An experiment being conducted by the Armstrong Laboratory at Brooks Air Force Base, Texas, is proving that concrete performs well under pressure--in this case, at pressures as high as 85 pounds per square inch gauge (psig). In the experiment, concrete post-tensioned in two directions was used to build a rectangular pressure chamber for hyperbaric oxygen (HBO) treatments, which require patients to breathe 100% oxygen in a pressurized environment. The prototype chamber is 18 feet wide, 30 feet long, and 16 feet tall. Although it was designed as a 29.4 psig chamber, tests have shown that the chamber can take pressures up to 85 psig without structural failure. The 625 monostrand tendons--each « inch in diameter--run in two directions through the chamber's 2-foot-thick floor, walls, and roof allow it to withstand the high pressures. The structure is also reinforced with two-way mats of #6, Grade 60 bonded rebar. ECONOMICAL ALTERNATIVE Most HBO chambers in use today are horizontal steel cylinders or acrylic tubes. However, the medical community wants larger, rectangular chambers to enhance patient comfort and provide more useable floor space. Using steel to manufacture a rectangular shape can be extremely costly. Post-tensioned concrete is an economical alternative. The time and cost required to build the concrete prototype were compared to those required to build a welded-steel chamber of equivalent size at Travis Air Force Base, Calif. The steel chamber was completed in about six months at a cost of $1.6 million. The prototype concrete chamber was built in less than two months at only 10% of the cost of the steel structure.