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Posted: Wed Nov 17, 2010 5:26 pm
by neil_forster
hello, im planning a concrete base for a timber workshop. approx 4.8mx 9m. i was planning on digging to formation level then 100-150mm compacted hardcore then 150mm concrete possibly fibre reinforced. i was going to get it ready mixed but was wondering if i will need some joints i a slab this size? if so how do you manage that- would i have to shutter and fill stages at a time calling the readymix lorry each time or is there a simple way??
thanks for any help.
Posted: Wed Nov 17, 2010 6:27 pm
by lutonlagerlout
i think at 150 depth you could pour that in 1 go
you would need a full load I.E. 6M 3 if you make the concrete 140mm thick
remember to blind the hardcore/type 1 with sand and lay a 1200 gauge DPM on this,also i would favour mesh over fibres,or even use both
I am sure the gaffer has a calc somewhere for size of slabs but with a timber workshop 140mm of reinforced C30 concrete should be ample
expect to pay between 5-600 quid for the concrete
regards
LLL 
Posted: Wed Nov 17, 2010 8:14 pm
by neil_forster
thanks for that, i was also going to put terram under the hardcore , do i support the mesh on bricks in the middle of the slab or is there something more technical? does the mesh need wiring or welding together??
i have not use reinforcement before,
thanks
Posted: Wed Nov 17, 2010 8:44 pm
by Mikey_C
this thread maybe useful also look on the main website about concrete hard standing and laying. the layers type 1, sand , dpm, concrete, mesh fibres are all explained in terms of there use, purpose and handling.
but the mesh needs to be tied with wire, blocks or bricks *can* be used to support the mesh, plastic or concrete seats/castles/stand/supports are better and detailed on the web site.
as is the precautions and PPE that need to be used when dealing with readymix concrete.
Posted: Wed Nov 17, 2010 8:59 pm
by GB_Groundworks
use 50-60mm concrete flags broken up or get some chairs or mesh mates when you get your a142 mesh, over lap 2 squares and fis together with steel fixing wire, if you haven't got any then cable ties will do for your slab. how you finishing for your workshop? brushed, trowled, rough tamped etc?
its a big slab for a diyer, but you'll get it good enough for a shed.
use concrete commons not clay bricks
Edited By GB_Groundworks on 1290027612
Posted: Wed Nov 17, 2010 11:31 pm
by Ted
I would just use mesh - fibres are a bonus if they don't cost you much more; but fibres don't add as much structural integrity to the slab as mesh does.
At 150mm deep, I generally wouldn't bother with a cut, but you could cut one in the following day to make two slabs of 4.8 x 4.5 and fill with silicone or something.
I cast my own chairs using sharp sand and cement or use broken bits of ceramic tile to raise the mesh in anticipation of pouring.
You should give a decent overlap to the mesh and tie the sheets securely to each other. Tie wire costs nothing and should be done well. If you have two sheets tied and pick one up, there should be no give where they are tied...
Posted: Thu Nov 18, 2010 12:27 am
by GB_Groundworks
tie wire cost nothing but normally gets thrown in free with big rebar orders but they give you like 3000ft rolls bit much for a little diy job.
travis do mesh mait plastic chairs, 10p each or we use ROM and get the concrete ones.
Posted: Thu Nov 18, 2010 9:30 am
by neil_forster
thanks for your help guys, im not looking forward to it, the last concreting job i did was to level the sloping floor of a 7x15m pig unit with screed, and it was an awful job, however im thinking that as this is outside and good access all round the formwork it should be simpler.- i hope!
ive got some old slabs i could crack up to support the mesh.
just out of interest how much would it cost to get a groundworks firm in to do the job?
Posted: Thu Nov 18, 2010 10:46 am
by Tony McC
The rule of thumb for slab span is "30 times the slab depth", so for a 150mm deep pour, the max span without a movement joint is 30 x 150mm = 4500mm, so you need a single central joint to split that 9m span into 2 bays at 4.5m
The fact that the slab width at 4800mm is 300mm over the max is not worth worrying about.