A. B. Harnik,' Ulrich Meier,' and Alfred RosW
Combined Influence of
Freezing and Delcing Salt on
Concrete—Physical Aspects
REFERENCE: Harnik, A. B., Meier, Ulrich, and Rosli, Alfred, "Combined Influence of
Freezing and Deicing Salt on Concrete—Physical Aspects," Durability of Building
Materials and Components, ASTMSTP 691, P. J. Sereda and G. G. Litvan, Eds., American Society for Testing and Materials, 1980, pp. 474-484.
ABSTRACT; The observation that the durability of concrete often is lower under the
combined influence of frost and deicing salts than under frost influence alone is discussed
with regard to several physical aspects and mechanisms, for example, hydrodynamic effect, capillary effect, supercooling, lowered melting point of water in smaller pores, layerby-layer freezing. It is concluded that some of the most detrimental factors with regard to
the durability of concrete are supercooling of water and aqueous solutions and a higher
degree of saturation of the concrete in the presence of salts. The differing effects of dry
application of deicing salts on snow-and-ice-covered concrete (temperature shock) compared to the preventive salt application on humid concrete (prevention of ice formation,
but with some negative aspects) are discussed.
KEY WORDS: capillary effect, cement paste, concrete, crystallization pressure, deicing
salts, durability (concrete), freezing, frost resistance, hydraulic pressure, layer-by-layer
freezing, porous media, preventive salt application, salt solutions, saturation, supercooling (water), temperature shock, building materials
For many years it has been observed that the resistance of concrete against
the combined influence of freezing and deicing salts seems to be generally
lower than its resistance to frost alone. The causes of this puzzling phenomenon are not yet fully known. Some considerations of physical aspects pertinent to this problem are discussed in this paper.
'Institut fUr Baustoffe, Werkstoffchemie und Korrosion, Department fUr Materialwissenschaften, Eidgenossische Technische Hochschule Ziirich, Zurich, Switzerland.
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HARNIK ET AL ON INFLUENCE OF FREEZING AND DEICING SALT
475
Freezing
Pore Sizes in Concrete
The physical properties of concrete and its resistance to frost depend greatly on the size of pores and other voids. These include gel pores, capillary
pores, and air voids as well as fissures, cracks, etc. The total void size range
covers more than a factor of 106 (Fig. 1).
Vapor Pressure
Water vapor condenses in a pore of a wettable material under a pressure
below saturation with respect to water in accord with Kelvin's equation
P
In—=
Po
2ff|g
-
r
§1
RT
where
R = gas constant,
T = temperature in Kelvins,
p andpg = vapor pressure of pore water and of bulk water,
ff/g = surface tension at the liquid-gaseous interface (water-air) (that
is, at the meniscus),
di = molar volume of water, and
r = curvature of the meniscus.
The curvature r of the water meniscus within a pore is determined by the
GEL PORES 1 CAPILLARY PORES
AIR
VOIDS
__, 1
^ 1
- 01
1^-^
10"^
,' 10"^
/q.
1
RADIUS,
mm
-1
-10
-100
-
7^
'4'
/*>
TEMP ,°c
FIG. 1—(top) Pore sizes in concrete, (bottom) correlation between pore size and melting temperature of water.
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476
DURABILITY OF BUILDING MATERIALS AND COMPONENTS
pore size and by the contact angle 9, that is, the angle at which the meniscus
meets the pore wall. In a cylindrical capillary with radius /?, the curvature r is
R/cosd.
The melting temperature of ice in contact with pore water is lowered because the vapor pressures of ice and water at the melting point (equilibrium
temperature) are equal. Therefore, the equilibrium (melting point) between
pore water and bulk ice (separated by a gaseous phase) is given by
"n "
r ' Ah
where T and r „ is the lowered and the normal melting point of ice and Ah the
molar heat of fusion.
In analogy, the equilibrium of a semispherical ice cap in a cylindrical capillary in contact with bulk water is given by
T„
r' ' Ah
where i9j is the molar volume of ice and r' the radius of the ice cap.
Freezable Water
When humid concrete freezes, not all of its water content changes into ice.
This is mainly due to two effects:
(fl)In gel pores and in most of the capillary pores the phase transition temperature of the pore water is modified by surface effects. Under normal atmospheric pressure, the temperatures of freezing and melting are below 0°C.
The amount of this temperature decrease depends on the dimensions of the
pores and the physicochemical nature of the inner pore surface. Generally,
the temperature of freezing is lower than the temperature of melting, that is,
there is a tendency for supercooling. This lowering of the melting point of ice
in a solution is a consequence of the vapor pressure lowering by the solute.
The straight line in Fig. 1 illustrates the correlation between pore size and
melting temperature. Specifically, the curve gives the theoretical melting
temperature of a semispherical ice cap in a cylindrical capillary in contact
with bulk water [1,2]^
(fe)The water in concrete is not chemically pure but contains several soluble substances, mostly in low concentration, for example, alkalies, free lime
2 The italic numbers in brackets refer to the list of references appended to this paper.
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HARNIK ET AL ON INFLUENCE OF FREEZING AND DEICING SALT
477
(CaO) or deicing salts (chlorides). Therefore, its freezing point lies below 0°C
(see also as follows).
Hydrodynamic Effect (Hydraulic Pressure)
During cooling, the specific volume of water attains its minimum at
+ 4°C, then increases rapidly by about 9 percent at the freezing point and
slowly decreases on further cooling (Fig. 2). Therefore, when water in
saturated concrete freezes approximately 9 percent of the volume of the water
has to be expelled into waterfree pores or to the outer surface. However, the
phase transition water-ice does not occur instantaneously or everywhere simultaneously; rather it takes place in two successive steps, the nucleation
and the growth of the formed ice nuclei.
The rate at which the ice-water interface propagates (the crystallization
velocity) determines the speed with which the expelled water is pressed
through the surrounding pores. The greater the crystallization velocity, the
greater will be the pressure generated by the process. The crystallization
velocity primarily depends on the cooling rate or on the supercooling (see as
follows). The phase diagram (Fig. 3) shows the highest pressure theoretically
possible during the freezing of water in capillaries if equilibrium is assumed.
However, in practice the speed of the ice propagation is limited and thus
the pressures are much lower. If this were not the case, hardly any concrete
structure would be able to withstand repeated natural frost-thawing cycles.
1,10
Sl.08
1
•
1
1
1
1
1
1
1
ICE
\
1
1
1
51.06
cc
DENSITY,
WATER
ICE
r
^1.04 —
o
us
a.
1,02
T
1
1
SPECIFIC
VOLUME,
q/cm3
cm'/g
0.9999
0.91674
1.00012
1.09052
-9%
1
1
—
1
+9%
WATER
> /
!
"A"
1.00
^ 1
-30
1
I
FM
0+10
0
1
1
1
1
1
+50
DETAIL A
2
4
6
1 1
+ 100
TEMP., °C
8+10
FIG. 2—Water-ice {H2O): anomalous behavior of the specific volume {anomaly of density).
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478
DURABILITY OF BUILDING MATERIALS AND COMPONENTS
100
PRESSURE , N/mri)2
200
300
400
-10
-40L
j
/
ICE I
p=1,18
TEMP., °C
FIG. 3—Phase diagram of water and ice with constant volume.
Furthermore, internal pressure also can be relieved through plastic flow of
the ice, since ice, like water, can be pressed into waterfree pores and air voids
or to the external concrete surface.
Nevertheless, quite large transient pressures can be generated locally,
because freezing does not proceed smoothly; rather the ice-water interface advances spontaneously into supercooled water as it jumps from one pore
"neck" to another [J]. The trapping of capillary water by the ice also can
represent a distinct localized source of high pressure, particularly if the
supercooling is large [4].
Supercooling
As a rule, water in cement paste and concrete can be supercooled, that is,
it freezes more or less below the melting temperature which is anyhow lowered in narrow capillaries (see Fig. 1). When freezing does begin at temperatures near the melting point, it is most probably initiated by some external
means, for example, by ice crystals on the outer surface of the porous body
(since water freezes spontaneously in contact with ice).
Recently, we have investigated the supercooling of water in cement paste
[4]. The external ice formation was suppressed by covering the specimens on
all sides with various substances, among them calcium chloride (CaCl2) solution and aluminum oxide (AI2O3) powder. Figure 4 gives an example of the
results. The specimen was water-saturated and covered with AI2O3. The
sharp peak on the upper curve signifies the rapid freezing of at least a portion
of the pore water at about — 12°C while the broad peak below 0°C of the
warming curve indicates the gradual melting. It should be noted that such a
sharp exothermal peak is generally observed in specimens with large pores; a
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HARNIK ET AL ON INFLUENCE OF FREEZING AND DEIGING SALT
479
EXOTHERMAL:
FREEZING
ENDOTHERMALl
MELTING
0
+10
TEMP. ,
"C
FIG. 4—DTA record of saturated cement paste on cooling (top) and warming (bottom).
more gradual freezing behavior pertains in specimens with fine pores dominant.
In our experiments, the water generally froze at temperatures below
— 10°C. The larger the supercooling of the pore water, the faster the ice front
in the waterfilled pores advances. Consequently, a large supercooling in the
order of 10°C and more should lead to higher hydraulic pressures than a
small one [4]. This hypothesis agrees with the observation by Radjy et al that
"freezing after too much supercooling fractured the specimen" [5].
Thus, supercooling of the pore water must be considered as another possible detrimental factor because it can intensify the destructive force of ice formation in cement paste and possibly also in concrete. At a given temperature, the supercooling of the yet unfrozen pore water will be most pronounced at a high degree of water saturation and a relatively low salt concentration, conditions that are at any rate recognized as dangerous.
Capillary Effect
The mechanisms related to an increase of the specific volume of water cannot be the single causes of expansion and frost damages in concrete and other
porous substances. This conclusion is reached in experiments where an expansion of the specimens was observed even when the pore water was substituted by liquids with normal contraction during freezing, for example by
benzene in cement paste [6].
It is the phenomenon of the capillary effect which contributes to an understanding of such a behavior. This effect occurs in porous bodies having different sized pores in which a solid and a liquid phase coexist. In saltfree
humid concrete, these two phases may be ice and water in the cement paste
matrix (Fig. 5). For example, assume that concrete is cooled until the water
in the larger capillary pores freezes. However, the water in the pores of the
surrounding gel still remains liquid, due to their much smaller size (see the
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480
DURABILITY OF BUILDING MATERIALS AND COMPONENTS
CEMENT GEL
CAPILLARY PORE
FIG. 5—Pressure generation by the capillary effect (schematically) [4]. Sample covered with
AI2OJ powder.
straight line in Fig. 1). In this case a thermodynamic disequilibrium will arise
between the unfrozen water and the newly formed ice. In order to balance
this disequilibrium, unfrozen water will move from the cement gel—which
consequently will shrink—to the ice in the capillary pores where it can freeze
immediately. However, the transport of water from the gel pores to the ice in
larger capillaries is only possible if the vapor pressure over the ice is lower
than that over the gel pore menisci; this may not be always true. Due to this
"fresh supply" the volume of ice increases steadily and the pressure upon the
surrounding solid boundary grows (for a detailed discussion see Ref 3). This
process may lead to the abovementioned increase in volume. It should be
noted that with the capillary effect, only the solid phase (ice) bears the increased pressure, in contrast to the hydrodynamic effect (see also Refs 7 and
8). By the way, an analogous mechanism is responsible for the formation of
ice lenses in soils with their devastating effect [9].
Litvan 's Model
According to Litvan (Ref 10) another transport mechanism plays a major
role if the pore water is free to evaporate. Due to the difference in vapor
pressure between the (supercooled) liquid in the pores and the bulk ice on the
surface of the body, there will be a migration of water to larger pores or to the
outer surface where it is able to freeze. This process leads to partial desiccation of the concrete. Mechanical failure occurs when "the rate of water transport out of the pores is significantly less than is required by the conditions.
Such a situation usually arises if permeability is low and porosity, the degree
of saturation, and the cooling rate are high" [10].
Combination of Freezing and Deicing Salts
The use of deicing salts in practice has negative as well as positive conseCopyright by ASTM Int'l (all rights reserved); Sat Nov 8 05:45:19 EST 2014
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HARNIK ET AL ON INFLUENCE OF FREEZING AND DEICING SALT
481
quences with respect to the durability of concrete. The effects depend on the
aggregate state of the salt (dry, humid, or in solution) and on the method of
application (on ice and snow, or preventively applied on humid or wet concrete).
Negative Consequences
Degree of Saturation—By the application of deicing salts, the degree of
saturation in concrete exceeds the amount normally attainable by pure water
alone. This behavior is due to the fact that salts and their aqueous solutions
are hygroscopical, that is, the vapor pressure of these solutions is lower than
that of water. Hence, water molecules in the air (vapor) have a greater tendency to condense into a salt solution than into water. It can be observed
readily in practice that a concrete surface containing salt is moister than
saltfree concrete.
The destructive effect of certain damaging mechanisms can partly take
place only if the degree of saturation has exceeded a certain minimum. Indeed, the damaging force becomes more pronounced as the degree of saturation is increased. For a detailed discussion of the influence of the critical
degree of saturation on the frost resistance of porous materials see Ref / / .
Supercooling Due to Preventive Salt Application—The prevention of ice
formation on the surface of concrete means that no external ice crystals are
present to initiate the freezing of the pore water at temperatures near 0°C.
When the supercooled pore water eventually freezes, however, the destructive
effect of the phase transition will be greater than with normal freezing.
Layer-by-Layer Freezing—A change of the salt concentration in the concrete leads to a corresponding change of the crystallization temperature of
the pore water. Therefore, if there is a salt concentration gradient in the concrete, the freezing of the pore water during cooling will, for a given temperature, be restricted to a certain concrete layer. When ice formation occurs in
such a layer-by-layer way, stresses can develop whose extent depends on the
dilatation difference between the frozen and the unfrozen layer. For details
concerning layer-by-layer freezing see Ref 12.
Temperature Shock—Heat is required for the melting of ice and snow.
When thawing takes place by means of salts this heat is extracted mostly
from the concrete. The great heat loss causes a rapid temperature drop and
shock-like cooling in the uppermost layer (only a few millimetres thick) at the
concrete surface (Fig. 6a). Thus, temperature gradients develop which can
cause internal tensile stresses of short duration, but which may reach the
order of magnitude of the tensile strength of concrete under unfavorable circumstances (thickness of ice 0.5 mm and more; high salt concentration). For
a description of our measurements and results see Ref 12. In contrast, the
natural melting process of ice or snow without salts (Fig. 6fo) or of frozen salt
solution (Fig. 6c) proceeds without any temperature shock, as well as the
natural freezing process of water and salt solution (Figs. 6d, c).
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482
DURABILITY O F B U I L D I N G MATERIALS A N D C O M P O N E N T S
TEMPERATURE
+
"'"I'^E
o-c-
^
( a ) MELTING OF ICE
WITH DEICING SALTS
.'
( b ) NATURAL MELTING OF ICE
WITHOUT DEICING SALTS
ICE MELTS :
TEMPERATURE SHOCK
+
o°c
-r^^
+
0°C
^
E MELTS
1
L
( c ) NATURAL MELTING OF
FROZEN SALT SOLUTION
1
^.--•'''''TROZEN
/ ^
SALT
SOLUTION MELTS
/'I
+
CiT
•.
1
\|
( d ) NATURAL FREEZING
OF WATER
V.
WATER FREEZES
\
1
0°C
^
•VSALT SOLUTION
^ " - . ^ FREEZES
1
( e ) NATURAL FREEZING
OF SALT SOLUTION
FIG. 6—Typical temperature changes on concrete surface in practice: schematical examples.
Displacement of 0°C Limit Due to Temperature Shock—A sudden temperature drop at the concrete surface displaces the 0°C boundary for a short
time period farther into the interior. By this process, inner parts and small
pores near the surface which normally do not freeze now run the risk of
becoming damaged.
Crystallization Pressure—The growth of salt crystals in larger pores of the
cement paste can also lead to pressure generation in the solid (salt) phase
[13]. A prerequisite is the presence of a supersaturated salt solution, produced either by evaporation of water from the solution (warming) or by freezing of a part of the pore water (cooling). The process corresponds to the
aforementioned capillary effect (Fig. 5): The formation of salt crystals starts
in the largest pores when the solution reaches its supersaturation. Analogous
to the water transport (see Fig. 5) a transport of salt ions takes place, namely
from the smaller pores (which hinder crystallization) toward the salt crystals.
If it is possible for the solution to penetrate between salt crystal and surrounding boundary, the crystal will be able to grow and exert an increasing
pressure on the cement gel.
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HARNIK ET AL ON INFLUENCE OF FREEZING AND DEICING SALT
483
The corresponding growth of ice crystals is probably a very minor contributor to damages, and is most effective in surface scaling failure.
Positive Consequences
The higher the concentration of soluble salts in water, the lower will be the
crystallization temperature of the aqueous salt solutions. Therefore, preventive application of deicing salts positively affects the durability of concrete
since the salt delays the ice formation. Even if some ice is formed, the salt
concentration in the residual solution will increase and in this way any further ice growth will be retarded. In addition, the capillary effect occurs only
at lower temperatures, and then to a lesser extent than by freezing without
salts.
The Most Dangerous Salt Concentration
It has been confirmed by many experiments that concrete is most exposed
to damages at a relatively low salt concentration. Extensive tests have
demonstrated that the greatest damages to the concrete surface occur at concentrations of about 2 to 4 percent [14]. The greatest expansion of NaClimpregnated cement paste during frost/thawing cycles was observed at about
5 percent concentration [15]. Clearly, these observations can be interpreted
as being the result of the opposing activity of the negative and positive effects
of deicing salts.
Conclusion
Essential negative effects of the application of deicing salt on concrete are
a higher degree of saturation of the concrete, internal tensile stresses in the
concrete due to temperature shock and layer-by-layer freezing, and supercooling of the pore water. On the other hand, the lowering of the crystallization temperature of the aqueous salt solution is a very positive effect.
However, we conclude that the negative effects of deicing salt on freezing and
deterioration of concrete far outweigh its positive effect.
Acknowledgment
This investigation was part of a research project "Durability of concrete
under the influence of frost and deicing salts." The work was supported by a
grant from the EidgenSssische Technische Hochschule ZOrich.
References
[1] Kubelka, P., Zeitung fur Elektrochemie, Vol. 38, 1932, pp. 611-614.
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484
DURABILITY OF BUILDING MATERIALS AND COMPONENTS
[2] Defay, R., Prigogine, L, Bellemans, A., and Everett, D. H. in Surface Tension and Ad
sorption, Longmanns, London, 1966, pp. 243-255.
[3] Haynes, J. M. in Low Temperature Biology of Foodstuffs, J. Hawthorn and E. J. Rolfe
Eds., Pergamon Press, London, 1968, pp. 70-104.
[4] Meier, U. and Harnik, A. B., Cement and Concrete Research, Vol. 8, 1978, pp. 545-551
[5] Radjy, F., Sellevold, E. J., and Richards, C. W., Cement and Concrete Research, Vol. 2
1972, pp. 697-715.
[6] Beaudoin, J. J. and Mclnnis, C , Cement and Concrete Research, Vol. 4, 1974, pp
139-148.
[7] Powers, T. C. in Proceedings, RILEM Symposium on Winter Concreting, Section C, Copenhagen, 1956, pp. 1-47.
[8] Powers, T. C. and Helmuth, R. A. in Proceedings, Highway Research Board, Vol. 32
1953, pp. 285-297.
[9] Jackson, K. A. and Chalmers, B., Journal of Applied Physics, Vol. 29, 1958, pp
1178-1181.
[10] Litvan, G. G., Cement and Concrete Research, Vol. 6, 1976, pp. 351-356.
[//] Fagerlund, G., Special Publication SP 47-2, American Concrete Institute, 1975, pp
13-65.
[12] Rosli, A. and Harnik, A. B., this publication, pp. 474-484.
[13] Hansen, W. C , in Proceedings, American Society for Testing and Materials, Vol. 63.
1963, pp. 932-945.
[14] Verbeck, G. J. and Klieger, P., Highway Research Board, Bulletin No. 150, 1957, pp
1-13.
[15] Litvan, G. G., Journal of the American Ceramic Society, Vol. 58, 1957, pp. 26-30.
DISCUSSION
Peter Schimmelwitz i {written discussion)—Do you think that temperature
shock is the only reason for damages caused by deicing salts? To avoid misunderstanding of this paper, it should be pointed out that there are other
reasons, too.
Alfred Rosli {authors' closure)—Of course there are all too many factors
affecting concrete durability during the attack of frost and deicing salts. Our
research was concentrated on the question of why deicing salts usually tend
to reduce the frost resistance of concrete. Our preliminary conclusion is that
this fact is due to certain factors, specifically temperature shock, supercooling, capillary effect, water saturation, and layer-by-layer freezing, respectively. In our paper, the temperature shock is taken as an illustrative example for these factors and therefore has been presented in some detail.
'Bundesanstalt fUr Materialpriifung, Berlin, Germany.
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