Permeability in concrete has direct relationship with the durability of the concrete. The lesser the permeability, the more durable the concrete will be. Permeability will make the concrete vulnerable to external media attack. These media include water, chemicals, sulfates, and etc. These external media, once capable of penetrating into concrete it will deteriorate the life span of that particular concrete structure. For example, water can cause corrosion to steel reinforcement bar in concrete. The permeability also reduces the resistance of concrete towards freeze-thaw action. Therefore, permeability is a very important characteristic of that need to be achieved in any application.
The permeability occurs in hardened concrete in two scenarios; firstly from the trapped air pockets from incomplete compaction, and secondly from the empty space due to loss of mixing water by evaporation. In both situations the air space is not good for concrete durability. It is therefore very important to make that any concrete placed in the formworks gets proper and adequate compaction. Some contractors might add water to the properly design concrete mix in order to make it more workable at site. This practice shall not be allowed at all for any reason. With so much development in concrete technology, now it is possible to do concreting without the need to compact it (or at least with minimum compaction). This type of concrete is known as self compacting concrete, which as it name sounds is capable of compacting on its own. It creates a very highly compacted and dense concrete; thus resulting in low permeability.
Since loss of mixing water is one of the reasons for higher permeability, simply adding water to concrete will create more air space after evaporation. Therefore, for any concrete to be watertight, the water-cement ratio shall be as low as possible. According to American Concrete Institute, for concrete exposed to fresh water the maximum water-cement shall not exceed 0.48, while this value shall be less than 0.44 if the concrete will be exposed to seawater.
Besides improper compaction and loss of mixing water, there are few other factors that affect the permeability in concrete. As the age of the concrete increases, the permeability will reduce. This is because concrete is material that will continue to hydrate over a long period of time as long as there is a presence of un-hydrated lime. So with the presence of water, the hydration products will fill the empty spaces in the matrix. Another factor that improves the permeability is the fineness of cement. Finer cement particles will hydrate much faster; thus creating the impermeable concrete faster.