When water hits Topmix Permeable concrete, it doesn't flow in all directions, slicking up the surface.

It flows in one direction - down. And it vanishes almost instantly.

Traditional concrete has to be permeable enough to let a minimum of 300 millimeters of water an hour through to the ground level. That allows it to safely handle a major storm event every 100 years.

Topmix Permeable, in contrast, lets through 36,000 millimeters of water an hour, or approximately 880 gallons every minute.

The seemingly magical product comes from Tarmac, a UK building materials and solutions company, which created Topmix Permeable to divert rainwater during storms.

While the company itself is only a few months old, the technology has been in development for the last six years, says product development manager Craig Burgess.

One of the main issues with permeable pavements is that they require heavy maintenance due to blockage, Burgess says. As water flows through the concrete, it can mix with the dirt underneath and harden inside the gaps, reducing the permeability.

Burgess says Tarmac avoided this problem with "paste control," an in-house technique used to mix the concrete that keeps it porous over time.

tarmac1Tarmac

Here's how it works.

Rather than use sand-based concrete, Tarmac uses something called no-fines concrete. It's made up of tiny pieces of crushed granite packed together. While Burgess says the mixture is extremely dry, the pieces are packed loosely enough to allow water to pass through.

The system can accommodate three designs: full infiltration, partial infiltration, and full attenuation.

  • Full infiltration refers to a system where all water goes through Topmix to flow into the soil underneath. It's particularly useful in wet areas that don't need to collect the rainwater.
  • Partial infiltration involves a semi-permeable barrier beneath Topmix that acts as a drainage system into nearby sewers or waterways - useful when the layer beneath Topmix can't pass the water through on its own.
  • Full attenuation uses a capture system to store all the water that flows through Topmix. This option is most useful in areas with unclean water and high recycling rates, since the captured water can be reused later.

Cities can choose between the three systems depending on their needs, Tarmac says.

Topmix Permeable fares well in all climates except extreme cold, according to the company. In tests, it performed best in driving conditions where the speed limit was 30 miles per hour or less, and traffic was moderate to light.

Topmix is currently limited to sales in the UK. In its brief life span, it's been installed on a car park and a golf course.

Though the one-time cost of replacing infrastructural systems may be high - given that existing roads would need to be totally gutted - over time the reduced maintenance costs could make Topmix a worthwhile investment in countries prone to flooding.

Or those that simply want to be more like sponges.