Wait a minute! I always thought that permeability is a negative feature when it comes about concrete! So why in the world are we talking about pervious concrete?
Well this questioni is…unquestionable…if we talk about structural concrete. The lower the permability the higher the durability and the strenght (you can read more in my post “Concrete durability”). But what if high pereability values are an expected performance? Well, in that case pervious concrete is exactily what you need!
Which are some of these cases? For example whenever one need to control the effect of water runoff on the roads, or avoid water stagnation, or minimize the heat in urban centers, or in case one needs to avoid depleting the water table, or wherever the hydrogeological balance is unstable, just to list some.
WHAT IS IT AND HOW IS IT MADE?
Pervious concrete is a mix that aims to maximize empty spaces or voids, interconnected with each other, so that the water can freely flow through the hardened conglomerate. The connected voids creates a network of channels with dimensions normally in the range 1-5 mm.
It’s evident that the aggregates plays a pivotal role on this special kind of concrete. The most appropriate sizes should be 10 to 20 mm. with almost no sand. Take into consideration that while small aggregates will grant more strenght and durability, they will, on the other hand, contribute to a less permeable structure. Moreover elongated and flattened aggregates must be discarted due to their shape that reduce the voids.
In order to have an effective pervious concrete the percentage of voids should be around 15-30%. This quantity is defined as the ratio between the volume of water released by gravity from a sample made out of perfectly saturated drainage material and the total volume of the sample.
A correct choice of admixtures is important as well, since they will reduce the w/c ratio (with obiouvs advanteges in terms of strenght and durability), prolong the setting time which tends to be shortened due to limited presence of paste and the high exposure of the components, and, if fibers will be chosen, it would be possible to minimize the superficial shrinkage of the cement paste and at the same time having a more strong bond between cement paste and aggregates.
Talking about compression strenght could be misleading since this is not the most important performance of this kind of mix, however it is virtually possible to have 25-28 N/mmq maximum whereas the permeability performance expected is not so demanding.
DELIVERING, CASTING AND CURING
Pervious concrete, due to its poor workability, cannot be pumped. Truck mixers should be almost half loaded due to the ”almost dry” condition of the concrete (obviously if premixer is used that last one is not applicable) . This condition will affect the downloading time significantly, moreover a lower load will preserve the mixing engine from excessive work preventing failure during the casting. The substrate, the layer under the casted pervious concrete, should be free of irregularities, which could negatively affect the concrete layer.
Consolidation must be done by rolling, preferably with roller screed. The compaction load should come only from the weight of the roller since as excessive pressure could cause the voids to collapse. This working process must be done with a good level of expertise since insufficient compaction could make the flooring less durable and more exposed to delamination.
The substrate must be thoroughly compacted beforehand so to have a stable and uniform surface. If clay is present (or other low permable terrains) that it would be necessary to scarify the surface, arrange a layer of non-woven fabric and then a layer of loose material with variable thickness according to the expected draining performance. This layer as well as the substrate when it is a relatively permeable one, must be wet before the casting, to prevent water suction from the fresh concrete.
I know, it could sound strange, but pervious concrete needs to be cured! One reason is that the correct hydration of the cement is essential in order to reach the best binding of the mix and the best durability of the structure. Pervious concrete is more exposes to evaporation since the surface exposed is greater. Therefore the curing process has to begin as soon as possible (see the post ”Concrete curing” for suggested methods).
As one can easily imagine, pervious concrete has low shrinkage ratio. However it is a good idea to design a clever net of joints in order to avoid unpredictable cracks which will not only be aesthetically orrible but will for sure compromise the solidity of the mass reducing drastically the durability of the flooring. You can find useful suggestions about joints on my post “Concrete Flooring” and on the brochure “Concrete flooring Tips and tricks” given for free to all the new members of practically concrete.