The Brew Cabin

Posted: **Wed Sep 14, 2011 10:38 pm**

Hi guys

I've nearly finished laying 50m2 of riven sandstone paving and will be pointing very shortly. All the flags (25-40mm thick) have been laid on a full bed (not spots) of 6:1 sharp sand/cement about 40-50mm thick (consistence slightly wetter than moist). I allowed a bed height of about 10-15mm for compaction. The sub-base, about 75-100mm thick, was fully compacted with a wacker plate.

All has gone very well except I've just noticed that when tapping the 900 x 600 flags with a wooden shaft of a hammer, quite a few sound hollow (all over) although there is no sign of rocking or sideways movement. The smaller surrounding flags, 600 x 600, 600 x 300 and 300 x 300, laid with exactly the same bedding material at the same time, all sound solid. All the flags were tapped down to the required level with a 2lb rubber mallet in the recommended positions.

Being paranoid, whilst laying, I lifted a couple of the 900 x 600 flags after tapping down to check that I had a fully compacted bed underneath. Both were fine (btw these were re-laid using fresh bedding).

What do you think has caused the hollowness and is it likely to be a problem in the future? Will pointing (4:1 building sand/cement) beef it up?

Posted: **Wed Sep 14, 2011 11:17 pm**

normally that hollow sound indicates a void or the flag delaminating
i think if you have full beds dont worry too much bruce
pictures please mate
LLL

Posted: **Thu Sep 15, 2011 8:57 am**

Many thanks LLL. Will post some pics soon of the partially complete job. Two small areas to go prior to laying the perimiter edge setts (single row to be bedded on 100mm of 10mm agg concrete then haunched). Obviously pointing to follow.

Posted: **Thu Sep 15, 2011 9:20 am**

it'll be the wooden shaft hitting the flag making the dull sound. Don't get paranoid about it as you seem to be doing everything to compliance.

Posted: **Sat Sep 17, 2011 8:38 am**

I encounter the hollow sound quite regularly, When using a a slightly moist mix its going to struggle to bond to the flag, not that it has to anyway, so i reckon the dull sound is the sound of an un-bonded flag.

May i add i only have this problem with indian sandstone, when laying 3x2 concrete council flags on a dryish mix, there no chance of them sticking to the bedding layer but they never sound hollow, Maybe it because they have more mass to hold themselves in place?

This particular subject had been a big thinking point for me in the past, but ive never had to go back to an indian sandstone job due to rocking or moving flags. I find SBR solves the hollow sound, or dare i say it laying flags on a 3-1 bricklaying mortar on a concrete slab - they realy do stick!

Posted: **Sat Sep 17, 2011 11:56 am**

DNgroundworks wrote:or dare i say it laying flags on a 3-1 bricklaying mortar on a concrete slab - they realy do stick!

ooohh you are naughty
:;):
LLL

Posted: **Sat Sep 17, 2011 1:34 pm**

ive done that once, and it was absolutely solid!

Posted: **Sat Sep 17, 2011 5:47 pm**

DNgroundworks wrote:ive done that once, and it was absolutely solid!

ever taken up a patio laid with building sand? very very easy..

Posted: **Sat Sep 17, 2011 9:36 pm**

the hollow sound under the patio? you may want to check if Trevor Jordache is still there

Posted: **Sat Sep 17, 2011 10:56 pm**

Trying to upload some pics but even after reading "How to upload photies" still can't do it. How do I set up a URL / http web address?? Your help would be much appreciated.

Posted: **Sat Sep 17, 2011 10:58 pm**

I use photobucket, upload there and copy and paste here.

Posted: **Sun Sep 18, 2011 12:42 am**

LLL - as requested please find a few pics of patio and path so far. I've included some pics of the small area done a couple of days ago showing the bedding of the first flag and the final job. Before anyone points it out, the main patio to the right of the small area starts at one course below dpc. The original patio was at this level for over twenty years with no signs of damp. The wall faces north-east so doesn't get a lot of rain. I've laid the flags to a fall of 1 in 50 and they seem to shed water very well (although still to be pointed).

With regard to the hollow sounding 900 x 600 flags, after much head scratching, I think it's due to excess water bleeding to the top of bedding on compaction caused by any one or combination of the following factors:

1) the moisture content of the bedding being too high

2) the surcharge being too high resulting in excessive tapping with the rubber mallet to achieve level

3) the bedding being too thick (if in excess of 50mm)

4) poorly graded sharp sand particularly if coupled with a lean mix (I'm using 6:1 by volume and the sharp sand is excellent quality)

As a consequence, a thin layer of water accumulates at the surface which, on evaporation results in voids and the flag becomes effectively de-bonded although it still remains firmly in position. De-bonding could also occur if the flags are moved too late after laying in order to adjust joint width.

Most of the hollow sounding 900 x 600 flags are in the path I laid first. I was still on a learning curve in the laying technique. The last few 900 x 600 I've laid, paying more attention to bedding moisture content and surcharge, are fine and solid as a rock. I've also reduced the bed thicknes by 10mm to 30-40mm (as originally recommended by Tony). The wife (in charge of the mixer) has got the bedding mix spot on - sand and cement being gauged in buckets so is consistent batch after batch!!

Apologies for rambling on but if I do a job I like to do it properly. This site and the help from you guys has been invaluable. Will post some more pics when the job is finished. One small area, edge setts and pointing left.

ps: many thanks haggistini for the photobucket info. Never heard of it before but have now registered. Hope the pics upload ok!!

Posted: **Sun Sep 18, 2011 1:05 am**

Brucieboy wrote:

With regard to the hollow sounding 900 x 600 flags, after much head scratching, I think it's due to excess water bleeding to the top of bedding on compaction caused by any one or combination of the following factors:

1) the moisture content of the bedding being too high

2) the surcharge being too high resulting in excessive tapping with the rubber mallet to achieve level

3) the bedding being too thick (if in excess of 50mm)

4) poorly graded sharp sand particularly if coupled with a lean mix (I'm using 6:1 by volume and the sharp sand is excellent quality)

to be honest i think you are looking for reasons that are not there, simply put, the hollow sound is due to a void under the slab as the boys have said, its not the end of the world. it acts like a drum

Posted: **Sun Sep 18, 2011 1:21 am**

local patios - just me being paranoid. With the assurances from you guys I'm now satisfied ther'e no problem. I'm glad I raised it as I wouldn't have relished the thought of having to take up big flags some time after pointing. Many thanks for all your help.

Posted: **Sun Sep 18, 2011 8:27 am**

nice job bruce
done well there
have you laid some of the flags over an existing concrete base?
LLL

The Brew Cabin

Hollow sound - Indian sandstone patio and path