mix and delivery of concrete

[boffintech]

Arrived site to observe concrete placement and help train a new concrete testing tech. Huge placement of 3yd3 today for a few piers.

The concrete was truck mixed and delivered in two 1 1/2yd3. High range water reducer was added at the site by the producer's QC tech. The producer's QC tech erred when adding the Super-P. The load being so small was way down at the bottom of the drum and he just dumped the Super-P in the at the top of drum. This simply got the inside of the drum moist with Super-P. To get the Super-P down to the load he (you guessed it) got the water hose a rinsed in down. This appeared to cause the mix to completely segregate.

Upon filling the oyster bucket with the mix, the water in the mix appeared to repel itself from the mix and immediately rise to the top. I have never seen anything like this.

The producer's QC insisted on a slump to which I said on what, that 6" of grey water on the surface there? I mean it was water, grey with no sand or rock. All aggregate has sunk directly to the bottom of the bucket.

The load was rejected. A few minutes later I asked the QC guy what the ETA was in the second truck to which he said the second 1 1/2 yd3 truck had been recalled so that they could add another 1 1/2 yd3 to that truck.

1) I looked in C-94 but could not find anything that precluded an already mixed truck from having more mix added to it. Is this practice restricted by an ASTM standard, by an ACI standard, or just by common sense.

2) I know the addition of water to the mix after the addition of SP is not allowed by spec but where does this restriction come from.

3) I thought I remembered that loads under 3 yd3 were not allowed. This particular job's spec book however did not list this restriction. I couldn't find this is C-94 either. Is it restricted by an ASTM or ACI code?

Thanks!

BTW, The second truck was rejected as well. This was the fist time I ever got a load rejected PRIOR to being mixed. How sweet is that! Also the placement was postponed until tomorrow just to make sure the plant didn't try to sleaze that bum load in on us. I mean they can't save it over night, right?

 

[BigH]

boffintech - they might try if they know how to tweak sugar spiking! I've not seen this either. Why was the super-P added at the site? Did the QC raise an NCR? What was the basis of rejection? Did he reject or did you? I would have rejected too (if I had the power) but . . . I presume you told him to stuff the pour and I agree with you. As for adding more to the mix, I think there is something about batching delays in a single "load" but I'll have to do a bit of looking for it. Did you have anything in the spec that said the concrete must be poured within a specific time? If so, this might be the way to have justified rejection of the second truck.

 

[boffintech]

Super=P was added at the site for producer convenience; the plant is a good distance from the site and often with heavy traffic.

An NCR?

The basis for rejection was that the mix could not be slumped due to extreme segregation. The producer is obligated to supply the mix at the specified slump for the specified time:

6.2 Concrete shall be available within the permissible range of slump for a period of 30 min starting either on arrival at the job site or after the initial slump adjustment permitted in 11.7, whichever is later.

Could have gotten them on the re-mixing because the first 1 1/2 yd3 would have been way over the 90 minute rule:

ASTM C-94 11.3 The mixing time shall be counted
from the time all the solid materials are in the drum.

ASTM C-94 11.7 Discharge of the concrete shall be completed within 1 1?2 h, or before the drum has revolved 300 revolutions, whichever comes first, after the introduction of the mixing water to the cement and aggregates or the introduction of the cement to the aggregates.

But they hadn't planned on telling us that the 3 yd3 load was really a 1/2 a load added to an old 1/2 a load. They were going to try to sleaze one by us. Their own QC guy spilled the beans.

I didn't reject the load; I only say that the mix appears out of spec and that XYZ Engineering recommends not placing out of spec concrete in the forms.

The contractor can decide for himself what to do but I have direct communication with the owner soooo....

 

[BigH]

NCR = Non-compliance Report. Typically, the contractor QC will need to issue it with respect to the improper material. It is a means of tracking the problems. You, or the RE staff would issue an NCR if non-compliant concrete was actually used - then the contractor would have to develop a method of remediation (i.e., removal, or . . .). The rejection of the load rightly is the choice of the contractor - but with potential repercussions!

 

[don45640]

The Super-P should have been added after bringing the mix up to the lip of the mixer. Slowly turn the bowl down and bring it back up again to add the rest of the plasticizer.Then mix it in thoroughly. That's how we do it,anyway.^) don45640

 

[bb29510]

all super p is added at the site, this is common practice, because of temp super p will only give you about 45 minutes. I really dont see nothing wrong. It might look like its segregated but its not, that grey water you see is the super p on top. super must be mix five minutes no matter what. If its too wet , supplier needs to cut back on the super.

 

[boffintech]

brownbagg, normally I'd agree with you, nothing unusual about adding the SP at the job site. However, it was the technique used to add it to such a small load as don45640 pointed out. don45640's idea was later reinforced by a higher up QC guy who came out later. The first QC guy tried to slump the mix but gave up when he discovered the stuff couldn't be scooped; it was impossible to keep mixed.

 

[henri2]

A little bit of info about super-Ps.

There are 2 types of superplaticizers. One is the basic high-range water reducer which complies with either ASTM C 494 Type F or ASTM C1017 Type 1. The other has ingredients which causes retardation of the mix....and complies with either ASTM C 494 Type G or ASTM C 1017 Type II.

The Type F super-P has been around longer....and is meant to be added at the jobsite.

The Type G super-P is meant to be added at the batch plant. Can you imagine a ready mix truck with a full load of soupy platicized mix driving up a steep road on the way to the job?

don45640's recommendation is the way to do it. It is important that the individual adding the super-P be properly trained.