Concrete block paving on a thin (mortar) bed?

[Robin]

What would be likely to go wrong (initially or over time) if 50mm
concrete block paving were laid on a *thin* (perhaps 20mm) bed of sand
over concrete? Or on a thin mortar bed over concrete? Only traffic
will be very light pedestrian - it's a short path in a back garden.

I ask as the path has grotty 450mm slabs laid on top of what looks like
a pretty solid, if old and rough, concrete base. Removing slabs and
mortar would be easy . Removing concerete and laying a conventional
base for blocks would be a pain. Even just sticking a thick layer of
sand on top of the base would I think leave the top too high.





--
Robin
reply to address is (meant to be) valid

[John Rumm]

I can't see any problem once you can get a decent sub base that is not
going to move about. Then the normal rules of block paving apply -
restrain the edges properly, and lay them on a compacted bed of git
sand. Laid with concrete under the paving will be impermeable, so will
need proper drainage.


--
Cheers,

John.

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[Phil L]

"Robin" <rb...@hotmail.com> wrote in message
news:mkukre$g1d$1...@dont-email.me...

If you lay the blocks on sand, the sand will wash away as the concrete is
impermeable, this will cause the path to become wobbly and uneven within a
short period of time.

You can lay the blocks on mortar, but you'll have trouble getting them all
level unless you use a dryish mix and trammel it first, then just place the
blocks on the screed without bashing them down.

To trammel the dry mix, you'll need two bits of wood the thickness of the
bed....say 20mm thick, place one on each side of the path on top of the
concrete, then spade in the dry mix.
You'll need a spirit level or a straight piece of timber the width of the
path and use this to drag the mix along, leaving a 20mm bed all along the
path, tamp it down lightly with a trowel and trammel again filling in any
voids until you have a section with a complete 20mm bed all along, then
place the blocks in whichever pattern you choose, leave to set for 24 hours
and brush kiln dried sand into the joints.


The blocks will need bracing at the sides as they won't be stuck down and
will move sideways ovedr time

[Robin]

John Rumm wrote:

> Laid with concrete under the paving will be impermeable, so will
> need proper drainage.
>

Thanks. Where the water goes with the joints not mortared is one of the
many, many things that worried me.

[Robin]

Phil L wrote:
> If you lay the blocks on sand, the sand will wash away as the
> concrete is impermeable, this will cause the path to become wobbly
> and uneven within a short period of time.

Ah yes. Thanks.

> You can lay the blocks on mortar, but you'll have trouble getting
> them all level unless you use a dryish mix and trammel it first, then
> just place the blocks on the screed without bashing them down.
>
> To trammel the dry mix, you'll need two bits of wood the thickness of
> the bed....say 20mm thick, place one on each side of the path on top
> of the concrete, then spade in the dry mix.
> You'll need a spirit level or a straight piece of timber the width of
> the path and use this to drag the mix along, leaving a 20mm bed all
> along the path, tamp it down lightly with a trowel and trammel again
> filling in any voids until you have a section with a complete 20mm
> bed all along, then place the blocks in whichever pattern you choose,
> leave to set for 24 hours and brush kiln dried sand into the joints.

Very many thanks for that.

Also, I forgot to mention the path slopes a bit - around 1 in 25. May I
ask if this means the kiln dried sand will need topping up more often
because water which cannot drain through the base will wash it down the
path?

> The blocks will need bracing at the sides as they won't be stuck down
> and will move sideways ovedr time

Understood, thanks.

[Phil L]

"Robin" <rb...@hotmail.com> wrote in message

news:ml0pgv$p8u$1...@dont-email.me...

1 in 25 isn't a lot, most of the water will drain off at the sides, I
shouldn't think the KDS will wash away - it usually gets replaced by moss in
a short period anyway, it's only job is to prevent lateral movement of the
blocks.

There's a new product now that you brush in and it sets, not very hard, but
enough to stabilise the joints, I'd try KDS first and if it needs topping up
regularly, get some of this and give it a shot.

[Robin]

Phil L wrote:

<snip clear, expert advice>

Many thanks again.

[tabb...@gmail.com]

Just one suggestion: avoid using kd sand! Sand filled joints turn what could be a nearly maintenance-free surface into needing regular clean out. I'd fill the joints with a dry sand/cement mix or possibly even bitumen. Epoxy sand mix is a bit expensive.


NT

[stuart noble]

For a thin layer I'd consider Gripfil. Worked very well for re-fixing
some quarry tiles on a concrete base