Workshop floor / concrete.

[Astro_Al]

Howdy all.

Anyone know anything about concrete? I need to put some down in my new workshop. There are rectangular areas which already have concrete flooring, and other rectangular areas which are just earth/chalk (what was there before has been removed with a JCB).

Now, who can tell me something about concrete? Whats the cheapest way to get the job done? (Mix it myself / get a lorry load delivered/poured in) I'm looking at between 50 and 100 square metres.

How thick should I make it to ensure it won't crack under a large machine tool?

How do I prepare the ground for the concrete? What type of concrete do I use (are there types?!) How do I prevent it from being 'dusty' like the floor in my current workshop - which powders away when you walk across it and everything gets covered in a fine layer of dust (or is that just the passage of time... ).

Cheers, Al.

[Les Henson]

When I built the base for my inventing shed I removed the top soil, then laid a hardcore base (bricks, etc), whacked it with a plate, then scalpings to level it off (again, whacked it). The edges had to take the weight of the walls, so they are 7-8 inches thick and the rest of the floor is 5-inches of 80 slump premix. I shuttered it up with scaffold planks, then poured the concrete, levelled it with an on-edge plank, then raised the fat with a concrete trowel and left it to dry for a few days. You can add plasticiser to the mix to kill the dust, or paint it afterwards. I let mine cure for a few hours, then brushed it with a soft broom to clean the top surface. My base was only 20' x 10' and after a couple of years it developed a hairline crack, which has never got any wider (I did the job nearly 20-years ago). You can buy reinforcing mesh for about £40 a sheet, which will prevent cracking - especially if you use a poker on the concrete. 50 - 100 metres is a big difference, so either get the calculator out, or phone the supplier and they will usually work it out for you. Mixing that much yourself - even with a barrow mixer will take ages. A lorry load is normally 6 cu metres.

Two types of concrete that I know off - Sulphate resistant, which is dark grey and more water resistant, and normal mix.

Les.

[Steve Hiatt]

The grade of concrete you want is C35, expect to pay at least £75 plus VAT per cubic metre based on a full 6 or 8 metre load. The slump refers to the amount of water in the concrete, 50 is usual but 80 as Les used will be more self leveling and easier.

Consider putting a membrane in and insulation, 50mm polystyrene. Lay it about 6" deep on a compacted hardcore base, a vibrating poker is the best way to ensure you get a good lay and a tamp for leveling. You may need to use a sprinkler on the concrete slab for the first day to prevent cracking as the surface dries. The slab will be giving off moisture for about six months so any coating put on in that period should allow for this.

If you are considering mixing it yourself a cubic metre weighs 2.2 tonnes of which about 340kg per metre is cement.

Have plenty of mates round to help.

[FridgeFreezer]

Al - if it helps, I'm very busy that day

Actually, I'm hoping to begin building my shed before too much longer so was wondering the same thing (but smaller), I basically need to add an extra ~1000mm to the front edge of an existing standing, and perhaps make the whole thing a bit smoother without gaining too much height. I'd already decided I couldn't be arsed mixing it myself though

[Orgasmic Farmer]

@ 2.2 tones to the cube you are looking at costs of at least £66 in raw material if you try to di it yourself (Thats without mixing it/collecting it etc. The readymix option is much more cost effective but it does rely to some degree on how far you are from a depot and it doesn't give you many options on the day of the pour. He comes, he dumps it and leaves. You need a lot of man power and you need all your preparation perfect as its too late to go fiddling with shuttering etc after he arrives!!

[jericho]

75 square meters @ 100mm depth is 7.5 cubic meters - more than 15 tonnes of material.

Don't even think of mixing yourself.

4" depth is barely thick enough for machinery - you would need a very good compacted base.

Book a delivery - three people will do it if the driver can pour straight on to the base area (not including the driver).

More people needed if you have to barrow it.

They normally allow 20 mins on site in the quoted price,and can charge more if you delay them beyond that,but most drivers are pretty relaxed about it,an would probably not moan about twice that time.

Reinforcing mesh is necessary on a slab that size.

The higher the slump,the higher the water content,the easier to level,and the weaker the finished concrete when cured.

Stick with 50 slump and vibrate it well.

[Steve Hiatt]

The higher the slump,the higher the water content,the easier to level,and the weaker the finished concrete when cured.

Only weaker when the water is added by the driver. If you order it at 80 the mix will be adjusted when batched to keep it within the spec

[jericho]

Only weaker when the water is added by the driver. If you order it at 80 the mix will be adjusted when batched to keep it within the spec

I stand corrected.

Synthetic fibres can be add to the mix for finished floor concrete.They reduce surface cracking/crazing,and dusting.

[Astro_Al]

Wow thanks for all the great info you guys.

John - if I help you with yours... Sounds fair to me! By the way - don't worry about being busy - I can wait until you're free.

Sorry for the vagueness of dimensions I promise I'll run around with a tape measure before I order the concrete! Its hard to see at the mo as its covered in smashed up concrete and steel. Sounds like mixing it myself is a total waste of time. The truck can get right to it, so no wheelbarrows necessary either. Nice. B)

Anyone know a decent place in Hampshire (near Winchester?) who come recommended?

Steve - what is C35? (Why do you sugggest that one?)

Some concrete-retard style question arise from reading that lot:

1. Do these synthetic fibres have a name? Do they add much to the cost? Do I need to specify some ratio of fibres to concrete, or is it just with/without fibres?

2. Water. So assuming the water content doesn't affect final strength, the only difference it makes is how easy it is to level, right? (Plus how long it takes to dry I guess). Just for clarity, an 80 'slump' means 80% water, 20% concrete? What ratio should I go for?

3. Mesh. So I chuck in what, 1 layer of square steel mesh? More? At what height? I guess it doesn't just sit on the ground at the bottom, so what do I support it on?

4. Depth. So if 4" isn't really going to stand up to some heavy things, what is the 'best' depth? Is there a way to figure out what a piece of concrete will stand up to?

5. When its all done, do I need to do anything while/after it dries (sprinkle it with water? For how long? Anything else? Chemicals?)? What is the best treatment for it? Brushing?

6. Prep. So if I scrape out earth down to the right depth, then what? Compact it? Chuck in some rock type stuff? Membrane? (Is that just to prevent water seeping up through the concrete?) Would insulation really handle the weight of big tools?!

7. How do you used one of these poker jobbers, and at what point (timewise). How much is enough? Do I need to use one of those vibrating plate machines?

8. What is shuttering? (Duh!)

Man thats a lot of questions! (Sorry).

Cheers, Al

[madmatt]

Shuttering is the Timber framework around the sides that suports the concrete and gives you a level to tamp off.

A concrete poker isn't to expensive to hire and is well worth it, makes all the differance.

C35 is what we use for bases don't ask me what it stands for thats what engineers are for! We usually put are steel (RA142) in the bottom third of the concrete supported either on the proper little legs you can buy or on bricks.

It sounds like you can back up and blast straight in so should be able to shoot out and have the driver away in 20 mins. Oh top tip avoid a Friday afternoon or late Saturday morning drop as even if you've paid extra standing time they won't be keen to hang about!

As for prep, dig out area lay in hard core base and compact then level with MOT1 40mm-dust and Compact then membrane and then pour.

[Steve Hiatt]

C35 is a grading of concrete, used for industrial floors etc. The spec relates to the strength of agregate and cement content.

Slump relates to how much a pile of concrete 'slumps', in simple terms. The wetter it is, the more it slumps.

Fibres cost £5 extra a metre and you just ask for C35 with fibres. They can help prevent cracking but will reduce the amount of time you have to work with the concrete.

A membrane will stop your floor being damp and insulation will keep it a little warmer, and you won't break 6" of C35.

[steve_d]

Fridge

As yours is just an extension of your existing floor borrow my electric mixer. That way you can do it at a time and pace that suits you.

Steve

[pete3000]

http://www.exactmix.co.uk/

Specify what you need and will arrive and mix only what you need and wheelbarrow it into the foundations for you, I used this for 3 cu metres 3 years back.

max lorry load 7-8 cu metres. Great for me as shed was no where near the road for access.

about £100 per cu metre. includes 3+ large blokes for 1hour to tip it just need to level concrete yourself

money well spent with no mess or bad back. Check delivery radius, though sure there must be branches/firms around the country.

[Kim Horsevad]

75 square meters @ 100mm depth is 7.5 cubic meters - more than 15 tonnes of material.

Don't even think of mixing yourself.

If you mix and level it yourself a normal able-bodied person should be able to do 20 - 25 square metre a day working alone. Pretty hard work though.

Finished my 130 square metre workshop in 2005. Foundations 40 cm wide and 150 cm deep. Concrete floor 20 cm thick, with two layers of 8mm mesh reinforments. Levelled with some kind of vibrating machine (which I dont know the name for in english) to remove air bubbles from the concrete and compact it as much as possible.

All concrete mixed in a 300litre forced-flow mixer. Took the most of my spare time for the better part of a couple of months.

This year I started making some concrete slabs at the areas around my workshop. Since febuary I have done a little over 220 square metres - and still need to do about 50 square metre before the project is finished. Not the most plesant way to pass the time of the day, but the end result is well worth it.

As to the reason for mixing it myself, the place where my house is is rather in-accesible for large lorries. Since I bought this house and started restoring it I have made well over 200 tonnes of concrete using that little forced-flow mixer.

So it is definately possible to mix yourself, but it is sooo much easier just to get it delivered...

[Kim Horsevad]

2. Water. So assuming the water content doesn't affect final strength, the only difference it makes is how easy it is to level, right? (Plus how long it takes to dry I guess). Just for clarity, an 80 'slump' means 80% water, 20% concrete? What ratio should I go for?

It does (affect the strenght)...

Mix three parts pre-mixed gravel (clay-free sand mixed with small stones) to one part cement. Use only as much water as nessecary to mix the concrete, level it with the vibrating machine. Then cover the finished slab and keep it wet for one or two weeks, letting it dry slowly.

This will give a very strong surface.

To finish the surface, rent a polishing machine after the concrete has cured completely. You can polish the whole slab in one day.

[africanpete]

If you are going to use a vibrating poker, don't leave it in the same spot for too long, just long enough to make sure the concrete has been able to get into all the voids but stop just before too much of the water comes to the surface as this will weaken the top layer of the concrete.

With an 80 slump you shouldn't really need a poker, a good tamp should do it. Make sure youre down to virgin soil (natural solid layer of earth) before you lay your aggregate otherwise whatever aggregate/concrete you put down will sink with time and if youre putting heavy machinery in the workshop that shouldnt take too long.

A damp proof membrane (DPM) is definately needed, you could just go for a heavy guage polythene, maybe 1200 guage? This should be laid over blinding ( a layer of clean soft sand put over the hardcore base to stop any penetration from your hardcore)

And as already said, I think you should have at least a layer of 50mm insulation, this is for comfort and energy effeciency more than anything else, make sure that what ever type you go for it designed to be used under concrete as they're not all the same.

Lastly, make sure everything is prepared before the concrete wagon arrives as it is a busy time, also make sure that you have cubed up the area correctly, there is no scope for error in your calculations as you will either have to take what concrete is left over or pay to have it taken away, both of those can be a PITA. The driver will also want to wash down his chute so make sure there is somwhere for him to do this, he has water on board, just get hold of some cheap polythene that you can lay down somewhere before he starts washing down, that way it's easier to clean up. Protect any paving lawns etc, concrete gets everywhere!!!

Sorry for the story, only wanted to tell you about the poker when I started this

[FridgeFreezer]

Anyone got any advice on joining a new bit of concrete to an existing bit?

I have a large slab already that I need to add ~1m to along one face, but unsure if I can just pour the new stuff next to it, or if I need to tie it together somehow?

[Les Henson]

Adding new to old - I used chemfix bolts with the bolts not fully wound in, so that the new concrete overflowed the heads of the bolts. I did this to add a swamp to my existing fish pond, and there has never been any seperation.

Les.

[africanpete]

Bolts with a resin/chemfix as Les said will work just fine, you could also use normal re-bar/dowls, also secured with a type of quick setting resin just make sure you drill them in deep enough, 2" to 4". If this slab extension is going to be at all load bearing then the usual steps apply i.e virgin soil, compacted sub-base, DPM etc. For a better bond you could scabble the face of the existing slab just to give the new concrete a better key.

Peter

[FridgeFreezer]

Going a stage further, the old concrete isn't level (puddles form on it), so I'm pondering either screeding (=£££) or just pouring some extra concrete over the top of everything to give ~20-30mm depth and one big, level, surface. Is 20mm (at the thinnest point) thick enough for a layer of concrete? I'm not averse to sticking some reinforcing mesh in if needed, it just seems cheaper and easier to have a delivery of reasonably runny concrete and spread it everywhere than lay a slab next to a slab and then spend a pile of cash screeding over the top.

[Les Henson]

Concrete is usually made from 20mm washed gravel (I think), so you could potentially have just a stone between the top of the new and the top of the old. Might make for a weak bond with that thickness. I think screed thickness should be 50mm.

Les.

[Guest WALFY]

20mm is too thin for using as a scab over the existing pad. Mesh hould be placed 50mm from the bottom edge and if the pad is thick enough then suspend from the top.

Runny concrete is not a good idea. Concrete will quick cure in 24hrs. But will be "green" for 28 days. Then it will have it's full strength. And be capable of it's full loading.

If you need to lay a skim over a pad then you must scabble the surface to allow it to bond. Scuffing with a wire brush on a drill is not enough. You need to break the surface of the existing concrete. So it looks like the surface of the moon.

If you need to join pads then you need to scuff up the edges if it is going to be load bearing in anyway.

My bro in law has just had a concrete pad laid in his garden. Mixer turns up and only mixes what you need. Much cheaper way of doing it. Charge by the 1/4m. The more you have the less the charge per m. Chute on the back had a 5m reach.

If your pad is to big you need to think about expansion joints if you think it will be taking direct sunlight at any point. You'd be surprised how much concrete expands and contracts.

HTH

[FridgeFreezer]

There goes that idea then, don't want to add more height than is really necessary, the shed is going to be tall anyway to accommodate the 109

Self-levelling screed can be as thin as 3mm it seems, and up to 30mm, the only real issue is the price which, according to some very quick googling, works out around £10/m2 or more, with my total area of slab being ~40m2 by the time it's done.

[Astro_Al]

You think thats bad Fridge - I just had a run around with the measuring tape, and I think I'm looking at about 40 cubic metres of concrete...

Al.

[Guest WALFY]

Screed is not concrete. So will have different working capabilitys. If you lay a skim of say 5mm, then whilst working on your truck drop something heavy on it, if your prep is not 110% perfect it will chip and the skab's integrity wil be broken.

You need to check the load bearing capacity of the screed before you go down that route. Every thin patch/skim I have seen has failed after time.