Concrete slab advice - it's not very strong.

[djaychela]

OK, after following advice here, I prepared my base for my shed (6mx4m) as such:

Plywood shuttering all round, providing a decent edge to work from and hold back the concrete.
Damp Proof Membrane
Steel Fabric, on the correct plastic supports (50mm).

And then I ordered the concrete from a local "ready mix" supplier - a small company who advertise a lot, and who had been helpful on the phone.

The stuff was C30, which I was told by them and others was the right stuff for a 4" slab for the use I wanted it for (a light-duty garage, essentially, but made of wood). I got it all barrowed in by me and some helpers, and smoothed it all off. It seemed VERY wet to me, but then I've only watched Grand Designs, never had any dealings with real concrete.

We tamped it and floated it as best we could (into the night), and it took a long time to set (about 18 hours? it wasn't below 8 degrees at any point during this), and was covered up with a tarp for 3 1/2 days to keep the rain off, but I kept it damp as I'd been told this was what was needed.

However, now two weeks later, I think it's a disaster. The concrete has no strength at all; I can rub the surface away with a bare finger, and drilling into it shows that deeper it's no stronger. It's more like dry sand than what I was expecting. Even brushing it clean removes a considerable amount of the top surface (which is quite fine), and below that it doesn't seem any stronger.

On looking at the receipt, it said that 375L of water was added to the mix "at customer's request" - I didn't ask for this, nor did any of the helpers.

The contractor is being less than helpful; I've phoned several times to get him to inspect it, and he still hasn't turned up.

I'm not an idiot (or so I thought!), and I thought I'd done everything right to ensure I'd get a decent slab - preparation, getting a professional to mix the concrete, etc. It seems I've spent about £600 all in to ruin a large patch of my back garden - from what I can tell, the whole lot would need to be dug out and removed, and then done again.

Any ideas? Suggestions? Would that much extra water ruin the amount of concrete I had (It was 3.5metres of it).

[lutonlagerlout]

thats a lot of water

sorry to say it but the problem was in you not knowing what you were doing

you should have refused such wet concrete

a slab like that you do NOT want very wet at all

and if it is wet you need to leave it till the laitence has gone

which is the water sitting on the surface

by *smoothing* it too soon you have pushed all the

aggregate down in the concrete

and made it weaker

sorry

LLL

[djaychela]

Thanks for replying.

Sorry to be picky, but do you think it would be fairer to day the problem was they gave me very wet concrete, and as a result I didn't know that was the case, and therefore did the wrong thing with it? I didn't ask for extra water - after all, I got the 'experts' in to give me the right concrete to work with, and they knew I'd never done this before, and didn't tell me I'd need to wait for the water to go (or even tell me that extra water had been added, or the reasons or consequences )

[Tony McC]

On looking at the receipt, it said that 375L of water was added to the mix "at customer's request" - I didn't ask for this, nor did any of the helpers.

This is an all-too common problem with small ready mix companies. They wet it up for *their* convenience, making it easier to pour and flow, so they can bugger off to the next job that bit quicker.

They will sometimes scribble what they think will be acceptable as a signature which they can then claim is their alibi for adding all that unnecessary and undesirable water.

You should be able to demand a strength test on the poured concrete. The onus should be on the supplier to prove that you, or someone with your accrediation, authorised the addition of water. This nonsense of a scribbled signature is wide open to abuse but, in my opinion, unless they can provide a sig which matches yours, then *they* are liable should the concrete prove to be below design strength.

Are they a member of QSRMC? It will say so, if they are, on the paperwork. If so, raise a query (NOT a complaint at this stage) with the scheme. If not, then raise a similar query with the supplier, namely that you feel the concrete is too weak and soft for purpose and you'd like to have it tested for compliance with the order criterion - ie C30

[lutonlagerlout]

djaychela

I dont think you are being picky at all ,but did you sign this receipt before you read it?

this kind of scenario is very common with DIYer s getting mugged off by suppliers

we use volumetric s and when they try and wet it up i refuse it

concrete should be almost able to hold its shape when poured ,if it flows like water its way too wet

for most applications

LLL

[TheRockConcreting]

If the readymix firm won't fix the problem, i would just spray it with sika proseal.

375L extra in 3.5m3 on a C30 mix would have near on put the mix at a 1-1 w/c which i think is a 50% strength loss, so you now have a C15 shed base, not the end of the world just don't try to convert it into a 2 storey man lab.

[djaychela]

Thanks for all your comments and advice, chaps.

In short:

Yes, I signed the receipt. I didn't notice what was written on it at the time; I was pretty knackered after wheelbarrowing in the concrete, plus was thinking about getting the concrete flat and so on; It didn't occur to me that I had anything else to check, as I didn't think anything other than to check that the amount was reasonable (it was a bit more than I'd calculated). So, that's completely on me, alas, and I guess they will be able to use that as a get-out; I feel a complete idiot for doing that as I'm usually on the ball for most things.

However, if someone came to me in my field of expertise (music and sound recording, or rally car preparation), and asked me to do something that would alter the end product in a negative way, I would strongly advise them against, it; I would think it should be the concrete supplier's duty to have informed me that this much water would weaken the concrete even if I -did- ask for so much extra water (which I didn't).

LLL - The concrete was very wet, but this didn't really become apparent until after we'd barrowed it all in, and it then sat with water/wet cement on the surface; we all thought this was odd, but as none of us had done this before, we again assumed we'd be given the right stuff.

Tony - they are not members of the QSRMC. In terms of testing, I guess this would be expensive? My own 'tests' have mostly been trying things and comparing them to the floor in the existing garage here - drilling, scraping, etc. The whole thing appears to be weak - the surface literally just brushes off, and underneath that it's rougher but still not strong. I can't see that it would be any use at all at present - it wouldn't even take floor paint, most of it would just come off on the brush.

TheRock - Thanks for the Proseal info, as well as the info on the concrete strength. Obviously I'm completely in the dark about the power of Proseal, but given how weak it is, do you really think it would do the job? My intention is for this to be a home workshop - it won't see a lot of traffic, but it will be having my own "project" cars in there - I've got a camper to work on, and I'm planning on doing my next rally car build in there after that. I'd really wanted to do this properly, and have a place I could work without issues, but also that I could take pride in having built properly - it sounds like you've used Proseal, so do you think it would be up to that, or should I dig it out and start again? Or dig out half depth and get the top half done properly?

Sorry to go on, but thanks again for all your advice so far.

[Brucieboy]

Can you clarify a few points:

1. When you say readymix, was it actually mixed in a large drum or via a mobile volumetric mixer.

2. What was the target consistence or slump stated on the ticket, i.e. S2 or 70mm or something similar?

3. What was the cement type stated on the ticket, i.e. CIIIA, CEMIIIA, CIIB-S, CEMI or something similar?

4. Does the ticket mention an admixture / addition, i.e. WRA. PLAS or something similar (or possibly a tradename)?

5. If not QSRMC certified, does the ticket display the BSI Kitemark to ISO 9001?. The latter is the alternative third party certified quality management system scheme to QSRMC.

6. You mention 375 litres of water was "added at the customers request". Is this is a box that actually states "additional or extra water added at customers request"? Is there another box that states "water added by driver to achieve specified consistence" or something similar. Presumably, no separate signature was required other than the one for acceptance of the load.

For a typical C30, 20mm aggregate, CIIIA, WRA mix, 375 litres of water would be the usual quantity added at the plant to achieve a consistence / slump of S2 / 70mm for 3.5 m3. If this quantity was actually added after arrival (without your knowledge), i.e. as extra water, the mix would have been absolutely flooded.

You say that you barrowed the concrete into place. Was the concrete visibly too wet at the time of barrowing or only after you'd tamped / trowelled it?

The reason I ask is that most modern readymix concrete mixer drums, even 8 m3 ones (4 axles), are only fitted with smallish water tanks of about 250 litres. To add an "extra" 375 litres after arrival seems a bit strange (not doubting what you say). Drivers don't like adding much than 150 - 200 litres to give them a reasonable quantity to wash down with.

Additionally, most drivers (not all) don't like concrete to be too wet when discharging into barrows because they can get more in (meaning less trips / less time) when the concrete is at a reasonable slump, say 50mm to 70mm.

Answering the questions might throw a bit more light on things.

[TheRockConcreting]

Have you got any photos of the slab?

I find it hard to understand from what your saying how wet it might have been... If 375L had been used, the mixed would have looked like pure water in the barrow but you ain't said anything about it sloshing out as its being moved (this is a common problem when moving wet concrete in a barrow) and you wouldn't have been rushing round levelling because it would have self levelled, this leaves me confused.

As for pro seal, yeah its the nuts. Saved many jobs. Times when you've had bad service all day and just want the job done then the driver turns up with his tap on! The load finishes up duster than the sahara, but bit of pro seal and its all good!

Its hard to say whats best/cheapest to do, pro seal ain't cheap nor is rip up and relay.

[GB_Groundworks]

Increasingly pissed off with concrete companies

We stopped using mini mix firms as they took the piss, I've got a wooden shutter 1m x 1m x 1m exactly for bases for a cabin and it's 3/4 full and they're telling me that's a cube of concrete erm no it isn't or a desire to wet it up and fire it out.

We use Cemex for big pours 100m3 in a day etc and their drivers take the piss as well claiming 30 mins waiting time when using a massive boom pump that's taking two trucks at once, putting false times on delivery notes or altering them after they've left.

Twice now I've gone to bat with the Cemex reps over waiting time charges of £400 or more on a £10k worth of concrete order I even supplied them with the security cabins in and out log showing their drivers being out by at least 20mins and offered them the CCTV

They backed down on that one but fed up of drivers passing me tickets with leaving time 15 minutes after the point I'm signing, when I confront them it's ohh is that the time or I need to wash off etc.

We've found a good volumetric set up in South Manchester and Peak District called r t Mycock and sons and they are sound done over 300m3 for us and always pitch in and help and couldn't ask for better guys. You ask for c35 you get c35 I've got 4 test moulds and I take cubes of pours now for record keeping.

[djaychela]

Can you clarify a few points:

1. When you say readymix, was it actually mixed in a large drum or via a mobile volumetric mixer.

It was delivered by a mobile mixing machine; it wasn't in a drum

2. What was the target consistence or slump stated on the ticket, i.e. S2 or 70mm or something similar?

On the phone he said 50mm slump.

3. What was the cement type stated on the ticket, i.e. CIIIA, CEMIIIA, CIIB-S, CEMI or something similar?

No cement type is mentioned on the ticket - the only description is C30

4. Does the ticket mention an admixture / addition, i.e. WRA. PLAS or something similar (or possibly a tradename)?

No.

5. If not QSRMC certified, does the ticket display the BSI Kitemark to ISO 9001?. The latter is the alternative third party certified quality management system scheme to QSRMC.

No, there's no BSI Kitemark.

6. You mention 375 litres of water was "added at the customers request". Is this is a box that actually states "additional or extra water added at customers request"? Is there another box that states "water added by driver to achieve specified consistence" or something similar. Presumably, no separate signature was required other than the one for acceptance of the load.

There's a box on the delivery note that says "Extra Water added at Customer request", and in there it says 375L. There's no other similar boxes.

For a typical C30, 20mm aggregate, CIIIA, WRA mix, 375 litres of water would be the usual quantity added at the plant to achieve a consistence / slump of S2 / 70mm for 3.5 m3. If this quantity was actually added after arrival (without your knowledge), i.e. as extra water, the mix would have been absolutely flooded.

You say that you barrowed the concrete into place. Was the concrete visibly too wet at the time of barrowing or only after you'd tamped / trowelled it?

It's difficult for me to say as my only experience of it has been watching pumped concrete on TV, and watching YouTube videos prior to doing it. When barrowing it, it seemed OK to me - it certainly wasn't like water, and took the barrow to be vertical to get all the mix out. There seemed to be a lot of water present during the tamping process. The only person out of us who had any experience was my father-in-law who said that it seemed a very wet mix once we started this (and the company had left site). We really noticed it when we had levelled it off.

The reason I ask is that most modern readymix concrete mixer drums, even 8 m3 ones (4 axles), are only fitted with smallish water tanks of about 250 litres. To add an "extra" 375 litres after arrival seems a bit strange (not doubting what you say). Drivers don't like adding much than 150 - 200 litres to give them a reasonable quantity to wash down with.

That sounds fair enough, but I unfortunately I'm going on what's on the ticket. When I phoned the company the other day to try to get someone to take a look, he quoted the figure, and said that it would "only have been added at [my] request", so I would have thought that, but from what you've said above, it sounds like that would be the total amount?

TheRock - I've taken some photos:


An overview - 6m wide x 4m deep. Shuttering is 105mm at the front. The base it was overlaid was a patio (discussed in another thread) that was solid and hadn't moved in 25 years, but was a little uneven with one corner now, hence the total being 3.5m of concrete. Damp proof membrane was put down, and fabric placed on 50mm stands.


This is typical of the surface - it's breaking up. The rough area in the picture is about 5mm or so below the flat surface, and the damage was created with a garden broom's bristles.


This rough patch was done this morning, by brushing the surface a couple of times with the broom. I've done similar with my fingers without hurting myself.


This is typical of the rest - the surface has just flaked off, and what's exposed doesn't seem very hard either - in one area I rubbed a 'trench' 15mm or so deep, again just using my finger.

Thanks again chaps.

[GB_Groundworks]

by drum he means this



as opposed to a volumetric like this

[djaychela]

Definitely a volumetric one - it was mixed on site by a machine like the bottom one.

[Brucieboy]

I suspected it was a volumetric mixer, hence some of the questions.

The 375 litres "added at the customers request" is almost certainly the total quantity of water added at the mixing stage to achieve a typical 50mm slump which, as said before, is normal for barrowing. You've more or less confirmed this by saying you had to tip the barrow vertically to empty it.

Because the company doesn't provide details of the constituent materials used on the delivery ticket, you're not sure what the concrete comprises. A QSRMC / BSI certified supplier would have to declare these materials. Additionally, under these two ISO 9001 quality management system schemes, the delivery ticket should also state that "the concrete conforms to BS 8500" - the British standard that supports EN 206 - the European standard for concrete.

Don't get me wrong, there are some very good volumetric suppliers in the market. However, some are not so good - particularly if they use high proportions, if not 100%, of recycled aggregates, the quality of which can vary from good to very questionable. Companies that are not subjected to third party certification / routine inspection by QSRMC or BSI are obviously self-regulating!!!

Most civil engineering specifications require concrete to be obtained from a QSRMC or BSI certified company to assure quality - not always achieved as even they get it wrong sometimes.

Unfortunately, the DIY market is open to anyone who wants to start up a concrete business.

It's difficullt to establish from what you've said / photographs the actual quality of the in-situ concrete. It really depends on the constituent materials used. A 50mm slump concrete can bleed (water rising to the surface), but it's not normally excessive.

Jay's suggestion of applying a hardener may beef the up the surface as long as all the scaly / loose material has been removed. If you're using the workshop to restore a campervan, you don't want a dusty surface.

[TheRockConcreting]

Looking at the photos the mix looks like a dark fly ash mix, this type of mix takes along time to harden. Does it say anything about CEM II/B on the ticket?

You might be having segregation problems aswell, this is a regular thing with volumetrics because the 30 seconds it takes to travel up the auger is crap.

Its not sandy anyway thats the main thing, just leave it for afew weeks then test it again, if its still weak i would get some pro seal.