Honolulu star-bulletin. [volume] (Honolulu [Oahu, Hawaii]) 1912-2010, 1915-12-20
City Engineer Advises No Less
Than Half-Foot Thickness
for City Streets

ing In any part OF the city, ana aovls-
ing the use of sand that may be Sc
cured here without importation from
the coast, Engineer L. NJ Whitehouse
last Friday submitted a detailed report
on concrete roads to the supervisors.
He suggests an experimental con
crete pavement on one Of the heavy
traffic districts in the business Sec
ton. The report, which was filed
with the road committee, is in part
as follows: :.
MaterisIs.
The materials available in Homo
lulu for the making of concrete are:
IN For the fine aggregate, imported
Portland cement, coral beach sand,
commonly called fWaianae' sand, and
crushed rock sand passing quarter
inch screen: l2l For the coarse az
gregate, crushed lava rock.
Several brands Of Portland cement
are used in Honolulu, all of which
meet the standard specifications Of
the American Society of Testing Ma
terials. No shipment Of cement, how
ever, should be allowed to go into any
structure, street pavement or other'
wise, before samples of the cement
have been subjected to the standard
tests.
.sWaianae' sand as a constituent
in the fine aggregate has been the
subject of much argument. The en-
gineers in charge Of the construction
of the Pearl Harbor dry dock elimin-
'ated this material from use In con-
crete on account Of its chemical un.
stability under the conditions prevail.
ing there. They used imported Puget
Sound sand with No. 4 crushed rock
sand in the proportion of IL.
aThe crushed rock available Is
thoroughly satisfactory and can be
obtained free from the socalled spuka
puka' rock, or disintegrated rock. 1t
is well established fact that the
fine aggregate and not the crushed
rock Is the controlling factor in the
strength Of concrete.
Tests.
There have been many tests made
in. Honolulu on the strength of con-
crete composed of the materials avail
able here. Thorough tests were made
by the navy engineers before COM
faencing the construction of the Pearl
Harbor dry dock. The College of Ha-
Waii testing laboratory has made nu-
'merous tests on the strength OF con-
crete composed of varying proportions
of cement, sand and crushed rock,
Three different sands were tested:
Imported Puget Sound sand, Valance
sand. and crushed rock sand. The
results of the College of Hawaii exper-
iments show that:
i a4i) sWaianae' coral sand gives the
same strength as the imported Puget
Sound siliceous sand. Although no
abrasion tests were made it is reas-
enable to assume that the silica sand
would have proved far superior to the
softer coral sand.
THE resistance to abrasion Is one
of the chief requirements of con-
crete pavement IL used with or with1
out q bituminous coating. An exam
nation of the samples of Waianae
sand and standard Ottawa silica sand
submitted with this report will show
conclusively the superiority OF silica
sand to coral sand in resisting 'abra-
sion. The silica sand is very firm and
hard while the coral sand is relatively
soft.
ss(2) Probably the best combination
of materials is one Cd part cement:
two ca to two and one-hah e Id
parts Of sand and four t4 parts OF
crushed rock. The sand Is composed
OF half coral sand and half crushed
rock sand. II was found In these ex-
periments that the above combination
Of the two sands gave greater strength
than either one used entirely alone.
The addition Of coral sand to the
crushed rock sand gives q concrete
which is much easier to handle and
which gives better appearance to
the completed work.
Thickness.
MANY theories have been developed
as to the requisite thickness OF con-
crete payements, all of which differ IN
the original assumptions and In the
results obtained. The only conclusive
proof as to thickness is based on ac-l
tuaI practise. It has been shown by
tests made by the California State
Highway Commission that a four "a
inch slab of IL liza concrete will
support the passing of ten (10) tons,
road roller over trench two feet
wide, without noticeable detection.
However, when the roller was stopped,
so that the load of the rear wheels
was concentrated at the edge of the.
trench, failure took place, Ode to in
sufficient area 70 carry the shear load.
Those who advocate q four (4) inch
are assuming a perfect or
ideal subgrade which takes the load of
traffic without causing anything but
stresses in the concrete.
AS a matter of fact no subgrade is al
together perfect and the concrete will
be called upon to bridge
certain weaknesses in the suog1sde,
thereby producing tensile and shear
ing strains which a thickness of four
(4l inches will not be able to resist
without ultimate failure. 1t may be
argued from California State Highway
practise that four (4) inches thickness
Is enough. This thickness has not
been open to traffic for period OF
time long enough to establish its suf.
fciency. It is not fair to assume that
the requirements of city street are
similar to those of country high.