Concrete Mix 1

[SteelPE]

I am in the process of reviewing a concrete mix design for an industrial building. A few weeks ago, we were submitted a concrete mix design that was outside our parameter for slump. I rejected the mix and then had a conversation with the GC in regards to what could be done and we came up with a compromise to reduce the slump of the concrete.

The GC submitted a new concrete mix design with a revised slump to conform to our phone conversation..... great..... until I compared it to the old concrete mix design and all the proportions of every single item were the same, they just changed the slump to conform to our conversation.

Am I wrong in the fact something should have changed in the concrete mix to conform to the new slump (decrease in water, or decrease in Water Reducer)? Seems like they are trying to pull a fast one.

 

[jayrod12]

We've stopped worrying about slump as long as the max W/C ratios, air content and strength all work out. There's lots of available things that can modify slump and desirable slump is dependent on placing techniques. A pumped mix can't be the same as a direct from chute to form. We leave the slump up to the contractor, but their requested slump must be noted on the batch ticket, and the tests must match their request.

 

[dik]

With WRDA admixtures and superplasticisers, slump is a lesser problem than the past. I still use it, depending on the mix requirements. It's as much a part of the mix design as is the strength and w:c ratio/

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[AskTooMuch]

Is there even an "approved" slump when water reducer/plasticizer is used which is almost always present?

 

[canwesteng]

As others have said, slump mainly relates to how easy the concrete to place now, and no longer can be related to concrete strength. One thing to watch out for is superP reducing the air content in the concrete, mainly when done on site.

 

[hokie66]

Temperature of the fresh concrete is more important than slump. If a slump cone is being used, so should a thermometer.

 

[RWW0002]

I like requiring slump to be measured and tracked (and put slump requirements on the drawings), however I use it more as a quality control measurement of consistency between mixtures. If we have several trucks come in @ 4" slump and suddenly one measures 8" it is a good time to take a closer look at that truck and make sure no water was added, or something else funny going on.

 

[hokie66]

What RWW0002 said...that's what slump control is for.

 

[AskTooMuch]

How would engineer know what desirable slump should be if super is used? Contractor should have a say because they know more the most ideal for wall or footing pour like Jayrod said.

You normally wouldn't even know the slump until they give you the concrete test report.

At that point what you're gonna do, tear up the whole concrete just because slump is 8" inches intead 4" (which you "blindly" require) if concrete strength and temperature are acceptable?

 

[hokie66]

You wouldn't 'tear up' concrete that has already been placed. The only place you would reject concrete because of slump or temperature variation is at the truck. And yes, the contractor always has a say.

 

[AskTooMuch]

What is your basis for you slump requirement if superplasticizer is used?

Assuming you put 4" and you got one of the trucks with 8", you're gonna reject the concrete which may result in cold joint?

 

[AskTooMuch]

If you are rejecting based on temperature, I understand. But you will reject based on slump when you dont even known what is acceptable if it has super?

 

[canwesteng]

What happened to get one truck with 8"? Indicates some kind of problem at the plant.

 

[dik]

You stipulate the superp requirements... it's easy.

Temperature in these northern climes, is not much of an issue, unless you are dealing with mass concrete.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[AskTooMuch]

Dik,
Majority of Structural Engineers are not familiar what particular brand of water reducer to use and how much to put per cubic yds in order to achieve certain slump. Not to mention different batch plant use different water reducer.You stipulate the slump, strength, temperature.. batch plant create a conrere mix.

 

[hokie66]

No. You wanted the contractor to be involved in the mix design, and I agree. But once the slump requirement is agreed with acceptable tolerances (not unilaterally stipulated by the engineer), then it should be adhered to. Only if/when an unacceptable mixer load arrives at site should it be rejected.

 

[dik]

ATM... if you're not familiar then you have a geotekkie (they do that here) review the mix...

for mix design, in Canada there are generally two options. If you know what you're doing, you can spec the mix design, else you leave it to the concrete supplier... not the contractor (he's likely less qualified). With the latter method, you have to realise that some mix designs are proprietary and suppliers don't want to divulge the design. That's why the mix design has to be included in the project specifications.

In addition concrete suppliers mix designs are often a compendium of several attempts to achieve the design requirements. Often with the first method, there are numerous attempts until you zero in on a mix.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik