Relaying slabs

surreygooner · 24 May 2014

Hi,

I'm having to relay my patio (didn't get the fall correct - schoolboy error)

I'm happy now with how to achieve a fall - but the slabs laid about a week ago came up very easily, just a spade under them and they popped up.

I guess what I'm asking is:

A) how to prep the base for good adhesion - it's a solid poured concrete base that is level and smooth
B) how to prep the underside of the slabs for good adhesion - they are 22mm calibrated sandstone
C) what mix of mortar/tile adhesive you'd lay these on

Thanks in advance

deandiy61 · 24 May 2014

on past similar circumstances...
chop or gouge the concrete to provide a key
apply exterior pva ( 5:1) to underside of slab
4:1 building sand to cement

surreygooner · 24 May 2014

Thanks, that's a great help.
Is it better to let the watery Pva go off on the reverse or put it down when damp/tacky
Also as they are only 22mm thick would you lay on dabs or a full bed of mortar?

deandiy61 · 24 May 2014

tacky pva. Never like dabs, I like a continuous bed of mortar.

surreygooner · 24 May 2014

Thanks again, 3 final questions:

Would you also Pva the concrete base after I've run the angle grinder on it to create a key?
Would you add any sharp sand to the mix or stick to building sand?
Would you add Pva or a plasticiser to the mortar?

Ta

r896neo · 24 May 2014

Don't use building sand use sharp sand.

Use SBR in the mix about 100ml half bag of cement. SBR will act as a plasticiser and will also aid adhesion more effectively.

The pukka way to do it is to use a bond bridge on the back of the slabs which should be a mix of sbr and cement.

Lots of good information on paving expert regarding bond bridge mixes etc.

A bond bridge is also a good idea as sometimes when laying on a concrete base rather than a free draining type 1 base you get mositure and staining problems because the water from the mix has nowhere to go but up through the slabs. The bond bridge will help alleviate that and also its worth using a leaner mix like 8:1 to reduce the chance of cement staining coming up through the flag.

Be aware that a leaner mix like 8:1 will not be as hard as quickly as say 5:1, so you will need to wait a full 36 hours before walking on it to point.

surreygooner · 24 May 2014

Cheers neo, don't have sbr but do have plasticiser and Pva admixture. Can I substitute if so what ratios?

Ta

rbdrifter · 25 May 2014

Listen to neo, that is exactly what you have to do. Sbr and sharp sand are your friend. You do not want to be using pva and building sand.

deandiy61 · 25 May 2014

As another respected member of the Forum once said of building/sharp sand, it's personal choice ( To regular readers, I mean Thermo and neo896 as the two respected members).

r896neo · 25 May 2014

My reasons for using grit sand are that it carries much less moisture when mixed to the same consistancy meaning it is easier to work when wet and in a deep bed.

Building sand at that depth turns into a soup and becomes unworkable

Secondly grit sand sets harder. try breaking a 40mm slice of building sand mortar and then try the same on a grit sand mortar. The difference is very distinct.

If you are using a semi dry mix for bedding building sand may be ok but for a wet mix its just too soupy.

deandiy61 · 25 May 2014

I believe building sand would give better adhesion here. the O.P. stated he had problems.( I laid the mortar 1" thick on top of the old concrete on my job, I wouldn't have fancied it at 2" thick. The concrete had no curved surface to accommodate).

r896neo · 25 May 2014

Everyone has their own opinion and experience but any of the large national paving mftrs will all recommend a sharp sand bedding mortar and all BS compliant bedding compounds are based on larger particle sizes such as 6mm and under.

There are a lot of boring BS standards regarding it too but they are only relevant to commercial works.

In reality for a patio as long as its well laid with a good full bed with enough cement most mixes will do whatever their sand but its always best to advise of industry standard best practise.

I hope you realise i'm not trying to get at you or disagree specifically with what you said just point out some of the reasons why grit based sands are prefered.

deandiy61 · 26 May 2014

hello r896, I've taken notice of your good advice long before my first Forum contribution.

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Relaying slabs

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