Chapter XI: Part II: Instructions to Inspectors on Street Paving Work (3)

Where a new foundation is required broken stone or macadam is most
frequently used for bituminous concrete pavements. Unless such
foundations are constructed in the same way and with about the same
care as is necessary for a macadam road it is liable to prove
unsatisfactory. Under the very heavy wheel loads that may occasionally
pass over the streets, imperfectly compacted broken stone is likely to
shift sufficiently to start incipient ruts which will enlarge and in
time necessitate expensive repairs. Such conditions are frequently
seen on bituminous concrete pavements subjected to heavy travel. These
pavements, like sheet asphalt pavements, require a foundation that
will be absolutely unyielding under travel. For this reason a concrete
foundation will generally be found more economical in the long run
than a broken stone foundation. The increased first cost per square
yard is not very great and this additional money will in most cases
prove a good investment. At the usual prices of material and labor a
square yard of 4 inch concrete should cost about 50 cents, while a
properly constructed broken stone foundation 6 inches thick (which
would not nearly equal in strength and rigidity 4 inches of concrete)
would cost about 45 cents per square yard. Considering the much
greater durability and lower cost of repairs of the pavement on the
concrete foundation, this small additional cost is not worth
consideration. While the specifications are made to cover the three
kinds of foundation, it is assumed that the kind of foundation to be
used will be decided in advance, and that the part of these
specifications relating to the others kinds of foundation will, in
actual use be omitted.

Footnote 58:

A bituminous cement composed largely of coal-tar pitch has heretofore
been most used in pavements of this character. It is not denied that
very good pavements have been, and can be built with this material,
but the superiority of the asphaltic cement here specified is so great
that it is true economy to use it. The difference in cost at
prevailing prices of material will be ten to twelve cents per square
yard. The greater durability and serviceability of the pavement made
with the asphaltic cement will, particularly on streets of
comparatively heavy travel, far more than justify this additional
cost.

Footnote 59:

Hydraulic concrete pavement is to be recommended only for country
roadways and for city streets of very moderate travel. While our
experience with this kind of pavement is yet limited there is reason
to believe from the nature of the material that it will not prove to
be a satisfactory or economical pavement for streets of heavy travel.
But in all cities and towns there are many residence streets where the
travel is very light, and yet where a permanent pavement is wanted and
warranted. For these, it is believed that a properly constructed
concrete pavement will prove very satisfactory and durable, and the
low cost at which it can be constructed should make it very attractive
to city officials and property owners. The author has advocated its
use under such conditions for many years (see Engineering News, July
21st, 1904). Like other composite pavements its utility and durability
will depend largely upon the good quality of the materials used and
the skill and thoroughness with which the work is done.

The specifications here offered are the result of the observation and
experience of the author, and it is believed that pavements laid in
accordance with them will give very satisfactory results.

Footnote 60:

A number of engineers advocate the construction of concrete pavement
in one homogeneous course, and quite a number of pavements have been
constructed in this way.

Like any other composite pavement, it is called upon to perform two
functions; to safely sustain the weight of loads passing over it, and
to resist wear and abrasion of its surface. A material and form of
construction that meets the first requirement may not meet the second.
Experience has proved that ordinary 1:3:6 concrete makes an entirely
satisfactory foundation for any pavement, but it lacks the hardness
and strength to successfully resist the surface abrasion of travel. To
secure this quality a richer and harder concrete is called for, but it
is unnecessary that the foundation should be equally hard. To
construct the pavement in two courses as here specified would seem to
be the logical way, especially as it decreases the total cost, and
should make a more durable pavement.

Footnote 61:

Some engineers advocate a greater total thickness of the pavement than
is here specified (6 inches). Considering that this pavement should
never be used on heavy traveled streets, a total thickness of concrete
of six inches will have ample strength to carry the loads to which it
will be subjected. If so, it is a useless waste of money to increase
the thickness of the concrete.

Footnote 62:

The use of limestone for the top course (unless it is of very superior
quality) is not advisable or economical unless the cost of trap rock
is so high as to be prohibitive, which, considering its superior
durability under the wear of travel, will not often be the case.

Footnote 63:

It is advisable to remove the screenings from this surface mixture for
two reasons: first, to secure greater uniformity of composition. If
the screenings are allowed to remain in the aggregate, there is danger
of segregation into patches of different sized aggregate and different
ratios of materials, which it is very important to avoid, and second,
the small fragments of stone are more likely to be crushed under the
concentrated weight of wheels than the larger masses, and to thus
start disintegration. Lack of uniformity in the composition and
homogeneity in this surface-course concrete is especially to be
guarded against, otherwise the surface of the pavement will wear
unevenly and depressions and ruts are likely to result.

Footnote 64:

The ideal composition of this surface-course concrete is one where the
stone forms the largest possible part of the mass consistent with
sufficient mortar to fill the voids and thoroughly bind the fragments
of stone together.

Footnote 65:

The importance of securing high quality and great uniformity in the
surface course cannot be urged too strongly.

Footnote 66:

This requirement must be strictly enforced. Otherwise there will be
danger that the two courses may not properly adhere to each other. It
is the writer’s experience that if this rule is observed there will be
no danger of the two courses separating.

Footnote 67:

The purpose of this rolling is mainly to evenly compress the mass and
thus secure its uniform density. It also produces a truer surface than
can usually be secured by ramming alone.

Footnote 68:

Among engineers there is quite a wide difference of opinion as to the
proper spacing of expansion joints, and, in fact, as to the necessity
or advisability of providing them at all. It has been suggested that
it might be better to omit them entirely, allowing the pavement to
form its own expansion joints by cracking along lines where natural
forces dictate. Such cracks by their irregularity give a bad
appearance to the surface, but observation seems to indicate that the
edges of these natural joints wear as well as those made by expansion
joints. Further observation and experience is needed in the matter. In
most concrete it is known that some contraction takes place during the
setting of the cement, regardless of temperature changes, and cracking
is probably due as much to this permanent contraction as to that
caused by low temperature. The coefficient of expansion of concrete by
heat is variable but so small that expansion joints ⅛ inch wide every
fifty feet along the street should provide for temperature changes.

Footnote 69:

If the expansion joints are not thus filled with bituminous cement
they will become filled and packed with incompressible stone, sand,
etc., that will not permit expansion.

Footnote 70:

The practical value of oiling concrete pavements has not yet been
determined by sufficient experience. There is reason, however, to
believe that the slight coating of bitumen will materially preserve
the surface from abrasion and that its benefit will thus be greater
than its cost. It will also tend to prevent the very slight dust that
might otherwise exist on the pavement.

Footnote 71:

In this class of pavements the contractor or promoter may properly be
required to assume responsibility for the character and utility of the
work produced, and the municipal authorities should assume no part of
such responsibility.

Footnote 72:

Upon the general subject of time guarantees of municipal work, see