Precast Under Live Fire

Bullet-absorbing precast concrete helps re-create a safe, realistic Iraqi village for live fire training.

By Fernando Pagés Ruiz

The U.S. Marine Corps Training Command Center at Twentynine Palms functions as the largest Marine Corps base in the world, covering a half-million acres of Southern California desert. Maj. Richard Doherty oversees the integration of new engineering systems for troop training, where the emphasis has shifted to design facilities for military operations on urban terrain (MOUT). “The war on terror unfolds in cities and towns,” Doherty said. “As more people move into urban centers, the world’s conflicts move into dense, populated environments.” More than military might, urban warfare requires experience in close-quarters combat and highly trained light forces to move from street to street and room to room, as well as the ability to distinguish between civilians and hostile forces.

Reconstructing an urban setting for military exercises might seem easy enough. But when it comes to live-fire training, rounds from deadly M-16s and AK-47s can fragment building materials and ricochet, creating a lethal salvo that makes live-fire training difficult to engineer. Until recently, soldiers have trained in urban settings primarily with laser rifles and paintball guns. A revolutionary precast concrete product now makes it possible for soldiers to conduct this type of training with live ammunition.

“Soldiers need live-fire training because paint balls and lasers do not replicate the emotional experience of a loaded gun,” Doherty said.

Nevertheless, concerns with “surface danger zones have hampered live-fire training in urban-like environments,” he said. “But now we have the means to safely prepare soldiers for battle.” The means comes with a precast shock-absorbing concrete, which absorbs and retains ammunition, eliminating the risk of ricochet or flying debris.

Creating ammunition-absorbing concrete
Jim Sigurdson was working as a consultant to the chemical industry when a friend told him about early experiments to produce bullet-absorbing concrete. Twenty years before, the Army Corps of Engineers began development on a concrete that could withstand live fire without fragmenting. Although research stopped before developing a viable product, Sigurdson decided to pursue the idea.

“I suggested we contact the Corps and see if they might still be interested in developing this concrete,” Sigurdson said. And they were. Two years later, Sigurdson’s startup, Ballistics Technology, shared a U.S. patent with the Corps for a revolutionary new precast shock-absorbing concrete, a low-density, fiber-reinforced, foamed concrete for use in the construction of urban-style live-fire training facilities.

A new precast concrete product
Precisely engineered to a density of 90 pounds per cubic foot, bullet-absorbing concrete soaks up the kinetic energy of projectiles in a controlled fashion without cracking or splitting the product. The material allows bullets to penetrate without ricocheting and then holds the munitions, providing a safe live-fire environment and an ecological medium for handling the lead. Encased in the concrete, the military can dispose of spent rounds in a landfill just as they would dispose of ordinary garbage.

Each batch mix must achieve certain density requirements calibrated for specific munitions. “While most concrete specifications require a minimum strength, such as 4,000 psi, we work in absolute densities, measuring the gravity of cement and sand as accurately as possible,” Sigurdson said. In terms of strength, the shock-absorbing concrete averages between 1,000 and 1,500 psi, but the mix demands more than specific strength – it requires almost perfect uniformity so that bullets can penetrate the concrete at any angle up to 130 degrees without ricochet. To achieve uniformity, the concrete is a mix of cement, selected aggregate and fiber along with special chemical additives. The resulting mix has very different working characteristics than standard concrete.

When Ballistics Technology International won a contract to construct two of the world’s largest mockup villages for MOUT training, Sigurdson knew he had to work with a precast manufacturer equipped to handle rigorous quality control and develop a new casting process. Building the first two five-acre Middle Eastern-style villages involved 2,200 wall panels, structural floor and roof panels, and specialty pieces quickly erected in the Mojave Desert – 50 miles from the nearest source of water. Sigurdson found the experience necessary in Mid-State Precast LP and Pankow Special Projects LP. Ballistics Technology manufactures the SACON precast concrete components in a plant in Wilson, N.C. The firm enlisted Mid-State Precast as a contract manufacturer. They joined to produce the products on a large scale and to build the most advanced live-fire training facility ever constructed. Pankow Special Projects managed the site preparation and the product installation.

Application challenges precast producer
As the plant superintendent for Mid-State Precast in Corcoran, Calif., Matt Burden is no stranger to highly engineered precast concrete applications. Mid-State has supplied precast concrete for specially reinforced buildings in seismic zones and experimental bridge pier components incorporating ductile, fiber-reinforced concrete. But he had to rethink everything when it came to shock-absorbing concrete. “Nobody had ever tried anything like this at such a large scale,” Burden said.

“We had to come up with a method to mix and pour this sticky blend of sand, cement, admixtures and fiber that resembled dough more than concrete. In fact, we used a planetary concrete mixer, a 10-foot diameter drum with paddles that churned the stuff like a cake mixer,” Burden said.

Since the concrete mix did not pour down a chute like regular concrete, workers had to shovel it into the
ready-mix truck for transport to the foaming station. Foaming is the key step in creating shock-absorbing concrete. At the foaming station, a concentrated foaming agent was mixed with water and pumped through hoses into the ready-mixed concrete truck “at a consistency like shaving cream,” Burden said.

A complex set of mathematical equations determined the amount of foam required, achieving a precise density of 90 pounds per cubic foot – much lower than the standard 145 to 150 pounds per cubic feet for concrete.

But the challenges did not end with the complex chemistry. After achieving the perfect mix, placing the gooey substance into conventional molds, removing the delicate pieces once cast and then curing them presented even bigger hurdles.

“We had to cast 70 molds a day to keep up with production,” Burden said. Since the mix did not flow out of the truck on a chute and into the molds as in standard pours, workers struggled to spoon the concrete out of the trucks and into the molds without disturbing the uniform blend. Because of its high viscosity, the concrete did not self-level, workers had to spread and then strike the concrete level very carefully. “We couldn’t use any standard vibration devices at all,” Burden said. Instead, workers “rodded” the concrete by hand, essentially poking it with shovel handles to achieve even uniformity. “We couldn’t trowel the finish either, which would create a slick, hard surface that could burst off when fired upon,” Burden said.

The shock-absorbing concrete contains no aggregates or steel reinforcement that might cause a bullet to ricochet. The manufacturer could not extract the hardened concrete pieces from their molds using conventional methods. “We ended up adapting a high-pressure vacuum device used in plate steel manufacturing to suction the pieces and extract them,” Burdon said. Since the suction cups designed for lifting steel could not easily form a vacuum in rough concrete, Burden used a film of water to complete the seal. “Since then we’ve found this vacuum device useful in many other applications,” Burden said.

After fabrication, the pieces underwent a rigorous wet-curing process. Within 15 minutes of releasing each piece from its mold, workers applied a coat of water and wrapped the piece in polyethylene sheathing. Once a full cure was obtained, tests determined whether the rigorous manufacturing process had produced the desired strength and density – no less and no more. The pieces that passed this final exam were loaded using foam-padded forklifts onto flatbed trucks and shipped out to the desert.

Meeting the deadline means more than the money
Tom Krajewski, group manager for Pankow Special Projects in Oakland, Calif., had the job to erect 2,200 precast shock- absorbing concrete panels for two MOUT training villages approximately 50 miles apart in the California desert. Marines would train in rapid assault and street-to-street fighting tactics in these installations. “These guys have to experience the adrenaline of danger, the sounds and smell of live fire before encountering it in the battlefield,” Krajewski said. Live- fire training represents the final and most critical stage of a soldier’s preparation. In this case, the urgency to build this project quickly had a compelling element.

“Every project we do has a tight schedule, and some have liquidated damages if the deadline is missed, but this time it meant a lot more. The major told us that every day we were not done, soldiers were dying for not having the proper training,” Krajewski said. “That motivated us more than any monetary concern ever could.”

Built to resemble Iraqi neighbor-hoods, the two villages included an installation of 14 buildings with a central plaza that Marines would use for convoy training and a dense, 28-building village where Marines could practice dismounts from helicopters and Humvees. Here Marines learn how to move from one building to another avoiding sniper fire and how to clear rooms, traverse hallways and communicate in the heat of battle.

Using satellite photographs and the recollection of experienced Marines returning from Operation Iraqi Freedom, the Pankow group helped design the layout of the two villages for safe and practical training while keeping true to the typical village setting. Because the precast concrete panels came in modular, interlocking 8-foot “T” shapes about 40 inches across and 30 inches deep, the construction proceeded quickly.

Pankow erected the panels over a granular bed as a footing. The interlocking “T” shapes prevented bullets from passing through construction joints. Since mechanized targets pop up at window and door locations, these riddled with bullets quickly. “In time when an area becomes riddled with rounds, the area is cut out with a carborundum blade and chipped out, then a precast concrete block gets siliconed in place to fill the void. No loss of structural integrity, and no worry of having to always move targets,” Krajewski said.

When the Marines saw the need for two-story buildings, Mid-State Precast had to develop a system to incorporate standard precast concrete structural planks with the shock-absorbing concrete. The precaster created a sandwich covering a structural plank with shock-absorbing concrete. To protect the edges, Ballistics Technology developed and cast edge blocks, which also served as parapets and other architectural features. Now, if an errant shot hits the ceiling or floor, no threat of ricochet exists.

In the end, despite unseasonable rainstorms, the unnerving effect of working while bombs detonate in the distance and the challenges of building something no one has ever constructed before, Krajewski and his crews delivered the project on time and much to the Marine’s satisfaction. Currently, Marines train in this state-of-the-art live-fire facility made possible by American ingenuity and the highly controlled, reliable performance of precast concrete.