Concrete slab to stop 8mm plate distorting

[Tin Can]

In a previous post i asked what was the best way to join 2 x 8mm plate (300mm x 500mm for on top of a black and decker workmate as a form of welding table) and got all the answers i needed.

Now i am about to clamp them up ready for mig'ing next weekend and 1 of the suggestions from the previous post was to clamp it to a concrete paving slab to stop it distorting

It just happens that i have a concrete slab exactly the right size but as i am a newbie i want to make sure that a concrete slab is ok as i am sure i have read somewhere that this shouldnt be done due to air or somthing trapped in the concrete 'popping' due to the heat build up

I hope the answer is yes as the 'concrete slab' suggestion is perfect for what i want to achieve

Thanks

Mark

 

[stormin]

It's moisture you'll have to watch out for! Any dampness in the concrete can boil with the heat from welding and cause the concrete to explode.

Keep the slab inside for a few weeks to ensure it's thoroughly dry and there shouldn't be a problem.

 

[Pete.]

Concrete aggregate explodes with heat, especially flint. It fractures and sprays everywhere and let me tell you it burns like fekk when it lands on you. Dunno about paving slabs though since they seem to contain very fine aggregate.

Basically, concrete and welding/burning temperatures don't mix well at all.

 

[chunkolini]

I have we;lded on top of concrete floors and can confirm the 'crack pop pow' effect, never had any burns but probably not the best idea.

Chunko'.

 

[Dune]

This is called explosive spalling. During hydration of concrete not all the water is used, this leads to small pockets of water which as has been stated boil and fly off. Its commonly found in fires such as the Channel Tunnel fire, where the firemen complained that it felt like they where being stoned.

The other causes are to do with the aggregate used. Some aggregates that contain flint will do that same without water.

Older concrete can absorb water and lead to the same effect.

My thesis at Uni was entitled Effects of short term rapid heating of concrete structures... It was some years ago but I got a first for it

 

[Tin Can]

Thanks guys for the replies so far

Dune - based on your accademic qualifications on the subject - would you do it??

Thanks so far

Mark

 

[brightspark]

i allways use the concrete floor for setting up and welding its nice and flat a good heatsink as well i wouldnt put a gas torch or welding torch anywhere near mind only ever done it once and it was nearly once 2 often

 

[Dune]

Thanks guys for the replies so far

Dune - based on your accademic qualifications on the subject - would you do it??

Thanks so far

Mark

Yes I would. If you are going to use something like a flag to weld on you may find it will split with the heat, something thicker would be preferable. If you have two sheets of fairly heavy metal butt jointed and cramped down together you wont get hurt by any parts that decide to fly off. I would tack weld the plates, then weld say 3 or 4 inches and then move to another part trying not to impart too much heat in to one area of the concrete.

In a previous life I used to build oily water seperators. Setting out cutting and welding was carried out on a concrete floor. We did have some small spalling episodes and never knew what was happening.

H&S at work

 

[Tin Can]

Thanks for your positve advise DUNE, i will give it a go this weekend and will follow your advice on controlling the heat transfer

Thanks Very Much

Mark