This Concrete Can Absorb a Flood

As Hurricane Joaquin and several other weather systems pounded the Eastern Seaboard, coastal communities have been faced with Biblical levels of rain. For some towns in South Carolina that meant 20 inches or more. Interstates were closed, cars were swept away by fast-flowing storm water, and at least six people died. As of Sunday night, flood watches were in effect from Georgia to Delaware.

Urban areas are particularly vulnerable to flash flooding. Manmade surfaces, such as asphalt, absorb much less water than dirt, and heavy rain can quickly overwhelm cities’ storm-water drainage systems. As the world continues to urbanize and the amount of the Earth covered in impervious surfaces increases, flooding is likely to become more of an issue.

This may be why a new product from a UK company has been generating so much buzz. Topmix Permeable, a fast-draining concrete, can absorb up to 1,000 liters of water per minute per square meter. Tarmac, the company that created Topmix Permeable, recently released a video of a truck pouring 4,000 liters of water onto a parking lot surface. The water is absorbed instantly, as if draining into a hidden hole.

Typically, road paving material is made of a mix of large and fine crushed stone held together by a binder. With Topmix Permeable, the fine crushed stone or sand is left out. This makes the resulting material porous enough to accept large amounts of water. A layer of Topmix Permeable concrete is installed on top of an aggregate sub-base of crushed stone, which generally sits on top of the soil. Rainwater drains through the top surface, collects in the aggregate layer, and is slowly released into the ground.

“We believe that the capacity is several times more than enough to deal with the heaviest rainfall we’ve ever experienced in the UK,” says Richard Stares, commercial director at Tarmac. The product's price, he adds, is similar to other concretes on the market. 

Topmix Permeable - The ultimate permeable concrete system

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The product can be used on roads, parking lots, walking and biking pathways, driveways and more. Tarmac says the material could help reduce flood damage, take pressure off aging storm water drainage systems and even reduce risk of water shortages by redirecting rainwater into natural aquifers. In times of extremely heavy rain, the pavement acts as a reservoir, its under-layer holding on to water and releasing it slowly at a pace the ground can handle. The system can also help filter contaminants, such as motor oil, out of water—the multiple layers of porous stone essentially act as a giant filter.  

Topmix Permeable isn’t the first pavement system of this kind. Permeable paving blocks have become an increasingly popular choice for driveways and pedestrian paths. Porous asphalt has been used with success in cities, including Portland, Oregon, where some parking lanes have been paved with the material. But porous asphalt is limited: it’s too weak for high-traffic areas, it needs regular cleaning to keep grit from clogging the surface (making it not suitable for places where sand is put down during snow and ice storms), and it allows oil and other pollutants to drain into the water system.

Topmix Permeable has some of these issues. It’s not a good choice for high-traffic areas, as heavy use will eventually start breaking up the surface layer. But it’s stronger than other products on the market, Stares says, due to the process of mixing the cement and the nature of the binders (a trade secret).

“We’ve been able to get this bond so strong," Stares says. “We’ve been able to make this product that has a very high void space.”

“Void space” is the industry term for the amount of air space in a paving product. Typical permeable concretes have void space around 20 percent, while Topmix Permeable’s is as high as 35 percent.

Tarmac's material has been used in several projects already, including a parking lot. Unfortunately for us Americans, especially all those South Carolinians who saw their cars inundated on roads full of standing water, it’s not yet available in the United States.

“In an ideal world it would be nice to get this technology to other parts of the world,” Stares says. “But it’s probably some ways off.” 

Emily Matchar | | READ MORE

Emily Matchar is a writer from North Carolina. She's contributed to many publications, including the New York Times, the Washington Post, the Atlantic and many others. She's the author of the novel In the Shadow of the Greenbrier.