is a 2% of porosity in it.  Then we reject the concrete, as it is not as per specifications.

2. Evaluating the permeability of already laid concrete structure:
Engineer have to check the Permeability property of concrete of older monuments or existing structures. This type of measuring requires a Non-destructive testings. 

Permeability test of concrete:

Permeability test of concrete, is more important in RCC.  We know that reinforcement undergoes to corrosion when it reacts with the water, after which it turn forms a layer around the reinforcement. This will causes an increase in the volume of concrete which ultimately leads to surface cracking. 

To resist the Corrosion of the reinforcement, the Permeability test of concrete is done. 
Not only the moisture, there are other atmospheric deleterious materials which results in corrosion of the steel.

Below  are different methods to measure the Concrete Permeability  here are most commonly used :

1. Rapid Chloride Permeability Test
2. Water Permeability by pressure
3. Ponding with a salt solution

1. Rapid Chloride Permeability test : 

As per the Specifications and the quality control purposes on site, we prefer a test that is simple to conduct as well as that can be performed in a short time. The rapid chloride permeability test [RCPT} meets these goals.

These test is covered by AASHTO T 277 or ASTM C 1202. it is the test for chloride ions. As test name mentions that, this test is performed to check the Concrete’s Ability to Resist Chloride Ion Penetration. 

This test is an Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration. 

This test helps to determination of the service life of concrete structures as well as for durability-based quality control purposes.

In this test, the constant voltage (V) is applied on a concrete specimen for 6 hours after that the current (i) passing through the concrete is recorded to find the coulombs.

What is coulomb?

Current is measured in amperes. A coulomb is an ampere – second;
This means that the 1 ampere passed through the concrete specimen in 1 second is a 1 coulomb.

Also, the charge passed in 60 seconds would be 60 coulombs. Atlast, The more permeable the concrete, the higher will be the coulombs; the less porous the concrete, the lower will be the coulombs.

Apparatus:

The Appratus require for these test are Rapid Chloride Permeability test equipment. These equipment consists of two reservoirs. 

One of them has 3.0% of NaCl solution and another reservoir has 0.3M NaOH Solution, Concrete having thickness 50mm as well as the dia 90-100mm is used as a test specimen.

Take help of the below image for more details:

Chloride test procedure:

1. The concrete specimen having dia 100mm and thickness 50mm is cast and saturated.
2. The concrete sample is placed in between the two reservoirs (which is called as a single cell) having NaCl solution in one reservoir and NaOH solution in the other.
3. These reservoirs are connected to DC supply and the voltage of 60V is applied to the concrete specimen at both the ends for 6 hours.
4. Now measure the current passing through the concrete at different time intervals.
5. The current passing through the concrete is determined by an LCD which is connected to the cell.

Formula of  RCPT Test :

For determination of Permeability of concrete  2-3 samples are taken from same batch of concrete mix and measured as mentioned,  the  average value is taken as a final reading. Permeameter can have 2-3 cells with separate LCD digital meter to determine 2-3 samples at a time.

Table to interpret RCPT test Results:

The total charge passed is determined by using the above formula and used to rate the concrete according to the below criteria.

Water Permeability test for concrete by Pressure :

This test is mostly suited and applied to the concrete having high permeability. We can also called these test as water penetration test of concrete.

Here in the above test, we check the concrete permeability based on chloride penetration. This test is not suitable for concrete having low permeability and takes a lot of time for results.

This test is carried out using a disc of concrete and involves water flowing out through the disc at a steady rate.

Apparatus:

The concrete disc having a thickness of more than 20 mm is cast as well as th saturated. 

For these test, the most important apparatus required will be a permeameter and water collection jar. 

For reference, Look at the following image for more details.

Procedure:

1.In these test,  the concrete having a thickness more than 20 mm is placed on permeameter.
2.After that, the permeameter is filled with the water.
3.For determination of the permeability of concrete the water passing through the disc is collected at the water collection unit.
4.To accelerate the process, the pressure is applied to the water from the top.

Formula for Water Permeability by pressure test :

Coefficient of Permiability “k” is calculated by using the below formula :-

Salt Ponding test :

The Salt Ponding test was first covered by AASHTO T259. This test helpful to measures the resistance of concrete to chloride ion penetration. 

The penetration of chlorides into concrete is very severe as we compared with the other deleterious substances. Due to these reason, the attention go towards the most of the researchers to find the permeability of concrete based on chlorides compared to other mechanisms.

In this test, the concrete sample is sealed on sides and the 3.0% NaCl Solution is ponded above the surface of the concrete.

Apparatus:

1. Concrete sample (300mm x 300mm x 75mm),
2. NaCl Solution

Procedure for Salt Ponding test:

1. Salt ponding test requires a large sample of concrete as compared with the other tests. 
2. We have to cast 3 concrete slabs of length & breadth 300mm x 300mm and thickness 75mm and cured for 14 days.
3. After that we have to kept in the drying room for 28 days with a 50% relative humidity environment before conducting the test.
4. We must have to seal each concrete slab on both sides and 3% of NaCl Solution is ponded on the top surface for 90days.
5. The bottom face of the concrete slab is left exposed to the drying environment.
6. The specimens we have to been maintained with a constant amount of chloride solution at top surface for 90 days.

The specimen after completion of 90 days have to be sliced as a layers having thickness 12mm each which means each specimen is cut into 6 segments.

Results :

Here, the chloride penetration in the layers is zero, if the concrete is impervious.

Also, the chlorides may penetrate up to the second or third layer from top, If the concrete is more permeable. 

For easy understanding of test, take reference of below video for easy understanding :

Video 1 :

Video 2 :

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