Some of the world’s experts in ultrahigh-performance concrete (UHPC) are working in Iran, a country regularly beset by earthquakes. They want to use the material, a mix of Portland cement, silica fume, quartz flour, fine silica sand, and either steel or plastic-reinforcing fibers, to build durable bridges, sewer pipes, dams, and other structures.
But U.S. military officials and others around the world are worried the Iranians will use the high-strength material to protect nuclear-weapons labs and to build other military bases and underground bunkers.
UHPC’s compression strength is on the order of 30,000 psi, while that of normal concrete is just 4,000 psi. And UHPC has a tensile strength of 1,000 psi, far above normal concrete’s 400 psi. Strength is critical for defensive structures. Tests of a 13-ton bunker-busting bomb, for example, showed it could penetrate 180 ft of ordinary concrete, but only get through 25 ft of concrete that was twice as strong. So it’s possible that the same bomb might only dent UHPC, with its compressive strength seven times that of normal concrete.
UHPC also resists chemicals such as salt at rates 100 times greater than that of normal concrete. And the steel or polymer fibers add ductility and strength. For example, when microcracks start in the brittle cement matrix, the fibers take up the load and prevent further slipping and cracking. In normal cement, the small cracks grow larger and weaken the structure. The fibers can also totally replace traditional rebar used to reinforce concrete.
Silica fume, or ultrafine silica powder, also adds several benefits to UHPC. On a chemical level, it reacts with calcium hydroxide released by the cement, turning the silica into a strong binder that helps hold together the UHPC mixture. And on a physical level, the silica particles, together with the quartz flour, fill in all the small spaces between cement grains and any coarser aggregate. This tight packing keeps out water and chemicals that can seep in and damage the concrete.
U. S. military leaders worry that Iranians are testing even smaller particles, nanoparticles, as additives to UHPC. Some material experts estimate that nanoparticles could boost UHPC’s already impressive strength by a factor of four.
The downside of UHPC is it is cost – 10 times that of normal concrete. It can also be more difficult to work with. To maximize its strength, for example, UHPC must be steam cured, a process that takes about 48 hr.