I've tested this with a voltmeter.
You can ONLY ground yourself when walking barefoot on grass or
sticking a metal rod in the ground and connecting to it.
You CANNOT ground barefoot on sand, dirt, concrete or asphalt.
You CAN ground yourself WITH shoes on by touching or holding to a plant, branch, or tree.

I am reading it works on pave stone, cement but not asphalt. Lots of conflicting info on this.

I've testes on paved stone and cement and I was not grounded.
We need to fully understand what grounding fully is.
Because stepping or toughing living matter works.
Tested this 100's of time.

I had heard that you were grounded on cement too, but questioned it as cement does not conduct electricity. I think you're right. Thought you would be grounded on sand and dirt though. Curious if you would be by walking on wood mulch. And have you tested all these places when they are wet like after a rain fall or a dewy morning? I have at times when having a sore back laid down in the grass, surprisingly or not, it works. Nice day, lay in the grass and look at the clouds, things we use to do as a kid... sometimes wonder if we get more stupid the older we get!