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Concrete


Photograph of forms and reinforcing rods ready for concrete to be poured

Forms and reinforcing rods ready for concrete to be poured.

U.S. Army. Via ibiblio.org

Concrete is a structural material widely used in buildings, roads, airfield runways, docks, and fortifications. It is inexpensive, being prepared from coarse gravel, sand, water, and Portland cement. The latter is manufactured by kilning limestone to drive off moisture and carbon dioxide to leave a mixture of alkaline oxides and silicates, with calcium oxide predominating. Concrete is relatively easy to work with, requiring that the ingredients be thoroughly mixed, poured into place, and shaped with simple hand tools. The gravel and sand (known as aggregate) provide most of the strength, with the Portland cement serving primarily to bind the aggregate together. When properly prepared, concrete is quite strong in compression, and becomes stronger with time as the concrete absorbs moisture and carbon dioxide from the air, which combine with the original alkaline silicates and hydroxides to produce an interlocked crystal structure.

Concrete comes in many formulations for diverse purposes, including compositions that can be worked and will harden underwater. Quality concrete requires a bit more care to mix and pour than suggested by the simple process described above, but even relatively unskilled laborers can lay concrete of adequate quality for ordinary purposes. Quality is improved if the concrete is kept wet for as long as possible after it is poured.

Concrete is not particularly strong in tension, and, where extra shear strength is needed, the concrete is reinforced by incorporating steel rods into its construction. Steel has much greater tensile strength than ordinary concrete. The strongest concrete is prestressed. Reinforcing steel cables are placed under high tension and the concrete is poured around them. These cables serve to compress the concrete after it has hardened, increasing its effective strength. However, while prestressed concrete was used extensively in Germany and to some extent in Britain, the very high quality of concrete and steel cable required meant that it was rarely if ever used in the Pacific.

The weakness of concrete in tension means that it has a pronounced tendency to spall when hit by large-caliber shells or a large explosive charge. The shock passing through the concrete tears fragments of concrete off the inside surface of the structure, and these can do significant damage. This can be prevented by reinforcing the innermost surface with chicken wire or some other wire mesh or by applying an antispall layer to the inside of the structure, but the more common practice during the time frame of the Pacific War was to line the inside of the structure with sandbags to catch spall. 

Concrete saw some unexpected uses. The U.S. Navy build a number of floating dry docks out of concrete, and these were surprisingly successful. The concrete used in their construction required about 8.4 bags of Portland cement per cubic yard and the water-cement ratio was five gallons per bag of cement, for maximum strength and durability. Cure time was 28 days and the cured strength was 4000 pounds per square inch (28 MPa).

Thick concrete is impervious to small arms fire and shell fragments, and can provide considerable protection from small caliber shells. Heavy artillery will destroy even reinforced concrete fortifications, but only with a direct hit. A 16" (406mm) armor-piercing shell from a battleship could penetrate 30' (9 m) of reinforced concrete.

The Japanese shipped Portland cement in 50kg (110 lb) steel cans that, when empty, were often pounded out to be used as sheet metal in field works. Reinforcing bar was usually low-grade steel rods 10-19mm (0.4 to 0.75") in diameter. Both were in short supply, but especially rebar, and the Japanese sometimes resorted to desperate improvisations such as using wire or even stout rope as a reinforcing material. The shortage of concrete meant that many fortifications consisted of local materials with a concrete cap, constructed by pouring the wet concrete on the roof of the fortification and allowing it to run down the sides to form a protective shell. American engineers were unimpressed with Japanese concrete construction methods, which left seams between separate pours, failed to overlap and tie in rebar connections, used poor quality aggregate such as coral or even crushed shells, and used too much water, which was often seawater. As a result the compressive strength of typical Japanese concrete was often only half that required in U.S. military specifications.

Typical concrete penetration

Projectile
U.S. concrete
Japanese concrete
37mm AP
18" (457mm)
31" (785mm)
75mm AP
24" (609mm)
42" (1060mm)
After Rottman (2003)

References

Bailey (2004)

"Building the Navy's Bases in World War II" (1947; accessed 2011-12-21)

Marrey and Grote (2003; accessed 2010-10-17)

Rottman (2003)



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