Permeability of Concrete | Factors Affecting Permeability of Concrete

Meaning of Permeability of Concrete

Permeability of Concrete means the ability of water (liquid) to pass out from the concrete. Best concrete is non permeable.

Why Concrete became Permeable?

About 38 percent water by weight of cement paste is required to fill the gel pores for complete hydration of cement.

If water is used more than 38 percent, the excess water will cause undesirable capillary cavities and concrete to convert into porous.

Porous cement concrete has a higher permeability

The pores in cement paste/slurry consist of gel pores and capillary pores. The pores in cement concrete because of incomplete compaction of concrete so that voids of larger size which gives a concrete honeycomb structure alternatively low strength of concrete, are not considered here. 

The gel is porous to the extent of 28 percent. But, the gel pores are so small that hardly any water can pass through under normal conditions.

The permeability of gel pores is about 7 x 10^(-16) m/s which is about 1/100 of that of paste. Hence the gel pores do not contribute to the permeability of cement paste.

The capillary pores constitute about 0 to 40 percent of the paste volume. They are too large that the gel pores. It follows that the permeability of cement paste is controlled by its capillary porosity.

Importance of Permeability

• In reinforced cement concrete, the entrance of water and air will result in corrosion of steel reinforcement leading to expansion.
• If concrete becomes saturated with water due to permeability, it is more vulnerable to frost action.
• The penetration of deleterious materials in solution may adversely affect the durability of concrete, for example, Ca(OH2) leaches out and aggressive liquids attack the concrete.
• The permeability is very important in any case of liquid retaining structures like water dams and tanks where water – tightness is essential.

Factors Affecting Permeability

Various factors permeability of concrete but I list out the main factors which are:

1. Water/cement ratio
2. Properties of cement
3. Aggregate
4. Absorption and homogeneity of the concrete
5. Curing
6. Use of admixture
7. Age of Concrete

Water/cement ratio

We have already written a detailed article on water-cement ratio so here we just provide some points.

• Permeability is proportional to the water/cement ratio.
• For the cement paste hydrated to the same degree, the permeability is higher with a higher w/c ratio or higher cement content.

Properties of Cement

• The properties of cement also affect the permeability of concrete.
• Coarse cement tends to produce a paste with higher porosity than finer cement, for the same w/c rato.
• In general, if the strength of cement paste is higher then the permeability of concrete is lower.

Also Check: Types of Cement Used in Building Construction

Aggregate

• The permeability of aggregate also affects the behavior of the concrete.
• If the aggregate has an extremely low permeability then it also helps to reduce the permeability because the presence of aggregate creates an obstacle to the flow of water.
• Further, since the flow path has to circumvent the aggregate particles, the effective path becomes significantly longer and the permeability decreases.
• For the same water/cement ratio, the large size of aggregate greater is in permeability. This is because of the relatively large voids. Well-graded aggregate decreases permeability.

Absorption and Homogeneity of The Concrete

• Absorption is measured by the volume of pore space in concrete.
• Absorption is a physical process in which concrete draws water quantity into its pores voids and capillaries.
• Absorption depends on the structural composition of the concrete. Non-homogeneity affects permeability.
• The absorption increase in concrete due to cracks in the structure, voids available in concrete due to segregation or honeycombs.
• The permeability can be reduced by a workable mix so that segregation is eliminated.

Curing

• Continuative hydration of the cement paste results in a decrease in the size of the voids which helps to decrease the permeability.
• Proper curing of concrete decreases also helps to reduce the permeability of concrete.
• Permeability of steam-cured concrete of normally higher than that of wet-cured concrete structural elements.

Use of Admixtures

• The use of waterproofing admixture decreases the permeability of lean mixes.
• Normally the use of extra cement will be more effective in reducing the permeability because the casing formwork is not totally waterproof so the cement paste is gone outside. I provide so many tips for civil engineering in my every article so read all of them.
• In the case of porous concrete, you can use surface treatment to decrease permeability.

Age of Concrete

• The age of concrete also affects its permeability of concrete.
• In the green concrete/fresh concrete paste the water flow is controlled by the size, shape, and concentration of the original cement grains.
• With the process of hydration, the permeability decreases quickly because the gross volume of gel is approximately 2.1 times the volume of the unhydrated cement.
• Gel gradually fills the original water-filled space.

Also Read: What is Haunch Bar? Why provided?

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