The reason was not hard to guess.\u00a0Iran was a leader<\/a>\u00a0in the new technology of\u00a0Ultra High Performance Concrete<\/a>,\u00a0or UHPC. Evidently, their latest was too much for standard bunker busters.<\/p>\n Dr. Stephanie Barnett<\/a>\u00a0of the University of Portsmouth in the U.K. is involved in developing stronger concrete to protect civilian buildings against terrorist attacks, pushing the envelope of ever-better protection. She has heard the story and understands the see-saw balance between offense and defense. While civilian audiences have been enthusiastic, she occasionally hears less positive responses from military personnel attending her presentations.<\/p>\n \u201cOne officer told me, \u2018If you make this stronger blast- and impact-resistant material, we need to think about how to get through it,\u2019\u201d Barnett tells\u00a0Popular Mechanics<\/em>.<\/p>\n Better concrete forces bunker busters to raise their game. In 2005,\u00a0Israel<\/a>\u2014which has had a longstanding interest in\u00a0Iran as a potential target<\/a>\u2014requested new, more powerful bunker-busting weapons from the U.S., and these were\u00a0eventually supplied in 2009<\/a>. The\u00a0GBU-28<\/a>\u00a0is a 5,000-pound bomb with roughly four times the penetrating power of the 2,000-pound GBU-31v3 previously supplied to the Israeli Air Force.<\/p>\n Now, Israel is raising the bar again\u00a0with a request<\/a>\u00a0for the United States Air Force\u2019s (USAF) new GBU-72 \u201cAdvanced 5K Penetrator\u201d bomb, which is not yet in service and was only\u00a0tested for the first time<\/a>\u00a0last October. Like the GBU-28, it\u2019s a 5,000-pounder, but with a significant improvement on the earlier weapon\u2014although the Air Force will not give details.<\/p>\n The development of the GBU-72 and Israel\u2019s urgent desire for it might be signals that in the quiet arms race between concrete and bombs, the concrete is winning.<\/p>\n Bunker Busting Basics<\/strong><\/p>\n The USAF introduced its\u00a0first modern bunker buster<\/a>\u00a0in 1985. General-purpose bombs have a thin steel casing filled with explosives; the bunker buster had a narrower profile, with a thicker casing and less explosives. This design concentrates all the weight on a smaller area, making it an ice pick rather than a hammer, so the bomb can smash through concrete or burrow through earth to strike deeply-buried targets.<\/p>\n While the same general-purpose bombs are being used as they were in the 90s, 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\u00a0Eglin Steel<\/a>, in association with steel specialist Ellwood National Forge Company.<\/p>\n Eglin Steel is a\u00a0low-carbon<\/a>, 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\u00a0USAF-96 steel<\/a>,\u00a0which boasts similar performance but is easier to produce and work with.<\/p>\n Materials scientists distinguish between the two qualities of toughness and hardness, and it is the balance between them that drives arms races between weapons and armor.<\/p>\n For example, when a soft\u00a0lead bullet strikes a Kevlar vest<\/a>,\u00a0the bullet crumples and deforms, losing energy because it lacks hardness. Give the bullet a hard steel jacket though, and the Kevlar gives way. The counter is to make armor harder, by adding\u00a0ultra-hard ceramic plates<\/a>\u00a0made out of materials like boron carbide. These are so hard that steel-jacketed bullets break up on impact. This led to special armor-piercing bullets tipped with hard tungsten. When these hit a ceramic plate, the plate breaks up in a process known as brittle failure.<\/p>\n The bunker-buster arms race is similar, but while the attackers have the advantage of steel, defense is based on concrete, which starts with a built-in disadvantage. \u201cConcrete is inherently brittle,\u201d says\u00a0Professor Phil Purnell<\/a>, an expert in concrete technology at the University of Leeds. \u201cIt is good at being squashed, not being stretched. The weakness is in its tensile capacity and toughness.\u201d Purnell notes that while some\u00a0modern concrete<\/a>\u00a0is actually stronger than aluminum, its brittleness is its Achilles Heel, and it gives way by cracking.<\/p>\n 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 \u201chigh strength,\u201d with the best going up to 10,000 psi. The new UHPC can withstand 40,000 psi or more.<\/p>\n The greater strength is achieved by turning concrete into a composite material with the addition of\u00a0steel<\/a>\u00a0or other fibers. These fibers hold the concrete together and prevent cracks from spreading throughout it, negating the brittleness. \u201cInstead of getting a few large cracks in a concrete panel, you get lots of smaller cracks,\u201d says Barnett. \u201cThe fibers give it more fracture energy.\u201d<\/p>\n Fracture energy is defined as the amount of\u00a0energy<\/a>\u00a0needed 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. More is better, but there is a limit. \u201cThe problem is that if you put in more than about one percent of steel fiber, it starts to clump together,\u201d says Purnell. \u201cThe clever trick [is] how do you mix in more than one percent of fiber into the concrete.\u201d<\/p>\n Various teams around the world have been working with techniques to mix fibers successfully. Much of this work is carried out by the\u00a0military<\/a>; but as Barnett notes, in her experience, the military sometimes come and ask questions to civilian researchers but never say anything about their own work. In the field of impact-resistant concrete\u2014a minority interest in civil work\u2014they may be some way ahead of their civilian counterparts.<\/p>\n When the USAF needed a new bunker buster in a hurry…<\/strong><\/p>\n In January 1991, when the U.S. was leading the operation to Kuwait, U.S. intelligence discovered something alarming. The Iraqis had built a series of new command bunkers around\u00a0Baghdad<\/a>\u00a0deep underground and protected by several feet of reinforced concrete, estimated to be invulnerable to the USAF\u2019s existing 2,000-pound bunker busters.<\/p>\n A\u00a0crash program was launched<\/a>\u00a0to build a new 5,000-pound bomb for the job. The USAF asked for ideas on January 18, and work commenced immediately at the Air Force Research Laboratory Munitions Directorate at Eglin Air Force Base, Florida. There was no time to fabricate\u00a0bomb<\/a>\u00a0casings from scratch, so surplus 8-inch howitzer barrels were used as the basis of the bomb body, hand-filled with explosive, with a new nose section added. The first prototypes were delivered to the USAF less than a month later; in a rocket-sled test, the new weapon penetrated over 20 feet of concrete.<\/p>\n Two operational bombs were flown to the theater on 27 February and delivered by F-111Fs. Six seconds after impact on one of the new Iraq bunkers, smoke poured from the entrance, showing the bunker had been breached and destroyed. The day was saved by a munition developed six weeks flat.<\/p>\n But will the Air Force win the next round of bunker-busting so easily?<\/p>\n Are There Really Invulnerable Bunkers?<\/strong><\/p>\n In 2012, the USAF launched a project to evaluate\u00a0the challenge posed by bunkers<\/a>\u00a0made of UHPC. The Air Force ended up developing its own version of UHPC, known appropriately enough as Eglin high-strength concrete, for the testing process.<\/p>\n While the results of the USAF study are classified, an\u00a0open-source Chinese study<\/a>\u00a0compared normal high-strength concrete with fiber-reinforced UHPC. Projectiles smashed through the reinforced concrete targets, but the UHPC targets survived with only minor cracking, and projectiles \u201ceither embedded in or rebounded from\u201d the targets.<\/p>\n The Air Force was already concerned that even its 5,000-pound bombs were not sufficient, and in 2011, it received the\u00a0Massive Ordnance Penetrator<\/a>\u00a0(MOP), a colossal 30,000-pound bomb. This was even bigger than the celebrated 21,000-pound Massive Ordnance Air Blast (MOAB, or \u2018mother of all bombs\u2019), to bust the deepest and toughest bunkers with sheer kinetic energy. MOP is about as big a bomb as you can fly\u2014only the\u00a0B-2 Spirit strategic bomber has the capacity<\/a>\u2014so smaller 2,000- and 5,000-pound weapons will still need to do most of the work against lesser targets.<\/p>\n After the concrete study, the Air Force upgraded the MOP. Then it upgraded it again. By 2018, it was\u00a0on its fourth upgrade<\/a>. Similar upgrades were made to smaller weapons.<\/p>\n The problem is that even the biggest bomb possible, made of the toughest material available, may no longer be able to hack it. Dr. Gregory Vartanov, of Toronto-based\u00a0Advanced Materials Development Corp<\/a>, claims high-grade UHPC is simply too strong for bombs made with existing steels. \u201cPenetrators with monolithic cases made from materials such as\u2026 Eglin Steel \u2026 cannot penetrate bunkers made from UHPC,\u201d Vartanov notes in a February 2021\u00a0piece in\u00a0Aerospace & Defense Technology<\/em>\u00a0magazine<\/a>, basing his claim on open-source penetration formulas.<\/p>\n But that is not the end of the story. UHPC is good,\u00a0but even better protection is already being tested in the laboratory.<\/p>\n Concrete Is Getting Even Stronger<\/strong><\/p>\n Recent work from China describes\u00a0Functionally Graded Cementitious Composite<\/a>,\u00a0or FGCC, made by layering different types of high-performance concrete with different properties. The thin outer layer is extra-hard aggregate-reinforced UHPC; beneath this is a thick layer of hybrid fiber-reinforced UHPC optimized to resist cracking. Finally, there is a layer of tough steel fiber-reinforced UHPC. As Purnell explains, each layer has a different effect.<\/p>\n \u201cYou\u2019ve got that hard outer layer to damage the projectile, then there is the thick layer with the mass to absorb its energy, and then the inner layer is there to catch the bits,\u201d says Purnell. This inner, anti-spall layer, ensures that if the concrete does crack, no fragments (or \u201cspalling\u201d) get through into the bunker beyond.<\/p>\n According to Chinese research\u00a0published in June<\/a>, FGCC resisted penetration and explosion far better than UHPC: \u201cpenetration depth, crater area, and penetration damage were decreased greatly by the synergistic effects of high-strength fibers and coarse aggregates.\u201d Barnett says that she has worked on a similar concept, and that this technique of layering materials with different properties could be more effective than any single material.<\/p>\n The latest research follows at least four years of Chinese study into layered concrete, with a particular focus on\u00a0absorbing impacts and blasts<\/a>. Expect new bunkers to be very tough nuts to crack.<\/p>\n Faster and Smarter Ways to Bust Bunkers<\/strong><\/p>\n There is limited scope for making bunker busters bigger and badder, but there are other approaches. The arms race may not continue along the same path but take off in a different direction.<\/p>\n \u201cHypersonic weapons<\/a>\u00a0offer a new potential mode of attack against hardened bunkers,\u201d says\u00a0Justin Bronk<\/a>\u00a0of the U.K. defense think tank RUSI. Hypersonics are missiles which travel through the atmosphere at speeds in excess of Mach 5. Equipped with tungsten penetrators, they could act as \u2018rods from god,\u2019 punching through layered concrete like an armor-piercing bullet. With no explosive warhead, such weapons do damage by kinetic energy alone.<\/p>\n Bronk notes also that it is not always necessary to actually destroy a bunker. You can damage the entrances, take out the aerials, and cut off communication to a command bunker with hits in the right places. In military terms, it might as well be a crater, even if the occupants are unharmed.<\/p>\n The USAF will understandably not discuss its current bunker-busting capabilities or how they stack up against potential targets in Iran,\u00a0China<\/a>, or elsewhere. And most of the military work on high-strength concrete is similarly classified.<\/p>\n The U.S. military relies heavily on airpower to hold targets at risk. Adversaries may try to hide their command headquarters or\u00a0nuclear facilities<\/a>\u00a0underground, but bunker busters give them no refuge. Incremental improvements in the prosaic field of concrete technology might have far-reaching strategic implications if they can blunt the edge of air power.<\/p>\n![\"\"](\"https:\/\/www.fie.undef.edu.ar\/ceptm\/wp-content\/uploads\/2025\/06\/usaf-f-15e-releases-gbu-28-16633.jpg\")
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