placing mixes in contact with each other

[boffintech]

The specs under section 3300 Cast-in-place concrete under High Range Water Reducers reads in part:

“Do not permit fresh concrete containing superplasticizers to come in contact with fresh concrete not containing superplasticizers.”

Case in point. On the job there is a foundation wall that requires 4000 psi (3”-5” slump) concrete that has several pilasters (columns integral with the wall) that require 7000 psi (6”-8” slump) concrete. The 7000 psi mix has high range water reducer to achieve the 6” to 8” slump.

The columns in questions were poured simultaneously with the wall by alternating 7000 in the column and then 4000 on each side so that the 7000 was “trapped” in the column. Clearly this method of concrete placement is disallowed by the specs. I admit that at the time I had not fully considered the point and now wish to fully cogitate upon it.

What are the consequences of this placement method? Why is this placement method disallowed by the specs?

 

[GeoPaveTraffic]

I would assume it is disallowed to prevent contamination of the higher strength mix by the lower strength mix during placement.

 

[boffintech]

So in the case I described there would be a zone on each side of the column (not in the column) in the wall that ranged between 4000 and 7000? OR could the mechanical vibrator cause the migration of SP into the 4000 mix on each side of the column having some deleterious effect on the 4000 mix?

 

[GeoPaveTraffic]

What I was trying to get at is that if the wall (4,000) mix got a little ahead of the column (7000) mix then the columns may not be at the specified strength. Given that the column mix is high slump, I'm not sure how it could be placed in advance of the wall mix without placing much more column mix than specified. Then you would have an uneven portion of the wall at a significantly higher strength than the remaining portions of the wall.

Not really my area, but I would think the great differences in strength could lead to strain incompatibilities and cracks.

 

[jheidt2543]

boffintech:

How could you tell what zone is where in the wall?

I agree with GeoPaveTraffic's comments. The difference in slump between the two mixes makes it impossible for the two not to mix when trying to place them simultaineously.

In my opinion, when a designer specifies an unusual combination like this he should also specify how it is to be placed. One could pour all the columns first with rebar dowels extending out of the columns, then place the walls or vice-versa. As engineers, our designs must be constructable.

It sounds to me like someone tried to take a short cut, now you don't know for sure what you have.

 

[DRC1]

The pilasters should have been bulkheaded. Period. The 4,000 and 7,000 psi material is now hopelessly ombined. the 4,000 was much wetter than anticipated, possibly due to super p or due to higher water in the 7,000 mix. If they were pouring the walls to "trap" the pilasters, the walls were not sufficently vibrated to get the concrete fully around the bars and eliminate voids. The pilasters probably were not vibarted either, so as not to distub the 4,000 mix. If you have 7,000 mix, it is a very fussy mix for redi-mix, and is hard enouh to get right with out doing something like this. I assume the 7,000 is needed to support high loads. I would strongly condone this practice. Nobdy likes to build and strip bulkheads, but they are necessary. If this is for a footing that is a foot or two high, you may be able to get this Rube Goldberg ideat to work. If this is for a wall of any height, you should seriously consider removing and replacing the wall.

 

[LCruiser]

I would think there would be shear plane problems due to differential curing - but in this case I'm not sure it's a big deal.

 

[boffintech]

DRC1 (wrote, "If they were pouring the walls to "trap" the pilasters, the walls were not sufficiently vibrated to get the concrete fully around the bars and eliminate voids. The pilasters probably were not vibrated either."

That was a darn fine prediction! In fact when the form-work stripped extreme honeycombing due to improper vibration was found. Voids and honeycombing were found all through the 7000 psi column and in areas of the wall from one side to the other.

As a result that placement was demo'ed. The rebar was scrapped, new dowels from the footing to wall had to be drilled and epoxied in, and the dowels from the pile cap to column had to be cut and reinstalled with mechanical couplings.

This despite the fact that we had had several conversations (inspector/owner/contractor/subcontractor) concerning improper use of the vibrator by inexperienced or unsupervised labor.

So they will try again later.

 

[jheidt2543]

"If there is not enough time to do it right the first time, where do you find the time to correct it?"

It seems like there has been a lot of wasted time and effort on something that should have been obvious to everyone involved.

 

[henri2]

IMO, the pilasters should have been constructed first utilising vertical construction joints with key-ways for the remaining concrete placement in between pilaster faces.

Please let us know what happened with the next placement.

 

[boffintech]

Although the three mixes were once again placed on the same day the next placement worked out OK. They used a light weight screen type material (approximately 3/8" holes) nailed in vertically to divide the members in placement. This eliminated the need to construct labor intensive wood bulkheads.