Reinforced Concrete - Covering a Hole

You need to be able to test it to well above its top load. How will you do that?

My highly inexpert thought is that these things don't usually just collapse catastrophically, without warning. However, even quite a small failure gets amplified for whoever is unfortunately at the top of the cherry picker at the time.

Earlier this year I needed to replace the cap on an old Well that we use for water to irrigate the garden. I thought about trying to cast something in situ but in the end went for a pre-fabricated concrete cover slab, from Cotterill Civils, that was 1450mm o/d and had a 600x600 hole - the price was about £175 and included delivery. I believe they do other sizes but be aware that this one weighed 455kg!!

That should be fine, but you could use 2 layers of A252 or even A142. Use some decent method of positioning the mesh fabric. Meshmen are cheap, and accurate.

It takes 28 days for concrete to achieve full strength (assuming you mix a proper C30? mix and pour it in, in one go). Don't go underneath it and bash out your formwork any earlier. Make sure your rebar extends over the edge of the cistern, because this is the point where there are some interesting shear stresses. We used to have a poster who was a BCO and wrote/sold a design package for structures called Greentram?. Haven't heard from him for ages since he emigrated to Oz.

Looks like he (Tony Bryer?) and 'Superbeam' are still going:

I looked at precast slabs but the handling issues looked difficult. A 2.38m diameter slab weighs over 2 tonnes which means a crane, its too heavy to be handled by anything but the very largest excavator.

Also 2.38m is a little large whilst a 2.07m diameter feels a little small.

Cheers,

Dale.

I fully agree on spacing - suitable proprietary spacers will be used to give a minimum spacing of 75mm from the bottom of the slab. Structurally I'm guessing it would be better for the mesh to be nearer the bottom face (e.g. 50mm from the bottom) but I'm worried about corrosion. I'm buying the concrete ready mixed so I'll have a chat with the supplier as to whether they've got any additives that will improve the protection of the steel.

Do you have a view as to whether there is any benefit in going for 2x A393 meshes - I'll end up buying a 4.8x2.4m sheet so I'll have enough for two layers? As its a small one off, my preference is for massively over-engineered if it doesn't add much to the cost.

Best Regards,

Dale

I remember him too. I think his expertise was in wooden structures but maybe I'm mis-remembering.

According to all the readymix suppliers, C40 is what is recommended for suspended applications. I'm having it delivered mixed so it will be a single pour. Having had readymix delivered before, my experience is that it will be a single very quick pour with the truck driver standing over me looking at his watch.

I have no intention of going underneath the slab until it has plenty of time to cure - being buried alive is one of my phobias so there will be an undue level of caution.

Cheers,

Dale.

It happens that Dazza formulated :

Could it not be in precast sections?

We moved it from where the HIAB lorry dropped it using fencing stakes as rollers - three people pulling and one acting as brake ... but that was

I was looking at page 20 of

Cheers,

Dale.

Mine would need moving 10m across a lawn and then dropping a foot or so into a depression in the ground. I did consider it but it felt like a challenge too far. I can get a readymix truck within 2m of the depression so I'm hoping that I won't even need to barrow the concrete.

Cheers,

Dale.

You should have 50mm minimum from all edges. If you're having readymix, be aware that this will be horsed in in a hurry, it will be very sloppy, and you need to make sure that your formwork and spacers are correct and will withstand the wash. Give it all a good jiggle as the concrete goes in, a bit of 2x2 or similar would be good. As for how thick the fabric mesh wires should be, I would not get too hung up about it, just make sure it's given a jiggle to get rid of air bubbles. Note that the sheet will be much more bendy than one might think, and it may have distortions from handling. You can buy "merchnt sheets" which are smaller. Mesh fabric is bery easy to trim with bolt croppers. Once the steel's properly encased in concrete, that's that, it should not corrode for A Very Long Time.

Thought it was 1 1/2m?

You are correct, my wrong choice of words. 1.5m span, 2.3m overall length (well diameter, its a circular hole) of material.

Cheers,

Dale.

Thanks for the comments.

I've done a ~100mm thick outbuilding floor before with A142 mesh and readymix with the 50mm concrete spacers which look like little castle towers. That was easy (and it helped that the concrete was sloppy) but that was on top of a DPM so should stay dry.

As this slab will be underground (and therefore in damp conditions), the recommendations I'm seeing suggest a minimum of 75mm cover around the reinforcement - do you think this is excessive? My comment in the post you quote was an observation that to my mind, in a 150-200mm thick slab you'd ideally want the reinforcement a bit nearer the bottom face from a structural perspective , but that doesn't fit with the cover recommendations and I feel a little torn between adequate cover and structural robustness. I'm probably trying to over-optimise here!

I'm guessing A393 will be more of an angle grinder job than bolt cutters, but that is not a problem.

Cheers,

Dale.

On 24/08/2021 22:52, Chris Bacon wrote: ...

The concrete fence posts in my garden, which had the reinforcement not far below the surface, lasted more than 60 years before they started to spall.

It probably wouldn't do any harm to put a sheet of DPC material down first and pour on top of that.

I would go with the advice to buy a pre-made slab, even if you might need to buy something a bit larger than you want. It is an instant fix and no need for form work. If you can get a ready mix truck to within

My thoughts too.