Page 1 of 1
Posted: Mon Dec 30, 2013 2:57 pm
by Paul.Able
hello all,
Any advice with regards to the following would be greatly appreciated....
Recently installed a new drive for a client. 90 square meters of block paving. Namely Bradstone woburn original. Laid to stretcher courses using 3 sizes of block. Here's the problem, the middle sized block (120mm x 120mm) is 2mm thinner than the other two sizes of block. And now the client has picked up on it. Basically these squares sit lower than the other two sizes of block and it has become noticeable. In addition these blocks don't have a rounded edge which compounds the problem...
Anyone had experience of this before??
I contacted bradstone at the time of laying and they told me that they have a 2mm tolerance with regards to block thickness and that it should be no problem once laid and compacted.
This drive was laid in the normal fashion, with a 25mm screed of 3mm grit on top of 200mm of 803 compacted with a twin drum roller. A light compaction of the screed and then blocks laid and final compaction with the plate and rubber foot. Standard stuff really. The problem is as above re the block thickness.
Has anyone experienced this themselves? And has anyone got any experience of throwing a problem like this back at the manufacturer and if so what result??
Thanks in advance for any thoughts you might have on this.....
Paul.
Posted: Mon Dec 30, 2013 3:23 pm
by msh paving
there is a british standard for block size tollorance , I thick im correct in saying +-3mm
25mm bed it to thin ,does not allow and more compaction after first compaction,35-40mm is ideal
this maybe why the blocks are not sitting flat, as the the block not having a chamfer/beval at top take it up with manufactures as there is possible a problem with the batch MSH 
Posted: Mon Dec 30, 2013 4:04 pm
by mickg
Disclaimer
no liability is accepted for the following advice
the issue you are seeing is no different to most paving block manufacturers where you can get a couple of millimetre different on the height as different batches of blocks can vary in thickness but the question I must ask is "why are you using a rubber mat on a tumbled paving block" ?
in my opinion and experience the rubber mat is only needed on riven surface blocks or blocks similar to Marshalls Argent Dark block paving where there is a surface finish to the paving and running the vibrating plate over this type of surface will cause damage
in your case this does not apply but before you compact the total area you should always test a small area first
what size and make vibrating plate are you using to compact the paving ?
Posted: Mon Dec 30, 2013 4:28 pm
by Paul.Able
Mick g and msh - thanks for replying.
The block is not a rumbled finish and it doesn't have a chamfer either.
Its up on the bradstone website as woburn original, not woburn original rumbled. I did try at the time to steer the client towards the rumbled but she was very specific about what she wanted.
To be fair I need to top up the joints a little which may eliminate a bit of the "shadow" on the joints....perhaps another pass with the plate might help too? By no means does it look terrible, I just hadn't had this prob before.
I use the metal round electrical conduit for screed rails. So the screed must actually be at approx 35mm cos they weren't tight to the deck when I screeded. ...
The plate is a belle pcx400
I'll snap a picture of some of the spare blocks and post it...
Cheers
Posted: Fri Jan 03, 2014 6:49 pm
by lemoncurd1702
When you say bedding of grit I'm assuming that's grit sand not stone dust.
Maybe a heavier wacker plate (pcx 400 should be heavy enough though) or try pushing down on the handle a little so that it is riding more on the rear of the plate.
Posted: Sun Jan 05, 2014 12:05 pm
by eastman landscapes
Paul.Able wrote:hello all,
Any advice with regards to the following would be greatly appreciated....
Recently installed a new drive for a client. 90 square meters of block paving. Namely Bradstone woburn original. Laid to stretcher courses using 3 sizes of block. Here's the problem, the middle sized block (120mm x 120mm) is 2mm thinner than the other two sizes of block. And now the client has picked up on it. Basically these squares sit lower than the other two sizes of block and it has become noticeable. In addition these blocks don't have a rounded edge which compounds the problem...
Anyone had experience of this before??
I contacted bradstone at the time of laying and they told me that they have a 2mm tolerance with regards to block thickness and that it should be no problem once laid and compacted.
This drive was laid in the normal fashion, with a 25mm screed of 3mm grit on top of 200mm of 803 compacted with a twin drum roller. A light compaction of the screed and then blocks laid and final compaction with the plate and rubber foot. Standard stuff really. The problem is as above re the block thickness.
Has anyone experienced this themselves? And has anyone got any experience of throwing a problem like this back at the manufacturer and if so what result??
Thanks in advance for any thoughts you might have on this.....
Paul.
i agree with the other comments and your screed being to mean is probably the culprit. what screed was used ?
Posted: Tue Jan 07, 2014 8:19 am
by Paul.Able
We use a 3mm grit. No fines and of an even size. The grit itself is crushed granite (its what we have down here in cornwall).
I am still wondering about the difference in block thickness though. We used 3 sizes of block for this drive and it was the square block (160x160) that was 2mm thinner. The other two sizes of block were exactly the same thickness throughout the packs. Coupled with the fact that none of these pavers have a chamfered edge to them. Surely if the screed is accurate and everything is compacted evenly then this difference between thickness will continue to show? As I said it is only the square blocks (and all of them at that) that sit lower....
Posted: Tue Jan 07, 2014 11:08 pm
by lemoncurd1702
Trouble with stone as a bedding layer is that It doesn't have that flexibility to allow the thicker blocks push further into the bedding layer as sand does.
Usually a stone dust or grano as some call it is used on large commercial projects where it will be trafficked by machinery before compaction.
I would always advise pre-compaction of a sand bed but not stone.
Have you sanded the joints? If not is it practical to lift the offending blocks and chuck a bit extra underneath and run wacker plate over again. Pain in the ass I know depending on area size.
Posted: Thu Jan 09, 2014 12:19 pm
by Paul.Able
Nightmare scenario mate, 3 packs of the square blocks. I'm going back to re whack it tomorrow and I'll see if that does it. Failing that it might be time to re lay.......
Posted: Tue Jan 21, 2014 9:50 am
by Tony McC
As many others have said, at ±2mm, the blocks are within the rather generous tolerance permitted by BS EN 1338 (can you tell that the standards cttee is stuffed with representatives of the manufacturers???) and so the chjances of a successful claim against either supplier or manufacturer is almost nil.
There are various reasons why such stark differences occur - mis-matched moulds or blocks from different factories are the most common cause - but it is a bloody nightmare to lay them when you get landed with a pack or two of over/under size blocks.
A huge part of the reason why we use a laying course is to accommodate such differences ( see the images on the Screeding page, but, if you check a few blocks when first delivered and before the whole pack/bale is opened, you often find most suppliers will take back packs which can be shown to be 'awkward to lay'. The fact that they then try to dump them on some other poor unsuspecting sod is unfortunate but not really your problem.