Pre-stressed v pre-cast first floor!

Salmon

Hi all,

Just looking to order a concrete first floor for a self build and am pricing around! Could anyone explain the difference between precast and prestressed? I think that one needs a structural screed and the other doesnt but Im not sure? Also would anyone have any idea of the price per m2 for each as well as the spans for each?

As always any recommendations or advice are more than welcome:) !!!

Put simply, Prestressed means that the steel reinforcing bars in the concrete are loaded (streached) while the lintel is being cast.
Precast just means that the concrete is poured into a mould and the steel laid in it.

Mellor

A prestressed floor will be able to take a higher load (weight) than a simply precast floor (or lintel the principal is the same). Or to look at it another way, to carry the same load a prestressed floor can be thinner in cross-section.
Its worth pointing out that a prestressed floor could also be considered precast.
Both will need a screed.
As for price, im not sure, prestressing obviously cost more to manufacture, but it uses less materials per m2, so i'm not sure how they relate offhand.
And dont worry about spans, as for a domestic situation you are fine. Floors like this are capable of spanning further than you will need in a house, under higher loads, think of the weight in some public buildings. If you still want to know, concast have span tables on their website. I only used them last month as I spec'd a prestressed floor in my thesis design

Salmon

Yeah I've been pricing around and I got a price of e31 m2 for the precast and e32 m2 for the prestressed from one company but they are telling me that there will be a much longer wait for the prestressed stuff! The maximum span I have is 5.5m but I will be putting in 4" block walls upstairs and just want to make sure that the precast is up to the job! The company I got the quote from has told me that the prestressed have a span of up to 6.5m and the precast(unstressed) up to 5.7m!

There shouldnt be any heavy loads acting on it other than the upstairs walls!

De_man

hello again salmon

You're catching up on me!!

i've installed 150mm pre-stressed solid core, i'm away for the weekend
let me look up the figures for 5.5m span...

did you take the loading of the precast slabs into account when doing the foundation?

From calculating the weight of the screed (3") (30:10); 150mm slabs,more
12mm re-bar and A142 mesh on my gaff we reckon total weight is in the region of 85ton on the walls.

Make sure you check this one with your architect/engineer and precast
supplier as i had to spend several grand extra in putting internal walls block
on flat, when i thought some of them could be block on edge.

Thankfully i had made provision for this just in case


Pm your exact Limerick location i'll give you supplier details

Johnniep

Mellor wrote:

And dont worry about spans, as for a domestic situation you are fine. Floors like this are capable of spanning further than you will need in a house

Now there's a very dangerous statement!!!!

De_man

Salmon wrote:

Yeah I've been pricing around and I got a price of e31 m2 for the precast and e32 m2 for the prestressed from one company but they are telling me that there will be a much longer wait for the prestressed stuff!

Sure what's the panic?, your foundation was only poured recently it's been so cold it still wouldn't be set properly, once your chimneys are started they come out, to make sure the measurements are correct..under no circumstances supply the measurements yourself, even the precast company
got mine wrong by 100mm and had to supply another slab..god knows what would have happened if i gave the measurements:eek: :eek:

The maximum span I have is 5.5m but I will be putting in 4" block walls upstairs and just want to make sure that the precast is up to the job!

You'll be choosing between 100mm and 150mm slabs...i'll get back to you
but it all depends aswell on the design of the house...be very careful you can't make a blank statement saying 100mm would be okay..it all depends on the loading and if you're going to use under floor heating
Personally, i found the 100mm a waste of time and kinda springy even though technically i could have got away with it..the 150mm much more stable...


There shouldnt be any heavy loads acting on it other than the upstairs walls!


okay...don't forget the weight of your screed, they'll want a 75mm screed on top...it all depends on the layout / span of the house...for starters HEAVY duty lentils or ordinary doubled up have to be put over all window openings over 700mm.. i used heavy duty placed on EDGE cost twice as much but at least it's safe..this also goes for all internal door openings,
etc..put the lentils on edge, if you need hangers use 203X133 RSJ i've one large opening which i used 254mm RSJ (what a job to put that in place!!!
but the slab fitted directly into the RSJ so i don't see any beam across the room) or if using Z hangers ensure that the 12mm rawl plugs are inserted a lot of them don't bother putting them in:(

This is very important...make sure the house is settled before the slabs are put up..some companies will install as soon as the blockwork is completed..
this could have disastrous consequences i reckon you should give it at least 28days,just to be safe, afterall a week could make all the difference.

Once the slabs are in place, get the brickies back to put down the soaps or stock brick to get you back up to the block level. (check with laser level).ensure all internal walls are done are done at this stage too, it makes it much easier to do the screed.

(My levels were out 5mm from the front to back of the house, the brickies did an amazing job and i'm extremely thankful )

You could wait until the roof is on...but it's an awful job to do it later...
i used conveyor trucks to put the concrete up, then using a makeshift stairs
used a 24" power floater (the larger one is too heavy to lift) i hope no health and safety is reading:D once the floor set..went around the internal
blockwork with the consaw to allow for expansion etc....

Plan your services now...you could insert the pipes at the screed stage
or later if you plan to run the pipes from below (thats what i'm doing at the moment whilst waiting for the roof to be sorted...

if doing that,,,you'll see the strands of steel on the edges of the slabs..mark
using a crayon where they are,,,this will make life much easier when drilling
to see where you can avoid the steel...and therefore eliminating the use of gas bottles to cut the steel (and you may damage the integrity of the slab)

gotta go ..i reckon go to the company now with your plans and ensure
you don't need block on flat...

why not draw a quick outline of your house at least then we all can see
where the difficulties may lie...

Overall, it's a great job and well worth the extra couple of quid.

hope this helps

Salmon

De man,

Thanks for your input! I'll attach the plans that I sent to the manufacturers! I have sent them to a lot of different places and I seem to be getting a price of between €32 and €45 per m2 plus VAT @13.5%. What kind of price did you get yourself? You can see on the plan that one of the internal walls have been laid on flat and the footings have also been laid on flat. I started looking for some quotes in June and one of the companies filled out my plans with a suggested slab layout so I have used that to put in my walls on flat! There seems to be a wait of around 3 weeks no matter what company I go with so I'll have to decide soon! I think that the last blocks on the ground floor should have at least 2 weeks maybe 3 before the slabs are placed on them so that should be an adequate amount of time for them to set in!:o . I'm going to have 4" blocks upstairs, but no UFH just to let ye know! I'll talk to my engineer later on to see what his view on the slabs is!

Regards,

Salmon

firespotter

<SNIP>
Dont spoil the thread which is as stated above

Viking House

We are using the Poroton middle floor system which is light, insulating and uses less polluting concrete. It works fine with Ecocem.

De_man

hi salmon,


the spans (precast) when using 4" upstairs with screed, A142 mesh etc

7 Metres.. 150mm Hollowcore
8 Metres.. 200mm Hollowcore


the house is great:D , the wall on flat between the lounge and kitchen is fine
but i'm concerned about the situation with the wall between the sitting room and utility room is that block on edge or flat?
it's a very long span.. what did the precast company say?

For the internal doors put heavy duty lentils on edge...

The external windows,,, on the inner leaf, ensure all the openings are
have two heavy duty lentils installed on top of each other and propped with ackrows before slab installation

I thought you were in limerick,,i'm afraid i don't know any of the companies in westmeath

Salmon

Viking house,
How does that system work? Its difficult to figure it out by the pic?

De_man,
The span looks fine from what you have listed above! The slabs in the sitting room will be laid from left to right as you look at the picture so the span will be 5.5m. Did you put block walls upstairs or studding? Is the info above for prestressed slabs or precast(not stressed)? Did you use spanwright for your slabs? and roughly what prices were you being quoted?

Thanks 2 all for the advice !!

De_man

hi salmon

i had "blinkers on" i thought you were running the slabs the other way..
yep that'll be fine imho..

I used 4" standard blockwork upstairs, no studs anywhere

I used pre-stressed 150mm Solid Wideslab, 2.4M and 1.2M widths
with a max span of 4Metres cost 36 Euro Per metre & vat...

I couldn't get my hands on Viking House product, as it's not available here in Cork i have heard very good reports on it though.


Don't forget the advice on the lentils and ackrows!!!

i'll try to upload a few photos of my own place tomorrow, if i can figure out how to do it:D

Viking House

How do you upload photos? Can I email someone photos that they can get up on the forum?

De_man

i forgot....for your 5.5 Metre Span

100, 125 & 150mm Wideslab ("normal" precast) will do the job fine for you

say you choose 150mm, once slabs installed, put down stock brick to get you back up to the block level (150+75mm screed = 225mm)

or if using 125mm slab use a "soap" 100mm to get you back to 225mm or finished block level



hope this helps

muffler

Viking House wrote:

We are using the Poroton middle floor system which is light, insulating and uses less polluting concrete. It works fine with Ecocem.

This has nothing to do with the question raised - pre-stressed v pre-cast slabs.
It is however a relatively new flooring system here and may be of benefit to other people so I will leave this post and any subsequent info that derives from this.

On a general note aimed at everyone posting in this forum please stay on topic - its fairly easy to start a new thread about something else rather than wander off.

Salmon

Hi folks,

I'm back again! The slabs are due to be installed on Sat so at least I can get the house moving again! Its frustrating when work is held up waiting on something but I suppose I'd better get used to it! In the end I had to settle for a mix of pre-cast and pre-stressed as the company I went with dont do the pre-stressed stuff anymore (they say they cant compete on price with the big guys!) but if they need some pre-stressed they purchase it off another company they have an agreement with! Anyway I got agreement that they would put 150mm prestressed in any area where there would be a span greater than 5.2m and 150mm pre-cast in the rest of the house.

Deman,

How is your build going at the min? what stage are you at now? A few quick questions if I may ! You said that you drilled out the service holes for your own house, did you have much hassle? My brother did this and it was an absolute nightmare! Took him about three days to do it and he broke a few expensive core bits! Im thinking of laying the pipes into the screed like I did with the finished floor downstairs. Has anyone any suggestions on the best way to go here? Also anyone got any advice on timber battons downstairs to run services if I go down that road? What about the steel hanging grid that has been mentioned before here somewhere, anybody got any advice?

I know there are a lot of questions above! But any advice would be greatly apreciated!!

De_man

How is your build going at the min? what stage are you at now?

Pulling my hair out at times:D overall it's not going
to bad.. roof just about finished now, scudding next week, windows in two weeks 1st fix electrics and plumbing then...but i did all the chasing for the electrics/comms upstairs/downstairs before the roof went on


A few quick questions if I may ! You said that you drilled out the service holes for your own house, did you have much hassle?
My brother did this and it was an absolute nightmare! Took him about three days to do it and he broke a few expensive core bits!

i got caught on one core...i ended it by getting the gas bottles out and cutting the wire strand, otherwise i got away with it so far
but you can get people who'll do it for 30euro per core (it works out expensive)

What you could do is, immediately after the slabs are installed you could
measure where the strands of rebar that are in the slabs (thats what i did) and mark them on a copy of your house plans - it worked for me
Also mark on the where your rebar/mesh will be!!


Im thinking of laying the pipes into the screed like I did with the finished floor downstairs.
might be the way to go, it's down to personal choice
i just feel if something should happen during the build your pipes will get damaged...

Now what i did...:rolleyes: i had to put a 75mm screed on the slab before
the brickies would come back...whilst it set me back ages waiting for a weather window, this imho was a good idea nothing worse in trying to do a finished floor upstairs later on!!:eek: i'll be running the pipes underneath the
slabs, must do some work:( i'll prepare a better answer later

De_man

Salmon

immediately after the slabs are installed get the brickies back to put in the "Stock bricks" where you have internal/external walls to get you back up to block level.
Don't forget to get the lads to do you stair opening area aswell, it'll make it much easier to do the screeding!!!

Install some mesh A142 and some rebar if you've got it on site

GROUT all openings with sand and cement it takes ages, for larger openings use insulation/caulking or whatever it takes to block the holes

I know the weather is forecast to be crap next week, but i reckon you should wait & put in a "finished floor" using a mix (30:10) it would be a complete nightmare doing the floors later on!!!:eek:

Cover allcavities with old insulation / papers etc cause murphys law will state that concrete will go down the cavity whilst doing the screed

on the ground floor cover all window openings etc with plastic just in case your conveyor truck "spills" concrete

Using a makeshift stairs carry 2' powerfloater it's much easier when compared to the larger 3' float.

it'll take the floor much longer to "go off" when compared to the ground floor!!

(most likely you could pour on a morning it'll still be okay for floating the following day)

install ackrows before pouring the screed or putting blocks on the slabs,
leave in position until upstairs blockwork nearly completed!!

also what are you going to do about building upstairs??? do you have a low loader? or how do you plan to get cement up for the lads?

when the screed goes off, i recommend you get a tracks machine or loader
to put up say 7 scoops of sand and let them mix the cement upstairs
spare sand will be used for making up the "chimney mix"


any other questions, just ask

let us know how you get on

jasgrif11

Guys

I'm thinking of going with McGraths PreCast flooring. These slabs a bigger then average, has anyone used them. I also think I'll go with the steel grid under the precast as this is laser levelled and it took a friend of mine ages to counter batten.
I plan on making 2 X 4" service holes in the precast to bring the electrics and central vac from the utility to the first floor (all first floor walls are 4" block). Plumping pipes I will run under the precast floor.

Anyone know anyplace installing the steel grid?

Cheers

Salmon

Hey JAsgrif11!

I was thinking about this one myself! Would be interested in finding out if anyone has used this system and how long it takes to install as well as pricing! Anyone know any companies that do this (in the midlands if possible). Will let ye all know how it went later!

De_man

jasgrif11 wrote:

Guys

I'm thinking of going with McGraths PreCast flooring. These slabs a bigger then average, has anyone used them. I also think I'll go with the steel grid under the precast as this is laser levelled and it took a friend of mine ages to counter batten.
I plan on making 2 X 4" service holes in the precast to bring the electrics and central vac from the utility to the first floor (all first floor walls are 4" block). Plumping pipes I will run under the precast floor.

Anyone know anyplace installing the steel grid?

Cheers

McGraths of tulla are top class to deal with! no BS

sorry jasgrif11 i haven't a clue about the steel grid

are you saying you install a steel grid on the blocks, then place the slabs on top?? why would you then need blocks on flat on the lower floor????

i'm wondering are you talking about the steel grid A142 mesh???, if so, this has to go on top of the slab

let me know more about this if you have a moment please,

Just make sure your brickies understand that the block work must be very accurate, the slabs will fit perfectly:D


i have the exact same set up but you'll need more than 4" holes for the services imho

jasgrif11

The Steel grid I heard about is for attaching your plasterboard onto the slab. Most people batten & counter batten under the precast slab, but there are companies that fix steel rods underneath the precast slab and laser level them before cutting them, then you attach your plaster board to these. It basically insures that your ceiling downstairs is perfectly level.
Someone told me that they can do a full house in about 1/2 a day.

Still looking for companies that do it

Salmon

Jasgrif11,

I also read on here somewhere that witht he steel grid you dont have to worry about the effect of heat on how level it is. Timber battens can contract and expand and this can result in the ceiling becoming unlevel!

salmon

De_man

jasgrif11 wrote:

The Steel grid I heard about is for attaching your plasterboard onto the slab. Most people batten & counter batten under the precast slab, but there are companies that fix steel rods underneath the precast slab and laser level them before cutting them, then you attach your plaster board to these. It basically insures that your ceiling downstairs is perfectly level.
Someone told me that they can do a full house in about 1/2 a day.

Still looking for companies that do it

ahh i see now:D afraid i'm stuck with the batten & counter batten job
i reckon the steel grid would be very expensive and i haven't a clue where i'd get someone to do it...

Ewan

Anyone have any experience with Ducon Concrete from Kanturk...?

Slates

I was thinking of using an 100mm prestressed floorslab with 100mm 35n10 screed with plumbing pipes buried in the screed, has anyone used this option ?

Would a 150mm slab with 75mm screed be more "solid" ie no bounce

muffler

Slates wrote: »

I was thinking of using an 100mm prestressed floorslab with 100mm 35n10 screed with plumbing pipes buried in the screed, has anyone used this option ?

Would a 150mm slab with 75mm screed be more "solid" ie no bounce

150mm non load bearing prestressed slabs would be the norm in these neck of the woods (depending on spans of course) but we would also want 100mm screed to ensure proper cover on the pipes.

I have seen quite a few cracks in the screeds where it is around 70 - 80mm.

De_man

muffler wrote: »

I have seen quite a few cracks in the screeds where it is around 70 - 80mm.

i used 75mm screed, thankfully no cracks;) i cut "expansion joints" with the consaw...mixed cement, stored blocks, loaded sand with the tracks machine etc up on the screed no probs..:D

regarding services all pipes were run underneath the floor slab just incase i need access to them at a later date.

Slates

De_man wrote: »

i used 75mm screed, thankfully no cracks;) i cut "expansion joints" with the consaw...mixed cement, stored blocks, loaded sand with the tracks machine etc up on the screed no probs..:D

regarding services all pipes were run underneath the floor slab just incase i need access to them at a later date.

Did you drill up through the floor with a core bit for the plumbing pipes ?

How would one put a wetroom in place on the first floor with a 150mm slab and 75mm screed...

De_man

Slates wrote: »

Did you drill up through the floor with a core bit for the plumbing pipes ?

How would one put a wetroom in place on the first floor with a 150mm slab and 75mm screed...

yes, i mainly used a 1" coring bit, had to use a gas bottle on one,
and got a guy into do laundry chute but the best job of the lot you can hire a diamond bit coring machine for about 160 per day it'll save you
a fortune in money & time (i discovered this a bit too late)


wetroom, i wouldn't see it being a problem let the tiler sort it out, i'm sure
someone here will answer that query (as i put my wetroom downstairs) i'm sure you can buy "wetroom floors" now, you only need 10mm or so of a fall for it to work

the precast floors are excellent but unfortunately they'll add cost to the
build in both terms of cost (foundations, lentils etc) and work schedule

Slates

De_man wrote: »

yes, i mainly used a 1" coring bit, had to use a gas bottle on one,
and got a guy into do laundry chute but the best job of the lot you can hire a diamond bit coring machine for about 160 per day it'll save you
a fortune in money & time (i discovered this a bit too late)


wetroom, i wouldn't see it being a problem let the tiler sort it out, i'm sure
someone here will answer that query (as i put my wetroom downstairs) i'm sure you can buy "wetroom floors" now, you only need 10mm or so of a fall for it to work

the precast floors are excellent but unfortunately they'll add cost to the
build in both terms of cost (foundations, lentils etc) and work schedule

I was of the understanding that there is no need to bulk up the foundations..........

Mellor

Concrete floor means an extrs loading on the foundations. Which will have to be bigger. The rule of thumbs for foundations may be enough to carry the extra loads, in this case they would of been oversized if a timber first floor was to be used. But of course, depending on loading, the engineer may need to spec founds larger than the rules

250882

When your putting in a wetroom, make sure of 2 things. There is adequate fall on the screed and that the floor and walls are fully sealed before the tiler starts. The floors should have a min 75mm screed (with hollowcore slabs it acts a levelling but can be reinforced to achieve greater spans) if its not structural then grade it as much as you can to the drain. The tiled floor should dry out fully within 20mins or there is something wrong.

pete6296

In relation to putting down screed on hollowcore, do people put down insulation before the screed, is this an essential requirement?

Yes you would place insulation here, assuming it's on the ground floor.

Mellor

There would rarely be hollowcore on the ground floor. In relation to first flor slab and hollow core, insulation is used when there is Under Floor heating. This would restrict the passage of heat to the room below. A plus for obvious reasons.

pete6296

Thanks for that Mellor, i reckoned that was the case. I will be using radiators so i will not be using insulation.
Thanks

Mellor

In using radiators it will have no affect what so ever. Well unless you could bulking up the floor.

Slates

I've seen a very good detail where the section of floor slab that sits on the external wall is "wraped" with a breather membrane to improve air tightness, the upper and lower section is the plastered into the wall to give a tight seal

Graaaaa

jasgrif11 wrote: »

Guys


I plan on making 2 X 4" service holes in the precast to bring the electrics and central vac from the utility to the first floor (all first floor walls are 4" block). Plumping pipes I will run under the precast floor.

Guys, You can't just make holes in precast floors, particularly prestressed as you run the risk of curring the prestressing cables - then it is not prestressed anymore!

You need to send the details of what you want to do to the precast suppliers to check their design. And try using a metal detector to locate the wire strands before drilling/coring so you avoid them.

Ideally the suppliers need information on opes in the floor before making the units.

Graaaaa

pete6296 wrote: »

In relation to putting down screed on hollowcore, do people put down insulation before the screed, is this an essential requirement?

The screed may or may not be structural. If it is structural then it needs to be bonded with the precast units so they achieve their overall thickness. You cannot put insulation between the precast and the structural screed. If you are installing underfloor heating on top of it and want to insulate also, then either fix the insulation to the soffit or lay it on the structural screed THEN another screed for your underfloor heating. I appreciate this is getting very deep, but these are questions you need to be asking the precast suppliers, they will not give/stand over a warranty if you deviate from their design brief. It's also quite reckless.
If a screed is not structural, just for finishing (i.e. precast units alson can take the live load and loads of any block walls) then you have no problems.

Graaaaa

Slates wrote: »

I've seen a very good detail where the section of floor slab that sits on the external wall is "wraped" with a breather membrane to improve air tightness, the upper and lower section is the plastered into the wall to give a tight seal

This would be fine for modest size rooms i.e. with plenty of buttressing walls to the load bearing and external walls. If you have a large open plan layout and/or 100mm load-bearing walls then there are two potential issues:

• The floor may need a better bond to the supporting wall – consult the precast supplier.
• The wall itself may have depended on a restraining action from the floor in order to take the vertical loads and against those severe storm wind loads. A DPM type material could reduce the friction between the floor and block, giving you less stability than expected.

You need to consult your engineer for these sort of situations i.e. non-traditional room sizes, non-traditional materials and most significantly – lots and lots of glass.

Graaaaa

De_man wrote: »

i used 75mm screed, thankfully no cracks;) i cut "expansion joints" with the consaw...mixed cement, stored blocks, loaded sand with the tracks machine etc up on the screed no probs..:D

regarding services all pipes were run underneath the floor slab just incase i need access to them at a later date.

When you cut 'expansion joints', were they at locations agreed with the precast floor supplier? If you cur through the steel mesh in the structural screed then it is no longer 'structural' and may allow more 'expansion' than you hoped for.
It's not so bad it the joints are parallel with the spans of the precast units, butr if you cut across them then you are partially negating the function of the screed as a structure. Would you knock through a window without a lintel?

De_man

this thread is old as the ark


all works carried out on my build was supervised by an engineer, architect tech and a structural engineer supplied by the precast flooring company, all metal bars were outlined on a drawing so we wouldn't core through them (one bar had to be cut - no choice - and was given approval by the engineer)

Graaaaa

True, old thread, but maybe points still relevant? I came across the thread in a search for something else and.. well maybe I have more time on my hands at the moment...

Fair enough, you did things the right way. No harm in making a few points that others may not have known or realised