Admixtures - When to add to the mix?

[Ibeam]

I'm reviewing a concrete specification that we're trying to use on an upcoming job. It's been in-place before my time and has had several shots/changes put to it, of which the history isn't really known. In the spec. section for admixtures, it states the following:
"High range water reducing agent (HRWRA) may be used provided they meet ASTM C49 & Type F or G requirements and shall be added at job site in the presents of the client's field representative."

Question:
Is this the standard way of adding HRWRA's? If it's part of the mix design submitted for approval, is it ever added at the plant?

 

[Laser28]

Some HRWRA's may be added at the plant, but for accurate control of the slump, its often better to add at the site. there should be no effect on the ultimate quality of the concrete.

 

[dirtsqueezer]

Ibeam,

I would have to agree. It's been my expience that HRWDs, or superplasicizers have a very short working time, only on the order of around 45 minutes. Adding at the plant takes out whatever may hang that truck up between the loader and the jobsite. I highly recommend the stuff, as it's a great improvement to the contractor adding water thus reducing compressive strength to improve workability. Most of the time, I usually see a representative with the concrete company on site to oversee the addition, as well as the inspection agency. Good choice, and good luck.

 

[Ibeam]

Thanks for the input from both. It does make sense to wait until making it to the site - it was the uncertainty of the quality control when adding it that had me wondering. I haven't seen a concrete company rep on-site outside of the driver in resent jobs, but maybe you mean in the particular case when a HRWRA is used. I still have time to add a line in the spec which would include the requirement if the circumstance appears.

 

[dollarbulldog]

Hi all,

It has been found that the efficiency of HRWRA type F,G are strongly influenced by the initial slump. For very dry mix where we try to push it to be super flow, HRWRA works well just a while depending of the content,main active components and the working condition like weather, wind a.s.o. Their slump loss rate is terrible in that case. However, for a bit wet concrete like initial slump 2.5cm-5.0cm, slump loss is significantly reduced. It is best to just put HRWRA before placing, but to make sure that it's mixed well with your concrete.

Bests,

 

[Ron]

Ibeam...with current technology it is not necessary to add HRWRA at the site. The pot life of these admixtures is long enough now to allow proper ready mix production and delivery. I'm not a fan of site-added admixtures as the admixtures should be carefully controlled and added at the same time and location as the mix water to provide proper distribution in the mix.

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