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Posted: Wed Nov 21, 2012 2:57 pm
by Adrian80
Can I ask another dopey question.
What is the laying course for?
If you can get a base nice and smooth do you need one.
Is it a drainage issue ?
Or is it cushioning for the block?
If its to thick does it increase the chance of rutting?
Would a 15mm sand bed be a Bad thing. I see the recommended is 35 to 50
Just interested in learning.
Cheers adrian
Posted: Wed Nov 21, 2012 4:34 pm
by Carberry
Adrian80 wrote:Can I ask another dopey question.
What is the laying course for?
If you can get a base nice and smooth do you need one.
Is it a drainage issue ?
Or is it cushioning for the block?
If its to thick does it increase the chance of rutting?
Would a 15mm sand bed be a Bad thing. I see the recommended is 35 to 50
Just interested in learning.
Cheers adrian
1. For laying the blocks on
2. Yes, always need one.
3. So the block is fully supported. If there are any voids underneath a block it makes the paving weak and susecptible to breaking / sinking.
4. Yes
5. Yes. Makes it more difficult to lay because of slight variations in thickness of blocks and depths of your sub base. Over 1m2 it might be easy enough to get a perfectly flat base compacted to an even 150mm but when you stretch that to an awkward shape over 100m2 then you're going to have a hell of a time keeping your sub base at a depth uniform enough to allow for only a 15mm bedding layer
Screeding FAQ
Posted: Wed Nov 21, 2012 5:23 pm
by Adrian80
Thank carberry,
It all seems a bit obvious, but i thought i would ask.
Cheers
A
Posted: Wed Nov 21, 2012 6:48 pm
by Carberry
Adrian80 wrote:Thank carberry,
It all seems a bit obvious, but i thought i would ask.
Cheers
A
It is obvious when you've been doing it for years. :laugh:
If in doubt always best to ask rather than plowing on blindly and making a mess of things.
Posted: Wed Nov 21, 2012 10:09 pm
by Adrian80
Ain't that the truth, nothing like a bit of practical experience, all the theory in the world is no substitute for time on the tools. 
Posted: Thu Nov 22, 2012 8:26 am
by Tony McC
The laying course is also a logical consequence of the 3,000+ year old principal of using progressively finer (ie:smaller) grain sizes as you approach the upper surface of a pavement. This gives structural integrity, which means a stronger, more interlocked, better performing pavement. We see this principal at play in almost every type of pavement construction, be it macadam, blocks or resins.
However, for block paving, the smaller grain size of the laying course allows us to create a more accurate profile and so the final level of the blocks is more true. It's easier to achieve a ±3mm tolerance using a gritty sand with particles of 4mm or less than it would be using a relatively chunky crushed stone with particles up to 30mm or more.
With the best will in the world, and even with years of experience, it's damned near impossible to achieve a tolerance of ±3mm using sub-base material in a reasonable time period, whereas it can be done in minutes using sand or grit.
There are other benefits: it *does* aid drainage and it *does* provide cushioning for the blocks. In particular, it's good at accommodating the minor variations in block depth that can occur with even the very best manufacturing techniques.
And as you suspect, too much can lead to rutting or channelisation, mainly because the smaller particles are more likely to behave like a fluid when saturated and under load. However, you thickness values are awry: the spec *used to be* 35-50 but we've since realised that there is less risk of rutting and other problems using a slightly shallower bed, so now the official spec is 25-40mm. Anything less than 25mm runs the risk of not providing adequate cushioning and being awkward to prepare, whereas once you get more than 50-60mm, the risk of unwanted movement is very real.
Posted: Thu Nov 22, 2012 2:43 pm
by Adrian80
Thanks Tony, Very useful words of wisdom.
I think these key points would be a very useful addition to the main site.
This is bloody marvelous resource.
Cheers A
Posted: Fri Nov 30, 2012 6:14 pm
by hitman
On the subject of laying courses, can you lay permeable block paving on a conventional sand laying course? I have a base that already prepared up to the sand layer. The subase contain land drainage/french drains. Is there any reason why the I can use permeable block paving. I'm not expecting the system to be fully permeable.
Posted: Fri Nov 30, 2012 6:45 pm
by Tony McC
As long as you accept that it will have initially limited and ultimately doomed permeability, then there's no strutural reason why they can't be laid on sand. After all, they are just concrete blocks with over-sized spacer lugs.
Other than by accident or incompetence resulting in it being taken up and re-laid correctly, I haven't see it done this way, but my best guess would be that the joints will become a veritable wildlife have for mosses and the like. In fact, you'll probably have to chase David Attenborough off the drive, as the open joints provide a propitious and safe haven for weed seeds while the sand beneath acts as a filter mat trapping all those lovely nutrients. They'll love it!
Posted: Fri Nov 30, 2012 10:01 pm
by lutonlagerlout
even the proper permeable paving ends up like this from what i have seen
LLL
Posted: Sat Dec 01, 2012 9:37 am
by Tony McC
Yeah, but it takes that much longer