Title page for ETD etd-71798-14046

Type of Document Master's Thesis
Author Alvarez, Juan C.
Author's Email Address jualvare@vt.edu
URN etd-71798-14046
Title Evaluation of Moisture Diffusion Theories in Porous Materials
Degree Master of Science
Department Mechanical Engineering
Advisory Committee
Advisor Name Title
Thomas, William C. Committee Chair
Dancey, Clinton L. Committee Member
Ellis, Michael W. Committee Member
  • moisture diffusion
  • transport
  • porous
Date of Defense 1998-08-07
Availability unrestricted
Moisture transport in building materials is directly responsible for structural

damage, as well as poor indoor air quality. For these reasons, the need to

understand transfer mechanisms and predict moisture transport through building

materials has increased over the last couple of decades.

Although moisture diffusion phenomenon in the isothermal regime has been

studied and explained extensively, there is no universally accepted model for

predicting the moisture diffusion in a nonisothermal situation. Several

diffusion models in the form of "Fick's Law" including ones based on gradients

of water-vapor pressure, chemical potential of water, moisture concentration and

activated moisture molecules have been proposed for predicting moisture diffusion

through porous materials. However, the lack of reliable experimental results,

resulting from the complexity of arranging accurate and repeatable measurement

techniques and slow moisture movement, has prevented any model from being universally

accepted. The present research addresses this modeling problem by evaluating

current diffusion models through a series of experiments performed on oriented

strand board (OSB), which is a wood-based material.

The present experimental apparatus, developed over the last three years, was

designed for the specific purpose of studying and developing an accurate method

to measure moisture transfer properties in porous materials under nonisothermal

conditions. The apparatus consists of a system of two environmental chambers

capable of achieving a wide range of temperatures and relative humidities.

Temperature and relative humidity can be independently controlled to within ±0.05°C and ±0.10 per cent R.H. of the set points. This apparatus is an alternative to the

ASTM "cup method" which is limited to isothermal conditions and discrete relative

humidities that correspond to those for various saturated salt-in-water solutions.

Unlike the cup method, the relative humidity within the chambers is controlled by

the direct removal and injection of distilled water. The system has forced

recirculating flow which reduces the time to reach steady state. The new forced,

direct control measurement procedure is denoted "ASHRAE FDC".

The results obtained from the ASHRAE FDC experiments, show that moisture diffusion

under nonisothermal conditions is governed by the gradient of the water-vapor pressure.

The moisture transfer must cease when the diffusion potential is the same on both sides

of the material for the validation of the diffusion model. The results show that the

water-vapor pressure model meets this necessary and sufficient condition. Furthermore,

a plot of the diffusion flux versus vapor-pressure difference was linear, within

measurement uncertainty bounds. This observation infers that the permeability is

approximately constant over the range of temperatures and humidities used in the


During the ASHRAE FDC experimental procedure a small difference in the static pressure

between the chambers was found. This pressure difference which was also observed in

ASTM cup tests, is believed to be caused by concurrent air diffusion. The bulk flow

of air governed by Darcy's equation balances the diffusion of air in the opposite

direction as a result of the gradient in the partial pressure of (dry) air. The air

permeability of an OSB specimen was measured and the results presented.

The operation and accuracy of the apparatus was validated by comparing results from

a series of isothermal tests to previously published results. The results obtained

from the isothermal test allowed the permeability to be compared to results obtained

from cup tests during the present investigation and to those previously published

using the same method. Good agreement was found between the new data from both FDC

and cup experiments and previously published results.

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  Appendix_A.pdf 27.24 Kb 00:00:07 00:00:03 00:00:03 00:00:01 < 00:00:01
  Appendix_B.pdf 32.37 Kb 00:00:08 00:00:04 00:00:04 00:00:02 < 00:00:01
  Appendix_C.pdf 18.18 Kb 00:00:05 00:00:02 00:00:02 00:00:01 < 00:00:01
  CHAPT3.PDF 178.32 Kb 00:00:49 00:00:25 00:00:22 00:00:11 < 00:00:01
  Chapter2.pdf 98.26 Kb 00:00:27 00:00:14 00:00:12 00:00:06 < 00:00:01
  Conc_Refe.pdf 15.10 Kb 00:00:04 00:00:02 00:00:01 < 00:00:01 < 00:00:01
  Intro.pdf 82.23 Kb 00:00:22 00:00:11 00:00:10 00:00:05 < 00:00:01
  Vita.pdf 4.45 Kb 00:00:01 < 00:00:01 < 00:00:01 < 00:00:01 < 00:00:01

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