Selecting and installing steel mesh for a concrete pour

[choochoo_balooParticipant @choochoo_baloo]

Following the helpful comments on my recent new workshop floor themed thread, I though it easier to start a new thread on getting advice on reinforcing concentre for a workshop floor.

I've received a quote (£500 for 4.8 m^3) from a well established and fully accredited concrete supplier. They confirmed that:

RC32/40 is suitable for a workshop floor, The floor must be reinforced with steel mesh for the machines to sit on (A structural engineer would provide the necessary detail for this)

I'm not prepared to add yet more money by getting a professional inspection from such an engineer, so I'm hoping some knowledgeable members can answer the following:

1. From the common meshes like A142, A393M, which is most appropriate?
2. For raising the mesh off of the hardcore, can any stones or brick pieces be used. Not sure if it'll affect the pour in some way.
3. Are there any other major considerations I've missed?

Thanks in advance.

[choochoo_balooParticipant @choochoo_baloo]

[Ian S CParticipant @iansc]

I would think that the local building regulations will specific the grade of reinforcing mesh required, that's the way it works here. The little plastic cones hold the mesh at the required height, placing the mesh on other things may not stop it falling off as the concrete is poured. Compaired to the price of the over all job, the price of doing it right is well worth it I would think.

Ian S C

[Gordon AParticipant @gordona]

For raising the mesh off the hardcore, maybe something like these:

https://www.ebay.co.uk/itm/8x-Concrete-Reinforcing-Mesh-Supports-Chairs-per-1m2-90-100mm-Gradeplate-spacer/111150236800?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D49130%26meid%3Dd7fe4895e7934ce193341c5dacee98b6%26pid%3D100623%26rk%3D5%26rkt%3D6%26sd%3D253664722570%26itm%3D111150236800&_trksid=p2047675.c100623.m-1

(No personal connection to the supplier)

[EmgeeParticipant @emgee]

RC32/40 and A142 are both suitable for your floor, you could use A393M but it is much heavier gauge and most likely OTT.

You haven't mentioned any damp proof barrier below the concrete, it may be advisable to have one.
If you are installing the mesh at 40mm or so from the base specially shaped concrete blocks are available to support the mesh during the pour and finishing process.

Emgee

[not done it yetParticipant @notdoneityet]

4.8m^3 seems a lot of concrete for a 5m x 3m base. Are you laying an apron outside it, as well? Or are they internal measurements? Something seems amiss.

I think I would opt for reinforcing fibres, if the sub-base is properly constructed.

[Bob AstillParticipant @bobastill76884]

when they did my base 30×10 they used concrete with fibres in and sheets of mesh twist wired together sat on bricks

Bob

[BazyleParticipant @bazyle]

If it's 9 ft wide every yard of length is a (cubic) yard of concrete. 5 yards long so you want 5 yards of concrete. Metricate the lot and you want 5m^3. That's about 3-4 bulk bags according to one online gravel calculator which might be a bit optimistic as I don't think you get 1m^3 per bag really. Bit tricky as it is sold by weight. As it is coming up to summer holidays have you considered a couple of nearby teenagers and a belle mixer?

142 is 6mm, 393 is 10mm but 252 is 8mm. I think 6mm will do for small machines. You might step up to 8mm if you are planning a T&K No2. There is a lot of variation in cost between suppliers and for going up in gauge also you will be doing some cutting up to fit your shape so might want to get the angle grinder sharpened.

You want the mesh at 1 in off the bottom so make your own spacers and pre-fit them every 2ft using the wire ties I mentioned on the other thread.

[JasonBModerator @jasonb]

4.8 cu meters would give a 300mm thick base, do you have very large machines?

Bulk bags are generally 1ton not 1 cu m. generally 1.6 ton to a cubic meter.

A142 will be fine for hobby size machines

[Raymond AndersonParticipant @raymondanderson34407]

Something not quite right here, If your base is 3 m x 5 m as in the opening post on another [ related ] thread then a depth of 150mm [ which is more than enough, unless its a DSG lathe and a K & T mill ] would only require 2.25 Cubic meters.. God knows where the 4.8 cubic meters comes in ! Anyway the numbers Rc 32 /40 is the strength in newtons [ first number ] and 40 is the slump [ which is the water content ] Don't know where some folks get this 1 inch off the floor for the mesh, structural engineers always specify 40/50 mm. It is exactly the same with a precast [ not prestressed ] lintol, the bottom bars are [ or should be ] 50mm up from the bottom. same for a floor. A 50mm paving slab is good for the spacers, no need for any plastic supports. just cut or break them into pieces about 2 inches square. lay them on top of your DPM about 500mm from each other in both directions, then lay your mesh onto your spacers,

[OldironParticipant @oldiron]

Totally agree with Raymond Anderson. Either you are having a base that will support the houses of parliament or someone has conned you on the required amount for your base. Its very basic maths here. ie length x width x height gives a cubic measurement. The builders who laid the floors for my extension stood the mesh on pieces of thin brick paviors about 45mm thick.

regards

[Gordon AParticipant @gordona]

If you are planning to fasten your machinery to the concrete floor, have due regard to where the reinforcement is situated in relation to the fixing points. Drilling reinforced concrete and hitting the steel reinforcement can be a real P.I.T.A. !

Gordon.

[Speedy Builder5Participant @speedybuilder5]

To avoid disappointment, do enquire about concrete curing and shrinkage cracks. My ready mix was like an endothermic experiment and in spite of covering with a plastic tarpaulin and flooding with water, several cracks appeared. Not a major problem for a garage base 20 square metre, but for a workshop !! And I laid 6mm weld mesh 20mm up from the damp proof membrane. I have seen concrete laid and sprayed with a PVA solution as soon as it was floated off – I wonder what that does ?
BobH

[brickyParticipant @bricky]

50 years of building experiance .Five inches of concrete will be adequate for your base with 142 reinforcement mesh set on 2" of broken concrete paving slabs set on top of the DPM ,also it would be a good idea to have expansion strips around the edges.

Frank

[BazyleParticipant @bazyle]

Hands up to a huge error in my first sentence in last post. 3x less volume than I said. The info for bulk bags given by Jason will be useful for me planning to mix my own.

Still going for the 1 inch off the bottom for the iron.
Think about it, The purpose of the reinforcement is to provide the strength in the part in tension. In a 4 in slab putting it 2 in up is sod all use as that is the neutral point. Structural engineers are dealing with 8in slabs on 24in beams in bridges and skyskrapers not tiddly little 4 in shed bases. There the extra inch makes no difference to strength but allows greater fire protection (a major consideration in big buildings), corrosion resistance in structures designed to last 99 years not your 15 years of retirement and lessens chance of voids where no attention will be given to pushing the slurry under the bars.

[choochoo_balooParticipant @choochoo_baloo]

Sorry chaps, my initial post was written in a hurry, and I now realise why confusion arose. I thought it best to post the amendments here rather then start another thread.

• The base will be 5 x 3 metres. Taking 5 inch depth givels volume = 2 m^3
• The site is bound by: one brick wall, gravel board fence, and two old treated oak sleepers (each set in concrete pads levelled and perpendicular).

 

Having also read the helpful comments above, I want to clarify the following:

1. Given the corrected volume, is A142 mesh still most suitable?
2. Is it preferable to use a plate compactor to get the hardcore as dense as possible?
3. Is there any reasons *not* to include a damp course?
4. Do 'expansion strips' refer to physical gaps between adjoining structures. Or is a spongey filler used?
5. Should I worry about control groves scored into the wet concrete, or is this size not going to require it for crack control?
6. I have some warehouse floor grade 2 part epoxy to use up. From experience, concrete gets dusty in time. To help the epoxy key, I assume I don't want an ultra smooth bull-floated surface?
7. Should I have a tarpaulin covering, so that it doesn't dry too quickly especially if sunny?

Thanks again for the help.

Edited By choochoo_baloo on 20/07/2018 01:39:33

[JasonBModerator @jasonb]

1. 142 OK

2. yes

3. Not really needed if standing a shed with its own floor on the slab but in your case use DPM

4 Foam "flexcel" or similar

5 Proabaly not

6 Use the correct primer

7 Sheet of poly will be less likely to mark the green concrete

 

8. In the other thread you mentioned pumping, your 40mm slump mix may well be too stiff, a pump mix will be more liquid and to get that without weakening the cement content is higher or plasticisers added which puts the cost up.

 

If anyone wants to see some impressive concreting then have a watch of the Super Sewer program that was on earlier this week, very impressive continuous pour of the massive vertical shaft lining.

 

Edited By JasonB on 20/07/2018 07:56:46

[JasonBModerator @jasonb]

Baz, the mesh is not used to make the slab into a beam, more to just stop it from cracking.

What if you put a heavy machine at each end of your workshop? Your mesh 1" up will now be in compression as the top of the slab will be in tension!

[Martin Johnson 1Participant @martinjohnson1]

I really would not go to the trouble of re-inforcing mesh. Put in 100 mm gravel, well tamped down. Put in 50 mm sand, well tamped down and raked level. Place 1 damp proof membrane over it and up the sides of any formwork or earth (if below ground level. Put in 100 mm of C20 concrete tamp down, strickle and allow to go "green" and then trowel off. Seal with proprietary sealer PVA or similar when dry.

At 3 m width, you might need a screed rail down the centre line to make strickling the concrete easier. Plus profile boards around the edges, of course.

That spec has supported my 1/2 ton lathe (Kerry AG) (plus a lot of metal in the cabinets below) plus a Tom Senior M1 and loads of junk in two workshops now for a total of around 30 years.

Martin

[not done it yetParticipant @notdoneityet]

Well done JB. You, too, can do simple maths. 4.8m^3 would give a 300mm average thickness. Of course, there should be a ‘toe’ around the base, which would add an extra, say about another 3/4 of a m^3. But the original thread was, to say the least, fairly woolly . 150mm thick should be more than sufficient.

Compression and tension? If the site is prepared correctly and the concrete laid properly, there should be no real issue with a single storey structure, even in brick. Reinforcement should, indeed be in the top half – that was one reason why I went for fibre reinforcement (s/s needles were an expensive option back then).

25 Newton concrete should be adequate for a structure such as this.

When I worked in the cement/concrete industry we regularly designed and checked concrete mixes for customers. Ordinary Portland cement, according to BS12, usually provided 28 day strengths of around 38N/mm^2. We never reported results far from that value, although the readymix and pre-cast customers would have known that these results were ‘massaged’. Simple checks on standard deviation of results, from the required certificates generated, showed a clearly insufficient range of results!

But all of the cement industry were in bed together on the subject of cement testing and concrete strength.. Each company supplied ‘average’ monthly cement samples, from each of their works, to other manufacturers for strength testing. After spending almost 20 years of my working life in the cement industry, on the technical side, I know a bit about cement production.

Building in the fens needs a different approach than on a granite rock. The OP gives no indication of the surface geology.

My sectional concrete garage has been standing for 25 years and an older one, in brick, for 40 years. There are no cracks in the bases. I did screed the floor of the sectional one, to prevent any water ingress at floor level, even though it did not need screeding across the full width – it provided a very smooth and level surface for the contents.

[MichaelRParticipant @michaelr]

I'll second Martins method it's all you want, I did basically the same method for my garage years ago and it is still sound, but if I did the job again I would hire and use a concrete poker (vibrate) it saves a lot of tamping and consolidates the mix. Mike.

Edited By MichaelR on 20/07/2018 09:06:09

[SillyOldDufferModerator @sillyoldduffer]

I agree with NDIY and JohnMcN (in the other thread) about the importance of the underlying geology. My back garden and the rest of the estate are built on an in-filled quarry. 'Made ground' is poor support and all the houses are built on hefty concrete rafts, not conventional foundations. The purpose of the raft is to distribute the weight. If more than 600mm of fill is under the floor, it should be designed to be self-supporting, ie as if nothing was supporting it underneath. The need for rafts was amply demonstrated some years ago when a large hole appeared in the road one night. About 5 metres deep and 10 metres across it was due to the collapse of an old van dumped and covered over 20 years before the houses were built.

On the other hand my mum's house is on a hilltop with solid rock about 500mm under the grass.

The weight of the machines might also an issue. My lathe and mill weigh about 270kg each, and the weight is distributed over about half a square metre. This is a moderate floor loading perfectly acceptable in an ordinary garage. (A garage floor has to support a ton or two of car with the weight concentrated under the wheels.) Much heavier machines, or heavy machines on a small foot-print, might well need a thicker floor. Another reason for extra thickness : choochoo mentions bolting his machines down, and the concrete should be deep enough to take the bolts without penetrating the DPM. Shallow bolts will stop machines moving, but if the purpose of bolting down is to soak up vibration, the bolts need to be bigger and deeper and the floor needs to be heavy, ie thicker. The floor also needs to be thicker if the load moves about (fork-lift truck?), or has a hammer action.

Is a DPM necessary? – maybe! They keep damp out and stop the concrete being attacked chemically in areas where the geology makes it necessary.

It's an interesting problem. Personally, if I wasn't confident about the ground and owned unusually heavy kit, I'd send for a structural engineer. But the hobby machines I own would sit happily on the sort of informal pad you might put under a garden shed – a few inches of stones with a few inches of unreinforced concrete slapped on top.

For most of us, a garage floor (at least 100mm thick) is more than adequate. If there's reason to think it might not be, send for an expert! No harm in over-engineering a floor; it will just cost more than necessary.

Dave

Edited By SillyOldDuffer on 20/07/2018 11:04:54

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