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Transactions of the American Society of Civil Engineers (Volume 81) online

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enced inspection is necessary to determine when the concrete has set
sufficiently for it to be safe to remove forms. Frozen concrete fre-
quently has been mistaken for properly set concrete.

3. DESIGN AND SUPERVISION.

The execution of the work should not be separated from the design,
as intelligent supervision and successful execution can be expected
only when both functions are combined. It is desirable, therefore,
that the engineer who prepares the design and specifications should
have supervision of the execution of the work.

The Committee recommends the following practice for the purpose
of fixing the responsibility and providing for adequate supervision
during construction.

(a) Before work is commenced, complete plans and specifications
should be prepared, giving the dead and live loads, wind and impact,
if any, and working stresses, showing the general arrangement and all
details. The plans should show the size, length, location of points of
bending, and exact position of all reinforcement, including stirrups,
ties, hooping, and splicing.

(6) The specifications should state the qualities of the materials
and the proportions in which they are to be used.

(c) The strength which the concrete is expected to attain after
a definite period should be stated in the specifications.



CONCKETE AND REINFORCED CONCRETE 1111

(d) Inspection during construction should be made by competent
inspectors selected by and under the supervision of the engineer, and
should cover the following:

1. Materials.

2. Construction and erection of the forms and supports.

3. Sizes, shapes, arrangement, position, and fastening of the rein-
forcement.

4. Proportioning, mixing, consistency, and placing of the concrete.

5. Strength of the concrete by tests of standard test pieces made
on the work.

6. Whether the concrete is sufficiently hardened before the forms
and supports are removed.

7. Protection from injury of all parts of the structure.

8. Comparison of dimensions of all parts of the finished structure
with the plans.

(e) Load tests on portions of the finished structure should be made
where there is reasonable suspicion that the work has not been properly
performed, or that, through influences of some kind, the strength has
been impaired, or where there is any doubt as to the sufficiency of the
design. The loading should be carried to such a point that the calcu-
lated stresses under such loading shall be one and three-quarters times
the allowed working stresses, and such loads should cause no injurious
permanent deformations. Load tests should not be made before the
concrete has been in place 60 days.

4. DESTRUCTIVE AGENCIES.

(a) Corrosion of Metal Reinforcement. — Tests and experience indi-
cate that steel sufficiently embedded in good concrete is well protected
against corrosion, no matter whether located above or below water
level. It is recommended that such protection be not less than 1 in.
in thickness. If the concrete is porous so as to be readily permeable
by water, as when the concrete is laid with a very dry consistency, the
metal may corrode on account of the presence of moisture and air.

(&) Electrolysis. — The experimental data available on this subject
seem to show that while reinforced concrete structures may, under
certain conditions, be injured by the flow of electric current in either
direction between the reinforcing material and the concrete, such
injury is generally to be expected only where voltages are considerably
higher than those which usually occur in concrete structures in prac-
tice. If the iron be positive, trouble may manifest itself by corrosion
of the iron accompanied by cracking of the concrete, and, if the iron
be negative, there may be a softening of the concrete near the surface
of the iron, resulting in a destruction of the bond. The former, or
anode effect, decreases much more rapidly than the voltage, and almost



1112 CONCKETE AND EEINFOKCED CONCRETE

if not quite disappears at voltages that are most likely to be encoun-
tered in practice. The cathode effect, on the other hand, takes place
even for very low voltages, and is therefore more important from a
practical standpoint than that of the anode.

Structures containing salt or calcium chloride, even in very small
quantities, are very much more susceptible to the effects of electric
currents than normal concrete, the anode effect progressing much more
rapidly in the presence of chlorine, and the cathode effect being greatly
increased by the presence of an alkali metal.

There is great weight of evidence to show that normal reinforced
concrete structures free from salt are in very little danger under most
practical conditions, while non-reinforced concrete structures are prac-
tically immune from electrolysis troubles.

(c) Sea Water. — The data available concerning the effect of sea
water on concrete or reinforced concrete are limited and inconclusive.
Sea walls out of the range of frost action have been standing for
many years without apparent injury. In many places serious disinte-
gration has taken place. This has occurred chiefly between low and
high tide levels, and is due, evidently, in part to frost. Chemical action
also appears to be indicated by the softening of the mortar. To effect
the best resistance to sea water, the concrete must be proportioned,
mixed, and placed so as to prevent the penetration of sea water into
the mass or through the joints. The aggregates should be carefully
selected, graded, and proportioned with the cement so as to secure the
maximum possible density; the concrete should be thoroughly mixed;
the joints between old and new work should be made water-tight;
and the concrete should be kept from exposure to sea water until it
is thoroughly hard and impervious.

(cZ) Acids. — Dense concrete thoroughly hardened is affected appre-
ciably only by acids which seriously injure other materials. Sub-
stances like manure, that contain acids, may injuriously affect green
concrete, but do not affect concrete that is thoroughly hardened.

(e) Oils. — Concrete is unaffected by such mineral oils as petroleum
and ordinary engine oils. Oils which contain fatty acids produce
injurious effects, forming compounds with the lime which may result
in a disintegration of the concrete in contact with them.

(/) Alkalies. — The action of alkalies on concrete is problematical.
In the reclamation of arid land, where the soil is heavily charged with
alkaline salts, it has been found that concrete, stone, brick, iron, and
other materials are injured under certain conditions. It would seem
that at the level of the ground-water in an extremely dry atmosphere
such structures are disintegrated, through the rapid crystallization
of the alkaline salts, resulting from the alternate wetting and drying
of the surface. Such destructive action can be prevented by the use of
a protective coating, and is minimized by securing a dense concrete.



CONCRETE AND REINFOECED CONCRETE 1113

CHAPTER III.
Materials.

The quality of all the materials is of paramount importance. The
cement and also the aggregates should be subject to definite require-
ments and tests.

1. CEMENT.

There are available for construction purposes Portland, Natural,
and Puzzolan or Slag cements.

(a) Portland Cement is the product obtained by finely pulverizing
clinker produced by calcining to incipient fusion, an intimate and
properly proportioned mixture of argillaceous and calcareous materials,
with no additions subsequent to calcination excepting water and
calcined or uncalcined gypsvma.

It has a definite chemical composition varying within compara-
tively narrow limits.

Portland cement only should be used in reinforced concrete con-
struction or in any construction that will be subject to shocks, vibra-
tions, or stresses other than direct compression.

(&) Natural Cement is the finely pulverized product resulting from
the calcination of an argillaceous limestone at a temperature only
sufficient to drive off the carbonic acid gas.

Although the limestone must have a certain composition, this
composition may vary within much wider limits than in the case of
Portland cement. Natural cement does not develop its strength as
quickly nor is it as uniform in composition as Portland cement.

Natural cement may be used in massive masonry where weight
rather than strength is the essential feature.

Where economy is the governing factor, a comparison may be made
between the use of natural cement and a leaner mixture of Portland
cement that will develop the same strength.

(c) Puzzolan or Slag Cement is the product resulting from finely
pulverizing a mechanical mixture of granxdated basic blast-fiirnace
slag and hydrated lime.

Puzzolan cement is not nearly as strong, uniform, or reliable as
Portland or natviral cement, is not used extensively, and never in
important work; it should be used only for unimportant foundation
work underground where it is not exposed to air or running water.

(



Online LibraryAmerican Society of Civil EngineersTransactions of the American Society of Civil Engineers (Volume 81) → online text (page 93 of 167)