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One for the civils – concrete
We are having some issues with the supply of concrete meeting the specification (its under strength) and I am curious as to why and how this could have been managed better. I don’t think the contracts are robust enough to manage the risk of getting it wrong as we appear to be dealing with the aftermath of it all rather than our subcontractor. Tail wagging the dog scenario. The questions goes out there as to whether anyone else has had any major dramas with their concrete not meeting the required standard and have you had any issues with your supply chain? If so, what has been done about it?
Repeat answers from the whatsapp group greatly appreciated 🙂
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Late trucks, incorrect slump, not reaching 28 day strength. We issued NCR then had a sit down meeting with the bigwigs and told them to sort it out.
PM sat down with the construction manager and regional director. BLUF sort it out or you won’t work in QLD again.
Doug
Am I understanding your response correctly?
A contractor held a concrete supplier to ransom in QLD?
In London there is so much demand that the concrete suppliers sort of have the upper hand.
From what I can gather it all depends how close your site is to the batching plant.
We are relatively close and therefore have not had many issues – still the usual late deliveries and occasional slump problems. But no failed cubes and nothing that has required big wig engagement
That’s pretty much it. Funnily enough we did not go for the plant that is 800 m away and went for one the other side of town.
Concrete quality so far has been fine both Euromix and Hansons are on the money with the mix specification and strength. We have had issues with availability though, especially when one of you gits has a big pour on a Friday and we’re only after a load plus.
We have hurdles for hauliers to jump through in terms of FORS bronze working towards Silver after 6 months – which has caused us to lose an aggregate haulier that can find quicker turnarounds elsewhere (drivers are paid by the load). The vehicle checks also have an effect on concrete turn around time – can be nearly an hour from plant to tipping and the plant is only 2 miles away.
So far though the concrete suppliers have been fairly helpful as they want to get in with us before the big D-Wall pours and supply our precast yard as well, but even with that we’re small fry compared to some of the jobs in London.
PS happy World Quality Day everyone!
I’m off to a presentation from Hansons very own Dr Concrete now so I might add some more later.
In fact Jo having re read your post I personally have an issue with our subcontractor using Euromix to test concrete supplied by Euromix. Clearly they have their lab license on the line but I wouldn’t be happy with this when we do some of the bigger pours. Shouldn’t this be an independent test. Anyone else have the same deal on their site?
Tony,
On our site it is stipulated that the QA must be carried out by independent labs with UKAS accreditation, although the sub-contractors may do the slumps and make the cubes themselves.
We also have one sub-contractor who gets the lab to attend site at the time of the pour to do slumps and make the cubes – this removes the requirement for concrete cube baths on site.
Both scenarios seem to work and all our SCs conveniently use the same lab to do the tests. There were some issues initially with getting the SC to make the cubes effectively – not leaving them out of the water for days on end and making sure the baths were temperature controlled etc.
Chris,
That’s exactly the way I would have expected us to be doing it. I think that our S/C is trying to cut corners everywhere having under priced their work, but i’ll check our works information to see what is stipulated.
However, Dr Concrete believes that the concrete industry should be trusted to self regulate as the conformity and duty to report shortfalls lies with the producer, and that tests done by the contractor are often poorly carried out (like your S/Cs initial testing) and can throw up confusing results. He thinks that identity testing should only be done by a contractor if there is doubt over the concrete quality, for striking purposes, spot checks, or if it is in the works information. I can see his point but I think a contractor will need a lot of trust in his relationship with the concrete producer.
Jo – do you have any more details on how your concrete was poor quality?
Not surprisingly, Hansons state that issues likely this are normally down to the contractor/designer either poorly specifying the concrete by wcr and slump (i.e. method and outcome) or by insisting on concrete on the “wet” end of the slump range for workability (i.e. trying to get an S3 while paying for an S2). Also when you say it failed the slump does this account for the permitted range and for the different limits for initial and main discharge? I can’t find the reference but the example I have in my notes is that the slump range for an S3 is 100-150mm but with the permitted deviation of -20/+30mm this becomes 80 to 180mm.
Tony, our concrete has been specified by performance. It must reach characteristic strength of 100MPa and associated elastic modulus of 42.2GPa in accordance with the Australian Standards. It has failed to achieve this. The concrete is pretty wet to ensure we have the consistence to pour and we haven’t as yet identified the exact cause for the low strength. It is probably a combination of some of the issues John has eluded to below. The engineers are interrogating the mix design but they feel it is probably more an issue with the aggregate, however no one seems particularly interested in visiting the batching plant to see what their processes are or checking the quality of materials they are using. They’ve just been told to sort it out. I think one of the major issues is they initially poured with only a 7 day strength. I cannot understand why the mix design wasn’t tested with sufficient time to get the 28 day results with time to adjust the mix design if required. We are where we are, four core pours down with under strength concrete. Rectification measures are taking place to some of the tower columns – load goes to stiffness and all that. Early results for the 5th pour appear higher with the tweaked mix, but we will have to wait out for the 28 day strength result to come in.
We use an independent tester who comes to site to take his cylinders every time we pour.
Of note, what is S2 and S3? I haven’t heard those terms.
I’m going to close my eyes to the “What are S2 and S3” comment and mutter Consistency, Slump test, Flow test, BS EN 12350 parts 2 and 5. Wainscot concrete labs…..
Not particularly my bag but , for the purposes of CPR debate:
Basic: Things that influence long term strength water/cement ratio; cement class; aggregate mineralogy ,grading and angularity; degrree of compaction
So we want high strength:
1 low w:c ( strength inversely proportional )
2 Lots of Tricalcium silicate and dicalciam silicate in cement ( and less t ricalcium aluminate and tetracalcium aluminoferrite)
3 Finely divided cement gives better hydration and a stronger binder
4 Since the aggregate is usually far stronger than the binder it is the grading of the aggregate and the uniform distribution of aggregate that is more important than the rock type. A crushed aggregate gives angular facet that it always stronger with the binder ( you get 15% form a rough aggregate surface) . A good grading = easier to get a low air voids content during compaction. Usually lower max agg size ( within reason) the greater the strength for a given w:c ratio
5 Low air voids content ( compaction – 6% reduction for a 1% increase in air voids)
On large jobs the things that militate against strength are:
1 The ambient temperature – low temperature gives low strength ( @ 5or so degrees you get about 60% 28 day strength by comparison with 20 degrees C)
2 Need for consistence – so either higher w:c OR the use of retarders and plasticizer additives OR use of uniformly graded agg
3 Need for good compaction pushes w:c up or use of plasticer up
4 Need to control thermal cracking in large volume pours – so the use of lower proportion of tricalcium silicate or pozzolan replacement ( so lower rate of hydration) which brings the short term strength down
Somewhere in that lot is an answer
Jo,
The S refers to the slump value and is one of the methods of classifying the consistence (workability) of fresh concrete listed in BSEN206:2013. Generally, the higher the number the more you pay for a specific strength, hence the tendency for contractors to request to be at the wet end of an S3 rather than pay for an S4. Everyone wants something for nothing. However, as John points out, the wetter the mix, the more risk there is that you don’t reach your required strength. I can’t see how to paste a table in to this comment but it’s table 3 to BSEN206:2013 if you want to look it up on IHS.
If yours is a designed concrete does that leave the liability with your Engineers rather than the supplier?
I’ll have a look at the BSEN. That classification is not used here, the slump is specified within the design to a tolerance.
The contract is a D&B so the engineers specify the strength, stiffness etc of the concrete and it is on the subcontractor (and their subcontracted concrete suppliers) to design the mix. The consultant engineers do not design the mix. They do however peer review the mix and test results and always give the standard non-committal comments that ends with ‘it is still your liability if you proceed’.
Remember your designers are interested in the final strength and the density of the matrix in terms of providing protection for durability and fire, usually encompassed by setting a max w/c ratio and min cement content. They might also have an interest in max agg size for ensuring bond. These are the parameters they will check when reviewing a proposed mix but they won’t want to get involved in squabbles about consistency, practical placement and curing that might erupt between contractor and supplier. The only other interest they might have is heat of hydration/contraction rates and rate of strength gain. Particularly the latter if early loading or prestressing are involved (think influence on creep coefficient).
The contractor, whilst wanting to deliver the designers requirements is more interested in cost per cubic meter, ease of achieving quality and rate of strength gain. Cost is for materials but also for placing and curing so consistency becomes important. This impacts ease of achieving quality and minimising remedial works and rate of strength gain is all about on hire costs for formwork and rate of progress. The result is the contractor adds requirements to those of the designer and passes this to the supplier to offer the best solution within their armoury of options.
QA is all about managing the risks. Liability may have been transferred through contracts but risks come home to roost.