Ground Beams — Shambles

[Jonm]

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

I have not physically used one myself, and It is a long time since I was based on site, but vibration is complete when air bubbles stop rising. Here is some advice, suggest you have a look at YouTube.

• Keep the vibrator vertical: As you work, hold the concrete vibrator in an upright position. This orientation lets it work most effectively. Horizontal motion can result in uneven compaction or cause the vibrator to snag on rebar. Be careful not to bend the vibrator — otherwise, it may sustain permanent damage.
• Prevent cold joints: Cold joints develop when insufficient bonding occurs between an old layer of concrete and a new layer. To prevent this issue, be sure to place the tip of the vibrator at least 6 inches into the previous layer of concrete.
• Vibrate with a regular pattern: When you’re using a single internal vibrator, be sure to place it at regular intervals throughout the concrete. Random insertions can result in uneven compaction and air bubble dissolution.
• Stop immediately when compaction is complete: Generally, you’ll know you’ve dissolved all the air bubbles when the concrete surface grows lustrous and air no longer escapes from it. Refrain from vibrating any further — doing so may cause damage.

 

[Jonm]

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

Steel specialist, elmley moor mast! The steel lattice one fell down, would that be the one? The replacement was reinforced concrete.

I will keep out of the “who worked on the biggest project” discussion.

 

[bansobaby]

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

An awful lot less time than you might think, and even less time than half the ops using a poker give it. For the job shown in this instance using a 2 inch high frequency poker, it would not be beneficial for it to stay in one place for more than 4 or 5 seconds. It's far more important to move it regularly, and not by any great distance, again to use this job as an example, to one side of the beam in to full depth and back out in about 3 or 4 seconds, repeat in the centre and the far side, then move along the beam about 18 inches to 2 feet and repeat.
The slump (sloppiness!!) of the concrete makes a difference, but not as much as you might think
A good operator on a poker will move further in a day than anyone else on the site.
If you leave the poker stationary for any length of time all the aggregate will sink to the bottom, followed by the sand, leaving the water on the top.
It really is a crucial aspect of concrete placement and its's scary how often it's very badly carried out.
To get an idea use the poker on a shallow pour with a sheet of mesh in it on spacers, you will soon see when the poker has done its job and when its time to move it...

 

[Jameshow]

Steel specialist, elmley moor mast! The steel lattice one fell down, would that be the one? The replacement was reinforced concrete.

I will keep out of the “who worked on the biggest project” discussion.

Reinforced concrete.

I used his little projects just as poster above did as a term of humility. He was a very humble man.

 

[dizjasta]

Surly steel fixing of this standard can not be accepted

 

[Superduner]

I think most of the advice on here is pretty good. A bit OTT maybe in parts, but not much. Concrete nearly always exceeds its design strength, and keeps getting stronger for years, incrementally.
I'd be more worried about steel cover and placement - plus curing properly to avoid the aforementioned shrinkage cracks which might let groundwater get to the rebars. Any corrosion would be invisible (and, I suppose, take years for it affect overall strength), but if it's worth doing, do it right - it doesn't take much more effort.

 

[Superduner]

One of the issues that I would pick up in the photo in post #45 is that all the tie wire ends are left poking towards the surface of the pour. Unless the wire is s/s (which I very much doubt) then it will rust, leaving another path for water to get to the rebar.
Ends should be tucked in towards the centre of the beam.

 

[johna.clements]

The wire is not stainless.

 

[dizjasta]

Looking at the white rectangle it looks like one of the piles will not be completely under the ground beam. Is this acceptable?

 

[Adam W.]

Looking at the white rectangle it looks like one of the piles will not be completely under the ground beam. Is this acceptable?
View attachment 143422

Pour concrete on it, it'll be OK.

Did you get help with your kidney stones btw ?

 

[dizjasta]

One of the issues that I would pick up in the photo in post #45 is that all the tie wire ends are left poking towards the surface of the pour. Unless the wire is s/s (which I very much doubt) then it will rust, leaving another path for water to get to the rebar.
Ends should be tucked in towards the centre of the beam.

Thanks for your explanation, informative. The image of the reinforcement cage seems to show 2 separate links at each location. Is this usual practice or a fix to overcome a bending error?

 

[ChaiLatte]

The image of the reinforcement cage seems to show 2 separate links at each location. Is this usual practice or a fix to overcome a bending error?

The structural analysis will tell the rebar detailer how many vertical legs of rebar are required. The detailer can provide those legs in any way acceptable to code. In this case, two identical links are used and that has the advantage of minimising the number of barmarks on the schedule. Where they have horizontal overlap is technically inefficient, so if the beam was five times as wide, you would detail in a different way. See, for example, Ground Beams in Construction | RPO This has a large outer link, a smaller inner link and two hook-and-bob bars in the centre, for six vertical legs (on five horizontal bars).

 

[dizjasta]

The structural analysis will tell the rebar detailer how many vertical legs of rebar are required. The detailer can provide those legs in any way acceptable to code. In this case, two identical links are used and that has the advantage of minimising the number of barmarks on the schedule. Where they have horizontal overlap is technically inefficient, so if the beam was five times as wide, you would detail in a different way. See, for example, Ground Beams in Construction | RPO This has a large outer link, a smaller inner link and two hook-and-bob bars in the centre, for six vertical legs (on five horizontal bars).

Once again thanks for your reply. The information provided should help members to understand construction aspects that they have observed but not been involved in. It is also possible that OP marcus will have an enhanced understanding on how his development should proceed.

 

[Superduner]

It's possibly worth mentioning that if there are joints in the longitudinal bars, there is a minimum lap length to consider. I've forgotten how much it should be in the UK but I'm guessing 48 X diameter of bars for high tensile steel in a beam? Joints should be staggered if possible.
You want OTT? That's probably OTT
Bars come in 12m lengths so in this case joints shouldn't be necessary.
Btw, overlapping links are ok in most cases

Edited to say that it looks as if the longitudinal bars are in one piece - so all ok.

 

[bansobaby]

One of the issues that I would pick up in the photo in post #45 is that all the tie wire ends are left poking towards the surface of the pour. Unless the wire is s/s (which I very much doubt) then it will rust, leaving another path for water to get to the rebar.
Ends should be tucked in towards the centre of the beam.

Concrete is porous (most of the time). Whilst I agree that the tying wire is not finished well, it's actually not that much of an issue. Tying wire is just to hold the rebar in place, and moisture is already present in the mix. Very large structural slabs are poured with steel (or composite) fibres in the mix. It's not possible to prevent them from being at the surface, which is why such slabs are never mirror finished with power floats.
You can specify waterproof concrete, but that's another ball game entirely...

But this job needs a good tidy up at the very least....

 

[Fergie 307]

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.

fair enough to protect it initially but it should be removed before the cages are placed, they should rest on the piles, not on the polystyrene. When the thing is done all that should be in contact with the polystyrene is concrete, with the steel embedded fully within in. Any steel left exposed will corrode and eventually weaken the beam.

 

[Fergie 307]

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?

I would absolutely agree with your point, unfortunately here the contractor doesn't appear to be following best practice, and so probably does need some oversight. And I don't think the advice given by yourself and others is OTT at all. If I were the OP I would be most grateful for it.

 

[Superduner]

Most of my concrete experience was on marine structures in the Arabian Gulf, where the sea water is particularly saline. We therefore had to take extreme care to avoid water getting to the rebar long term.
As far as I remember, all of my contracts were very specific about tie wires pointing inwards, because as it rusts away over time it provides a clear path for ingress of salty water. We didn't use fibre reinforcement for marine structures, but as it does not rust away then there would never be a clear path to the rebar.
One thing we did use - a lot - was microsilica. This is a very, very fine powder added to the mix to fill up the small voids between sand grains and aggregate, making a much denser concrete. One of the tests when determining the mix for a project was a porosity test where a pressurised water column was applied to the surface of the concrete. After a set period of time, the sample was cut open and the penetration measured.
If you want true protection to steel, I could ramble on for a while about cathodic protection where a very small electrical current is applied between the rebar cage and the concrete surface and monitored by computer. Now that can be fun to install, but quite possibly overkill on this project.

 

[sometimewoodworker]

1. How long would you normally expect two blokes working together to take install the steelwork in the picture (I mean not to do the digging, but just to put in the armature once the digging is done (it is 4m2, give or take).
2. Is it normal to assemble the armature above the hole and then lower it in, as they have been doing?
3. It is normal/sensible to install the formwork after the armature is in, as they are planning to do?
4. When it is finally ready, would it be reasonable for them to mix the concrete for this in a cement mixer on site? That is what is being planned, and to me it seems risky (poor quality control, what if the cement mixer breaks down before it is finished, risk of cold joints, and so on.).

1) 1 day possibly less
2) it’s a rebar reinforcement; and yes
3) yes
4) yes, but you need enough labour to get it done without stopping. This may mean that a RMC truck and barrow is a cheaper choic.

 

[AJB Temple]

Some very interesting stuff here from concrete experts.

However, this is a rather small domestic building and significant risk of over analysing. Definitely get them to tidy the site up much better than this, and fit the rebar correctly to spec. Definitely get a correct spec ready mix. Definitely get building control to inspect pre-pour and if you have any concerns get his or her opinion - they will have seen more domestic slabs than you can shake a stick at.

Personally I would want the OSB out as I would not want a rot void at the bottom of my foundations with eventual risk of slight subsidence. If that is not as per spec then you have a good excuse to make them take the steel out and refit it properly this time, and do a decent job of the shuttering.