Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
It has been found that upon curing of a portland
cement mortar or concrete composition containing a film-
-forming polymeric modifier, e.g. a polymer latex, a
three-dimensional network of polymeric film is formed
throughout such material. This film seals the material
and prevents the evaporation of moisture therefrom by
shielding such composition from the deleterious effects
of low humidity, wind and excessive temperatures. Under
extreme drying conditions, such as high wind and/or high
temperature con~ined with low humidity conditions, the
modified concrete may develop crazing or random sur-
face cracking approximately one-tenth to one-half of an
inch deep. Such cracking is believed to be caused by the
following sequence of events~ the surface layer of
the latex modified cement mortar or concrete loses water
rapidly through surface evaporation, (2) coalescence of
the polymer modifier (latex) is induced by drying causing
the polymer in the surface layer to form a film network
before the cement paste, which is the matrix of such net-
work, has had a chance to develop mechanical strength
through the hydration reaction, (3) the hydration reac-
tion proceeds to a point wherein the cement paste phase
undergoes volume shrinkage exerting tensile stress, and
(4) since the polymer modifier has already formed a network
in the surface layer and resists shrinkage stress, when the
stress exceeds the tensile strength of the polymer network
such stress is relieved by cracking of the surface layer.
While numerous liquid curing membranes have
been proposed to prevent the excessive loss of moisture
from portland cement mortar and concrete compositions,
~b
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none of these compounds have proven to perform satisfac-
torily with a portland cement mortar or concrete composition
containing a film-forming polymeric modifier. The primary
reason for such failure is that all existing curing com-
pounds are designed to be applied after the initial set
of the portland cement mortar or concrete surface. This
occurs primarily because most of the compositions contain
a water-immiscible solvent which is incompatible with free
water on the surface. In polymer modified portland cement
mortar or concrete, however, the cracks begin to form
before the initial set.
It is, therefore, a primary object of the
present invention to provide a means for the prevention
of surface cracking of portland cement mortar or concrete
containing a film-forming polymeric modifier.
The present invention resides in a method for
preventing surface cracking of portland cement mortar or
concrete containing a film-forming polymeric modifier, com-
prising applying onto a fresh, non-cured mortar or concrete
surface a co~ting of from 0.06 to 1 lb/yd2 of said surface
of polyglycerine, glycerine or mixtures thereof.
It is believed that the advantage described
herein is attributable to the ability of the prescribed
coating materials to retard rapid coalescence of the
film-forming polymeric compound contained in the portland
cement mortar or concrete so as to prevent premature
formation of the polymeric film layer.
Exemplary of portland cement mortar or concrete
compositions containing film-forming polymeric modifiers,
which compositions are applicable in the practice of the
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present inventlon, are those specific compositions des-
cribed in U.S. Patents Re. 28,722 (reissue of U.S. Patent
3,043,790), 3,228,907 and 3,239,479. More particularly,
preferred polymer modifiers used herein are polymer
latexes wherein the polymer constituent is a copolymer
of styrene-butadiene-1,3 having a styrene to butadiene
weight ratio of 30:70 to 70:30 and, more particularly,
such polymer latexes containing, based on the weight of
the styrene-butadiene copolymer, (a) 2-10 percent of
nonionic surfactant, (b) 1-7.5 percent of anionic sur-
factant, at least 15 percent of which is a sodium alkyl
sulfate in which the alkyl group contains 9-17 carbon
atoms, and (c) .1-5 percent of polyorganosiloxane fluid
surfactant, the sum of (a) and (b) not exceeding about 11
percent by weight of said copolymer and the weight ratio
of (a) and (b) being within the range of 0.7:1 to 10:1.
Other preferred polymeric modifiers are polymer
latexes containing an interpolymer where vinylidene chloride
is present in proportions of 50 to 90 percent by weight
being interpolymerized with 50 to 10 percent by weight
of at least one interpolymerizable material composed of
at least one monomer having the general formula:
2,
~5 wherein "R" is hydrogen or the methyl group and "X" is
selected from -CN, halogens of atomic nurrbers 9-35, or
ester forming groups such as -COOY, wherein "Y" is a
primary or secondary alkyl group containing up to and
including 18 carbon atoms.
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It is to be understood, however, that the method
of the present invention is applicable to any portland
cement mortar or concre~e system containing a film-forming
polymeric modifier, which system would be compatible with
the treatment of the present invention.
The coating compounds applicable in the prac-
tice of the present invention are selected from polygly-
cerine, glycerine or mixtures thereof. Polyglycerine
herein referred to are the compounds produced during the
hydrolysis or finishing steps in the production of syn-
thetic glycerine and are in essence the still bottoms
obtained in the manufacture of glycerine. These compounds
are humectants for polymeric latex coatings, i.e., they
act to prevent premature drying of the polymer latex
coating prior to the setting of the cement base. Such
compounds are also miscible with water and as such are
capable of penetrating the water phase of the latex,
thereby preventing rapid latex particle coalescence.
The coating compounds may be applied to the
cement mortar or concrete surface by any convenient means.
A preferred method of application, however, comprises
spraying such compounds onto the cement mortar or concrete
surface in the form of a water miscible composition con-
taining at least about 80 weight percent of the prescribed
coating compound. In this regard, suitable diluents for
such sprayable compositions include water; alcohols such
as methanol, ethanol, isopropanal; or glycols such as
ethylene glycol.
The coating compounds contemplated herein may
also be admixed with conventional pigments and/or
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dyestuffs if desired. In this regard, pigments are
generally used in concentrations of less than ~bout 5
weight percent based on the weight of the coating compound;
and dyestuffs are generally used in concentrations of less
that about 10 ppm based on the weight of the coating com-
pound. Such compounds may also be admixed with small
amounts of humectants such as calcium chloride.
The rate of application of the prescribed coating
materials on the finished polymer latex-modified portland
cement mortar or concrete surface is from 0.06 to 1 lb/yd2
of cement mortar or concrete surface, preferably from 0.2
to 0.5 lb/yd2, which incidentally, covers the range of
thickness of fresh paint when brushed or sprayed on. These
values are converted into alternative units in the following
table.
TABLE I CONVERSION
Thickness Coverage Dosage*
(thousandth of 2 2
an inch) (ft. /gallon) (lb./yd )
1 1600 0.06
Z0 3 500 0.20
8 200 0.50
16 100 1.00
* for a liquid formulation of
specific gravity about 1.3
The invention is further illustrated by the
25following examples.
Example 1 - Control (For Comparison)
A concrete plate (100 sq. inches of surface)
composed of a portland cement modified with a latex
containing 67 weight percent styrene/33 weight percent
butadiene/2 weight percent anionic surfactant/5 weight
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percent nonionic surfactant (having a latex to cement
ratio of 0.15 and a water to cement ratio of 0.37) was
exposed to a temperature of 120F, a 20% relative humi-
dity, and a wind velocity of about 10 mph for a period
of 24 hours. Weight losses due to the evaporation of
water from the concrete plate were calculated after being
exposed to these conditions for certain periods of time.
After 30 minutes exposure 21.1 grams of water had evapora-
ted. 29.1 grams had evaporated after 1 hour; 41.2 grams
after a 3 hour period, and 50.3 gram loss was realized
after 24 hours. Surface cracks started to appear at about
15 minutes after the mortar had been finished.
Example 2 - Control (For Comparison)
Another concrete plate was exposed to the same
conditions outlined above. This plate, however, was
modified with a polymer latex blend containing a buta-
diene-styrene copolymer and a 75 percent vinylidene
chloride/20 percent vinylchloride/3 percent ethyl acry-
late/2 percent methyl methacrylate interpolymer latex
dispersion. The latex to cement and water to cement
ratios were the same. Water losses of 20.2, 27.2, 37.9,
and 47.3 grams were realized at 1/2, 1, 3, and 24 hour
time intervals along with surface cracking of the cement
plate.
Example 3 - The Invention (Polyglycerine)
By way of comparison, a concrete plate having
the same constituents as Example 1 was exposed to the
same conditions. This plate, however, was coated with
0.15 lb/yd2 of surface of an 80% polyglycerine solution
in water to prevent water loss and surface cracking.
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After 1/2 hour a water loss of only 9.2 grams was reali-
zed, followed by water losses of 14.5, 30.0, and 49.5
grams at the 1, 3 and 24 hour time intervals. No cracking
was observed in the plate.
Example 4 - The Invention (Polyglycerine)
Under the same conditions, a plate having the
same cement composition as Example 2 was coated with
O.23 lb/yd of surface of the polyglycerine solution
of Example 3. Here weight losses of 9.5, 15~6, 31.0,
and 59.0 grams were experienced with no surface cracking
occurring.
Example 5 - The Invention (Glycerine)
A plate as in Example 1, coated with 0.10
lb/yd2 of surface of 96~ glycerine was exposed to the
same conditions as above. Weight losses of 11.4, 22.4,
39.8, and 57.3 grams were experienced with no surface
cracking occurring.
Example 6 - The Invention (Glycerine)
A plate as in Example 1 was coated with 0.15
lb/yd2 of surface of 90% glycerine. Weight losses of
9.3, 16.6, 36.6, and 52.7 grams were experienced with
no surface cracking occurring.
Example 7 - The Invention (Glycerine)
A plate as in ~xample 2 was coated with 0.23
lb/yd2 of surface of 90% glycerine. Weight losses of
8.6, 13.5, 27.5, and 56.9 were experienced with no surface
cracking occurring.
Example 8 - The Invention (Glycerine)
A plate as in Example 2 was coated with 0.46
lb/yd (1% by weight) of surface of 90~ glycerine. Weight
..
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losses of 3, 9, 27 and 53 grams were experienced at 1/2,
1, 1 1/2 and 24 hour intervals, respectively, with no
evidence of surface cracking.
Example 9 - For Comparison
A concrete plate was prepared as set forth in
Example 8 except that the glycerine was admixed into the
cement formulation, instead of being sprayed onto the
preformed plate. After 1/2 hour, a water loss of 18 grams
was realized, followed by water losses of 26, 37, and
58 grams at 1, 1 1/2 and 24 hour time intervals. Surface
cracking of the cement plate was observed following the
initial 15 minutes of exposure.
This example illustrates that the required
results of the present invention cannot be obtained by
addition of glycerine to the cement mix.
Example 10 - For Comparison
By way of further comparison, the cement formu-
lation of Example 1 was mixed with IPANEX (curing com-
pound marketed by IPA Systems, Inc.) at 1% by weight of
cement according to IPA recommendations. Weight losses
of 23.4, 32.0, 45.3, and 56.1 grams were experienced
with surface cracking occurring.
Example 11 - For Comparison
A cement plate as in Example 1 was coated with
0.15 lb/yd2 of surface of ~ethocel~ E-15. Weight losses
of 20.9, 37.8, 56.8, and 66.5 grams were experienced
with surface cracking occurring.
Example 12 - For Comparison
A cement plate as in Example 1 was coated with
0.16 lb/yd of surface of a 33~ calcium chloride (CaC12)
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solution. Weight losses of 19.0, 30.0, 62.7, and 85.8
grams were experienced with surface cracking occurring.
Example 13 - For Comparison
A cement plate as per Example 1 was coated with
0.13 lb/yd2 of surface of l-octadecanol (10% in ethanol).
Weight losses of 13.1, 35.9, 61.4 and 73.4 were experi-
enced with surface cracking occurring.
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