>Pardon me if I'm outa line here
>but isn't w/c ratio directly correlated to
>design mix strength ? If your w/c ratio is getting thrown for a loop
>somewhere it will definitely show up in the compression test results.
Doh! I guess I was thinking too hard and missed the obvious.
>Additionally, if they've submitted data in accordance with ACI 301 you
>should know the design average strength and the running standard deviation.
>Am I missing something ? No mater what admixtures are added you should have
>some past test results to go off of.
We are using CSA codes. Usually they are similar. I could not find where
they have to submit this. When we ask for the batch mixes we get them, but
that's about it.
>Even if you parked yourself beside the truck, what could you do. The driver
>delivers the concrete to the buyer, the contractor. The contractor tells
>driver to add water. Is the driver going to say, "No?" Not on your life!
>You can't give the driver directions as he/she is not under your control.
>The only thing that the driver (and you) can do is record the added water.
If we see it, we can, and have told them to send it back if we deem it to
be unacceptable. That is providing we make it clear beforehand that the
driver is not the batcher and no water will be allowed.
>The thing that concerns me more than the adding of water is the mixing (or
>lack thereof) that is done after the water is added. Early in my career
>I was inspecting a pour (placement, if you will) I walked to the counter to
>get the counter reading and noticed that rubber or leather finger on the
>was missing. I called this to the attention of the job superintendent and
>later I got a call from the concrete supplier --- they had only two trucks
>that had working counters. I have never seen the driver get the counter
>number to make sure that they had 100 revolutions at mixing speed after
>adding water. I have never seen a delivery ticket that had the counter
>number on the ticket when cement was introduced into water. Some of the
>clumps that I have seen come out of a truck would indicate that maybe they
>had about 10 revolutions at mixing speed and the rest at agitating speed.
>I wasn't at the chute when the clumps came out, and pointed to them, I would
>guess that they would have gone in the forms.
Of course they would have. Most of my inspection time was four summers
inspecting concrete pours of curbing and sidewalk. Yes I was almost hit by
a stray 8" diameter cement ball when taking a sample for the air test.
>Don't you require that the design mix be provided to you prior to the
>concrete delivery? Don't you provide your inspectors or observers with the
>various weights/volumes of materials that would be in 5-, 6-, 8-, 10-yard
>trucks so that they can quickly compare what is on the delivery ticket and
>what proportions should be in the mix?
Full time inspection is not typical for normal projects. Government
projects yes. Private almost never. The inspection there is more to confirm
intent of design. We ask for design mixes. Usually get them ahead of time.
>Without the use of retarders, concrete would not be placed between June 1st
>and October 1st here in the southwest. It would flash set and plastic
>shrinkage cracks would develop before the finishers could get on it.
Sure. I don't know how you guys in Arizona, Texas, etc. take that heat. In
our case the real heat (meaning >30 deg C or >86 deg F) can usually be
avoided. If not then other means will have to be taken as mentioned before.
David Handy, P.Eng
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