More on the Midnight Hammer raid

I mentioned reading an article on Iranian concrete technology in the comments on the BDA column – here is some more on that subject.

On Saturday, June 21, 2025, following a spate of unprecedented aerial attacks that Israel carried out against Iran just days before, the United States joined the war and used bunker-busting bombs to strike three key Iranian nuclear sites and their underground bunkers: the Fordow fuel-enrichment plant, the Natanz nuclear facility, and the Isfahan nuclear technology center.

Other than the leaked DIA preliminary assessment, all concerned on our side consider the raid an unqualified success. Here is some more data:

If history serves as any indication, there is a chance Iran’s underground nuclear facilities could be partially or wholly intact. That’s because up until now, in the quiet arms race between concrete and bombs, the concrete has been winning.

In the late 2000s, for instance, rumors circulated about a bunker in Iran struck by a bunker-buster bomb. The bomb had failed to penetrate—and remained embedded in—the surface of the bunker, presumably until the occupants called in a bomb-disposal team. Rather than smashing through the concrete, the bomb had been unexpectedly stopped dead. The reason was not hard to guess: Iran was a leader in the new technology of Ultra High Performance Concrete, or UHPC, and its latest concrete advancements were evidently too much for standard bunker busters.\

Seems counterintuitive – 30,000 pounds – 15 TONS of high explosive – you would think would take out anything. (Might point out our command center under Cheyenne Mountain is supposed to be able to resist pretty much anything for a while, it was built half a century ago. Would modern weapons like our bunker-busters hurt it nowadays? Hmm…)

While the same general-purpose bombs from the 1990s are being used today, bunker busters had to go through several generations of upgrades. In the early 2000s, the Air Force even developed a special type of steel for the purpose, known as Eglin Steel, in association with steel specialist Ellwood National Forge Company.

Eglin Steel is a low-carbon, low-nickel steel with traces of tungsten, chromium, manganese, silicon, and other elements, each contributing a desirable property to the whole. Eglin Steel is the gold standard for bunker-busting munitions, although in recent years it has been supplemented by new USAF-96 steel, which boasts similar performance but is easier to produce and work with.

Sounds like whatever it hits, it will penetrate, right?

“Concrete is inherently brittle,” explains Phil Purnell, Ph.D., an expert in concrete technology at the University of Leeds. “It is good at being squashed, not being stretched. The weakness is in its tensile capacity and toughness.” Purnell notes that while some modern concrete is actually stronger than aluminum, its brittleness is its Achilles’ Heel, and it gives way by cracking.

However, this changed with the advent of the type of concrete known as UHPC. 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.

Fracture energy is defined as the amount of energy needed to split a material open. The concrete absorbs the incoming kinetic energy of a projectile as it fractures, slowing it right down and stopping it from penetrating. Naturally, researchers have been experimenting with finding the optimal mix of fibers for UHPC.

The article goes into more detail on the evolution of bunker-busting bombs – and of the concrete bunkers we’re trying to kill. Worth noting is that the current evolution of bunker-busters, the MOP bombs used Saturday, are already so big only a U.S. B-2 can carry one. They’ll have to become more effective without gaining much weight, to penetrate lower depths and take out harder barriers.

Gregory Vartanov, Ph.D., of Toronto-based Advanced Materials Development Corp, claims high-grade UHPC is simply too strong for bombs made with existing steels. “Penetrators with monolithic cases made from materials such as … Eglin Steel … cannot penetrate bunkers made from UHPC,” Vartanov notes in a February 2021 piece in Aerospace & Defense Technology magazine, basing his claim on open-source penetration formulas.

And a last comforting quote from the article, about an even more resistant Chinese development called FGCC::

According to Chinese research published in 2021, FGCC resisted penetration and explosion far better than UHPC: “penetration depth, crater area, and penetration damage were decreased greatly by the synergistic effects of high-strength fibers and coarse aggregates.”  Popular Mechanics

Extremely interesting non-technical read – check it out.

Category: Iran, Science and Technology