Science Buddies: "Ask an Expert"

Hi, im doing a project where i will use a van de graaff generator to pass a spark through concrete, causing thermal expansion, and shattering it.
I have two questions:

1. What would have the largest effect on the needed expansion, tensile strength or compressive strength? or could it be both?
2. And what effect would rebar have? would the concrete crumble around it, or hold it together?

Thanks in advance, any help is appreciated.

Do you have any evidence that any spark from a van de graaff generator will break concrete? That's a pretty impressive experiment if so! (though please be careful and use safety glasses, or better yet a face mask, and proper insulating gloves at all times).

Honestly, I don't know how that would work, so I have a hard time visualizing whether tensile or compressive strength would be most important.

Rebar would ground out the spark, so I doubt it would travel past that point no matter what you did. Exactly what this would do to the concrete is a bit imponderable. I suspect it would make it *much* harder to break it.

I'd really love to hear more about your experiment... set up, procedure, variables, etc. as well as the results!