Smooth finish on concrete surfaces 1

[WARose]

Something occurred to me the other day (not for the first time): how do cast in place reinforced concrete members like beams & columns (whose surfaces are covered by forms) get that surface that doesn't show any aggregate? With a surface not covered by forms (like mats) you can get that look with trowel work.....but I've seen some really good finishes on non-exposed surfaces.

 

[Compositepro]

In any molding or casting process, the finished surface texture is almost entirely determined by the mold surface (the forms). Aggregate will not be seen at the surface unless there is an air pocket that displaces the cement. Vibration can remove air pockets by allowing it to rise out of the concrete. If the cement shrinks a lot during cure you can get an effect called "print through" where you can see the aggregate pattern. Where a piece of aggregate touches the form there will be no shrinkage, but in the cement filled void between aggregate pieces the cement will shrink away from the form. You generally need a glass finish to be able to see this. Light reflecting off of glass smooth surfaces will reveal even micron size changes in flatness. Cement particles are larger than a wavelength of light so you cant really get true glass-smooth surfaces without some other coating.