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A client has asked whether a container can be placed on an existing unreinforced concrete foundation of depth 100 mm. The grade of the concrete is unknown. How can the capacity (in kPa) of this plane concrete foundation be evaluated?

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In order to determine the capacity, you will need to know a couple of things.

  1. Dimensions of the pad and container
  2. Bearing capacity of the soil under the slab. You would normally receive this from a geotechnical report.
  3. The strength of the concrete. This can be determined either through core testing or something called a "schmidt hammer". The schmidt hammer can give you a ball park idea of the strength of the concrete. However you need to calibrate it against something of a known strength first.
  4. You will want to check the concrete pad for crushing, shear. You local building code should provide guidance on capacity checks for both bearing and shear.
  5. You will also want to look at unbalanced loads from the container, and potentially the container not being centered on the pad.

Note that many parts of the world will consider plain concrete (unreinforced) to be non-structural in nature. That does not mean that it can't carry a load, just that its relatively weak compared to the structural loading reinforced concrete tends to take.

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    $\begingroup$ assuming the pad is evenly supported by the soil is also a possible issue... some could have been washed away over time... $\endgroup$ – Solar Mike Sep 5 '19 at 11:21
  • $\begingroup$ I was once coring a concrete pier/wharf in Port Stanley, Ontario. The concrete deck was close to 500 mm thick. I nearly lost the core as the ground beneath at that location had settled about that same distance. The core fell into the space and started to fall over and got wedged in the bottom of the core hole. Thankfully the whole concrete pier/wharf was supported on piles and the ground settlement underneath the deck was a non issue, but yes, you can definitely get voids beneath concrete slabs and footings over time $\endgroup$ – Forward Ed Sep 5 '19 at 12:16

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