The San Francisco call. [volume] (San Francisco [Calif.]) 1895-1913, 1895-05-25

| IN some constructions every barrel IS tested
before being allowed to go into the work,
rms cement-testing is an extremely
| important feature; and to the complete
absence of it here is to be attributed many
OF the giaring defects of our public work.
| Inere is no other important city where
TINS matter is so completely dlsregarded as
| 1t is In San Francisco. Even the best
| brands OF cement will sometimes spoil,
| and there is no way to detect the defect
| except by actual test. When it is remem-
oered that defective cement has about the
| same efficacy as brick dust, that It is often
reground and sold to unscrupulous con-
tractors for less than half the price OF good
cement, that no difference whatever is APP
parent to the eye or touch between good
and bad cement, and that no difference
can be detected except by the tests above
| referred to, 1t will be seen how urgent 1T IS
that we should give some heed to this
matter.
1.-The broken stone should be OF good
hard rock, free from loan, clay or earth
| 1t should be made up of particles from the
size OF pea to that which will pass through
| ring two inches in diameter. Trap, gran
lite, hard limestone, quartz rock and meta
morphlc sandstone make good concrete.
sometimes bituminous concrete,
wherein asphalt is the cementlng mate
| rial and where the other ingredients are
SAND and broken rock, is used; but this
ANA0Ne1Oii 40 llOI dS IISlu do ehO oAdoliN
cement concrete, and it is not therefore in
great favor.
Having disposed of the foundation the
next consideration is the wearing surface.
The enlightened experience of older COM
munltles has narrowed the choice down to
two materials-stone blocks and asphalt.
Other materials, such as vitriied brick and
wooden blocks, are also used; but the best
practice adheres closely to stone blocks for
the very heaviest traffic and asphalt for
everything else.
"Vitrified brick pavements are largely
used In parts of the Mississippi Valley and
IN the Southern States on account of the
abundance OF good clay in those sections;
but the impossibility of burning the brick
evenly renders it difficult to produce first
class pavement out of that material. 1t is
not as noisy as stone blocks, nor as expen-
sive; neither is it as difficult to keep clean,
but 1t is not as durable as stone. 1t is used
to good advantage on steep grades, where
asphalt would be too slippery.
Experience in San Francisco with
wooden pavements has been no more salts
factory than that of our fellow-experi-
mentors on the shores of the Great Lakes,
where cedar blocks have been so largely
used on account of cheapness. Such pave
ments have never been successful, except in
rare instances and under extremely favor
able conditions.
1.-THE best materials for block pavement
are traprock and basalt, both of which are
hne-gralned, hard and tough. Granite and
metamorphic sandstone are often used, but
they are friable and not so durable as
basalt. The blocks are usually four inches
in width, six inches deep and at least eight
inches long, and should be cut fairly true.
They should be laid in regular courses run
ming the width of the street, each block
breaking joint with its neighbors in the
courses adjoining.
"a cushion of clean sand two inches
thick is first laid on the concrete founda-
ton. On this the blocks are set and clean,
sharp sand is brushed into the joints. The
blocks are then thoroughly rammed into
place with heavy paving-rammer. The
excess of sand is then swept up and
groutlng of hot asphaltic cement contain
ing as much fine sand or pulverized lime
stone as it will hold without becoming too
thick CASUALLY 60 per cent limestone to 40
per cent pure bitumen) is poured into the
joints, thoroughly saturating the sand that
has been previously brushed into these
joints. The whole is then given final
ramming with heavy paving-rammers.
"The trouble with our present basalt-
paved streets is the lack of foundation and
the irregularity of the size and shape of
the blocks. Eastern specifications allow
no more than a half-inch joint between
blocks; on some of our San Francisco
basalt pavementS there are joints two
inches in width. As to foundation, know
of no concrete under basalt anywhere in San
Francisco, except on some of the cable-car
lines. The hydraulic cement grouting that
is sometimes used between stone blocks,
while it makes the pavement rigid and
stable, renders it at the same time very
slippery under the feet of draught animals.
1t is therefore not to be encouraged.
LET us now come to the discussion of
the asphalt wearing surface, which is
1,cr,o1lxr 1oiH dirocflu ha ofo
foundation. 1t is generally two and half
inches thick after compression, except in
lightly travelled streets, where it is some
times laid only two inches thick. There
is practically no asphalt pavement in San
Francisco to-day, nearly all of our smooth
pavements being of SO-called bituminous
rock. Almost all of the fine, smooth pave
ments of other cities of the Union are of
asphalt, the supply being obtained largely
from the island of Trinidad in the West
Indies. The surface so laid is an artificial
mixture of asphalt, sand and powdered
limestone, and in appearance very much
resembles some of our bituminous rock.
cit is extremely difficult to find btu
mlnous rock that is uniform In compost
ton. 1t is natural mixture of bitumen
and sand; and adjoining parts of the same
deposit will vary extremely both in pro-
portion and in quality of the contained
bitumen.
the term bltumen, by winch IS meant
the cementing substance of the bituminous
rock, is broad one and comprises range
of substances extending from fluid and
extremely volatile oil to an exceedingly
brittle and pliable solid. If bituminous
rock contains the volatile bitumen, it will
soon go to pieces in the pavement through
loss of its cementing material into the air.
If, on the other hand, its bitumen be of the
brittle solid kind, the pavement will soon
be ground to a powder under the action of
the traffic that passes over it. 1t is there
fore not sufficient to specify that the mate
rial must contain 10, Is or 20 per cent of
bitumen the desired quality of the con-
tanned bitumen must be specified. What
is that quality? Recent chemistry has de-
veloped some exact tests on that subject;
and it is now quite possible to define the
desired quality of bitumen with precision.
The matter is somewhat too technical for
discussion here, and l shall therefore pass
it for the present, trusting, however, on
another occasion to enlarge thereon.
From perusal of the foregoing it will
become apparent why bituminous rock
often wears so poorly. In brief, different
parts OF the same quarry have different
bitumen percentages, and the bitumen in
different parts varies greatly in quality.
The resulting pavement is bound to wear