Raft detail confusion

[deuce22]

Hi.

 

As I have never laid a raft foundation before (only strip foundations), I didn't know the exact way of doing it. I was told by a few people that the concrete is poured in 2 stages, so that you can shutter the 2nd pour, and can create a step (cavity tray). For this reason I was planning to use either Jacksons Atlas system or Isoquick. 

 

I have now been told by Nudura that you do not need a cavity when using ICF, so 1 pour of concrete is the correct way. Is this correct?

 

I also wanted to know if there is a way of adding insulation into the concrete and then finish it or do you have to insulate on top and then screed?

 

Thanks.

 

[ToughButterCup]

1 minute ago, deuce22 said:

[...]

I have now been told by Nudura that you do not need a cavity when using ICF, so 1 pour of concrete is the correct way. Is this correct?

[...]

 

As I imperfectly understand it, not having a cavity is one of the attractions of an ICF.

 

1 pour - how high?  A reasonable expectation for 1 pour is - roughly - head height.

Beware here, across the ICF fraternity, there's a lot of willie waving about having poured a skyscraper's worth of building in just one pour. All of it sh!te.

 

Nudura have some very good training videos on YooChube.

[ToughButterCup]

7 minutes ago, deuce22 said:

[...]

I also wanted to know if there is a way of adding insulation into the concrete and then finish it or do you have to insulate on top and then screed?

[..]

 

The answer to that question should be in your raft design detail.

[deuce22]

I wasn't that concerned about how high you can pour the ICF fill. As I understand you can go to floor level each time, which is what I will be doing.

 

It is more about the raft detail. As I've never done one, I'm unsure of how it is constructed. The engineer also stated that it is poured in two stages and hasn't had much experience with ICF. I've received a detail from Nudura and my engineer will obviously have to speak with them. I will receive a design detail off of him, but just wanted to know what others using ICF have done.

[Alexphd1]

We used a eps raft with nudara icf. The raft was done in one pour and the icf in two pours.

 

Our biggest mistake with the raft was not sorting out how doors and windows would sit on the eps upstands. 

Edited May 19, 2020 by Alexphd1

[deuce22]

Which company did you use for the raft?

 

I was told that, the benefit of using something like Isoquick was that it only needs one pour, whereas a traditional raft is two pours. However the guy from Nudura has stated that it can be poured in one amount as there's no need for a cavity tray around the perimeter.

 

It's been pretty difficult as I've only built 1 property about 15 years ago and that was on a strip foundation. The architect is also the engineer and not that helpful, he is a friend of the family and I feel like I'm the one doing all the research and coming up with different ideas.

[PeterW]

17 minutes ago, deuce22 said:

whereas a traditional raft is two pours

 

..??

 

A traditional raft is a single pour with thickened areas such as the perimeter and any strengthening beams across the slab.

[ToughButterCup]

Beat me to it @PeterW

[Adrian Walker]

27 minutes ago, deuce22 said:

Which company did you use for the raft?

 

I was told that, the benefit of using something like Isoquick was that it only needs one pour, whereas a traditional raft is two pours. However the guy from Nudura has stated that it can be poured in one amount as there's no need for a cavity tray around the perimeter.

 

It's been pretty difficult as I've only built 1 property about 15 years ago and that was on a strip foundation. The architect is also the engineer and not that helpful, he is a friend of the family and I feel like I'm the one doing all the research and coming up with different ideas.

 

Just a single pour.  The last one I did used Cemex Evolution concrete which is self leveling/finishing. - https://www.cemex.co.uk/evolution-self-compacting-concrete

[Conor]

I think your being fed conflicting and incorrect info.

 

There are two types of ground bearing raft foundations. The more traditional, a concrete raft is poured directly on top of the excavation (with appropriate compacted material etc), then walls are built on top of that. After that, insulation is then fitted on top of the raft and finally a screed finish.

 

The other type is an insulated raft, where EPS forms are laid and concrete pour also forms the finishes floor (and needs to be appropriately floated)

 

Both can be done in a single pour. The variable will be the total loading from the supporting walls, you may need a thicker ring beam around the edges. In our case, we will have 400mm thick ring beams and a 200mm thick main slab. To do this, we are  excavating down to the required depth, putting down 100mm of sub base, compacting. We'll add extra stones to the main floor part, so we only need the 200mm of concrete.  We then build 400mm high formwork, lay our services, tanking membrane, mesh and pour concrete.

 

Similar for an insulated raft, but the EPS forms are thinner at the load bearing parts, forming deeper channels for the concrete.

 

Be aware that there are quite low loading limits for EPS. We only found out at the end of the design process that an insulated raft wouldn't work, as no matter how we designed it, we were always outside the safety limits of the EPS, so have gone for a traditional raft. It's working out needing less steel and labour, if a little more concrete. On balance, faster and cheaper.

 

Make sure your engineer knows enough to get their job done, ours didn't!!

[deuce22]

Thanks guy's for that info.

 

I've got a better understanding now and thinking that an insulated raft may not work.

 

The house has 3 floors, with wall plate height of 6.6m and ridge height of 10m. I will also have a retaining wall built in to the rear and side wall, so I would imagine that this is going to be a lot of weight.

 

Thanks.

[deuce22]

2 hours ago, Adrian Walker said:

 

Just a single pour.  The last one I did used Cemex Evolution concrete which is self leveling/finishing. - https://www.cemex.co.uk/evolution-self-compacting-concrete

How did you find using the self levelling concrete?

 

Do you just pour it in and leave it?

 

 

 

 

[Alexphd1]

Dont count it out just yet. My 2nd build is 3 stories inc 1 being a basement and the engineer is happy with a insulated raft foundation. 

[Adrian Walker]

2 hours ago, deuce22 said:

How did you find using the self levelling concrete?

 

Do you just pour it in and leave it?

 

 

 

 

It worked very well and needs very little help to get in all the places it needed to be.  It eliminates power floating which can be time consuming and noisey. I will see if I can find a better photo that shows the finish. This was an ISOQUICK insulated slab with UFH.

[Conor]

13 hours ago, Alexphd1 said:

Dont count it out just yet. My 2nd build is 3 stories inc 1 being a basement and the engineer is happy with a insulated raft foundation. 

Pretty much same design here. We had issues with breaking the compressive limit of the eps300 around the edges. Only way it would have worked was with a massive ring beam and loads of steel. I think at one point we had 1200mm wide ring beam. It was just going to be massively expensive. Interested to know how your engineer has got past the loading safety limit issue of the EPS.

[Gone West]

16 hours ago, Conor said:

Be aware that there are quite low loading limits for EPS. We only found out at the end of the design process that an insulated raft wouldn't work, as no matter how we designed it, we were always outside the safety limits of the EPS, so have gone for a traditional raft.

That's quite surprising. What types of EPS are you talking about.

[Conor]

2 hours ago, PeterStarck said:

That's quite surprising. What types of EPS are you talking about.

EPS 300... Safety margins etc mean the max design loading is only 140.  It was doable, but the extent of the ring beam was huge. Was also the additional loading as the basement walls were acting as retaining walls.

[Alexphd1]

I haven't the raft spec close by hand. To be honest its basement, ground floor and 1st floor in roof but still high gables. In  my 1st build the 1st engineer had a ridiculous size of ring beam in the 1 and half floor icf house, the same local engineer washed his hands of the project when I stuck to icf when he wanted to change to single cement block on side with ewi. The 2nd engineer cut everything in spec.

[Gone West]

5 hours ago, Conor said:

EPS 300... Safety margins etc mean the max design loading is only 140.  It was doable, but the extent of the ring beam was huge. Was also the additional loading as the basement walls were acting as retaining walls.

I see, I hadn't appreciated the basement and retaining walls aspect.

[Conor]

24 minutes ago, PeterStarck said:

I see, I hadn't appreciated the basement and retaining walls aspect.

One other thing. The rear wall of the house has a 5m wide and 6m high glazed wall... Apparently this was causing point load issues at either end ?