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Help to select and lay concrete reinforcing mesh

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choochoo_baloo07/08/2018 14:04:32
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Following on from my previous thread about a concrete workshop floor I'm closet to pouring, I'm after some clear advice on (a) choosing and (b) laying steel mesh in the concrete.

After some browsing, I'm getting all materials from Bradfords, https://www.bradfords.co.uk/building-materials/builders-metalwork/steel-reinforcement

1. I know others recommended A142 in the previous thread, but given the increased choice of meshes, I wanted to double check this is still most suitable for a 3 x 5 metres by 5 inch deep workshop floor.

2. I'll need approx 1.5 meshes to cover this floor area. This leads me to ask if there's a technique for how/where to cut mesh (I have a cutting disc fitted angle grinder for that)?

3. How should the two mesh piece be joined, if at all?

4. Does the mesh touch the shuttering, or is there a specified offset in from the shuttering?

And advice gratefully received.

Edited By choochoo_baloo on 07/08/2018 14:05:33

Edited By choochoo_baloo on 07/08/2018 14:05:52

richardandtracy07/08/2018 14:54:30
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I cannot claim to be a structural engineer, but this is what I did when re-building a Victorian outhouse whose foundations had failed.

I used two sheets of the 6mm bar stuff (SRF001). these I trimmed to length with an angle grinder & cutting disc. I put them into the concrete after pouring and knowing the concrete would be in bending with tension on the top face, so I pressed the steel in less than 1/3 the concrete depth. Did not tie the steel together, but overlapped it by 4 holes in the mesh - about 2 ft. Edge distance, knowing the edges were likely to be wet, was 3". Less than that could lead to enough oxygen to be present for corrosion.

All in all the slab ended up thicker than expected because the truck had 2.5m^3 when we'd ordered 1.5, and was about 14" thick, so could have done without any steel. However, the old building had lasted 120 years on a 2" thick lime mortar foundation, so how long will this version last with a 14" steel re-inforced foundation? At least as long I hope.

Regards,

Richard.

Zebethyal07/08/2018 14:58:45
188 forum posts

The guys who did the floor for my pre-fab garage 14' x 21' ( 4.26 x 6.4m ) did the floor in two halves and put two pieces of A142 mesh in each half. The pieces were 3.6 x 2m in size and they just lay them in fairly roughly on top of the hardcore then poured the concrete. The mesh had a small amount of overlap, but the pieces were not tied together, nor did they touch the shuttering.

JasonB07/08/2018 14:58:52
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A142 will be fine

Angle grinder to cut

Overlap sheets 150mm and a few tying wires to keep put or nylon zip ties will do.

40-50mm cover around the edge

bricky07/08/2018 17:04:04
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As previously stated buy a concrete slab and break it into small pieces and prop 142 mesh on this.A word of warning about cutting with an angle grinder make sure you are clear of any windows as I have had to replace window glas when the red hot metal sticks to it.The wind can carry them a long way which I know to mt cost.

Frank

choochoo_baloo13/08/2018 20:21:53
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Thanks all for the info.

New question please: is it acceptable to overlap damp proof membrane (DPM), or should I cut a single piece from a bigger roll (which would obviously be very wasteful)?

Assuming overlapping is fine for the finished floor

(a) is 150mm overlap correct?

(b) how should I stick the overlap together?

JasonB13/08/2018 20:33:39
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150mm minimum lap and tape with this

richardandtracy13/08/2018 20:46:18
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Consider buying a sheet that's big enough. Quite a few sizes are available. For our 12x 13 ft building we got a sheet 14ft X 16 ft. Was available as a cut size from our local builder's merchant.

Regards,

Richard.

bricky14/08/2018 00:28:19
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Just lay one mesh to lap the other you don't need to cut the mesh or tie it together the concrete will do that job.

Frank

bricky14/08/2018 00:29:15
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Just lay one mesh to lap the other you don't need to cut the mesh or tie it together the concrete will do that job.

Frank

Speedy Builder514/08/2018 07:43:09
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Sorry, but Ignore Richard and Tracy advice - That is wrong. The steel needs to be in tension and at the bottom of the slab ie: put some spacers (broken thin paving slab) on top of the DPM (Overlaped at the joint) . Put your mesh on top of the spacers (Overlap the mesh at the joints) and wire together if you want with bare iron wire (Not essential but keeps things tidy). Pour your cement and tamp down well. Now make sure the cement sets slowly. If you want a smooth surface, polish it with a float once the initial set starts - you will have to walk on the wet cement to do it, so it needs to be hard enough to do that. Professionals use a power float, but you won't need that.

You may have to cover the cement to stop it drying out too fast and cracking - even flooding it with water if its hot. Cement will cure under water, so don't be afraid to use plenty.
Apologies to Richard
BobH

not done it yet14/08/2018 08:16:22
3248 forum posts
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Nay, ignore the posts which are not referring to damp proof membrane. Damp proof membranes are plastic sheets, not steel in any form.

Farmboy14/08/2018 09:21:34
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Over half a century ago I spent a lot of time digging out and concreting a barn floor and a large area of yard, working alongside a farm worker who never broke into a sweat but had a phenomenal workrate I couldn't hope to match. For the 45ft x 20ft barn floor we hand-mixed with shovels; this is probably why I now suffer chronic lower back pain sad

We didn't use reinforcing mesh anywhere but we did prepare a good solid hardcore sub-base. None of this area has any signs of serious cracking despite heavy traffic, including tractors and lorries. The base preparation is the critical thing . . . my father was a civil engineer.

Mike.

P.S. I seem to remember buying cement at 3/6 ( 17.5p ) for a hundredweight bag and a 15 ton load of ballast for £1 per ton crying 2

Edited By Farmboy on 14/08/2018 09:27:28

richardandtracy14/08/2018 10:32:30
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Posted by Speedy Builder5 on 14/08/2018 07:43:09:


Sorry, but Ignore Richard and Tracy advice - That is wrong. The steel needs to be in tension and at the bottom of the slab ie: put some spacers (broken thin paving slab) on top of the DPM (Overlaped at the joint) . ....
Apologies to Richard
BobH

BobH, in my situation it was not wrong. I made a hole in the ground the full size of the building footprint and the slab went under the walls, Being supported in the middle of the floor with minimum weight there and maximum weight at the edges will make my slab hog should there ever be sinkage due to the extra pressure from the walls, so the centre of the slab will be in tension. I did know what I was doing, I simply had not fully explained the situation because it was not relevant to the point I was making. I did not advocate putting the mesh near the top of the slab, just said that was what I did.

When it came to the DPC, I lined the hole with it, then tucked it under the first layer of bricks when building the wall. Also put a further DPC 3 bricks up the outer skin of wall. When it comes down to it, in my case having a building with any form of slab will be better than what was there - and that lasted at least 100 years. In fact a number of the bricks that we re-used had paint inside the wall we dismantled, indicating those bricks are now in their third building at the very least.

Regards,

Richard.

choochoo_baloo15/08/2018 23:55:07
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A new question chaps:

Should I lap the DPM so that its edge is flush with the top of the concrete slab? I've sketched what I mean below:

dpm lapping.jpg

JasonB16/08/2018 06:59:42
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Turn it over the top of the sleeper and tack it down. If you are not building the walls off the sleeper then trim it off once concrete set. If you are building off the sleeper then leave in place and put DPC on top.

If you try to cut it flush before concreting it will more than likely move down with weight of concrete.

 

Where do the walls actually come as may affect where you turn it up.

Edited By JasonB on 16/08/2018 07:01:12

Martin Johnson 116/08/2018 09:48:22
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Your sketch does not show a layer of blinding sand. If you use sharp hardcore, the weight of the concrete will puncture the DPM on sharp bit of hardcore - leading to damp patches which will be impossible to fix.

As to turning up edges of DPM up and trimming after the concrete has set, I second Jason B's recommendation.

Martin

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