Concrete permeability refers to how easily liquids or gases can move through it. Essentially, it measures the amount of water, air, or other substances that can enter concrete through its tiny pores. These pores act as pathways for substances to penetrate the concrete structure.
The primary cause of concrete deterioration, particularly due to the corrosion of embedded steel, is permeability. When steel within concrete comes into contact with water or moisture, corrosion occurs. The resulting rust can be up to 5 to 6 times the volume of the steel-oxide layer. This process leads to the development of cracks in concrete and eventual spalling.
To prevent corrosion of reinforcement and spalling, it’s crucial to make concrete impermeable. However, there are cases where permeable concrete is intentionally used, such as in pavements for parking areas, light traffic zones, residential streets, and walkways.
Various permeability tests are conducted to assess the permeability of concrete, especially in Reinforced Cement Concrete (RCC) structures. Let’s delve into one such test – the Rapid Chloride Permeability Test (RCPT).
This test evaluates a concrete sample’s ability to resist the penetration of chloride ions. These ions are electrically passed through the concrete sample, and the resulting current readings are crucial.
| Equipment | Solution Composition |
|---|---|
| Rapid Chloride Permeability Test Equipment | 3% NaCl, 0.3m NaOH |
Q=900×(I0+2×I30+2×I60+2×I120+…+I360)Q=900×(I0+2×I30+2×I60+2×I120+…+I360)
| Charge (coulombs) | Chloride Permeability |
|---|---|
| >4000 | High permeable concrete |
| 2000-4000 | Moderate |
| 1000-2000 | Low |
| 100-1000 | Very low |
| <100 | Negligible |
Another method to assess concrete permeability is the Water Permeability Test, which involves passing water through a disc-shaped concrete sample under pressure.
k=QLtAhk=tAhQL
Where,
k=Coefficient of permeabilityk=Coefficient of permeability
Q=Discharge of water into collection unitQ=Discharge of water into collection unit
t=Elapsed time in secondst=Elapsed time in seconds
L=Top length of concrete specimenL=Top length of concrete specimen
A=Top area of concrete specimenA=Top area of concrete specimen
h=Applied pressure head in metersh=Applied pressure head in meters
