deciphering mix placement requirements 1

[boffintech]

OK, got a shearwall 1' thick and and 28' long capped on each end by 24"x32" symmetrically placed columns. The shearwall requires 5K concrete and the columns require 10K concrete. There is a note stating that the columns and walls must be placed monolithic.

I asked the EOR (out of sheer curiosity, no pun intended) how this could be accomplished. He said that was means/methods which, of course, means it's up to the contractor to figure out.

This requirement would appear to be further complicated by a pesky line in the project manual forbidding fresh SP mixes to come in contact with fresh non-SP mixes.

The contractor evades the requirement by placing the whole thing with 10K. But what if they wanted to place 5k and 10K, could it be done with that mesh form?

 

[dik]

Bad detail... things should have been proportioned to use 5ksi concrete throughout with or without more rfg... looks like an oversight in spec'ing monolithic.

Is there a real need for the concrete to be monolithic? or for the columns to be 10ksi at the ends of the wall.

I guess it's possible to cast the 10ksi concrete for the columns and letting the concrete flow into the wall form and supplement this with 5ksi stuff... tricky and I think your contactor using 10k for all has chosen the best solution... still think it's a bad detail.

 

[concretemasonry]

The key or operative word may be "fresh". After the initial placement and vibration, the concrete may not be considered "fresh".

In Russia, prior to 1995, precast concrete was the only concrete used in construction. Even after that, much of the technology was used despite a shift away from precast toward cast in place and masonry.

I have seen structural walls panels made from normal weight concrete (6000-8000 psi) and lightweight concrete (4000-5000 psi) using vertical dividers/bulkheads that were stripped upward immediately after the concurent concrete placement and vibration. Granted, this is a factory process, but moving bulkheads could be used the same way on site.

They even routinely carried the same process on to several types of concrete sandwich panels (normal concrete, polystyrene, lightweight concrete and normal concrete, lightweight aggregate, lighweight concrete) and even to precasting individual housing modules.

The tesing of the actual complete fabricated panels and modules was impressive and done in detail.

It was an impressive process, but the practical applications were something to be desired - Want to buy several hundred 20+ story apartment buildings that are in the process of failing without a viable method of repair?

Dick

 

[Ron]

10 ksi concrete is a bit difficult to control in the field for a variety of reasons. I agree with dik...proportion all for 5ksi and cut down the brain damage. Poor specification on the part of the structural engineer/architect.

But, to your question...yes it can be done. Place the 10 ksi in the columns with the wall form/column form separated by a slice gate and simultaneously fill the wall forms with 5 ksi. Fill at same rate and raise the slice gate a little behind the concrete placement so that the two concrete bond and slightly mix at the interface. Putting a vibrator on the slice gate as it is raised (yeah, I know you should vibrate the forms!) will enhance the mixing.

....more trouble than it's worth.

 

[boffintech]

Thanks for the info. As for it being a bad detail, don't think so as I have been reminded of it SERVERAL times by the guy who wrote it. Don't know why he is so fond of it or understand why they have to be monlithic. Would like to hear possible explanations.

These 24"x32" columns are heavy with 20 #10 verts and #4 ties @ 5". The SW is just #4 @ 12" EW.

 

[dik]

I don't think there is a good reason! If a higher strength is necessary for the columns, wanting to minimize the size or whatever... then it's possible to create a keyway in the column and dowel to the wall using shear friction. If the shear is high, then you have to drill the forms to accommodate the larger dwl bars. else you can use z-bars in the key and bend them down.

Thanks, Ron... was aware of the problems with high concrete strengths and never thought to include this be... 10ksi can be a little tricky and there is likely quite a difference in colour... so hope there are no architectural issues...

Dik

 

[mitchelon]

Let’s analyze the reasons for this “monolithic” construction. Flexure? Concrete is not as good as steel in flexure. Shear, shear friction, increase the wall’s stiffness? Nothing you cannot fix with the proper amount of reinforcement. Engineers and contractors seem to be confused about the real definition of monolithic. Steel is what makes a column-beam connection monolithic, for example, not a continuous cast. Here is ACI’s definition, make your own conclusions: concrete, monolithic—concrete cast with no joints other than construction joints.

 

[henri2]

mitchelon, indeed, that is the ACI definition, right out of ACI 116-Cement and Concrete terminology. An example of the use of this term is ACI 318 Section 6.4.6. The commentary in R6.4.6 is consistent with your interpretation. However, is that what the EOR on this particular project had in mind, when the term "monolithic" was used?

 

[mitchelon]

Well, since the EOR maybe has no idea why he had specified it had to be “monolithic” and possibly cannot even prove why it has to be that way, one has to reference the real definition of monolithic and prove the EOR that the chosen method (perhaps a construction joint) meets the requirements of the specification. The specification reads monolithic, not a continuous pour as one typically assumes.

 

[boffintech]

mitchelon - on a day when I haven't tested his patience with a bunch of other questions, I'll ask him about this ACI116 definition. Good stuff.

 

[boffintech]

So I looked it up and see why I was confused:

concrete, monolithic—concrete cast with no joints other
than construction joints.

monolith—a body of plain or reinforced concrete cast or
erected as a single integral mass or structure.

The exact spec reads:
"Place scheduled columns monolithic with shearwall."

I thought that the intent of the spec was to require a "seamless" piece: no construction joints between columns and shearwall. Now I'm thinking it must mean that the reinforcing (the horizontals) is continuous through the members.

column/shearwall/column O----O

The ACI-116 term "Monolith" appears to imply seamlessness. The ACI-116 term "Monolithic Concrete" obviously allows construction joints which appear to me to be seams!

If he had wanted it seamless maybe he should have written:
"Place scheduled columns and shearwall as a monolith."

 

[dik]

ya... and designed it as 5ksi concrete...

Dik

 

[mitchelon]

You got it. It is never good to get controversial with the EOR about this sort of things, but it is always important to clarify things that are not clearly defined. The benefits of providing a monolith connection, seamless, at that location are none.