placing mixes in contact with each other
placing mixes in contact with each other
(OP)
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?
“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?





RE: placing mixes in contact with each other
RE: placing mixes in contact with each other
RE: placing mixes in contact with each other
Not really my area, but I would think the great differences in strength could lead to strain incompatibilities and cracks.
RE: placing mixes in contact with each other
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.
RE: placing mixes in contact with each other
RE: placing mixes in contact with each other
RE: placing mixes in contact with each other
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.
RE: placing mixes in contact with each other
It seems like there has been a lot of wasted time and effort on something that should have been obvious to everyone involved.
RE: placing mixes in contact with each other
Please let us know what happened with the next placement.
RE: placing mixes in contact with each other