PERMEABILITY OF CONCRETE

When the excess water in the concrete evaporates, it empties into the concrete element, creating capillaries directly related to the porosity and permeability of the concrete as shown in Fig. 1.0. The amount of moisture passing through the concrete depends on its permeability. Permeability is controlled by porosity, which is a direct result of the water-cement ratio of the concrete mixture.

Fig 1.0

The capillary porosity of the paste and permeability of concrete

Almost non-covered concrete can be obtained by proper selection and mixing of materials and following good construction practices. The enhanced image of the concrete slice shown in Fig 1.0 (b) indicates that the fine-grained aggregate mixture is locked to a matrix of hardened cement paste.

Fig 2.0

Compressive strength vs. capillary porosity and permeability coefficient vs.
capillary porosity

A well-packed aggregate reduced the amount of space-filling with water and cement paste. This has helped improve the pore structure of the concrete and, therefore, its permeability.

The study of permeability of concrete is important for the following reasons:

1. Penetration of materials in the solution can adversely affect the durability of concrete, eg, Ca (OH) 2 leaks and invasive liquids attack the concrete.

2. In the case of reinforced concrete, the inflow of moisture and air causes the corrosion of the steel, leading to an increase in the volume of the steel and the cracking and spalling of the concrete coating.

3. The penetration of moisture depends on the permeability and is more vulnerable to frost action if the concrete is saturated with water.

4. Permeability is of interest in relation to the water tightness of the fluid retaining structures and the hydrodynamic pressure problem within the dams.

The flow of water through concrete is the same as flowing through any perforated body. The pores in the cement paste contain gel pores and capillary pores.

You may also like: Corrosion of steel in concrete

As a result of incomplete compaction, the pores in the concrete are very large, which results in the formation of the honeycomb structure of low-strength concrete. Such holes are not considered here. Because capillary pores are larger than gel pores and 20 to 100 times more permeable than cement paste gels, the permeability of the cement paste is controlled by the capillary porosity of the paste as shown in Fig.1.0 (a). )

In rocks, the pores are smaller in number, but the larger the size, the greater the permeability. The permeability of the cement paste varies with the age of the concrete or with the level of hydration. with age, the permeability decreases because the gel gradually fills the space filled with original water.

Also, watch for a better understanding:

 

For hydrated pastes to the same level, the permeability is lower with a lower water-cement ratio or higher cement content. For the same water-cement ratio, the permeability of the paste with coarse cement particles is higher than that of fine cement.

Generally, the higher the strength of the cement paste, the less permeability there is. Durable concrete should be relatively insoluble. Permeability can be measured by a simple test by measuring the amount of water flowing through a certain thickness of concrete at a given time.

dq/dt⋅1/A=k×Δh/L

Using the Darcy equation, the drop in hydraulic head is (dq / dt) flow rate (ml / s), A is the cross-sectional area (mm2), Δh is the drop in the hydraulic head (mm), L is the thickness in millimeters, and the k permeability coefficient (mm / s) ).

You may also like: Durability of concrete