I need to make some single-use concrete forms and would, therefore, like to subsequently use the wood from the forms as part of the above-ground structure. I know that form release oil is the normal choice for concrete forms but is there some other treatment that wouldn’t leave the wood looking oily but yet still allow them to peel away from the cured concrete?
TIA
Replies
Parrafin sprayed hot?
Depending on the surface area and type of aggregate you can wrap the form wood in anything from .6 mil poly to saran wrap. Cove and crown molding wrapped with aluminum (aluminium for our Canadian friends) foil can make some interesting designs. Whatever you use for wrap has to withstand the placement of the mix. Some old work I've seen from the 1930's used sheathing boards covered with red resin paper but it's obvious that the cement was hand placed. Aggregate slinging down a chute will make a mess of anything not strong enough to resist a tear. Maybe a suitable use for tyvek?
I just poured my footing last Sunday, the form was lined with 6 mil poly. The poly made a positive impression on the building inspector, it's a quality way to go. In addition to keeping the wood free of concrete residue, it also puts an absolute barrier between the concrete and everything else, so the mix doesn't lose water into the lumber or gain water from the ground. When you line forms, leave yourself some extra slack at the bottom and tuck it under the sides to make sure that the poly is never under tension when the concrete hits it. It stands up pretty well if it has a solid surface on the other side and no tension on it. One side of my form is earmarked to become the shearwalling for the cripple wall.
-- J.S.
When fixing a broken piece of curb, I used clear packing tape - the kind 2" wide -to protect the wood form. An interesting side effect is that the finish on the concrete was extremely smooth. This could be important.
-Peter
I have used Johnson paste wax on forms for porch slabs that had decorative edges. the wax helped to fill in small holes and other imperfections.
This may or may not apply to what you are doing, but I have been making my own 'bricks' for several years now, using wood molds/forms. Mind you, mine are just homemade stepping stones, so here goes:
I 'grease' the form with boiled linseed oil before adding concrete. After the concrete has firmed up a bit to hold it's shape (this is where you might want to stamp a design in the top), I loosen the form - lift it up & down from each corner to dislodge the concrete. Then I take the form off after about another 1/2 hour of sitting in the sun. However, when I first started doing this, I didn't bother to oil. If I don't loosen the mold, the oiling has no effect anyway (I am hoping it keeps the concrete from deteriorating the wood too much. Once you've oiled, then used it, the concrete has basically sucked everything back out of the wood). Then I'd have to wait a 1/2 to whole day for the concrete to firm up enough so that removing the form wouldn't crack the sides. Plus this allows the wood to shrink back after it had absorbed the moisture and expanded from the concrete. Needless to say, I don't worry about leaving my molds on for 24, 48, or whatever hours, unless I got lazy and didn't loosen them. I do let them cure for at least 2 days before I use them, tho. Any coloring agent will tint the wood, too.
Hope this helps.
Would making the forms with a taper or rake help to release the mould from the concrete be helpful? Say about 3-5 degrees?
Edited 6/15/2002 10:22:46 PM ET by Larry