What is permeability?
Permeability is the extent to which a material allows liquid or gas to flow through it under a pressure difference. In construction the concept turns up in three places — in soil, in concrete and masonry, and in finishes such as paint and membranes.
An important distinction: permeability concerns flow through the pores of the material. It is not the same as capillary suction and not the same as vapour diffusion. All three transport mechanisms can occur in the same material at once, and they are often confused.
In soil mechanics
For soil, permeability is expressed as the coefficient of permeability k, in metres per second. The order of magnitude varies enormously by soil type: gravel and coarse sand let water through very quickly, fine sand and silt considerably more slowly, and clay is for practical purposes effectively impermeable.
| Gravel and coarse sand | Let water through very quickly |
|---|---|
| Fine sand and silt | Let water through considerably more slowly |
| Clay | For practical purposes effectively impermeable |
That difference determines in practice:
- whether a soakaway in the garden works or stands full after every downpour;
- how much water enters an excavation and how heavy the dewatering has to be;
- how quickly the groundwater level responds to rain or to water level management.
To find out for your own plot, run an infiltration test: dig a hole roughly 30 by 30 centimetres and 50 centimetres deep, fill it with water, let it drain, refill and measure how many centimetres it drops per hour. If it barely drops in an hour you have clay or loam, and infiltration is of little use without additional measures.
In concrete and masonry
In concrete, low permeability is the key to durability. Water entering the concrete brings chlorides and carbon dioxide with it, which in time leads to reinforcement corrosion and concrete rot. The main levers: a low water-cement ratio, good compaction and above all careful curing, so the outer centimetres do not dry too fast. Choosing a polymer-modified PCC mortar for repairs is in part a choice for lower permeability.
In finishes
For masonry paint, render and membranes, the usual measure is vapour permeance, expressed as an sd value in metres. The same principle applies: a wall finished too tightly traps moisture in the masonry instead of keeping it out.
Related terms
- Capillary action
- Vapour diffusion
- Groundwater level
- Infiltration
- Concrete rot
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