oak tree tarmaced around base

[Matthew Storrs]

thats a very good point... I suppose the roots must be reaching down to the water table?

[Daniël Bos]

Tarmac is porus and flexibable and I think the saying "Grass doesnt grow on a busy road" might have a relevence to this topic, even pedestrians walking on Tarmac will stop weeds growing through but left un touched it will soon start to sprout weeds from the sub base.

The bigger issue is compaction from the traffic either motor or human using said tarmac

 

Grass grows on virtually no soil at all if needed, on low or dis-used tarmac, soil will build up in the pores, this is enough for some grasses. It grows on the tarmac, not through it.

 

 

There is now a porous tarmac available, but it's not (yet) in widespread use, There appears to be a British company claiming this as new and their invention? It just uses the larger substrate when mixed, and not the fine stuff, leaving some gaps between for water to either drain through or drain out. It is used on Dutch motorways, as there is no surface water, so much less spray= better safety. For it to work well long term it needs to be used though, the Dutch hard shoulders need to be "hoovered" ocasionally, to remove the build-up of detritus that would hinder drainage. The need for this was found after some very slippery roads in winter, caused by water stuck in this "open" tarmac, unable to drain away because of the blocked shoulder froze, and thus rose. que: Best tarmac road turning into ice-rink!

Edited November 5, 2011 by Daniël Bos

[Fungus]

There is now a porous tarmac available, but it's not (yet) in widespread use It is used on Dutch motorways, as there is no surface water, so much less spray= better safety. For it to work well long term it needs to be used though, the Dutch hard shoulders need to be "hoovered" ocasionally, to remove the build-up of detritus that would hinder drainage. The need for this was found after some very slippery roads in winter, caused by water stuck in this "open" tarmac, unable to drain away because of the blocked shoulder froze, and thus rose. que: Best tarmac road turning into ice-rink!

 

Daniel,

You're referring to ZOAB (= very open asphalt concrete), that is not porous and doesn't let through water, I presume ?

[Daniël Bos]

Yep, but zoab is used only for it's buffering capabilities in NL. Rain goes down into it, and flows through it to the side of the road. They can also make it so it's open all the way down.

[Xerxses]

There has been a porous asphalt available for years, but its not very hardwearing so it is mainly used on parkinglots where wheelturn and other sideforces is limited. The picture shows how it can be used in combination with porous plantingpits (underneath the asphalt there is a fullloading capacity porous plantingpit with airation and waterstorage). The parkinglot has a combination of "normal" asphalt in the "road" and porous in the lots. Picture taken just after rain has stopped.

[Xerxses]

http://youtu.be/gXCcC4iH8IQ

 

Usdes a lot in the States for stormwater managment.