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?

  • $\begingroup$ The word forceplate completely new to me. Is it the same as a baseplate/endplate/faceplate? Bolting a steel baseplate to a concrete base is common. In this case the normal procedure is to cast the concrete base with anchor sockets in it. Once the concrete is cured, the baseplate is bolted in with a wet mortar bed between the plate and the concrete. This ensures a completely smooth contact surface. See fhwa.dot.gov/bridge/images/sign12.gif. (I know this doesn't actually answer the question you asked, but I hope it may be useful.) $\endgroup$ – AndyT Jul 10 '15 at 9:11
  • $\begingroup$ @AndyT a forceplate is an instrumented (strain gauges) platform to measure ground reaction forces, typically of gait or other movements across a floor. See here bertec.com/products/force-plates $\endgroup$ – willpower2727 Jul 10 '15 at 12:11
  • $\begingroup$ Thanks for link. From the look of the image, it does appear that the forceplate will need to be bolted down, hence hopefully my comment is of relevance. If not, let me know as I may as well delete it. $\endgroup$ – AndyT Jul 10 '15 at 13:01

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.

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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:

  • 15 or 20 degrees if fine or coarse (1)

  • 40-44 degrees (1)

  • 39 degrees (2)

  • Can't trivially be defined (3)

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  • $\begingroup$ @ wasabi There is a mold, and the final product is a prismatic shape with perpendicular sides. By "without any special procedures" I mean no machining or polishing to improve the surface finish. I want to have an idea of what the flatness will be if I did the easiest cheapest thing. $\endgroup$ – willpower2727 Jul 8 '15 at 15:26
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    $\begingroup$ @Wasabi is right - if you just dump something in one spot, it will take the shape of a cone. The slope of the cone is known as the angle of repose (the link he gives is a useful read). With a 15 degree angle, you still should trowel the cement to form a trowel finish, which is usually defined as no more than 1/4" standoffs between high and low spots when measured w/ a 10' straight edge. $\endgroup$ – Mark Jul 8 '15 at 16:26
  • $\begingroup$ @Mark just to be clear, a trowel finish is flat within +-0.25" across a 10' distance? that's the kind of spec I'm looking for. Did I understand that correctly? $\endgroup$ – willpower2727 Jul 8 '15 at 17:03
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    $\begingroup$ That's correct. The specification is ACI 301-72, Secion 11.7.3. $\endgroup$ – Mark Jul 8 '15 at 17:29
  • $\begingroup$ @Wasabi thanks, I realize that my question is poorly asked, and I see what you mean about the cement piling up close to it's dry angle of repose. If I had known more about cement pouring I would have asked the flatness of the cement using nothing more than a trowel. I apologize for the mix up. $\endgroup$ – willpower2727 Jul 10 '15 at 12:19

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