The Hidden Race Between Drill Bombs and High-Tech Concrete (UHPC)
The Hidden Race Between Drill Bombs and High-Tech Concrete (UHPC)
The military is constantly upgrading the protection of concrete bunkers while upgrading the bombs that destroy them, but concrete (UHPC) may be winning the race. In the late 2000s, there were rumors of an Iranian bunker being hit by a drill bomb. The drill bomb failed to penetrate the bunker and remained stuck on the surface of the bunker until the bunker's occupants called in a bomb disposal team. The drill bomb stopped unexpectedly and did not smash through the concrete. It's not hard to guess why; Iran is a leader in the new technology of Ultra High Performance Concrete (UHPC). Apparently, their newest bunkers have enough protection against standard drill bombs.
Dr. Stephanie Barnett of the University of Portsmouth in the UK has worked to develop stronger concrete for protecting civil buildings against terrorist attacks and to promote improved envelope protection. She has heard the story and understands that offense and defense are two ends of a tug-of-war balance. Listening to her talk was exciting for the general audience, but the response from military personnel was sometimes less positive. “One officer told me, 'If you make this stronger blast and impact resistant material, what we need to think about is how to penetrate it.'”
Barnett told Popular Mechanics. Better concrete forces bunker busters to improve their craft. Israel, which has long viewed Iran as a potential target, asked the U.S. in 2005 for new, more powerful drill bombs, eventually receiving the 5,000-pound (2,270kg) GBU-28 drill bomb in 2009. The GBU-28 roughly quadruples the penetration capability of the 2,000 lb (908 kg) GBU-31v3 drill bomb previously supplied to the IAF.
Now, Israel has raised the bar again by requesting the new U.S. Air Force (USAF) GBU-72 “Advanced 5K Intruder” round. The bomb is not yet in service, having been tested for the first time last October (October 2021, that is). Like the GBU-28, the GBU-72 is a 5,000-pound drill round, but with significant improvements - although the Air Force would not provide details.
The development of the GBU-72, and Israel's urgent need for it, seems to reveal a signal that in the quiet arms race between concrete and drill bombs, concrete is winning.
In 1985, the U.S. Air Force had its first generation of modern drill bombs. While general-purpose bombs are filled with explosives in a thin steel casing, drill bombs have a slimmer profile, thicker casing, and less explosives. This design concentrated all the weight into a smaller area, making it more like an ice pick than a hammer, and allowed the bomb to penetrate concrete or drill into the ground to hit deeply buried targets. The general-purpose bombs used today are the same as those used in the 1990s, but the drill bombs have gone through several generations of upgrades. In the early 2000s, the Air Force also worked with Ellwood National Forge Company, a specialized steel company, to develop a special type of steel called Eglin Steel. Eglin Steel is a low-carbon, low-nickel steel that contains trace amounts of tungsten, chromium, manganese, silicon and other elements, each of which contributes to the desired overall performance. Eglin steel is the steel used in the counterpart of the drill bomb, and in recent years Eglin steel has been replaced by the new USAF-96 steel, which has similar properties to Eglin steel but is easier to produce and process. Materials scientists identify the two qualities of materials by their toughness and hardness, and the balance between the two drives the arms race between weapons and armor (spears and shields). For example, when a soft lead bullet hits a Kevlar (aramid fiber) bulletproof undershirt, the bullet crumples and deforms, losing energy due to lack of stiffness. If the bullet is fitted with a hard outer shell, the Kevlar undershirt will not be able to protect against it. The way to deal with this was to add super-hard ceramic plates made of materials such as boron carbide, and bulletproof undershirts were fitted with even harder armor, so hard that steel-cased bullets shattered on impact. This, in turn, led to the invention of special armor-piercing bullets, when the armor-piercing bullets equipped with hard tungsten tips hit the ceramic plate, the ceramic plate will be broken, that is, brittle failure.
The arms race for drill bombs is similar, but when the attacker has the advantage of steel, the defense is still based on concrete, which originally had an inherent disadvantage. Prof. Phil Purnell, a concrete technologist at the University of Leeds, says: “Concrete is naturally brittle; it's good at compression, not tensile. The weaknesses are tensile strength and toughness,” Purnell says, pointing out that while some modern concretes are actually stronger than aluminum, brittleness is their Achilles' heel and they can fail through cracking. However, things have changed with the advent of the type of concrete known as UHPC. Previously, concrete with a compressive strength of 5,000 pounds per square inch (psi) (34.5 MPa) was rated “high strength,” preferably up to 10,000 psi (69 MPa). New UHPC can withstand 40,000 psi (276 MPa) or more. Gaining higher strength can be accomplished by adding steel or other fibers to turn the concrete into a composite material. These fibers hold the concrete together, preventing cracks from expanding in the concrete and reducing brittleness. “Instead of a few big cracks in the concrete slab, there are a lot of small cracks,” Barnett says. “The fibers give the concrete more fracture energy.”
Fracture energy is defined as the energy required to split a material. Concrete absorbs the kinetic energy of an intruding projectile, causing it to break, slow down, and stop the penetration. Of course, researchers have been experimenting to find the best fiber composition for UHPC. The more the merrier, but there are limits. “The problem is that if you add more than one percent steel fibers, the fibers start to clump together,” says Purnell. The technical trick is how to mix and disperse more than one percent of the fibers into the concrete.”
Various teams around the world have been working on techniques that mix fibers well. Much of this work has been carried out by the military; but as Barnett points out, in her experience, the military will from time to time turn to civilian researchers for advice, but keep quiet about their own work. In the field of impact-resistant concrete, which has received little civilian engineering attention, the military may be somewhat ahead of its civilian counterparts.In January 1991, while the United States was leading operations in Kuwait, U.S. intelligence discovered something alarming. The Iraqis had constructed a series of new command bunkers deep underground around Baghdad, protected by several feet of reinforced concrete, estimated to be indestructible by the USAF's existing 2,000-pound drill bombs. For this reason, an emergency program to build a new 5,000-pound drill bomb was initiated.The USAF requested it on January 18, and the Air Force Research Laboratory at the Quartermaster General's Office at Eglin Air Force Base, Florida, began work immediately. There was no time to make the bomb shells from scratch, so the bodies were made from surplus 8-inch howitzer barrels, hand-filled with explosives and fitted with new warheads. Less than a month later, the first prototypes were delivered to the U.S. Air Force; during a rocket skid test, the new weapon penetrated concrete more than 20 feet (more than 6 meters) thick.On February 27, two operational bombs were flown into the theater and delivered by F-111F fighter jets. Six seconds after a new bunker in Iraq was hit, smoke poured out of the entrance, indicating that the bunker had been breached and destroyed. The day was memorialized in the archives by a munition developed over six consecutive weeks. But would the Air Force win the next round of bunker breaches so easily?In 2012, the U.S. Air Force initiated a program to assess the challenges posed by bunkers built by UHPC. The Air Force eventually developed its own version of UHPC, appropriately called Eglin High Strength Concrete, for pilot testing. The results of the U.S. Air Force study are classified, but a publicly available Chinese study compared ordinary high-strength concrete to fiber-reinforced UHPC. Projectiles penetrated the reinforced concrete targets, but the UHPC targets resisted with only minor cracking, and the projectiles “embedded in or bounced off the targets. The U.S. Air Force has feared that even a 5,000-pound bomb would not be enough, and in 2011 received a huge 30,000-pound (13,620 kg) bomb, the “Large Military Penetrator” (MOP). This is even larger than the famous 21,000 lb (9,534 kg) Massive Ordnance Air Blast (MOAB, or “Mother of All Bombs”), which destroys the deepest and hardest bunkers with purely kinetic energy. The MOP is the largest bomb that can fly - only the B-2 Phantom strategic bomber is capable of flying it - so the smaller 2,000- and 5,000-pound weapons would still be needed against most smaller targets.
After a specific study, the Air Force upgraded the MOP and then upgraded it again. By 2018, the MOP underwent its fourth upgrade. Smaller weapons underwent similar upgrades. The problem is that even the largest bombs that can be built may no longer be able to penetrate bunkers built of the toughest materials. Dr. Gregory Vartanov of Advanced Materials Development, based in Toronto, claims that the high-grade UHPC is simply too strong for bombs made from existing steel. “Monolithic Shell Intruders made from ...... Eglin steel ...... cannot penetrate bunkers constructed of UHPC,” Vartanov noted in a February 2021 article published in the journal Aerospace & Defense Technology, basing his claim on open-source intrusion formulas.
But that's not the end of the story.UHPC is good, but better protection has been tested in the lab. Recent research in China describes Functional Gradient Cementitious Composites, or FGCCs for short, consisting of layers of different types and properties of high performance concrete. The thin outer layer is a UHPC layer reinforced with super-hard aggregates; a thick layer below is a hybrid fiber-reinforced UHPC layer optimized to resist cracking; and finally, there is a tough steel fiber-reinforced UHPC layer. As Purnell explains, each layer serves a different purpose. “You have a hard outer layer that destroys the projectile; then a thicker layer that absorbs a lot of its energy; and after that, an inner layer that catches the fragments,” Purnell says. That inner layer is an anti-spall layer, ensuring that no fragments (or “spalls”) enter the bunker even if the concrete cracks. According to Chinese research published in June, FGCC's resistance to penetration and blast is far superior to that of UHPC: “The synergistic effect of the high-strength fibers and coarse aggregates results in a significant reduction in penetration depth, crater area, and penetration damage,” says Barnett, who has researched similar concepts, and that this technique, which involves the layering of differently characterized materials, may be more effective than any single material. techniques that may be more effective than any single material. This latest research comes after at least four years of layered concrete research in China, specifically aimed at absorbing impact and blast. The new bunkers are expected to be very hard to crack nuts. There is limited space to make drill bombs bigger and more destructive, but there are other ways. The arms race may not follow the same path, but move in a different direction. “Hypersonic weapons offer a potential new mode of attacking hardened bunkers,” says Justin Bronk of RUSI, a British defense think tank. Hypersonic weapons are missiles that travel through the atmosphere at speeds in excess of Mach 5 and are equipped with a tungsten penetrator, which acts as a “God's staff” and penetrates layered concrete like an armor-piercing bullet. The weapon has no explosive warhead and causes damage only through kinetic energy. bronk also points out that it is not always necessary to physically destroy the bunker. You can damage the entrance, get rid of the antennae, and hit the right spot to cut off communications with the command bunker. In military terms, it could also be a crater, even if the bunker's occupants are not harmed.
Understandably, the U.S. Air Force will not discuss its current bunker-destroying capabilities or how they might be used against potential targets in Iran, China, or elsewhere. Most military work on high-strength concrete falls under a similar level of secrecy. The U.S. military relies heavily on air power to control targets at risk. Adversaries may try to hide their command headquarters or nuclear facilities underground, but drill bombs deprive them of sanctuary. Incremental improvements in the area of bland concrete technology could have far-reaching strategic implications if they diminished the dominance of air power.