Stones not adhering during paving process

[charriso]

Hi,

I am in the process of laying Solhoffen-like irregular sandstone "crazy paving" on a patio. In fact, I am being greatly assisted by a neighbour who is a mason, as I have no idea what I am doing. (He's Portuguese, I'm Australian, and we live in France — you can imagine the scene...)

I don't know how much info you would need to answer the question I have, so I'll throw it out first, and fill in the details afterwards:

Some of the stones have not stuck down, and I would like to know what the best way to stick them back down is, before we get to pointing the lot.

Details:

We're laying the stones over a purpose-laid concrete slab, with what appears to be a "moist mix" bedding: grit sand and cement — I have been unable to determine the proportions, as my mason mate does everything by eye! If I had to guess, however, i would say that the ratio of cement to sand would be no more than 1:3 (or maybe 1:2.5). We lay and level about 2 sq. m of this bedding at a time (= 1 x 35kg bag of cement), then sprinkle it with cement (which immediately moistens into the surface), and proceed to position and tap down the individual stones. Working one mix takes us about 1.5 hours.

We're now about 1/3 of the way through (working roughly 3 hours per day — evenings — and not every day).

Today I noticed that a handful of the 100 or so stones we have already positioned have not adhered. I could simply lift them off the bedding, which is now quite hard and solid. Most of them seem to be solidly anchored, but just a few have not hung on at all.

Some of the stones are exfoliating (delaminating) a little, but this does not seem to be why they have not held on: all but one of the 9 that have come loose appear to have clean, stable surfaces.

So, the question (again): what would be the best way to stick these recalcitrant stones back down again, and I guess while I have your attention, I might also ask whether there is anything wrong with this whole approach? Should I expect more of them to give up in the future? Or will the jointing hold them down...?

Many thanks for your help, and well done on a superb website!

Colin

[lutonlagerlout]

if you are laying onto a concrete base you should be using a wetter mix
TBH there was no need for a base you could have bedded straight onto the wet concrete
you need to use a bond bridge on the bottom of your slabs
this si a mixture of SBR and neat cement painted on to the bottom of each flag prior to laying
this aids adhesion
cheers LLL

[simeonronacrete]

A mix lasting 1.5 hours? Too long in my opinion, especially in the warm. But then at Ronacrete we're purists.

Neat cement on the surface? Not a good idea; it's unlikely to mix with sufficient water to achieve anything other than produce a weak layer. That could contribute to the debond.

Ideal mix - in my opinion:

1 cement
3 parts medium sharp sand
Gauge with 1:1 Ronafix : water (add sufficient of the diluted Ronafix to the cement and sand to achieve a suitable workability.

Damp the base and the underside of the block with clean water. Remove any standing water. Prime the base concrete and the underside of the block with a primer of Ronafix and cement (1:1) and painted on with a brush. Place the primed block into the wet mortar and compact.

The system and mix I've described work every time without fail - first applications in the '60's and '70's.

Give us a call if you'd like to ask any questions or order some Ronafix.

Happy bonding!

[local patios and driveway]

for me i would remove the loose slabs and chip out what has been laid in the area, then a moist (not wet, very nearly dry) 4 to 1 mix, then use a slurry to bond the stone to the bed. slurry needs to be a bucket of water to 1 shovels worth or cement. you wont get that stone up the next day without busting it so get it right the first time. im not sure if this method is documented on this site but its my favorite method to laying slabs acuratley and without come backs. not so ideal on cut stone due to thicknesses but per slab it'll do the job

[charriso]

Wow, thanks for these very rapid replies.

I forgot a rather important detail, which is I believe what simeonronacrete is referring to. We are including in the bedding mix a bonding agent called "Sikalatex", although in considerably weaker concentrations than 1:1 (product:water)... This product claims to enhance bonding and waterproofing (I'm translating...) of cement mortars. Maybe I'll increase the dosage. (Oh, and I'm sure I'm the reason a mix takes 1.5 hours ??? - with an experienced side-kick, my mason mate would undoubtedly be able to move faster!)

As to the water content of the bedding mix, (LLL's comment) before laying the bedding mix, we have been wetting the slab and sweeping some of the bedding mix into the standing water, spreading it a round to make a kind of slurry that covers the area to be worked, and then spreading the (drier) bedding mix over the top. When I say it is a "moist mix" I really have no idea, as I have never done this before. It seems to correspond best to the description of the "moist mix" given on this site.

"Local patios and driveways", thanks for the advice. I will try that today.

Cheers,

Colin

[London Stone Paving]

Take care when you are chipping out the old mortar. Otherwise you may end up loosening the surrounding slabs in the process. Use a grinder if possibble to cut away the old mortar around the edges.

[charriso]

London Stone Paving,

Again, thankyou.

As the loosened slabs were perfectly levelled, I was thinking of chipping out the bed in such a way as to leave three points of the old surface intact... That way, I cannot possibly depress the slabs beyond level when I replace them... bad idea?

As the stones have not been pointed yet, then there will be a space (the joint space) between that which I have to chip out, and the surrounding slabs.

Colin

[London Stone Paving]

BAD IDEA

You need to remove all the old mortar. Its important achieve full contact with the mortar bed and the stone, especially in light of loose slabs.

It wont be too difficult to replace the slabs to the correct level as you will have all the surrounding slabs to work to.

Follow the previous advice given re bond bridge and you wont have any problems.

P.S Any labouring jobs going over there?

[charriso]

Right! Bad idea! I tried it on just one stone this morning, and indeed, it is very hard to guarantee the contact... So now I know. I'll do the rest properly!

Cheers.

[GB_Groundworks]

a bolster chisel will do, grinder messy and pain

i'd be using a wetter mix with sbr in the mix and a bond bridge, keep the mix in the mixer with a moist sack over the opening if its in the sun keep it moving, we go through a 400 litre mix, about 4 barrows worth in 20 mins between 2 of us laying on a full bed.