A friend had a low brick garden wall built with slabs on top for seating.
A couple of slabs came loose after very little use and it is evident that
none of them are fixed properly - a sharp tug will make them come off.

Builder is blaming friend, for having the temerity to sit on the seating
area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang
allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any prep
on the underside of the slabs? They are smooth-ish - think slate texture -
but fairly thick.

On 19/02/2021 13:38, Scion wrote:
A friend had a low brick garden wall built with slabs on top for seating.
A couple of slabs came loose after very little use and it is evident that
none of them are fixed properly - a sharp tug will make them come off.

Builder is blaming friend, for having the temerity to sit on the seating
area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang
allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any prep
on the underside of the slabs? They are smooth-ish - think slate texture -
but fairly thick.


The problem is not just the mortar mix (which was probably too weak)
but the differential thermal expansion and contraction of wall and
slabs.
Also 'slate texture' slabs won't have the suction required to
create a proper cement bond, which could be the major issue which
you won't cure even with a stronger mortar mix.
I presume the mortar is still well adhered to the wall but the slabs
have parted company with the mortar ?. You might have to consider a
chemical adhesive which allows some flexibility as the slabs expand
and contract.

On Fri, 19 Feb 2021 14:35:04 +0000, Andrew wrote:

On 19/02/2021 13:38, Scion wrote:
A friend had a low brick garden wall built with slabs on top for
seating. A couple of slabs came loose after very little use and it is
evident that none of them are fixed properly - a sharp tug will make
them come off.

Builder is blaming friend, for having the temerity to sit on the
seating area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang
allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any
prep on the underside of the slabs? They are smooth-ish - think slate
texture -
but fairly thick.


The problem is not just the mortar mix (which was probably too weak)
but the differential thermal expansion and contraction of wall and
slabs.

I hadn't considered thermal expansion but wasn't aware it was an issue in
brick/stone.

Also 'slate texture' slabs won't have the suction required to create a
proper cement bond, which could be the major issue which you won't cure
even with a stronger mortar mix.

Hence the 'prep' question. Wondering if roughening the underside would
help.

I presume the mortar is still well adhered to the wall but the slabs
have parted company with the mortar ?.

I haven't seen it first hand, I'll ask.

You might have to consider a
chemical adhesive which allows some flexibility as the slabs expand and
contract.

I'll have a Google. As I assumed the mortar was too weak and a stronger
mix would cure it I hadn't considered other options. A flexible adhesive
might be fine - the overhang would hide it so it doesn't matter if it
doesn't match the rest of the brickwork.

On 19/02/2021 13:38, Scion wrote:
I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any prep
on the underside of the slabs? They are smooth-ish - think slate texture -
but fairly thick.

You won't do this I know, but after repeated failures a friend of mine
did this with long screws and blue plugs. A complete fix.

Bill

On 19/02/2021 13:38, Scion wrote:
A friend had a low brick garden wall built with slabs on top for seating.
A couple of slabs came loose after very little use and it is evident that
none of them are fixed properly - a sharp tug will make them come off.

Builder is blaming friend, for having the temerity to sit on the seating
area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang
allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any prep
on the underside of the slabs? They are smooth-ish - think slate texture -
but fairly thick.


Might help to know the height and width of the wall and the width of the
slabs. If there's a really big overhang I wondered if it might be worth
thinking of (so as to speak) widening the wall to support the slabs.
Possibly galvanised steel angle screwed to the wall if they'd be low
enough to be hidden?

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

On 19/02/2021 15:17, Scion wrote:
Also 'slate texture' slabs won't have the suction required to create a
proper cement bond, which could be the major issue which you won't cure
even with a stronger mortar mix.
Hence the 'prep' question. Wondering if roughening the underside would
help.


Also, if the slabs were laid so that they were tightly butted up
to each other then thermal expansion will break the cement bond.
This is why the end brick of the top course of a wall frequently
has an L-shaped bit of galvanised metal embedded in the mortar
and turned up at the end to provide lateral support to the end
brick.

If they were laid 'tight' then re-lay but leave a 5 mm gap between
them and get some exterior mastic, suitably coloured and fill the
gaps with this.

On 19/02/2021 13:38, Scion wrote:
A friend had a low brick garden wall built with slabs on top for seating.
A couple of slabs came loose after very little use and it is evident that
none of them are fixed properly - a sharp tug will make them come off.

Builder is blaming friend, for having the temerity to sit on the seating
area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang
allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any prep
on the underside of the slabs? They are smooth-ish - think slate texture -
but fairly thick.

For TENSILE strength use epoxy mortar!


--
€œThe ultimate result of shielding men from the effects of folly is to
fill the world with fools.€�

Herbert Spencer

a few bloody great bolts might be of more use. Cement is not good when
being pulled apart as it is on a slab on a wall with lop sided pressures.
Brian

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"Scion" wrote in message
...
A friend had a low brick garden wall built with slabs on top for seating.
A couple of slabs came loose after very little use and it is evident that
none of them are fixed properly - a sharp tug will make them come off.

Builder is blaming friend, for having the temerity to sit on the seating
area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang
allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any prep
on the underside of the slabs? They are smooth-ish - think slate texture -
but fairly thick.

On 19/02/2021 16:19, Robin wrote:
On 19/02/2021 13:38, Scion wrote:
A friend had a low brick garden wall built with slabs on top for seating.
A couple of slabs came loose after very little use and it is evident that
none of them are fixed properly - a sharp tug will make them come off.

Builder is blaming friend, for having the temerity to sit on the seating
area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang
allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any prep
on the underside of the slabs? They are smooth-ish - think slate
texture -
but fairly thick.


Might help to know the height and width of the wall and the width of the
slabs.Â* If there's a really big overhang I wondered if it might be worth
thinking of (so as to speak) widening the wall to support the slabs.
Possibly galvanised steel angle screwed to the wall if they'd be low
enough to be hidden?

In the past, I've solved similar problems by drilling into the underside
for a couple of plugs and then inserted screws which stick out and
provide a key for the mortar to hang on to. They were standard concrete
slabs, so might not be advisable with the stuff you're working with.
John M

On 19/02/2021 16:19, Robin wrote:
On 19/02/2021 13:38, Scion wrote:
A friend had a low brick garden wall built with slabs on top for seating.
A couple of slabs came loose after very little use and it is evident that
none of them are fixed properly - a sharp tug will make them come off.

Builder is blaming friend, for having the temerity to sit on the seating
area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang
allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any prep
on the underside of the slabs? They are smooth-ish - think slate
texture -
but fairly thick.


Might help to know the height and width of the wall and the width of the
slabs.Â* If there's a really big overhang I wondered if it might be worth
thinking of (so as to speak) widening the wall to support the slabs.
Possibly galvanised steel angle screwed to the wall if they'd be low
enough to be hidden?

In the past I've solved this problem by drilling into the underside for
a couple of plugs, then inserted screws which stick out and provide a
key for the mortar to hang on to. Those were standard concrete coping
stones though so might not be applicable to the stuff you're working with.
John M