Different Mix Designs in the Same Slab 1

[epitome1170]

I have a situation in which I have a suspended slab (24" deep) for an aquarium. In general, I have a 3/4" aggregate for the 5ksi mix design. However, due to some heavy rebar congestion, the contractor would like to use a self-consolidating mix design in a small portion of the slab (only where the heavy congestion is). In doing so, he would pour all of the slab at one time and have the single truck of self-consolidating mix on site for the area that it needs. Basically, all of the mixes would be plastic at the same time for a monolithic pour and where the two mixes meet a vibrator would be used for consolidation.

My question: is there something I am overlooking by allowing for this?

If both are 5ksi mixes, then strength should not be an issue unless the aggregate interlock at this "mixture" line is not sufficient.

 

[Ron]

The different mixes will have different coarse aggregate sizes, aggregate distribution and water-cement ratios. Correspondingly, the mixes will have different rates of strain under stress (not a bunch, but some). Further, they will shrink differently on curing, thus introducing bond stresses that can lead to cracking.

I have seen this tried in some post-tensioned beams, without much success. Although this was not a primary reason for failure, this structure had failed during construction resulting in a death. There were numerous other defects that contributed.

As for your situation, the contractor probably has the best of intentions, but concrete placement, even on a good day, can be filled with surprises. I wouldn't let this be one of them. Further, don't let claims of a mix's performance substitute for good placement practices.

"Self-consolidating" mixes (very fluid with lots of plasticizer)also tend to segregate more than conventional mixes, thus compounding some of the difference issues.

Have the contractor pay close attention to the slumps as the trucks come in, have a qualified concrete inspector there to observe the placement and properly use vibrators to achieve adequate consolidation.

 

[epitome1170]

Thanks, Ron... I actually did not think of different shrinkage rates, although minimal cracks are not a concern due to the waterproofing liner that will be applied to this slab.

I do agree and have noted that the concrete placement will need to have the utmost scrutiny though. I was just hoping for additional insight of potential problems that I may not have thought of.

 

[hokie66]

Reinforcement congestion preventing thorough consolidation should be avoided. I don't see "self-consolidating" concrete as a solution to this issue. Like Ron, I would be concerned with segregation.

As to two different mixes being used in the same slab, this is sometimes done where the transmission of column force through the slab requires a much higher strength than the slab. Called "puddling".

 

[TXStructural]

Puddling is very common, as Hokie points out for columns. I would not hesitate to allow the practice where cosmetic differences can be tolerated. I would make sure the mixtures are compatible and the mixtures are placed at nearly the same time. Place the stiffer mix around the congested area, then the second mix, then vibrate to consolidate and assure that there isn't a break between the two mixes.

I would discourage true self-consolidating concrete since it is difficult to produce correctly and can be expensive. True SCC doesn't segregate excessively, but does require close control of water, admixtures, and aggregate mixtures. You can use a normal pump mix with high range water reducers and viscosity modifiers to get good placement rates and consolidation. Viscosity modifiers will prevent segregation and will allow the concrete to flow into congested areas. The cost for the admixtures is not excessive, and would probably allow use of one mix for the whole slab.

 

[JAE]

I'm going to get real technical on you all here - and suggest a concern that comes to mind - wouldn't the SCC sort of get "squished" out of the way if they put the "regular" concrete on top of it? Sort of like dropping some thick mud in a bucket of water and the water squirts out of the sides of the bucket.

OK - no more technical jargon - back to general speaking.

 

[Ron]

JAE...yes. One of the many concerns with mixing the like.

 

[dik]

Don't do it... re-examine the reinforcing congestion and use a superplasticiser with the mix. If the rebar congestion can be remedied, you might consider using a larger aggregate.

Look at a 3" slump with 5" to 6" with superP.

Dik

 

[Teguci]

I have reviewed SCC for use in precast beams and it is a great product. The difference with what you are doing are two fold:
1. A contractor with minimal experience with SCC (my assumption)will be casting it at the site under a very small time frame and considerable coordination.
2. You are mixing the SCC with another non-SCC concrete.

You specifically asked regarding item 2 but I believe item 1 is enough to disqualify the option. As for item 2, yes there will be a higher level of shrinkage of the SCC concrete. Although I do not believe that will cause failure it is a potential problem, especially for concrete retaining water. I would suggest cylinders with half and half be tested prior to the field pour.

 

[a2mfk]

I am no concrete guru, so maybe I am behind the times. Is SCC being used a lot in structural applications such as epitome's?? I thought it was mainly used in pavements and dams...

 

[a2mfk]

I withdraw my last question, sorry, brain fart. I was thinking of low/no slump concrete. Basically the opposite of SCC.