What is permeability of concrete?
The word “permeability” refers to the ability to allow liquids or gases to pass through it. While the term “porosity” refers to the amount of air-filled space inside the material.
So, the permeability of concrete is its ability to resist penetration by liquid, gas, or chlorine ion due to its pores.
Connected pores with a diameter of a minimum of 120nm-160nm are the major reasons for causing permeability. Isolated and very tiny pores, pores filled with water, or big pores with narrow entrances don’t support permeability.
Why is it mandatory to know the permeability of concrete?
In reinforced concrete, the entrance of water and air results in corrosion of steel leading to expansion, cracking, and disruption of steel.
Concrete would not be able to resist frost action if concrete becomes saturated with water because of permeability.
Determining the permeability of concrete becomes significant for water retaining structures like water tank, dam where water tightness is necessary.
Factors affecting permeability of concrete:
1) Curing of concrete
Since we have already discussed isolated pores filled with water doesn’t encourage permeability. So, during the process of curing, we provide sufficient water to the concrete to prevent it from dehydration.
As a result, the water sprayed is used by concrete for its reaction to strengthen such that pores get reduced. So, due to the presence of lower pores(voids), there will be low permeability.
Hence, curing of concrete utterly boosts permeability of concrete or decreases permeability in concrete.
2) Water to cement ratio
Water cement ratio is responsible for the porosity of the hardest cement paste. The lower the w/c ratio the lower the permeability in concrete.
This is because for a lower w/c ratio there will be more quantity of cement portion. So, when cement gets mixed up with water it makes more paste and this paste gets filled up so that extra pores get easily filled up.
For the high w/c ratio particles of cement and aggregate wouldn’t be as compacted as in the case of a lower w/c ratio. Ultimately, it creates pores which are not filled with hydration. Hence, concrete would be permeable when free water leaves the pores due to evaporation.
Permeability is practically nil for water cement ratio of 0.4.
3) Age of concrete
Concrete permeability increases or permeability in concrete decrease
as the age of concrete increases because with the increase in the age of concrete the pores would be filled with water due to repetitive process of hydration for long period of time.
4) Other factors
Compactness in concrete, Use of mixtures, size of aggregate in concrete, absorption, and homogeneity in concrete, properties of cement.
Since we already know water is not only responsible for permeability in concrete.
3 ways to determine the permeability in concrete.
- Water permeability by pressure
- Ponding with a salt solution
- Rapid chloride permeability test
1) Rapid chlorine permeability test
For this experiment, a water-saturated, 50-mm thick, 90-100mm diameter concrete specimen is subjected to a 60V applied DC voltage for 6 hours using the apparatus which has two big reservoirs at both ends.
The concrete sample or test sample is placed right in the middle of the apparatus and same distance apart from both the reservoirs.
In one reservoir is a 3.0% NaCl solution and in the other reservoir is a 0.3 M NaOH solution. Since a voltage source is provided, the charges pass through one reservoir to another.
Examine the current flow overtime for 6 hours and finally get the charge that has passed, in columns. The current that passed through the concrete and traveled from one reservoir to another is determined by LCD which is connected to a 6V cell.
2) Water permeability of concrete by pressure:
This method is best for concretes with very high permeability. The test is carried out using a slab or disc of concrete and water is poured in it from its upper face and we measure the rate of flow of water through concrete at its lower end. To quicker the process the water is applied with extra pressure.
The thickness of the concrete-disc should not be too thin as the experiment could take a lot of time. The concrete with a thickness of more than 20 mm is used.
If the permeability in concrete is very low, then water may not come out from the lower end at all.
After the water is flowed continuously for time ‘t’ in the disc. Remove the test sample i.e. concrete-disc from the apparatus and measure and record the mass of the disc before pouring water and after pouring water nearest to the 0.1g.
Assure that the temperature of the apparatus was the same throughout the process. Then the coefficient of permeability can be calculated by using the following formula:
K= permeability of concrete (m/s).
Q=volume of water in m3 coming out of the concrete.
L= Length of test samples in meter.
T= Elapsed time in second over which Q is measured.
A= Area of the test sample in m2.
H=Applied pressure heads in meters of water.
3) Salt Ponding test
In this test permeability of a concrete regarding concrete’s ability to resist chloride ion is calculated. The penetrating action of chlorine ions in concrete is very bad or unpleasant in comparison to other materials. This test is also used as specification writing.
Concrete with 3.0% NaCL on its surface.
In this method, a thick film of 3.0% NaCl solution is placed at the surface of the test concrete slab. For this, each side of the concrete is sealed leaving the bottom face of it open to the environment.
This apparatus is maintained with a constant amount of chlorine solution at the top surface for 90 days.
After 90 days, the concrete slab is sliced in equal parts and the extent of penetration in each slice is observed with the order.
If the chloride ion has penetrated up to 1/3rd or up to 1/2nd part of the concrete, from the upper surface, then the given concrete slab is permeable. If there is not any penetration by chlorides into the slab, then the concrete is impermeable.
The magnetic permeability of concrete: It seems pretty intriguing but magnetic concretes can also be constructed or prepared by replacing the sand and gravel with coarse and fine magnetic fillers.
This permeability of magnetic material could be advantageous by compacting the concrete with reinforcement giving rise to very strong reinforced cement concrete.
The magnetic permeability in concrete is also increased by adding soft iron powder and stainless steel fiber, shown by recent research.
The magnetic permeability in concrete should be encouraged because with the human eager to develop smart cities. We obviously should propel wireless power technology.
And to speed up the wireless networking we obviously need a medium to let the magnetic field pass through it, which is provided by magnetic permeability in concrete.
Remedy to solve the problem of permeability in concrete
- Waterproofing concrete to keep waters out and keeping concrete structures dry by using admixtures.
- Always maintaining the water-cement ratio low for the formation of strong paste.
- Concrete should be adequately compacted using the hand compaction method or machine compaction method.
- Finer cement or cement of high quality should be selected for preparing concrete.
- Always fine-sized solid aggregate should be used for concrete. Also, the aggregate should be hard-solid, not porous.
- Curing of concrete should be done properly and it is best to cure the concrete for 21 straight days.
I hope this article on “permeability of concrete” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Mohan Khadka