Permeable Concrete

Over the last decade or so, the United States mainland has experienced a multitude of severe hurricanes that left devastating wind damage in their wakes. But the real long-term devastation is caused by the flood damage following the storm. Post-hurricane flooding has the power to turn entire communities into lakes, forcing crews to wait until the flood waters slowly recede before rebuilding efforts can commence in earnest.

Such flood damage hits urban areas especially hard. The main reason is the non-porous nature of surfaces like concrete and asphalt, which cover large percentages of big cities. After a major storm, drainage systems can become quickly overwhelmed by the volume of water, resulting in flash flooding and untold pollutants ending up in the groundwater.

However, permeable or semi-permeable concrete is now readily available to enable excess water to seep through it and safely into the thirsty soil below. Both give a much-needed reprieve to city drainage systems, and potentially mitigate disastrous flooding conditions. What is more, they allow the soil to naturally filter pollutants instead of slowly dumping them into our water supply.

UK-based sustainable building materials company Tarmac has developed Topmix, a permeable concrete that can absorb a whopping 880 gallons of water per minute, according to Business Insider. You can see a rather dramatic video of Topmix in action right here. The applications for permeable concrete like Topmix are many, and include residential roads and parking lots, bicycle and pedestrian walkways, road shoulders, patios, driveways, tennis courts, and many more surfaces subject to flooding from overburdened drainage systems, according to Tarmac.

The secret is in utilizing a material called no-fines concrete, as opposed to traditional sand-based concrete. No-fines concrete is composed of tiny pieces of crushed gravel compressed together with cement, resulting in a dry mix that water can seep through.

Topmix offers three primary design options for its permeable concrete:

System A: Full Infiltration allows all water hitting the pavement to permeate through the surface layer, passing through the lower pavement layers and into the ground below. The pavement serves as a reservoir that retains some groundwater before distributing it into the sub-grade for absorption.

System B: Partial Infiltration is best used for locations where the ground itself is semi-permeable. It incorporates outlet pipes in the sub-base layer to divert excess water that cannot be absorbed by the soil into other drainage systems like sewers.

System C: Full Attenuation is used in areas where the water reaching the surface may be contaminated, where repurposing of the water is desired, or where the ground below the concrete might be less permeable or weakened if subjected to a prolonged deluge. It incorporates the same outlet pipes as the Partial Infiltration design, but adds an impermeable layer above the native subgrade to enable easy storage of the water for recycling or safe disposal.

Coastal areas covered with a high percentage of impermeable asphalt and concrete ultimately divert storm water into oceans and local water supplies, including all of the toxic chemicals like oil and other pollutants swept up in the natural flow of the water, according to a recent lecture by Geoffrey Scott, a Medical University of South Carolina researcher.

It would be a costly undertaking to replace the world’s concrete and asphalt surfaces with permeable concrete, but it is an investment that would certainly pay long-term dividends by mitigating flood damage and slow pollutants from entering water supplies.