Page 1 of 1
Posted: Thu Feb 01, 2007 4:50 pm
by Wildfang
Tony, LLL (anyone else)
Starting a new string as this is not really related to my last questions. Nearly ready to "walk the walk", but...
..I was planning to use a dry bed depth of 50 mm as that is what is used in the calculator on this site (15 m2 at 50 mm = 1.1 T). HOWEVER, more often than not a depth of 40 mm is referred to on the main site AND in the replies I got from Tony to my last posting.
Assuming 40 mm will be OK (better for me!), what quantity of sharp sand will I need for 15 m2 coverage?? Will this now bring me to the magic 850 kg number required for purchase of a single bulk bag
cheers as always
Posted: Thu Feb 01, 2007 7:09 pm
by Ted
4/5ths of 1.1 is just over 0.85.
Posted: Thu Feb 01, 2007 8:07 pm
by lutonlagerlout
when the bed gets too thin wildfang theres much more chance of a rogue crouton in the mix making your slabs go crack! crack!crack!
no point sinking a battleship for a ha'penny worth of tar
i would ver over rather than under mate
regards LLL
Posted: Sat Feb 03, 2007 1:24 pm
by Tony McC
Over the last 18 months, the 'thinking' behind bedding depth has been revised, and the general consensus of opinion now suggests that slightly less gives slightly better results in the long term, so, as an industry, we now recommend 40mm of bedding rather than 50mm.
However, due to other things happening in my life, I've not had chance to catch up on all the references on the site and update them, but I am in the process of so doing, along with re-building the whole of the Flag laying section to reflect the different techniques now used for patio flags, small element flags and larger elements. 10 years ago, when I started this site, such a task might have taken a couple of days. Now, with all the cross-links and the sheer scale of it all, it's likely to take me at least 8-10 days of work, and finding 8-10 days when I've nowt better to do is not all that easy, but it is happening, a bit at a time.
On a slightly different subject, earlier this week I updated the Block Laying pages to reflect the changes in the lastest edition of 7533:3, especially regarding compliance checks and jointing, and to provide a 'sequence of operations' flowchart. As with all trades, our methods and techniques are constantly under review and we are always on the lookout for ways to improve our trade, all of which means my work on this site will never be completed.
Posted: Sat Feb 03, 2007 9:23 pm
by Suggers
I'm amazed that 10mm can be seen to make any difference in the bedding layer, given the materials we're using - does this tie in with our slabs getting thinner?
Posted: Sat Feb 03, 2007 11:07 pm
by Tony McC
The thinning of flags is largely economic; the reduction in laying course depth is more to do with the behaviour of a partially compacted granular material. Reducing laying course depth by 10mm might not make a huge difference, but it has been shown to result in less settlement over larger areas over longer periods of time.
Think about it this way: let's assume for the sake of this explanation, that a laying course will settle by 5% in the first 12 months after the paving has been laid. So, if we have a 50mm laying course, that means 50 x 5% = 2.5mm. A 40mm laying course would result in 40 x 5% = 2mm. Not a huge difference as I said, but it is and improvement.
Obviously, the less laying course that's used, the less settlement will occur, which is why, for Class I and II block pavements (the real heavy duty stuff) we now aim for laying courses of 25-30mm depth, but there's a balance to be found. Too little laying course means it's impossible to accommodate the variation in paver thickness and an uneven distribution of laid onto the sub-layers, while too much, as explained, means excessive settlement post-construction.
If you continue to use 50mm laying courses, the earth will not stop turning on its axis, but it is now considered "best practice" to aim for 30-40mm.