is
exceedingly strong. But it is difficult and expensive to work up into
various forms. Concrete has been avoided for making beams, slabs and
thin walls, just because its deficiency in tensile strength doomed it to
failure in such structures. But if a concrete slab be "reinforced" with
a network of small steel rods on its under surface where the tensile
stresses occur (see fig. 1) its strength will be enormously increased.
Thus the one point of weakness in the concrete slab is overcome by the
addition of steel in its simplest form, and both materials are used to
their best advantage. The scientific and practical value of this idea
was soon seized upon by various inventors and others, and the number of
patented systems of combining steel with concrete is constantly
increasing. Many of them are but slight modifications of the older
systems, and no attempt will be made here to describe them in full. In
England it is customary to allow the patentee of one or other system to
furnish his own designs, but this is as much because he has gained the
experience needed for success as because of any special virtue in this
or that system. The majority of these systems have emanated from France,
where steel concrete is largely used. America and Germany adopted them
readily, and in England some very large structures have been erected
with this material.

[Illustration: FIG. 1.--Expanded Steel Concrete Slab.]

[Illustration: FIG. 2. Expanded Metal.
Section through Intersection.]

The concrete itself should always be the very best quality, and Portland
cement should be used on account of its superiority to all others. The
aggregate should be the best obtainable and of different sizes, the
stones being freshly crushed and screened to pass through a 7/8 in.
ring. Very special care should be taken so to proportion the sand as to
make a perfectly impervious mixture. The proportions generally used are
4 to 1 and 5 to 1 in the case of gravel concrete, or 1:2:4 or 1:2-1/2:6 in
the case of broken stone concrete. But, generally speaking, in steel
concrete the cost of the cement is but a small item of the whole
expense, and it is worth while to be generous with it. If It is used in
piles or structures where it is likely to be bruised the proportion of
cement should be increased. The mixing and laying should all be done
very thoroughly; the concrete should be rammed in position, and any old
surface of concrete which has to be c