TRHUMIDADE - A water diffusivity model to predict moisture content profiles

Figures & data

Table 1. Most common procedures to determine moisture content profiles in porous building materials

Destructive methods	Non-destructive methods
Slice–dry-weigh-method: to obtain the moisture content profiles the specimen is quickly sliced into discs that are weighed, dry and weighed again. This is one of the most accurate methods, because the actual amount of water in the specimen is measured (Janz, 1997).	Electrical methods:
	Resistance method: is based on electrical conductivity variations. The electrical conductivity is measured with two electrodes and the water content is evaluated through a calibration curve of the material under study (Janz, 1997; Roels et al., 2004).
	Capacitance method: is based on electrical capacitance variations. The tested material is used as a dielectric and the capacitance variations as a measure of the water content in the material (Janz, 1997; Roels et al., 2004).
Thermal imaging: is based on the decrease of the temperature when the water starts evaporating. Every time the measurement of the moisture content profile is required the specimen is split into two halves. The temperature distribution on the split surface is measured with an infrared camera and the moisture content profiles can be consequently obtained (Janz, 1997).	Neutron radiography: in this method a neutron beam is used to interact mainly with the hydrogen nuclei of the materials under study. The attenuation of the neutron beam is directly related to the total water content of the material. To perform a neutron radiography test safety measures and trained operators are required (Pel, 1992/1995; Pel, Ketelaars, Adan, & Van Well, 1993; Pleinert, Sadouki, & Wittmann, 1998).
	Nuclear magnetic resonance (NMR): in this technique an exterior permanent magnetic field is applied to the material tested, causing the absorption of some energy. The amount of energy absorbed is proportional to the number of hydrogen nuclei, which is a measure of the water content in the material. (Pel, 1992/1995; Roels et al., 2004).
	Computer tomography: measures the intensity loss of a narrow beam of X rays that goes through the specimen. The amount of absorbed radiation depends on the material density and is evaluated by using the CT-value. The difference in the CT-value between air and water is an indicator for measuring the water content in the material (Baker et al., 2007; Janz, 1997).
	Microwave bean: uses a microwave beam to measure the moisture content profiles. During the measurement the sample is placed between a transmitter and a receiver and the attenuation of the beam is measured. The magnitude of the attenuation is related to the water content of the material under study (Janz, 1997).
	Thermal conductivity: the sample is submitted to a known amount of heat and then the temperature vs time curve is calculated, which is a measure of the thermal conductivity of the material. In this way, the moisture content can be calculated if the relation between the moisture content and the thermal conductivity is known (Janz, 1997).
	Gamma-ray attenuation: in this method a narrow beam of gamma-ray photons crosses through the specimen and interacts with the orbital electrons of the material which will be absorbed or scattered. The intensity loss of the emitted radiation is used to determine the water content of the material. The moisture content profiles are measured in an almost continuous way. The equipment is rather expensive and special safety arrangements are necessary due to the radioactivity involved in the process (de Freitas, 1992; Roels et al., 2004).

Figure 1. Operating scheme of the gamma-ray measuring equipment.

Figure 2. Position of the red brick sample in the gamma-ray equipment.

Figure 3. Characteristics of the red brick samples tested (dimensions in mm).

Figure 4. Experimental moisture content profiles of the red brick provided by gamma-ray attenuation method.

Figure 5. Experimental moisture content profiles of the autoclaved cellular concrete (ACC) provided by gamma-ray attenuation method.

Table 2. Hygrothermal properties of the materials used in the WUFI Pro simulations

	Red brick		Autoclaved cellular concrete (ACC)
ρ (kg/m^3)	1,925		525
C 0 (J/(kg.K))	920		1,050
ξ (%)	28		66
Moisture storage function*** W (kg/m^3)	W	RH%	W	RH%
	0	0	0	0
	7.7	80	9.975	10
	17.325*	95	18.375	70
	250.25**	100	24.15	90
			145.95*	95
			273.00**	100
λ (w/(m.K))	1		0,14
D ws (m^2/s)	W	DWS	W	DWS
	0	2.50E-09	5.775	8.06E-08
	9.625	2.02E-09	8.400	6.27E-08
	19.250	1.54E-09	16.275	8.90E-09
	38.500	2.64E-09	32.550	1.44E-09
	77.000	6.53E-09	82.950	5.39E-10
	134.750	2.21E-08	142.800	5.20E-09
	173.250	5.61E-08	203.700	1.16E-08
	211.750	1.37E-07	247.800	2.72E-08
	231.000	1.78E-07	262.500	9.84E-08
	250.250	3.91E-07	268.275	1.26E-07
			273.000	1.49E-07
μ	77		7,5

* Critical moisture content, W _cr (kg/m³).

** Saturation moisture content, W _sat (kg/m³).

*** The hysteresis between absorption and desorption is not considered in this program.

Table 3. Hygrothermal properties of the materials used in the TRHUMIDADE simulations

	Red brick				Autoclaved cellular concrete (ACC)
ρ 0 (kg/m^3)	1,925				525
C 0 (J/(kg.K))	920				1,050
ξ (%)	28				66
Sorption isotherm** U (kg/kg)	U		RH%		U		RH%
	0.000		0		0.000		0
	0.004		80		0.019		10
	0.009*		98		0.035		70
					0.046		90
					0.278*		98
D w (m^2/s)	U		D W		U		D W
	0		2.50E-09		0.011		8.06E-08
	0.005		2.02E-09		0.016		6.27E-08
	0.010		1.54E-09		0.031		8.90E-09
	0.020		2.64E-09		0.062		1.44E-09
	0.040		6.53E-09		0.158		5.39E-10
	0.070		2.21E-08		0.272		5.20E-09
	0.090		5.61E-08		0.388		1.16E-08
	0.110		1.37E-07		0.472		2.72E-08
	0.120		1.78E-07		0.500		9.84E-08
	0.130		3.91E-07		0.511		1.26E-07
					0.520		1.49E-07
D T (m^2/(s.K))	U		D T		U		D T
	0		4.95E-12		0.011		2.01E-13
	0.005		6.30E-12		0.016		2.46E-13
	0.010		1.03E-11		0.031		2.91E-12
	0.020		1.53E-11		0.062		8.36E-12
	0.040		1.88E-11		0.158		2.14E-11
	0.070		1.77E-11		0.272		3.18E-11
	0.090		1.56E-11		0.388		3.81E-11
	0.110		1.08E-11		0.472		3.75E-11
	0.120		7.85E-12		0.500		3.28E-11
	0.130		4.95E-12		0.511		1.95E-11
					0.520		1.56E-12
λ (w/(m.K))	U	λ (ºC)			U	λ (ºC)
		10	30	50		10	30	50
	0	1.00	1.00	1.00	0.011	0.140	0.141	0.144
	0.005	1.05	1.05	1.05	0.016	0.142	0.144	0.148
	0.010	1.09	1.10	1.10	0.031	0.149	0.153	0.161
	0.020	1.18	1.19	1.20	0.062	0.163	0.171	0.187
	0.040	1.36	1.38	1.39	0.158	0.211	0.241	0.277
	0.070	1.55	1.58	1.60	0.272	0.278	0.345	0.430
	0.090	1.69	1.72	1.73	0.388	0.313	0.400	0.565
	0.110	1.84	1.86	1.87	0.472	0.337	0.414	0.611
	0.120	1.92	1.93	1.93	0.500	0.345	0.419	0.618
	0.130	2.00	2.00	2.05	0.511	0.348	0.421	0.620
					0.520	0.351	0.422	0.617

^* Critical moisture content, U _cr (kg/kg).

^** The hysteresis between absorption and desorption is not considered in this program. For an easy numerical treatment, the sorption isotherm must be provided to the program on a linear shape, which justifies the small number of points defined for each curve.

Figure 6. Comparison of the moisture content profiles on red brick obtained by gamma-ray (γ-ray), WUFI Pro (WUFI) and TRHUMIDADE (TRH).

Figure 7. Comparison of the moisture content profiles on autoclaved cellular concrete (ACC) by gamma-ray (γ-ray), WUFI Pro (WUFI) and TRHUMIDADE (TRH).

Figure 8. Calculation procedure to obtain the total amount of water absorbed (kg) per m² of material (t ₁ and t ₂ represent the different hours at which the moisture content profiles were obtained).

Janz, M. (1997). Methods of measuring the moisture diffusivity at high moisture levels ( Licentiate thesis). Institute of Technology, Division of Building Materials, Lund.

 Google Scholar

Roels, S. , Carmeliet, J. , Hens, H. , Adan, O. , Brocken, H. , Cerny, R. , ... Pel, L. (2004). A comparison of different techniques to quantify moisture content profiles in porous building materials. Journal of Thermal Envelope and Building Science , 27 (4). doi:10.1177/1097196304042117

 Google Scholar

Pel, L. . (1992/1995). Moisture transport in porous building materials ( PhD thesis). FEUP, Porto.

 Google Scholar

Pel, L. , Ketelaars, A. A. J. , Adan, O. C. G. , & Van Well, A. A. (1993). Determination of moisture diffusivity in porous media using scanning neutron radiography. International Journal of Heat and Mass Transfer , 36 (5), 1261–1267.10.1016/S0017-9310(05)80095-X

 Web of Science ®Google Scholar

Pleinert, H. , Sadouki, H. , & Wittmann, F. H. (1998). Determination of moisture distributions in porous building materials by neutron transmission analysis. Materials and Structures , 31 , 218–224.10.1007/BF02480418

 Web of Science ®Google Scholar

Baker, P. H. , Bailly, D. , Campbell, M. , Galbraith, G. H. , McLean, R. C. , Poffa, N. , & Sanders, C. H. (2007). The application of X-ray absorption to building moisture transport studies. Measurement , 40 (9-10), 951–959. doi:10.1016/j.measurement.2006.10.016

 Web of Science ®Google Scholar

de Freitas, V. P. (1992). Transferência de humidade em paredes de edifícios—Análise do fenómeno de interface ( PhD thesis). FEUP, Porto.

 Google Scholar