Ground Beams — Shambles

[Superduner]

Water sitting on top of setting concrete is not a problem. In my days of supervising some pretty large pours in the Middle East (up to 10,000 m³), it was compulsory to put a sand berm around the top surface and keep it filled with water for a few days. Either that or keeping wet hessian over the surface. Concrete needs water to harden properly without shrinkage cracks appearing.

 

[Jones]

If they really can't get ready mix in then make sure they use a bucket gauge with the mixer. You need lots of buckets, fill one with cement and six with aggregate then load the mixer with them and keep repeating. If they try and just count shovel fulls you always end up with a weak mix with not enough cement.

 

[Jameshow]

If they really can't get ready mix in then make sure they use a bucket gauge with the mixer. You need lots of buckets, fill one with cement and six with aggregate then load the mixer with them and keep repeating. If they try and just count shovel fulls you always end up with a weak mix with not enough cement.

Best off with a mixer with barrows job.

They did 10m3 for me in a couple of hours.

 

[Jonm]

would it be reasonable for them to mix the concrete for this in a cement mixer on site?

I have only skimmed the replies so may be repeating what others have said. This is a structural element, totally different to normal ground bearing foundations. Ready mix is best, then you get a guaranteed mix. The problem is the speed of delivery and getting it in place, if access is difficult then pumping is best.

I can see why the builder wants to use a mixer on site, given the quantities. The questions to know/ask are
1. What strength of concrete is specified
2. What mix is proposed, ie course Aggregate proportion and size, fine aggregate proportion, cement proportion, water content.
3. how do they propose to measure the materials. The proportions are generally by weight, so if doing it by volume then the density of the materials is required and suitable measuring containers required.
4. Using a shovel as a measure is not acceptable.

The other issue is compaction of the concrete and removal of air bubbles. A dense concrete is required which protects the reinforcement, adheres to the reinforcement and provides the required strength. This is usually achieved using a vibrating poker. Hand compaction is unlikely to achieve this. Note, the concrete must not be over vibrated as this can cause the materials to separate.

One check you can do is measure the volume of the beams being cast. Then work out the weight of cement needed and hence the number of bags. Then count how many bags are used. It is best to be there when the work is undertaken.

 

[johna.clements]

I have only skimmed the replies so may be repeating what others have said. This is a structural element, totally different to normal ground bearing foundations. Ready mix is best, then you get a guaranteed mix. The problem is the speed of delivery and getting it in place, if access is difficult then pumping is best.

I can see why the builder wants to use a mixer on site, given the quantities. The questions to know/ask are
1. What strength of concrete is specified
2. What mix is proposed, ie course Aggregate proportion and size, fine aggregate proportion, cement proportion, water content.
3. how do they propose to measure the materials. The proportions are generally by weight, so if doing it by volume then the density of the materials is required and suitable measuring containers required.
4. Using a shovel as a measure is not acceptable.

The other issue is compaction of the concrete and removal of air bubbles. A dense concrete is required which protects the reinforcement, adheres to the reinforcement and provides the required strength. This is usually achieved using a vibrating poker. Hand compaction is unlikely to achieve this. Note, the concrete must not be over vibrated as this can cause the materials to separate.

One check you can do is measure the volume of the beams being cast. Then work out the weight of cement needed and hence the number of bags. Then count how many bags are used. It is best to be there when the work is undertaken.

1. That should be on the drawings!. I would assume 40N/mm2 cube design strength by the design engineer. But with site mixing I would go for 50N/mm2 mix design as the quality control will be ****.
2. It will be difficult to control the water content accurately because they will not know how wet the ballast is. There is a pile of ballast behind the bricks in the photos in the op. Obviously not enough and stored on the mud. another reason to want the strength increased. I would assume they will just get a dumpy bag of all in ballast and maybe some 25kg bags if not close to a full bag.


I would ask which testing house they are using to take cubes for testing to confirm that their mix is up to the design strength

 

[Daniel2]

I am beginning to almost feel sorry for the builders...

 

[Spectric]

Maybe people have overlooked the fact this is not a block of flats or a base for a nuclear reactor but an extension on a house which means a different approach.

 

[Jameshow]

Maybe people have overlooked the fact this is not a block of flats or a base for a nuclear reactor but an extension on a house which means a different approach.

Still a structural addition to a house though.

If a shed base I'd say wing it but not a extension...

 

[johna.clements]

Maybe people have overlooked the fact this is not a block of flats or a base for a nuclear reactor but an extension on a house which means a different approach.

Looks like a two story extension. It has needed piles to support it. As others have noted more than a shed.

 

[Fergie 307]

I would be concerned that the steel appears to be sitting directly on the osb. Not sure what the osb is doing there anyway, but the steel should finish up completely encased in concrete. If it is not raised off the bottom then it will be exposed and will corrode

 

[Spectric]

The piles are because of the clay which apart from being horrible for gardeners has depending on it's type a habit of expansion and contraction but look how much of london housing was built without piles.

 

[Spectric]

but the steel should finish up completely encased in concrete.

And only supported by the piles, not the ground.

 

[baldkev]

Note, the concrete must not be over vibrated as this can cause the materials to separate.

Hi, on a geniunely interested note, how can you tell when it has had enough poker time?
I'm considering doing a shuttered concrete wall with reinforcing cages in customers garden and can borrow a poker

 

[Superduner]

I'd say when the level of the concrete stops going down, showing that it has fully gone into the spaces between the rebar. It's easy to tell the difference between this and the natural spreading of the material (and vibrating should not be used to move concrete around within the shutter)

 

[johna.clements]

I would be concerned that the steel appears to be sitting directly on the osb. Not sure what the osb is doing there anyway, but the steel should finish up completely encased in concrete. If it is not raised off the bottom then it will be exposed and will corrode

The OSB is there to protect the expanded polystyrene from damage when it is walked on and the cage placed.
It also seems to be hiding that they have cut up the polystrene into bits rather than putting down one piece.

 

[Jonm]

Maybe people have overlooked the fact this is not a block of flats or a base for a nuclear reactor but an extension on a house which means a different approach.

As a young site engineer working for a contractor, many years ago, I installed hundreds of metres of reinforced concrete ground beams of similar cross section to the ones here. They were for single storey industrial units. There we’re constructed to the same standards any other reinforced concrete, cast in situ element. Good quality shuttering avoiding loss of grout, cover to steel, cleanliness of steel, concrete quality, compaction etc. And we did not have resident engineers on site, crawling over everything.

The beam will take a triangular shaped load from the building so the height of building is fairly immaterial, other than wall thickness.

If you were installing a precast concrete lintel and it arrived with honeycombed weak concrete and exposed steel, would you install it? This ground beam acts in the same way as a lintel and could be like that, but it cannot be seen.

 

[Superduner]

Nothing wrong with Resident Engineers crawling over everything Actually my inspectors did most of the crawling while I tried to keep the client happy.
Dubai Marina was one of my little projects.

 

[Jonm]

Thanks Garden Shed Projects, I was just thinking that I will drop the engineer a line today ...

Marcus, these ground beams have to be constructed correctly and I think your best course of action is to get the engineer involved. Really he needs to certify that the beams have been constructed in accordance with his design and specification. This will no doubt cost you money but will give you peace of mind.

 

[Jonm]

Nothing wrong with Resident Engineers crawling over everything Actually my inspectors did most of the crawling while I tried to keep the client happy.
Dubai Marina was one of my little projects.

I agree, my point was that the contractor was following good practice without the oversight of a resident engineer.

Do you think that the advice being given here is OTT as some have suggested?

 

[Jameshow]

Nothing wrong with Resident Engineers crawling over everything Actually my inspectors did most of the crawling while I tried to keep the client happy.
Dubai Marina was one of my little projects.

Love it - had a freind a steel specialist from Sheffield who said Humber + seven bridge, elmley moor mast, Conway tunnel were his little projects...!