entrained vs entrapped air
entrained vs entrapped air
(OP)
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?
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?






RE: entrained vs entrapped air
I would question the EOR about this and get his direction.
RE: entrained vs entrapped air
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.
RE: entrained vs entrapped air
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".
RE: entrained vs entrapped air
RE: entrained vs entrapped air
The parking deck is open. The remainder of the building 20 levels of condos.
RE: entrained vs entrapped air
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.
RE: entrained vs entrapped air
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".
RE: entrained vs entrapped air
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?
RE: entrained vs entrapped air
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?
RE: entrained vs entrapped air
A gap in the specs!? Blasphemy!
RE: entrained vs entrapped air
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.