On wall form moulds we routinely run a 3/4 in 45° chamfer at every external arris to ease stripping and keep the concrete from spalling, but does anyone know when/why 3/4 became the default instead of 1/2? I cut miles of it on the table saw and only ever saw 20 mm specified once in 2019 — was there an old spec or tie-head clearance issue that set the size?
I’ve always figured 3/4 stuck because that’s what form suppliers standardized — plastic/wood chamfer strips in 3/4 survive multiple strikes and rough stripping, where 1/2 tends to bruise tilt-up and column corners… Building on @george_hansen23, I’d do a quick mockup: pour two panels with 1/2 and 3/4 and strip at 24–48 hrs; you’ll usually see more paste loss and spalls on the 1/2. If anyone’s got a pre-’80s Symons/Doka catalog that says “3/4 default,” I’d love to see it.
And i think 3/4 stuck because it matches the ‘actual 1x’ thickness and plywood, so you can rip 45° chamfer strips fast, and the about 1.06 in flat also plays nice near corner hardware and cones. If you want to test the tie-head idea, check your cone OD against a 1/2 in chamfer face (about 0.71 in) and see if you’d be happy with that margin. For clean architectural interior edges I’ll still spec 1/2, but outside it gets chewed up.