Opes in hollowcore flooring

[Moggaman]

Hi . I Want to design in the opes required in my HC rather than having to core them out after. 
 Can you let me know if have I missed anything please in yer opinions.....

- 300x300 ope for MVHR transition to upstairs

- heating pipes — ope size???

- electric cable transition from floor to floor???

-waste from WC upstairs going out through external wall ... no need for ope.

-possible laundry chute!

 

Thanks 

[Adam2]

Some suggestions: water supply, hot return, network cables (can add up to a fair few), TV coax, CCTV cables

It's always 1 large hole more than you expected ? am just doing the exact same for my build - though we have internal waste so a few more holes to plan for

 

[ProDave]

Is it just me? or what is an OPE?  Google does not help me. 

[jack]

3 minutes ago, ProDave said:

Is it just me? or what is an OPE?  Google does not help me. 

 

Based on context, I assume "openings".

[Moggaman]

2 hours ago, ProDave said:

Is it just me? or what is an OPE?  Google does not help me. 

Yes ope, short for opening ...

[Gus Potter]

Hello Moggaman.

 

I take it that by HC you mean a hollowcore slab?

 

You may be familiar with the design of these but if not.. and for all.

 

Small occasional service penetrations (say less than 50mm dia) will normally be ok so long as you keep away from any additional rebar, main slab tendons.

 

Once you go beyond this small opening size you need to look really carefully at what you are doing as you may start to get close to the steel and significantly compromise the shear / bending capacity of the slab. It needs careful thought and detailing. Also, once you start to take chunks out of these slabs you need to check the lifting points are still ok. They are not forgiving if they fall during lifting!

 

It's worth doing some background reading on this, get familiar with the terms and talk to the manufactuer at an early stage.

 

 

 

 

 

 

[Conor]

The slabs work it's easier to have a "slot" than squares as you want to minimise the number of cables you lose. Your slab company will suggest what's possible. In our case, we left a few strategic gaps between slabs, and where we had a slab run parallel to a wall, leave the side of the slab short a couple hundred mm. When we've all our services though, we'll shutter the underside and pour some concrete in. I've left a gap like this along one wall of the main bathroom, so all the waste pipes, vent stack and MVHR ducts to the first floor will come though here. I'll then build a false wall in front of the gap to hide the services.

 

Coring a 116mm or smaller hole in a 150 slab is easy enough, you just need to avoid the cables which are normally 150mm centre to centre.

 

Edited April 7, 2021 by Conor

[Iceverge]

5 hours ago, Moggaman said:

Hi . I Want to design in the opes required in my HC rather than having to core them out after. 
 Can you let me know if have I missed anything please in yer opinions.....

- 300x300 ope for MVHR transition to upstairs

- heating pipes — ope size???

- electric cable transition from floor to floor???

-waste from WC upstairs going out through external wall ... no need for ope.

-possible laundry chute!

 

Thanks 

 

We managed to take all services apart from wastes from the downstairs utility to an upstairs hot press through a 600 x 150 opening in the Slab.this required a bit of planning but it worked fine. The electric cables and MVHR all went straight up to a service void of 88mm under the roof trusses and eventually dropped vertically into the sockets and outlets.

 

I drilled 2x150mm holes for the wastes through the slab after with an SDS and a chisel (i love suffering). My time would have been better spend pinpointing the opes to the supplier and letting them do it initially.

 

 

PS our supplier limited us to 300mm from a 1200 slab so the opening had to be across 2 slabs. 

 

 

 

 

Edited April 7, 2021 by Iceverge

[Moggaman]

Hi. I have read a few threads on the wrapping of the precast slab sitting on the inner leaf .A lot of the time it refers to hollowcore.. I will be using wideslab slabs and as a self builder , I am looking for input on the detail.
I believe the wide slab doesn’t have the same air issues as HC... generally I have been told that builders in my area bed the slab in mortar and don’t use membrane. For me it seems, bedding the slabs, then taping the underside of slab to external wall and airtight painting the extent of the wall down to just below the suspended ceiling might be the plan?. Upstairs , probably tape joint as well although I will be pouring a screed .
I presume u cannot bed and use membrane at the same time, logistically it would not work with the slabs on-site?
Thanks

[Thorfun]

I've made the decision to go for block and beam instead of hollow core slabs now. Reason is I just know I'll make a mistake with the openings/penetrations and will forget something and the thought of drilling through the slabs fills me with dread. at least with block and beam I can remove a block to run pipes and cables

 

I'm hoping block and beam will be cheaper as well.

[Ronan 1]

We use alot of hollow core slabs for apartment blocks we are building as it seems common place in Ireland instead of an in situ slab once above transfer slab. 

I wouldn't and won't worry too much about punching or maybe scoring is a better word holes as small as 150mm through them.

 

The installers hack chunks out of them with great abandon when they have forgotten opes and not a bother sign8ng off their warranty in them. So unless you are planning multiple on a single slabs of worry not. 

[Thorfun]

14 minutes ago, Ronan 1 said:

We use alot of hollow core slabs for apartment blocks we are building as it seems common place in Ireland instead of an in situ slab once above transfer slab. 

I wouldn't and won't worry too much about punching or maybe scoring is a better word holes as small as 150mm through them.

 

The installers hack chunks out of them with great abandon when they have forgotten opes and not a bother sign8ng off their warranty in them. So unless you are planning multiple on a single slabs of worry not. 

it's more the tools required to create those holes rather than whether the holes would be allowed for me.

[Ronan 1]

Fair point , we have a team on some site at least twice a month and often weekly so they come with big saws and core rigs because there is ALWAYS opens missing.

[MortarThePoint]

Interesting reading. My experience has taught me two things:

1. At spans under 5m you can have big holes knocking out two steels with no issue. 

2. When they install them it is easy for the planks to drift out 50mm or so in the direction perpendicular to span, putting your opening in slightly the wrong place. OK for services a pain for something like a chimney (I had a 700mm chimney passing through the join in two planks and had to shrink the chimney to 650mm). Make a checklist of dimensions to be checked as they install. I'd have been all over them if it hadn't been for Covid.

3. You'll forget some holes. Haven't tried drilling anything larger than 25mm, but that was very easy.

 

[MortarThePoint]

You also end up with some unintended openings. I have to work out how to close this one (200mm wide) within the 40mm of screed depth that is going over the top.

 

[tonyshouse]

Holes in my concrete planks were designed in - they came with perfect sized holes in the perfect places for ducts and pipes 

[Conor]

1 hour ago, MortarThePoint said:

You also end up with some unintended openings. I have to work out how to close this one (200mm wide) within the 40mm of screed depth that is going over the top.

 

We had a few of those. Prop a scaffold board up underneath and shovel in some concrete and couple lengths of rebar.

[MortarThePoint]

43 minutes ago, Conor said:

We had a few of those. Prop a scaffold board up underneath and shovel in some concrete and couple lengths of rebar.

 

Yes that could work nicely. If I tucked the rebar into the core hole on the right hand side and onto the beam on the left hand side it would all hold well.

 

Alternatively, I am thinking of leaving it to have OSB3 and SLC over the top and using the hole as a possible 'conduit' to route things along the landing as there is a smaller hole at the other end too. The SLC would be minimum 20mm thick which meets the requirements of most over timber (e.g. Weberfloor Flex or Weber 4310). Covered in carpet any small cracks at the SLC/screed interface wouldn't matter. Not so sure what I'd send along there though.

[MortarThePoint]

1 hour ago, tonyshouse said:

Holes in my concrete planks were designed in - they came with perfect sized holes in the perfect places for ducts and pipes 

 

Your installers did better than mine then. Like I said, I chalked this up as a Covid consequence though as I'd have been all over it otherwise.

[tonyshouse]

I was my installer and ordered my own planks 

[Adam2]

This is timely - I have my planks coming next week. We carefully planned the cut-outs so will see how accurate we can get the install - though mostly we have some flexibilty. Just hoping the steels get installed on Friday else we'll be re-planning ?

 

[Thorfun]

21 hours ago, MortarThePoint said:

2. When they install them it is easy for the planks to drift out 50mm or so in the direction perpendicular to span, putting your opening in slightly the wrong place. OK for services a pain for something like a chimney (I had a 700mm chimney passing through the join in two planks and had to shrink the chimney to 650mm). Make a checklist of dimensions to be checked as they install. I'd have been all over them if it hadn't been for Covid.

this is what my Groundworker said. he said if you're having hollow core then he'll go around after and core the openings as they're never in the right place in his experience. 

 

40 minutes ago, tonyshouse said:

I was my installer and ordered my own planks 

if you want a job done properly....do it yourself. ? 

[saveasteading]

Agreed, buy whole planks and cut on site UNLESS there need to be cut-outs to miss columns. Even then they are likely to be in the wrong places, but is easy to chip off some more.

Big holes are made at the factory by a worker with a shovel, and a tape measure if you are lucky.

I much prefer T beams where possible. No cranes or big forklifts. On small spans with block infills, it is very easy to form a hole unless it has to be precise.

Cutting through a T is a bad idea.

 

T usually much cheaper if the spans are small, all factors considered.

Beware the camber on precast slabs. They vary by span (long slabs bend up further). It can cause a lot of extra screed, and eat up the headroom.

If more than one load of slabs, of different sizes (or with cut-outs) check they are delivered in the right order.

[kxi]

Re hollowcore grouting, what's the best way to grout the planks if the gap (the keyway) is variable and a bit wide at the bottom where planks havn't been pushed together, e.g. between 1-15mm gap at the base?

 

So far options suggested have been:

 

1. Stuff some backer rod down to the bottom of the cracks then can pour a wetter mix in

Seems to be used in the US

 

https://www.youtube.com/watch?v=hVs8cq3ZBA8

 

2. Shutter underneath

Seems a lot of effort as there are a lot of joints

 

3. Use a dryer mix so it doesn't slop out of the bottom

?

Edited May 18, 2021 by kxi

[Conor]

@kxi you don't need to worry about gaps unless they are more than 25mm or so. If you use a drier mix and are gentle with the first few shovel fulls, it'll most stay in place. For the bigger gaps we used a sheet of OSB propped up with a couple scaffold boards. Takes a few minutes.

 

As we were doing an ICF build, we were able to do all the grouting with the left over mix.