Q: I'm working on a project for a rocket engine test facility where the exhaust goes down a concrete-lined trench. Originally a radiator-like shield sprayed water into the exhaust to cool it, but that has been removed. Now the heat is making the concrete lining blast apart, even though the test firings last only thirty seconds; upcoming test fi rings will be as long as three minutes. How can we make the concrete more heat resistant?
A: Standard concrete holds up well against fire temperatures below 400° F. Between that temperature and about 800°, differential thermal movement between the cement paste and the aggregate can lead to problems. Using lightweight aggregates can help, but only to a point.
In the case of rocket engine exhaust, which is much hotter, the rapid temperature increase adds to the concrete's distress. But to hold up where temperatures are above 1000°, you'll need a refractory concrete. These materials often use calcium aluminate cement rather than portland cement. They have been around for many years, but their use in the U.S. space program provided the impetus for their further refi nement from the 1960s through the 1980s.
The ACI committee on refractory concrete was very active in the 1980s, then dissolved in the late 1990s. Tim Fowler, a now retired professor from the University of Texas at Austin, served as editor of the committee's publication ACI 547.1R-89, a stateof-the-art report called "Refractory Plastics and Ramming Mixes."
Back then, he recalls, AP Green, Mexico, Mo., was one supplier of refractory materials. They're now part of ANH Refractories, as is Harbison-Walker, which was referred to in the "Design handbook for protection of launch complexes from solid propellant exhaust," March 1966, NASA. One current source for refractory-grade calcium aluminate concrete is Kerneos (www.kerneosinc.com), based in Chesapeake, Va.
Fowler also says it was useful to include in the concrete stainless steel fibers, which would take the heat better than plain steel. Rather than drawn wire, the fibers were a cast material, RA-330, a product of Rolled Alloys (www.rolledalloys.com).
Yours isn't the only rocket-oriented facility facing such problems, by the way. The May 31, 2008, launch of the space shuttle Discovery did some signifi cant damage to the Launch Pad 39A fl ame trench at the Kennedy Space Center. Crews are using Fondue Fyre--a calcium aluminate cement concrete that is referred to in the 1966 NASA handbook--to repair areas where more than 5000 fire brick were blown away by the exhaust. For more details, visit www.nasa.gov and search for "shuttleflametrench."