extracted from the concrete, to its detriment.

It has been found on removing boxing that the portion adjacent to the
wood was frequently friable and of poor quality, owing to the fact
just stated. It is usual to face or plaster concrete work after
removing the boxing. On breakwater work, where the writer was engaged,
the wall was faced with cement and flint grit, and this was found to
form a particularly hard and lasting protection to the face of the
work.

Batches of concrete should be placed in position as if they were
stones in block masonry, as the union of one day's work with a
previous is not by any means so perfect as where one batch is placed
in contact with another which has not yet set. A slope cannot be added
to with the same degree of perfection that one horizontal layer can be
placed on another; consequently, where work must necessarily be
interrupted, it should be stepped, and not sloped off.

Experience in concrete work has shown that its true place is in heavy
foundations, retaining walls, and such like, and then perfectly
independent of other material. Arches, thin walls, and such like are
very questionable structures in continuous concrete, and are on record
rather as failures than otherwise. This may to a certain degree be due
to the high coefficient of expansion Portland cement concrete has by
heat. This was found by Cunningham to be 0.000005 of its bulk for one
degree Fahrenheit. It is a matter which any intelligent observer may
remark, the invariable breakage of continuous concrete sidewalks,
while those made in small sections remain good. This may be traced to
expansion and contraction by heat, together with friction on the lower
side.