Q.: Can a concrete wall be made more resistant to intense heat by vacuum dewatering?

If so, could ordinary concrete be used to build the walls and floor of a soaking pit for steel ingots?

Has anyone worked out the methods?

A.: Suitable techniques are available but you will have to be the judge of whether the concrete would have suitable heat resistance for your purpose. An example of the use of vacuum dewatered concrete for heat resistance is the 8-inch-thick faces of the ducts under the space shuttle launch pad at Vandenberg Air Force Base in California. The walls and floor of these ducts are exposed directly or indirectly to the blast of the rocket flames when the shuttle lifts off.

The method used was to build the flame ducts or tunnels of reinforced concrete 6 to 9 feet thick. The interiors of the ducts are 50 feet high, 70 feet wide, and 330 feet long. The walls and floors of the first 100 feet of each duct, were covered with a layer of vacuum dewatered concrete made with Type II cement 8 inches thick on the floor and 6 inches on the walls.The floors were dewatered by the standard techniques.

To dewater the walls, 10-foot-high forms were first built for use in placing the 8-inch-thick facing. A hole was drilled in the center of each form to connect to the vacuum manifold. A self-adhering rubber gasket was placed around the perimeter of the form face to seal it against the concrete, and vacuum filter pads were stapled to the form within the gasketed area. A nylon mesh was then stapled over the filter pads, and a silk cloth, soaking wet, then stapled over the nylon mesh. The assembly was set aside to dry 24 hours, leaving a taut silk surface to contact the concrete.

Forms were tightly butted, 6 inches out from the existing wall, and concrete was placed and vibrated. When the concrete reached a level of 2 to 2 1/2 feet in the forms the vacuum was applied and maintained until half an hour after concreting had been completed. Then the forms were tightened to compensate for the compaction that had occurred during dewatering.As to the resistance to heat and blast, it is expected that the dewatered concrete will withstand six to eight space shuttle launches before extensive repairs are necessary. Without the blast, though with the heat, your soaking pit ought to last longer.