air in sidewalk

[carlcap]

is 3% air entrainment acceptable for a sidewalk that is breaking at over 9,000PSI?

 

[JAE]

Is it meant to have some minimum air content for freeze protection? What was specified?

 

[carlcap]

well the standard 5-7% air is specified? my question is from an engineering standpoint is 3% sufficient when the sidewalk is over 9000PSI

 

[phamENG]

As with anything - it depends. If it's in South Florida, then you're probably fine. If it's in Minnesota, ice will win, even if the concrete has a compressive strength of 9000psi.

Air entrainment creates little bubbles in the concrete that allows water in concrete to expand when it freezes. If that space doesn't exist, tension will develop. While tensile capacity of concrete is roughly correlated to compression capacity, it's still really small - even if you're breaking at 9000psi. Water always wins, especially when it freezes.

 

[carlcap]

its southern Virginia

 

[phamENG]

As a structural engineer practicing in southern Virginia, I'll say that I've seen plenty of sidewalks spalled by inadequate air entrainment. Brand new library at my alma mater...sidewalks looked like trash within a year.

Just meet the specs.

 

[carlcap]

thanks for everyone's input

 

[dik]

even if you're breaking at 9000psi. Water always wins, especially when it freezes.

Freezing water can develop a compression force of 2ksi. 9000 ksi concrete has a tensile capacity of about 1/10 the f'c or about 900 psi... as you note, guess who wins. In Winnipeg (quite a bit different than Florida) I usually spec 5% to 7%.

The air also helps with the workability.

3% is the upper end of air entrainment without admixtures.

 

[emmgjld]

Colorado, desert & mountain areas - assuming 3/4" aggregate, moderate curing before hard freezes - I have never seen obvious freeze-thaw problems if the concrete strength is over 4500 psi, unless the concrete is fresh (recently placed, less than 45 days old) and has not 'dried' out, usually more than 90 days old. If the concrete is normally saturate, a strength of 5000+ psi seems to be required. This also assumes hard freezing and usually cyclic freeze/thaw.

 

[dik]

I have never seen obvious freeze-thaw problems if the concrete strength is over 4500 psi,

It's not the strength, it's the air entrainment...