Sugar is too obvious for such sabotage as the concrete will just not harden. What you need to do is add more water into the mix to make it weaker, and then slop off the surface water to force it to cure too quickly. Salt in the mix is an accelerant and will induce cracking/spalling.
If I had control over the total mix I would try to substitute a contaminated aggregate (e.g., pyrites).
No not really.