entrained vs entrapped air 4

[boffintech]

Specs for a particular job state that normal weight concrete equal to or greater than 5000psi shall have entrained air content of 3+-1%.

Contractor has been using a 7000psi "non-air" mix which usually has 2% or less entrapped air.

This is for exterior columns in a parking deck in the southeast US.

What are the ramifications, if any, of this?

 

[jike]

I would expect a higher air entrainment requirement for columns with moderate exposure. This is dependent upon maximim aggregate size. Refer to ACI Table 4.2.1. Note that the air requirement can be reduced 1 % for concrete strengths greater than 5000 psi. Perhaps there was a typo in the spec.

I would question the EOR about this and get his direction.

 

[Ron]

boffintech...if your project is in an area that freezes at all, it's a good idea to have some air entrainment. Entrapped air and entrained air are two completely different things and each can have a separate effect on the concrete.

Increasing the compressive strength of the concrete helps because as the concrete deteriorates from exposure, the remaining cross section is stronger than it would have been otherwise. That only works up to a point though. Increasing the strength helps to increase durability, but moreso from abrasion and chemical attach than from freezing/thawing. The higher density of stronger concrete also helps to mitigate moisture intrusion and migration, so you can less moisture available in the concrete to freeze.

Entrained air, as you know, is chemically induced. The chemical causes very fine, highly dispersed "bubbles" in the cement paste. These "bubbles" or air voids allow the moisture in the concrete around them to expand and the expansion effect is elastically absorbed by the fine air voids. This reduces the spalling that occurs when the moisture in concrete freezes and "blows" out the surface (even in tiny increments...it eventually deteriorates the concrete).

Entrapped air is caused by the mechanical mixing of the concrete and results in random, usually large, air voids that are so infrequent and poorly formed that they do nothing to cushion the moisture expansion during freezing. In fact, those voids become reservoirs for water as it condensed in the concrete as the temperature falls, thus the free water that collects in those larger air voids will freeze, expand and spall the concrete.

 

[boffintech]

Well I wrote it up as a discrepancy on the last 3 days of inspection reports and added it to the discrepancy log.

The contractor claims that "it's an approved mix". It's an approved mix, OK, but according to the specs not for the location where it's being placed: exterior columns. For the interior columns, now that might be a different story.

BRW, the mix design submittal has a stamp indicating EOR review with a clear hand written note that reads, "approved as noted, revise and resubmit with a schedule of use". I'll give you one guess on who was unable to find any evidence of a resubmittal with the required "schedule of use".

 

[miecz]

The "as Noted" part of "Approved as Noted", Revise and Resubmit" demands some explanation to the Contractor as to what needs to be revised. Asking for a schedule of use is vague at best. At the same time, the Contractor (who should have known better) doesn't have a fully approved submittal. Although you didn't say so, I'm assuming that your parking deck is unheated, and that the columns will see freezing temperatures. If so, the mix is wrong as Ron explained above, and both parties are partially responsible. What about the rest of the structure?

 

[boffintech]

The contractor doesn't have a fully approved sunmittal. Exactly. This is why I wish that during the review process the EOR would stamp each individual mix design. The contractor handed me this stack-O-stuff and said here these are all approved. Three of the mix designs had a big red X from top to bottom on them. I said how's this giant red X mean "approved". In my language when an EOR uses a fat magic marker to hand write a giant red X that means "rejected".

The parking deck is open. The remainder of the building 20 levels of condos.

 

[henri2]

boffintech, both jike and Ron have brought up the issue of durability in a freeze-that environment. Is this the case here? Why does the EOR require air-entrained concrete?

It is very possible that the requirement for air-entrainemnt is an error..and that a revised non-air-entrained mix design might be acceptable to the EOR if air-entrainment is really not necessary.

 

[boffintech]

henri2, the mix is being placed in exterior parking deck columns and shearwalls. The structure is located near Atl., GA. Do these columns/walls need or not need entrained air? Don't know. Not my call.

The specs state:

1) no entrained air is required for normal weight concrete placed in foundations, slabs, walls, and columns with interior exposure

2) normal weight concrete shall have entrained air content of 5+-1.5% UNO

2) normal weight concrete equal to or greater than 5000psi shall have entrained air content of 3+-1%

Based on the info given I think the current use of this non-air mix is a deficiency that should be reported and put on the Deficiency Log.

I just find it interesting that the contractor argues that the mix is "approved". I'm not arguing that it's not "approved". I'm asking "approved" for what? The specs require a Schedule of Use and none has been submitted.

Ooops, I'm re-reading the specs just now and I think I've found another deficiency. The specs give a definition of "agreesive environment" which includes "parking floors". Later in the section on Air Contect it states: for concrete within agressive environments entained air contect shall be 3 to 5% for normal weight concrete less than or equal to 5000psi.

To date a non-air mix has been used for the parking deck slabs. I guess it's OK because that mix has been "approved".

 

[Ron]

boffintech...Atlanta is in an area that typically needs air entrainment on exposed concrete. I would interpret the same way you have.

At the least, it is in the gap where an RFI to the Structural Engineer is required.

Was there a pre-placement conference on the concrete? If not, obviously would have been helpful to sort such out.

Is your office in Atlanta or one of the burbs?

 

[miecz]

boffintech-

Item 3 of your last post has the EOR specifying a non air-entrained mix (in my mind, 3% is non air-entrained) for concrete over 5000 psi. Has the EOR approved a 7000 psi non air-entrained mix for aggressive environments? If so, then the Contractor has an approved mix for the columns. Same for the deck, if he used a 7000 psi mix.

What mix was used for the exterior beams for the garage levels?

 

[boffintech]

Yes, in Atlanta or one of the burbs.

A gap in the specs!? Blasphemy!

 

[boffintech]

jmiec, Has the EOR approved a 7000 psi non air-entrained mix for aggressive environments?

That seems to be the question of the day. There is a 7000psi non-air mix design in the stack of so-called "approved" mix designs. However, the submittal is marked "revise and resubmit with a schedule of use per spec section ...."

There are plenty of columns on levels that are interior. Maybe the 7000psi non-air mix was approved for those levels. It may be an approved mix, but not for this location if the EOR wants 1/2% more air than he's been getting and if he wants tiny chemically induced bubbles and not fat entrapped bubbles.

FYI, a 5000psi non-air mix went into the parking deck slabs (no beams, just PT).

At any rate all the AC results to date are low. The entrapped air is never higher than 1.75% and is usually 1.5%.

I agree, BTW, that 3+-1 is a low requirement; however, as this job demonstrates you can never count on entrapped air to get you 3% air and especially with 7000psi or 5000psi/he. Plus the spec doesn't state "provide concrete with entrapped air of 3+-1%". It says provide concrete with entrained air and probably for the reasons Ron cited.