Let it sink in: The inner workings of porous concrete

Permeable pavements, including porous concrete, are one of the solutions mentioned in the London Sustainable Drainage Action Plan. They are filled with little openings that allow rainwater to seep through to the soil. This should help reduce flooding. However, intentionally riddling a building material with holes is bound to produce some challenges. Engineers must balance how much water is allowed to flow through (conductivity) with the strength of the material.

How Porosity Affects Conductivity

The non-linear relationship between porosity and conductivity.
Source: Montes and Haselbach

Montes and Haselbach related the amount of holes in concrete (porosity) and conductivity. They used 17 samples of three different concretes to examine different porosities. Porosity was measured by comparing the weight of the concrete when it was dry to when it had been submerged in water for 30 minutes. They used a falling head permeameter to find the conductivity (a video of a similar experiment can be found here). They found a non-linear relationship between conductivity and porosity with a threshold at 15% porosity. Concrete with porosity lower than 15% didn't allow any flow.


Huang et al. used latex, sand, and fibers in concrete mixtures with aggregates (the little rocks in concrete) sizes of 12.5 mm, 9.5 mm, and 4.75 mm. They also used a falling head method to measure conductivity (aka permeability), but they used a vacuum seal to measure porosity.

Mixing concrete with other fillers cuts the permeability. Source: Huang et al.

Montes and Haselbach's curve predicted about a 20-60 mm/s conductivity for the latex, latex and sand, and latex, sand, and fiber mixes. The regular porous concrete should have had a conductivity of 80 mm/s. The actual permeabilities were consistently, considerably lower. The similar conductivities for the 3 add-in mixes shows that porosity should not be the only variable in Montes and Haselbach's relationship (equation on the 1st picture).

Creating Strength Despite The Holes

The fillers added by Huang et al. strengthen the concrete at the expense of a little conductivity. A mixture of latex and sand had both high permeability (compared to other add-ins) and high strength. Although, Huang et al. point out that their fiber wasn't thoroughly mixed.

A latex and sand mixture is both strong and permeable Source: Huang et al.

Yang and Jiang added silica fume (SF) and superplasticizer (SP), organic polymers, to their concrete. They measured a water penetration coefficient, similar to conductivity, to find out how quickly the concrete could absorb 20 mm of water. (They poured 200 mm of water and timed between 160 and 140 mm.) After performing a 28-day compression test, as opposed to Huang et al.'s 7-day test, Yang and Jiang produced an outdoor-cured  SF and SP concrete that had a strength of 58.9 MPa. The water penetration coefficient was only 2.9 mm/s. This demonstrates the potential strength of porous concrete, but the cost of the polymer may make SF and SP an unrealistic add-in. I foresee Huang et al.'s use of cheaper materials will make theirs more applicable, despite its weakness.

Porous concrete can help prevent pavement from becoming a river. Artist: Edgar Müller