Cementing help?

[brettdel]

Just doing a bit of cementing around our place and just wondering if anyone could give some advice on the theory behind:

Edging
Floating

Edging: the use of the edger is to give a rounded edged so as when you pull the form-work away you don't crack the cement - is this correct?

Floating: What is the theory behind floating the surface? Is it - you use the float to compress the concrete a little to squash out, and bring to the surface, some of the water/cement to bind the surface together to stop the slag from separating?? or........

Sorry for the lack of woodworking content but you are such a knowledgeable lot I though someone could offer much needed advice


TIA Brett

[journeyman Mick]

Brett, edging is for decorative purposes only, it's not used where slab edges aren't seen. Your concrete won't (or shouldn't) crack when you strip the formwork. Floating, which is done using a wooden or magnesium float, brings the "fines" to the surface and forces the aggregate below the surface. This is followed by trowelling which compresses and strengthens the top surface.

Mick

BTW the term is concreting, cement is one of the raw materials involved.

[brettdel]

thanks Mick,

If I'm just doing paths etc how much should you float?

I remember reading ages ago, somewhere, that you should not overwork the finished product? Good advice?

Brett

[journeyman Mick]

Brett, when working with concrete it's all a matter of timing, which you can only really learn about through experience. However here are some tips that may help:
Mix as little water into your concrete as possible.
You must place steel in your path and give it at least 50 mm bottom and side cover.
Place concrete in formwork and screed.
You can then immediately start to float. You only need to float it enough to bring the fines to the surface and force the aggregate down. If you keep floating it after this you will bring excess water to the surface.
You will then need to wait a while (unless you're working very slowly or in extreme heat) to trowel the surface. It needs to be quite firm, only just wet enough so that the trowel compresses the surface, but not quite so firm that you burnish the surface. You should trowel the surface even if you are planning to give the concrete a rough broom finish (preferred finish for outdoors) as it makes for a stronger, denser top surface and minimises surface cracks.
Once the top starts going off, you can cover it with plastic and keep it moist for a week or so as this will result in an ultimately stronger slab with less surface cracking.

If all of this sounds a bit difficult, it's because it is. Concreting is simple (especially if you watch a pro at work) but mastering the timing is difficult. I recently poured the slab for my shed (75M2) and paid a concretor $330 to place and finish the concrete - he was worth every penny. Start on a small section that's not too visible, ggod luck!

Mick
Good Luck!

[Sir Stinkalot]

Originally posted by journeyman Mick
I recently poured the slab for my shed (75M2) and paid a concretor $330 to place and finish the concrete - he was worth every penny.

Mick,

What was the cost of the concrete per m2 out of interest. Was the $330 for the labour only? The reason that I ask is we are looking at either paving an entertaining area and paths or concreting. Thanks

Stinky

[Pulpo]

[ I recently poured the slab for my shed (75M2) and paid a concretor $330 to place and finish the concrete - he was worth every penny]

Mick

Exactly what did the contractor do?

I take it you leveled the area.

Did the contractor supply the formwork?

If this was a days work that's a great price.

I've been quoted for labour including formwork, no materials $2,700 for 84m2. [not likely]

Its a job I keep avoiding.

I agree with Mick its simple but extremely easy to stuff up, experience is everything, something I have not had for many years.

Start with a small section divide the total slab into smaller sections, you will need to have contraction joint liners between the first slab/section to the next. Its like a spongy foam strip, many hardware stores should have them. Otherwise the concrete will crack.

As for pulling away the formworks so no cracks or big chips fall away you can spray a chemical on the wood to stop the concrete adhering but this is not necessary if using the right plywood; more used for concrete walls.

If it was for house foundations, really should get a pro.

I have a reasonable book "Working with Concrete" by Rick Arnold
but its American so has some gaps for the Aussie user.

More helpers the better.

Good Luck.

Pulpo

[journeyman Mick]

Total price for 13.4M3 of concrete delivered from batching plant 30km away $2195.51 (trade). Add: concrete pump & two operators $346.50 and cost of roll of visquene (plastic), 8 sheets of F62 mesh, 9 lengths trench mesh, 15 lengths Y12 bar. Can't remember how much I paid for all this other stuff, but it would be around $400. Can't find my calculator so you'll have to do the maths Stinky (I thought you archyteck guys knew all the M2 costs of stuff, or do you not subscribe to Cordell's?) Bear in mind though that this slab had thickening beams in it and extra steel for the retaining walls which are built along 2 edges, your entertainment area and paths will be substantially cheaper.
I supplied, erected and levelled the formwork and sandfilled, laid plastic and placed steel. The pump & two operators placed the concrete (and lifted the reinforcement where neccesary). The concretor screeded, floated, trowelled and edged the concrete. He also managed to get me a pump after I had unsuccessfuly rung every operator in town. I could have screeded and floated the slab myself as I have a screed and bull float but it would have cost me $80 to hire a helicopter for the day (plus 1 hours drive each way to pick up & return) and there was no way I was going to hand trowel a 75M2 slab. He made more money than I generally do in a day but he did it in way less time than I would have, so I paid less for the pump, plus he got it perfectly level and beautifully finished even though it was raining. Like I said, worth every cent.

Mick