Concrete tabletop question

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i have some serious industrial bases that I would like to use on a dining table with a concrete top, but have some concerns.
A cast concrete top will be very (very) heavy. Therefore I am thinking of using a substrate with a concrete overlay.

The substrate- I am thinking plywood with an apron and cross member to prevent flex. Should I consider using putting a layer of kerdi board over the plywood?

The cement overlay- looking for suggestions of what product to use and how I should treat the edges.
Thank you.

  5 answers
  • V Smith V Smith on Jan 06, 2018
    Couldn't you use ultra light cement In a form?

    • John Biermacher John Biermacher on Jan 07, 2018
      I am thinking overlay becaus I am concerned about the weight of regular concrete. The table will be 7 to 8 ft long and aboit 40 inches wide. I will try to find numbers for the weight of ultra light concrete. Thank you.

  • 71729639726 71729639726 on Jan 06, 2018
    You can make a form the size you need the table top to be. Seal the frame so no cement leaks out. Get a piece of flat foam sheet. Cut it 1/2" smaller than the frame. Drill holes of 1/2" diameter every 4-5" apart . Make sure that the foam is only a third as thick as the finished product. Pour half of the cement into the mold vibrate the cement into the corners. Before the cement starts to setup add the foam. Rocking it into place. Be sure when you add the rest of the cement to fill all the holes in the foam and have at least 1/2" to 3/4"s of cement on the top. That way it will be a 1/3 less weight . You could also use corigated cardboard to fill in some of the mold.

    • John Biermacher John Biermacher on Jan 07, 2018
      Please see my reply to V Smith. I will need to calculate if using a foam core and light concrete might work. Thank you for the option.

  • Suzette Suzette on Jan 07, 2018
    Hi John, Here's a how to video. He's making a coffee table, but the principle is the same! https://www.youtube.com/watch?v=BygJGz3TC3o

    Good luck!

  • John Biermacher John Biermacher on Jan 07, 2018
    I did some calculations and the dining tabletop I am thinking of doing would be between 200 to 250 pounds even using a foam core and lighter weight concrete. I think I have found an alternative process (https://www.hometalk.com/diy/bathroom/vanities/diy-concrete-countertop-28896411) that I will give a try (come spring). I will keep this question open, should someone have an idea/opinion and I want to thank everyone for their input.