Previously, a yield strength of 5,000 pounds per square inch (psi) was enough for concrete to be rated as “high strength,” with the best going up to 10,000 psi. The new UHPC can withstand 40,000 psi or more.
The greater strength is achieved by turning concrete into a composite material with the addition of steel or other fibers. These fibers hold the concrete together and prevent cracks from spreading throughout it, negating the brittleness. “Instead of getting a few large cracks in a concrete panel, you get lots of smaller cracks,” says Barnett. “The fibers give it more fracture energy.”
Yeah, if the concrete is 40’ thick and they’re only getting 10’ of penetration with the explosives, then this isn’t going to do much. But if it’s 20’ thick and they got through the first 12 with HE, the remaining 8 are going to have a lot of cracks to admit slow liquid death.
I have zero information on what the reinforcers are in the concrete, so shot in the dark is about right. Glass might be tough - unless you could deliver hydrofluoric acid effectively. Metals - we’re not going to want to wait for iron to oxidize, looks like hydrogen embrittlement with HF again - so maybe that’s the magic sauce. Nasty stuff, but that’s what weapons manufacturers are good at handling and packaging: nasty stuff.
2000 lbs of HF poured on the surface isn’t going to do much to the buried chamber, but 2000 lbs of HF delivered into the freshly stressed and heavily cracked concrete layer under all the dirt - that could be a problem for future use of the facility.
Unfortunately (?) HF is a gas.
inste-edit: I know I’m being pedantic and a reasonable concentration of hydroflouric acid is what you were talking about. There was an article somewhere about a 1 tonne spill on chlorine triflouride
There was a “etch your VIN in your car window glass” campaign around here years back, they had a liquid solution of HF that they were wiping across printed stencils to do the glass etching. The fact that its natural state is a gas just makes it all the nastier to handle - and possibly even more effective at diffusing through the cracks to cause hydrogen embrittlement of any steel reinforcement it may come into contact with.