For the moment, anyway:

I designed a 2”x2” concrete cube with a compressive strength of 16,000 psi [pounds per square inch] at 28-days, a relatively high strength as standard concrete is on the order of 3,000 psi, typically. Now, The University of Tehran made several cubes between 50,000 to 60,000 psi, and possible stronger! I thought the aggregate to be made from quartz, and I also remember some steel fibers in the mix. These cubes exploded at failure, finally damaging the compression machine on the third or fourth cube (that machine was substantial, made for much larger samples). So, keep in mind this is unreinforced concrete (save the steel fibers) at an early age. Concrete becomes stronger, sometimes by orders of magnitude, over time.

I’ll make you follow the link to see why that may pose a problem for not only our biggest conventional bunker buster but even one with a nuclear payload. Possible solution: Deep Digger, a.k.a. the earthquake bomb.