Concrete New Homes

Waffle Slab Inspection With Engineer’s Commentry

G’day. This is Matt Cornell from Cornell Engineers. This is a waffle slab inspection that we did a little while ago and I wanted to go through and put some words to it to let you know what we’re looking at during this inspection.

So here we go coming in from the front I’ll just pause it there and explain some of these things.

The white things that you can see are the polystyrene waffle pods. They are the voids in this soon to be poured concrete waffle slab. The waffle pods are about one-metre x one-metre squares. They’re sort of semi-hollow underneath. They’re not solid polystyrene but on top they’re solid. The space between waffle pods is about 110mm wide.

In between the waffle pod, void formers are the ribs of the waffle slab. When this concrete slab gets poured the spaces will be filled with concrete. The tops of the waffle pods all will be covered in concrete and the ribs will be the strength of the waffle slab – these beams or ribs that occur at about 1.2-meter centres.

You can also see a few other things that are interesting in this shot. Around the outside edge, there’s an extra heavy bar – much bigger
or thicker than all the rest of the bars in the middle – so this is some crack control around the edge of the slab.

These things are bar chairs they keep the mesh up off the slab and the timber in the front is the formwork that’s the side of the concrete when the concrete gets poured and this timber gets removed so that’ll be the outside edge of the concrete.

So let’s get a little bit closer and have a look here. OK. So now we can see the bar chairs closely. We can see that really looking right up the middle of that 110mm wide rib and we know it’s that wide because there are some pod spaces down in the bottom holding these pods apart.

So let’s zoom around this way. So looking round to the front. Maybe this is the entry in behind here.

These pipes sticking into the air – they’ll be toilets and sinks and basins. They’re connected to the pipes are in the ground that are running underneath the waffle slab.

You can see here there’s a gap in waffle pods and you can see there’s a gap around the front between the formwork and the first waffle pod. This gap is about 300 millimetres wide. This is the strong part where the walls is going to be built on.

The code in Australia, AS2870, is the code for Australian residential footings and slabs and it says that you have to have a certain amount of strength and thickness in underneath the outside walls so that’s what this is through here.

This big area through here is the stiffness that’s required to support a little bit of footing that comes out in front of us here to the right. So the space where there’s no waffle pods will be all concrete.

In this waffle slab, this area will all be concrete and it’ll continue out through the middle here.

Coming up to the left-hand side we get an opportunity to have a good close look at what’s in the bottom of the footing around the outside edge.

There’s these three bars, one two three. There’s that extra bar up on the top – don’t forget it’s hanging off the slab mesh – but these three bars in the bottom are the bottom reinforcement.

In waffle slab design the slab mesh is actually the top reinforcement so this is the strength when the load is pushing down and the slab mesh is the strength when there’s an uplift load or hogging we call it when part of the slab is cantilevering.

Coming around this way, in this area you can see the shape of the waffle. The waffle pods have been cut. The pods didn’t fit exactly within the slab shape and this is what it looks like when a waffle pod is cut through. You can see there’s thin legs of polystyrene inside the waffle pod.

This area even though this is meant to be void obviously concrete’s going to be able to flow into this space. That’s okay. It’s a little bit of extra concrete. They probably didn’t allow for it but it’s okay. It’s good.

It’s really hard. You can’t get waffle pods to fit every house so they get adjusted on-site by the concreters or by the carpenters.

Just stepping back here through this area is where the slab goes from inside to outside. This would be the outside patio area. There’s a strong beam through here. The extra reinforcement on the top and then this is probably the patio slab in the back.

The slab surface is a little bit lower – these pods are actually set a little bit lower in the ground and this is part of the problem that we have with waffle slabs – that the waffle pods have to be the same height for strength but the fact that they’re set a little bit lower in the ground.

So on our website, we talked a lot about drainage under waffle slabs and this is the area that we’re talking about where the pods are actually set lower in the ground. It actually allows water to get in underneath the house and can’t get away because the ground has been cut lower like a swimming pool.

Coming around the back edge of the house we’ve got some cut waffle pods again, some ribs, and then a series of ribs all at one-metre centres because of the shape of the pod.

Bar chairs holding the mesh up and they’re doing a great job.

Okay here’s some more ribs. So coming to the end of the inspection we’ll see as we scroll past we’ll see some of these pipes have been set up for termite protection to stop termites coming up beside them. They’ve got discs that are installed by the termite installer.

I don’t think I’ve mentioned already that’s the bottom reinforcement that runs through the ribs. So the engineering drawings tell the builder how big these bars need to be.

A couple of things that control how strong this slab is is the height of the waffles of the waffle pods – the higher they are the thicker the concrete is the higher the mesh is from the ground and the bottom reinforcement. The higher they are the stronger the slab is so, in this case, I can’t remember how high this was maybe 310 millimeters high overall and the waffle pods are set up to account for about 85 to 100mm thick slab and they sit straight onto the dirt.

That takes us to the end of our waffle slope inspection.

I hope you’ve picked up something from this. A couple of things that we actually look for structurally as structural engineers is to make sure that these bars, the steel bars, aren’t touching the formwork and this is not a bad shot you can see that there’s a bit of space there between the formwork and the first bar and you can see there’s a bit of space there between the reinforcement and the waffle pods. That means when this gets filled with concrete the concrete is going to be all around these bars and they’ll be protected from the moisture and the outside environment by the concrete and that’s what stops them from rusting. So that’s good.

There’s one thing missing that we haven’t spoken at all about and that is the fact that there’s a step between the where the bricks are going to sit so there is going to be a rebate or a step in here and that’s going to be installed in the next day or so after this video. So they won’t actually put in a horizontal piece of formwork they’ll probably put in a vertical piece that is supported over this outside formwork. That becomes the top part of the step and this is the vertical face that you can see on the outside when the job is all done and the bricks will sit on this space in here.

All right. Thank you very much for joining us. That makes up the end of our engineer’s waffle pod slab inspection. I hope you took something away from it.