concrete porosity?

[Wade Lippman]

I went for a long walk yesterday and it started to rain lightly. In one neighborhood the gutters had been repaired about 8 years ago. The old concrete formed puddled immediately, but the newer concrete absorbed the water as fast as it fell. I figured that the pores on old concrete must fill up with oils from the road. Then I got to another neighborhood where the gutters were repaired 2 years ago. Both the old and new sections repelled the water equally. What is going on here? Why did the 2 year old and 30 year old concrete repel water, while the 8 year old concrete absorbs it?

[Don Phillipson]

In your place, I should ask at the local public works (road maintenance) office. It seems seldom difficult to find local experts, where they exist, and most are keen to share their specialized knowledge.

[Tim Mulvey]

There is a type of concrete that is designed porous to allow water to drain through it. It is placed on a subgrade with good drainage such as 6" to 8" gravel. If I remember correctly it is mixed without sand and is not floated or troweled but placed, vibrated, and rolled. Stormwater is then filtered on its way to the water table through the concrete and gravel.

Tim