Does anyone have a feel for the flatness of Portland Cement when poured in a standard/typical way? Say, for example, that I wanted to pour a cement "cube" (20"x 18"x 19.25") in a mold. How flat would the top surface be without any special procedures?
Background: I am going to mount a forceplate on top of this cement block and I am trying to determine if I need to specify the flatness tolerance in the drawing.
Is there somewhere I can look for this information?
If you live in a country that uses ACI standards, you might consider specifying flatness using an FL or FF number. ACI 117, Commentary Table R4.8.4 has this information, with flatness ranging from "Conventional" to "Super Flat." For the dimensions you have listed, specifying anything beyond "Flat" is probably overkill since your forceplate will likely cause the underlaying slightly-unflat cement to consolidate to the face of the plate anyway.
However, what I said above may be overkill for what you're doing. I'd just tell them to overfill and then strake it off using the top of the form as a guide, just like what you'd do scooping flour for a cookie recipe. That should get you what you want.
And to clarify - if the cement is a dry mix, my comment about the consolidation of the cement is valid. However, if this is a cement mix with water that is allowed to cure, the consolidation comment is not strictly applicable.
Understanding "without any special procedures" as meaning simply dropping the cement in a pile, it would probably end up with an inclination approximately equal to its angle of repose. Unfortunately, this value is highly variable, and a quick search has resulted in many different values:
