g used in buildings of first
importance, the aggregate being very carefully selected, and in many
cases the whole mixture coloured by the use of pigments. Care must be
taken in their selection, however, as certain colouring matters such as
red lead are destructive to the cement. One of the great objections to
the appearance of concrete is the fact that soon after its erection
irregular cracks invariably appear on its surface. These cracks are
probably due to shrinkage while setting, aggravated by changes in
temperature. They occur no less in structures of masonry and brickwork,
but in these cases they generally follow the joints, and are almost
imperceptible. In the case of a smooth concrete face there are no joints
to follow, and the cracks become an ugly feature. They are sometimes
regulated by forming artificial "joints" in the structure by embedding
strips of wood or sheet iron at regular intervals, thus forming "lines
of weakness," at which the cracks therefore take place. A pleasing
"rough" appearance can be given to concrete by brushing it over soon
after it has set with a stiff brush dipped in water or dilute acid. Or,
if hard, its surface can be picked all over with a bush hammer.

Resistance to fire.

At one time Portland cement concrete was considered to be lacking in
fireproof qualities, but now it is regarded as one of the best
fire-resisting materials known. Although experiments on this matter are
badly needed, there is little doubt that good steel concrete is very
nearly indestructible by fire. The matrix should be Portland cement, and
the nature of the aggregate is important. Cinders have been and are
still much favoured for this purpose. The reason for this preference
lies in the fact that being porous and full of air, they are a good
non-conductor. But they are weak, and modern experience goes to show
that a strong concrete is the best, and that probably materials like
broken clamp bricks or burnt clay, which are porous and yet strong, are
far better than cinders as a fireproof aggregate. Limestone should be
avoided, as it soon splits under heat. The steel reinforcement is of
immense importance in fireproof work, because, if properly designed, it
enables the concrete to hold together and do its work even when it has
been cracked by fire and water. On the other hand, the concrete, being a
non-conductor, preserves the steel from being softened and twisted by
excessive temperature.

Cost.

On