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Posted: Fri May 01, 2009 9:29 pm
by henpecked
Right, now you all can get an idea of how much I dont know :p
Im doing a 'Tommy Walsh' and building the ultimate workshop 
On marking out my new base, theres an already laid slab (8x4) in the middle. My new base will be 7 x 2.4 x 3.7 mtrs (i know, but its an odd garden) Now , when I pour the new slab (125mm) this will be covered with a 2" layer of concrete.If I put rebar mesh in it, will this be enough depth to cover? Is it likely to crack? is there any solutions you can see to get me over this one?
Its a sound base with no movement, I really don't want to have to break it out as:
1, its v v thick
2, I can only get a mini mix wagon up the back, with this slab in the base will keep my order under 3 cube.
3, Will be a barrow job to skip for hard core
so fellas .... what do you think?
HP
Ps Ive heard you have to sledge the base to break it up, is this a definite?
Posted: Fri May 01, 2009 9:42 pm
by flowjoe
Did you use the terms `Tommy Walsh` & `Building The Ultimate Workshop` in the same thread ?
Build a bunker HP, there is a load of sh1t heading your way from this forum :;):
Posted: Fri May 01, 2009 10:07 pm
by henpecked
Posted: Sat May 02, 2009 8:31 am
by worldofpaving
HP,
50mm caps are a little problematic basically due to shrinkage and curl - concrete shrinks as it "dries" (hydrates) and does so from the edges in (a bit like an old piece of bread) and it's the weight of a 75mm cap that helps to minimise this.
Do not try to bond the old and the new slab - pouring on poly as a bond breaker, especially in the warmer weather, will be better. Get the ready mix people to use as high a cementitious replacement percentage as possible, good quality 20mm aggregate and a good superplasticiser to keep the water / cement ratio as low as possible.
Some A147 mesh will be fine, also use ask the RMC for some polypropylene fibres, they will help a lot.
Finally, try to pour on a cloudy, dry, windless day and you should be OK.
Oh, and don't sledge it. (Sorry, Tommy).
Best,
WOP
Posted: Sat May 02, 2009 12:56 pm
by henpecked
Cheers WOP,
I always knew Tommy was a bit of a bodger 
Thats a real concise reply, thanks alot
HP
Posted: Sat May 02, 2009 2:10 pm
by henpecked
just a little point. You said the caps a prone to curling.This slab (I want to keep) is in the middle of the new slab, so there will be at least 125mm depth for a good 600mm all the way around the sides and for 1000/2000mm front and back. Still 50mm on top of the slab, will the fibre additive stop curling/cracking if I go ahead at this depth?
Does the thicker slab all round the existing make a difference?
Cheers
HP
Posted: Sun May 03, 2009 2:37 pm
by henpecked
Anyone know if 50mm cap on existing base will be less likey to crack if the mix has fibres and the base is 125mm all round the old one?
Cheers
HP
Posted: Sun May 03, 2009 6:16 pm
by worldofpaving
HP, just to clarify, do I understand correctly that you are making a continuous pour the bulk of which will exceed 50mm and that the 50mm will just be on the old base?
Thin pours are all about minimising shrinkage, fibres help with this by promoting a steady and even bleed, combine this with avoiding classic shrinkage weather and high w/c ratio mix and you will have a good chance of all being OK especially in a relatively undemanding application.
Best,
WOP
Posted: Sun May 03, 2009 6:38 pm
by henpecked
Ah!
Thats what I wanted to know
Just occurred that the perimeter of the pour would be much thicker and quite wide (comparatively) just thought it may have made a difference with the cap depth (would have negated cracking/shrinkage due to weight)
Cheers WOP, you cant find this type of query anywhere on tinternet
Cheers!
HP
Posted: Mon May 04, 2009 11:00 am
by worldofpaving
HP, your instincts are spot on, for slabs on grade it's customary for the edges to be thicker than the centre for just this reason.
To generalise, concrete is very strong in compression but weak in flexural strength so much of the slab depth usually specified is to overcome this and, as previously discussed, curl - as you are pouring on an existing very solid base, this will provide the flexural strength needed so minimise curl and preferably keep the slab floating (ie. not bonding to the existing slab) and you should be in business.
You are really not looking for huge compressive strength here so get your RMC to supply a mix with as little cement and as much replacement as possible, this will minimise the tendency to curl.
Glad to help,
Best,
WOP
Posted: Mon May 04, 2009 12:20 pm
by GB_Groundworks
i thought rmc got bought out by the mexican cemex group?
Posted: Mon May 04, 2009 4:34 pm
by worldofpaving
Used as an acronym by many for ready mixed concrete, GB, but it wouldn't matter if you rang the same number, they still make concrete......
