Waffle Slab Inspection with Engineer's Description
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- Опубліковано 19 жов 2024
- This is an inspection of a waffle slab that Cornell Engineers completed. Brisbane structural engineer, Matt Cornell, describes the different parts of a waffle slab what he looks for when inspecting waffle slabs before they are poured.
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Don't forget, this was not the inspection required for a Form 16 inspection certificate and we weren't checking the slab against the drawings - that inspection had already been done by the design engineer's inspector.
This was an inspection of the waffle slab on ground to AS2870 that Cornell Engineers carried out for the owner's peace of mind. We checked the waffle slab construction for good practice in the assembly and make up of the waffle slab before it was poured.
So during the inspection we talk about the pods themselves, the ribs that are formed between the waffle pod void formers, the edge beams of the waffle slab, the slab reinforcement, the beam reinforcement, penetrations, cover, (which is the amount of concrete around the reinforcement once it has been poured) and edge reinforcement.
if you're building with a volume builder and you get a chance to take a video of your slab before it is poured I strongly recommend doing it. A video is better than 1000 pictures and a video like this can help rule out problems with the slab if problems ever arise. I guess this video will help you know what to take a video of!
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Thanks - I needed an explanation. It appears to be what the builders are using behind us on our friend's sloping clay block. I had my doubts about the strength. Building inspections down here are a thing of the past. Future house prices will be reduced by the lack of quality.
This is the best video i have ever seen, very detailed and professional for all the points, thump up
Hi
bao Robin
You just made my day! Thank you for the feedback.
Matt Cornell
Awesome. More reo inspections would be great!
Thanks Ryan. That's a good idea!
Super informative! Cheers!
I think there’s a fundamental problem with waffle slabs as the only two friends I know who have waffle pod houses, are both are having serious problems with movement. When your house starts to break in two because of poor foundations there’s got to be a systemic problem. The engineer, builder, concreter and inspector have all failed to do their job. How does that happen? Like someone asked what happens when water soaks in under the house? Won’t the buoyancy cause problems? Either way, 100% of my sample has had issues so I’ll stay clear of this design thank you.
In terms of scientific investigations I think two samples is a fairly small subject pool. :) But yes, at 100% problem rate it does make you wonder.
I haven't heard of houses floating away som let's rule out buoyancy issues for now and have a chat about reactive clays, screw piers and site drainage.
Wait.
We already have. We have a series of videos right here on our channel that cover off on these isses!
There's the potential for a lot of finger pointing when houses move so your friends need to educate themselves, do everything they can to show they have complied with the engineer's design drawings relating to drainage, tree planting, leaking taps, garden watering so they don;t become scapegoats.
Watch our videos and hopefully they make sense of all this.
Then prepare for a battle because you are right. The builder blames the soil tester. The engineer blames the certifier. The soil tester blames the plumber. The council blames the developer. And somehow you have to make sense of this massive blame game.
Or, if you read this and watch the videos in time (before you build) you might decide to skip all this heartache, take lots of photos during construction (as per another of our videos) and think very carefully about which builder you invest with.
I hope we caught you all in time. In any case - take lots of photos!!!
Matt Cornell
Taking lots of photos of the formwork and the day of pouring is the best advice
When slab mesh got cut due to vertical service pipe, some engineers specify "provide a 600*600 off cut of mesh, lap and tie to the underside of slab mesh". Just wondering does this off cut mesh has to be installed to the underside of the slab mesh or it can be located on top of the slab mesh?
Could you please show me some spec for spacers and how to estimate it? To be honest, we don't use this type of slab to build houses here (Southeast Asia), so I don't have any sources to learn about it clearly. Thank you!
Great video. Thank you for explaining this in detail. Just wanted to know more about piers and edge beams. Do these increase waffle pod strengths? What about Concrete strength? Looks like most builders these days use the minimum recommended strength of 20mPa. Do you reckon that a 25 or even a 35 mPa concrete will make a waffle pod function better? What function does the re-entry mesh serve? How about waterproofing and sanding the surface before laying out the pods? What about vapour barriers and damp proofing? Sorry to ask so many questions but with builders cutting corners for maximum profits, I think home owner's only remaining hope is an honest engineer :)
Wow. Abhishek Vora thank you for all those questions. The easy one to answer is that 20MPa concrete is all that is required under AS2870 - the code for footings and slabs. In theory 25 and 32 MPa standard concrete strengths have some advantages when it comes to crack control but have more problems with workability - the screeding and finishing process. The extra concrete grade improves deflection control marginally but when an Australian standard specifies a minimum complying limit there are some that manage to just hit that line and some that prefer to jump right over it.
Let me ponder the other questions and I may do a follow up video to answer. If any other commenters want to chime in, please do so below!
@@CornellEngineers You guys are awesome. Thank you for getting back I will wait for other answers.
+1
My off the plan house has concrete piers in & with waffle raft. Is this better than the floating waffle pods? My builder is only offering 6 years structural warranty, which raises concern. It would be great if Mr. Cornell can suggest what should be done as a purchaser in this case.
Thanks for the video but some conclusions at the end would be great. Has that particular slab been done properly? Different height was mentioned for patio. Is it a really big issue? Worth talking to the builder and making them redo half of the slab or not?
Fair call Alexander and I shouldn't have presumed the answer would be clear if I didn't say anything.
In this case, the slab reinforcement and cover complied with the engineer's drawings. No major issues were identified.
So pretty much good news on this occasion.
In fact, most concreters put a waffle slab together in accordance with a set of drawings pretty well.
We do see issues with pods floating within the wet concrete from time to time. This results in a thinner slab than specified. The last time I heard about this the builder identified it by accident when a hammer was dropped on the slab. A dull, hollow-sounding slab when struck might be a reason to investigate the slab thickness further.
The bigger issues come from poor site drainage and we're about to upload a video specifically on drainage issues for waffle slabs and this new video will cover off on the patio slab setdowns a bit more.
Matt
PS: This is the link to our drainage video for waffle slabs ua-cam.com/video/kO5GDPhQexI/v-deo.html
Thanks for the Video Matt. I'm no expert but watch your video, it seems like that plastic sheeting will also act as a funnel for surface water from the backfill next to the slab. And isn't polystyrene going to float if there's water underneath it? Isn't this a terrible design which promotes slab failure/heaving? I'm looking at building a home in Qld and just seeing so many predatory project sales tactics is an awful experience
Hi Jarrod
Thanks for your comments.
I know slab edge heave can be an issue but I don't necessarily attribute this to polyethylene membranes under the slab.
I've not heard of polystyrene floating after the concrete cures but if the concrete isn't poured carefully the individual pods can move and float as the concrete is poured.
The idea of void formers within a concrete slab isn't without merit but it needs careful protection against uneven soil moisture conditions - but then so do raft slabs.
As far as predatory sales tactics - call the companies out. Social media and forums like #homeone can be a great place to share experiences. Eventually the careful, considerate builders will float to the surface. When the other builders manage to silence bad situations by muting individual customers then the building industry suffers Why should shoddy builders be able to get away with it?
Matt Cornell
@@CornellEngineers Thanks for the reply Matt, you’ve given some good advice. Today I spoke to a national ‘premium’ spec home builder and I asked about their slab options. It was either straight waffle on cut/filled ground, if it unfavourable soil: waffle on piers (30-90cm wide up 1m-3m depth x 40-60 piers) with an option to vary the pod height. The waffle slab then rests on the levelled piers but aren’t attached to allow movement. Approx $50k for the piers + waffle pods. What are your thoughts on this method? Would you mind me asking why the waffle design can lead to failure? And would the pier + waffle method mitigate the issues?
@@Tigerratcat Waffle slabs on piers where the ground conditions vary and/or are soft is a solution to the specific problem of uneven differential settlement/consolidation of soils. Having piers throughout the dwelling (under internal and external footings) is a great idea - the whole house is then founded on a similar soil stratum
For an explanation of why the waffle slab system can lead to failure check out my video on waffle slabs and drainage. ua-cam.com/video/5Cu8PXN3Wl4/v-deo.html
The solution? A compliant engineered design based on a careful site classification built by a builder that cares enough to inspect and record photographic evidence of correctly-installed plumbing, concrete work, landscaping, and framing backed up with in-person construction phase inspections by competent and experienced engineers and certifiers.
Matt Cornell
Great video!
Thank you!
I have off the plan house coming up, It is approved by structural engineer for M-class slab on P classed site. It is waffle raft with concrete piers. Is there anything I can do to bring it to builder attention as the structural warranty is only minimum 6 years.
Check out my answer here: ua-cam.com/video/5Cu8PXN3Wl4/v-deo.html
nice video, thanks mate
Glad you enjoyed it Alberto.
Is it possible to build a two storey home on this kind of waffle pod system? And is it possible to use a cut and fill method on slope sites with a waffle pod system?
Hi Stefan
Thanks for your questions. Yes. It is quite possible to build a two-storey house on a waffle slab system but with limits according to AS2870. For example the code does not allow for double storey masonry veneer construction on highly reactive (Class H2) sites. An engineer can over-ride these standard designs, but at their own peril.
Cut and fill is almost mandatory on waffle slab sites. The slab system requires a super-flat site so that all the polystyrene pods can be laid out at the same height (except for set down areas). Those flat sites are long gone so any sloping site has to be 'made flat' and the cheapest way to do that is cut to fill. Of course, with filling comes the chance of subsidence so that too will need an engineer to sign off.
Hope that helps.
Matt Cornell
Cornell Enngineers
@@CornellEngineers Thanks Matt that was very helpful info!
Do the water pipes get tested before the pour?
Hi SkodaPANDA. I’m not sure. Maybe a plumber can answer this question?
In theory yes in practice no.
I do inslab heating and cooling and all pipes must be tested.
But plumbers never do.
Drainage needs inspection before pour.
I’m no fan of waffle pods.
Great marketing by the polystyrene companies.
Is it possible to use waffle to have a slab for a shed raised 1.3m off ground level?
Hi Commissarius
I'm guessing it is a sloping site? Unfortunately, waffle pods don't come that high and it isn't usual to stack them. The other issue you will have is forming up the side of a slab that high. So at that height, there are a few factors working against you. Your engineer will have some ideas on how to overcome them.
You didn’t mention the shrinkage control buddy
These are not great on highly reactive clay are they? Would you have any clue why builders are still using this kind of slab when it's not recommended to build on specific types of land?
Hi Discotheque
Yes, it's a controversial issue and I guess the answer is that it comes down to construction cost, the buying power of builders who are willing to look for engineers that will sign off on waffle slabs on reactive clays and ignorance of the soil moisture maintenance issues that must be addressed by builders, plumbers and homeowners to achieve satisfactory performance. Queensland has started to attack this issue by introducing requiring special plumbing designs for reactive sites. The next step is to educate plumbers to ensure this information makes it through to construction. Eventually, I expect the cost savings that waffle slabs offer will be dissolved by the extra compliance, extra plumbing cost and possible extra insurance overheads that come with floating a house on reactive soils. But really, consumers need to become more informed so they can choose for themselves whether they can really achieve the required soil moisture maintenance specified by the engineer - sometimes at risk of not having gardens or trees anywhere near the building.
Matt Cornell
Cornell Engineers
Is that still the case of you use piers?
What is the trimmer bar doing structurally under slab mesh?
Hi Al Jafari, do you mean the diagonal trimmers?
@@CornellEngineers At 3:27 you are explaining the extra bar on top below the slab mesh. What is that bar doing? Thanks
@@aljafari2364, the design engineer specified it. It's top reinforcement so that bar makes the slab slightly stronger. From memory this slab was poured on concrete piers so top reinforcement is also required over piers according to the code. Finally some engineers provide extra top reinforcement to try and prevent cracks forming at the edge of the slab.
Hope this helps.
Matt Cornell
@@CornellEngineers That makes sense. Thanks Matt
no over lap in the top mesh in many parts of the job 😅
these waffle slabs can crack up pretty badly conventional slab on ground is much better the Styrofoam also attacks termites rats mice etc 😅
:)
the steel fixing is pretty average
you should have 300 mm over lap in top mesh everywhere 600 mm in trench mesh minimum 😅😅😅
1200 centres rib to rib buddy. I would of failed that job just on the chair spacing let alone the rest of that mess. Doesn't sound like you know what your talking about, must be an inspector acting like a real engineer
LOL. Oh Jacob you crack me up!
@@CornellEngineers what would I know right, just a concreter that lays pods almost everyday chair spacing should never exceed 800mm spacings tell me I'm wrong
@@jacobedmonds2076 Yes, you are right about the 1200 centres for pod ribs.
The 800mm spacing requirement for chairs is shown on most engineers drawings (including ours) but this number doesn't come from AS2870. Cement Concrete and Aggregates Australia specify 800mm to 1000mm centres. 800mm is a good target, especially for light mesh, but I haven't seen anyone fail a slab for bar chairs being spaced out a bit further. Have you?