American Society of Civil Engineers.

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

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Forms should be substantial and unyielding, in order that the con-
crete may conform to the design, and be STifliciently tight to prevent
the leakage of mortar.

It is vitally important to allow sufficient time for the proper harden-
ing of the concrete, which should be determined by careful inspection
before the forms are removed.

Many conditions affect the hardening of concrete, and the proper
time for the removal of the forms should be determined by some com-
petent and responsible person.

It may be stated in a general way that forms should remain in
place longer for reinforced concrete than is required for plain or
massive concrete, and longer for horizontal than is required for vertical
members.

In general it may be considered that concrete has hardened suffi-
ciently when it has a distinctive ring under the blow of a hammer,
but this test is not reliable, if there is a possibility that the concrete
is frozen.



1130 CONCRETE AND REINFORCED CONCRETE

CHAPTER VI.

Details of Constructfon.

1. JOINTS.

(a) In Concrete. — It is desirable to cast an entire structure at one
operation, but as this is not always possible, especially in large struc-
tures, it is necessary to stop the work at some convenient point. This
should be selected so that the resulting joint may have the least pos-
sible effect on the strength of the structure. It is therefore recom-
mended that the joint in columns be made flush with the lower side
of the girders, or in flat slab construction at the bottom of the flare of
the column head; that the joints in girders be at a point midway
between supports, unless a beam intersects a girder at this point, in
which case the joint should be offset a distance equal to twice the
width of the beam; and that the joints in the members of a floor sys-
tem should in general be made at or near the center of the span.

Joints in columns should be perpendicular to the axis, and in
girders, beams, and floor-slabs, perpendicular to the plane of their sur-
faces. When it is necessary to provide for shear at right angles to
the axis, it is permissible to incline the plane of the joint as much as
30° from the perpendicular. Joints in arch rings should be on planes
as nearly radial as practicable.

Before placing the concrete on top of a freshly poured column a
period of at least 2 hours should be allowed for the settlement and
shrinkage.

Shrinkage and contraction joints may be necessary to concentrate
cracks due to temperature in smooth even lines. The number of these
joints which should be determined and provided for in the design will
depend on the range of tem.perature to which the concrete will be sub-
jected, and on the amount and position of the reinforcement. In
massive work, such as retaining walls, abutments, etc., built without
reinforcement, contraction joints should be provided, at intervals of
from 25 to 50 ft. and with reinforcement from 50 to 80 ft. ; the smaller
the height and thickness, the closer the spacing. The joints should be
tongued and grooved to maintain the alignment in case of unequal
settlement. A groove may be formed in the surface as a finish to
vertical joints.

Shrinkage and contraction joints should be lubricated by an appli-
cation of petroleum oil or a similar material, to permit a free move-
ment when the concrete expands or contracts.

The movement of the joint due to expansion and contraction may
be facilitated by the insertion of a sheet of copper, zinc, or even tarred
paper.

(6) In Reinforcement. — Wherever it is necessary to splice tension
reinforcement the length of lap should be determined on the basis of



CONCKETE AND KEINFORCED CONCEETE 1121

the safe bond stress, the stress in the bar and the shearing resistance
of the concrete at the point of splice; or a connection should be made
between the bars of sufficient strength to carry the stress. Splices at
points of maximum stress in tension should be avoided. In columns,
bars more than | in. in diameter not subject to tension should have
their ends properly squared and butted together in suitable sleeves;
smaller bars may be lapped, as indicated for tension reinforcement.
At foundations bearing plates should be provided for supporting the
bars, or the bars may be carried into the footing a sufficient distance
to transmit the stress in the steel to the concrete by means of the bear-
ing and the bond resistance. In no case should reliance be placed upon
the end bearing of bars on concrete.

2. SHRINKAGE AND TEMPERATURE CHANGES.

The stresses resulting from shrinkage due to hardening and con-
traction from temperature changes are important in monolithic con-
struction, and unless cared for in the design v^ill produce objectionable
cracks; cracks cannot be entirely prevented, but the effects can be
minimized.

Large cracks, produced by quick hardening or wide ranges of tem-
perature, can be broken up to some extent into small cracks by placing
reinforcement in the concrete; in long, continuous lengths of concrete,
it is better to provide shrinkage joints at points in the structure where
they will do little or no harm. Reinforcement is of assistance, and
permits longer distances between shrinkage joints than when no
reinforcement is used.

Provision for shrinkage should be made when small or thin masses
are joined to larger or thicker masses; at such places the use of fillets
similar to those used in metal castings, but proportionally larger, is
recommended.

Shrinkage cracks are likely to occur at points where fresh concrete
is joined to that which is set, and hence in placing the concrete, con-
struction joints should be made, as described in Chapter VI, Section 1,
or, if possible, at points where joints would naturally occur in dimen-
sion-stone masonry.

3. FIRE-PROOFING.

Concrete, because incombustible and of a low rate of heat con-
ductivity, is highly efficient and admirably adapted for fire-proofing
purposes. This has been demonstrated by experience and tests.

The dehydration of concrete probably begins at about 500° Fahr.
and is completed at about 900° Fahr., but experience indicates that
the volatilization of the water absorbs heat from the surrounding
mass, which, together with the resistance of the air cells, tends to
increase the heat resistance of the concrete, so that the process of dehy-



1122 CONCRETE AND REINFORCED CONCRETE

dration is very much retarded. The concrete that is •actually affected
by fire and remains in position affords protection to that beneath it.

The thickness of the protective coating should be governed by the
intensity and duration of a possible fire and the rate of heat con-
ductivity of the concrete. The question of the rate of heat conduc-
tivity of concrete is one which requires further study and investiga-
tion before a definite rate for different classes of concrete can be fully
established. However, for ordinary conditions, it is recommended
that the metal be protected by a minimum of 2 in. of concrete on
girders and columns, 1^ in. on beams, and 1 in. on floor-slabs.

Where fire-proofing is required, and not otherwise provided in
monolithic concrete columns, it is recommended that the concrete to
a depth of li in. be considered as protective covering, and not included
in the effective section.

The corners of columns, girders, and beams should be beveled or
rounded, as a sharp corner is more seriously affected by fire than a
round one; experience shows that round columns are more fire resistive
than square.

4. WATER-PROOFING.

Many expedients have been resorted to for rendering concrete
impervious to water. Experience shows, however, that when mortar
or concrete is proportioned to obtain the greatest practicable density
and is mixed to the proper consistency (Chapter TV, Section 2 d), the
resulting mortar or concrete is impervious under moderate pressure.

On the other hand, concrete of dry consistency is more or less
pervious to water, and, though compounds of various kinds have been
mixed with the concrete or applied as a wash to the surface, in an
effort to offset this defect, these expedients have generally been dis-
appointing, for the reason that many of these compounds have at best
but temporary value, and in time lose their power of imparting
impermeability to the concrete.

In the case of subways, long retaining walls and reservoirs, pro-
vided the concrete itself is impervious, cracks may be so reduced, by
horizontal and vertical reinforcement properly proportioned and
located, that they will be too minute to permit leakage, or will be
closed by infiltration of silt.

Asphaltic or coal-tar preparations applied either as a mastic or as
a coating on felt or cloth fabric, are used for water-proofing, and
should be proof against injury by liquids or gases.

For retaining and similar walls in direct contact with the earth,
the application of one or two coatings of hot coal-tar pitch, following
a painting with a thin wash of coal tar dissolved in benzol, to the
thoroughly dried surface of concrete is an efficient method of prevent-
ing the penetration of moisture from the earth.



CONCRETE AND REINFORCED CONCRETE 1133

5. SURFACE FINISH.

Concrete is a material of an individual type, and should be used
without effort at imitation of other building materials. One of the
important problems connected with its use is the character of the
finish of exposed surfaces. The desired finish should be determined
before the concrete is placed, and the work conducted so as to facilitate
securing it. The natural surface of the concrete in most structures
is unobjectionable, but in others the marks of the forms and the flat
dead surface are displeasing, making some special treatment desirable.
A treatment of the surface which removes the film of cement and
brings the aggregates of the concrete into relief, either by scrubbing
with brushes and water before it is hard, or by tooling it after it is
hard, is frequently used to erase the form markings and break the
monotonous appearance of the surface. Besides being more pleasing
in immediate appearance, such a surface is less subject to discolora-
tion and hair cracking than is a surface composed of the cement that
segregates against the forms, or one that is made by applying a cement
wash. The aggregates can also be exposed by washing with liydro-
chloric acid diluted with from 6 to 10 parts of water. The plastering
of surfaces should be avoided, for even if carefully done, it is liable to
peel off under the action of frost or temperature changes.

Various effects in texture and in color can be obtained when the
surface is to be scrubbed or tooled, by using aggregates of the desired
size and color. For a fine-grained texture a granolithic surface mix-
ture can be made and placed against the face forms to a thickness of
about 1 in. as the placing of the body of the concrete proceeds.

A smiooth, even surface without form marks can be secured by the
use of plastered forms, which, in structures having many duplications
of members, can be \ised repeatedly; these are made in panels of
expanded metal or wire mesh coated with plaster, and the joints made
at edges, and closed with plaster of Paris.

CHAPTER YII.

Design.

1. massive concrete.

In the design of massive or plain concrete, no account should be
taken of the tensile strength of the material, and sections should
usually be proportioned so as to avoid tensile stresses except in slight
amounts to resist indirect stresses. This will generally be accom-
plished in the case of rectangular shapes if the line of pressure is kept
within the middle third of the section, but in very large structures
such as high masonry dams, a more exact analysis may be required.
Structures of massive concrete are able to resist unbalanced lateral



1124 CONCEETE AND EEINFOKCEl) COJ^CIKETI;

forces by reason of their weight; hence the element of weight rather
than strength often determines the design. A leaner and relatively
cheap concrete, therefore, will often be suitable for massive concrete
structures.

It is desirable generally to provide joints at intervals to localize
the effect of contraction (Chapter VI, Section 1).

Massive concrete is suitable for dams, retaining walls, and piers
in which the ratio of length to least width is relatively small. Under
ordinary conditions, this ratio should not exceed four. It is also
suitable for arches of moderate span.

2. REINFORCED CONCRETE.

The use of metal reinforcement is particularly advantageous in
members such as beams in which both tension and compression exist,
and in columns where the principal stresses are compressive and where
there also may be cross-bending. Therefore the theory of design here
presented relates mainly to the analysis of beams and columns.

3. GENERAL ASSUMPTIONS.

(a) Loads. — The forces to be resisted are those due to:

I 1. The dead load, which includes the weight of the structure

and fixed loads and forces.
2. The live load, or the loads and forces which are variable.
The dynamic effect of the live load will often require con-
sideration. Allowance for the latter is preferably made
by a proportionate increase in either the live load or the
live-load stresses. The working stresses hereinafter recom-
mended are intended to apply to the equivalent static
stresses thus determined.

In the case of high buildings, the live load on columns
may be reduced in accordance with the usual practice.

(b) Lengths of Beams and Columns.— The span length for beams
and slabs simply supported should be taken as the distance from center
to center of supports, but need not be taken to exceed the clear span
plus the depth of beam or slab. For continuous or restrained beams
built monolithically into supports, the span length may be taken as
the clear distance between faces of supports. Brackets should not be
considered as reducing the clear span in the sense here intended,
except that when brackets which make an angle of 45° or more with
the axis of a restrained beam are built monolithically with the beam,
the span may be measured from the section where the combined depth
of beam and bracket is at least one-third more than the depth of the
beam. Maximum negative moments are to be considered as existing
at the end of the span as here defined.



CONCRETE AND REINFORCED CONCRETE 1125

When the depth of a restrained beam is greater at its ends than at
mid-span and the slope of the bottom of the beam at its ends makes
an angle of not more than 15° with the direction of the axis of the
beam at mid-span, the span length may be measured from face to face
of supports.

The length of columns should be taken as the maximum unstayed
length.

(c) Stresses. — The following assumptions are recommended as a
basis for calculations:

1. Calculations will be made with reference to working stresses

and safe loads, rather than with reference to ultimate
strength and ultimate loads.

2. A plane section before bending remains plane after bending.

3. The modulus of elasticity of concrete in compression is

constant within the usual limits of working stresses. The
distribution of compressive stress in beams is therefore
rectilinear.

4. In calculating the moment of resistance of beams, the tensile

stresses in the concrete are neglected.

5. The adhesion between the concrete and the reinforcement

is perfect. Under compressive stress the two materials
are therefore stressed in proportion to their moduli of
elasticity.

6. The ratio of the modulus of elasticity of steel to the modulus

of elasticity of concrete is taken at 15 except as modified
in Chapter YTII, Section 8.

7. Initial stress in the reinforcement due to contraction or

expansion of the concrete is neglected.

It is recognized that some of the assumptions given herein are
not entirely borne out by experimental data. They are given in the
interest of simplicity and uniformity, and variations from exact con-
ditions are taken into account in the selection of formulas and working
stresses.

The deflection of a beam depends upon the strength and stiffness
developed throughout its length. For calculating deflection, a value
of 8 for the ratio of the moduli will give results corresponding
approximately with the actual conditions.

4. T-BEAMS.

In beam and slab construction an effective bond should be provided
at the junction of the beam and slab. When the principal slab
reinforcement is parallel to the beam, transverse reinforcement should
be used, extending over the beam and well into the slab.



1126 CONCRETE AND REINFORCED CONCRETE

The slab may be considered an integral part of the beam, when
adequate bond and shearing resistance between slab and web of beam
is provided, but its effective width shall be determined by the following
rules:

(a) It shall not exceed one-fourth of the span length of the

beam;
(&) Its overhanging width on either side of the web shall not

exceed six times the thickness of the slab.

In the design of continuous T-beams, due consideration should
be given to the compressive stress at the support.

Beams in which the T-form is used only for the purpose of
providing additional compression area of concrete should preferably
have a width of flange not more than three times the width of the
stem and a thickness of flange not less than one-third of the depth
of the beam. Both in this form and in the beam and slab form
the web stresses and the limitations in placing and spacing the
longitudinal reinforcement will probably be controlling factors in
design.

5. FLOOR-SLABS SUPPORTED ALONG FOUR SIDES.

Floor-slabs having the supports extending along the four sides
should be designed and reinforced as continuous over the supports.
If the length of the slab exceeds one and one-half times its width,
the entire load should be carried by transverse reinforcement.

For uniformly distributed loads on square slabs, one-half the live
and dead load may be used in the calculations of moment to be
resisted in each direction. For oblong slabs, the length of which
is not greater than one and one-half times their width, the moment
to be resisted by the transverse reinforcement may be found by using
a proportion of the live and dead load equal to that given by the

I
formula, r = - — 0.5, where I = leno:th and h = breadth of slab. The
b ' °

longitudinal reinforcement should then be proportioned to carry the
remainder of the load.

In placing reinforcement in such slabs account may well be taken
of the fact that the bending moment is greater near the center of
the slab than near the edges. For this puri:)0se two-thirds of the
previously calculated moments may be assumed as carried by the
center half of the slab and one-third by the outside quarters.

Loads carried to beams by slabs which are reinforced in two direc-
tions will not be uniformly distributed to the supporting beams,
and the distribution will depend on the relative stiffness of the slab
and the supporting beams. The distribution which may be expected
ordinarily is a variation of the load in the beam in accordance with



CONCRETE AND REINFGECED CONCEETE 1137

the ordinates of a parabola, having its vertex at the middle of the
span. For any given design, the probable distribution should be
ascertained and the moments in the beam calculated accordingly.

6. CONTINUOUS BEAMS AND SLABS.

When the beam or slab is continuous over its supports, reinforce-
ment should be fully provided at points of negative moment, and the
stresses in concrete recommended in Chapter YlII, Section 4, should
not be exceeded. In computing the positive and negative moments
in beams and slabs continuous over several supports, due to uniformly
distributed loads, the following rules are recommended:

(a) For floor-slabs, the bending moments at center and at support
should be taken at —— for both dead and live loads, where w
represents the load per linear unit and I the span length.

(h) For beams, the bending moment at center and at support for
Ulterior spans should be taken at and for end spans it

should be taken at — — for center and interior support, for
both dead and live loads.

(c) In the case of beams and slabs continuous for two spans only,

with their ends restrained, the bending moment both at the
central support and near the middle of the span should be

taken as .

10

(d) At the ends of continuous beams, the amount of negative

moment which will be developed in the beam will depend
on the condition of restraint or fixedness, and this will
depend on the form of construction used. In the ordinary

cases a moment of may be taken ; for small beams

16 ■^

running into heavy columns this should be increased, but

, ivP

not to exceed .

12

For spans of unusual length, or for spans of materially unequal
length, more exact calculations should be made. Special considera-
tion is also required in the case of concentrated loads.

Even if the center of the span is designed for a greater bending
moment than is called for by (a) or (b), the negative moment at
the support should not be taken as less than the values there given.



1128 CONCRETE AND EEINFORCED CONCRETE

Where beams are reinforced on the compression side, the steel
may be assumed to carry its proportion of stress in accordance with
the ratio of moduli of elasticity, Chapter VIII, Section 8. Eein-
forcing bars for compression in beams should be straight and shoiild
be two diameters in the clear from the surface of the concrete. For
the positive bending moment, such reinforcement should not exceed 1%
of the area of the concrete. In the case of cantilever and continuous
beams, tensile and compressive reinforcement over supports should
extend sufficiently beyond the support and beyond the point of inflection
to develop the requisite bond strength.

In construction made continuous over supports, it is important
that ample foundations should be provided; for unequal settlements
are liable to produce unsightly if not dangerous cracks. This effect is
more likely to occur in low structures.

Girders, such as wall girders, which have beams framed into one
side only, should be designed to resist torsional moment arising from
the negative moment at the end of the beam.

7. BOND STRENGTH AND SPACING OP REINFORCEMENT.

Adequate bond strength should be provided. The formula herein-
after given for bond stresses in beams is for straight longitudinal
bars. In beams in which a portion of the reinforcement is bent up
near the end, the bond stress at places, in both the straight bars and
the bent bars, will be considerably greater than for all the bars
straight, and the stress at some point may be several times as
much as that found by considering the stress to be uniformly dis-
tributed along the bar. In restrained and cantilever beams, full
tensile stress exists in the reinforcing bars at the poiiit of support,
and the bars should be anchored in the support sufficiently to develop
this stress.

In case of anchorage of bars, an additional length of bar should
be provided beyond that found on the assumption of uniform bond
stress, for the reason that before the bond resistance at the end of
the bar can be developed the bar may have begun to slip at another
point, and "running" resistance is less than the resistance before
slip begins.

Where high bond resistance is required, the deformed bar is a
suitable means of supplying the necessary strength. But it should
be recognized that, even with a deformed bar, initial slip occurs at
early loads, and that the ultimate loads obtained in the usual tests
for bond resistance may be misleading. Adequate bond strength
throughout the length of a bar is preferable to end anchorage, but, as
an additional safeguard, such anchorage may properly be used in
special cases. Anchorage furnished by short bends at a right angle
is less effective than bv hooks consisting of turns through 180 degrees.



CONCRETE AND EEINFORCED CONCRETE 1129

The lateral spacing of parallel bars should be not less than three
diameters from center to center, nor should the distance from the
side of the beam to the center of the nearest bar be less than two
diameters. The clear spacing between two layers of bars should be
not less than 1 in. The use of more than two layers is not recom-
mended, unless the layers are tied together by adequate metal con-
nections, particularly at and near points where bars are bent up or
bent down. Where more than one layer is used, at least all bars above
the lower layer should be bent up and anchored beyond the edge of
the support.

8. DIAGONAL TENSION AND SHEAR.



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