John C. (John Cresson) Trautwine.

Concrete online

. (page 6 of 23)
Online LibraryJohn C. (John Cresson) TrautwineConcrete → online text (page 6 of 23)
Font size
QR-code for this ebook


35. Use dry cone when it is to be heavily loaded at once. Tests indicate
that wet and dry cone will be equal in strgth within a year.

36. Wet cone bonds better to old work than does dry cone. Excess of
water increases efflorescence and laitance.

37. Rule for percentage, W, of water. H. P. Gillette, Cost
Data, p 266.

Let S = parts of sand to 1 part cem. Then

W = (8 S + 24) -T- OS + 1).
This gives

when S = 1 1.5 2.0 2.5 3.0 3.5 4.0

W = 16 14.4 13.3 12.6 12.0 11.5 11.2

Falk finds that mortars, thus proportioned, adhere well to steel.

38. Slag cement requires plenty of water for its proper hardening.
Therefore, if used in air, slag cem mortar should be kept damp.

Setting and Hardening.

39. Setting, or the loss of plasticity, usually occurs within a few hours
(sometimes within a few minutes) after mixing cem with water; whereas
hardening and increase of strength (which appear to result from a
different set of chemical processes) often proceed for months or even years.

63



CEMENT MORTAR.

40. Molded blocks of Portland cone, of even 50 tons wt, can
generally be handled and removed to their places in from 1 to 2 weeks

Initial and Final Set.

41. Initial and final set are stages of the setting process, arbi-
trarily distinguished by means of the resistance, of the mortar, to penetra-
tion by cylindrical wires, of standard diams and loaded with standard wts,
the blunt ends of the wires resting upon the surf of a pat of the mortar,
formed in a flat cylindrical mold on a glass plate. See H 8, p 943.

Determination of Set.

42. CJeiil Totten, (Genl Q. A. Gillmore, Limes, Hydraulic Cements and
Mortars, p 80,) at Fort Adams, R. I., prior to 1830, used a Vi 2 inch wire,
loaded with 0.25 Ib, and a ^34 inch wire, loaded with 1 Ib; initial and final
set being taken as the conditions when these wires, respectively, failed to
make an impression upon the mortar.

43. Vicat used but one wire, or "needle." The A S C E (see specifica-
tions, p 943) prescribes, for this needle, a diam of 1 mm (0.039 inch) and a
load of 300 grams (10.58 oz). Initial set occurs when the end of the needle,
penetrating a pat of mortar 4 cm (1.57 ins) deep, can no longer approach
within 5 mm (0.2 in) of the glass plate; and final set when the needle fails
to sink visibly into the mortar. The mortar, under the setting test, must
be of *' normal consistency," or such that a cylindrical rod, 1 cm
(0.39 inch) in diam, loaded with 300 grams, its end resting upon the mortar,
penetrates 1 cm into it.

Speed.

44. Speed. Some of the best cems are the slowest setting. A layer of
very quick-setting cem may partially set, especially in warm weather, before
the masonry is properly lowered and adjusted upon it, and any disturb-
ance, after setting has commenced, is prejudicial. On the other
hand, quick-setting cements are best in certain cases, as when exposed
to running water, etc. They may be rendered slower by adding a bulk of
lime paste equal to 5 or 15 % of the cement paste, without weakening them
seriously. Nat cems usually set quickly. Slag cem sets slowly.

45. In general, setting is accelerated by high alumina and by
soda and potash in the cem, by freshness and fineness of the cem, by the use
of warm water and warm sand in mixing, and by warm weather. Set-
ting is retarded by excess of lime and silica in the cem, by the presence
of sand, by wetness of mixture, by cold, by retempering, by salt or sulfuric
acid in the mixing water, by the presence of 1 or 2 % of lime sulfate, either
hydrated (gypsum) or anhydrous (plaster of Paris) or of slaked lime, in some
cases by hard burning, and, in general, by the age of the cement, but the
storage of new cem in warm places accelerates setting.

45 a. Oypsiim. CaSO4. Time of setting (initial and final) increased
rapidly with additions of gypsum up to about 2 %, and remained constant,
or increased slightly, up to 4 %. E. Candlot, "Ciments et Chaux Hydrau-
liques."

45 b. Time of setting (initial and final) increased, up to about 1.5%
gypsum, but then decreased, as the gypsum was increased to 7 %. Knis-
kern and Gass, Sibley Jour of Engng, '05, Jan.

45 c. Calcium chloride, CaCl2. A weak solution retards, but a
concentrated solution accelerates, the setting of Port cems. Thus, with 10
to 40 grammes per liter, the time of setting reached 500 to 850 mins ; while,
with 200 to 300 grammes per liter, it was reduced to from 2 to 25 mins.
Cems with very high or very low alumina are but little affected by CaCl2.
A weak solution (30 to 60 grammes per liter) may render sound a cem con-
taining free lime, by facilitating the hydration of the lime. E. Candlot,
"Ciments et Chaux Hydrauliques. "

45 d. From % to 1% % dry CaCl2, ground with cem clinker and made
into pats of normal consistency (See Tests, If 7, p 943) increased the
time of initial set from 2 to 167 mins, and that of final set from 52 to 275
mins. With 6 % , the times were 68 and 145 mins respectively, Kniskern
and Gass, Sibley Jour of Engng, '05, Jan.

46. Setting is attended by an increase of temperature. In quick
Betting, this increase may amount to 10 C (18 F) or more.



MORTAR. 947 h

47. Slow setting 1 cems are apt to harden more rapidly than quick
setting.

48. In warm air, setting cem, in drying, loses the moisture upon
which the operation of hardening depends. It therefore sets without
hardening-. In hot weather every precaution should be taken against
this.

49. Cems of the same class differ much in their rapidity of harden-
ing. At the end of a month one may gain nearly one-half of what it will
gain in a year, and another not more than one-sixth; yet at the end of a
year both may have about the same strength. Hence, tests for 1 week
or 1 month are by no means conclusive as to the final comparative merits
of cements.

50. Many years are required to attain the greatest
hardness : but after about a year the increase is usually very small and
slow, especially with neat cem. Moreover, any subsequent increase is a matter
of little importance, because generally by that time, and often much sooner,
the work is completed and exposed to its max loads.

51. Cems which are slow-setting when made, are apt to become quick-
settiiig (or "flashing") when stored, especially in warm places,
and if the cem is underlimed. This is attributed to disintegration of the
particles and consequent increase in fineness. The change sometimes takes
place very quickly. This difficulty can usually be overcome, without
reducing the strgth, by storage in cool places and by adding 1 to 2% of
slaked lime. On small jobs, a few lumps of lime may be added to each
bbl of mixing water.

52. The requirement, not uncommon in specfns, that a certain percent-
age of increase of strength must take place between 7 and 28
days, tempts the mfr to grind the cem coarsely, or to adulterate it with
inert material, in order that it may not gain too much of its strgth within
the first 7 days.

Properties.
Soundness.

53. U ns oundness,'in cem mortar, is the tendency to expand, contract
or disintegrate in air or water, or under heat and cold. See Specifications.

54. Cem, of any established brand, will seldom be found deficient in
strength; but may be deficient in soundness, upon which durability depends.

55. Unsoiindness is generally due to excess of free lime, arising

, or coarseness
e presence of
Paris is favorable
Unsound cem is improved by storage.

56. Change of dimensions during hardening of concrete.
Cone, placed in air, shortens or shrinks during the first two or three
months; while cone, in water, expands during about the same time.
These changes are greater with those cones having the larger proportions
of cem.

57. Shrinkage of mortar set in air.

per cent. ins. per 100 ft.

Neat cement,* ........................ 0.132 to 0.140 1.58 to 1.68

Mortar, 1:1,* ........................ 0.080 to 0.170 0.96 to 2.04

Lean mortars.t ....................... 0.030 to 0.050 0.36 to 0.60

The expansion in water is somewhat less than the contraction in air.
The total change in dimensions is the algebraic sum of that due to setting,
and that due to temperature changes.

58. Cone shrinks less when'it sets under pressure. Fineness oi
sand is conducive to shrinkage.

* Trans. A S C E, 9! xvii, 1887, p 214.

t Considere. Experimental Researches on Reinforced Concrete. Trans-
lation by Moissieff , p 87.



from incorrect proportioning, overburning, lack of seasoning, or coarseness
of grinding; the latter preventing perfect hydration. The presence of
lime sulphate (gypsum plaster of Paris) is favorable to soundness.



947 1



CEMENT MORTAR.



Strength.

59. Cem mortars are usually tested (by means of briquets) for tensile
strength.

60. Factors affecting strength. The strengths of samples,
under test, are much affected by the temperature of the air and water, as also
by the force with which the cem is pressed into the molds; by the extent
of setting before being put into the water, and of drying when taken out;
and still more by the pres under which it sets, which increases the strength
materially. On this account, cems, in actual masonry, may, under ordi-
nary circumstances, give better results than in tests of samples. The
causes named, together with the degree of thoroness of the mixing, the
proportion of water used, and other considerations, may easily affect the
results 100 % or even much more. Hence the discrepancies in the reports
of different experimenters. Specimens of the same cem, tested under
apparently similar conditions, may give widely diff results.

61. Personal equation. In connection with the building of the
Croton Aqueduct, New York, one set of testers, testing 835 briquets, ob-
tained an av strgth of 62.3 Ibs per sq in; while another set of testers,
testing 2434 exactly similar briquets by the same methods and under the
same circumstances, obtained an av strgth of 85.2 Ibs per sq in, or 36 %
greater.

62. Owing to such uncertainties, a series of tests, to be of value, must
cover a large number of specimens, in order that the accidental
diffs may be averaged.

63. Diffs in comparative results with diff materials may be due to one
or other of several diffs betw the materials. Thus, in comparing mortars
made with clean and with dirty sands, the strgths may be more affected
by diffs in density than by the diffs in cleanness of the sand.

64. Effect of age. The diagram,* Fig 1, illustrates approx the
strengths of av Portland and of av nat cems, neat and with 2 and 3 parts



900




14234 G
Weeks Months

Fig 1. Age and Strength of Mortar.

of sand, up to an age of two years. Tests may readily vary 10 per cent
or more eitherway from the average.

* See Richard L. Humphrey, in "Cement," Chicago, May, 1899.



MORTAR.



947 j



65. Fig 2* shows, approximately, the effect of sand.

in diff proportions, upon the strengths of Portland and natural cements, at diff




O 1 2345678
Parts of Sand to l.Part' Cement

Fig- 2. Effect of Sand upon Strength.



ages from 1 week to 1 year. The four solid curves represent average Port-
land cements, and the four dotted curves represent average natural cements.
For each kind of cement, the curves represent ages of 1 year, 6 months, 1
month and 1 week, respectively, beginning at the top. The curves for
natural cement are carried only to 5 parts sand.

66. The compressive strengths of cem mortars, in cubes, appear
to be about 8 to 10 times their tensile strengths, and their shearing strgths
about V4 their tensile strgths.

67. The adhesion of eem mortars to bricks or rough
rubble, at diff ages, and whether neat or with sand, may be taken at an
av of about % the tensile strength of the mortar at the same age. If the
bricks and stone are moist and entirely free from dust when laid, the ad-
hesion is increased; whereas, if very dry and dusty, especially in hot weather,
it may be reduced almost to nothing. The adhesion to very hard, smooth
bricks, or to finely dressed or sawed masonry, is less than the adhesion to
rough and porous surfs.

68. Dr. Bohme, Berlin, found tensile strgth -=- adhesive strgth = 10,
with 1 : 3 and 1 : 4 mortars, and = 6 to 8, with neat and 1 : 2 mortars.

Finish.

69. Lime mortar and cems, when used as mortar for brickwork, often
disfigure it, especially near sea-coasts, and in damp climates, by white
efflorescence, which sometimes spreads over the entire exposed face of
the work, and also injures the bricks. This occurs also, to some extent,
with Portland cems; also in the mortar joints of stone masonry, but to a
much leas extent. It injures only porous stone. It is usually a hydrous
carbonate of soda or of potash, or sulfate of lime (Epsom salts) often with
other salts. As a preventive, General Gillmore recommends to add, to
every 300 Ibs (1 bbl) of the cem powder, 100 Ibs of quicklime, and from
8 to 12 Ibs of any cheap animal fat; the fat to be well incorporated with
the quick-lime before slacking it, preparatory to adding it to the cem.
This addition will retard the setting, and somewhat diminish the strength
of the cem. It is said that linseed oil, at the rate of 2 gals to 300 Ibs of dry
cem, either with or without lime, will, in all exposures, prevent efflorescence;
but, like the fat, it greatly retards setting, and weakens the cem. See also
Bricks, p 929.

70. For pointing, the best Portland cem should be used, and is best
used neat, but it is often used with from 1 to 2 parts of sand. Mix under
shelter, and in quantities of only 2 or 3 pints at a time, using very little
water; so that the mortar, when ready for use, shall appear rather incoherent,
and quite deficient in plasticity. The joints being previously scraped out

* Compiled, by permission, from Prof. Baker's "Masonry Construction."



947 k CEMENT MORTAR.

to a depth of at least half an inch, the mortar is put in by trowel; a straight-
edge being held just below the joint, if straight, as an auxiliary. The
mortar is then to be well calked into the joint by a calking-iron and hammer;
then more mortar is put in and calked, until the joint is full. It is then
rubbed and polished under as great pressure as the mason can exert. If
the joints are very fine, they should be enlarged by a stonecutter, to about
} inch, to receive the pointing. The wall should be well wet before the
pointing is put in, and kept in such condition as neither to give water to,
nor take it from, the mortar. In hot weather the pointing should be kept
sheltered for some days from the sun, so as not to dry too quickly.

Behavior in Water.

71. I^aitance. "When cone is deposited in water, especially in the sea,
a pulpy gelatinous fluid exudes from the cem, and rises to the surface. This
causes the water to assume a milky hue; hence the French term, laitance.
As it sets very imperfectly, and, with some varieties of cems, scarcely at all,
its interposition betw the layers of cone, even in moderate quantities, will
have a tendency to lessen, more or less sensibly, the continuity and strgth
of the mass. It is usually removed from the inclosed space by pumps,
which must be used cautiously, to avoid disturbance of the C9nc by currents.
The proportion of laitance is greatly diminished by reducing the area of
cone exposed to the water, as by using large boxes, say from 1 to 1.5 cu
yds capacity, for immersing the cone." (Gillmore, "Limes, Hyd. Cems &
Mortars.")

72. Authorities differ as to the effect of sea water. H. LeChatelier
(Internatl Assn for Testg Materials, Procs, 1906), finds that the active in-
gredients of cem (lime, aluminates, silicates) are decomposed by the magne-
sium salts of sea water, yielding soluble calcium chlorides and lime sulfates.
The latter, with lime aluminate, forms a compound whose crystallization
tends to swell and crack the material.

73. In view of the notable puddling- effect of percolating water,
it would appear that sea water especially, with its numerous salts, ought
shortly to block its own passage into the cone.

74. The substitution of iron for alumina, in cem, is found to
remove one of the most active reagents in the deteriorating effects of the
salts in sea water.

See Cement, ^ 30, p 933.

75. The disintegration of cone in water (salt or fresh) ap-
pears to be due less to action of the water itself than to the repeated action
of frost where the cone is alternately exposed to freezing temps between
high and low water.

76. Mortar of puzzolano and lime has remained in perfect condition for
15 to 20 centuries in Italian harbor works.

77. At the dock at Kobe, Japan, to avoid possible injury, the salt water,
inside the dam, was replaced with fresh water, which entered at the surface,
while the heavier salt water was pumped out from the bottom.

For Concrete, see pages 1084, etc.



ABBREVIATIONS. 947 /

Abbreviations, symbols and references, in general use in the
articles on Cement, Sand and Mortar, pp 930-947 k t and on

Concrete pp 1084-1210.
For references to specifications, see pp 1184-5.

agg aggregate

ASTM American Society for Testing Materials

ASCE American Society of Civil Engineers

Assn Eng Socs. . .Association of Engineering Societies

cem cement .

cone concrete

constr construction

c c cubic centimeter

d day

elas elastic

EN Engineering News

E R Engineering Record

expt experiment

h, hr hour

Instn C E Institution of Civil Engineers

Jour Journal

kg kilogram

km kilometer

m meter

mm millimeter

mo month

mod modulus

mom moment

nat natural

Port Portland

Procs Proceedings

reinfd reinforced

reinfmt reinforcement

specf n specification

standd standard

surf surface

T & M Turneaure and Maurer, "Principles of Reinforced Con-
crete Construction," 1907.

T&T Taylor and Thompson, "Concrete, Plain and Reinforced,"

1905.

Trans Transactions

transv transverse

U. S. A Report, Chief of Engrs, U. S. Army.

wk week

/ per

D square

D" square inch

> greater than, more than

< less than

> not more than, equal to or less than.

< not less than, equal to or greater than, at least.



1084 CONCRETE.

CONCEETE.

For Cement, Sand and Mortar, see pages 930, etc.

For abbreviations, symbols and references, see p 947 I.

AGGREGATES.*

Constituents.

1. Order of value. (1) Trap, (2) granite, (3) gravel, (4) marble,
(5) limestone, (6) slag, (7) sandstone, (8) slate, (9) shale, (10) cinders.

2. The strgth of cone, with good sandstone, is about 0.75 X strength
with trap. With slate, less than half strength with trap. Good cinders
nearly equal to slate and shale. Hardness of agg increases in importance
with the age of the cone "because, as the cem becomes hard, there is greater
tendency for the stones themselves to shear thru, and the hardness of the
agg thus comes into play." (Sanford E. Thompson, E R, '06/Jan/27, p 109.)

3. The choice of agg is of course a matter of cost, as well as of strength,
&c, of product. Thus, with gravel sufficiently cheap, as compared with
broken stone, it may be economical to use the gravel, or a mix of gravel &
stone, obtaining the reqd total strgth by using a larger mass of cone. In
foundations, on weak ground, this is advisable because it distributes the
load over a greater area.

4. In many cases, the choice of sand and agg depends largely upon
what material can be had, and upon its distance from the work.

5. Where cem is cheap, it may be economical to use materials nearest
at hand, and to depend, for quality, upon excessive use of cem.

6. Stone which breaks into nearly cubical fragments packs better than
that which splinters into long pieces, and the fragments are less apt to
break in the finished work.

7. Good broken stone is usually preferred to gravel. The roughness
of the stone particles is believed to give better adhesion. Gravel cone
cannot well be tooled.

8. Cinders are sometimes used for the agg. They are ordinarily those
resulting from the burning of bituminous coal under boilers. The
material is mostly a fine ash, containing considerable unhurried coal.

9. Anthracite cinders are less extensively used, the supply being
less abundant.

10. Cinder cone, weighing only from 80 to 100 Ibs per cu ft, is of
advantage where lightness is reqd. Broken stone or gravel cone weighs
from 140 to 145 Ibs per cu ft.

11. Clay or loam, adhering to gravel particles, destroys or weakens
the adhesion of the mortar to the stones. The Boston Transit Commission,
Report for 1901, page 39, found the ratio of strength, betw cone with clean
and dirty gravel, about 60 : 45.

See "Clay and Loam," under "Sand" and "Accidental ingredients,"
p 1135.

Size.

12. In beams, arches, &c, the size of aggregate should not exceed
1.5 to 2 ins on any edge; but, if it is well Treed irpm dust by screening
or washing, and if the mortar completely fills the voids, all sizes, from 0.5
to 4 ins. on any edge, may be used in mass work, as foundations, dams,
piers, etc.

13. With large agg, coarse sand should be used, and vice versa.

14. It is usually economical of cem, to screen sand from gravel, or
fine material from crusher stone, and then remix in the required propor-
tions.

Density.

15. When a solid body is reduced to a mass consisting of broken pieces
separated by voids, the increase in bulk is due solely to the voids, and is

* By "aggregate," we mean the solid materials of cone, other than the
cem and sand. The term "aggregate" is sometimes used as including the
sand also.



PLAIN CONCRETE. 1085

equal to the space occupied by them. Hence the ratio, betw the Increase
of bulk, or " swelling 1 ," and the original bulk, is that of the voids
to the original, and not to the final bulk. Thus, if a solid cu yd of stone,
after being broken into pieces, occupies twice as much space as before,
then the increase in bulk, or the space occupied by the voids, is = that
occupied by solid pieces = half that occupied by the entire broken mass.

16. In sharp and angular broken stone, having all its pieces of nearly
uniform size, about 50 per cent of the vol, when measured loose, will
be voids. If the sizes of the stones vary betw somewhat wide limits,
as from 2 ins down to % inch, the vol, occupied by the voids, will be less, often
as little as from 28 to 30 % of the whole.

17. Tests by Mr. Wm. Hall (Trans A S C E, Vol 42, 1899, p 132) of voids
in crushed Green River blue limestone, 2.5 inch, screened; very clean Ohio
River gravel, 1.5 inch, and mixtures of the two, resulted as follows:
Percentage of stone 100 80 70 60 50

" gravel 20 30 40 50 100

" voids 48 44 41 38* 36 35

These are ays of a number of tests of several bargeloads of materials,
but there was little variation betw the mixtures.

18. Stone Crushers. See Price-list, p 992.

Cyclopean Concrete.

19. "Cyclopean" cone, consisting of large, rough stones ("dis-
placers" or "plums") laid in cem mortar, is largely, economically and ad-
vantageously used in mass work, especially in dams, where wt and hor
shearing strgth are desiderata. The stones need not be flat. They are
usually dropt into the wet mortar, without other bedding than that due
to their fall and wt. Wet cone facilitates the bedding of the stones, and
bonds better with them than does dry cone.

20. At Chandiere water power dam, Canada, the "plums" were
obtained from hard ledges in the river bed, in good shape for bedding.
Their agg vol av'd betw 25 and 30 % of the vol of the dam; max, 40 %

21. At Transmere Bay Development Works (Procs Inst C E, Vol 171,
1908, p. 145) the "plums were of sandstone, 9 ins apart hor'y. Near the
bases of the walls, they weighed a ton or more. The proportion of plums
decreased, with wall thickness, from 10 to 7 % of the whole mass.

22. Unnecessary restrictions, imposed upon contractors, may
eliminate the profit due to the use of "plums." See U 19.



1086 CONCRETE.

PLAIN CONCRETE.

1. Cement Concrete is composed of broken stone, gravel, cinders,
slag, shells, or other hard and inert * material (the aggregate), held together
by cement mortar, composed of cement and sand.

Advantages.

2. The principal advantages of cone are the convenience with
which it may be placed, particularly in otherwise difficult situations or
under water; its availability for subaqueous work; its cheapness, due
largely to convenience of placing and to its use of stone too small for masonry;
and its fire-resisting qualities, as compared with limestone (which calcines)
and with granite (which splinters).

3. The availability of C9nc has been very greatly extended by the practice
of reinforcement, which permits its use (heretofore often impracticable^
in members subject to tension as well as to compression, as in beams, in


1 2 3 4 6 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Online LibraryJohn C. (John Cresson) TrautwineConcrete → online text (page 6 of 23)