When concrete is permeable it can cause corrosion in reinforcement in presence of oxygen, moisture, CO2, SO3- and Cl- etc. This formation of rust due to corrosion becomes nearly 6 times the volume of steel oxide layer, due to which cracking develops in reinforced concrete and spalling of concrete starts.
So, if the concrete is made impermeable, the corrosion and ultimately spalling of concrete can be prevented.
Fig. Process of corrosion
Permeability Test on Concrete
1. Three specimen of concrete each of 200mm dia and 120mm height are cast.
2. After 24 hours, the middle portion of 100mm dia is roughened and the remaining portion is sealed with cement paste.
3. The specimen are cured for 28 days and then water pressure is applied on the middle roughened portion so that water can penetrate inside the concrete. The water pressure is maintained as given below:
1 bar (1kg/cm2) for 48 hours.
3 bars for next 24 hours.
7 bars for next 24 hours.
4. After this, the specimen are split to know the penetration of water. The specimen are split in compression machine by applying concentrated load at two diagonally opposite points slightly away from central axis.
The average of three maximum values of penetration is calculated. The depth of penetration of water should not be more than 25mm otherwise the specimen are considered to be failed in permeability test.
Permeability of concrete can be minimized by adopting low water-content ratio, ensuring proper compaction and curing of concrete as shown in the table below.
