FREEZE-THAW DETERIORATION

OF CONCRETE PAVEMENTS

By Dan F. Adkins,1 Associate Member, ASCE, and Vance T. Christiansen2

ABSTRACT: Under winter exposure conditions, solar radiation effects along with the presence of de-icing salts have a pronounced impact on the deterioration of concrete pavements. The rapid changes and differences in temperature between the pavement's dry surface and saturated subsurface can cause both thermal and hy­draulic stress. As subsequent cycles of freezing and thawing occur, a subsurface plane of cracks will form in which ice lenses can grow parallel to the surface and eventually cause failure of the subsurface paste. Once the subsurface has failed, the surface will spall off in sheets. This thin brittle layer will then crumble under traffic, and leave the aggregate exposed. By using both laboratory and field data this paper discusses concepts of how de-icers and freeze-thaw cycles limited to the upper portion of the concrete pavement contribute to scaling by setting up thermal gradients and lowering the immediate surface moisture content.

INTRODUCTION

In cold northern climates, a major expense for state and local governments centers on repair and replacement of existing concrete highways and side­walks. A considerable portion of the need for this repair results from de­terioration due to saturated freezing and thawing. This deterioration most often occurs as a separation of a thin layer of the top surface from the body of the concrete and is usually called scaling or spalling of the surface. This thin brittle layer then crumbles under traffic and leaves the aggregate ex­posed. In recent years, surface scaling of concrete has been aggravated by the widespread use of de-icing salts. The areas showing the greatest dete­rioration are usually where ponding of water and salt solutions or continual wetting has occurred during repeated cycles of freezing and thawing (Fig.

Although considerable fundamental research has been conducted (Cordon and Merrill 1963; Hansen 1963; Litvan 1971, 1975; Powers 1945; Woods et al. 1962), a review of the literature indicates that there is limited infor­mation about the actual conditions existing within the scaled concrete sur­faces or about the disruptive action of the microclimatic changes occurring during freeze-thaw cycles. The mechanisms of the destructive processes in­volved in freeze-thaw deterioration are complicated and must be broken down into interrelated factors that directly or indirectly affect the internal pressures created by the expansion of ice beneath the surface of the concrete. The following discussion of the thermal and hydraulic stresses involved in sur­face scaling is intended to present a viewpoint as to how concrete may ac­tually freeze and thaw under natural conditions.

'Civ. Engr., P.O. Box 246, 164 East 2nd South, Hyde Park, UT 84318. 2Prof., Civ. and Envir. Engrg., Utah State Univ., Logan, UT 84322. Note. Discussion open until October 1, 1989, To extend the closing date one month,

a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on June 14, 1988. This paper is part of the Journal of Materials in Civil Engineering, Vol. 1, No. 2, May, 1989. ©ASCE, ISSN 0899-1561/89/0002-0097/S1.00 + $.15 per page. Paper No. 23510.

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MECHANICS OF SURFACE SCALING

By definition, portland cement concrete pavement is a material which con­sists essentially of a binding agent and aggregate filler. The binding agent is the hydraulic portland cement, which hardens by reacting chemically with water. The process, known as hydration, combines water and cement to form a rock-like material composed mostly of calcium silicate (Akroyd 1962; Lea 1971). The actual amount of water needed to produce complete hydration is less than one-third the weight of the cement (Cordon 1979). Although it takes a relatively small amount of water to complete hydration, a concrete mix with a low water content is stiff and unworkable. Excess water must be added because of the difficulty in manufacture and placement of too stiff a concrete mix.

As excess water not required for hydration bleeds to the fresh pavement surface, it dilutes the surface paste and weakens the gel structure by sepa­rating the calcium silicate matrix; i.e., adding pore space. The excess water also weakens the concrete just below the surface. The water, channeling its way to the top of the pavement, brings air-entrained bubbles to the surface (Ropke 1982) where they dissipate into the atmosphere. Further, these chan­nels create capillaries where moisture at later dates may enter and cause freeze-thaw deterioration within the concrete.

Additionally, finishing is very important if concrete is to display a high degree of frost resistance. If concrete is finished prior to initial set, excess fines will be brought to the surface. If overfinishing or the addition of water to aid in finishing occurs, excessive mortar and water will bleed to the sur­face causing it to have a greater water-to-cement ratio. This surface will then be particularly vulnerable to the action of freeze-thaw scaling (Concrete Manual 1975). Excessive floating or finishing of the surface further weakens scaling resistance by depleting entrained air from the surface paste. This inferior mortar layer, having a high water-cement ratio, will have a tendency during saturated freezing and thawing to separate from the concrete mass beneath.

Freezing within the Paste During the evaporation of the water not required for hydration, air voids

are left within the hardened paste. These voids lower the strength of the

FIG. 1. Surface Scaled Concrete Sidewalk

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concrete and affect the permeability (Taylor 1972). This, in turn, dictates freeze-thaw durability. When the mature concrete is saturated and the water begins to freeze, ice crystals start to form and expand within the capillary voids. As the water continues to freeze, hydraulic pressure is developed by the expansion of water as it is converted into ice. This expansion, if unre­strained, is about 9% (Woods et al. 1962). The attendant hydraulic pressure then either forces the remaining unfrozen water out of the capillaries and into the surrounding paste or causes dilation of the capillary cavities along with microcrack formation.

As the temperature continues to fall, the difference in entropy between the ice in the capillaries and the water molecules in the nearby unfrozen pore spaces will cause the gel water to migrate back into the capillaries. This water then freezes and adds to the growing system of ice within the capil­laries. As the ice crystal-water network continues to grow, the pressure within the capillaries becomes high enough to stress the surrounding paste beyond its elastic limit, causing microcracks to form and thus further weakening the pavement's subsurface layers (Cordon 1966).

Ice removal agents contribute to surface scaling in several ways, the most important one being that strong concentrations of de-icers in the capillary voids tend to increase the osmotic pressure arising from the difference in salt concentrations between the capillaries and gel pores. As water in the unfrozen pores begins to diffuse back into the frozen capillary cavities, a pressure differential develops in the direction of flow. When the void be­comes full of ice and solution, any further growth of ice will cause dilation and stress in the paste (Brown and Cady 1975; Reading et al. 1977).

An additional mechanism of de-icer scaling is believed to be caused by the transfer of heat during the melting process of the ice covering the surface of the pavement. The resulting rapid changes in the temperature of the upper layers of the concrete is thought to produce stress in the frozen paste im­mediately below the surface of the pavement (Cordon 1963; Lea 1977).

Entrained-air content plays an important role in surface scaling from the concept that intentionally entrained air bubbles act as reservoirs in accom­modating the volume increase that occurs during the freezing of water within the concrete. Unlike entrapped-air voids, which can be large and irregular in shape, bubbles of intentional air-entrainment are less than 4 millionths of an inch in diameter (100 (xm) and as many as 300 billion closely spaced bubbles may be present in a single cubic yard (400 billion/m3) of 6% air-entrained concrete (Design and Control 1979). These discrete cavities help to relieve the hydraulic pressure that develops in the channels of the hard­ened paste and during the process of freezing the voids prevent or limit the growth of microscopic bodies of ice within the cement (Hansen 1963; Rod-way 1979).

TEMPERATURE AND SATURATION DIFFERENTIALS

In the field it is commonly observed that during cold winter weather one feels warmer when the sun is shining and suddenly colder during periods of alternating cloudiness, although an actual temperature drop has not occurred. This is caused by direct absorption of the sun's rays (radiant energy) by the body. The theory is that the same thing happens with concrete. The surface of the concrete pavement absorbs enough radiant energy to cause the tem-

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perature to rise above freezing in the upper layers, while the bottom portion of the pavement and surroundings remain below freezing. Thus, solar ra­diation contributes to the thawing process and causes many more freezing and thawing cycles than would occur due to convection from the ambient air alone (Ritchie and Davidson 1968).

When the sun's energy is no longer available to thaw the surface of the concrete and the ambient air temperature is below 32° F (0° C), freezing may occur from the top downwards while at the same time freezing takes place from the frozen bottom upwards. The resulting freezing temperature at the top and bottom of the concrete may converge upon a saturated nonfrozen zone directly below the surface of the pavement. The boundary between the frozen and unfrozen concrete represents the zone of highest stress and results in a region where microcrack formation may be initiated.

Additional gradients of subsurface saturation can be introduced with the application of de-icer salts. Salt solutions have a lower vapor pressure than water (Reading et al. 1977). This tends to cause the subsurface of the pave­ment to dry at a slower rate than would normally occur. Additionally, im­mediately after the application of de-icers, the concentration of salt is highest at the top of the pavement, which results in moisture being drawn from the immediate surface paste. This, along with normal surface drying or subli­mation, can result in a difference in water content between the surface and subsurface of the concrete. With less moisture at the surface, the concrete forms a thin top layer which is less susceptible to disintegration as a result to frost action and tends to hold together better. The saturated concrete im­mediately below this layer can then form lenses of ice which cause subsur­face failure and spall the dryer surface off in sheets.

Further, normal drying during cycles of partial thawing and complete freezing can cause variations between the surface and subsurface moisture content. If the slab is partially thawed, drying can take place only in the top layer but not in the frozen subsurface paste.

Laboratory Study To establish the validity of the theory that surface scaling could result from

freezing temperatures converging upon a zone of greater saturation, a series of laboratory tests were devised. Saturated laboratory specimens were ther­mally insulated on the sides and bottoms; then cold air was blown over the exposed top surfaces until the entire block was completely frozen. After complete freezing, the upper layers of the specimens were partially thawed by the use of heat lamps and then refrozen by again blowing cold air over the top surfaces.

Preliminary test results from a laboratory specimen can be seen in Figs. 2 and 3. With the use of thermocouples, temperatures at the surface, 0.25 in. (6 mm) down from the surface, the middle, and the bottom of the slab were taken. As indicated in Fig. 3, the temperature of the upper portion of the slab rose rapidly during the 15 minutes of partial thawing. The temper­ature then leveled slightly during the phase change from solid to liquid of the ice frozen within the upper portion of the slab. At the same time, the bottom temperature dipped as heat was transferred from the lower portion of the block. As refreezing began, the surface temperature dropped sharply. Then the middle temperature leveled while the bottom temperature continued

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FIG. 2. Surface Scaled Laboratory Specimen

to rise as conductive heat transfer occurred between the lower and upper portions of the specimen.

As refreezing progressed, the surface temperature became colder than the temperature of the middle of the slab. The surface temperature continued to

Laboratory Simulation

TIME (MINUTES)

FIG. 3. Typical Variations in Temperature of Laboratory Specimen Subjected to Cycles of Complete Freezing and Surface Thawing

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Field Concrete

2 4 6 B 10 12 U 16 18 20 22 TIME (HOURS)

FIG. 4. Typical Daily Variations in Temperature for Concrete Structure in Field on Cold and Sunny Day

utes. In this way, it is believed that a lens of solution was trapped below the concrete's surface, which upon freezing caused stress and weakened the concrete. After 10 freeze-thaw cycles, the failed specimen, while still fro­zen, was saw cut and a transitional layer of microcracks and ice was ob­served.

The most severe scaling occurred in saturated specimens with no air-en-trainment and a high water-to-cement ratio. Their immediate surfaces were allowed to dry slightly between repeated applications of de-icer solutions and cycles of complete freezing and partial thawing. It is believed that this caused a lower degree of saturation in the immediate surface layer of the concrete and a greater degree below it. As dilation and failure of the capillary walls will not occur if the concrete is not critically saturated (Maclnnis and Beau-doin 1968), the immediate surface layer was less susceptible to frost action than the saturated layer beneath it. Failure then occurred in the subsurface paste and spalled the dryer surface off in brittle sheets.

It is believed that salt solutions further aggravated this process not only with osmotic and thermal stress but by drawing water from the immediate surface. At the same time, the de-icers, by having a lower vapor pressure, slowed the rate of internal drying as it took place from the top surface down­wards. With the surface being dryer than the subsurface during repeated drop until it reached the temperature of the bottom of the concrete. With the middle of the slab being the warmest part of the specimen, the concrete was subjected to partial thawing and then refreezing along two approaching paths, as represented by the cross over of points at 17.5, 22.5, and 25 min-cycles of partial thawing and complete freezing, a subsurface ice layer was formed that caused scaling.

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Field Experiments A sound argument for this theory can be related to independent field mea­

surements. During the winter of 1984, without prior knowledge and inde­pendent of the laboratory experiment, the University of Windsor in Ontario, Canada implanted thermocouples into the bottom south face of an existing barrier wall that had shown rapid deterioration in the form of surface scaling. From January through March, temperatures of the surrounding air, surface, and at a 0.25 in. (6 mm) depth were measured. Typical fluctuations in the temperature of the concrete are shown in Fig. 4. The figure indicates that direct sunlight played an important role in temperature change. As soon as the sun's energy was available to thaw the concrete's top surface, the tem­perature of the upper layer rose rapidly above the ambient temperature. When the sun's rays were no longer available to thaw the surface of the concrete, freezing occurred from the top surface of the structure downwards into the j interior of the wall.

When the concrete's surface was exposed to sunlight and the surrounding temperature was below freezing (32° F, 0° C), a temperature difference of 10° F (5° C) was recorded between the surface and a 0.25 in. (6 mm) depth. Depending on the weather, the surface of the concrete became alternately warmer and colder than the 0.25 in. (6 mm) depth. Since scaling can be attributed to thermal and hydraulic stress, conclusions can be drawn from the results obtained in the field that indicate that the concrete scaled due to rapid changes between the temperature of the saturated subsurface and sur­face layer, saturation levels of the structures were reported by Hudec et al. | (1986). The similarities between field observations, Figs. 1 and 4, and the j laboratory results, Figs. 2 and 3, should be noted. i

r l;

CONCLUSIONS JI < l | I

A strong argument can be presented that during the winter season solar J radiation effects with the presence of de-icing salts have a pronounced im- j-pact on the surface of concrete pavements. The rapid changes and differences j in temperature between the dry surface and saturated subsurface of the con- ! crete can set up thermal and hydraulic stresses. When the saturated subsur- < face layer begins to freeze, ice crystals will form in the water-filled capillary voids. As failure of the capillary walls progresses, cracks form, in which j ice crystals will continue to grow into lenses by drawing water from the f surrounding gel pores. J

Solar radiation can contribute to many more cycles of freezing and thaw- I ing of the concrete than would occur due to changes in the ambient tern- I perature alone. The increased number of freeze-thaw cycles causes stress in the pavement's subsurface, which will initiate the formation of cracks be- j tween the boundary of the frozen and unfrozen concrete as freezing pro-gresses. J

The primary force component exerted by this expanding lens of ice will { be perpendicular to the top of the pavement and along a subsurface plane j of weakness parallel to the surface. As subsequent cycles of complete freez- * ing and partial thawing occur, ice will continue to form at the same level, causing further dilation of the capillary voids and eventual failure of the subsurface paste. Once failure of the subsurface-paste occurs, the less sus- * ceptible surface layer will spall off in sheets. Traffic will then crumble this

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scaled surface and leave the aggregate exposed. Salt solutions and freeze-thaw cycles limited to the upper portion of the pavement contribute to scaling by creating temperature and moisture gradients between the surface and sub­surface of the concrete.

ACKNOWLEDGMENTS

The experimental laboratory work was carried out at the Department of Civil Engineering at Utah State University, and was supported by depart­mental funds and a William A. Cordon Concrete Fellowship. The writers wish to express their gratitude to Dr. Mosongo Moukwa of Northwestern University for making the field data available.

APPENDIX. REFERENCES

Akroyd, T. N. W. (1962). Concrete properties and manufacture. Pergamon Press, New York, N.Y.

Brown, F. P., and Cady, P. D. (1975). "Deicer scaling mechanisms in concrete." Durability of concrete, Publ. SP-47, Amer. Cone. Inst., Detroit, Mich., 101-119.

Concrete manual. (1975). 8th Ed., U.S. Government Printing Office, Washington D.C.

Cordon, W. A., and Merrill, D. (1963). "Requirements for freezing and thawing durability for concrete." Proc. Amer. Soc. for Testing Materials, 63, 1026-1036.

Cordon, W. A. (1966). Freezing and thawing of concrete—mechanisms and control. Monograph No. 3, American Cone. Inst./Iowa State Univ. Press, Detroit, Mich.

Cordon, W. A. (1979). (1979). Properties, evaluation, and control of engineering materials, McGraw-Hill Book Company, New York, N.Y.

Design and control of concrete mixtures. (1979). 12th Ed., Portland Cement As­sociation, Skokie, 111.

Hansen, W. C. (1963). "Crystal growth as a source of expansion in portland-cement concrete." Proc. Amer. Soc. Testing Materials, 63, 932-945.

Hudec, P., Maclnnis, C , and Moukwa, M. (1986). "The capacitance effect method of measuring moisture and salt content of concrete." Cement and cone, res., 16, 481-490.

Lea, F. M. (1971). The chemistry of cement and concrete, 3rd Ed., Chemical Pub­lishing Co., Inc.

Litvan, G. G. (1972). "Phase transition of adsorbates: VI, effect of deicing agents on freezing of cement paste." J. Amer. Ceramic Soc, 58(1-2), 26-30.

Litvan, G. G. (1975). "Phase transition of adsorbates: IV, Mechanics of frost action in hardening cement paste." J. Amer. Ceramic Soc, 58(1-2), 26-30.

Maclnnis, C , and Beaudoin, J. (1968). "Effects of the degree of saturation on frost resistance for mortar mixes." J. Amer. Cone Inst., 65, 203-207.

Powers, T. C. (1945). "A working hypothesis for further studies of frost resistance of concrete." J. Amer. Cone Inst., 41.

Reading, T. J., et al. (1977). "Guide to durable concrete." J. Amer. Cone. Inst., 74. Ritchie, T., and Davidson, J. L. (1968). "Moisture content and freeze-thaw cycles

of masonry materials." J. Materials, 3(3), 658-671. Rodway, L. W. (1979). "Void spacing in exposed concrete flatwork." Cone. Int. 1,

83-89. Ropke, Jon C. (1982). Concrete problems causes and cures. McGraw-Hill Book

Company, New York, N.Y. Taylor, H. F. W. (1972). The chemistry of cements, Vols. 1 and 2. Academic Press,

New York, N.Y. Woods, H., et al. (1962). "Durability of concrete in service." J. Amer. Cone. Inst.,

59, 1772-1816.

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