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Publisher’s version / Version de l'éditeur:

ASTM Special Technical Bulletin, 589, pp. 123-132, 1975

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Testing the frost susceptibility of bricks

Litvan, G. G.

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CONSEIL NATIONAL DE RECHERCHES DU CANADA

TESTING THE FROST

SUSC'EPTIBILIPY

OF BRICKS

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A m e r i c a n Society f o r Testing a n d Materials Special Technical Publication 5 8 9 , 1 9 7 5

P. 123-132

Research Paper N o . 6 4 5 o f t h e

Division o f Building Research

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ESSAIS DE LA SENSIBILITE DES BRIQUES AU GEL

A l'heure actuelle l'ivaluation de la durabiliti des briques se fonde principalement sur la ditermination du coefficient de saturation. Une Ctude de l'essai a montri que pour une brique donnie la valeur est reproductibie d moins de k0.0 15. Toutefois, il risulte d'importantes incertitudes de la variation de la valeur d'un Cchantillon d

l'autre mime s'il provient du mCme lot de production. La valeur numerique du co- efficient peut dipendre largement des dimensions de l'e'chantillon et peut ne pas Ctre uniforme dans l'ensemble d'une mCme brique. L'auteur decrit bri&vement la thiorie ricente de l'action du gel et en Ctudie quelques ripercussions. La durabiliti n'est pas une propriitC matCrielle et n'est pas une quantiti qui se mesure par un seul essai. La valeur de plusieurs caractiristiques matirielles determine la capaciti de risister d la longue aux conditions inconnues du milieu. On peut Cvaluer la probabiliti de survie dans des conditions donnCes, par un essai de 50 cycles de gel- dCgel ou en diterrninant l'aire superficielle d'azote B.E.T., qui semble bien con- corder avec les conditions d'utilisation, d'aprks les risultats obtenus de 27 produits cornmerciaux.

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Authorized Reprint from Special Technical Publication 589

Copyright

American Society for Testing and Materials

1916 Race Street, Philadelphia, Pa. 191 03

1975

G. G.

Litvan

'

I

x ~ ~ r u z ~ a

Testing the Frost Susceptibility

of Bricks

REFERENCE: Litvan, G. G., "Testing the Frost Susceptibility of Bricks,"

Masonry: Past and Present, ASTM STP 589, American Society for Testing and Materials, 1975, pp. 123-132.

ABSTRACT: Analysis of the saturation coefficient test results concluded that while the reproducibility is better than k0.015, considerable variation exists within a given brick and among bricks originating from the same pro- duction lot. T h e value of the saturation coefficient was also found to b e strongly dependent on the size of the specimen. The test suffers not only from uncertainties but is based on assumptions inconsistent with the recently developed theory of freeze-thaw durability. The freeze-thaw test was found to yield greatly improved results when dimensional changes were monitored. The prediction of frost resistante by another test based on the measurement of the nitrogen surface area correlates well with field performance.

KEY WORDS: masonry, bricks, testing, frost action, freeze-thaw durability

The ASTM Specification for Building Brick (Solid Masonry Units Made from Clay or Shale) ( C 62-69) covers 3 grades of clay or shale bricks: SW, MW, and NW. For satisfactory performance under the most severe conditions, Grade SW bricks must be used. These bricks have the following characteristics: compressive strength greater than 2 500 psi; less than 17 percent water absorption in a 5-h boiling test; and, a saturation coefficient smaller than 0.78. These requirements are waived, however, (a) if the compressive strength exceeds 8 000 psi, or if water absorption, after 24-h submersion, is less than 8.0 percent; and, ( b ) if there is no breakage or weight loss greater than 1 percent after 50 cycles of freezing and thawing. Although these requirements usually ensure satisfactory performance, quite frequently bricks conforming to these specifications fail in service for no apparent reason.

1 Research officer, Building M'aterials Section, Division of Building Research, Na- tional Research Council of Canada, Ottawa, Canada K I A 0R6.

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Such occurrences are indicative of a need for better understanding of the freezing-thawing phenomenon, and thus a study a t the Division of Building Research was undertaken. Before presenting the results, how- ever, I should like to describe very briefly some of the shortcomings of the present testing method. Finally, some recently developed tests for durability will be reported.

Saturation Coefficient

The basis of the saturation coefficient test is that resistance t o freeze- thaw action will be satisfactory if the brick does not become completely saturated under ordinary conditions, that is, some spaces remain empty. The ease of water uptake is assessed by a 24-h cold water submersion test; the proportion of unfilled volume is determined b y the 5-h boiling test.

The saturation coefficients of 7 bricks, originating from the same pro- duction lot, were determined (Tablc 1 ) . From the results, the following conclusions can be drawn.

TABLE 1-Effect of sllccessive division on rlle satllration coefficient.

Brick Speci- men Half 1 , 4 tests Half 2, 2 tests Quarters, 4 tests Eighths, 2 tests

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1. The reproducibility of test is good, k 0 . 0 1 5 or less.

2. Considerable variation exists among the bricks, for example, the values of half bricks range from 0.88 to 0.70.

3. Variation within a given brick is quite significant.

4. The value of the saturation coefficient decreases with decreasing specimen size. In the case of Brick 34, the average value for the half brick was 0.78. When it was halved again, the quarter sections were found to have saturation coefficients less than 0.74, with an average of 0.716.

Because only approximately 70 to 90 percent of the available volume fills up during immersion for 2 4 h in cold water, the steadily increasing absorption on continuous submersion, as indicated by Fig. 1, is as expected. More surprising is the finding that, on repeated boiling, water uptake in- creases. But even after 4 boilings (of 5 h each), the values are far below the ultimate water uptake determined by vacuum saturation (Fig. 2 ) .

From the values obtained from the tests (given in Table 2 ) , very in- teresting inferences can be drawn.

Brick 30, for example, absorbed so much water during 24-h submersion

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T A B L E 2-Percc~~~lngc ~ b ~ o r p l i o ~ ~ , sa1~ruli011, U I I ~ srrllc?.alio~l coetficie~ll o n c o ~ l l i ~ l u o u s cold wuler i ~ n ~ ~ l e r s i o ~ ~ and repeated 5-h

h)

boilitlg 1esl.v. 0.

Cold Water Immersion Repeated 5-h Boiling

Vacuum

Saturation

I

GI

0,

14.48 5

1 day 2 days 3 days 4 days 5 days 6 days 7 days 8 days Brick Specimen 30 absorption, % saturation, % saturation coefficient Brick Specimen 3 1 absorption, % saturation, % saturation coefficient Brick Specimen 32 absorption, % saturation, % saturation coefficient Brick Specimen 33 absorption,% saturation, % saturation coefficient Brick Specimen 34 absorption, % saturation, % saturation coefficient Brick Specimen 35 absorption, % saturation, % saturation coefficient Brick Specimen 36 absorption, % saturation, % saturation coefficient Brick Specimen 37 absorption, % saturation, D/o saturation coefficient

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7 . 5

V A C U U M ' O I L I N G S A T U R A T I O N

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that 66.09 percent of the total available volume became filled; and, this value increased to 70.30 percent after contiriuous immersion for 8 days. After the 5-h boiling test, the water content iricrcascd to 80.63 percent of the maximal value. It is most significant that: ( a ) aftcr 5-h boiling, 19.37 percent of the total available volume is still cmpty; and, ( b ) the saturation coefficient, calculated on the basis of these values, is 0.82. This brick, therefore, has to be judged as not meeting the requirements of Grade SW. On repeated boiling, saturation increased to 84.26 percent, and, corre- spondingly, the saturation coefficient decreased to 0.78. Taking 24-h sub- mersion and vacuum saturation values as a basis, thc saturation coefficient was found to be 0.66.

Values for the other samples (as given in Table 2 ) prove that this behavior is by no means an isolated occurrence.

Based on these facts, we arrived at 2 conclusions.

1. Although for a given specimen the saturation coefficient can be rc- produced and easily determined, the value of the figure obtained in charac- terizing the production batch from which the specimen originates is uncertain T h e numerical value of the coefficient may depend to a great extent on the size of the specimen and may not b e uniform within 1 brick. 2. The assumption that frost damage is caused by the 9 perccnt volumc expansion of water on freezing is inconsistent with the finding that a brick which contains approximately 2 0 percent cmpty pore space after 5 h of boiling cannot accommodate the 9 percent extra volume as shown by its moderate frost resistance (Brick 3 0 ) . At the same time, Brick 3 7 with an excellent service record has 9 0 percent of the total pore volume filled after the 5-h boiling test (Table 2 ) .

Mechanism of Frost Action

It was found'in these studies that water contained in the pores of a solid body, such as brick, remains in a liquid-like, glassy state when cooled below 0°C." Presumably, the freedom of the water is so restricted by surface forces that it prevents the rearrangement necessary for crystal formation. As a consequence, a noncquilibrium condition develops be-

2 Litvan, G. G., "Pore Structure and Frost Susceptibility of Building Materials,"

Proceeclings, Rkunion Internationale des Laboratoires d'Essais et de Recherches sur les MatCriaux et les Constructions/International Union of Pure and Applied Chem- istry Material Symposium on Pore Structure and Properties of Materials, Vol. 2,

18-21 Sept. 1973, Prague, pp. F17-F30.

3 Litvan. G. G., Journalof the Anwricarl Ceramic Society, Vol. 5 5 , No. 1, Jan. 1972,

pp. 38-42.

Litvan, G. G., Materials and Struct~ires, Vol. 6 ? No. 34, July-Aug. 1973, pp. 293- 298.

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cause the metastable liquid in the pores and crystalline ice in the environ- ment have different vapor pressures and, normally, cannot coexist. Equilib- rium is restored by partial desiccation of the brick; the liquid remaining in the pores will be under a concave meniscus thus reducing its vapor pressure to that of bulk ice; and, the exuded liquid forms crystalline ice at the external surface. Frost action, which causes the irreversible destruc- tive deformation of a brick, was found to occur when the process leading to the establishment of equilibrium is prevented or greatly hindered from taking place.

If follows then that any property that facilitates or lessens the need for water flow is beneficial. These properties are low water content, low poros- ity, high permeability, and high rate of evaporation. On the other hand, glaz- ing is beneficial only in as much as it reduces rain penetration, and thus water content, but is detrimental since it impedes outward flow.

New Tests for Durability

It is of fundamental importance to realize that frost resistance is not a material property and therefore is not a measurable quantity; life expectancy cannot be assessed by a single test. The performance of a brick depends on the severity of the environmental conditions and the value of several material properties which determine the ability of the solid to survive the freeze-thaw cycles.

In such situations the best, and perhaps only, avenue open has been to compare performance under standardized conditions. This is the aim of the 50-cycle freeze-thaw test. It bears repeating that in this test frost resistance

per se is not evaluated but rather resistance to frost action under a set of imposed conditions. It is hoped that the behavior manifested in the test is related to that shown under different field conditions. This hope is, of course, not always realized.

It should be mentioned that the sensitivity of the freeze-thaw test can be greatly increased if dimensional changes are also measured. Surprisingly, the additional expenses in labor and equipment are very modest. The gage can be bought for approximately $1 0 0 0 and the measuring pins can be affixed with epoxy cement in minutes. The plot of the number of cycles producing 0.02 percent expansion (an arbitrarily selected value) against the firing temperatures of specimens fabricated from the same clay is shown in Fig. 3. Firing temperature is closely related to frost resistance. This method has the great advantages of reducing testing time and providing quantitative results.

The electromicrographs shown in Fig. 4 are of the same specimens as used in previous tests. It is instructive to examine the differences in the

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F I R I N G T E M P E R A T U R E O F B R I C K . 'C

N o t e : N O E X P A N S I O N AFTER 1 1 5 C Y C L t S F O R S P E C I M E N F I R E 0 A T 1 0 7 5 ° C

FIG. 3-Number o f cycles producing 0.02 percent expatlsion o f bricks.

porous network caused when the temperature is raised by 50°C. The photo- graph on the top shows that at 1000°C (1832°F) the texture is rough, the surface area, and porosity quite high, and a large amount of loose material is present. In contrast, at 1050°C (1922"F), the contours are smooth, the surface area decreased, the clay has obviously been vitrified. The superior resistance of the vitrified, well-fired brick is quite under- standable.

Further progress was made by using a pragmatic approach based on the following working hypothesis.

Notwithstanding the complex nature of the frost action phenomenon, some parameters must be of dominant importance. If this is true, values characterizing the pore structure are most probably such parameters. Accordingly, the nitrogen Brunauer-Emmett-Teller surface areas of 27 commercial products were determined. Some of the samples were obtained from bricks in the field which failed in service, others from unexposed or exposed bricks used locally. The assessment of frost resistance was based on the reputation of the particular product considering the frequency of failure in service that was observed. Thus, products were classified as dura- ble (no incidence of failure), questionable (some known cases of failure of product), and unsound (sample originating from brick that failed in service). In Table 3, the results are arranged in order of increasing surface area values.

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132 MASONRY: PAST A N D P R E S E N T

TABLE 3-Slrrfnce area of bricks related t o frost drcrnbility.

Brick Specimen Area tn2g-I Frost Durability Q 26 0.21

+

23 0.35

+

19 0.45

+

13 0.52

+

3 0.57

+

17 0.63

+

7 0.76

+

18 0.78 ? 15 0.85

+

22 0.98

+

9 1.01 14 1.01

+

8 1.11

+

6 1.14 - 12 1.25

+

4 1.35 - 2 1.44 - 11 1.46 ? 2 1 1.50 - 1 1.65 - 27 1.68 - 2 5 2.00 - 10 2.65 - 16 2.88 - 5 3.00 - 24 5.60 - 20 8.25

-

+

= durable, ? = questionable, and - = unsound. Concluding Remarks

A better understanding of the mechanism makes it possible to identify the parameters most important from the freeze-thaw phenomenon point of view. Although the test based on the measurement of surface area, residual expansion, or pore size disribution are by no means infallible, they can be developed as a much more meaningful method for the prediction of per- formance than the presently used ones.

Acknowledgments

This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.

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This publication is being distributed by the Division of Building Research of the National Research Council of Canada. It should not be reproduced in whole or in part without permission of the original publisher. The Division would be glad to be of assistance in obtaining such permission.

Publications of the Division may be obtained by mailing the appropriate

remittance (a Bank, Express, or Post Office Money Order, or a cheque, made

payable to the Receiver General of Canada, credit NRC) to the National Research Council of Canada, Ottawa. KlAOR6. Stamps are not acceptable. A list of all publications of the Division is available and may be obtained from the Publications Section, Division of Building Research, National Research Council of Canada, Ottawa. KlAOR6.

Figure

TABLE  1-Effect  of  sllccessive  division on rlle  satllration coefficient.
FIG.  1-Cold  water absorption  of  bricks.
FIG. 2-Water  absorption  on repeated  5-h  boiling  test o f  bricks.
FIG. 3-Number  o f  cycles producing  0.02  percent  expatlsion o f  bricks.
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