The Manchester journal. [volume] (Manchester, Vt.) 1861-current, 1915-09-09

In the past efforts have frequently
been made TO overcome these objec-
tions to a certain degree by covering
the concrete pavement with bitum-
inous wearing surface. At the pres
ent time. the specialists in the depart
ment hold that this can not be eco
nomicalIy justified, although II Is pos
sible that future investigation may
change the situation in this respect.
In the present state Of road science,
however, il seems that where traffic
conditions are such that bituminous
surface on concrete road is practi-
cable a bituminous-surface macadam
road would be equally practicable and
certainly cheaper Where traffic is too
heavy for macadam road the bitumin-
GUS surface is likely TO give way and
the uneven manner in which it fails
tends to produce excessive wear on
portions of the concrete.
For successful concrete road,
hardness, toughness, and uniformity
are the most essential qualities. These
can be secured to a great extent by
care in the selection Of the constituent
materials and the proportions in
which they are mixed. Sample speci-
fcations are included in the bulletin,
No. 249, Portland Cement Concrete
Pavements for Country Roads. These
specifications are believed to typify
the best engineering practice as it
has been developed up to this time.
They cover such points as materials,
grading, subgrade, and construction.
The cement, it is said, should always
conform to some standard specifica
tions for Portland cement, such as
those issued by the United States
Bureau of Standards or the American
Society for Testing Materials. The
sand should not contain more than 3
per cent of foreign material, and sand
with more coarse than fine grains is
to be preferred. The coarse aggregate
may consist Of either crushed stone or
gravel In either case it is very de-
sireable that the coarse aggregate be
well graded in size between proper
limits.
The proportion of cement to the
sand and coarse aggregate combined
should not be less than about / to 5,
and the proportion of sand to coarse
aggregate not less than 11a to 3, nor
greater than 2 to 8. A useful formu-
la when gravel is used as coarse ag-
gregate is l part of cement to 12
parts of sand to 3 parts of gravel.
When crushed stone is used, IL
parts of sand may be substituted in
place Of "a parts.
In addition to discussing the engin-
eering details of construction, the bull
letin already mentioned calls attention
to the fact that ordinarily from one
third to one-half of the total cost of
constructing concrete pavement is
for labor after the materials are de-
livered. This emphasizes the import
ance Of efficient organization and
proper equipment. Failure to take
these factors into consideration fire.
quently results, it is said, in adding
from 10 to 20 per cent to the cost of
a concrete pavement.
The most economical method is to
have the work of mixing and placing
the concrete as nearly continuous as
practicable. The work should be plan
ned with primary view of keeping
the mixer going full time. The drain
age structures, the grading, and the
subgrade should, therefore, be COM
pleted we'll in advance of the mixer
and provision made for obtaining all
of the necessary materials without de-
lay. A common error is the failure to |
make adequate provision for deliver
ing water on the work, and the |
amount which given stream is ca
pable of supplying is frequently over
estimated.