Incorrect Concrete Dilemma

[nexusone]

Hi - I am in the process of building 2 small industrial sheds. 32MPa concrete was specified for the slabs, which the concretor quoted. They were poured in the last 2 days.

HOWEVER - I found out today that only 25MPa concrete was ordered and used (I have confirmed this with the concrete supplier). I found out from some delivery dockets left lying around on the site.

I don't think the concretor was trying to rip me off, and suspect he just had brain disengaged for the day.

I am hopping mad. Too many times I have used tradespeople who do not do what you ask them!

What suggestions do you have for how I deal with this? I have not paid him yet obviously.

Steve

[dai sensei]

The impact of 32 vs 25 MPa depends on how big your shed is, how it is reinforced, and what founding the slab has - speaking as a structural engineer myself. 25 is not generally used as structural concrete, more for the slab that don't matter (back yard sheds & walkways etc).

Ask your builder to provide evidence from a structural engineering certifyer that the slab will in fact be satisfactory for the intended use.

You should at least not pay the full price, 25 concrete is cheaper, if anything at all.

[Barry_White]

I think you will find that 25mpa concrete is adequate structurally for a shed however if you are going to be running forklift trucks on the surface you will find that it will deteriate quicker than 32mpa concrete.

You could always extend your footing bolts with threaded rod and threaded rod joiners and pour a four inch slab on top and get the concretor to pay for it provided you had submitted the request in writing.

Spent several years selling and organizing construction of industrial sheds and our engineer who specialized in industrial sheds only ever specified 32mpa concrete whenever the shed was to be used with forklifts.

[Gaza]

This often happens in commercial projects, that is the reason samples are taken and tested as pours take place. (U know the cylinder's that are sent away to be tested and also slump test).

I would contact your structual enginer as you will require to get this desigh change certified, also you can contact the concrete association in vic. (NSW has one)

Prepare a letter and send it off ASAP to the builder and concretor advising them of this issue with your proof and that you will be with holding money to this is resoved. This stops them from taking action against you in the short term for non payment.

It maybe ok most enginers go over the top with designs but in factories pallet racking has very large point loads from the unrights.

[seriph1]

nexusOne I would get a NexusSix to visit them and explain in brain crushing detail the subtle differences and implications of their actions

:D:D:D:D

Alternatively do what the others said. What a huge pain in the *****

[Pulpo]

Structually I have no idea as to the difference between 25mpa and 32mpa.

I recently ordered concrete for a shed [75m2] and specified 30mpa.

This slab was not engineered, just spoke to a few.

The concrete place suggested only 25mpa required, although I insisted on 30mpa for delivery.

The difference in cost was $10 per cubic metre, bugger all.

If price is a guide I cannot immagine a great deal of difference.

However depends what you will place on the slab.

One reason why I ordered the concrete knowing the order was right.

But considering that I never did a slump test, how would anyone know, how would anyone claim on the manufacturer?

MPa is obviously important but just as important how it was laid.

In truth I would be annoyed but not devastated.


Good Luck
Pulpo

[Gaza]

Slump test does not calc the Mpa but is a mesure of the workability of the mix, ie stiff etc or wet. If the mix is to wet then this can cause big problems too.

All of the big brand concrete co.s have quality systems in line with AS2001 in order to maintain this they must supply the mix as ordered. Ie if an add mix is required or smaller agg, . For a small job they would not want to risk there certification.

[Carpenter]

One little piece of advice, the only thing I could recommend you do to ensure maximum strength out of your 25Mpa slab is to keep it wet/damp for as long as possible. Concrete will continue to cure whilst it remains wet/damp. Soak it down & cover it with plastic to prevent drying by evaporation, wet it every day for a couple of weeks. I have noticed road building contractors spray bitumen on freshly poured concrete, this encapsulates the water & helps attain maximum strength. This will help, & aside from the topping suggestion, its about all you can do.

[Wildman]

I am with Barry on this one, Roughen up the existing slab (and soak it thoroughly with water before you pour so it doesnt suck moisture from fresh concrete on top) tie another layer of mesh on the top and thicken the slab. 75mm would be enough depending on the coverage you require (indoors, no liquids - especially acids, then 25mm will protect the steel fine). Get 32Mpa on the top if you need more surface hardness but the extra thickness would more than compensate for 25Mpa for the whole slab.

If this floor is going to be trafficked heavily, especially with steel castors or the like, or if it going to be subjected to oil (eg mechanics oil spill), there is a product from Cementaid (Diamite I think) that you spray onto cured concrete (min 14 days) that soaks in about 5mm and converts any remaining unreacted cement into a synthetic quartz like material giving 100-150MPa surface hardness as well as making the concrete far more resistant to oils and liquids and eliminates slab dust. I have seen sump oil wiped off with almost no evidence remaining. It is not cheap, a 205 litre drum (400 square metre coverage) was around $1100 when I used it 10 years ago but it is sensational for slab longevity. (make sure you drill dyna bolt holes before you treat as it makes it difficult to start a hole in the right place, the drill skips a bit!). It may save the contractor some time and money though if the only reason the 32MPa was ordered was due to it being run over by heavy traffic rather than structural requirements.

Cheers
Ben