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Concrete without Aggregate

Concrete without Aggregate

Concrete without Aggregate

(OP)
Okay, here's a new one for me. A contractor has submitted a mix design for the proposed "concrete" on one of my projects. Their mix design consists of water, cement, and superplasticizer... No aggregates at all. This is being used to retrofit an existing railroad bridge, so they are proposing to mix all of this on site. They are casting a sizable amount of concrete. A 5' x 8' x 21' concrete collar around some existing steel piles. Has anyone seen something like this before? Below are some of my concerns.
  • It's not concrete, as called out in the plans. ACI, and every other document indicates concrete is a mixture of cement, water, and aggregates.
  • No test data was provided to confirm mix design strength or slumps.
  • With this much cement and no aggregate as a heat sink, I have notable concerns about excessively high heat of hydration.
  • I question long term durability as well.
I'm planning on rejecting the mix design, but I'd like to have a firm foundation while doing it. Any additional insights are appreciated.

RE: Concrete without Aggregate

You should insist a cylinder compressive test. Coarse aggregate provides better bond strength. The proposed mix can be proven to be quite brittle, given the compressive strength is constant.

RE: Concrete without Aggregate

Concrete shear strength is significantly reduced as there is no aggregate interlock, so that part of the design would have to be reviewed.

You probably cannot do cylinder strength tests on it as cylinders do no work with grout (fail in shear not compression), you will have to do cube tests and convert the values. Depending on the strength, the conversion is about cylinder = .8 cube.

RE: Concrete without Aggregate

Quote:

fail in shear not compression

That's a cylinder test can tell - fail in shear prior to compression (cone or crumbling).

RE: Concrete without Aggregate

Clutching at straws...but maybe the contractor is using 'pre-placed-aggregate'. As the name/technique implies, pre-place all the graded aggregate in the form and then place grout (cement/water/admixture). However, PPA it not normally used for the application your describe, but I have used it for retrofit grouting projects with success.

RE: Concrete without Aggregate

(OP)
Thanks for the replies. I asked the contractor what aggregate he is planning on using and he said none. That's when I decided to come here and confirm.

Reduced shear capacity and brittleness is more than enough justification. Thanks for the replies all!

RE: Concrete without Aggregate

Sounds very strange. Contractors normally push for cheaper alternatives. Cement is more expensive than aggregate, so what's his explanation?

RE: Concrete without Aggregate

You might need to be very careful about heat of hydration and the effects it has on thermal cracking.

RE: Concrete without Aggregate

You asked for concrete, they provided something that's not concrete. No aggregate = no concrete.

Its a no brainer for me. "Rejected without review"

Even it if were feasible, would you trust some untested concoction rather than getting what you designed?

RE: Concrete without Aggregate

(OP)
To circle back around on this, MotorCity, that's close to what I did. They then said whoops, not what we meant to send for the concrete mix design.

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