HT_1E: Moisture transport characteristics

 

Following the requirements of the international agreement on material characterization and those of the engineering model of material characterization (Grunewald at al 2003, Grunewald and Haeupl, 2002; Grunewald and Bomberg, 2002) two basic transport characteristics are to be measured. First, the water vapour permeability is measured at the lowest end of moisture content and the second, capillary water conductivity is measured at the highest end of the moisture content field. Alternatively, rather than measuring the primary transport characteristic i.e., water conductivity (permeability) coefficient, one may measure a property that is closely correlated with it namely the water absorption coefficient. We shall start with the latter transport descriptor.

 

HT_1H. Current method to measure the water absorption coefficient

The water absorption from free water surface into a porous material is a two stage process. In the first stage, the water flow form water surface into the specimen is a result of the capillary forces within the material. The cumulative water flow into the specimen is expressed as a function of square root of time (water absorption- or A-coefficient). The water absorption coefficient, kg/(m2s1/2), is defined as a ratio between an increase in the cumulative water flux at the material surface to the difference in the square root of time for which this increase was measured.

The second stage of the water absorption starts when the water front has reached the upper surface of the specimen or the maximum capillary rise. During the second stage, the changes in moisture content are much slower. They are now governed by the rate of dissolution and removal of air entrapped in the continuous water field.

A review of measurements of water absorption coefficient and the capillary moisture content measurements that were performed at the Technical University of Dresden (TUD) during the HAMSTAD project is complemented with additional tests performed during the summer 2003 when the SU researchers were working at the TUD facilities (Bomberg and Plagge, 2004). The A-coefficient measurements were performed both with an automatic balance and manual measurements. Measurements performed on clay brick and calcium silicate board (see Roels et al 2004) will be used to estimate the contribution of experimental errors in the overall variability of results.

 

1.1.       Period used for the calculation of the A-coefficient.

 

The A-coefficient is measured under one-directional, free water intake test and it is defined as a ratio of the cumulative water flux entering the bottom surface of the specimen to the square root of the period of water absorption. As the specimen is typically placed in a vertical position, the test is often called a water uptake, instead of a free water intake, or a water imbibition test. To calculate the A-coefficient one assumes that all requirements for the Boltzman transformation are satisfied, i.e.,

1.       the water front has not yet reached the opposite surface of the sample

2.       the moisture content at the surface in contact with water is constant, and

3.       the height of the specimen is only a fraction of its maximum capillary height for the tested material.

 

Nevertheless, many testing laboratories that are not familiar with the limitations of this approach use a long testing period e.g., 24 hours which may significantly increase the imprecision of this test method. Long tests durations not only put to question the third requirement but also cause changes in air pressure conditions on both the lower and the upper surfaces of the material.

Furthermore, the A-coefficient calculation assumes a developed moisture profile, i.e., the test may not be started at time zero. This posses a question as to what constitutes a valid duration for the initial period prior to starting the calculation.

1.1.1. Initial time used for the calculation.

Descamps (1997) demonstrated that initial surcharge of water under small water head (typically immersion depth is between 2 and 3 mm), causes an increase of air pressure ahead of the waterfront. While the air pressure peaks rapidly and decays within a few seconds, the rate of decay in the overpressure of air depends on the air permeability and height of the specimen. It is, therefore, necessary to disregard the initial period and to start A-coefficient determination after an initial period. To this end, one must introduce a concept of the initial time (to) into the A-coefficient calculations. Furthermore, one must examine how the length of the initial period changes when testing different materials.

1.1.2. Final time used for the A-coefficient calculation.

The maximum duration of the testing period depends on the uniformity of pore structure and the height of the specimen. Because of a simplification in its mathematical definition, the water absorption coefficient can be measured precisely only during the initial stage of water inflow into a dry specimen. Yet, this short time may not be practical for materials with limited capillary suction as it must be accompanied by higher precision in the laboratory work. Therefore, some material standards decided to postulate a 24-hour immersion process.

 

Such a long experimental period may introduce two significant drawbacks, namely:

(1)     Moisture content at the bottom surface of the specimen increases above the capillary moisture content as a result of air bubbles from the material being removed by the flowing water. When the test is carried over an extended period, the transition layer at the water ingress face (where the moisture content exceeds the capillary moisture content) becomes thicker and the result of the measurement may become deviated from the theoretical solution. 

 

(2)     For porous materials with multiple pore-size distribution (e.g., aerated autoclaved concrete, AAC), the slope of the moisture flux against the square root of time may change with time so significantly that the A-coefficient measured during short period may be significantly different from that measured during long period of water absorption.

 

Water absorption of AAC was measured under the HAMSTAD project and shown to have a systematic deviation from the assumed dependence on the square root of time. This material is therefore not included in the following paper.

 

1.2. Initial and final times as measured on clay brick and calcium silicate

Figures 1, 2 and 3 show the increase in cumulative moisture flux over a period between two consecutive measurements presented as a function of square root of time for two materials. The selected specimens represent the median A-coefficient values selected from 10 measured replicas.

 

 

Figure 1 A derivative of A-coefficient measured in the clay brick HS B 33 specimen with a 1957 kg/m3 density representing a median A-coefficient.

 

 

 

 

 

Figure 2. A derivative of A-coefficient measured on the calcium silicate Ak 33 specimen with a density of 319 kg/m3. The specimen represents a median value of the A-coefficient.

 

 

Figure 3. A derivative of A-coefficient measured on the calcium silicate Ak 33 specimen with a density of 288 kg/m3. The specimen represents a median value of the A-coefficient.

 

 

1.3. Discussion on the period of measurements recommended for the tested clay brick and calcium silicate products

 

For clay bricks, disregarding 2 initial points from the measured results i.e. starting from 3 minutes into the absorption process provides a constant A-coefficient. Effectively, 3 minutes can be considered as the starting point for determination of A-coefficient. For calcium silicate, the corresponding value is much smaller and the initial time of 1 minute appears sufficient. Table 1 lists maximum, minimum, and median values for the beginning and the end of period that can be used for calculation of the A-coefficient.

 

Table 1. Beginning and the end of period (in square root of seconds and minutes) that can be used for calculation of the A-coefficient during the free water intake tests on clay brick and calcium silicate.

 

Specimen code

Minimum time,

s1/2, (min)

Maximum time,

S1/2, (min)

HS-B26-minimum A

15 (3.8)

45 (34)

HS-B27-median A

10 (1.7)

55 (50)

HS-B33-median A

15 (3.8)

50 (42)

HS-B32- maximum A

15 (3.8)

60 (60)

HS-AK30

5 (0.5)

47 (37)

HS-AK29 -median A

5 (0.5)

42 (30)

HS-AK28-median A

5 (0.5)

50 (42)

HS-AK33

5 (0.5)

48 (38)

 

An interesting observation was made with respect to the transition from the first to the second stage of the water absorption process. Figures 1 through 3 indicate the presence of a transition region between a constant water flux associated with the water absorption process and the second stage associated with the redistribution of air. An assumption can be made that a non-uniform (finger-like) water flow has already reached the upper surface of the specimen. During the transition period only the unfilled pore space between the water fingers are being filled (i.e., dispersion is the main component of moisture movement during this stage). Surprisingly enough, the water redistribution within the specimen continues for a much longer time.  Figure 3 shows the waterfront reaches the top of the specimen in 42 minutes (45 s1/2) in the HS-B33 clay brick specimen, representative of the median result. This same specimen demonstrates a transition zone, which continues approximately until 70 minutes into the test, well after the water-front reaches the top surface. Such a significant difference in the time to fill the specimen volume (here 83.4 cm3) makes a direct comparison between the end of the first stage of water absorption and the capillary moisture content. To give an example, the first stage of water absorption for a 54.9-mm long specimen was determined in the experiment at 42 minutes. Yet, the beginning of the second stage of water absorption (where the capillary moisture content is determined) is reached at 70 minutes.

 

1.4. Comparison of different methods for determination of A-coefficient

 

Different methods of determining A-coefficient were examined using calcium silicate specimens. These comparisons included the effect of manual or automatic measurement of water mass in the specimen, influence of operator, size of the specimen and the type of side protection used. First, the manual and automatic methods of determining mass increase in the tested specimen were compared. Figure 4 shows calculations obtained from the manual measurements with those determined using an automatic balance. The derivative of A-coefficient for the first stage of the water absorption process performed with Ak 28 specimen shows a higher variability than that of manual measurements. It is clear that performing manual measurements is as good as using an automatic experimental set-up. 

 

While, there are differences in the measured rate of liquid inflow, this difference can be attributed to changes in the specimen’s geometry. Nevertheless, it is clear that the water absorption process for manual and automatic measurements is similar.  

 

 

Figure 4. The derivative of A-coefficient vs. square root of time as determined from manual (marked SU) and automatic measurements of mass increase in calcium silicate (the latter marked TUD). 

 

The effect of the operator on the measured results was also examined.  To this end, a separate set of measurements was performed manually by the two operators. Figure 5 shows a good agreement when different operators performed the water absorption test in the same manner. 

The next test series examined the effect of initial moisture content on the measured results.  Following an oven drying, one set of specimens was enclosed in an airtight desiccator containing a 500 g of desiccant ensuring that the specimens remained dry prior to testing.  The second set of specimens was enclosed in an airtight desiccator containing a salt solution that maintained air at approximately 80% RH. The specimens were stored in each condition for three weeks to ensure that the equilibrium moisture content has been reached.  Figure 6 shows the water absorption process performed on the specimens with different initial moisture contents.  The graph indicates that the results obtained are in excellent agreement.  The initial moisture content has no effect on the water absorption process in calcium silicate.

 

 

Figure 5. The derivative of A-coefficient vs. square root of time measured for calcium silicate by the two operators.

 

 

Figure 6. The derivative of A-coefficient vs. square root of time measured by a single operator with calcium silicate specimens having initial moisture content in equilibrium with near 0% and 80% RH environment.

 

Finally, the effect of side protection on the measured moisture inflow was also examined. Two types of side protection were used to provide one dimensional moisture transport. In the first instance, a paint coating was used. In the second instance, a combination of two component epoxy resin, and self-adhesive strips of aluminum tape were utilized in sealing the sides of the specimens. Aluminum strips were applied onto the coated sides once the epoxy begun to set.  The strips were pressed and smoothed to remove any air pockets.

 

 

Figure 7. The derivative of A-coefficient vs. square root of time shows a good agreement between specimens with different type of side protection.

 

The results from parametric tests indicated that manual measurements were in good agreement with automatic measurements.  Either method can be used in the determination of A-coefficient.  The manual method provides an advantage since several specimens can be tested at once.  With the automatic method a single specimen is tested at a time, and thus this alternative is time consuming.  The other factors examined including the use of different operator, different initial moisture content, and two methods of specimen sealing had no effect on water absorption process in calcium silicate.

 

1.5.       Proposed improvements to the test method to determine the A-coefficient

 

The Ao-coefficient for clay brick and calcium silicate was now determined as the mean of the values measured between time to and t1, between 3 and 45 minutes for both materials. A correlation between this value and the capillary moisture content is to be established. The selection of the time to determine a value of the capillary moisture content is, however, uncertain. This characteristic is calculated using two different assumptions:

 

(1)  Capillary moisture content is measured immediately when the transition to stage two ends (no time for air redistribution),

 

(2)  Time for the air and water redistribution was set as equal to the period in which the water front reached the upper surface of the specimen.

 

In the first case the period is approximately 60 minutes and the second case is approximately 90 minutes. Table 5 shows results for clay brick specimens and Table 6 shows results for calcium silicate.

 

Table 5. Clay brick – Comparison of Aw and Ao coefficients and capillary moisture content determined for two cases discussed in the text.

 


 

            Table 6. Calcium silicate – Ao coefficient and capillary moisture content.

 

 

 

63 s0.5

 

Ao

water content

Specimen

[kg/m2s0.5]

[m3/m3]

HS - AK 26

1.210

-

HS - AK 27

1,243

-

HS - AK 28

1,201

0,825

HS - AK 29

1,153

0,820

HS - AK 30

1.239

0.826

HS - AK 31

1,089

-

HS - AK 32

1,171

-

HS - AK 33

1,137

0,838

HS - AK 34

1,227

-

HS - AK 35

1,223

-

 

 

 

Figure 9. The correlation between the capillary moisture content Wc and the Ao-coefficient for clay brick. This correlation includes the period of air redistribution (case 2) and the Ao-coefficient measured within the specified time interval.

 

Comparing Figures 8 and 9 highlights the effect of improved precision in determining the Ao and Wc. For clay brick, a material with a significant variability between specimen the correlation coefficient changes from 1% to 13%. Though this is a weak correlation (similar to the level observed for other two tested materials), the difference appears significant enough to warrant the need for improving the test precision.

The above discussion highlights the need to improve A-coefficient tests. These measurements must be performed during an initial stage of water absorption process. It is necessary to use a defined test period i.e., restricting both the initial and the final times. It is also advisable to introduce the quadratic progression of time steps into the testing schedule. Given an appropriate preparation of the specimens, the measurements of water absorption coefficient determined either automatically or manually can yield reproducible and repeatable values. 

 

2. Recommended methods for the water absorption coefficient and capillary moisture content determination

 

The first requirement for the A-coefficient test is the stability of the water level. One can use either one of two approaches:

·         a large tank with enclosure to limit evaporation

·         the overflow in a tank with a constant water circulation.

 

The stability of the water level has to be quantified for both short and long term as follows:

(I)                   0.1 mm per 1hour, and

(II)                 1 mm per 24 hours.

 

The tank must be equipped with specimen supports e.g., cylindrical, non-corroding rods with diameter between 10 and 20 mm. These rods are to be leveled to the precision of 2 mm per 1 m i.e., limiting a difference in the immersion depth on the 100 mm square specimen to the maximum 0.2 mm difference. The geometrical difference between any point on the top surface of the supporting rods and the surface of the overflow edge should be within 1.0 + 0.5 mm. The overflow should be designed so that the difference between the water level during the overflow and any point on the top surface of the support rods does not exceed 2 mm. 

 

If an automatic balance is used, it must be provided with two devices. One device must allow for mounting of a specimen so that it is leveled and in conformance with the above requirements. The other device must allow movement of the water tank providing 1.5 + 0.5 mm immersion into the water tank at time zero.

 

Independent from the specifications on the allowed variation in the mean immersion depth, each testing apparatus must be verified to prove the repeatability of precision (uncertainty of the mean value when the tests are done in the same laboratory by different operators) such that the average of three tests ensures the water immersion depth to be within 0.5 mm. 

 

The test specimens can be either square or round with the side or diameter of:

 

1)       100 + 10 mm, or

2)       50 + 5 mm

 

Specimens should be cut and finished smooth, though the presence of a third material adhered to the surface is not allowed when the specimen is brought for testing. The minimum number of specimens used for one measurement is either 3 large or 5 small.

 

Two initial conditions are recommended.  Procedure a) requires the specimen to be oven dried (i.e., dried to a constant weight), while procedure b) accepts conditioning at 32 + 2 %RH (e.g., using MgCl2 solution) to achieve required equilibrium moisture content. The procedure b) is used for materials that cannot easily be dried to the constant weight e.g., wood or wood based products. The specimen weight must be determined prior to and following the application of side protection. In both procedures, the mass of the specimen must be determined prior to and after the application of side protection. Acceptable materials for use as a side protection include of epoxy resins, paraffin/wax mixtures (note temperature limits) or liquid applied coatings if they are proven to be non-reactive with the material, and proven to adhere to the material’s surface. All kinds of adhesive tapes are disallowed. Large size specimen should be used if the coating penetrates into the specimen’s pores (e.g. epoxy resin coating).

 

If the overhead balance is used, the space in which the free water intake test is conducted must be enclosed. The relative humidity of the air in the enclosed space above the specimen should not fall below an average 95%. If the manual method is selected, a plastic film should be used to protect the upper surface of the specimen and avoid evaporation of water from the specimen’s upper surface. To avoid build-up of air overpressure the film should be punctured with a single pinhole. Typically, the space separating the film and the specimen’s upper surface should not exceed 1-mm in thickness.

 

One of the more controversial parts of this test procedure is the selection of the specimen’s thickness. While varying thicknesses can be used to determine the water absorption coefficient, the determination of the capillary moisture content requires a more narrowly defined experimental duration.  It is recommended, as much as possible, that the period in which water reaches the upper surface of the material be limited to 2 hours. Continuation of the measurements is, however, required for the subsequent establishment of the capillary moisture content.

 

Typically, the following rules of thumb can be used:

 

·         for a material with A-coefficient above 0,5 kg /(m2s½)                                    60 – 70 mm

·         for a material with A-coefficient between 0,1 kg /(m2s½)  and  0,3 kg /(m2s½)   40 – 60 mm

·         for a material with A-coefficient between 0,01 kg /(m2s½) and  0,1 kg /(m2s½)  10 -20 mm

 

In such a case the recommended sequence for the collection of raw data is presented in Table 7.

 

Table 7. The recommended times for weighing or collecting data from the automatic readout.

 

s 0.5

Seconds

Minutes

Hours

5

25

0.42

-

15

225

3,75

-

20

400

6,67

-

25

625

10,42

-

30

900

15,00

-

35

1225

20,42

-

40

1600

26,67

-

45

2025

33,75

-

50

2500

41,67

-

55

3025

50,42

-

60

3600

60,00

1hour

75

5625

93,75

1h - 34 min

90

8100

135,00

2h - 15min

120

14400

240,00

4h

150

22500

375,00

6h - 15min

 

 

 

 

and for wcap

(as needed)

 

 

294

86400

1440

24 h

415.7

172800

2880

48 h

509.1

259200

4320

72 h

 

Determining the end of the transition stage requires calculation of the increase in cumulative moisture flux over a period between two consecutive time measurements presented as a function of square root of time. Several measurements must also be performed in the second stage of the water absorption process. If the water front has reached the upper surface of the material within 2 hours, reading obtained 24 hours from the beginning of the test ensures that the second stage of the water absorption test is well described. If the waterfront has not reached the upper surface within the first 4 hours the test must be extended to 48 hours period or longer. After 24h period all readings should be performed with a 24 hours frequency.

 

NOTES: In the manual test procedure, the immersed surface of the specimens must be wiped with a moist cloth to remove the remaining water droplets. Note that use of a dry paper towel is disallowed (as it may suck some water out of the specimen). The weighing and the replacement of the specimens in the water tank should be performed within a short time, typically less than 10 seconds.

3. Recommended method for determination of the capillary moisture content

A few measurements must also be performed in the second stage of the water absorption process. If the waterfront has reached the upper surface of the material within 2 hours, a 24 hours period would describe well the second stage of the water absorption. If the waterfront has not reached the upper surface within 4 hours the test must be extended to 72 hours or more and the frequency of readings should be appropriately reduced.

Figures 1 through 3 displayed a period of constant increase that was followed by a transition zone. Only at the end of this transition zone, the further increase of mass is stabilised to a small and almost constant value indicating that the capillary moisture content has been reached. At his point (t2= 63 s1/2 in Figure 3), starts the second stage of the free water intake process. Traditionally, the second stage of this process has been determined as a cross point between tangents approximating the first and the second stage of absorption. Depending on the precision of the graph used in representing the cumulative water flux into the specimen, this intercept could vary. Such an approximation is valid only for those cases where the entire free water intake process can be approximated as a straight line e.g. for a calcium silicate but not for materials such as AAC (autoclaved aerated concrete).

Figure 10 compares time required for water to penetrate across the entire height of the specimen (measured as a change of the electric resistance) in comparison to the end of the free water intake process (measured as the change in the mass increase).


 

Figure 10. Time when water front reaches the uppers surface measured as a change of the electric resistance on the surface (about 48 s½), and end of the free water intake process measured as the mass increase (about 56 s1/2).

 

In the case of calcium silicate, the difference between moisture content at the end of water penetration and at the capillary saturation is significant, namely 48 s1/2 and 63 s1/2. Even larger are differences measured on clay brick materials (see Table 5).

It is therefore required that when reporting the water absorption coefficient and the capillary moisture content one should indicate the following:

(1)     time to, at which A-coefficient readings were considered as constant

(2)     time at which the front of water intake has reached the upper end of the test specimen, i.e., end of the A-coefficient validity range,

(3)        time at which the capillary moisture content was measured and the definition that was used (tangents or a real time estimate).

4.         Dry cup water vapour transmission

While a water vapour permeability coefficient, corresponding to other heat and mass flow transport coefficients, is used in North America, another physical parameter, m [-] is often used in Europe. It is based on a traditional German system of measurements. It is defined as the ratio of the water vapour resistance of the material to a resistance of the water vapour in an air layer of same thickness and same temperature. Based on Fick´s law and the general concept of state variables for ideal gases, the water vapour flux can be described with the following equation:

J = D/ (RT) grad (rv)

Where: D is the water vapour diffusion coefficient in air, R is the universal gas constant for water vapour and T the absolute temperature. Independently of the manner in which results are reported, measurements of the water vapour diffusion are the same.  The rw gradient is described by the difference in the partial pressures of water vapour. The material samples are mounted and sealed to a PVC-holder that is placed between two environments with a constant temperature and RH.

5.  Narrow range high relative humidity water vapour transmission

While for the engineering model one only needs the dry cup permeability, to enhance the precision of the model, water vapour transmission in the upper end of the hygroscopic range can also be measured. The test is performed within a narrow range of relative humidity and with the use of salt solutions. Typically, the specimen is placed between two environments with 85 and 96% RH respectively and increase of mass is measured on the side with a lower vapour pressure.

 

Refernces and selected literature

 

Bomberg M., J. Carmeliet, J. Grunewald, A. Holm, A. Karagiozis, H. Kuenzel, S. Roels, 2002, Position paper on material characterization and HAM model benchmarking, Nordic Building Physics Symposium 2002, vol. 1, p.143-150

 

Bomberg M., R. Plagge, and M. Pazera, 2004, An analysis of water absorption coefficient measurements performed during HAMSTAD project, Journal of Thermal Envelope and Building Science, in print

 

Descamps F, 1997, Continuum and discrete modelling of isothermal water and air transfer in porous media, PhD-dissertation, K.U. Leuven, Belgium

 

Grunewald J. and M. Bomberg, An engineering approximation of material characteristics needed for input to Heat, Air and Moisture transport simulations, 11th Building Physics Symposium, Dresden Sept 26-28, 2002 pp.272-285

 

Grunewald, J. P. Haeupl and M. Bomberg, 2003, Towards an engineering model of material characteristics for input to HAM transport simulations – Part 1: An approach, Journal of Thermal Envelope and Building Science, Vol. 26, p343-366

 

Grunewald J., Häupl P. 2002. Ein Modell zur Beschreibung der feuchteabhängigen Dampfleitfähigkeit kapillarporöser Materialien. 1. Bauklimatisches Symposium an der TU Dresden Sept 26-28, 2002 p286-294.

 

Häupl P. and H. Fechner, 2003. Hygric properties of porous building materials, Journal of Thermal Envelope and Building Science, Vol. 26,#3, pp259-284

 

Pel L., 1995, Moisture transport in porous building material, Ph.D. thesis, T.U Eindhoven

 

Roels S, 2000, Modelling unsaturated moisture transport in heterogeneous limestone, Ph.D. thesis, KU Leuven,

 

Roels S, Carmeliet J and Hens H, 2003, HAMSTAD, WP1: Final report Moisture transfer properties and materials characterisation, February 2003, K.U.LEUVEN, Belgium.

 

Roels S, O Adan, H Brocken, J Carmeliet, R Cerny, Ch Hall, H Hens, K Kumaran, Z Pavlik, L Pel, R Plagge, 2004, Interlaboratory comparison of the measurement of basic hygric properties of porous building materials, Journal of Thermal Envelope and Building Science,  in print

 

Carmeliet J., Roels S. 2002. Optimal determination of the moisture capacity of porous building materials. Journal of Thermal Envelope and Building Science, January 2002 p167-188