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kxi

Early thermal cracking in newly poured external concrete slab

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About 150m2 getting an external concrete slab. Nicely divided into reasonable sized, mostly squarish bays. Several of these have gone in fine.

 

But one of the bays poured on Thursday (i.e. 48 hours ago) has now developed lots of small cracks, which I assume are early contraction cracks as the slab heats, cools, dries out, and whatever else concrete does as it cures.

 

My concern is - how much do these matter? Is any remedial work worth it? I'm assuming this is annoying but probably fine to leave as is?

 

This slab:

- Farm roadway to take occasional heavy traffic

- 200mm thick

- Roughly 5x4m

- Single layer of thickish reinforcement mesh (basalt fibre - more on that another time) at about 130mm from base i.e. in upper half

- Layer of poly at the bottom

- Compacted subbase of crushed concrete

- Laid on hot sunny day, some shading

- Something was sprayed on top - I think to slow down evaporation

 

All the other slabs have been totally fine, including two others done same day. I wasn't on site to see this one, but the usual groundworker foreman was on holiday, and apparently there was a dog that ran into the mix as it was being poured which caused some disruption.

 

This one:

 

image.thumb.png.42703ace9f9013f3441056906fda34ac.png

 

 

Examples of cracks:

image.png.d93f69426769cf37c6fe8728c8d11d5d.png

 

image.png.72669b9809e4709c4fc41fe15fb9d754.png

 

image.png.0aef176d9bc1a4e8608f02dbb1d94ac7.png

 

image.png.bb3a23dd4b1fa6e2c1e45d38a78c74b3.png

 

 

 

 

 

 

 

 

 

 

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Looks like cracking from too warm and not protected after pouring in this really hot weather concrete would really need damp Hessian at a minimum.

Don't look like they'll be too much of a worry but I'd war h them and see. If caught early you can brush some raw cement in which will help seal some.

 

Biggest problem with small cracking like them is it's a route for water which can cause larger problems with the freeze thaw affect in winter time

 

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You have two choices 

break it out and re do it, or leave it alone, there is nothing remedial you can do. 

As said above, cover with wet hessian next time or polythene sheet. 

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As your paying to have this done if your not satisfied (not fit for purpose) ask the contractor what he proposes 🤔

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Concrete too wet when laid, then dried too quickly. And esp too much wet paste at the top.

Probably inappropriate reinforcement, and lack of debonding at joints.

Hot day and fast evaporation. 

(The difference between curing and drying is for another essay).

This is a very common problem and I am constantly amazed that groundworkers do not know this. I can only assume that they have not been docked for it before.

 

Adding extra water makes it much easier to move around and level. But then that water evaporates and leaves voids, where there should be concrete.
In any ready-mix delivery there is a space on the chit to record added water, and the driver will fill it in and want it signed, because the supplier knows this can cause problems, and they  don't want the blame.

A concrete mix is an exact science. When 'just right' for strength it is awkward to shovel, so they will add water, without asking anyone in authority..

 

They may well have sprayed some sealer on top, supposedly to keep it from evaporating, but it hasn't worked, probably because it has just sat on the free water.

polythene/wet hessian are more fool-proof.

 

It may not fall apart, or maybe it will. As stated above it will let water through the cracks, and that will freeze, and that will break the concrete.

I think the top surface of paste (no stone) will abrade quite quickly.

 

Also, all concrete cracks, and it is a matter  of controlling it to a minimum and to multiple invisible microcracks.

The reinforcement , and debonding at earlier edges, helps with that, and I can't comment on what you have in that regard. whether it was appropriate, and whetehr it reached the edges/ overlapped etc.

The choice of appropriate steel mesh is well proven, but I have no idea with 'thickish basalt' mesh. Certainly plastic fibres don't work as well as was originally trumpeted, or as steel mesh. More info would be interesting.

I would worry that a small mesh would reduce  bonding at the layer. But it is new to me and I don't know.

 

Depends what you have paid really. If you have employed a bunch of labourers who don't know the science of it ( and simply have not been paying attention) then perhaps it is your problem, and is decent value, as you have saved 25% oh and p, and management. 

If there is a specification or a known performance requirement then you have an argument.


If this was for a formal contract it would  have to  come out  and be redone.

as a private job for yourself it is up to you.

 

Whatever, keep the whole roadway damp throughout the warm weather and for a month in total, as the cement needs that time being damp to do its job.

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You may be able to scabble the top off and lay a thin layer, but that is expensive too, and not a proper job.  A rebuild is the proper solution if it is a problem to you.

Otherwise, plan to tarmac over it in a few years.

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Thanks all.

 

The guys have hitherto done an excellent job, and already laid maybe 300m2 perfectly. This was the first slab without their usual very-experienced foreman, as he was on holiday this week. I was apprehensive about this, and turns out with good reason. This slab is exactly the slab config as 6 others laid in the roadway (inc. same 8mm basalt mesh reinforcement), including 2 others laid that same day, so I doubt the reinforcement etc is to blame. The cracks are all over it, whereas the other slabs have none at all. So I suspect something happened with this one.

 

Shame, since they have been on site for nearly a year (lots of groundwork...) and with a few weeks to the end it's the first mistake.

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29 minutes ago, kxi said:

Thanks all.

 

The guys have hitherto done an excellent job, and already laid maybe 300m2 perfectly. This was the first slab without their usual very-experienced foreman, as he was on holiday this week. I was apprehensive about this, and turns out with good reason. This slab is exactly the slab config as 6 others laid in the roadway (inc. same 8mm basalt mesh reinforcement), including 2 others laid that same day, so I doubt the reinforcement etc is to blame. The cracks are all over it, whereas the other slabs have none at all. So I suspect something happened with this one.

 

Shame, since they have been on site for nearly a year (lots of groundwork...) and with a few weeks to the end it's the first mistake.

When the foreman is back and if he's a good un , have a chat and I'd imagine once he sees it he'd be happy enough to discuss relaying this section it doesn't look very big and won't be a big effort or cost to take out and relay.

If it was me or one of mine I wouldn't want a couple of cube of concrete tarnishing a good job otherwise.

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16 hours ago, saveasteading said:

(The difference between curing and drying is for another essay).

Post up that essay, there are similar problems in the composite plastics industry that are not well understood by many.

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Done that essay.

 

I have a friend who posted the most awful domestic slab up on facebook, asking about cracking. It was very much worse than this discussion.

The answers and suggestions coming back from public and supposedly 'experts' (my husband is a screeder and says....) were concerning.

 

Re the current post. I once had trouble getting concreters for a big industrial floor. A gang I knew from before said they could do it, without their ganger man who 'wasn't necessary'.

Oh yes he was. I couldn't believe how these people who had indeed poured lots of slabs, could not do it properly without their ganger. I had to do a lot of pointing and instructing myself, which isn't the idea, then I can't remember if I sacked them or found a ganger.

 

This happens in all trades....the second and third in command do not tend to think much of the whys and wherefores of what they do every day.

 

So yes, your ganger may well agree that this is not acceptable and has to be sorted...

Please, ....not by concealing the cracks with some cement paste, as it will only be a few mm thick, and the cracks will still be there.

 

Trying to be optimistic, it is possible that only the surface paste has a severe problem. They have allowed too much paste to come to the top, and that may just come off and leave a hard surface, but a few mm step down.

 

Keep all these slabs wet.

 

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4 hours ago, saveasteading said:

Done that essay.

Yes, read it.

Basically the same as most resins curing. One problem is that the exothermic reaction can cause extra evaporation of the styrene.

Also if the top cures faster than the base, problems can occur that are not obvious at first.

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Is this for thick resins? In what sort of use?

 

In concrete there is significant heat generation too, but it isn't much relevance in everyday building. Dams and bridges under construction may have chilled water and aggregate in the mix, and cooling pipes running through.

 

Concrete slabs curl in curing too, but not in everyday, controlled circumstances.

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Posted (edited)
34 minutes ago, saveasteading said:

Is this for thick resins

Not really. Can be a problem with just a few millimetres. There is also a problem with different shrinkage rates between the side that is in contact with the mould, with or without a gel coat, and the side exposed to the air.

It was a serious problem with Carbon and Kevlar fibres in the early days, part of the reason vacuum bagging was developed.

But if you lower the pressure too much, you get greater evaporation and that changes the chemistry of the resins.

 

I used to know a guy at Sheffield University that was very knowledgeable about concrete, we used to call Ewan, Dr. Cement.

The place I did my apprenticeship at made material testing machines, was always fun watching then push, pull, shake, cycle, freeze and bake stuff to destruction. When I went back to university to start my Masters, one of the older lecturers asked if I was Mo or Joe (an Instron joke), he knew his stuff and was over excited to meet me. 

Material testing people are a bit strange.

Edited by SteamyTea

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22 hours ago, kxi said:

very-experienced foreman, as he was on holiday

Good luck with the discussion.

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6 hours ago, saveasteading said:

Done that essay.

 

Saveasteading. Thanks for posting that and your other info, enjoyable and informative reading for me. Keep going if you can as this type of info, presented in an easy to read format as you have done really adds to the quality of this forum. You give some great practical tips too!

 

Interesting that Steamy Tea mentions carbon fibre and it's behavoir. There are few few posts on BH about folk using bassalt type meshes for crack control in slabs but as it's more elastic " stretchy" athough with a high ultimate strength then many of the old rules of thumb need careful appraisal before swapping say a traditional A142 / 193 mesh out for a carbon derived mesh.

 

You also mentioned slab "curling". I've got a bit of this on my own house. I have a quasie "structural" ground bearing slab.. I'll leave it at that as I was experimenting. I put the day job hat on so cured it properly but I hand batched it so less quality control over the water cement ratio etc..but you have touched on this already. The slab is thin ~ 100mm with A142 mesh and the UF heating pipes are attached to this. It curled before the UF heating went on though. If you give the corners of the slab a "dunt" you can feel it's not fully resting on the perimeter supports at the corners but only by one or two mm so when it cracks the crack width should not be enough to cause a problem for my application.

 

It's not called for example "concrete technology" for a laugh! There is a fair bit to this remarkable mixture as you say.

 

On 24/07/2021 at 14:53, kxi said:

About 150m2 getting an external concrete slab. Nicely divided into reasonable sized, mostly squarish bays. Several of these have gone in fine.

 

 

kxi. The joint locations look respectable, the aspect ratio (length of each panel / width of panel < 2.0) looks ok too. I would imagine you have some extra loose rebar diagonally at the corners of the manhole covers as if not you could expect some cracking springing from the corner points of the manholes due to the stress concentration.

 

 

 

 

 

 

 

 

 

 

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I had not heard of basalt reinforcement before joining this forum. Fibreglass basically I think.

 

If anybody can pass on success or problem stories then that would be great.

Where do we buy it even? If only from ebay and alibaba then it is not for me, yet.

 

why are people buying it?  Price? Availablity? Or known properties?

 

I can find remarkably little about its uses and properties, so am sceptical.

Shame because buying it in rolls is so much more convenient than great 4.8 x 2.4m sheets.

 

I can see that it might be very appropriate for internal concrete floors. The tendency to stretch would not be  a problem (might even help distribute micro-cracks) , as long as the bond is good....it doesn't look great by  sight.

Steel mesh has more texture, is less shiny, and even the rust helps grip quite a lot.

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Posted (edited)
On 24/07/2021 at 14:53, kxi said:

About 150m2 getting an external concrete slab. Nicely divided into reasonable sized, mostly squarish bays. Several of these have gone in fine.

 

But one of the bays poured on Thursday (i.e. 48 hours ago) has now developed lots of small cracks, which I assume are early contraction cracks as the slab heats, cools, dries out, and whatever else concrete does as it cures.

 

My concern is - how much do these matter? Is any remedial work worth it? I'm assuming this is annoying but probably fine to leave as is?

 

This slab:

- Farm roadway to take occasional heavy traffic

- 200mm thick

- Roughly 5x4m

- Single layer of thickish reinforcement mesh (basalt fibre - more on that another time) at about 130mm from base i.e. in upper half

- Layer of poly at the bottom

- Compacted subbase of crushed concrete

- Laid on hot sunny day, some shading

- Something was sprayed on top - I think to slow down evaporation

 

All the other slabs have been totally fine, including two others done same day. I wasn't on site to see this one, but the usual groundworker foreman was on holiday, and apparently there was a dog that ran into the mix as it was being poured which caused some disruption.

 

This one:

 

image.thumb.png.42703ace9f9013f3441056906fda34ac.png

 

 

Examples of cracks:

image.png.d93f69426769cf37c6fe8728c8d11d5d.png

 

image.png.72669b9809e4709c4fc41fe15fb9d754.png

 

image.png.0aef176d9bc1a4e8608f02dbb1d94ac7.png

 

image.png.bb3a23dd4b1fa6e2c1e45d38a78c74b3.png

 

 

 

 

 

 

 

 

 

 

What you are seeing their is shrinkage, either from direct sun, high temps, or a breeze blowing over it.

 

I would not be too worried about this given the application, the cracks will not be all the way through. 

 

 

Edited by Carrerahill

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38 minutes ago, saveasteading said:

had not heard of basalt reinforcement before joining this forum. Fibreglass basically I think

I had heard of it, may have to do a little research when I get time. Holiday season now so not got much free time.

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25 minutes ago, saveasteading said:

I had not heard of basalt reinforcement before joining this forum. Fibreglass basically I think.

 

I'm using both the light rolls and the bigger mesh, mainly for anti-corrosion aspects, and plan to post about my perceived pros and cons when I get a chance.

 

Previous posts:

 

 

Quick comments:

- 'Better than' glass fibre since doesn't degrade in alkaline environments (i.e. concrete)

- Buy it in UK from Orlitech (who resell the Galen product) unless you want vast quantities in which case can go direct

- Still very unknown in UK, and barely used, but more widely talked about/used in US and China. Perhaps because they have more of a culture with fibre-rebars

- Plenty of published research on the properties, but it's heavy going, and perhaps some question marks over whether the stuff you buy always meets standards (I don't know...)

- The bigger stuff is sanded so has a very good grip

- Was at one point (and perhaps still is?) cheaper than 'equivalent' steel 

- Needs 'full consideration' e.g. use with non-metal spacers if going for a zero-steel approach, how it will be walkable on etc (this was a problem for us, but now solved)

 

It will be interesting to see whether the Surfside collapse prompts further consideration of non-steel rebar such as BFRP. Even if rebar corrosion isn't found as the ultimate cause (e.g. if was a sinkhole) it was telling that a common reaction was roughly 'well of course the structure would be weakened because of rebar corrosion - that's what happens to steel rebar especially in coastal conditions or with poor design'. I.e. an implicit assumption that steel rebar can be problematic.

 

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Thanks re Basalt. I will keep looking for info.

If your original road design was for 2 layers of A393, that seems very heavy structural construction. Presumably the SE had nothing to do with your choice of a crack-control amount of Basalt mesh instead.

 

As you say, there appears to be an assumption that 'of course steel rusts'.

This makes a good story for non-technical journalists and public. There is seldom an interview with a Structural Engineer, because the answer would not aid the panic....

I should have some articles in the pile of technical press I haven't got around to opening.

 

He would say 'It is not that simple, and proper dense concrete does not normally fill with salt water and rust to failure.

There are dams and sea walls that have stood for decades.'   so they don't ask.

 

We saw the exposed steel in Miami, but I expect that could have been sorted in good time.

I have seen a seaside (ie 10m) concrete building completely disintegrated in the Caribbean. I suspect there was salt in the concrete mix, and a series of other issues like insufficient cover to the bars.

 

Would other reinforcement perform any better?

 

I am working on design of 400m2 of floor, in a conversion, so would be interested in using basalt mesh.

It will go through the door, and be easy to place.

There is insulation and screed to go on top, so no real risks.

Hence my interest is real as well as academic.

 

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9 minutes ago, saveasteading said:

Basalt

Just had a quick read of the wikipedia page about it.

https://en.wikipedia.org/wiki/Basalt_fiber

 

Seems to be used in a similar way to glass fibres.

The thing to look up, in my opinion, is what the mesh is mixed with i.e. binding agents, coatings, water repellents etc.

Some fibres have proved very hard to process, so a Silane coating is added, this can, depending on the chemistry, be a help or a hinderance.

Chemists can sometimes be quite clever and design polymers that have one end that is attracted to a material and the other end that can be attracted to another material.  But as a clever Dr. of Chemicals I worked with once (actually worked with 3 in the past, all a bit weird, well one was just Scotch) said, 'you get the properties you want, and something else'.  It is that 'something else' that catches us all out.

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