Permeability is an important characteristic that should be achieved in concrete applications, but too much may lessen the concrete’s durability. Concrete, an integral material in construction, should be impervious enough to last for many years. The less permeable the concrete is, the more durable and less vulnerable it will be to external media attacks.
For instance, if water seeps into the concrete, it will cause the steel reinforcement bar to corrode, which will reduce the resistance of the concrete. Here are major factors that contribute to the permeability of concrete.
Trapped Air Pockets
When the concrete hardens, you will notice that there are air pockets on the surface. Improper compaction forms these air pockets. Before placing concrete in formworks, it is highly important to achieve the proper compaction. These air pockets will allow external media attacks, such as water, sulfates or chemicals, to seep into the concrete and speed up its deterioration. Sulfate and alkali-aggregate attacks make the concrete expand, while hydration causes concrete minerals to dissolve.
Evaporation of Mixing Water
In some construction sites, workers add unmeasured water to the concrete mix to make it more workable, but this practice is not advisable. Water contributes to the forming of empty spaces in the concrete. When the concrete hardens and the water evaporates, it leaves air spaces and results in high permeability. Total Scan & Survey suggests that when adding water, the water-cement ratio should be low, not exceeding 0.48 for concrete exposed to fresh water and 0.44 for seawater.
Age of the Concrete
The age of the concrete is also a contributing factor to its permeability. Over time, concrete will continue to hydrate, thus reducing its permeability. This is why concrete should be watertight the moment it hardens. Over a long period of time, weather phenomena, such as freeze-thaw actions, reduce the resistance and integrity of concrete.
To improve the permeability of the concrete, the cement mix should be absolutely fine. This is because finer cement particles hydrate much faster, making the concrete more resistant to cracks. There are many current developments in concrete technology that make concreting possible without the need for compaction.
Self-compacting concrete creates highly dense and compact concrete without the need for water. Today, many tradesmen add superabsorbent polymers, a new class of admixture that, when added to the mix, absorbs and stores water and releases it during the hydration of cement. These processes reduce micro-cracking tendencies and make concrete more impervious.