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EVALUATION OF MOISTURE CONTENT MEASUREMENTS IN STONE

BORIS SIZOV

State Research Institute for Restoration, Moscow, Russia.

INTRODUCTION The detennination of actual moisture content in building materials is an important tool for the diagnosis of the pathological situations in monuments. It is frequently used for the attribution of the water to different sources as e.g. rising damp or condensation . The study of water content and moisture distribution is carried out in different seasons of the year, i.e. at the different temperatures. The value of water content is estimated on the basis of analysis of the water vapour adsorption isotherm (w.v. adsorption isotherm) of respective porous building materials. At the same time it has been known that the value of w.v. adsorption depends on temperature of material and surround air. This is in agreement with results of our previous investigations. On the fig. (1,2) the curves of w. v . adsorption are shown under t = 15° C and t = 25°C for the brick from the Church of Intercession in Fily of XVII c. (Moscow) and fresco plaster from the Church of St. George in Old Ladoga of XII c. (near St.­Petersburg).

0,04 ~--------------......,

0,035

Ci :! 0,03 Cl

.II: ~.025 c .l!! c 0,02 0 CJ

~.015 . -"' == 0,01

0,005

--+-25° c

·---15° C '

01---------------~

0 20 40 60 R.H.[%]

80 100

Fig.1.Water vapour adsorption isotherms for brick from the Church of Intercession in Fily (XVII) .

0,025

Ci 0,02 --+- 25° c .II:

Ci ---15· c , =.. 0,015 --- ----~ 'E Cl)

'E 0,01 0

CJ '-Cl)

iQ 0,005 ==

0 0 20 40 60 80 100

R.H.[%]

Fig.2 .Water vapour adsorption isotherms for the fresco plaster of the Church of St. George in Old Ladoga (XII c. ).

However the experimental definition of curves for the w.v. adsorption under more low temperatures is

very complicated task and takes rather extended time.

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BACKGROUND OF METHOD.

In the theory of "moisture potential" suggested by Prof. V. Bogoslovsky, for the characterisation of moisture state of investigated material the water content of filter paper contacted with this material is taken, similar to the height of mercury column is used for characterisation of material's temperature. The moisture potential (8) is evaluated by conventional unit - 0 8(1 ).

In the investigative works of Prof. V.Bogoslovsky the correlations are obtained for the water content of filter paper vs. moisture potential (8) under different temperatures (quadrant II on fig.4), as well as the interrelation between moisture potential (8), temperature (t) and relative humidity of air (R.H.) (quadrant Ill on fig. 4) . The experimental test performed by us have verified, that the interrelation between water content of material and water content of filter paper as standard one practically doesn't depend on the temperature (fig.3).

0,045 iU 0,04 ·;::: Cll - O,Q35 · ns E 0,03 0 a; O,Q25 - ..II: c: - O,Q2 Cll Cl - ..II: c: .....

0,015 0 (,,) ... 0,01 Cll iQ 0,005 ::

0

0

1• 25 ·c

I• 15° c I ·--~

••

0,05

• •

• •

••

0, 1 0,15

water conteent of filter paper [kg/kg]

Fig.3. Water content of filter paper vs. water content of brick.

0,2

The fig.3 is drawn by way of just graphical juxtaposition of w.v. adsorption isotherms for material and filter paper, obtained with exsiccator method.

water content of limestone r"g/kgJ

~ c -0

"" ~ 0. '· -0 ~· ;::: 0 = c u c .:;; 8· ,., - ..... 0

~

~

moisture potential (0 8)

Fig.4.Construction of water vapour isotherms for the limestone from portal of the Nativity of Virgin Cathedral in Suzdal (-_ ~)

quadrant I - correlation water content of limestone vs. water content of filter paper;

quadrant 11 - correlation water content of filter paper vs. moisture potential(0) under different temperatures

quadrant Ill - correlation relative humidity of air vs. moisture potential (0) under different temperatures

quadrant IV - w.v. adsorption isotherms obtained by graphical construction. 0

- experimental points for w.v. adsorption isotherm under 20"C, obtained by exsiccator method .

......... defines the values of adsorption water content of limestone under different temperatures (R.H. = 65%).

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According to the main postulate of "moisture potential" theory [1), two body ( in our cases investigated

material and filter paper) in moisture equilibrium with third one (air) are in moisture equilibrium with each other. In this case we can estimate the water content of material under different temperatures on the basis of the experimental correlation of water content of material vs. water content of filter paper for one temperature (e.g. standard t = 20°C).

On the fig .4 the scheme of w.v. adsorption isotherms construction is given for the limestone from portal

of the Nativity of the Virgin Cathedral (XIII c.) in town Suzlal. For the comparison (in quadrant IV) the experimental points are shown for the isotherm under 20°C, obtained with using exsiccator method.

' APPLICATION OF METHOD

The described method was used for the evaluation the moisture condition for limestone of west portal of the Nativity of the Virgin Cathedral .

The moisture distribution in material of portal in autumn 1995 and in spring 1996 are presented in fig.5

0,06

0,05 'ijj .:.: Ci 0,04 ~

r-·---~

1~10.10.95

~-~5.04~?~-c: .! 0,03 c 0 u ~ 0,02 . t=5°C .! "' :r:

0,01 t=20°C

0

0 50 100 150 200 250 300 350

height [cm)

Fig.5. The moisture distribution in portal in autumn (1995) and spring (1996) in comparison with real adsorption water content of limestone (quadrant IV in fig.4).

The average temperature of air and stone in periods of water content determination amounted to :::: 5° C, and R.H. of air was 60 - 70 % on the average.

If we compare the measured water content of portal limestone with the value of w.v. adsorption for this limestone under 20° C (fig.5), we must propose the availability of any sources of dampness. The comparison of w.v. adsorption under 5 ° C shows that material of portal higher 1,0 m is in air - dry condition, that more corresponds to the facts.

Monthly data

Para- I II Ill IV v VI VII VIII IX x XI XII

meter

Moscow t (° CJ -10,5 -9,7 -4,7 4,0 11 ,7 16,0 18,3 16,3 10,7 4,1 -2,5 -7,5

climate R.H.(%] 84 82 79 67 60 61 65 71 75 79 83 85

0 (0 BJ 8 8 10 44 50 46 52 48 51 60 10 8

Museum t (° CJ 18,0 18,0 18,0 18,0 18,0 18,0 18,0 18,0 18,0 18,0 18,0 18,0

conditions R.H.(%] 60 60 60 60 60 60 60 60 60 60 60 60

(constant) 0 (0 BJ 22 22 22 22 22 22 22 22 22 22 22 22

Chosen t (° CJ 5,0 5,0 7,5 15,0 18,0 18,0 18,0 18,0 18,0 15,0 7,5 5,0

conditions R.H.(%) 35 35 40 55 60 60 60 60 60 55 40 35

0 (0 BJ 8 8 10 17 22 22 22 22 22 17 10 8

Table 1. Data of Moscow climate and microclimate parameters for calculating .

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The moisture potential (8) permits to describe the moisture state of material both in sorption and in

supersorption fields. That is why the further possible employment of the "moisture potential" is the choice (definition) of the parameters of the microclimat for preservation of architectural surfaces (e.g. mural

painting). In this case, the minimisation of moisture streams through the internal wall surface and

minimal change of water content of the material on interior wall surface are assumed as a criterion. In table 2 the results of calculation carried out for different parameters of interior microclimat (table

1) of the Church of Intercession in Fily in Moscow are presented.

Water content of interior wall surface [kg/kg] Yearly moisture stream [kglm2J

max. min !l. [%]

Museum 0.0116 0.0105 7.8 0.32 conditions

Chosen 0.0114 0.0112 1.7 0.22 conditions

Table2. Calculated water content and the values of moisture stream for the interior wall surface under different conditions.

CONCLUSIONS

- for the correct diagnosis of dampness causes of the monuments it is necessary to evaluate the data of the moisture distribution having regard to value of water vapour adsorption for materials under different temperatures;

- for estimating of adsorption water content of porous materials under different temperatures the method has been proposed with using "moisture potential" (8 );

- two types of microclimate parameters are theoretically assessed with regard to suggested criterion: the minimization of moisture stream and constant water content on the interior wall surface.

BIBLIOGRAPHY

[1] V. BOGOSLOVSKY (1983): "Grundlagen der Heizung, Luftung und Klimatechnik", Verlag fur Bauwesen, Berlin.