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. It 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,
wth the Portland cement serving primarily to bond 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 more mundane 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
shearing strength is needed, it is reinforced by incorporating steel rods into the liquid
concrete. Steel has much greater tensile strength than ordinary
concrete. The strongest concrete is prestressed. Reinforcing rods are
placed under high tension and the concrete is poured around them. These
rods serve to compress the concrete after it has hardened, increasing
its effective strength.
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.
The Pacific War Online Encyclopedia © 2009 by Kent G. Budge. Index