Hollow sound in fully bedded flags

[Wiggy9]

I laid a dozen or so Indian Sandstone 600x300 flags yesterday to see how it would go.

I was happy with the way they went down. I used a dry 8:1 mix of sharp sand and cement, laid as a full bedding.

I was satisfied that nothing was rocking when they were in place, and it looks pretty good to me today.

I can't do much today due to rain, and so I was just cleaning an edge border with a wall where I was a little messy. I noticed that one of the flags sounded hollow when I tapped it with my knuckles. I tapped a few more, and it sounded like a frigging kettle drum orchestra -- loads of different notes.

I stood on every flag to test for rocking when I was laying them. I layed each one at least 4 times trying to get it right.

There is obviously something seriously wrong with my bedding technique.

Can anyone tell me what I am doing wrong?

[digerjones]

hi wiggy, i would think your problem is having a dry mix, you need slightly damp mix ,wet enough to form a ball in your hand, also wet the back of the flags. its all in the main site.

[lutonlagerlout]

even a dry mix has some water in it 3-4 pints
wet mix would be like a gallon
LLL

[Wiggy9]

Thanks for your responses guys.

I was going by the advice on the main site "A semi-dry mix contains no added water (or very, very little when the aggregates are particularly dry and dusty). It relies on natural moisture within the sand, in the sub-base or sub-grade, and in the atmosphere, to initiate curing in the cement content."

My sand is pretty moist, and it seems to be curing fine.

I would like to persist with this dry laying approach if possible, as I find it clean, and not too difficult to level the flags.

I'm using a pretty small mall, shaft size is the same as a standard claw hammer. Should I be looking at investing in one of those big malls?

What kind of force should I be using with the mall to tamp down 600x300 sandstone flags?

Thanks again for taking the trouble to help me guys - I really do appreciate it.

[Bob_A]

From one DIYer to another I used one of these.
http://cgi.ebay.co.uk/Silverl....249ac13
Difficult to describe how much force I used but the mix I used was moist. Probably more moist than most people would use but it suited me.
I found that I need to give them a reasonable clout but never bashed the daylight out of them.
Once the bedding layer has reached consolidation it won't consolidate more. Only option is to lift the flag/slab, remove some bedding layer, fluff it up and try agian.
I used this tool as described by henpecked.
http://ext.pavingexpert.com/cgi-bin....hl=rake
It helped me but it is definitely a DIYer's tool (toy), not one you'll find a pro use

[Wiggy9]

I think I might have found the reason for this.

I was supplied with plastering sand (see my other thread) which I couldn't use and so had to source a couple of bags of sharp sand on a bank holiday Saturday. The sharp sand I got was from B&Q, and was much coarser than plastering sand.

I've now located a supply of what's labelled Paving Sand, and it's much coarser again.

I reckon that this will compact much better when I tamp it down, due to the larger grain size.

Sound plausible?

[seanandruby]

.
I used this tool as described by henpecked.
http://ext.pavingexpert.com/cgi-bin....hl=rake
It helped me but it is definitely a DIYer's tool (toy), not one you'll find a pro use

Don't know about that, i would maybe use it to scratch my bal.... er, i mean back :laugh:

[TheVictorianCobbleCo]

Not to hi-jack this thread - but here goes - re your hollow sounds. I did a 250sqm area in Crowborough with 600 x 600 x 38 slabs and found this method to be the ONLY successful one. I lay rails about 3m apart at the correct finished height. I used a combination of sharp/building sand 50/50 and cement with a cup of PVA and a dash of plasticiser, in a WET mix - stiff mashed potato type. Each slab was brushed and wetted on the backside, and laid on a "Serrated" base of mix - vvvvvvv , and then tapped into place using a straight edge running across the rails. All exposed sides were fully buttered before laying the next slab. Using the back of the trowel, each corner of the slab was tapped to get a ringing sound - which meant they were properly seated. In the one or two cases where I picked up a hollow sound on a corner I mixed a slurry of kiln dried sand and cement1/1, and worked this in until the cavity was filled. 3 years on, solid as a rock.
Now I've had to supervise a job in Cape Town, South Africa, where the method commonly used is to compact the base, lay a offdry mix of sand and cement, tamp that, screed it flat, and then place the slabs on this, with a tamp or two of a rubber mallet to seat them. Then they mix a grouting slurry and sweep this in with copious amounts of water. The slabs and grout are then sponged clean. Within weeks the grout starts to crack and movement is seen in the paving.
Typically this method is used for pool surrounds and patios. I would like to show the client independent responses to this method and so would appreciate your comments re this method, as I am now having to go back to redo the paving, and she wants to be certain we do it correctly/solve the problem.

[cookiewales]

Not to hi-jack this thread - but here goes - re your hollow sounds. I did a 250 sqm area in Crowborough with 600 x 600 x 38 slabs and found this method to be the ONLY successful one. I lay rails about 3m apart at the correct finished height. I used a combination of sharp/building sand 50/50 and cement with a cup of PVA and a dash of plasticizers, in a WET mix - stiff mashed potato type. Each slab was brushed and wetted on the backside, and laid on a "Serrated" base of mix - vvvvvv , and then tapped into place using a straight edge running across the rails. All exposed sides were fully buttered before laying the next slab. Using the back of the trowel, each corner of the slab was tapped to get a ringing sound - which meant they were properly seated. In the one or two cases where I picked up a hollow sound on a corner I mixed a slurry of kiln dried sand and cement1/1, and worked this in until the cavity was filled. 3 years on, solid as a rock.
Now I've had to supervise a job in Cape Town, South Africa, where the method commonly used is to compact the base, lay a off dry mix of sand and cement, tamp that, screed it flat, and then place the slabs on this, with a tamp or two of a rubber mallet to seat them. Then they mix a grouting slurry and sweep this in with copious amounts of water. The slabs and grout are then sponged clean. Within weeks the grout starts to crack and movement is seen in the paving.
Typically this method is used for pool surrounds and patios. I would like to show the client independent responses to this method and so would appreciate your comments re this method, as I am now having to go back to redo the paving, and she wants to be certain we do it correctly/solve the problem.

i would say the sand to cement ratio is to weak have seen this method of laying in ireland 4 to 1 sharp sand cement 70mm granite then slurry pointed thousands of people every day on them no probs i lay the same only in rows so i dont need to walk on them cheers cookie :;):ps only use sbr outside or anywhere in contact with water :;):



Edited By cookiewales on 1276257349

[lutonlagerlout]

i use a wetter mix than most pavers and generally use a string line rather than a piece of wood
just dont like dry/semi dry mixes,clean yes, but too many fails
LLL