A brief overview of…


Pervious concrete (porous concrete/ gap-graded concrete) is a pavement with a typical void content between 15% and 25%. It is a new movement towards the green era of construction. Voids allow rainwater to flow through the pores of the concrete and return to the earth. Pervious Concrete helps restore groundwater as well as the reduction in the amount of retaining ponds, thus allowing for more efficient land use. Many environmental engineers consider the placement of pervious concrete a best management practice. Based on research conducted at Middle Tennessee State University, pervious concrete can drain 5 gal/min between 30 seconds and 1 minute depending on void content of the mix.

Pervious concrete is a mixture of cement paste and large aggregate, with little or no sand. This mix design, along with proper placement and finishing techniques, allows rain water to permeate to the sub-base of the slab and return to the earth. With a high percentage of voids and the low cement paste content, pervious concrete is considerably lower strength than normal concrete.

Pervious concrete must be designed for its specific application or use. Most applications of pervious concrete are for low traffic pavements such as parking lots, alleys, and driveways. It is also practical to use along the shoulders of heavy traffic roads, such as highways and interstates, or as a sub-base to other pavements. These heavy traffic areas are graded towards the shoulder, directing the water towards the pervious pavement where it then percolates back to the earth.  All in all, it reduces the need for storm water drains.

Because of it’s insulating characteristics, pervious concrete is now being used in many structures. It acts as a filter and retains contaminants such as oils and other fluids that leak from automobiles onto the pervious pavement. These same contaminants on other pavement surfaces can be washed away and introduced to waterways. Within the pervious concrete, these chemicals are broken down by naturally occurring bacterias and fungi.

Although pervious has many advantages in  construction and environmental practices today, one disadvantage is that it can become clogged rather easily with sediment. The pavement must be flushed and cleaned out when this happens.

When laying a section of pervious concrete, the sun and wind can be detrimental. The plastic state of pervious is very sticky and resembles sticky pea gravel. Adding water to the concrete will only make the mix creamy and will make the surface impermeable. Rolling the surface is the preferred method over troweling and floating the surface. This method is more effective for keeping the void content consistent, and it does not seal the surface like toweling or floating.

Because pervious is a relatively new science in the concrete industry, there are many questions and technical problems that are still prominent.  However, the strong surge in the economy for green practices is helping pervious concrete gain popularity