When concrete exhibits permeability, it can lead to corrosion of the reinforcement when exposed to oxygen, moisture, CO2, SO3-, Cl-, and other elements. This corrosion causes the formation of rust, which can expand up to six times the volume of the original steel, resulting in the development of cracks in the reinforced concrete and ultimately leading to spalling of the concrete surface. However, by making the concrete impermeable, it is possible to prevent corrosion and spalling of the concrete.
Fig. Process of corrosion
Permeability Test on Concrete
Three concrete specimens, each measuring 200mm in diameter and 120mm in height, are cast. After 24 hours, the middle portion of each specimen, measuring 100mm in diameter, is roughened while the remaining portion is sealed with cement paste. The specimens are then cured for 28 days.
Once cured, water pressure is applied to the middle roughened portion of the specimens in a controlled manner. The water pressure is maintained at different levels for specific durations as follows: 1 bar (equivalent to 1 kg/cm2) for 48 hours, followed by 3 bars for the next 24 hours, and finally 7 bars for another 24 hours.
After the water pressure test, the specimens are split in a compression machine by applying a concentrated load at two diagonally opposite points slightly away from the central axis. The maximum penetration of water is measured at three different points on each specimen, and the average of these maximum values is calculated. The depth of penetration of water should not exceed 25mm, as per the permeability test criteria. If the penetration depth is greater than 25mm, the specimens are considered to have failed the permeability test.
The table below illustrates how the permeability of concrete can be minimized through three key measures: adopting a low water-content ratio, ensuring proper compaction, and providing adequate curing.
Firstly, using a low water-content ratio in the mix design of concrete can significantly reduce its permeability. This means that the amount of water used in the mixture is minimized, resulting in a denser and less porous structure. As a result, the passage of water through the concrete is impeded, reducing its permeability and making it less susceptible to moisture penetration.
Secondly, proper compaction during the placement of concrete is crucial in minimizing its permeability. When concrete is not thoroughly compacted, it can contain voids and air pockets, which can create pathways for water to pass through. However, by ensuring that the concrete is properly compacted, these voids and air pockets are minimized, resulting in a more dense and impermeable structure.
Lastly, adequate curing of concrete is also essential in reducing its permeability. Curing is the process of maintaining a favorable environment for the concrete to properly set and gain strength. This process helps in preventing premature drying and cracking of the concrete, which can lead to increased permeability. By providing sufficient curing, the concrete develops a denser and more durable structure, resulting in reduced permeability.
In summary, adopting a low water-content ratio, ensuring proper compaction, and providing adequate curing are effective measures in minimizing the permeability of concrete, making it less susceptible to moisture penetration.
| Water-cement ratio | Coefficient of permeability |
| 0.32 | 1 x 10-3 |
| 0.50 | 10 x 10-3 |
| 0.65 | 100 x 10-3 |