high air entrainment

[Prestressed Guy]

On a current project, I received a concrete test report from the special inspector that indicates 12% air-entrainment on an exterior elevated topping slab. This slab is cast over 1.5” steel deck supported by HSS joists.

The general notes called for 5% +- 1.5% so this is nearly twice the max specified air. The steel deck will carry the required loads so I am more concerned about the durability of the concrete. The slab is exposed aggregate with a sealer.

What is the long-term affect of this much air in the mix.

I haven’t yet received an answer from the special inspector as to why he didn’t reject the truck.

 

[Teguci]

How does the concrete perform under compression?

I'd be concerned with the permeability of the concrete. If the concrete is exposed to salts and it is reinforced then a sealer might not be good enough.

The final air content may be less if it was tested at the truck and pumped to the elevated slab (losses of 1 to 3% are typical).

 

[lsmfse]

Read my post under concrete engineering, general discussion. We had a similar situation recently.

 

[emmgjld]

Given high air contents (at or above the 'normal' specification limits for exterior concrete)I have had more problems with delamination of the surface after troweling than the ultimate strength being insufficient. In the last 4 to 6 years, the problem has been getting worse, at least in my Rocky Mountain area.

Please note that a concrete surface which is subject to delamination will be adversely affected by freeze-thaw and deicing chemicals. Also note that freeze-thaw and deicing chemicals have only exposed a condition which was already present.

Please also note that ACI does not recommend air entrainment for interior slabs, which are most likely to be moderately to heavily troweled.

 

[Ron]

Here's the link to the thread that lsmfse referenced...it is very similar to your conditions....

http://eng-tips.com/viewthread.cfm?qid=257382&page=1

 

[Prestressed Guy]

Thanks for the link Ron.

This is not a case of strength but of durability. The concrete in question is an exterior exposed aggregate slab on the shoreward side of a building approximately 200’ from the ocean. The climate is subject to freeze/thaw.

My concern is for durability of the slab and corrosion of the rebar and decking.

 

[Ron]

Haydenwse...durability will certainly be affected by the high air content. Air content up to about 6 percent helps with freeze-thaw resistance. When the air content goes much higher, the permeability is affected, thus creating more problems with freeze-thaw than lower air content. Further, scaling might be a problem as the finishing process might have trapped more air just below the surface. This creates separation planes in the concrete, that ultimately can cause scaling issues, particularly if the floor is subjected to traffic (forklifts, vehicles, hard wheeled carts,etc.)

Good luck.

 

[hokie66]

I agree with Ron, but durability concerns with the steel deck would have been an issue even without the concrete problem.

 

[Prestressed Guy]

Thanks for the comments.

I think that the bottom side is fairly well protected. I have specified a G90 galvanizing with an epoxy paint top coat so it should be okay. It is not in a constant wet condition and if it is kept clean will perform very well.

The surface in contact with the concrete is G90 but doesn’t have the paint. I am just worried that if the concrete is more porous, it can get to a permanently wet condition which might compromise the galvanizing.

 

[hokie66]

Concrete is not watertight, so I would assume the deck does get wet. Concrete decks on steel form are notorious for durability issues.

 

[SkiisAndBikes]

For applications where the concrete deck is exposed to moisture we specify the use of vented steel deck. Moisture will penetrate through the concrete and/or cracks and end up sitting on the deck. Vented deck will allow this moisture to slowly dissipate through the underside of the deck. With some older rehab projects, we have added venting afterwords in an attempt to slow the corrosion rate, however, this is not ideal.

Agree with the others, such a high air content will affect durability, i.e. long term performance of the concrete.

 

[TXStructural]

Yes, I think the 12% air is going to be a problem, particularly in light of the salt exposure. Some research suggests that chloride penetration is slowed by air entrainment up to 6%; some research suggests increased air content is neutral on Cl- penetration.
You are in the ACI 318 C2 severe chloride exposure zone; Table 4.3.1 requires the concrete to have w/cm of 0.40 or less, and 5000 psi minimum strength. This application also warrants consideration of increased cover per 7.7.6 (two inch minimum for slabs.)

The concrete slab will need to support itself once the deck has failed. You mention rebar, so I assume it was designed for this.

The rebar would ideally have been protected from corrosion by one or more means (epoxy, galvanizing, cathodic protection, or use of stainless rebar) to extend the life of the structure.

You might consider a traffic membrane coating, or a trowel-applied epoxy-aggregate surface, to reduce water+Cl- intrusion.

 

[boffintech]

C 231 – 04 3.3 The air content of hardened concrete may be either higher or lower than that determined by this test method. This depends upon the methods and amount of consolidation effort applied to the concrete from which the hardened concrete specimen is taken; uniformity and stability of the air bubbles in the fresh and hardened concrete; accuracy of the microscopic examination, if used; time of comparison; environmental exposure; stage in the delivery, placement and consolidation
processes at which the air content of the unhardened concrete is determined, that is, before or after the concrete goes through
a pump; and other factors.