Hollow sounding slabs - Hollow sounding slabs

[BZK]

Hi,

I've got a newly laid patio. The slabs are Indian sand stone. When you walk over them they sound distinctly hollow. I believe they were laid on a full bed, with compacted scalpings as a sub bed. I've lifted a couple and they do seem to have a full bed. Under the scalpings there's rubble from what looks like old pebbledash - largest pieces about 3" x 1".
Question is - is this going to cause me trouble in the future...?

Thanks

[lutonlagerlout]

the fact that you are lifting them is not good

what have they been laid on?

a picture would be handy

LLL

[London Stone Paving]

Does sound a bit ropey if you are finding 3x1" lumps of aggregate in the subbase. A proper subbase should be Type 1 50mm to dust. Look forward to seeing a few piccies

[BZK]

Hi,

There is the 40mm to dust stuff on top, it's just underneath that where the bigger pieces are. Here are some pics:

https://www.dropbox.com/sh/cf7uur5onqnos0l/KZM4w-CjFo

Thanks

[lutonlagerlout]

without seeing the whole job its hard to say but those flags look well laid
why are you chiseling up the bedding?
LLL

[BZK]

I took up the stones to try to find out why they sound hollow when tapped...

[BZK]

Bit quiet - anyone got an opinion on whether this sub base is going to cause me trouble long term?

[rimexboy]

Tony has already said...

When u took the slab up did it have a full bed of mortar,



Any better pictures of the whole patio...

Also that slab to the right in the picture looks like its from another place... Is it??

[London Stone Paving]

I dont think you have much to worry about from what I can see. The bed looks like its been done properly and the subbase seems to be substantial. The subbase is definately not responsible for the hollow sounds. It could be the odd slab delaminating.

[seadub]

I've also just had a 90m2 of tumbled sandstone laid on a full mortar bed on a sub-base of 100mm concrete and a lot of the slabs sound hollow when tapped.The contractor says this is probably due to ridges in the mortar underneath as he assures me they are fully bedded and bonded. Should I be concerned about de-lamination (they came from Ajeers)?

[lutonlagerlout]

if there are ridges in the mortar then it NOT fully bedded
to my mind fully bedded means 90% + of the flag in contact with mortar
LLL

[London Stone Paving]

I have not had a lot to do with ajeer for quite a while but from what I can remember they were a decent company. Sounds like the issue is with your mortar bed, not your stone. If it has been laid on a full bed and properly compacted there should not be any ridges, like Luton says

[lutonlagerlout]

seadub why was the concrete being overlaid?
we come across this time and time again and its bad practise
100mm type 1 ,40-60mm bedding then the flag is sufficient
LLL

[Tony McC]

I wonder if this is a matter of perception? I went to look at a patio yesterday, just before the heavens opened, and the homeowner was adamant the flagstones sounded hollow, but to my hear, when clonked with my walking stick, they sounded fine.

Thinking about this last night, the only reason I could fathom for why the flags gave a higher-pitched sound when tapped than was the case with the reclaimed yorkstone laid elsewhere to his property was the thickness of the stone itself. The yorks will be at least 2" (50mm) thick, and give a dull 'thud' when tapped, whereas the imported sandstone has a more resonant 'tonk'.

I brutally forced a hollow-sounding 300x30mm square from its bed and, lo and behold, it was a full bed. Building sand mortar, which is very naughty, but a full bed nonetheless.

[seadub]

seadub why was the concrete being overlaid?
we come across this time and time again and its bad practise
100mm type 1 ,40-60mm bedding then the flag is sufficient
LLL

It was a sloped site so the contractor had to scrape off a layer of soil & turf from one half, build up the other half with tons of hardcore then MOT. He said a mass fill of readymix concrete, 120mm thick would be the best long term solid solution. The paving was laid approx. 10 days after the concrete was poured.