This year winter didn’t get the memo, and we seem to proceed directly from non-stop rain storms to glorious sunshine; perhaps a sign and a symptom of the changing climate.
Where I live, the ground went from a boggy mess to a cracked, hard-baked moonscape, with grass struggling to get going where the soil has been churned up. Low river levels also indicate the ground water has quickly receded. Counterintuitively, these dry conditions can actually increase the risk of flash flooding. This got me thinking about how liquid water interacts with soil, and what that can tell us something about how our buildings work.
It turns out I’m far from the first person to have these thoughts – in fact, we have advances made in agricultural science to thank for our current understanding of moisture storage and transport in building materials. It is of course really important to understand how water moves around in soil for plant health, and the research on this topic was transposed to building materials decades ago.
What has this got to do with flash flooding? The absorption of liquid into porous materials is governed by two counteracting effects: capillary suction and viscous friction. Capillary suction is the meniscus effect – the phenomenon where the surface of a molecular substance such as water curves up when it touches another material. But it’s the meniscus effect writ large (or, if you like, very, very small). Consider a cup of tea. A small force due to the interaction between the tea and cup exists around the circumference and lifts the edges of your tea up the sides of the cup, maybe a millimetre or two. Imagine the diameter of the cup shrinking until the curve of the meniscus meets in the middle; the surface area reduces faster than the circumference (and hence the total lifting force). Keep shrinking the tube and that force starts to overcome the weight of the water. Keep reducing that diameter to the sub-millimetre scale and the force on the tube of water becomes enormous compared to the area of the tube, so a large pressure is created, sucking liquid into the pores. This is capillary suction and it is the reason brick, stone and other porous building materials suck up water.
On the other hand, there is viscous friction slowing things down – it’s the reason your honey is runny (but less so than water), and is the subject of the world’s longest continuously running laboratory experiment. It’s harder to drink lemonade through a narrower straw. At the sub-millimetre scale of the pores in building materials, the viscous friction of water is considerable, and increases as pore size reduces.
These effects are therefore in direct competition, both increasing in magnitude, but in different directions, as pore size reduces. They are both non-linear effects, and there is a sweet-spot at around one micron, where the balance between the two effects results in the fastest transport. It just so happens that the pores of lime-rich mortars are mostly in of this size, hence their very useful moisture transport properties. Many stones and bricks have larger pores, which hold onto water less tightly, so it tends to move from the masonry unit into the mortar. Cement-based materials have mostly very small pores (and significantly fewer of them – they are denser) in which viscous friction dramatically slows transport, so moisture drying is much slower.
So, if a material is relatively dry, the liquid transport will be driven by the behaviour of the smaller pores – that is where the water prefers to hang out. The liquid transport in these pores is dominated by viscous friction, and is therefore relatively slow.
There are other, more important reasons why dry soil doesn’t absorb water quickly – the plate-shaped particles of clay form a less porous matrix when dry, and damp surfaces are more hydrophilic. But all together these factors mean a heavy shower is more likely to overwhelm the short term absorption capacity of dry soil because it can’t suck it up fast enough, potentially resulting in run-off. Somewhat moist soil will absorb more readily, but of course if the ground is saturated this will also cause run off. This raises questions about how our changing climate will influence run-off; severe storms after longer dry spells is bad news for the soil and the areas within its watershed.
The shift towards more extreme weather is very relevant in building materials, not least via the increasing risks of flooding. This is neatly captured by the Four Cs proposed by Chris Sanders and the late Neil May. All of us working with buildings, but particularly those including porous materials, should be aware of the likely changes in rainfall patterns, and what these might mean for our buildings, as well as our agriculture.