Unstable mix after addition of HRWDA 6

[LKPRECAST]

Goal is to not have to vibrate our precast forms. Max w/c is .44 for our vaults. After the addition of Adva 575 HRWRA at 5oz per 100wt of cement the mix is unstable and segregates. A grace rep said get the batch to a 1" to 3" slump before adding the HRWR, however to achieving a 2" slump exceeds our max .44 W/C. Is it acceptable to use a WRA and a hwrda together? As in reach a higher slump with the WRA before adding a HRWRA?

Current mix:

1yd

Cem 650 lbs
3/4 chip 788 lbs
3/8 chip 788 lbs
Sand 1400 lbs
H2o 285 lbs
Fiber mesh 1lb

Additives:
26oz per yard Grace 575 Advacast HRWRA
2oz per yard Air for 5 to 6 percent air entrainment.

Pulling my hair out getting this stuff stable and eliminating voids

We also have access to Grace WRA 27 and VMAR 3.

Thanks

 

[concretemasonry]

LKPRECAST -

I assume you are trying to come up with a practical mix design compatible with your production facility, production limitations, locally available aggregates and using an admix to provide the adjustments for those limitations. That is a very real challenge to monitor, depending on the testing facilities you have and how minor variations will affect the product. It is not as simple as specifying a result as a design engineer can do.

You are dealing with angular aggregates (probably crushed), where small variation can create a very large difference in results because the shape variations can have a larger than the gradations, depending on the screens (square or slotted) used in production and testing.

On top of that, you are concerned with appearance that is a visually, non-scientific measure of quality/properties.

Look to alternate solutions from others in the concrete products industry (producers, suppliers, associations, etc) for a solution to your personal situation.

I came to the concrete products industry after experience in high-tech concrete and concrete use followed by heavy industrial construction. My next new position with a firm was to go to a five day national convention to learn, never take a free meal from a supplier and to have unlimited expenses to eat/treat and talk with suppliers, similar producers to learn. The next assignment was to go to every national association meeting, be on every national committee association possible (ASTM, ACI, etc.) and to gain more experience. Your situation may not offer the same benefits, but the concept of using others in an industry that have the practical, proven procedures and controls are worth pursuing.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[karma134]

@ Hoaokapohaku

I would like to quote what you said earlier in this post "Our problems with surface voids is probably a fine aggregate gradation problem. This happens a lot with crushed sand. It is caused by over-packing the mix with more of a certain size aggregate than will fit in the overall aggregate void structure, usually the size passing a #4 and retained on #8 sieve (4.75 mm x 2.36 mm)"

I also have the same problem at my site. And my question is what is the limit to passing a #4 and retained on #8 sieve? Also the fineness modulus limit.

Thank you in advance

 

[Hoaokapohaku]

@karma

I can't give a definitive answer to your question because it varies depending on the particle shape and texture of the aggregates, both of the size in question, and the larger particles that create void space for those particles. Flat and elongated particles, particularly in the 4.75-2.36mm size, compound the problem dramatically. However, from what I've gathered reading other's experience, my situation is pretty typical. Here, less than about 82% passing the #8 or an FM higher than 3.0 spells trouble. Since we are talking sand, 100% passes #4. I aim for 85% passing #8 with an FM of 2.9. I would aim a little higher than 85 if I could, but certain local specifications make that problematic.

 

[Hoaokapohaku]

LK...I've been wondering how it's going with your mix. Update?