Paving fool - Laying over concret existing base

[bridger]

First off, great website. This has been the best designed site I've used in a long time.

Down to business.

I'm laying an 18 sq m paved area with bradstone slate paving. Part of this will cover a smaller area of concrete approx 4cm thick. We live on a hill and the existing slab was not level. My plan was to initially to cover the whole area to a depth varying between 35mm and 60mm with the 10:1 mix to a new level and compact using a plate compactor. Conversations with various paving suppliers raised concerns about the existing slab causing drainage problems and so I've since taken a sledgehammer and extensively cracked the whole slab to allow any trapped moisture to drain through. I'm now somewhat concerned about the comapcted sand/cement getting washed throug h the cracks into any voids underneath. Am I being unduly paranoid? All my trusty relatives say the cracked concrete is an ideal base.

[Tony McC]

If the bedding layer is to be the 10:1 mix, then by virtue of the fact that it is cement-bound will prevent it running down into any gaps, but, if you are really concerned, you could lay a permeable geo-textile over the cracked slab before placing the bedding mix. This would further help ensure nothing is lost to those cracks.