d with great advantage. They have the property of entering into
chemical combination with the lime, forming a hard setting compound, and
increasing the hardness of the resulting concrete.

The commonest aggregates are broken stone and natural flint gravel.
Broken bricks or tiles and broken furnace slag are sometimes used, the
essential points being that the aggregate should be hard, clean and
sound. Generally speaking, broken stones will be rough and angular,
whereas the stones in flint gravel will be comparatively smooth and
round. It might be supposed, therefore, that the broken stone will
necessarily be the better aggregate, but this does not always follow.
Experience shows that, although spherical pebbles are to be avoided,
Portland cement adheres tightly to smooth flint surfaces, and that rough
stones often give a less compact concrete than smooth ones on account of
the difficulty of bedding them into the matrix when laying the concrete.
In mixing concrete there is always a tendency for the stones to separate
themselves from the sand and cement, and to form "pockets" of
honeycombed concrete which are neither water-tight nor strong. These are
much more liable to occur when the stones are flat and angular than when
they are round. Modern engineers favour the practice of having the
stones of various sizes instead of being uniform, because if these sizes
are wisely proportioned the whole mixture can be made more solid, and
the rough "pockets" avoided. For first-class work, however, and
especially in steel concrete, it is customary to reject very large
stones, and to insist that all shall pass through a ring 7/8 of an inch
in diameter.

The water, like all the other constituents of concrete, should be clean
and free from vegetable matter. At one time sea-water was thought to be
injurious, but modern investigation finds no objection to it except on
the score of appearance, efflorescence being more likely to occur when
it is used.

Sometimes in massive concrete structures large and heavy stones as big
as a man can lift are buried in the concrete after it is laid in
position but while it is still wet. The stones should be hard and clean,
and care must be taken that they are completely surrounded. Such
concrete is known as _rubble concrete_.

Proportions.

In proportioning the quantities of matrix to aggregate the ideal to be
aimed at is to get a concrete in which the voids or air-spaces shall be
as small as possi