Safe transportation and storage of liquefied natural gas (LNG) require that the materials used to contain the fluid must withstand sub-arctic temperatures as low as -265 degrees F, the temperature to which the gas must be cooled in order to become liquefied. Pre-stressed concrete composite storage containers have been built for this purpose. One such design consists of two concentric concrete cylinders. Each cylinder is lined with 9 percent nickel steel, and the space between cylinders is filled with insulation. The inner cylinder is covered with a concrete dome and the outer cylinder with the steel lining. This lining acts as an impermeable gas barrier, while the concrete provides structural rigidity. Concrete used at such low temperatures is called cryogenic concrete.
The compressive strength of concrete increases as temperature decreases, with the increase greater when the concrete has a higher moisture content. Moisture content contributes to strength by forming load-bearing ice crystals in the pores of the cement paste. The continued increase in strength below -94 degrees F has been attributed to internal prestressing by the ice, which has a coefficient of contraction of about 29 x 10 -6 per degree F. The tensile strength of concrete also increases with decreasing temperature, but the increase slows down gradually below -94 degrees F.
For everyday operating safety, a single prestressed concrete tank with internal insulation is the most economic system. While leakage will occur at cracks and construction joints, it is usually considered tolerable. Experience shows that prestressed concrete is not noticeably weakened when small pores weep. In fact, visual inspection and repair of the outside are made easier, and the possible rupture of an inner steel vessel is eliminated. These advantages, plus concrete's resistance to corrosion and fire and its low maintenance costs, make concrete attractive to the LNG tank designer.