placed when the thermometer was 5° above zero. When the thermometer was
below the freezing point both sand and stone were heated by wood fires
in large pipes under the supply piles; the temperature of the mix was
taken frequently, and was kept above 40 degrees. Numerous tests made
while the work was in progress showed that, while the temperature fell
slightly soon after the concrete was deposited, it was always from 2° to
5° higher at the end of 2 hours. The face and back of the concrete were
prevented from freezing by a liberal packing of salt hay just outside
the forms.

A vertical hog trough, 24 in. wide and 9 in. deep, was placed in one
end of each section, for its full height below the bridge seat, into
which the next section keyed, and, when the temperature at the time
of concreting was below 50° Fahr., a compression joint was formed by
placing a strip of heavy deadening felt, 2 ft. wide, on the end of the
completed section next to the face and covering the remainder of the
end with two ply of the felt and pitch water-proofing; the one ply of
deadening felt near the face was about the same thickness as the two ply
of water-proofing, and was used to prevent the pitch from being squeezed
out of the joint to the face of the wall.

The excavation for the retaining walls in 31st and 33d Streets were in
all cases made of sufficient width to receive the sewers, which were
laid as soon as the back-fill, carefully rammed and puddled, had reached
the proper elevations; the back-filling was then completed, and the gas
and water mains were afterward laid in separate trenches.

[Illustration:
Fig. 8.
SKETCH SHOWING FORMS AND BRACING FOR NINTH AVENUE WALL]