Water Absorption in Paving: Why It Matters
Water absorption in paving is an important property of a paving slab or paving unit because it tells us how much water the material can absorb into its pore structure.
In simple terms, the more porous a material is, the more water it can potentially take in. However, different paving materials naturally have very different absorption rates, so the figure always needs to be considered alongside the type of material and its intended use.
Why Water Absorption Matters for Paving Durability
One of the main reasons is durability.
When water enters the pores of a paving unit, it can become particularly important during cold weather. If the material becomes sufficiently saturated and the temperature falls below freezing, the water within the pore structure can freeze and expand.
Repeated freeze-and-thaw cycles can place stress on the material. If the paving is not sufficiently resistant, this can eventually contribute to surface scaling, spalling, cracking or other deterioration.
This does not mean that a paving slab with higher water absorption will automatically suffer frost damage. Freeze-thaw resistance depends on several factors, including the pore structure of the material, its degree of saturation and the actual durability of the stone, concrete or clay product.
Water absorption can also affect staining and moisture movement.
A more absorbent natural stone, for example, may take up moisture from the bedding mortar or surrounding construction more readily. This can contribute to temporary darkening, moisture staining and the movement of dissolved salts through the stone.
It is also one of the reasons why the bedding system and drainage beneath paving are so important. Even a good-quality paving unit should not be installed in a construction that unnecessarily traps water around or beneath it.
How is Water Absorption Measured?
For natural stone paving, water absorption is normally expressed as a percentage by mass.
The stone is tested under controlled conditions to establish how much water it absorbs compared with its dry mass. Natural stone slabs for external paving are covered by BS EN 1341, with water absorption determined using the test method in BS EN 13755 where the property is required to be declared.
Concrete paving also has requirements relating to weathering resistance. Under BS EN 1339, water absorption is one method used to classify the weathering resistance of concrete paving flags, while more severe exposure involving frost and de-icing salts requires specific freeze-thaw testing.
What Does a Low Absorption Figure Mean?
Generally, a low water absorption figure tells us that relatively little water is entering the accessible pore structure of the paving unit.
Dense stones such as granite and slate can have very low absorption figures, whereas some sandstones and limestones can absorb considerably more water.
But we should not simply say that lower absorption always means better paving.
Some perfectly suitable natural stones have higher water absorption than others. What matters is whether the particular paving material has the correct declared performance and durability for the environment in which it is going to be used.
Water absorption is therefore another useful piece of the paving jigsaw.
Alongside flexural strength, freeze-thaw resistance, abrasion resistance and slip resistance, it helps us understand how a paving material is likely to perform once it is installed outside and exposed to water, frost and the British weather.
