overed should be cleaned and coated
with fresh cement.

[Illustration: FIG. 3.--Hennebique System.]

The reinforcement mostly consists of mild steel and sometimes of wrought
iron: steel, however, is stronger and generally cheaper, so that in
English practice it holds the field. It should be mild and is usually
specified to have a breaking (tensile) strength of 28 to 32 tons per sq.
in., with an elongation of at least 20% in 8 in. Any bar should be
capable of being bent cold to the shape of the letter U without breaking
it. The steel is generally used in the form of long bars of circular
section. At first it was feared that such bars would have a tendency to
slip through the concrete in which they were embedded, but experiments
have shown that if the bar is not painted but has a natural rusty
surface a very considerable adhesion between the concrete and steel--as
much as 2 cwt. per sq. in. of contact surface--may be relied upon. Many
devices are used, however, to ensure the adhesion between concrete and
bar being perfect. (1) In the Hennebique system of construction the bars
are flattened at the end and split to form a "fish tail." (2) In the
Ransome system round bars are rejected in favour of square bars, which
have been twisted in a lathe in "barley sugar" fashion. (3) In the
Habrick system a flat bar similarly twisted is used. (4) In the Thacher
system a flat bar with projections like rivet heads is specially rolled
for this purpose. (5) In the Kahn system a square bar with "branches" is
used. (6) In the "expanded metal" system no bars are used, but instead a
strong steel netting is manufactured in large sheets by special
machinery. It is made by cutting a series of long slots at regular
intervals in a plain steel plate, which is then forcibly stretched out
sideways until the slots become diamond-shaped openings, and a trellis
work of steel without any joints is the result (fig. 2).

[Illustration: FIG. 4. Hennebique System.]

The structures in which steel concrete is used may be analysed as
consisting essentially of (1) walls, (2) columns, (3) piles, (4) beams,
(5) slabs, (6) arches. The designs differ considerably according to
which of these purposes the structure is to fulfil.

The effect of reinforcing _walls_ with steel is that they can be made
much thinner. The steel reinforcement is generally applied in the form
of vertical rods built in the wall at intervals, with lighter horizontal
rods which cross the v