_Principles involved_
Concrete and steel render valuable assistance to each other in the
support of heavy burdens. On a solid foundation, loaded from above
and thus under direct pressure, a concrete column will withstand the
strain of an enormous load. A much smaller load so placed as to cause
stretching or bending toward one side of the same column may cause
it to snap off, for concrete is strong, but brittle. On the other
hand, steel is tough and elastic. In the form of rods or wire, steel
withstands massive loads that tend to stretch it, and thus displays
a kind of strength directly opposite to that of the plain concrete
column. In modern construction these two valuable properties of
concrete and steel are utilized by combining them in what is called
reinforced concrete. With steel properly buried in the concrete, the
column withstands not only the load which might otherwise snap it, but
one many times larger, and even though it is applied at any place along
its length.
Reinforcement, therefore, is steel in the form of rods, bars or wires,
buried in concrete to take up and to withstand the strains which tend
to stretch or to bend the concrete. A concrete fence post is merely a
small concrete column. Reinforced, it easily stands the strain from
usage in a fence line.
[Illustration]
The value of reinforcing concrete posts properly may readily be seen
in the figure. If a load (L) is raised so that its weight is supported
on one side by a wooden post, the post will bend. The fibre in the
wood on the side away from the load may be tough and elastic enough to
prevent the post from breaking, and when released the post will spring
back into its former position. In the third figure a No. 9 wire (W)
is fastened securely to the wooden post at the top and at the ground
surface, and is supported along its length by the struts (S). If the
same load is applied, the post will not bend, because the wire takes
up the bending or stretching strain. This is precisely the case with
the reinforcement in a concrete post. Supported along its length by the
concrete, the wire (W) or steel in other shapes takes up the bending or
stretching strains. Since the load which causes bending or stretching
may come from any direction, concrete posts are reinforced on every
side; otherwise they might break in a manner somewhat similar to that
in which the wooden post bends when the reinforcement is not on the
proper side of the post.
In the effort to be safe it is a common fault to insert more
reinforcement than is absolutely necessary. This adds needlessly to the
cost, for concrete becomes stronger as it grows older.