Reinforced Concrete - Covering a Hole

Look into precast concrete lintels.

An angle grinder will cut them to size, and then you can lay a top layer of concrete on top

I cast a similar slab as that by dint of simply mixing buckets of concrete in situ from cement and ballast bags.

I gave away my concrete mixer a few years back.

Yes, it took a couple of days instead of a couple of hours, but involved no heavy machinery and apart from dragging 15kg bags of ballast and cement around, no hard work,

I'd really like to do this but I'm worried about access. The gap in the wall is quite tight and the road is not wide. The longest vehicle I can get to the site is basically the size of a traditional UK skip lorry or a small (~6.7m long) cement truck. I've not seen something that size with a suitable long reach Hiab. It needs a reach of at least 4m, probably 5m and safely lifting 2.5 tonnes at that radius needs more counterbalance than the weight of a small truck chassis.

It could be done with a small crane but it would need a series of lifts to work the slab up the garden. A (seriously) big crane would be an option but I'd need to put it in the middle of the road (it if fitted) which I suspect the neighbours / council might have a view on, not to mention the hire cost.

I'd also need to cast a suitable ring to rest the precast slab on - that is probably equivalent work to building the formwork for something cast in situ.

As you might guess from the above, I have spent quite a long time thinking about the logistics of a precast slab because it really appealed to me too.

Cheers,

Dale.

Not sure I understand that. You're overlaying a circular hole with a rectangular cover? Have you got any pictures of this? What're you going to do for the falsework. You could get your supplier to mix in a recommended waterproofing admixture. If you're going to have 75mm of cover, remember that the lower 75mm therefore will be unreinforced.

I'm basing the coverage on things like

I attempted to understand Eurocode 2 but I don't have enough knowledge to understand its coverage requirements (let alone the structural ones which go right over my head).

I'd want to get 50 years out of it to be certain that it won't cause me problems.

Best Regards,

Dale.

You really are over thinking this... and this is coming from a structural engineer. I assume the thing you are covering is just a limited depth brick lined pit, not the top of something complex.

If you have been quoted £4k for this by several companies then you have either confused them, missed out something important on here, be universally hated by them or asked them to fly over from Japan to personally build it for you. They might be thinking you are capping an old mine....?

If you really want an engineer then it sounds like a £200-£300 bit of work for a local engineer to produce you a sketch scheme.

The simple option would be the precast biscuit, subject to access. You say you want to span onto the soil, why cant it just sit on the brick walls? Are they too deep to raise up or are they shot to pieces. If the latter, then surely you need to fix that to get your intended extra 50 years? If you want to place it on the surrounding soil, you dont need a concrete pad, soil is fine if its competent, if not, dig the soft stuff out and refill with type 1 or similar. Make sure there is a gap between the top of the brick walls and the bottom of the precast.

If you want to do it insitu, then 150mm deep would be fine unless it's buried more than say 600mm below the ground. Cherry pickers are not that onerous, well not the sort of thing you will have in your garden. 1 sheet of A393 is plenty, provide 50mm cover to the bottom and sides. The 75mm cover you quote is to allow for tolerances in the soil cut line. As you can ensure min 50mm cover then it is fine and will give you 60+ years. Use C30/37 mix,, and you dont want it too wet, but that means you might need to vibrate the concrete to get out the air (I have used a hammer drill to vibrate the rebar on a small thing I cast). Too much water leads to weaker concrete and a shorter life before the rebar starts to corrode. You certainly dont want a waterproofing admixture. Expensive and totally unnecessary. Put a plastic sheet under the concrete just to keep in the water and fines while it cures. Your biggest fun thing will be building the formwork so it doesn't load the chamber in the long term.

Seems to be well less than 1m3 of concrete - cant believe that mixing that yourself is not an option if you have access to a cement mixer. Premix will be expensive for a part load. And then they go and wash out on your drive...

IME, with any problem like this, the people who know best how to solve it are the ones supplying what you are buying, whether that is a pre-cast slab or, as was usual in my case, a large piece of machinery. You won't be the first person with difficult access and they know what equipment they have and whether it is up to the job. Ask for a quote for delivering and placing the slab and tell them about the access problems. They won't charge for a quote but they will get a chance to assess the site with a practised eye. They may agree with you or simply say not a problem.

Except that way you would not need to remove the formwork from inside the hole underneath a recently cast slab.

Sorry for being unclear. I have a circular hole of 1.5m diameter and ~2m depth from "normal" ground level. I have a circular depression of 2.3m diameter and around 350mm depth from "normal" ground level on the same centre which the cast slab is going to fill to a depth of 150-200mm. The rest of the depth will be used for the patio finish. "Normal" ground level will be lowered by ~250mm and then built up again with sub-base, screed, slabs, etc. The area of the slab will have a reduced (or no) sub base laid over it as the slab itself will provide the base.

The falsework will be a circle of 2x thickness ~20mm ply or OSB supported from below with something like 3 or 4 6x2 joists which in turn will be supported on 4 off 4x4 vertical timbers or acrow-props. I'll probably go for the 4x4 vertical timbers since I can then diagonally brace between them. Whilst the height from the base of the cistern to the formwork is only 1.5m, I'm keen to make sure that it can resist the dynamic forces during the pour. Collapse would be embarrassing.

I note your comment about the bottom 75mm not being reinforced, that is why I'm fretting about reinforcement coverage vs. structural strength.

Thanks for your help,

Dale.

Stop fretting. (1) 75mm cover is not needed and just a waste of concrete. 50mm is fine. And it's supposed to be to unreinforced - that's why it's there, to protect the rebar!

Why's that, for interest? It's not that expensive? But I agree it's unneccessary.

Waterproofing admixtures (Pudlo, Caltite etc) are expensive, and if you want to waterproof concrete then it is much better to use a low water/cement ratio and use a plasticiser.

But waterproofing concrete is about forming a dry basement etc, not about protecting rebar. The best way of protecting rebar is surrounding it in dense concrete (ie well vibrated, no air pockets from lack of vibration or evaporation of excess water) with highish cement content and sufficient cover. Rebar corrodes when the carbonation layer reaches it and set up a corrosion cell (I'm not going to pretend to understand the chemistry). 50mm cover is absolutely fine in this case - greater is only really needed for 4 hour fire protection, places with abrasion and really corrosive environments.

Yes, hence the remark about 75mm being a waste.

OK. I'll admit to over-simplifying, I was wary of presenting an exam question to people who respond to posts on the internet for fun. The cistern is frustum shaped (smaller at the top) of about 1.9m diameter at the base and 1.6m diameter at the top of the frustum, about 1.2m above its base. The remaining ~0.4m height is a brick dome (made of half bricks in concentric rings) with a hole in the middle, springing off the top of the frustum at around 45 degrees to horizontal, reducing the 1.6m diameter to around 1m (it was around 600mm originally but parts of some of the inner brick rings were missing so I've broken out the damaged rings on the basis that partial rings in a dome aren't good structurally). I suspect, but do not know, that the overall structure is half brick thick. The whole structure (well, at least the top bits I can see) is surrounded by ~200mm of thick clay, presumably to ensure the water stays inside.

There are also drainage pipes (which cannot easily be moved) at the same height the slab would be which means that there is a ~1.5m long chord across the edge of the 2.3m diameter circle in one location - this is one of the things discouraging me from a pre-cast solution.

I was working on the basis that I'd cast the slab over the existing remains of the dome with the aim that only the very edges of the dome carry any real loading, propping the dome for the pour as well as propping the board necessary to close the hole and cover the inner dome rings. As you can see from the above, there isn't really a robust "top" of the brickwork to rest the slab on, hence my plan to support it on the surrounding soil (which will in turn pass some of the load down to the sides of the frustum). Once the slab is cast then the narrow gap between the top of the brick dome and the slab would be filled with mortar to tidy up. If the dome cracks because I've left some of the weight of the new slab resting on it then I don't see it as a big issue - the slab will be the structural element and most of the dome underneath could be broken out if necessary.

Both quotes I received had come from the engineers' drainage departments and looked like they had thrown a number at it to discourage me, both quotes were accompanied by a slew of assumptions which might be appropriate if it was a job for the council in the middle of a park but didn't work for this project - e.g. "A full set of Architect?s drawings in dwg format will be available prior to the commencement of our design. This information will include the level of the plots, car parking, and setting out information." and "A full tree survey of trees on the site and adjacent to it will be provided to [...] prior to our design.". I'd previously used one of those for some other structural stuff around the house so I was a bit disappointed in the response, an email back to them questioning the cost and assumptions yielded no response.

Thankyou for the comments on cover, I'm happy I can guarantee at least 50mm with little variation so I'll go with that.

I agree, the formwork needs care, the biggest difficulty I have is minimising the loading of the inner rings of the dome. Part of me was wondering about breaking out the dome further to simplify things but then I've got a larger hole in the ground to fill with formwork and the need to rebuild a wall around the cistern between the top of the remaining brickwork and the underside of the slab.

I'm lazy and if 0.8m3 of concrete (rough guess) can be delivered and poured for £200 then that beats spending a day with a shovel and a cement mixer (which I'd need to hire). Previously they've washed out into some of those large builders mixing trugs so no mess on the drive (not that it would matter if there was, its rather tatty aggregate at the moment).

Sorry again for the drip feeding.

Thanks for your help,

Dale.

Yes still around after 32 years of selling SuperBeam but I dropped out of following this group because of the noise.

RC design (except simple beams) is a bit outside my field of knowledge sadly.

....

Is the radical option of demolishing the cistern and replacing with a much smaller installation viable and a better long term option?

Instead of using temporary formwork underneath, would it be possible to span the hole with a couple of sheets of corrugated iron, just have formwork for the access hole, and cast your concrete on the corrugated iron (still with the rebar mesh of course). Then cut out the sheet in the access hole region. Maybe set a couple of Acrow Props and a good piece of timber in the middle, perpendicular to the corrugations, to support the sheets until the concrete sets. Then remove via access hole.

Sorry for being unclear. I have a circular hole of 1.5m diameter and ~2m depth from "normal" ground level. I have a circular depression of 2.3m diameter and around 350mm depth from "normal" ground level on the same centre which the cast slab is going to fill to a depth of 150-200mm. The rest of the depth will be used for the patio finish. "Normal" ground level will be lowered by ~250mm and then built up again with sub-base, screed, slabs, etc. The area of the slab will have a reduced (or no) sub base laid over it as the slab itself will provide the base.

(Sorry about the lack of quotation above - it insisted on making the first line about half the paragraph long, so I had to copy and paste).

I see that you have quite poor access for heavy items so, without knowing the volume of the hole, how about a truck-load of small rubble and a couple of blokes just to fill it in?

Ideally I'd want something bigger, so no.

The "new" option would be to dig out the brickwork and install a suitable modern GRP tank. Whilst the reinforced concrete is a reasonable job, digging a 2m+ deep hole (with all the side support necessary to stop it killing me) would be a different order of magnitude.

Cheers,

Dale.

But I'm using it (garden watering)?

The inadequate cover was identified when the decking over it was removed in preparation for the patio.

Cheers,

Dale.