What happens if there is a problem with the mix? Who to assign blame to? Just curious. What happens if one supplier allows water to be added and the other doesn't?
One supplier is the client, the other supplier is a local supplier from town. I don't think we're going to let either supplier add water on site, only super plasticizer.
As Dik mentioned, if for some reason after placement the breaks come back and they don't meet the spec you will get into a sticky mess on who pays for what if the concrete needs to be torn out and replaced. That said, I believe it is actually common for large pours to get trucks from multiple batch plants and suppliers.
It is a not good idea, but for large area and massive concrete volume, the use of multiple suppliers is possible, or sometimes bot avoidable. The plants shall use the identical mix design, with source of materials specified, to narrow the difference in products. QA is of paramount importance. Also, consider to divide the suppliers in different areas with distinct boundaries will help to minimize the confusion in trouble shooting.
The inspector takes his samples from a single truck and notes the truck ID, batch time, source, where poured etc., so you have a good Perry Mason built in.
My experience is that unless you know what the specific object being cast is, you cannot normally determine the 'exact' location of the load. This is more likely to occur if you have a large pour. You may have better 'record keepers' than I'm used to.
A suggestion as a quality record, setup some cameras to record the pour so there is at least some idea of which batches went where.
My thoughts on the original question is that if the mix design is the same then the same level of performance being achieved is the intent from either batching plant if they are certified to the same supply standard. Write the specification in a way that whoever designed the mix becomes responsible for any issues from both suppliers. If required the side responsible could supply personnel to oversee the batching in the other plant to ensure the same standard is being achieved. Or just use one batching plant but utilise the concrete trucks from both companies.
I don't think for large pours that it is that unusual to have multiple suppliers involved.
One thought though, suppliers are usually fairly protective of their mix designs, if the two companies are usually competing for work. As engineers in this part of the world they are even reluctant to give us the mix design and it is subject to strict confidentiality agreements when they do release them. I cannot imagine they will want to willingly hand over their competitive advantage in the form of their proprietary mix designs to a competitor.
I'm curious about whether the client is usually a concrete producer (eg a precaster), or the client is just dabbling. Also to what level of detail the mixes are the same: are the aggregates from the same source, or just meeting the same minimum requirements but actually could be different etc.
I still don't think there's an inherent problem, but of course there's a problem if you assume someone's concrete is bad.
Same experience at Agent666 here that suppliers don't give out all their secrets so maybe the mix design is by the client? One time a supplier was required to provide a client's mix rather than one of their own proven mixes, the supply contract was on the basis of 'all care, no responsibility'.
Coming from an industrial background where most concrete is simply spec'd as 4500 psi compressive strength I have never seen any concrete supplier have an issue with supplying the mix design that they use (locations across the US). Are they protective with all of the mix designs or just some of the more specialized mixes?
I have also only ever seen one job where the concrete breaks didn't meet spec, however they were still deemed to be acceptable. With everyone in this thread cautioning about the potential issues if there is a problem with the mix, I am curious how often this is actually an issue for people?
dauwerda - I have heard tell of a time in the early 2000's (think economic bubble) when the demands on the batch plants in my area were so high that they had to get a little creative in material sourcing. The result was less than ideal. Engineers started specing 500 to 1000 psi more than was used in design just to make sure they got acceptable breaks.
I only had the issue once myself, and it turned out to be a failure of the testing lab to maintain proper controls for sampling and testing. I got a 28 day break of 1700 on a 4000 mix. We had to take core samples to verify everything was good. It was, but had it not been and the source was from multiple plants I could certainly imagine the headache it would cause.
The client is mining, they often have batch plants on site. They would be sharing mix design, and ultimately some bad breaks wouldn't really matter unless they were truly bad (like 2000 psi instead of 4000). I'm not sure how we could split the areas of the pour up, but we will know which plant supplied which cylinders.
For small job that one batch plant can handle, why mix more than one together? For large job, there is always a way to break it into smaller areas with different crews and concrete suppliers.
It's more important to know what area the cylinders came from... you will not likely run into any problems... but, if you do it could get very interesting. If the client is so set up, why not have him provide all concrete?
When I was a Concrete inspector on BARTD, we had to record the exact placement location of the concrete we sampled. I haven't checked ACI but suspect this is a requirement.
