Forum: Flags and Slabs
Topic: Pointing riven flags with sbr in the mix
started by: Back Yard Bill
Posted by Back Yard Bill on 07 July 2019,13:07Hi, have a smallish patio down with 2ft square flags / riven edges on a very solid base.
Due to the riven edges & the variable depth of the base underneath(not uniform) it loks like i'd need at least 2+ tubs of brush in jointing compound, which makes it very expensive for what it is.
So i gather a 4:1 sand:cement mix that's barely wet is the way to go. But reading around it, I gather some SBR will beef up the mix and make it more durable.
So a big bottle later, I have some to add to the mix. My question is, will the SBR 'work' as part of the liquid to make it barely wet?
And so do i push it in as per the standard 4:1 mix & brush off or do i have to be very careful with the SBR in about leaving it due to staining?
Any other issue to be aware of plse?
Thanks in advance oh wise ones.
Also, i assume it's a standard building sand, not a grit sand to use?
Posted by Tony McC on 09 July 2019,10:48SBR will indeed 'beef up' the mortar, but it alsop makes it incredibly tricky to use, as any minor spill, smear or spolsh WILL stain the paving, and stain it permanently, unless it is meticulously cleaned up immediately.
As a contractopr, we oonly ever used SBR in the jointing mortar of commercial projects where we could expect vehicle overrun and/or thousands of pedestrains per day. This is because, without such traffic loadings, there's no need for SBR, and also because of the need for meticulous cleanliness.
For a residential patio, a well-mixed 4:1 sand/cement mortar with a simple plasticiser is more than adequate for the jointing. No need whatsoever to over-engineer the job.
Building/soft sand will give a smoother, finer mortar, while grit/sharp sand will give a coarser finish. It's not a matter of one being structurally better than the other: it comes down to which you prefer to look at.
Posted by Back Yard Bill on 09 July 2019,14:19Thanks Tony.
I only got it due to reading on here of all the people re-doing their poor grouting and (at least thought I) read also that it would also strengthen it so it wouldn't need re-doing.
Oh well. So any sploshes etc, just a clean with copious amounts of water or something stronger (sure there's some info on that, so I'll find it).
I'm foreseeing sploshes as I'm certainly no pro. As evidenced by these two further questions:
Does plasticiser just make it more workable as a mix?
Does anyone want to buy some SBR?
Posted by TimW on 09 July 2019,18:40May I re-ask a question I've asked elsewhere...
What makes a sand/cement pointing mix turn out to be white? I need it to be grey!
Posted by Tony McC on 11 July 2019,09:00Bill, the whole purpose of a plasticiser is to make the mortar more 'plastic' more workable, usable, easier to fold and smooth and play with in general.
Tim: the question uyou posed was answered. Why do you need it repeating
Posted by TimW on 11 July 2019,10:22Sorry, I hadn't seen your reply, Tony.
Posted by Back Yard Bill on 13 July 2019,00:04
So, if I promise to be extra careful and not splosh everywhere and add some SBR for longevity, can I also add in a plasticiser as well?
And as I'm trying to use the 'dry' mix method (very little water somit can brush away) will the quantities of either / or both a plasticiser or SBR be so small as to be negligible - especially given the small atch of mortar i'll be using?
Posted by Tony McC on 16 July 2019,09:46With a "dry mix" a plasticiser is completely bloody useless and SBR will render it no longer dry!
Posted by Back Yard Bill on 17 July 2019,23:14
I thought even a 'dry' mix had a little bit of liquid in surely?
And, my thought was that part of that 'little bit' could be the SBR?
Agree on the platiciser though.
Posted by Tony McC on 18 July 2019,09:19As soon as you mix even a few drops of SBR to a dry sand/cement mix, it starts to 'clump'.
Dry cement grouts are weak and friable at the ebst of times; adding SBR achieves nothing - it needs the moisture of a wet mix to distribute it evenly and to initiate the curing process.
Posted by Back Yard Bill on 19 July 2019,09:39
I have obviously been watching the wrong videos then.
Is there one (or very clear, idiot-proof* instructions) about on how to do it properly then please anywhere?
* me obviously ; )
Posted by Tony McC on 19 July 2019,10:17Whole series of pages about how to use cement mortars for pointing, starting < here >
Posted by Back Yard Bill on 25 July 2019,09:59Thanks Tony.
I guess I need this one:
< http://www.pavingexpert.com/point04.htm >
(but without the dye)
3:1 sand : cement
but definitely no SBR as part of the limited liquid content added for the semi-dry mix?
I guess that would just make it all too sticky and to not brush off correctly / easily etc?
Posted by Tony McC on 31 July 2019,19:03You really don't need the SBR.
Posted by Back Yard Bill on 05 Aug. 2019,08:35OK, OK, I'm betting the hint thanks.
I did a test area with no SBR using a grit sand 3:1 and it looks great alongside the riven flags.
So looks like that's the way to go.
Thanks for your help Tony.