Permeability of concrete is a measure of the amount of water, air, and other substances that rise through concrete, which is the ability of concrete to resist the penetration of any substance that enters the concrete matrix, the concrete has pores that can allow these substances to enter or depart.
Concrete has small pores whose diameter varies from 0.01 to 10 μm in cement pastes whereas it may be between 1 mm to 10 mm when the cement paste is placed on the aggregate.
The commonly used methods for testing the permeability of concrete include water permeability, air permeability, soaking chlorine pools and electric quantity.
Function of Permeability:
- Permeability controls the rate of moisture penetration.
- Higher permeability will have lower durability.
- Also, if the permeability is high, the water-tightness will be less.
- If good quality aggregates are used then permeability is low.
Factors affecting permeability of concrete:
It is necessary to provide sufficient material to make impermeable concrete.
The permeability of concrete is low with the fineness of cement.
Quality of Aggregate:
If good quality aggregates are used then permeability is low.
If more porous aggregates are used, the permeability of the concrete is higher.
Larger sized aggregate also increases the permeability of concrete.
The decreasing amount of paste is beneficial in decreasing permeability.
Permeability is very low at and below porosity of 30%.
The porosity above 30% causes a high increase in permeability.
Water cement Ratio:
The water mixture is indirectly responsible for the permeability of the hydrated cement paste.
Loss of water mixture responsible for high permeability.
Age of concrete:
As the age of concrete is more, permeability will decrease.
Degree of Compaction:
Permeability of concrete arises due to incomplete condensation and voids.
It is therefore important that concrete should be reasonable and adequate compacted.
Adequacy of treatment:
The curing will continue the process of hydration, which remains in the pore space making the concrete more impermeable.
Concrete is more permeable with steam curing.
Importance of Permeability:
- In reinforced concrete, ingress of water and air will cause steel erosion causing the concrete to expand, crack, and disintegrate.
- Entry of solute into the solution may adversely affect the durability of concrete.
- Ca (OH2) leaches and aggressive fluids attack the concrete.
- If the concrete becomes saturated with water because of permeability, it is extra susceptible to frost action.
- It is essential within the case of liquid retaining structures like water tanks and dams where water tightness is critical.
Measures to reduce the permeability of concrete:
To reduce the permeability of concrete, the following conditions should be considered as follows:
- Use of pozzolana or slag cement.
- Precast members should be given preference, plastering should be avoided.
- Avoid the use of broken bricks, soft limestone, or other porous aggregates.
- Increase the cover over rebars.
- A concrete mixture with a low water/cement ratio will produce concrete with low permeabilities.
Also read: Creep of Concrete, Curing of Concrete & Segregation of Concrete
The permeability of concrete is a function of cement paste, aggregate, and translucent transition zones.
Theoretically, the permeability of these components is in turn related to porosity.
The porosity and permeability do not need to be directly related, the interconnectedness of the pores is usually responsible to the high permeability.