Different Mix Designs in the Same Slab
Different Mix Designs in the Same Slab
(OP)
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.
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.






RE: Different Mix Designs in the Same Slab
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.
RE: Different Mix Designs in the Same 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.
RE: Different Mix Designs in the Same Slab
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".
RE: Different Mix Designs in the Same Slab
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.
RE: Different Mix Designs in the Same Slab
OK - no more technical jargon - back to general speaking.
RE: Different Mix Designs in the Same Slab
RE: Different Mix Designs in the Same Slab
Look at a 3" slump with 5" to 6" with superP.
Dik
RE: Different Mix Designs in the Same Slab
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.
RE: Different Mix Designs in the Same Slab
RE: Different Mix Designs in the Same Slab