Concrete control joints?

[cutter12]

Hi,

I am going to create a concrete slab for an open workspace outside the shed:

• 
  3m sq.
  100mm Concrete
  100mm Sub Base.

I will hire a mixer for the weekend and will have someone helping me.

I am also going to do a path 13m x 0.5m

I am getting a bit mixed up regarding what material is used Control/Expansion joints? I have read you can use 'hardboard' but I am thinking that will rot pretty quick? Or would I be better just cutting control grooves? Any suggestions?

Also what is the maximum area that I could realistically mix, pour and level in one go? Do I need to divide up the slab into separate bays?

Any insight into this would be greatly appreciated. :?

 

[John15]

A 3m2 slab shouldn't need expansion/shrinkage joints provided it's reasonably square and not a narrow strip. If you do the path in the summer I would cast it in alternate bays about 2m long then cast the infills a few days later. No need to use expansion material as you are casting the concrete at the warmest time of the year - it can only shrink as the weather gets cooler. If you construct the path in the winter then the concrete could theoretically expand so introduce a 5mm expansion joints.
To avoid shrinkage cracking during the first few days only add sufficient water to make the concrete workable and keep the finished work wet to allow the cement to fully hydrate and reach sufficient strength.

John

 

[cutter12]

Hi John,

Thanks for the reply. Very helpful.


As I am hiring the mixer I would like to do all the concreting over the weekend. Therefore I would only be able to wait 'overnight' if I do alternate bays. Do you think that a day would be long enough before taking out the intermediate formwork?

Is it possible that cracks may occur in the Winter due to shrinkage?

Do you think it is realistic to try and mix/pour a 3m square slab in one go before it starts to go hard? There will be just two of us working on it.


Jon

 

[owen]

Is it 3m2 or 3m x 3m so 9m2?

 

[Lons]

Hi Jon
Your slab ( 3m x 3m ? ) is less than a cube so really shouldn't be a problem for 2 of you to handle, I've done it many times but there are some factors which might make it more difficult.

Do you already have the supports in place and levelled and the hardcore base ready and consolidated?
Are the gravel and sand materials next to the base or will you need to mix further away and wheel the concrete?
Do you have a tamper prepared? ( A straight length of timber will do ).

I assume one of you will handle the mixing while the other lays and tamps.

 

[John15]

Jon,
Sorry, I misread your post and replied for a 3sqm slab rather than 3msq. A 3m x 3m x 100mm thick slab is still a bit small for movement joints but will be more at risk of cracking if not cured properly. Spray with a membrane or keep wet for 7 days.
Any slight shrinkage in winter will not cause cracking.
As Lons says you can easily do a 3m x 3m slab in a morning with 2 people as long as you know what you are doing. Finish off as you go and try not to do it on a very hot day!!
Hope that helps.

John

 

[cutter12]

Thanks once again for all the replies

Is it 3m2 or 3m x 3m so 9m2?

3m x 3m

Hi Jon
Your slab ( 3m x 3m ? ) is less than a cube so really shouldn't be a problem for 2 of you to handle, I've done it many times but there are some factors which might make it more difficult.

Thanks Lons, that's reassuring. I will definitely have all the prep done before I get mixing. Have been digging out the last couple of days and so glad that it has rained a bit to soften the ground.

Thanks again John15, I plan to cover the concrete with plastic and keep wet like you suggest. When you say 'Finish off as you go ', do you mean tamp and level the concrete as we progressivley mix/por the slab?

One last thing..... ........the path runs up/downhill on a slight gradient of '1 in 33'. Do you think that will cause any problems?

 

[John15]

Thanks once again for all the replies

Is it 3m2 or 3m x 3m so 9m2?

3m x 3m

Hi Jon
Your slab ( 3m x 3m ? ) is less than a cube so really shouldn't be a problem for 2 of you to handle, I've done it many times but there are some factors which might make it more difficult.

Thanks Lons, that's reassuring. I will definitely have all the prep done before I get mixing. Have been digging out the last couple of days and so glad that it has rained a bit to soften the ground.

Thanks again John15, I plan to cover the concrete with plastic and keep wet like you suggest. When you say 'Finish off as you go ', do you mean tamp and level the concrete as we progressivley mix/por the slab?

One last thing..... ........the path runs up/downhill on a slight gradient of '1 in 33'. Do you think that will cause any problems?

'Finish as you go' - as soon as you have placed a half metre or so you should tamp and screed the concrete to level then float the surface with a wood float before, during and beyond the initial set and continue this process as you place the freshly placed concrete. Finishing to a good standard isn't easy for the novice and you'd be wise to get someone who knows what he's doing.
Regarding the gradient 1 in 33 isn't a problem but keep the mix fairly stiff to avoid the possibility of slumping.
With respect it sounds as if you don't know much about concreting so I would advise you recruit someone with experience otherwise it could end as an untidy mess!

John

John

 

[Lons]

'Finish as you go' - as soon as you have placed a half metre or so you should tamp and screed the concrete to level then float the surface with a wood float before, during and beyond the initial set and continue this process as you place the freshly placed concrete. Finishing to a good standard isn't easy for the novice and you'd be wise to get someone who knows what he's doing.
Regarding the gradient 1 in 33 isn't a problem but keep the mix fairly stiff to avoid the possibility of slumping.John

Jon, Do you understand what John means in the above post? If not then I'd agree when he says........

With respect it sounds as if you don't know much about concreting so I would advise you recruit someone with experience otherwise it could end as an untidy mess!

.
Concrete isn't difficult to do but it is a skill that needs practice to do well, needs a correct and consistent mix of materials and water and must be correctly tamped and then finished if you want a strong level base with a decent top finish. It's easy for people like me to say "simple no problems" forgetting that if it's your first time then it isn't that simple at all.
It looks like being hot this weekend as well and if you don't have a sore back already you might have after the weekend. Wear wellies or old boots, gloves and I'd advise safety glasses in your case as concrete burns the skin so wash splashes off as soon as you can.

However: If you don't try, you'll never learn so good luck with it Jon.

 

[cutter12]

Thanks again for the replies. Very helpful.

Jon, Do you understand what John means in the above post? If not then I'd agree when he says........

With respect it sounds as if you don't know much about concreting so I would advise you recruit someone with experience otherwise it could end as an untidy mess!

..

Yes, I am understanding. It does probably sound like I am asking some obvious questions. I can get a bit caught up on the details, I just want to be well prepared.

I am definitely going to give it a go and unfortunately don't know anyone with concrete experience. I usually embark on these projects alone so I try to research it as much as possible beforehand (books, youtube, internet and forums ).

I am never really that confident doing anything but I do challenge myself and things generally work out ok.

Thanks once again for all your advice and support. (if it goes well I might post a picture...if not then I will just close my account and create a new username (only joking :lol: )).

 

[John15]

All the best Jon

John

 

[Lons]

Hope it goes well Jon

I recommend Voltorol gel for your bad back afterwards. :lol: