How does absorbent concrete ‘drink’ so much?

With heavy rain comes water pooling on roads – and the chance of an accident skyrockets (as any driver who’s lost control as their car tyres slip on a cushion of water knows). What if water could be absorbed by the road, keeping your wheels firmly on the surface? Enter absorbent concrete. Where conventional concrete lets relatively little water through, absorbent concrete appears to drink it down. Its secret: a network of gaps that allow water to slip through to the ground below or to pipes which whisk it away.

The conventional type is made from a mixture of cement, made from heated limestone and clay, water and an aggregate such as sand, rock or gravel to provide structural stability and stop the mix shrinking too much. 

It’s a formula used in various forms since the Egyptians mixed mud and straw with gypsum and lime to build the pyramids around 5,000 years ago. 

The resulting dense concrete is very strong. And, by regulation, concrete must be able to allow some water through to the ground underneath but this takes time, leaving the surface wet and slippery.

So UK company Tarmac developed Topmix Permeable concrete. Each square metre can absorb up to 1,000 litres each minute. 

It is able to do this because unlike the regular kind, around a third of Topmix Permeable’s volume comprises gaps between aggregate. 

It may sound simple, but the first permeable concrete was developed 50 years ago, and only now is it tough enough to be certified for the weight of motor vehicles.

The compressive strength of residential concrete is usually around 17 megapascals. Tarmac claims Topmix Permeable’s compressive strength is 10 to 20 megapascals.

Once through the permeable layers, stormwater can be collected by an attenuation layer, which directs the water to pipes for distribution or storage, allowing even more water to funnel through. 

The water can also pass straight through into the ground below if it is deemed safe to do so. 

There is a drawback – if water sitting in the gaps freezes and expands, it can crack the concrete. 

Related reading: Concrete, clean thyself

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