BenB
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| posted on 25/9/09 at 01:34 PM |
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OT: laying a concrete slab
I'm planning some underfloor heating in a single room in my new house and I've got to lay a slab. Not too big. About 3m*3m at 75mm thick.
Trouble is it's at back of the house and there's no access from the back. So how do I get the concrete in? I don't really want to
start mucking around with concrete pumps. But can I mix it in 85L batches (capacity of a 240v HSS mixer) and dump it in batches? HSS say you can mix
85L in 7 minutes so there wouldn't be a long wait. Or does it need to be laid in one go?!?!? When I laid my garage floor I did it in multiple
loads (bucket at a time over a few months) and no problem so far. But if my garage floor breaks up it's no biggy, I'd be a bit more upset
if it was my kitchen!!
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designer
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| posted on 25/9/09 at 01:36 PM |
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Idealy concrete floor have to be laid in one go to keep the unit as one piece and solid.
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richardlee237
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| posted on 25/9/09 at 01:49 PM |
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If you have a mate to keep loading the mixer, then no problem. I have laid shed and garage bases using a mini mixer and they haven't broken up.
Keep finishing as you go and don't let it dry out. These are good months for laying concrete
You could build the whole garage for the price of the ready mix and a concrete pump !! Well, almost
Quote Lord Kelvin
“Large increases in cost with questionable increases in performance can be tolerated only in race horses and women.”
Quote Richard Lee
"and cars"
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BenB
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| posted on 25/9/09 at 01:55 PM |
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Yes, I was looking at readymix screeds and they're blooming expensive......
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richardlee237
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| posted on 25/9/09 at 02:13 PM |
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Here in Yemen, the only construction done with ready mix are bridges and big civil works.
The average house is built from concrete pillars and re bar with the concrete being supplied from a mixer and a lot of guys pushing wheel barrows.
Most of them seem to remain standing. (Though not necessarily vertically !)
If all you need is to screed the floor with the underfloor pipes in it, within the existing walls, then you should be perfectly Ok.
Quote Lord Kelvin
“Large increases in cost with questionable increases in performance can be tolerated only in race horses and women.”
Quote Richard Lee
"and cars"
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will121
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| posted on 25/9/09 at 02:31 PM |
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are you insulating under as well? if doing as team work with next batch mixing whilst others laying should be ok, just like it was done in the good
old days!
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hughpinder
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| posted on 25/9/09 at 02:32 PM |
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I have cast 4m*4m*100mm using a mixer and barrow many times (bases for stables). That size takes about 3 hours. Lay a large polythene sheet uner the
mixer and so the work area is covered as its much harder if you have to try to be tidy! Get bulk bags of agregate, position the mixer so you can
shovel straight from the bag into the cement mixer and then turn the mixer 90 degrees to drop in in the barrow. I put in about 2/3 of the water
needed, then half the cement,half the agregate, the rest of the cement, the rest of the agregate, and top up the water whenever the mix looks
dry(you'll have to experiment for the first batch). Final adjust of the water at the end. When properly mixed, tip into the barrow, and load up
the next mixer load so its mixing while you barrow the first load away. Dont try to do this with buckets- its too slow and too hard. When shovelling,
if you're right handed, you will probably find it easier to stand with the mixer to your left, and the big bag of agregate directly in front,
and shovel the agregate in by a right to left scooping action. When you empty the mixer into the barrow, make sure it is right up against the mixer
and tip fairly slowly or the whole load will go suddenly - remember the concrete moves up the drum acording to the direction of rotation. I've
never had a problem with floors cracking or going off too quick doing it this way. I usually roughly level the floor every 4 or 5 barrowloads, leaving
a batch mixing.
Lastly, if you have to go over soft ground beg borrow or steal some planks of ply as a barrow ful of concrete is hard enough without the wheel sinking
into the ground too.
Regards
Hugh
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hughpinder
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| posted on 25/9/09 at 02:32 PM |
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I have cast 4m*4m*100mm using a mixer and barrow many times (bases for stables). That size takes about 3 hours. Lay a large polythene sheet uner the
mixer and so the work area is covered as its much harder if you have to try to be tidy! Get bulk bags of agregate, position the mixer so you can
shovel straight from the bag into the cement mixer and then turn the mixer 90 degrees to drop in in the barrow. I put in about 2/3 of the water
needed, then half the cement,half the agregate, the rest of the cement, the rest of the agregate, and top up the water whenever the mix looks
dry(you'll have to experiment for the first batch). Final adjust of the water at the end. When properly mixed, tip into the barrow, and load up
the next mixer load so its mixing while you barrow the first load away. Dont try to do this with buckets- its too slow and too hard. When shovelling,
if you're right handed, you will probably find it easier to stand with the mixer to your left, and the big bag of agregate directly in front,
and shovel the agregate in by a right to left scooping action. When you empty the mixer into the barrow, make sure it is right up against the mixer
and tip fairly slowly or the whole load will go suddenly - remember the concrete moves up the drum acording to the direction of rotation. I've
never had a problem with floors cracking or going off too quick doing it this way. I usually roughly level the floor every 4 or 5 barrowloads, leaving
a batch mixing.
Lastly, if you have to go over soft ground beg borrow or steal some planks of ply as a barrow ful of concrete is hard enough without the wheel sinking
into the ground too.
Regards
Hugh
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BenB
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| posted on 25/9/09 at 02:43 PM |
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Cool, cheers for the hints. Glad to hear it might be okay. Yes, I'm laying some nice insulation first.... That's one of the main reasons
I'm going for UFH. It's a 1920s kitchen extension with 3 external walls and a concrete floor. At the moment it's got no radiators
either. It's like a freezer  So I've got to dig down to put down the insulation anyway and seeing as there's nowhere to put a
radiator UFH seems a nice idea...
So it's out with the heavy duty kangor hammer down a good 125mm followed by the DPM, insulation, UFH pipes and screed. Sweet  That'll be
a fun weekend!!!
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Xtreme Kermit
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| posted on 25/9/09 at 02:44 PM |
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make sure you insulate under your floor otherwise the heat just keeps on going down.
We put underfloor heating in the extension in my old place. As I remember it, it had a insulation, covered by a poured concrete block. The pipes were
then tied to some squares of reinforcing bar and dry-ish screed barrowed in over the top. Took a few days for the screed to dry - but was a good
job.
The heating took a good couple of hours to heat up, but because of the screed and the concrete sub floor it had enormous mass and continued to keep
the back of the house warm for a good part of the day.
I miss it
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blakep82
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| posted on 25/9/09 at 03:01 PM |
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to be honest, i haven't read all the replies, so i don't know what people have said already 
earlier this year, i laid a step, only 1mx1m, but sits almost entirely on top of the ground (about 4" under, 6" over. sorry about mixing
my units lol)
anyway, i did mine over a couple of days since i ran out of cement and ballast. first layer came to 3" over ground, laid the next lot 2 days
later. all knit together nicely, you can't even see the join really.
in short:
if you lay it all in 1 day, there'll be no probs at all
________________________
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don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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rallyingden
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| posted on 25/9/09 at 04:21 PM |
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Under floor heating .......... its brill
The best thing I ever did when building my extension.
Our 10 year old can often be found lying on the kitchen floor in winter :
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