Soft concrete - Concrete a bit soft on top?

[Parched]

I had a concrete suspended floor laid for my extension sometime in July (it's November now). It was a very hot day and shortly after it was laid the sun was on it. I'm told there was some kind of inhibitor mixed in with it which made it a bit gloopy. There was no covering and no water sprayed on top afterwards. The next day there were some small surface cracks and dry cement was brushed in. Yesterday though I discovered the surface (at least 1cm deep) is easily dug up with a screwdriver. I've been assured that's because it's still relatively new and it takes a few years to go properly hard. Do you think there's a problem with the concrete or am I just being fussy?
https://www.flickr.com/photos/111609225@N06/15726727452/
https://www.flickr.com/photos/111609225@N06/15724922405/

[Brucieboy]

There are a number reasons but before further comment what were the mix details as described on the delivery ticket:

Designation: e.g. RC??, C?? - denotes strength or was any other description used
Cement type: e.g. CEMI, CIIIA, CIIB-V or any other description
Minimum cement content (kg/m3): e.g. 260, 300, 320 or any other value
Water/cement ratio: e.g. 0.65, 0.60, 0.55 or any other value
Consistence or Slump: e.g. S3, 120mm, S2 or any other description
Admixture or additive: e.g. PLAS, plasticiser or any other description

If nothing like the above, how was the mix described?

Finally, was it delivered via a rotating drum mixer truck or by volumetric lorry (like a tipper with a small mixer at the back)?

[Parched]

I don't know, I wasn't given those details - I guess the builder might know but he tells me it's ok. It arrived on a volumetric lorry. These are the pics from the day after it was laid showing the cracks:
https://www.flickr.com/photos/111609225@N06/15727334702/
https://www.flickr.com/photos/111609225@N06/15723853401/
It was laid 6" thick with reinforcing mesh in the middle, on top of DPC membrane, on top of Celotex, on top of levelled sand, on top of the ground.

[Brucieboy]

I was a bit concerned with the colour of the concrete from the first pics - seemed quite brown - but from the second set it appears to be the more normal grey.

If the cracks were evident within the first day of laying, they are classed as "plastic shrinkage" which generally occur if the surafce has'nt been protected thus allowing the water to evaporate quickly causing a change in volume. Although not ideal, it's general practice to brush in a cement slurry to close them - being plastic shrinkage they don't normally extend the full depth of the slab so the structural integrity is not compromised.

With regard to the softness of the surface and your ability to scratch it easily with a screwdriver, this could depend on one or combination of a number of factors such as the strength designation of the concrete, the cement type, cement content and if an excess of water was added. Without knowing these details, it's difficult to be certain.

From the second set of pics the concrete doesn't look too bad (I've seen a lot worse) but if you're still in doubt ask the builder to provide you with a copy of the delivery ticket.

[Brucieboy]

ps - when you say the levelled sand was laid on top of the ground, do you mean soil / clay or was it on a properly compacted base such as Type 1?

[lutonlagerlout]

looks like plastic cracking july was hot
we had an oversite go like this in the heat
LLL

[Captain Concrete]

After 28 days the concrete should be up to the specified strength so, the soft top is worrying. Its possible its the bleed water coming to the surface and that it was to wet, many builders like it too wet as it makes it easy to lay, this would make the suface dusty as well. As for the cracks to me that just looks like normal concrete contracting, how big was the area? was it a hot day? I often refer my customer a very good section on this very site
http://www.pavingexpert.com/concjnt1.htm

also Iam not sure about your builder doing this
"The next day there were some small surface cracks and dry cement was brushed in" was that over the whole slab?
What is the slab for is it a finish?
AC

[Parched]

Thanks for your replies guys, I appreciate it.
Levelled sand was laid on top of the ground, soil / clay, whatever war there before, no type 1 or compaction, but I guess being suspended it's unimportant.
It's a garage, so yes, final finish (bar some floor paint).
I was speaking with a civil engineer yesterday, and he said what the architect specified, C35, should be rock hard after 28-days. Apparently the crack of 1.5-2.0mm is big, and constitutes more than cosmetic hairline cracks. It shows rapid drying and probably would have been avoided if the concrete had been covered with plastic sheet to allow it to cure properly. If left as is then over time road salt from the car is likely to seep into the crack and corrode the internal reinforcement compromising the integrity of the suspended slab. There are 3 main issues:
1) is the whole slab strong enough (can be determined by digging down to hard concrete, and asking the architect or an engineer if the reduced thickness is sufficient for purpose, or by a core test - drilling out a core & sending off, cost ~£150, but would need to avoid reiforcement!). Also need to determine if the problem is all over the slab or localised.
2) the durability of the finished surface. If it's only soft for the first, say 20mm, then it's possible to remove that layer and repair with appropriate preparation & more concrete of 10mm aggregate), cost, no idea.
3) if it's deemed strong enough and the surface left as-is, then at a minimum the cracks need to be repaired, properly.
Flippin' nightmare.

[Captain Concrete]

What has your builder said? Have you companied to the company that supplied it? Is there any spare floor to ceiling height? could you go over the floor with some Proper concrete?

[Brucieboy]

If it's a suspended garage floor slab as opposed to a house floor slab, the cracking is a bit more of an issue for the reason pointed out by the civil engineer, i.e. potential corrosion of the reinforcement by road salts over time.

Firstly, how certain are you that a C35 was actually supplied? – you’ll need to ask the builder to provide you with a copy of the delivery ticket if you are in doubt.

However, even if the delivery ticket does state C35 you still won’t be absolutely certain the concrete in question has achieved the required strength for a number of reasons including the possibility of batching errors, was excess water added over the target design quantity etc, bearing in mind your concern about the softness of the surface.

You will have more security of the above if the supplier is third party certified by either QSRMC or BSI - the logos of either of these organisations will be shown on the delivery ticket. You could also ask the supplier to provide batch records for the load(s) in question.

If you go down the coring route, one core is generally not enough. You'll need at least two (preferably four) to give an overall strength assessment of the slab. Avoiding reinforcement is possible with the use of a covermeter (suggested core size 100mm). Coring is not cheap - I think circa £150 is an underestimate for the combined drilling and testing costs. If you decide to proceed you should inform the builder and supplier beforehand and preferably get their agreement - who is going to pay for it depending on the outcome?

Coring will not provide an assessment of the quality of the surface of the concrete as the top and bottom sections of the drilled cores are diamond-sawn off to ensure their surfaces are flat and parallel for capping prior to testing. It’s a bit of a tricky one to determine suitability as it’s quite subjective.

If by whatever means you are happy with the quality of the concrete, the cracks may need to be filled with a proprietary material depending on the severity – 2mm is quite wide and whilst they may not extend the full depth of the slab, they may reach the reinforcement – for a suspended slab this will be nearer the bottom. I have seen cracks on externally exposed slabs filled by brushing in a cement slurry – it all depends on how fussy the client is. Some are even left to fill up with rubbish – not ideal though.

If you go for a concrete overlay (assuming working height is not a problem) , the minimum thickness is 75mm to avoid curling at the edges. Thin toppings are available but these are generally expensive and require removal of the surface by scabbling or similar.

You really need to discuss the problem in more detail with your builder, the supplier and the architect before making any decisions. Concrete is not a forgiving material and costly for someone when it goes wrong. However, with further discussion/investigation it might be resolved with minimal expense and aggravation.

[lutonlagerlout]

those pictures dont look like C35 unless they *pissed* it up

which would be crazy

LLL

[GB_Groundworks]

Sounds like they didn't know what they were doing and treated like a typical over site subfloor.

Concreting is an art and takes years to perfect.

Those cracks look way bigger than any plastic cracking I've ever seen.

What finish was it looks a roughy tamp? We magnesium or wood bull floated then later steel float/brush or power float.

On big pours over 60m2 our supplier cemex send a surveyor to take slump cones and we have cube moulds and take cubes on structural concrete now.

[TheRockConcreting]

To be able to make the hole in the concrete with the screw driver like that, it would put the slab at around a C10/C15, way to much water added to the mix epic fail.

[Parched]

Thanks for your replies, it's much appreciated; useful info. The builder admits it's 'sunburnt' and is prepared to 'put it right', which helps. I asked the concrete supplier what was supplied and he said he'd get back to me. Meanwhile the builder asked me why I had asked the concrete company that (and I hadn't told him I had phoned them!). C35 apparantly, but I'm not convinced. 'Putting it right' comprises taking the top layer off, filling the cracks with poarable 2-part resin and applying a topping, but having spoken to people like Ardex the floor top needs to be 'sound' beforehand, which seems to be somewhat subjective.
A flooring guy came round this week and took off the surface with a grinder (that broke half way through) which shows up the extend of the cracking (see https://www.flickr.com/photos/111609225@N06/sets/72157649199591387/ - ignore the ads). What concerns me is that the top is still scratched off easily, even with my finger nail in places (and I have vaccumed it). Perhaps it's strong enough, just not very durable? What bothers me is if a thin topping is really going to stick properly. It could end up even worse in the long-run. Ultimately I have to pay up when the jobs finished, because I can't withold payment on the basis that I think it's rubbish underneath, and in the long term what is the likelyhood of him coming back to fix any problems ...

[Captain Concrete]

You have 2 options
1: cover the floor with an really good covering here's a link, talk to Ashley or Craig there both very good.
http://www.remmers.co.uk/Product-catalogue.159+M55546b9274f.0.html#bot
2: take up floor and re-lay c35 with an S3 slump

Good luck