How to Eliminate air pockets at surface??

[FrankF]

Am trying to pour an outdoor fireplace, grill and pizza oven. What shooting for a smooth clean surface, but after stripping forms was disapointed by the air pockets on the surface of all the vertical surfaces.

How to eliminate these. I've seen commercial work where the walls are uniform and smooth. What are we doing wrong?

Used a 6 sack mix with 3/8" rock and polyheat to help it flow better. Mix had a 4" slump. It was pumped into the forms. Forms were made from HDO form plywood. The mix was vibrated as it was filled, and we tapped on the forms as they were filled with hammers.

 

[dik]

It's pretty difficult to tell, not seeing the formwork required and/or the reinforcing. Was the concrete restricted by rebar? Did the formwork require venting holes? By airholes, do you mean 'bugholes' tiny holes? or rough surface called 'honeycombing'?

Did you apply the vibrator to the body of the mix? or lightly to the concrete as it entered the form? 4" slump doesn't sound too bad, but may be a bit stiff for irregular forming. The 3/8 aggregate sounds about right. Entrained air makes the concrete mix a little more fluid. A superplasticiser may have helped by increasing the apparent slump to 8" or so.

Just some comments.

 

[Rjeffery]

The technique used by the guy with the vibrator is critical. generally, it is best to leave the vibrator verticaly in the mix near (not touching) the formwork until bubbles stop forming at the surface of the mix. quickly stabbing the tip of the vibrator into the concrete smoothes the surface but does little to allow entrapped air (as opposed to entrained air) to escape. "fishing" or dragging it across the surface also does not promote good consolidation.

my $0.02 (US)

 

[BigH]

Rjeffery has hit one good point on vibration - most guys I've seen (Asia region) just do not understand the need to keep the vibrator "in place" for a period of time. I usually see no more than 3 seconds per vibration. Had a lot of honeycombing on some curb foundation concrete until I showed the guy how to count to 8 (minimum). Found much much better results!