Slump, bond bridges, and joint balancing

[42Neil]

As posted in another thread, I am a DIYer about to attempt laying some 20mm thick quartzite paving and have been reading this site and forum to establish best practice for laying them. However, it seems that there are a number of differing opinions on various aspects:

The main site suggests semi-dry as a preferred slump for a laying course, as it is cleaner to work with and has a longer working life. However, on this forum however, I have read several posts where people have followed the advice of using a semi dry mix, but have had problems with adhesion and they normally get told that they should have used a moist or a wet mix (and a bond bridge if they didn’t use one).

As for adhesion, the main site suggests that adhesion between the laying course and flags isn’t actually important, but the forums are full of people with flags that aren’t adhering being told what they did wrong.

Regarding Bond Bridges, the main pages of the site provide a recipe for a self mixed bond bridge of SBR and cement, and states that SBR costs £3-5 per litre. However, on a forum post where someone had used Cementone SBR (approx £4 per litre) to mix up a Bond Bridge, they got told that the better SBR costs £20 per litre.

After reading the above and being thoroughly confused, I decided to try laying a test flag (using a small 15x15cm sample). I used a semi dry mix and no bond bridge, to test adhesion of this paving. The bed set fine, but the flag lifted right up. So based on this I think i do need to do something different to get better adhesion.

Do you think just using a wetter mix would be enough to promote adhesion, or do I need a bond bridge? Also, given that the chap in the forum who used Cementone SBR got told that he should have used a “better� SBR, does anyone have any recommendations for a better one to use (one that is still fairly easy to source though)?

My main concern at the moment about using a bond bridge is the working life. The patio I am laying is pretty big (circa 4m x 10m) and is being layed using a random layout, and will therefore need some joint balancing. I get the impression that the bond bridge will go off pretty quickly, potentially causing me headaches when it comes to joint balancing.

Any advice (other than get a professional in!) on how to get proper adhesion but still have enough working life to do the joint balancing?

[42Neil]

After posting the above, I went out to throw away the test patch and I noticed that it is pretty crumbly. It held together initially, but crumbled pretty easily when struck with a lump hammer. Does this seem normal for a set 8:1 grit sand/cement mix, or have I not mixed it well enough?

[lutonlagerlout]

neil most on here use 5:1 - 6:1
I always use a wetter mix for thin stuff and this has worked well for me for 20 years
we add approximately 100ml of SBR into every mix in the mixer

so far the failure rate has been less than .01 %

never laid quartzite so no knowledge specific to that

hope this helps

LLL

[42Neil]

Thanks for the reply.

Do you also tend to use a bond bridge applied to the flags as well as SBR in the mix, or is the SBR in the mix usually sufficient to give good adhesion?

Any thoughts on working life and the time it gives for joint balancing?

[lutonlagerlout]

I just add it to the mix
a wetter mix gives more adhesion anyway through suction if that is the correct terminology

a gauge from a mixer lays roughly 1 m" and we do not have trouble with it going off in a roughly 30 minute time frame

cheers LLL

[seanandruby]

Back in the day we would brush the back with water.