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1.6 PROPERTIES OF FLUIDS IN SOLIDS

1.6 PROPERTIES OF FLUIDS IN SOLIDS. 1.6.1 Relevance 1.6.2 Viscosity 1.6.3 Water and water vapour 1.6.4 Porosity 1.6.5 Water in pores 1.6.6 Condensation in pores 1.6.7 Drying of materials. Applications of Moisture Movement. Waterproofing buildings The effect of frost on building materials

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1.6 PROPERTIES OF FLUIDS IN SOLIDS

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  1. 1.6 PROPERTIES OF FLUIDS IN SOLIDS • 1.6.1 Relevance • 1.6.2 Viscosity • 1.6.3 Water and water vapour • 1.6.4 Porosity • 1.6.5 Water in pores • 1.6.6 Condensation in pores • 1.6.7 Drying of materials

  2. Applications of Moisture Movement Waterproofing buildings The effect of frost on building materials Damp in buildings Penetration of salt into materials Shrinkage and swelling

  3. 1.6 PROPERTIES OF FLUIDS IN SOLIDS • 1.6.1 Relevance • 1.6.2 Viscosity • 1.6.3 Water and water vapour • 1.6.4 Porosity • 1.6.5 Water in pores • 1.6.6 Condensation in pores • 1.6.7 Drying of materials

  4. Viscosity Shear Stress  Fluid velocity V1 y The viscosity e is defined as: e = y Pa s v1 - v2 where  is the shear stress Fluid velocity V2 Shear Stress  Bingham Fluid Gradient of this line = viscosity (e) Newtonian Fluid yield stress  Shear rate

  5. 1.6 PROPERTIES OF FLUIDS IN SOLIDS • 1.6.1 Relevance • 1.6.2 Viscosity • 1.6.3 Water and water vapour • 1.6.4 Porosity • 1.6.5 Water in pores • 1.6.6 Condensation in pores • 1.6.7 Drying of materials

  6. Definition of Humidity

  7. Water and air in equilibrium at 20oC Air at atmospheric pressure (100 kPa) made up of nitrogen (approx 70kPa), oxygen and other gases and water vapour (approx 2kPa) 100% RH Vapour pressure = 2kPa Saturation vapour pressure = 2kPa Relative Humidity (RH) = 100% Water

  8. Half of the gas is replaced with dry air Vapour pressure = 1kPa Saturation vapour pressure = 2kPa Relative Humidity (RH) = 50% (typical for inside a building) Air at atmospheric pressure (100 kPa) water vapour pressure 1kPa (This will increase to 2kPa with time) Water

  9. The temperature is increased rapidly to near 100oC (total pressure held constant) Air at atmospheric pressure (100 kPa) water vapour pressure 1kPa (this will increase to 90kPa with time) Vapour pressure = 1kPa Saturation vapour pressure = 90kPa (reaches atmospheric pressure at 100 oC) Relative Humidity (RH) = 1.1% To get over 100oC without boiling the pressure must be raised (as for autoclaved concrete) Water

  10. The temperature is decreased below 20oC (total pressure held constant) Air at atmospheric pressure (100 kPa) water vapour pressure 1kPa Vapour pressure = 1kPa Saturation vapour pressure = 0.5kPa Relative Humidity (RH) = 100% (cannot go above this. Will cause condensation or fog) Water

  11. Effect of temperature on water content in air Example A shows air at 100% RH and 20oC heated to 40oC to give 31% RH Example B shows air at 50% RH and 20oC heated to 40oC to give 15% RH

  12. 1.6 PROPERTIES OF FLUIDS IN SOLIDS • 1.6.1 Relevance • 1.6.2 Viscosity • 1.6.3 Water and water vapour • 1.6.4 Porosity • 1.6.5 Water in pores • 1.6.6 Condensation in pores • 1.6.7 Drying of materials

  13. Porosity The porosity (p) is defined as: p = volume of pores 100 % Bulk volume The bulk volume is measured by measuring the dimensions of the solid and multiplying height  width  depth.

  14. Pore size Distribution Pore volume (bars) Cumulative pore volume (line)

  15. Helium PyconometerMeasures porosity

  16. 1.6 PROPERTIES OF FLUIDS IN SOLIDS • 1.6.1 Relevance • 1.6.2 Viscosity • 1.6.3 Water and water vapour • 1.6.4 Porosity • 1.6.5 Water in pores • 1.6.6 Condensation in pores • 1.6.7 Drying of materials

  17. Water in Pores • Most materials will swell when they get wet. • The amount of swelling varies from 1000 microstrain for weak mortars and wood across the grain down to 100 microstrain for strong bricks. • Materials with finer pores will swell more and strong materials will swell less. • Apart from causing an item, such as a door, not to fit swelling may damage materials when it occurs unevenly. For example if the outer surface of a solid wets and swells but the inner core does not it may be damaged.

  18. 1.6 PROPERTIES OF FLUIDS IN SOLIDS • 1.6.1 Relevance • 1.6.2 Viscosity • 1.6.3 Water and water vapour • 1.6.4 Porosity • 1.6.5 Water in pores • 1.6.6 Condensation in pores • 1.6.7 Drying of materials

  19. Condensation in pores The conditions under which water will sustain a meniscus in small pores are given by: r Ln(RH) = - 2 m s  R T  where: r is the pore radius in m Ln(RH) is the natural log of the relative humidity m is the molecular weight of water = 0.018 kg s is the surface tension of water = 0.073 N/m  is the density of water = 1000 kg/m3 R is the gas constant = 8.3 J/mol/oK T is the temperature = 290 oK (at 20 oC)

  20. Water in Concrete • A pore of radius 3  10-9 m will fill with water at humidities over 70%. Thus in a moist atmosphere concrete will absorb a large amount of water.

  21. 1.6 PROPERTIES OF FLUIDS IN SOLIDS • 1.6.1 Relevance • 1.6.2 Viscosity • 1.6.3 Water and water vapour • 1.6.4 Porosity • 1.6.5 Water in pores • 1.6.6 Condensation in pores • 1.6.7 Drying of materials

  22. Drying of materials • The rate of evaporation will depend on temperature, the relative humidity near the surface, the chemistry of the pore solution, and the exposed surface area. • The relative humidity near the surface will be controlled by air movement (wind or convection). • Materials with salt in the pores will tend to attract moisture and are described as "hygroscopic". • The exposed surface area will be the porosity the area.

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