Bug-holes 1

[ajk1]

1. Does anyone know of a form release agent that has been shown to sigificantly reduce bugholes at formed surfaces?

2. Does self-consolidating concrete generally give a somewhat sandy surface finish? I have been told by a forming contractor that if I do a round column with plastic-lined sonotube type form or steel form, it will not give the normal "shiny" type finish that is obtained with normal concrete (he says though that he has used a specially designed self consolidation concrete that did address this).

 

[Ron]

ajk1...

1. No. It has more to do with the development of laitance at the formed surface, a function of the concrete mix, not the form release agent.

2. Self-consolidating concrete has a higher sand ratio, so stands to reason that you might get a more sandy texture; however, against formwork, there should be no real texture other than the smoothness (assuming smooth formwork) of the concrete and a reflection of the forming surface. Excessive laitance (wet mix, overvibration) will cause a shiny surface, particularly with plastic lined forms.

 

[ajk1]

Ron - I seem to recall that there was a paper published, perhaps in ACI CI, about research study with trials with different form release agents, in which it was found that certain types produced more bug holing than other types. Maybe I should see if ACI has a forum that might lead me to that.

In any event, is there anything specific that we can specify about the concrete itself, or the placement method, in vertical elements (columns and walls), to reduce bug-holing?

 

[shobroco]

I designed a building that had site-cast walls with decorative (split-faced vertical ribs) plastic form liners. The ready-mix supplier convinced the owner to buy a self-consolidating mix instead of the standard mix that I had specified. The finish was beautiful, smooth & glossy, and the concrete shrank so much in the first couple of months that the caulking lines on the cracks look like another decorative feature.

 

[ajk1]

Shobroco - I appreciate the actual experience - ok, but this would likely not happen with a column

 

[Ron]

ajk1...if I recall correctly, the problem with some form release agents was a surface reaction creating air bubbles...wouldn't want that. I'll see if I have that paper.