U.S. patent application number 10/305869 was filed with the patent office on 2003-06-19 for methods of protecting concrete from freeze damage.
Invention is credited to Chen, Jeffrey, Scherer, George W., Valenza, John.
Application Number | 20030110984 10/305869 |
Document ID | / |
Family ID | 26829482 |
Filed Date | 2003-06-19 |
United States Patent
Application |
20030110984 |
Kind Code |
A1 |
Scherer, George W. ; et
al. |
June 19, 2003 |
Methods of protecting concrete from freeze damage
Abstract
A method of protecting a cementitious mixture from freeze damage
is provided. The method consists of incorporating an entrainment
air composition into the cementitious mixture to form air voids in
the concrete, and further adding an effective agent for nucleating
ice, preferably, in the air voids, such that upon the freezing of
concrete formed from the cementitious mixture, ice is nucleated in
the air voids. In one embodiment, the air entrainment composition
includes ceramic shells, which could be impregnated with an agent
for nucleating ice such as metaldehyde.
Inventors: |
Scherer, George W.;
(Pennington, NJ) ; Chen, Jeffrey; (Evanston,
IL) ; Valenza, John; (Lambertville, NJ) |
Correspondence
Address: |
Michael R. Friscia
Wolff & Samson
5 Becker Farm Road
Roseland
NJ
07068-1776
US
|
Family ID: |
26829482 |
Appl. No.: |
10/305869 |
Filed: |
November 26, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10305869 |
Nov 26, 2002 |
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09562213 |
Apr 28, 2000 |
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6485560 |
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60131447 |
Apr 28, 1999 |
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Current U.S.
Class: |
106/672 ;
106/681 |
Current CPC
Class: |
C04B 2103/0068 20130101;
C04B 28/02 20130101; C04B 28/02 20130101; C04B 28/02 20130101; C04B
38/08 20130101; C04B 20/1022 20130101; C04B 28/02 20130101; C04B
20/1022 20130101; C04B 14/12 20130101; C04B 2111/29 20130101; C04B
14/301 20130101; C04B 38/08 20130101 |
Class at
Publication: |
106/672 ;
106/681 |
International
Class: |
C04B 020/00; C04B
020/06; C04B 022/00; C04B 038/08 |
Claims
What is claimed is:
1. A method of protecting a cementitious mixture from freeze damage
comprising incorporating an amount of air into the cementitious
mixture to form air pores in the cementitious mixture, the
cementitious mixture including an amount of an air entrainment
agent and an effective agent for nucleating ice in the air voids
upon the freezing of the cementitious mixture.
2. The method of claim 1 wherein the water nucleating agent
comprises metaldehyde.
3. The method of claim 2 wherein the metadehyde consists of
tetrameric units (CH.sub.3CHO).sub.4.
4. The method of claim 2 wherein the air entrainment agent contains
a surfactant.
5. The method of claim 2 wherein the metaldehyde is ground to
expose the crystal planes that are most effective for nucleating
ice.
6. The method of claim 2 wherein the air entrainment composition
includes ceramic shells impregnated with metaldehyde.
7. The method of claim 2 wherein the amount of air is from about 3%
to about 8% by volume.
8. A method of protecting a cementitious mixture from freeze damage
comprising incorporating an amount of air into the cementitious
mixture to form air voids in the cementitious mixture, the
cementitious mixture including an amount of an air entrainment
composition and an effective amount of an agent for nucleating ice,
selected from the group of metaldehyde, acetoacetanilide,
p-bromoacetphenone, coumarin, m-nitoaniline, pthalic anhydride, and
2,4,6-trichloraniline for nucleating ice in the air pores.
9. The method of claim 8 wherein the air entrainment composition
comprises ceramic shells impregnated with an agent for nucleating
ice.
10. The method of claim 8 wherein the air entrainment composition
comprises glass ceramic shells impregnated with an agent for
nucleating ice.
11. The method of claim 8 wherein the air entrainment composition
comprises shells of kaolin impregnated with an agent for nucleating
ice.
12. The method of claim 8 wherein the air entrainment composition
comprises clay shells inpregnated with an agent for nucleating
ice.
13. A method for protecting concrete from freeze damage comprising:
mixing an air entrainment composition into a cementitious mixture;
adding an agent for nucleating ice to the cementitious mixture; and
allowing the cemetitious mixture to form concrete.
14. The method of claim 13 wherein the agent for nucleating ice is
added to the air entrainment composition before mixing the air
entrainment composition with the cementitous mixture.
15. The method of claim 14 wherein the agent for nucleating ice
comprises metaldehyde.
16. A concrete composition including pores and air voids comprising
a nucleating agent within the air voids for nucleating ice.
17. The composition of claim 16 wherein the nucleating agent
comprises metaldehyde.
18. A method of forming porous shells for use as an air entrainment
composition for concrete comprising: suspending particles in a
slurry; atomizing the slurry to form droplets; drying the droplets;
and sintering the dried droplets to form shells with porous outer
walls.
19. The method of claim 18 wherein the particles comprise ceramic
material.
20. The method of claim 18 wherein the particles comprise clay.
21. The method of claim 18 wherein the particles comprise aluminum
oxide.
22. A method of protecting concrete from freeze damage by providing
an air entrainment agent comprising mixing an amount of hollow
porous shells into a cementitious mixture and allowing the
cementitious mixture for form concrete, wherein the hollow porous
shells allow water to pass thereinto.
23. A concrete composition formed of a cementitious mixture with
porous hollow shells mixed therein for allowing water to pass
thereinto, to prevent freeze damage to the concrete
composition.
24. The composition of claim 23 wherein the porous hollow shells
are formed of a ceramic material.
25. The composition of claim 23 wherein the porous hollow shells
are formed of clay.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of provisional
application U.S. Serial No. 60/131,447 filed Apr. 28, 1999. This
application is incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] This invention relates to a process whereby the nucleation
of ice within concrete is controlled. It is current practice to
protect concrete against frost damage by introducing air voids,
which are generated by adding surfactants with the cement paste.
Those voids protect against one mechanism of damage (hydraulic
pressure), but not against crystallization pressure. By introducing
nucleating agents into the voids, ice can be forced to occur only
in the voids, and this will further reduce frost damage.
[0003] Concrete, like all porous media, has the ability to retain
and absorb moisture. Under freezing conditions, ice can grow within
the concrete pores, leading to significant internal cracking of the
cement matrix and/or scaling of the concrete surface. While the
precise mechanisms of frost action are not known, concrete
deterioration is believed to result from three important forces:
crystallization, hydraulic and diffusion/osmotic pressures. These
mechanisms are thought to produce flows of metastable water in the
concrete pores that generate sufficiently high stresses to induce
fracture of the cement matrix. To reduce the internal pressures,
air-entrained voids are often placed within the cement matrix to
provide escape boundaries for the flow of unstable water.
[0004] From experimental evidence, properly air-entrained concrete
samples have given consistently good results in terms of the ASTM C
666 standard freeze-thaw tests. However, in practice, the technique
of air entrainment has several disadvantages such as
inconsistencies in spacing factors (means half-distance between
voids) and uncertainties in bubble stability. Both issues have
caused frequent discrepancies between expected and actual frost
durability
[0005] Numerous references in this area are discussed in the
Detailed Description section of this application.
OBJECTS AND SUMMARY OF THE INVENTION
[0006] It is a primary object of this invention to protect concrete
from freeze damage.
[0007] It is a further object of this invention to provide a
simple, inexpensive, and easy to use method of protecting concrete
from freeze damage.
[0008] It is another object of the present invention to add an
effective amount of nucleating agent to a cementitious mixture to
nucleate ice in concrete.
[0009] It is a further object of the present invention to provide a
nucleating agent in concrete which can be added during mixing.
[0010] It is even a further object of the invention to provide
porous ceramic or clay shells for air entrainment in concrete, and
to provide a method of making such shells.
[0011] These objects and others are achieved by the method of
protecting a cementitious mixture from freeze damage according to
the present invention. The method comprises incorporating air into
a cementitious mixture to form air pores in the cementitious
mixture, including an air entrainment agent, and adding an
effective amount of, preferably, metaldehyde, or an equivalent
nucleating compound for nucleating ice in the air pores upon the
freezing of concrete. The nucleating agent is added to the
cementitious mixture during the normal mixing process. Other
nucleating agents may be used. Preferably the air entrainment
composition contains a surfactant. Because ice nucleating agents
are hydrophobic, when mixed with a surfactant, which is normally
used for forming air voids, the metaldehyde particles associate
themselves with the surfactant and become incorporated within air
voids formed in the concrete. Optionally, the air entrainment is
achieved by using porous ceramic shells, which could be used alone
or which could be impregnated with metaldehyde or another ice
nucleating agent. Preferrably, the metadehyde consists of
tetrameric units (CH.sub.3CHO).sub.4, rather than polyacetaldehyde
chains.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Other important objects and features of the invention will
be apparent from the following Detailed Description of the
Invention taken in connection with the accompanying drawings in
which:
[0013] FIG. 1 is a schematic of approximations of pore geometry in
cement paste in: (a) Straight channels; (b) sloping channels; (c)
irregular sized pores with necks that connect to larger pores.
[0014] FIG. 2 is a graph of theoretical minimum pore radius that a
growing ice crystal can penetrate as a function of
undercooling.
[0015] FIG. 3 is a graph of tensile stresses induced by
crystallization pressures at increasing undercoolings.
[0016] FIG. 4 is a graph of critical pore length as a function of
pore radius and super-undercooling.
[0017] FIG. 5 shows typical air-entrainment compounds used in
practice today.
[0018] FIG. 6 shows a body-centered tetragonal unit cell of
metaldehyde showing the columnar arrangement of the tetramers.
[0019] FIG. 7 shows an axial view of the packing arrangement of the
metaldehydes tetramers revealing the steric effects of the bulky
methyl groups.
[0020] FIG. 8 is a graph of DSC comparison of the impregnated and
unimpregnated Vycor glass sample.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Structure of Cement and Concrete
[0022] The character of the cementitious porous network is deeply
integrated with the concrete's freezing properties. Any change in
the former necessarily dictates the behavior of the latter. A large
part of the reason for uncertainties in frost deterioration stems
from the complex microstructure of concrete. Due to the nature of
the hydration reaction, voids range from the nanometer (gel pores)
to the millimeter scale (capillary pores). Setzer, M. J.,
"Interaction of Water with Hardened Cement Paste" in Ceramic
Transactions, Vol. 16: Advances in Cementitious Materials, Ed.
Sidney Mindness, The American Ceramic Society: Westerville, Ohio,
1990, devised a three-tier classification scheme that includes the
structured surface water, the capillary condensed water and the
macroscopic bulk water.
1 Pore classification scheme indicating the broad range of pore
sizes present in the cement matrix. Name Upper radius Pore water
classification Macro Capillaries 2 mm Macroscopic bulk water Meso
Capillaries 50 .mu.m Macroscopic bulk water Micro Capillaries 2
.mu.m Macroscopic bulk water Mesopores 50 nm Condensed water
Micropores 2 nm Structured surface water
[0023] The pore size distribution in concrete is not a fixed
parameter. It will vary with chemical composition, aging and the
water to cement ratio, Mehta, P. K., Concrete: Structure,
Properties and Materials, Prentice-Hall, New Jersey, 1986.
[0024] An analysis of the freezing behavior in cement paste will
require some approximations of the geometry of the porous network.
FIG. 1 shows some plausible simplifications of the pore geometry,
and shows ice formed therein. As will be discussed in detail, the
geometry and connectivity of the porous network will have a
profound influence on the durability of concrete.
[0025] Freezing within the Pores of Concrete
[0026] It is well known that the freezing properties of a liquid in
a porous medium are significantly altered. The phenomenon is due to
the interaction between the liquid (absorbate) and the solid pore
surface (absorbent). The nature and intensity of the interactions
is a function of the chemical and geometrical features of the pore
wall and of the distance between the absorbate molecules from the
absorbent surface. Collectively, these forces have the effect of
depressing the freezing point of the pore liquid. Thermodynamic
expressions relating the freezing point depression to the geometry
of the pore are well established, Defay, R. and Prigogine, I.,
Surface Tension and Adsorption, Wiley: New York, 1966; Scherer, G.
W., "Freezing Gels," Journal of Non-Crystalline Solids, V. 155,
1994, pp. 1-25; Marchand. J., Plea, R. and Gagn, R., "Deterioration
of Concrete due to Freezing and Thawing," in Materials Science of
Concrete IV, 1991.
[0027] Theoretical considerations imply that the smaller pores will
have lower melting temperatures than the larger pores. Thus, the
crystal will only invade the pore when the crystal has undercooled
to a lower temperature, and is able to adopt the required radius of
curvature of the small pore. Furthermore, theoretical
considerations predict that freezing will occur in a progressive
manner; that is, freezing is initiated in the larger pores and then
penetrates into smaller ones as the undercooling increases
(Scherer, G. W. 1999, "Crystallization in pores", Cement and
Concrete Research, Vol. 29, pp. 1347-1358). This has been confirmed
experimentally by various low temperature investigations of cement
paste by either calorimetry or nuclear magnetic resonance (NMR)
imaging techniques (Banthia, 1989, Badger, D., "Ice Formation in
Hardened Cement Paste Part I--Room Temperature Cured Pastes with
Variable Moisture Contents," Cement and Concrete Research, Vol. 16,
pp. 706-720; Badger, D., "Ice Formation in Hardened Cement Paste
Part II Drying and Resaturation on Room Temperature Cure Pastes,"
Cement and Concrete Research, Vol. 16, pp. 835-844; Badger, D.,
"Ice Formation in Hardened Cement Paste Part III--Slow Resaturation
of Room Temperature Cured Pastes," Cement and Concrete Research,
Vol. 17, pp. 1-11; Prado, P., Balcom, B., Beya, S., Bremmer, T.,
Armstrong, R. and Grattan-Bellew, P., "Concrete Freeze/Thaw as
Studied by Magnetic Resonance Imaging," Cement and Concrete
Research Vol. 28, No. 2, 1998, pp. 261-270). This
progressive-freezing phenomenon can be seen graphically in FIG. 2,
which implies that for an undercooling of .DELTA.T=7.degree. C.,
ice will not be present in pores smaller than .about.10 nm.
[0028] We can picture a growing ice front initiating at the surface
and then penetrating into the tortuous, interconnected porous
network. At a given undercooling, ice will advance until the ice
front is impeded by smaller pores that require a greater
undercooling. The ice front will percolate through the entire body,
only when able to penetrate a critical breakthrough radius,
r.sub.BT. Once entering the breakthrough pore at a characteristic
undercooling, .DELTA.T.sub.BT, the ice front will be able to travel
unimpeded in pores of radius.gtoreq.r.sub.BT.
[0029] The breakthrough radius, r.sub.BT (or equivalently,
.DELTA.T.sub.BT), is equivalent to the characteristic pore size
that controls the permeability of porous bodies (Katz, A. J., and
Thompson A. H., Journal of Geophysical Research, Vol. 92, No. B1,
1987, pp. 599). Thus, highly permeable materials should have
correspondingly larger breakthrough radii than less permeable
materials. In many porous materials, the r.sub.BT lies near the
inflection point in a mercury penetration curve, thus corresponding
to the mean pore entry radius. Various r.sub.BT with their
respective .DELTA.T.sub.BT (governed by eq. 3.7) are shown in Table
3.2.
2 Breakthrough radius, r.sub.BT, with respective breakthrough
undercooling, .DELTA.T.sub.BT. Breakthrough Breakthrough radius,
r.sub.BT [nm] undercooling, .DELTA.T.sub.BT [.degree. C.] 5 13.3 10
6.7 15 4.4 20 3.3 25 2.7 30 2.2
[0030] It is difficult to obtain a definitive r.sub.BT for cement
paste since the pore structure, and hence the breakthrough
conditions, will vary depending on parameters such as the water to
cement ratio (w/c), temperature, age and additive concentration.
From mercury intrusion curves generated for cement pastes with a
range of w/c ratios, a 0.4 w/c ratio paste is expected to have a
r.sub.BT of .about.15 nm to .about.20 nm.
[0031] The cement paste in concrete will have higher porosities
than plain hydrated paste due to the presence of highly permeable
interfacial transition zones (ITZ) surrounding aggregates (Winslow,
D. N., Cohen, M. D., Bentz, D. P, Synder, K. A. and Garboczi, E.
J., "Percolation and Pore Structure in Mortars and Concrete,"
Cement and Concrete Research, Vol. 24, No. 1, 1994, pp. 25-37). The
r.sub.BT for cement paste in concrete is therefore higher than the
corresponding r.sub.BT for plain cement paste. However, common
additives for concrete such as silica fume, with particle size
.about.3 orders of magnitude smaller than that of cement particles,
will significantly decrease the permeability of a concrete by
reducing r.sub.BT.
[0032] Crystallization pressure can be defined as the pressure of
the growing ice crystal on the pore wall. Theoretical calculations
and considerations, as shown in FIG. 3, indicate that
crystallization pressures will potentially exceed the concrete
tensile strength (about 3 MPa) at undercoolings greater than or
equal to 5.degree. C. and pore radii less than or equal to 13.3
nm.
[0033] To cause fracture, the generated tensile stresses must act
on the flaws in the pore wall. At the breakthrough temperature,
T.sub.BT, most of the pore volume has frozen, implying that all of
the flaws (including the most damaging large flaws) in the body
should feel the stress at that point. Hence, crack propagation is
expected to strongly correlate with the propagation of the ice
front at T.sub.BT.
[0034] After ice has propagated and cracking has initiated in the
body, the effect of crystallization pressures at lower temperatures
important. As long as there is water in contact with the ice,
crystals will form at sufficiently low temperatures, and
crystallization pressures will be present. There are generally
small isolated pockets of unfrozen water even after percolation.
Thus, when the temperature drops below T.sub.BT there is
crystallization pressure as the ice front is penetrating into the
smaller pores of the unfrozen pockets. The generated stresses could
be quite high (>10 Mpa) but likelihood of failure is dependent
on the whether stresses are brought to bear on flaws in the small
pores.
[0035] It is highly probable that the percolation event at T.sub.BT
will accelerate crystallization pressure damage by amplifying
stresses on the largest flaws in the body. It is desirable that
nucleation in the air voids occur at a temperature above the
T.sub.BT; in that way, the freezeable water is removed from the
pores before stresses are applied to the largest flaws in the body.
Moreover, it is preferred that nucleation occur above about
-5.degree. C., thereby confining the ice growth to the air voids
before stresses can theoretically exceed .about.3 MPa.
[0036] Water has the unusual property that the liquid phase
(.rho.=1.00 g/cm.sup.3) is more dense that the solid phase
(.rho.=0.92 g/cm.sup.3). This property has very important
repercussions in the freezing of porous media since ice necessarily
undergoes a 9% expansion. The volume change forces water ahead of
the growing crystal thus creating a pressure gradient in the
pore.
[0037] Based on theoretical consideration, FIG. 4 shows the maximum
length the displaced water can travel before generating tensile
stresses exceeding that of concrete. Thus, if an ice crystal is
growing in a 10 nm pore with a .DELTA.T* of 1.degree. C., cracking
of the pore wall will potentially occur if the displaced water does
not reach an escape boundary by the time it travels .about.470
.mu.m. FIG. 4 also suggests that ice growth in small pores with
large super-undercoolings will be the most damaging (i.e., having
the lowest critical pore lengths). Furthermore, with increasing
.DELTA.T*, the slope of the critical pore length curve will
necessarily decrease, causing larger pores to enter into the
probable vulnerable zone. As the percentage of the "nondurable
pores" increases, the resistance to hydraulic pressure damage
necessarily decreases.
[0038] A crystal front must be growing (possessing a finite
velocity) for hydraulic pressures to generate. However, if the
temperature has not reached the breakthrough conditions, the ice
front will most likely be in a stationary state, pressing against
the pore walls, and hence, creating negligible hydraulic pressures.
Only when the breakthrough conditions have been met (namely
.DELTA.T.sub.BT and r.sub.BT) will an ice front be able to grow for
extended lengths, and thereby create significant hydraulic
pressures. Now, we can picture a crystal just penetrating a pore of
size r.sub.BT at an undercooling of .DELTA.T.sub.BT. As the crystal
front percolates through a network of pores with radii greater than
or equal to r.sub.BT at a temperature of T.sub.BT, the
super-undercooling, .DELTA.T*=T.sub.BT-T, will change depending on
the size of the pore. Hence, the ice front will pulse along the
percolation path with the greatest velocities occurring in the
larger pores. Moreover, since the tensile hoop stress in the pore
wall is a function of .DELTA.T* (power dependence of 1.7) and
r.sub.p (inverse square dependence), the stresses generated will
also vary with changes in pore sizes.
[0039] The freezing rate in nature is low with a maximum rate of
6.degree. C./hr. Thus, one can reasonably assume that the
temperature remains constant at the breakthrough temperature,
T.sub.BT, throughout the entire percolation event. For a modest
super-undercooling of .DELTA.T*=1.0.degree. C. at breakthrough
conditions (thus the front is free to travel long distances) an ice
front can move .about.6 m in one hour. With this speed, concrete
slabs will fully crystallize before the temperature drops far below
the breakthrough temperature, T.sub.BT.
[0040] Theoretical considerations predict the damaging effect of
inducing hydraulic pressures in a very fine porous network
(possessing a small characteristic r.sub.BT). For a r.sub.BT of 10
nm, stresses can approach devastating stresses of .about.30 MPa
when invading larger pores. Furthermore, such theoretical
considerations contradict the idea that concrete will necessarily
be less prone to frost damage if it is less permeable. It is
certainly true that concrete will be completely protected if there
is absolutely no freezeable water present in the pores. However,
this level of impermeability is very difficult to achieve even with
very fine porous networks. Moreover, theoretical considerations
confirm the generally accepted notion that high strength
(compressive strengths>.about.100 MPa) and silica fume concretes
are very susceptible to damage in winter climates (Mehta, P. K.,
"Durability--Critical Issues for the Future," Concrete
International, July, 1997, pp. 27-33). Most likely this
susceptibility is due to the low characteristic breakthrough radii
for these very fine concretes.
3 Maximum tensile stresses generated from hydraulic pressures for a
specific r.sub.BT with the assumption that the maximum pore size is
.about.50 nm. Breakthrough radius, r.sub.BT [nm] Maximum tensile
stress [MPa] 5 433.6 10 27.3 15 4.9 20 1.3 30 0.1
[0041] As mentioned earlier, the r.sub.BT for cement paste in
concrete should vary from sample to sample depending on the curing
environment. Assuming a maximum pore size of 50 nm as before, it
can be calculated that stresses are expected to be greater than 3
MPa only when r.sub.BT is less than about 16.7 nm. The r.sub.BT was
estimated for plain cement pastes to be .about.15 nm to .about.20
nm, so it is not unreasonable to assume that concrete can possess a
r.sub.BT of 16.7 nm, which would imply potentially damaging tensile
stresses.
[0042] Up to this point, it has been assumed that ice is present in
the cement pores prior to reaching the breakthrough temperature.
Equivalently, this assumption implies that heterogeneous nucleation
has occurred near 0.degree. C., presumably at the surface where
foreign catalysts are most probable. While there have not been any
extensive studies on surface nucleation for concrete in the
environment, there have been some studies on laboratory quality
samples. Calorimetry experiments by Badger and Banthia et al.
revealed an initial freezing peak near -10.degree. C. and either
one or two peaks between -20.degree. C. to -40.degree. C. Both
papers agree that the initial peak corresponded to the nucleation
of the ice at the surface of the sample. Badger further cited that
the initial peak could be shifted towards higher temperatures by
adding AgI (effective ice nucleant at T=.about.-4.degree. C.) on
the surface of the sample. Of course, the laboratory samples
studied in these calorimetry experiments will probably not contain
potential natural ice nuclei such as some active bacteria which are
known to induce crystallization on non-coniferous plants as high as
-2.degree. C. (Vali, 1971). However, it is also doubtful that there
will always be a high enough concentration of these effective
nucleating agents on exposed concrete surfaces.
[0043] Thus, if a concrete surface is "clean," nucleation on the
surface could very well be delayed to -10.degree. C. as in the
laboratory samples. The most important implication of a delayed
surface nucleation is that percolation can now occur at
temperatures lower than T.sub.BT. If nucleation occurs at a
temperature, T.sub.N, which is lower than T.sub.BT , the
percolation event can occur at the lower temperature, leading to
higher .DELTA.T*, and hence, higher tensile stresses.
[0044] Besides the initial expansion of concrete at the onset of
freezing, concrete undergoes considerable shrinkage during freezing
if held at a constant sub-zero temperature. It has been
hypothesized that the ice crystals that were initially formed in
the larger pores could feed off the unfrozen water in the
neighboring nanosized gel pores (Powers, T. C. and Helmuthm, R. A.
1953, "Theory of volume changes in hardened portland-cement paste
during freezing", Proc. Highway Res. Board, Vol. 32, pp. 285-297).
This ice accretion mechanism is thought to be a result of the free
energy gradient between the crystal and the unfrozen gel water. At
the onset of crystallization, the ice and gel water are in
equilibrium. As the undercooling increases, the gel water (having
greater entropy) should gain free energy at a faster rate than the
crystal. Thus, to regain equilibrium, the gel water migrates to the
growing crystal and is allowed to shrink.
[0045] Osmotic pressure theories were later added to account for
the shrinkage of concrete during prolonged freezing periods. The
origin of osmotic pressures is that salt is highly insoluble in
ice. Consequently, very steep salt gradients accumulate at the
ice/water interface. Moreover, since ice will tend to initiate near
the surface of concrete structures (as a result of minimum
temperatures), the highest salt gradients should occur near the
surface. Amplifying the effect is the use of surface deicer salts
on concrete roads. The net result is a migration of the dilute gel
water to the high salt concentration at the surface and shrinkage
of the interior concrete layers. The combination of the shrinkage
of the interior gel layers and the expansion from freezing in the
saturated surface layer produces potentially destructive
stresses.
[0046] Air Entrainment Agents
[0047] Introducing a nucleating agent directly in the air voids
initiates ice growth in the large air voids and minimizes the
internal pressures created by the metastable water (whether from
hydraulic or diffusion/osmotic mechanisms).
[0048] The purpose of an air-entrainment agent is not to entrain
air bubbles, which is done mechanically in the mixer, but to
stabilize the bubbles in the cement matrix. The role of the
air-entrainment molecules is to stabilize the air-water interface,
reduce the surface tension of water (by as much as .about.20%), and
to bind the air bubbles to the cement particles. Most
air-entrainment compounds are aqueous solutions of ionic or
nonionic surfactants, implying the presence of hydrophilic heads
and hydrophobic tails. Air-entrainment molecules stabilize air
bubbles by adsorbing at the air/water interface with their
hydrophobic ends protruding into the air-void itself and their
hydrophilic ends remaining in the aqueous phase.
[0049] Commercial air-entrainment products are typically dilute
aqueous solutions (5% to 20% by weight) of surfactants (Rixom, M.
R. and Mailvaganam. N. P., Chemical Admixtures for Concrete,
E.&F.N. Spon.: London, 1986). In practice, there are five basic
groups of surfactants suitable for concrete use (shown in order of
probably decreasing use):
[0050] (a) Abietic and pimeric acids salts (neutralized wood
resins)
[0051] (b) Fatty acid salts
[0052] (c) Alkyl-aryl sulphonates
[0053] (d) Alkyl sulphates
[0054] (e) Phenol ethoxylates.
[0055] The chemical structure of a representative of each group can
be seen in FIG. 5.
[0056] What is interesting about the different air-entrainment
compounds seen in FIG. 5 is that they possess varying degrees of
freeze-thaw resistance at a given total air-content. This implies
that there is a chemical interaction taking place in the air-voids
between the air-entrainment and water molecules. Kreijer. C. I.,
"Effect of Admixtures on the Frost Resistance if Early-Age
Concrete," in RILEM-ABEM International Symposium on Admixtures for
Mortar and Concrete, Brussels, pp. 235-244, 1967, showed that for
an air content of .about.5%, sodium oleate produced the best
freeze-thaw resistance while phenol ethoxylate showed very little
improvement over the non-air-entrained control sample. The
reasoning for this discrepancy is that an air void can obviously
not serve as a sink for displaced water if it is already full of
water. The water-free void will be ensured if the tails of the
air-entrainment molecules are highly hydrophobic. This explains
partly why non-hydrophobic molecules such as phenol ethoxylate
(capable of hydrogen-bonding on oxygens) yield poor freeze-thaw
results while the hydrophobic oleaetes, sulphates and resins
perform much better.
4 Varying freeze-thaw resistance of several air-entrainment
admixturers. Dosage Air Relative (m./50 kg content freeze-thaw
Air-entrainment admixture cement) (%) Resistance.sup.1 None
(control) 0 2.0 5 Sodium oleate (10% sol.) 353 5.6 86 Sodium lauryl
sulphate 18 5.8 46 Pine resin 15 5.2 57 Phenol ethoxylate 75 5.2 7
.sup.1An average of several techniques measuring changes in
compressive strength and modulus after freeze-thaw cycling.
[0057] A discussion of the chemistry of air-entrainment molecules
was presented since it is desired to introduce ice nucleating
agents into the air voids. When selecting air-entrainment
compounds, the chemical interaction between these molecules and the
ice nuclei should be understood. A nucleating particle is thought
to contain active sites and planes where nucleation is favored. If
these active sites strongly interact with the air-entrainment
molecules and "poison" the nucleating surface, the activity of the
nucleating particle will correspondingly decrease. However, since
it is desired to concentrate the nuclei in the voids and not in the
cement pores, there needs to be some attractive forces present
between the nuclei and air-entrainment molecule to ensure that the
two settle in the voids after hydration. This attractive force must
be sufficient to maintain the bond between the two compounds even
after mixing of the concrete. But again, it must be remembered that
the attractive force should not annul the nucleating properties,
implying that a compromise must be established.
[0058] Nucleation of Ice
[0059] Although ice melts consistently at 0.degree. C., pure liquid
water can supercool to as low as -40.degree. C. (Weissbuch, et al.;
and Hobbs). The induction, or catalysis, of the freezing point to
higher temperatures has many important consequences in nature.
Vonnegut, B., Journal of Applied Physics, Vol. 18, 1947, pp. 593.
was the first to identify that silver iodide could induce ice
nucleation in atmospheric clouds at .about.-4.degree. C. His
finding has spurred much research on other inorganic, organic
(Fukata N., "Experimental Studies of Organic Ice Nuclei," Journal
of the Atmospheric Sciences, Vol. 23, 1966, pp. 191-196; Garten,
1965) and bacterial nuclei. (Maki. L. R., Gaylam, E., Chang Chien,
M. and Caldwell, D. R., Applied Microbiology, Vol. 28, pp. 456-459;
Gurian-Sherman, D. and Lindlow, S. E., "Bacterial Ice Nucleation:
Significance and Molecular Basis," The FASEB Journal, Vol. 7,
November 1993, pp. 1338-1343; Wolber, P. K., "Bacterial Ice
Nucleation," Advances in Microbial Physiology, Vol. 34, 1993, pp.
203-237; Pattnaik, P., Batish, V. K., Grover, S. and Ahmed, N.,
"Bacterial Ice Nucleation: Prospects and Perspectives," Current
Science, Vol. 72, No. 5, Mar. 10, 1997, pp. 316-320. Fukuta, N.,
"Ice Nucleation by Metaldehyde," Nature, Vol. 199, 1963, pp.
475-476; Fukuta, N., "Some Remarks on Ice Nucleation by
Metaldehyde," in Proceedings of the International Conference on
Cloud Physics, Aug. 26-30, 1968, Toronto, pp. 194-198, found that
metaldehyde nucleated ice as high as -0.4.degree. C. from the vapor
phase. Frost inducing bacteria (Vali, 1971) was discovered to be
the cause for much of the wide spread damage to nonconiferous
plants due to nucleation of ice as high as -2.degree. C. Recently,
much research has been devoted to the nucleating properties of
monolayers of amphiphillic alcohols (C.sub.nH.sub.2n+1OH). It has
been found that C.sub.31H.sub.63OH (n=31) could nucleate ice as
high as -1.degree. C. (Gavish, M., Popovitz-Biro, R., Lahav, M. and
Leiserowitz, L., "Ice Nucleation by Alcohols Arranged in Monolayers
at the Surface of Water Drops," Science, Vol. 250, 1990, pp.
973-975; Popovitz-Biro, R., Wang, J. L., Majewski, J., Shavit, E.
Leiserowitz, L. and Lahav, M., "Induced Freezing of Supercooled
Water into Ice by Self-Assembled Crystalline Monolayers of
Amphiphillic Alcohols at the Air-Water Interface," Journal of the
American Chemical Society, Vol. 116, 1994, pp. 1179-1191).
[0060] The basis of the theory of nucleation of new phases was
established long ago by Volmer M. and Weber, A., Zeitschrift fuer
Physikalische Chemie, Vol. 119, 1325, pp. 277 and Becker, R., and
Doring, W., Annalen de Physik (Leipzig), Vol. 5, No. 24, 1935, pp.
719, and remains virtually unchanged today. Within a supercooled
liquid or a supersaturated vapor, there are transient groupings of
the parent molecules with the structure of the stable phase (ice,
in this case). These fortuitous embryos are unstable and are
continuously being created and destroyed by thermal fluctuations in
such a fashion that a Boltzman distribution in energy is
maintained. The free energy barrier (.DELTA.G*) associated with the
formation of a stable embryo has a maximum value at a certain
critical embryo size. Once the embryo reaches this size (or
equivalently, when the embryo contains a critical number of water
molecules) crystallization occurs spontaneously.
[0061] There are two mechanisms of ice nucleation commonly
recognized (Fletcher, N. H., "Chemical Physics of Ice," Cambridge
University Press, Cambridge, 1970, pp. 73-103). Homogenous
nucleation refers to the spontaneous nucleation of ice
crystallization in pure supercooled water. Nucleation by this
mechanism requires overcoming a high free energy barrier due to the
large surface free energy requirements. Heterogeneous nucleation
involves the binding of supercooled water molecules to foreign
particles to initiate nucleation. The presence of the particles
promotes nucleation since it reduces the surface energy investment,
and hence, the free energy barrier to nucleation.
[0062] Since homogeneous ice nucleation can only take place at
temperature below -35.degree. C. (Franks, 1985), most ice
transformations in nature must occur by heterogeneous nucleation.
This phenomenon can be explained by the high probability for
foreign particles in naturally occurring liquid or vapor phases. An
efficient nucleating agent is one that has a good lattice match, or
structural fit, with the ice crystal (Fletcher, N. H., "Nucleation
and Growth of Ice Crystals Upon Crystalline Substrates,"
Australian, Journal of Physics, Vol. 13, 1960, pp. 108-419;
Fletcher, N. H., "Chemical Physics of Ice," Cambridge University
Press, Cambridge, 1970, pp. 73-103). Fletcher, N. H. has discussed
several other factors affecting nucleation, including the contact
angle (between the ice crystal and substrate), nucleant size
(Fletcher, N. H., "Size Effect in Heterogeneous Nucleation," The
Journal of Chemical Physics, Vol. 29, No. 3, 1958, pp. 572-576),
effects of topographical imperfections (Fletcher, N. H., "Active
Sites and Ice Crystal Nucleation," Journal of the Atmospheric
Sciences, Vol. 26, 1969, pp. 1266-1271) and an entropic
consideration of the induced dipoles in the water molecules
(Fletcher. N. H., "Entropy Effect in Ice Crystal Nucleation," The
Journal of Chemical Physics, Vol. 30, No. 6, 1959, pp.
1476-1482).
[0063] The two most well-known nucleating substrates for ice are
AgI and PbI.sub.2 (identified by Vonnegut).
[0064] Experimental results show a wide range in the onset
temperatures for various nuclei. The activity spectrum can be
attributed to the distribution of "active sites" upon the surfaces
of the nucleating particles (Fletcher, 1969). Active sites refer to
the particular sites on the nucleating particle that have the
highest probability of forming a stable embryo. For clarification,
when saying that AgI has an onset nucleation temperature of
-4.degree. C., we are actually quantifying the nucleating ability
of the active sites. Not all particles are active at -4.degree. C.;
indeed, to achieve 100% activity for AgI particles.gtoreq.100 .ANG.
in radius, one must lower the temperature to -22.degree. C. (Mussop
S. C. and Jayaweera, K. O. L. F., "AgI-NaI aerosols as ice nuclei,"
Journal of Applied Meteorology, Vol. 8, pp. 241-248).
[0065] There are several general requirements for an efficient
active site. First, the contact area between the embryo and the
nucleus must be comparable with the total surface area of the
embryo if the nucleus is to be effective. If the low energy site is
too small, only a few water molecules will be captured and the
resulting embryo will not be stable. Fletcher (1969) estimates that
freezing nuclei must be on the order of 200/.DELTA.T [.ANG.] if
nucleating at -.DELTA.T [.degree. C.]. Also important is that the
interfacial free energy of the particle-ice interface must be as
low as possible. Thus, the chemical nature is clearly important
since it dictates the bonding between the substrate and overgrowing
ice crystal. The crystallographic nature of the substrate has an
equally important role in the energy of the interface due to the
specific alignment of the surrounding water molecules.
[0066] The residual entropy of the ice crystal structure influences
the nucleation process. Fletcher (1959) was the first to explore
the consequences of the randomly oriented water dipoles and claimed
that there is an entropic penalty if a heterogeneous catalyst
orients the dipoles parallel to the catalyst surface. The reasoning
is that if the dipoles are ordered (as would be the case on a
surface of uniform charge) the entropy of the ice structure would
be reduced and the resulting free energy barrier would increase.
Consequently, the uniformly charged surface would be a poor
nucleating agent and require a larger undercooling for inducing
crystallization.
[0067] From these theories, Fletcher predicts that the uniformly
charged (either +1 or -1) basal surfaces {0001} of AgI and
PbI.sub.2 should be poor nucleating planes as a result of the
entropic penalty. However, the prism faces of these crystals,
having an equal distribution of positive and negative charges, will
not orient the dipoles parallel to the surface (probably in the
plane of the surface) and hence, be better nucleating planes.
Another implication of Fletcher's theory is that the steps on basal
and prism planes are not necessarily equivalent in terms of
nucleating activity. Steps on basal planes, exposing prism faces,
are good nucleating sites while steps on prism faces expose basal
planes, and hence, are not expected to be good nucleating
sites.
[0068] Fletcher's predictions were first confirmed experimentally
by Edwards, L. F. and Evans, G. R., "Effect of Surface Charge on
Ice Nucleation by Silver Iodide," Trans. Faraday Soc., Vol. 58, pp.
1649-1655, who found that AgI was most active at its isoelectric
point. Isono, K. and Ishzaka, Y., Journal de Recherches
Atmospheriques, Vol. 6, 1972, pp. 283, showed that the (111) face
of pure .gamma.-AgI and the (1010) face of pure .beta.-AgI (where
both Ag.sup.+ and I.sup.- are present) were more active than the
(0001) face of .beta.-AgI (where either Ag.sup.+ or I.sup.- are
present). Pruppacher, et al., (1975) etched a ferroelectric
substrate creating adjacent positively and negatively charged
domains and founds that ice preferentially nucleated on the
boundaries rather than within a uniformly charged domain. It was
concluded that by nucleating on the boundaries between the domains,
the water molecules could randomly orient in the plane of the
substrate, thereby eluding Fletcher's entropic penalty.
[0069] From overwhelming experimental evidence, it is seen that the
most efficient ice nuclei are insoluble in water. Fukata (1958)
showed that water soluble salts with ice-like lattice parameters
(such as CdI, NH.sub.4F, CaI) could not exceed onset temperatures
higher than .about.-11.degree. C. The difficulty in nucleation is
thought to be a result of the instability of the ice embryo caused
by the diffusion of water molecules and substrate molecules across
the embryo surface. For this reason, it was thought that an
efficient nucleus should be hydrophobic in nature. The hydrophobic
surface can be thought of as forcing the surrounding water
molecules into an "uncomfortable" state, thereby making
crystallization energetically more favorable than the supercooled
phase. Furthermore, the increased energy of the substrate/liquid
interface (.gamma..sub.SL) will reduce the contact angle with the
ice crystal and thereby favor nucleation.
[0070] While most of the classical theories of heterogeneous ice
nucleation were developed with inorganic compounds such as AgI,
there is research on organic crystal nuclei from the 1960's. A
major incentive for using organic nuclei is the possible lower
costs when compared to those of inorganic nuclei. Fukata
(1963,1966) tested 329 organic compounds and found that metaldehyde
could nucleate ice as high as -0.4.degree. C. when exposing
particles (less than 13.mu. in diameter) to water vapor. Six other
compounds, acetoacetanilide, p-bromoacetphenone, coumarin,
m-nitroaniline, phtalic anhydride, 2,4,6-trichloroaniline showed
ice nucleation thresholds almost as high as metaldehyde
(-1.5.degree. C. to -1.degree. C.) when exposing the freshly ground
samples to water vapor.
[0071] One of the main differences between organic and inorganic
nuclei is the fact that the former can participate in hydrogen
bonding with the water molecules. Head, R. B., Journal of Physical
Chemistry--Solids, Vol. 23, 1962, pp. 1371, was the first to show
that hydrogen-bonding is essential for organic ice nucleation.
Garten, et al., (1965) expresses the idea of hydrogen bonding group
(HBG) density by implying that the most efficient organic nuclei
have HBG densities (3-4 per 100 .ANG..sup.2) which are lower than
those of ice (5-7 per 100 .ANG..sup.2). When the density on any
plane exceeds the latter figure, the substrate becomes hydrophilic,
and for reasons expressed above, the nuclei become ineffective.
Garten later adds that the effect of excess HBG's is to stabilize
the denser structure of liquid water rather than that of ice to
higher undercoolings. Molecular symmetry, as Fukata (1966) points
out, is also important. It was claimed that organic molecules with
rotational symmetry are better nuclei than non-symmetrical
molecules since the former cannot avoid exposing their active HBG's
at the surface. Non-symmetrical molecules, on the other hand, will
tend to point their HBG's inward since the hydrogen bond is
energetically costly and a minimum surface free energy is
desirable. Consequently, the contact angle between the ice embryo
and the HBG's on the non-symmetrical molecules will be high,
inhibiting nucleation.
[0072] A preferred nucleating agent of this invention is
metaldehyde. Metaldehyde (CH.sub.3CHO).sub.4, the cyclic tetramer
of acetaldehyde, still possesses the highest nucleation temperature
for crystalline substances at -0.4.degree. C. (Fukata 1963). The
structure, as deduced by Pauline, L., Journal of the American
Chemical Society, Vol. 57, pp. 2680, produces some interesting
crystal properties. First, the tetragonal lattice parameters of
a.sub.0=10.40 .ANG. and c.sub.0=4.11 .ANG. of the unit cell are
close to that of ice, hence making it a potential ice nucleant. The
puckered 8-member ring (C-O distance=1.43.+-.0.03 .ANG., C-C
distance=1.54.+-.0.03 .ANG.) creates a negatively charged plane of
oxygens (facing down) and a positively charged plane of hydrogens
(facing up). Consequently, when packing these tetramers, the
molecules will stack in weakly-interacting columns oriented in the
c-directions. This can be understood by the fact that the
oppositely charged faces cause strong attractive forces in the
c-direction while the inactive, bulky methyl groups shield the
columns from each other. The methyl groups are approximately
equidistant from each other, having two groups a distance of 3.90
.ANG. away, four at 4.03 .ANG. and two at 4.11 .ANG. (directly
above and below). This arrangement yields a packing radius for the
methyl groups of 2.01.+-.0.06 .ANG.. As a result of the steric
effects of the methyl groups, metaldehyde will form long bundles of
easily cleaved fibers if allowed to recrystallize slowly in any
suitable solvent. (THF or chloroform). A body centered tetragonal
unit of metaldehyde is shown in FIG. 6, and an axial view of the
packing arrangement of metaldehyde tetramers is shown in FIG.
7.
[0073] Another interesting phenomenon is the fact that the
c.sub.o/2 translation in each column relative to its four nearest
neighbors brings the molecular dipole into an electrostatically
stable configuration in which the oppositely charged poles are
arranged as nearest neighbors.
[0074] Fukata (1968) refers to this dipole stabilization as a
pyroelectric effect. It was further cited that this effect is
beneficial in cloud seeding since it allowed the metaldehyde smoke
particles to induce polarization and attraction of the water
droplets.
[0075] Metaldehyde, like other organic and inorganic (including
AgI) nuclei, is known to be photosensitive. Fukata (1963) showed
that after exposing metaldehyde to sunlight for more than one hour
at temperatures higher than 55.degree. C., the nucleating property
completely disappears. The exact cause of this phenomenon has not
been confirmed. It may be due to some free-radical, photo-oxidative
process as exhibited by phologlucinol and .alpha.-phenazine (Garten
1965). The vanishing nucleating properties could also be due to the
decomposition of metaldehyde to paraldehyde (cyclic trimer of
acetaldehyde) at 80.degree. C. Regardless of the cause the
temperature range of the specific nucleation application for
metaldehyde needs to be taken into account.
[0076] Metaldehyde (and organic crystals in general) have lost
their research appeal as heterogeneous nuclei since the 1960's and
have given way to inorganic AgI-based compounds. One explanation is
that metaldehyde, while much cheaper than AgI and better in terms
of onset nucleation temperature, is not as active as AgI at higher
undercoolings; specifically, the number of active nuclei per gram
of substrate is almost four orders of magnitude lower for
metaldehyde than AgI (Garten 1965). This relative specific
inactivity in metaldehyde is probably more a concern in cloud
seeding (which was the application in mind) since individual smoke
particles have to nucleate independently. Another possible reason
for the decline in interest in metaldehyde (and other organics) is
its toxicity problem. Metaldehyde is widely used as snail poison
and may pose a risk to humans if consumed (Morgan D. P.,
"Miscellaneous Pesticides, Solvents, and Adjuvents," in Recognition
and Management of Pesticide Poisonings, 4.sup.th ed., Chapter 15,
Environmental Protection Agency, March 1989). This toxicity issue
would be important if metaldehyde were used as a cloud seeder (as
it was intended to be); however, if immobilized in a cementitious
matrix, the toxicity issue becomes less important. If metaldehyde
did spread to natural environments, contamination would be minimal
since metaldehyde would depolymerize to acetaldehyde and oxidize
eventually to harmless levels of acetic acid.
[0077] In order to place the metaldehyde in the air voids, the
commercial grade metaldehyde particle size has to be reduced,
because the average air void is roughly 100 microns in diameter. In
addition, a mechanism for transporting the metaldehyde into air
voids had to be established. To reduce the particle size we
initially tried ball milling the metaldehyde. However, the
ball-milled metaldehyde showed no effective nucleating abilities.
Next we tried grinding the metaldehyde, and found that the ground
metaldehyde effectively nucleated freezing around -1.degree. C.
[0078] To account for the difference between the ball-milled and
the ground metaldehyde's nucleating ability, we performed x-ray
diffraction on each of the samples. The results illustrate a
predominance of the most effective nucleating plane (viz., the
(110) crystallographic plane) exposed in the ground sample, while
the milled sample showed a more uniform distribution of exposed
planes. This result leads to the conclusion that vigorous
ball-milling randomized the exposed planes, while the subtle
grinding catalyzed cleavage predominately along the nucleating
plane. In addition, this result confirms Fukata's suggestion that
the (110) plane is the effective plane of nucleation.
[0079] Two types of metaldehyde-containing cement paste samples
were produced: metaldehyde with and without air entrainment.
Incorporation of metaldehyde into the air voids could only be
accomplished in the sample with air entrainment. To accomplish this
deposition, the air entrainment agent is thoroughly mixed with the
ground metaldehyde before adding the solution to the cement
paste.
[0080] The influence of metaldehyde on the freezing behavior of the
cement paste was evaluated using the DMA (Dynamic Mechanical
Analyzer). The DMA measures dilation in a sample as a result of
freezing. After analyzing both samples it appeared that the
metaldehyde worked effectively: samples with and without air showed
a higher freezing point and gradual dilation. For reference a plain
paste sample was also run, and showed an abrupt dilation at roughly
-9.degree. C. The elevated freezing point shows that metaldehyde is
effectively nucleating freezing around -3.degree. C.
[0081] Effectiveness of Air Entrainment
[0082] The effectiveness of entrained air voids in protecting
concrete from freeze/thaw damage is well known. According to most
microscopic theories of frost action, air voids act as escape
boundaries for the rejected water, whether originating from
hydraulic, diffusion or osmotic mechanisms. Properly entrained
concrete has shown good resistance to internal cracking and scaling
in the laboratory under standard ASTM C 666 freeze-thaw tests which
subject samples to 300 freezing and thawing cycles typically at a
rate of 6.degree. C./h to 8.degree. C./h.
[0083] Experiments have also shown that the most reliable measure
for frost protection is the air void spacing factor, {overscore
(L)}; although recognized as a useful parameter, it is not easy to
measure. The main cause for this difficulty arises from the fact
that air-voids are randomly distributed in the cement paste.
Microscopical evaluation of polished concrete samples (as outlined
in "Standard Practice for microscopical determination of air-void
content and parameters of the air-void system in hardened
concrete," Annual Book of ASTM Standards, ASTM, Philadelphia, Pa.,
1990) is the only direct measurement of the spacing factors;
however, this procedure is time consuming and certainly not
applicable for on-site assessments. It would be ideal if there were
a way to predict and consistently control the air-void network.
Current ASTM C 457 procedure provides simple equations as
guidelines but they tend to grossly oversimplify the random
air-void network.
[0084] Numerous studies have been conducted to better approximate
the random distribution of air voids. Attiogbe, E. K., "Mean
Spacing of Air-Voids in Hardened Concrete," ACI Materials Journal,
Vol. 90, No. 2, March.-April. 1993, pp. 174-181; Attiogbe, E. K.,
"Predicting Freeze-Thaw Durability of Concrete--A new Approach,"
ACI Materials Journal, V. 93, No. 5, 1996, pp. 457-464, for
example, accounts for the randomness of the voids by coupling the
idea of a mean factor, {overscore (s)}, and the parameter F, which
represents the fraction of the total paste volume within the radial
distance {overscore (s)} from the edges of the air-voids. However,
it is questionable whether any mathematical model can accurately
account for the instabilities that are inherent with entraining air
in concrete.
[0085] Despite fairly good predictability of freeze-thaw behavior
in the laboratory, concrete structures still suffer from frost
damage in practice. The problem of air-entrainment can essentially
be reduced to two questions: (i) What are the required air void
characteristics (e.g., spacing factors) for the specific concrete
system in use? (ii) How can the air-void system be preserved in a
reliable and consistent manner during the setting of cement?
[0086] In terms of the first question, the chemistry of the cement
binder has a direct influence of the pore structure, and hence, the
freezing properties of the concrete. As previously discussed, the
permeability (quantified by r.sub.BT) will significantly affect the
resistance to induced internal pressures. The lower the
permeability, the more resistance the displaced water will
experience, and thus, higher stresses are generated. To compensate
for low-permeability concretes made using admixtures such as
water-reducing agents and silica fume, a greater air content must
be entrained in the concrete to ensure a shorter spacing length.
This practice is disadvantageous in two ways. First, increasing the
entrained air necessarily decreases the strength of the concrete,
thus, possibly negating the benefits of the admixtures altogether.
Second, and probably more important, is that the spacing factor
will vary depending on the characteristics of the cement paste in
the concrete. This makes standardization of building practices very
difficult, if not impossible.
[0087] Taking all types of cements into consideration, it is
generally thought that a spacing factor of 200-250 .mu.m represents
an adequately frost resistant concrete (Marchand). As mentioned
earlier, the real problem is the reproducibility of generating an
air-void network with a desired spacing factor. Since there is no
direct technique to measure spacing factors onsite, our national
standards only require the measurement of total air content (a
readily measurable parameter) rather than spacing factors. It is
generally believed that air contents in the range of 5% to 8% by
volume correlate with frost protected spacing factors (on the order
of 200 .mu.m). However, experience has shown that this assumption
is not a valid one. In fact, it was seen that spacing factor can
vary considerably with a given air content. Specifically, a 6% air
content can yield a spacing factor of 100 .mu.m to 400 .mu.m
(Saucier, F., Pigeon, M. and Cameron, G., "Air Void Stability--Part
V: Temperature, General Analysis and Performance Index," ACI
Materials Journal, Vol. 88, 1991, pp. 25-36). This discrepancy
could easily make the difference between a durable, frost resistant
concrete and a frost prone concrete.
[0088] The reasons for the poor correlation between air content and
spacing factor are believed to result from three sources of bubble
instability: buoyancy, coalescence and dissolution effects. The net
effect of these instabilities result in an increase in the spacing
factors, and hence, a decrease in frost resistance. Buoyancy
effects refer to the phenomenon of the tendency for larger bubbles
to rise to the surface and to be expelled from the paste. This
phenomenon results from the fact that the buoyancy force
(proportional to volume, .pi.d.sup.3/6) for larger bubbles can
overcome the shearing frictional force (proportional to .pi.d). The
coalescence of air bubbles results from the drive to reduce the
free energy of the system by decreasing the interfacial surface
area of the bubble. For two identical volume air bubbles with the
same surface tension, the coalescence of the two bubbles result in
21% reduction in energy of the two bubble system (Marchand).
Dissolution of air is the third source of instability and it comes
from the fact that the solubility of air increases with pressure,
and the pressure inside an air bubble is inversely proportional to
its diameter (Kelvin's law). Thus, small air bubbles have a
tendency to collapse due to the solubility effect.
[0089] These bubble instabilities are even more pronounced during
the transportation and pumping of concrete where significant air
losses can occur. It would be ideal if a void system could be
produced in concrete that has a controllable spacing factor.
Moreover, it would be advantageous to minimize the spacing factor
to .about.100 .mu.m or less. This is not a feasible solution for
the air entrainment technique since the inclusion of more air voids
will necessarily reduce the strength of concrete and allow for
further cracking of the matrix.
[0090] To avoid the problems of instability and unreliability of
air voids produced using air-entrainment agents, we prefer to use
porous ceramic shells for air entrainment. The preparation and use
of such shells is described in the following section.
[0091] Hollow Shells as Alternative to Air Entrainment
[0092] Hollow shell technology has attracted attention in
applications such as low dielectric constant materials, fiber-optic
microsensors, light weight composites and impact resistant
materials (Wilcox, D. and Berg, M., "Microsphere Fabrication and
Applications: An Overview," in Materials Research Society Symposium
Proceedings, Vol. 392, Hollow and Solid Spheres and Microspheres:
Science and Technology Associated with Their Fabrication and
Application, 1994, pp. 3-13). One method of preparing well-shaped
hollow and spherical particles is by spray pyrolysis (SP). The
technique differs from the well-established technique of spray
drying in the use of solutions (rather than slurries), the process
of precipitation or condensation within the droplet, and the use of
significantly higher temperatures (.about.>300.degree. C.).
During SP, the solution is continuously atomized in a series of
reactors where aerosol droplets experience solvent evaporation and
solute condensation within the droplet, drying, thermolysis of the
precipitate to form a microporous particle, and finally sintering
to achieve full density.
[0093] Either solid or hollow spheres can result from SP depending
on droplet size, solute concentration, precursor supersaturation
and evaporation rate (Charlesworth, D. and Marshall, W., Journal of
the American Institute of Chemical Engineering, Vol. 6, No. 9,
1960; Leong, K., Journal of Aerosol Science, Vol. 18, pp. 511,
1987). From modeling of the evaporation phase of SP, Messing, G.,
Zhang, S. C. and Jayanthi. G. V., "Ceramic Powder Synthesis by
Spray Pyrolysis," Journal of the American Ceramic Society, Vol. 76,
No. 11, 1993, pp. 2707-26, predicts that hollow shells of different
thickness can be obtained depending on the concentration gradient
at the onset of precipitation. If the precipitate shell is
sufficiently permeable, the remaining solvent can be removed and
the hollow shell structure can be preserved.
[0094] The properties of the precursor solution, including thermal
characteristics, must be known because they can profoundly effect
the particle morphology during the various SP stages. In general,
SP studies have been confined to aqueous precursor solutions of
highly soluble metal chlorides and oxychlorides as well as other
water-soluble metal salts such as nitrates, acetates and sulfates
(Messing 1993). Very few studies have dealt with colloidal
precursors, as would be required for hollow clay shells. An
understanding of the colloid chemistry of the dispersion is
required for shell processing, especially concerning the stability
of the dispersion at the high operating temperatures during SP.
[0095] Hollow shells with porous walls can be produced economically
by processes such as spray drying and SP; moreover, they can be
prepared with mean diameters on the order of 50-150 microns.
Therefore, such shells could be used to provide air entrainment in
mortar or concrete without the use of chemical air-entrainment
agents. The shells could be introduced as an ingredient in the
concrete mix, so that the quantity and size of the air voids would
be guaranteed. The shells could be pretreated to impregnate them
with a nucleating agent, such as metaldehyde, to enhance their
effectiveness for frost protection.
EXAMPLES
[0096] Metaldehyde possess one of the highest nucleation
temperatures for ice nuclei in the vapor phase (.about.0.4.degree.
C., Fukata 1963); however, little is known about the freezing
capabilities (ice nucleated from the liquid phase) of metaldehyde.
Furthermore, from the cited literature, the mode of preparation and
molecular structure of an ice nucleant is known to have a drastic
effect on the nucleating properties. Thus, an effort to isolate the
critical parameters affecting the freezing nucleation capabilities
of metaldehyde was conducted. Freezing experiments were done on
metaldehyde-impregnated Vycor glass samples having uniform 100
.ANG. pores.
[0097] Procedures
[0098] (a) Materials
[0099] The metaldehyde (C.sub.2H.sub.4O).sub.n used in this
investigation was manufactured by Fluka Chemika. The chemical
formula of metaldehyde is often denoted as (C.sub.2H.sub.4O).sub.n
since there is a strong tendency for the tetramer units
(CH.sub.3CHO).sub.4 to form long fibers (as discussed earlier).
Metaldehyde should not be confused with polyacetaldehyde (Natta,
1961) which has the same unit but an entirely different
head-to-tail arrangement (C-O-C connectivity). Metaldehyde was
prepared by several different methods (crushing in mortar and
pestle, washing on a Buchner funnel with water, dissolving the THF
and then precipitating in different solvents) to determine optimal
nucleating properties. Coumarin (C.sub.9H.sub.6O.sub.2), or
2H-1-Benxopyran-2-one, was also tested for its nucleation
properties since it was cited as a potential freezing nuclei
(Fukata 1966).
[0100] (b) Vycor Glass Experiments
[0101] Commercial brand Vycor glass from Corning Glass Works was
used during the experiment. Vycor glass is prepared by melting a
homogeneous mixture of sodium borosilicate liquid and then
quenching the mixture to a temperature below the coexistence and
spinodal curve where it phase separates into two interpenetrating
phases. The boron rich phases is leached out leaving behind a
silica skeleton with a known distribution of pores sizes. The
average pore size is .about.100 .ANG..
[0102] The Vycor glass samples were first crushed into pieces small
enough to fit into the 50 .mu.l DSC pans. The samples were immersed
in a 30% hydrogen peroxide solution and heated up to
.about.70.degree. C. for several hours to remove any organic
impurities absorbed by the glass. A soak in distilled water was
then conducted followed by drying in a 50.degree. C. oven until the
samples were clear.
[0103] Prior to conducting freezing experiments, dry Vycor samples
were submerged in distilled water for .about.1 hour to ensure
complete saturation. Water absorption capacities of Vycor samples
were estimated by heating saturated samples in a Perkin Elmer
Thermal Gravametric Analyzer (TGA 7) and noting the stabilized
weight loss. Vycor samples were impregnated with metaldehyde by
soaking in a .about.6.5 mg metaldehyde/mL THF solution at a
temperature of .about.60.degree. C. for .about.1 hour and then
precipitating in a 0.degree. C. water bath (while stirring). When
testing the saturation capacities of the impregnated samples in the
TGA, maximum heating temperatures were .about.40.degree. C.
Metaldehyde was visualized in the Vycor samples by viewing in a
Nikon SMZ-U Zoom 1:10 optical microscope with polarizing
filters.
[0104] (c) Differential Scanning Calorimetry (DSC)
[0105] DSC scans were performed on a Perkin Elmer Pyris 1
Differential Scanning Calorimeter with a cooling and heating rate
of 1.degree. C./min. Calibration was provided by melting pure water
and n-decane samples.
[0106] For nucleation experiments with metaldehyde, approximately 1
mg of sample was placed on the bottom of the pan. A single drop of
water (diameter .about.1 mm to 2 mm) was then pipetted over the
metaldehyde and gentle rearrangement of the metaldehyde was done to
ensure intimate contact between the water droplet and the nuclei
particles. The weight of the sample pan was recorded before and
after the DSC run; if any weight loss occurred, the test was
discarded.
[0107] (d) Surface Morphology
[0108] The surface morphology of the metaldehyde samples was
investigated by scanning electron microscopy (SEM) on a Philips XL
30 FEG-SEM. Samples were carbon coated with a thickness of
.about.20 nm prior to viewing.
[0109] Results/Discussion
[0110] (a) Ice Nucleation by Metaldehyde
[0111] As developed in the theoretical section of the
investigation, the properties of ice nuclei will depend on several
factors including size, surface contamination, exposure of active
sites and age. To isolate some of these factors and their effects
on the nucleation capacity of metaldehyde, several different
preparation methods were devised (see table).
5 Description of the different preparation routes for metaldehyde.
Sample name Description As-received Metaldehyde taken from the
bottle. Care was taken not to damage the needles while placing the
samples in the DSC pans. Crushed Metaldehyde crushed in a mortar
and pestle. Testing of the sample in the DSC directly followed the
crushing. To test the effects of age on the crushed samples, the
crushed samples was stored in a sealed glass scintillator bottles
for .about.1 month. Water washed Metaldehyde washed on a Buchner
funnel with a vacuum. Approximately 1 L of distilled water was used
per gram of metaldehyde. Washed/crushed Metaldehyde washed as above
then crushed in a mortar and pestle. THF ppt/0.degree. C. Crushed
metaldehyde dissolved in a .about.6.5 mg metaldehyde/ml THF
solution at .about.60.degree. C., then precipitated in a 0.degree.
C. distilled water (while stirring). Sample was collected on a
Buchner funnel. THF ppt/25.degree. C. Same as above but
precipitated in 25.degree. C. distilled water.
[0112] During cooling, in the DSC, the sample will crystallize at a
temperature, T.sub.C, corresponding to the onset of the exothermic
freezing peak. The temperature was determined by the Pyris 1 DSC
software, which takes the intersection of the tangent at the
inflection point of the freezing exotherm with the baseline of the
curve. It is important to remember that the nucleation process is a
statistical event (based on the fortuitous groupings of water
molecules) so multiple runs of a specific sample will give a range
of onset temperatures scattering around a mean onset
crystallization temperature, T.sub.C,avg. The table below lists the
DSC results for the metaldehyde samples, the as-received and
crushed coumarin samples and distilled water.
6 Onset temperatures obtained from DSC scans for metaldehyde (MA),
coumarin (CO) and distilled water. Number of Range of
Crystallization Sample samples activity [.degree. C.] temperature,
T.sub.C,avg As-received MA 5 -11.5 to -7.6 -9.1 Crushed MA 8 -3.8
to -2.6 -3.3 Aged crushed MA 3 -4.2 to -3.4 -3.8 Water washed MA 10
-9.8 to -7.0 -8.3 Washed/crushed MA 6 -3.7 to -3.2 -3.4 MA THF
ppt/0.degree. C. 6 -6.5 to -1.8 -5.0 MA THF ppt/25.degree. C. 5
-8.1 to -4.3 -6.1 As-received CO 3 -15.4 to -7.4 -11.1 Crushed CO 5
-7.8 to -4.0 -6.1 Distilled water 3 -15.5 to -15.3 -15.4
[0113] It seems that the largest effect in improving nucleation
temperatures is to crush the sample. Crushing will increase the
surface area, and more importantly, will increase the density of
sites (presumably the basal planes in metaldehyde) in the sample.
Both freshly crushed metaldehyde (crushed and crush/wash) samples
possessed the highest onset temperature at .about.3.3.degree.
C.
[0114] Aging showed little, if any, change in the nucleating
properties of the crushed sample. A discrepancy of only
.about.0.5.degree. C. separated the aged crushed and crushed
samples.
[0115] Washing the impurities away with water does not seem to have
a large effect on metaldehyde as evidenced by similar onset
temperatures between the unwashed and washed samples. The slight
increase in onset temperature of the water washed compared to the
as-received sample could very well be due to inadvertent crushing
of the metaldehyde when collecting the sample off the Buchner
funnel.
[0116] Allowing the crushed metaldehyde to recrystallize in a warm
THF solution will decrease the onset temperature. Quickly
precipitating in a 0.degree. C. water bath seems to possess better
nucleating potential than the slower 25.degree. C. precipitate.
Even though the T.sub.C,avg for the two different precipitates only
differ by .about.1.degree. C., it is noteworthy that the 0.degree.
C. precipitate was the only sample to possess an onset temperature
greater than -2.degree. C. Furthermore, the amount of precipitate
collected from the 25.degree. C. water bath was very small, as most
of the metaldehyde tended to stay in solution. The 0.degree. C.
precipitate, however, dropped out of solution almost
instantaneously in much larger quantities.
[0117] Coumarin followed the same trends as metaldehyde in the
sense that the crushed samples yielded significantly higher onset
temperatures than the as-received. Solubility in ethanol and hot
water (not attempted) certainly makes coumarin attractive in terms
of ease of impregnating porous shells; on the other hand, coumarin
has suspected toxic and carcinogenic properties.
[0118] (b) Morphology of Metaldehyde
[0119] Depending on the mode of preparation, metaldehyde will have
varying surface morphologies. Estimated form optical microscopy and
SEM images, the approximate characteristic dimensions of the
various forms of metaldehyde can be seen in the table below.
7 Dimensions of various metaldehyde forms estimated from optical
microscopy and SEM. Sample Length Thickness As-received 500 .mu.m
to 1 mm .sup. 50 .mu.m to 100 .mu.m THF ppt/0.degree. C. 100 .mu.m
to 200 .mu.m Less than 10 .mu.m Crushed* Less than 50 .mu.m Less
than 5 .mu.m THF ppt/25.degree. C. Less than 10 .mu.m Less than 5
.mu.m *Most of the crushed sample was much smaller than the upper
limit of 50 .mu.m and 5 .mu.m for the length and thickness,
respectively. These upper limits represent the dimensions of the
residual as-received crystals that were only partially broken down
during the crushing process.
[0120] (c) Porous Glass Experiments
[0121] At full saturation, the cleaned and dried Vycor glass was
measured to absorb water up to .about.49.5% of its weight (0.495 g
of water/g of dry Vycor). This measurement was made from heating
samples in a TGA (for greater precision) on several Vycor glass
samples submerged in distilled water for several days (ensuring
full hydration). Furthermore, for the small DSC Vycor samples
(.about.3 mg), it was found that only 20 minutes was required to
fully saturate the sample. Thus, since the degree of saturation
will effect the locations of the freezing and melting peaks in
scanning calorimetry experiments (Takamuru, T., Yamagami, M.,
Wakita, H., Masuda, Y. and Yamaguci, T., "Thermal Property,
Structure, and Dynamics of Supercooled Water in Porous Silica by
Calorimetry, Neutron Scattering and NMR Relaxation," Journal of
Physical Chemistry B., Vol. 101, 1997, pp. 5730-5739), submersion
times were at least 30 minutes to ensure consistent water
contents.
[0122] It was found that metaldehyde could be precipitated in the
100 .ANG. Vycor pores by precipitating a saturated Vycor sample
(containing a warm metaldehyde/THF solution) in a 0.degree. C.
water bath. The metaldehyde can clearly be visualized in an optical
microscope by a brownish, grain-like Vycor interior. Taking
advantage of metaldehyde's crystallinity, polarizing filters can
induce a dramatic scattering effect in the impregnated sample.
[0123] Since metaldehyde is a hydrophobic material, the absorption
capacities of the impregnated samples were analyzed. Results showed
no evidence of a repulsion effect, and in fact, the samples showed
an increase in absorption capacities to .about.65.8% (g water/g
Vycor). The increase in absorption for the impregnated samples is
thought to be a result of the damage of the porous network due to
the extra dying cycles required for the impregnation procedure.
This idea is supported by the finding that repeated submersion and
drying of the impregnated samples showed further increases in
absorption to .about.94.2%.
[0124] Cooling a fully hydrated Vycor sample from 3.degree. C. to
-30.degree. C. and heating from -30.degree. C. to 5.degree. C.
shows the presence of one sharp freezing peak and a double melting
peak (or sometimes a single uneven broad peak. The difference
between the onset freezing peak and the melting peak is the
characteristic nucleation undercooling, .DELTA.T.sub.c. Freezing a
metaldehyde impregnated Vycor sample (by precipitating the warm THF
solution in 0.degree. C. water) also reveals one characteristic
freezing and a double or broad melting peak. The location of the
melting peak was fairly consistent (indicating fairly uniform pore
sizes) for all samples. The freezing peak, however, was shifted to
higher temperatures by .about.10.degree. C. when metaldehyde was
present in the pores of the glass. The average undercoooling for
the impregnated Vycor was .about.7.1.degree. C. while the
unimpregnated samples scattered around a .DELTA.T.sub.c of
17.1.degree. C. See FIG. 8.
8 A comparison of the nucleating properties in the pores of
unimpregnated and impregnated Vycor glass samples. Freezing,
Melting, Average Sample T.sub.c [.degree. C.] T.sub.m [C.]
.DELTA.T.sub.c [.degree. C.] .DELTA. T.sub.c [.degree. C.]
Unimpregnated -15.0 -1.8 13.2 17.1 -21.5 -1.8 19.7 -21.5 -2.4 19.1
-18.8 -2.4 16.4 -18.9 -2.2 16.7 Impregnated -8.5 -1.8 6.7 7.1 -10.3
-2.4 7.9 -9.1 -2.4 6.7
[0125] It is surprising that the metaldehyde in the 100 .ANG. pores
of the Vycor sample (.DELTA.T.sub.c=7.1.degree. C.) behaves
relatively similar to the metaldehyde precipitated directly from
the warm THF solution in 0.degree. C. water (.DELTA.T=5.0.degree.
C.). First of all, the size of the metaldehyde crystals in the 100
.ANG. pores will certainly be significantly smaller due to the size
constrictions. (Fletcher predicts that the nucleating capacity of a
spherical catalyst will decrease precipitously when radii are less
than .about.100 .ANG.. The surface morphology of the precipitate
should also be altered since the quenching rate will be slower
(controlled by the diffusion of water into the pores. Despite these
potential hindrances to nucleation in the 100 .ANG. pores, the
confined metaldehyde only sees a depression of .about.2.degree. C.
from the freely precipitated sample.
[0126] (d) Implications of Metaldehyde Nucleation on Frost Action
in Concrete
[0127] For impregnated microshells to show any noticeable frost
action improvements, the shells must initiate ice growth before
significant crystallization and hydraulic pressures can be
generated. Crystallization and hydraulic pressures should only
induce tensile stresses over 3 MPa when temperatures are
.ltoreq.-5.degree. C. (corresponding to a r.sub.BT of .about.13 nm)
and -4.degree. C. (r.sub.BT of .about.16.7 nm), respectively. The
freely precipitated metaldehyde from 0.degree. C. water with an
average onset temperature of about -5.degree. C. and a maximum near
-1.8.degree. C. can certainly compete with these onset
temperatures. The freely precipitated metaldehyde, rather than the
impregnated metaldehyde in the Vycor glass, is used in the
comparison since it was argued that the onset temperature of the
100 .ANG. metaldehyde in the Vycor glass was depressed due to the
effects of the confined geometry in the 100 .ANG. pores. The shells
will presumably be .about.100 .mu.m, thus minimizing any geometric
effects in nucleation. Furthermore, it is most important for the
metaldehyde to nucleate before T.sub.BT, or the point where
crystallization and hydraulic pressures begin to become potentially
dangerous. With an onset nucleation temperature of -5.degree. C.,
the impregnated shells should be able to significantly improve the
frost action of pastes with a r.sub.BT.ltoreq.13 nm by confining
the ice in the voids and removing the percolation event (i.e., the
progressive invasion of ice through the pore space) altogether.
This implies that the metaldehyde-impregnated shells should have
the biggest impact on very fine pastes with low characteristic
breakthrough radii. For pastes with r.sub.BT>13 nm, percolation
through the cement body could occur; however, the crystallization
and hydraulic pressures that are generated will probably be below 3
MPa, causing little damage to the concrete.
[0128] The impregnated shells would also be integral in the event
of a delayed surface nucleation. Since tensile stresses generated
from delayed surface nucleation can reach as high as .about.8 times
that of the stresses generated from spontaneous nucleation
(assuming a surface nucleation temperature at -0.degree. C.), it is
important to remove the freezeable water before the surface
nucleation takes place. If the impregnated shells can consistently
nucleate at -5.degree. C., there should be very little freezeable
water by the time the concrete reaches -10.degree. C. Hence,
potentially devastating stresses of .about.20 MPa should be avoided
with use of the shells.
[0129] (e) Ceramic Microshell Design and Function
[0130] Ideally, the ceramic hollow shells will be relatively strong
in tension (greater than 3 MPa), sufficiently porous (to allow for
liquid flows) and able to nucleate ice at maximum temperatures
(ideally before T.sub.BT).
[0131] The strength and porosity of the shell will be strongly
influenced by the firing temperature of the ceramic material. Being
relatively inexpensive, kaolin is an attractive candidate for the
shell material. Kaolin clay consists of mainly ordered kaolinite
(Al.sub.2SiO.sub.5(OH).s- ub.5), with some mica and free quartz.
The firing temperature will presumably be in the range between
.about.700.degree. C. and .about.1000.degree. C., thereby ensuring
the increased porosity of metakaolin (Al.sub.2O.sub.3.2SiO.sub.2)
derived from the dehydroxylation of kaolinite at .about.520.degree.
C. and the added strength due to partial densification. At
980.degree. C., metakaolin goes through a series of transformations
as it rearranges into a spine and then into small mullite
crystals.
[0132] The increased strength of the shells will impart benefits
since crystals will be able to grow in voids where the surrounding
walls can support higher tensile stresses before fracture. Thus,
with shell tensile strengths potentially greater than 3 MPa,
crystallization induced failure should be delayed to higher
undercoolings. Furthermore, inclusions of the shells could also
improve concrete mechanical properties such as fracture toughness
and impact resistance provided that the shells have a high aspect
ratio (defined as the ratio between the outer radius and the wall
thickness) and a larger Young's modulus compared to the matrix
(Liu, J. G. and Wilcox, D. L., "Design Guidelines and Water
Extraction Synthesis Capabilities for Hollow Icrospheres for Low
Dielectric Constant Inorganic Substances," in Hollw and Solid
Spheres and Microspheres: Science and Technology Associated with
Their Fabrication an Application, Materials Research Society
Symposium Proceedings, Vol. 372, Materials Research Society:
Pittsburgh, 1995, pp. 231-237).
[0133] As mentioned before, increasing the strength of the shells
implies a decrease in porosity. The shells must be sufficiently
porous to allow water to contact the metaldehyde. If water cannot
penetrate into the interior of the voids, the entire purpose of the
shells, that is to remove the freezeable water from the cement
pores, will be lost. Ideally, the metaldehyde will line the inner
walls of the shell. However, with the intended impregnation by
soaking the shells in a warm THF/metaldehyde solution followed by
precipitating in a 0.degree. C. water bath, there will inevitably
be metaldehyde in the pores of the wall. The hydrophobicity
problem, that is the concern of hydrophobing the shell to the point
of total repulsion of water, will probably not be an issue (while
not confirmed definitively) as suggested by the impregnated Vycor
glass experiments. Even if there is a repulsion effect, there are
bound to be pores where metaldehyde is absent, thus allowing water
to freely penetrate into the void.
[0134] The size of the void space in the shells should be large
enough to avoid any undercooling effects (dictated by the
Gibbs-Thomson effect). This would allow for maximum nucleation
temperature by the metaldehyde. The shell size will be on the order
of an air void, .about.100 .mu.m in diameter, so size effects will
be negligible. With the diameter known, the concentration of shells
can be calculated by requiring a total void space volume at least
equal to the volume of theoretical ice in the paste (calculated
from the amount of freezeable water in the paste).
[0135] Once nucleation occurs in the void space, there should be a
draining of the water from the paste into the void. Each shell
(like an air-void) will have a characteristic sphere of influence.
The net volume of paste intercepted by the individual spheres
should cover the entire paste to ensure total protection.
Furthermore, the growing crystal creates suction in the liquid
(which is responsible for migration of the water to the void), and
that reduces the risk of cracking by putting the surrounding
concrete into compression.
[0136] It is understood that the embodiments described herein are
merely exemplary and that a person skilled in the art may make many
variations and modifications without departing from the sprit and
scope of the invention. All such variations and modifications are
intended to be included within the scope of the invention.
* * * * *