CHOSEN PROPERTIES OF BUILDING MATERIALSCHOSEN PROPERTIES OF BUILDING MATERIALS

Properties of building materials – introduction and summary

• basis elements of buildings materials represent atoms (corpuscles) – elements having mass range 10-22 – 10-24 g (diameter range from 62pm (He) to 520 pm (Cs)

• molecules – sufficiently stable units at least of two atoms in definite arrangement (see Lecture No. 2) held together by chemical bonds

• elements – pure chemical substances composed of atoms with the same number of protons

• compounds – combination of two or more molecules (dimension in the range of nm to high molecular compounds – they excess of 1000 atoms)

• formation of compounds from the elements, their trnaformation and processes of elements insulation are chemical processes denoted as chemical reactionschemical reactions

within the chemical reactions there are formed bonding forces between the particular atoms and are rising and destroying chemical bonds (see Lecture No. 3)

Properties of building substances• properties of building material are determined by the type and size

of bonding and cohesive forces, that take place among particularatoms, ions and molecules

Basically, we can distinguish:Basically, we can distinguish:1) Inorganic materials – metals

non-metallic (ceramics, glass, inorganic binders, surface coatings, mono-crystals – NaCl, diamond)

2) organic materials – wood, sheep wool, plastics, paper, straw3) composite materials – matrix with reinforcement (combination in

minimum of two materials), glass-ceramics, reinforced concrete, glass-cement

• With respect to the scale of investigation, texture texture and structurestructure of materials can be distinguished. Texture describes spatial distribution of particles and pores onmacroscopic levelmacroscopic level (from 0.1 mm).

Structure characterizes type and composition of particular phasewithout relation to spatial distribution. This characterization is done typically on microscopic levelmicroscopic level ( ⟨1μm).

Fig. 1: Idealised microstructures.

A) poly-crystallic with grains of different dimension, B) poly-crystallicwith grains of similar dimension, C) poly-crystallic with oriented grains (microtexture), D) microstructure with small pores, E) microstructure having pores dimension equal to dimension of grain, F) microstructure with big pores, G) microstructure having two phases – crystalline and glass (dashed line), H) microstructure of two phases whereas the crystalline phase has not direct bound

Scheme of the origin of microstructure of ceramics

Mixture of raw materials

(disperse system)

_______________________

mixture of minerals grains of different grain size (phases)

transient microstructure (moulding, dried moulding)

_______________________

forming and connection of mineral grains (phases) by inter-molecular forces, the presence of pores among the grains

final microstructure of clinkered ceramics

_______________________

orientation and connection of grains by reaction products, change of grains and pores (phase composition)

pressing, consolidation

clinkering, compacting

Scheme of the origin of microstructure of thehardened mortarMixture of raw materials(disperse system)_______________________

mixture of mineral grains of different size, one or several minerals

Mortar microstructure

compact (e.g. clinker) incompact (e.g. cement)

Formation and connection separated and free grains

of mineral grains, pores clusters of mineral grains

Microstrcture of the hardened mortar

_______________________

grains of hydration products of fresh mortar forming the continuous solid phase

thermal processing, clinkering

+ H2O, setting, hardening

Scheme of the Portland cement production

The rotary cement kiln

Typical clinker modules

1400 – 1450°C

Properties of building materials

- material composition (type of substances, solid, liquid, gaseous and amorphous substances)

-- type of materialstype of materials – porous materials, high-density materials, homogeneous and non-homogeneous materials, isotropic materials, anisotropic materials, (ortho-tropic materials – composite materials)

-- heterogeneous materialsheterogeneous materials – there are some areas in the materials that are having defined bounds and differ in properties or composition from their environment – typical for the most of solid substance (except of some alloys) – typical heterogeneous area represent pores, composites and powder substances

- eeffectffect of environmental conditions on the material propertiesof environmental conditions on the material properties(temperature, liquid moisture, relative moisture, pressure) !!!

Properties of building materials – development of intelligent materials – materials engineering

- materials as well as the complete structures that can in dependence on the change of their external environment properly and efficiently change their performance and properties

- e.g. vapour-permeable foils with variable water vapour resistance

- interior plasters that allows in sunny and hot days accumulation of heat, glasses with the reactive change of their colour (possibility to use them for heating – intelligent facades)

- under development there are also materials that change their properties due to the specific artifical signal – e.g. glues with phero-fluid nanoparticles on the basis of ferric oxide (these particles act as antennas that can record microwave radiation and on its action heat the glue on required temperature – glueing at command)

- self-cleaning roof, self-cleaning linings – coated by specially type of coatings that repellent dirt, smear and water

Properties of building materials – development of intelligent materials – materials engineering

- progressive development e.g. in outdoor equipment – intelligent intelligent textiles on PCM (phase change materials) principlestextiles on PCM (phase change materials) principles

- these textile materials react on the actual climatic changes as well as on heat and moisture produced by wearer(as high is the activity of wearer/ as high is the vapour premeabilityof material), this action is physically based on variable structure of hydrophilic polymer membrane that reacts on heat changes produced by wearer

-- materials with recovery (in shape), polymer foams materials with recovery (in shape), polymer foams –– definition of definition of strength, driven density etc.strength, driven density etc.

-- electrically conductive polymerselectrically conductive polymers – combine electrical performance typical for semi-conductors together with material properties that enable simple processing - they can change their structure and in consequence their physical properties (In 2001 Noble price winners -Alan J. Heeger, Alan G. MacDiarmid, and Hideki Širakawa) Necessary presumption for the advanced progress of intelligent materials represent mastery of nanonano--technological operationstechnological operations that will make possible design and development of new advanced well-considered material structure in molecular range!!!molecular range!!!

Properties of building materials

Basic material properties

Hygric and diffusion properties

Mechanical properties

Thermal properties

Acoustic properties

Radioactivity

Chemical properties

Basic physical properties

• properties that require for their determination only measurement of the weight of the sample and the dimension or the volume of the sample

• properties that in certain way material characterize and the other properties are with them in clear relation

• bulk density

• matrix density (specific density)

• relative density

• porosity

• granularity,grain size distribution etc.

Bulk density, matrix density

• physically defined as a ratio of elementary mass to elementary volume

• (within the matrix density determination, only the volume without pores is taken into account whereas for the measurement of bulk density, the total volume is considered)

[kg/m3] for the homogeneous material we can written

where ρv is bulk density defined as a ration of mass of the whole sample to sample volume including pores

dVdm

=ρ

vVm ρρ ==

Experimental determination of bulk density and matrix density

• Gravimetric method – from the measured dimensions of the sample and its mass, the bulk density can be calculated

• Vacuum water saturation method - from the measured dry mass of sample ms, mass of fully water saturated sample mv and from the mass of immersed saturated sample ma (so-called Archimedes weight) the sample volume can be calculated according to the following equation

where ρl is density of liquid water

Basic material properties as saturation moisture content wsat and material matrix density ρmat can be determined by the following equation

l

av mmVρ−

=

0v s

sat vm mw

Vψ ρ −

= = ( )01s

matm

Vρ =

−Ψ

Experimental assesment of matrix density

• Experimental assessment of volume of un-shapely building materials samples is very difficult. On this account, the indirect pycnometric method is used for the matrix density measurement.

• Pycnometer is special vessel having stopper containing capillary for the overflowing liquid. Hence, the pycnometer volume is always the same. The matrix density of material can be then calculated using following equation

[kg m-3]

where m1 is mass of dry sample [kg], m2 mass of closed pycnometer with sample and liquid [kg], m3 mass of pycnometer with stopper fulfilled by liquid [kg], ρl density of used liquid [kg m-3] (for water at 20°C cca 998.205).

( )1

3 2 1mat l

mm m m

ρ ρ ρ= = ∗− −

• On the other hand, building materials are mostly not homogeneous

they are porous, whereas very often are formed by mixture of several components (homogenization principles)

• generally, for most of studied materials certain volumetric homogeneity can be considered (samples of representative volume)

- on this account, the bulk density is used in technical practice for the basic characteristic of building materials

- bulk density is dependent on matrix density of particular components and on porosity

- for loose materials (aggregates, sand) and compressible materials (mineral wool, glass wool etc.) also on their compressibility – compression (powder density – involves whole volume of the grains icluding the gaps and interspace among the particular grains).

For example, for the porous aggregates, four different quantities can be distinguish:

• Powder density in freely state (e.g. 400 kg/m3)

• Powder density for partially vibrated aggregates (e.g. 600 kg/m3)

• Bulk density of grains (e.g. 850 kg/m3)

• Matrix density (e.g. 2550 kg/m3, according to the type of aggregates)

! Bulk density will be also changed due to the moisture Bulk density will be also changed due to the moisture content rising, because the porous space can be fulfilled by content rising, because the porous space can be fulfilled by water.water.

! Bulk density is quantity important not only from the Bulk density is quantity important not only from the mechanical reasons but affects also several thermal mechanical reasons but affects also several thermal material parameters like thermal conductivity, thermal material parameters like thermal conductivity, thermal diffusivity, specific diffusivity, specific heaheatt capacity, acoustic and hygric capacity, acoustic and hygric parameters.parameters.

Relation between bulk density and porosity of inorganic materials

Relation between bulk density and porosity of inorganic materials

Relative density

• Describes, how the material volume is fulfilled by the solid (defined only by solid matters)

• Mathematically is defined as a ratio of solid phase volume to the total volume or as a ratio of bulk density to matrix density

• in practice is given as a decimal number or in %

• For the loose materials, the rate of compression is introduced

(ratio of powder density at specific compression to the powder density at ideal compression)

h v

mat

VhV

ρρ

= =

Porosity

• Porosity is basically defined as a ratio of pores volume to the total volume of the porous body

[-], [%]

Open porosity part of total porosity involving the open pores - pores having the direct connection with the surface of material

- Open pores are usually formed by the gases released during the material production (light-weight materials), by water evaporation from the materials (concrete, ceramics, plasters, cement based composites), by wilful aerating and foaming (light-weight concretes, perlit).

VVo=ψ

The open pores due to the direct connection with external environment affect the following material performance and properties:

• moisture absorption and evaporation

• gases and liquids diffusion

•Sound-proofing properties (sound absorption)

• Thermal insulation properties (possibility of heat transport and accumulation).

• Mechanical properties (very serious – one of the objective and otcomes of materials engineering)

Closed porosity part of total porosity including closed pores (they are not connected with the material surface – they do not take part in transport processes)

- the closed pores cab be formed e.g. by clinker of ceramics and do not allow the air humidity ingress into the porous structure

- the pores are not simple capillaries, their shape if difficult and variable

- on this account, the porosity of materials is described by poresdistribution curve, which represents pore size as well as their volumetric representation

- for pores distribution curve measurement, different methods and approaches can be used – e.g. Mercury porosimetry, gas adsorption porosimetry, electron microscopy, optical microscopy, water suction etc.

- porosity has clear relation to the specific surface, that can bemeasured e.g. by nitrogen adsorption on the basis of BET method

100*(1 )vc

mat

ρψρ

= −

Suitable for micro and mesoporous matters(pore radius < 25 nm)

determination of: specific surfacemesopores size distribution

Gas adsorption porosimetry

Sorptomatic 1990

Gas adsorption porosimetry

Mesopores volume distribution of metakaolin:

Specific surface (BET):11.3 m2 g-1

Mesopores volume:0.021 cm3 g-1

Mercury intrusion porosimetrySuitable for meso and macroporous matters

(pore radius > 2 nm)

determination of: meso and macropores distributionspecific surfaceparticle size distribution

Pascal 140+440

Material Porosity [% vol.]

Ceramic brick 20 - 37

Cement mortar 31

Lime mortar 41

Gypsum 51 - 66

Sand 39

Fine aggregates 42

Marble 2 - 3

Sandstone 1 - 31

Limestone 31

Slate 1,5 – 2,5

From the point of view of transport processes, the porous substances are classified according to the – pores distribution curve

- The pore size influence the fulfilling of the pores by water dueto the effect of absorption and capillary forces

Example of pores distribution curve (dominant are capillarypores)

10000Ř / nm

1 1000010 10000100 100001000 10000

0.08

V Por

e / c

m3 g

-1

0.00

0.02

0.04

0.06

0.08

·10-3

dV/dŘ

/ cm

3 nm-1

g-1

0.0

0.5

1.0

1.5

2.0

·10-3

Dimensions of pores

• Ultracapillary pores - radius < 10-9 m, the dimension of these pores is comparable with the dimension of molecules – the water can not be transport through these pores

• Capillary pores – radius 10-9 – 10-3 m, water and gases are acting as in the system of capillaries – the moisture movement is induce by surface tension

Capillary pores:• capillary micropores: 2 . 10-9 – 2 . 10-6 m

• capillary mesopores: 2 . 10-6 – 60 . 10-6 m

• Capillary macropores: 60 . 10-6 – 2 . 10-3 m

• Macropores and air pores – the capillary forces are not present because of the big size of the pores, the effect of gravitation is dominant

Voids• property assessed for loose materials• ratio of volume among the grains to the total volume

of loose substance

• depending on powder density

Vh – material volume except the pores, gaps etc.

Vp – pores volume

ρv – bulk density

ρs – powder density

v

sphphm

VVV

VVVV

VVM

ρρ

−=−

−=−−

== 11

Grain size distribution and specific surface

• one of the basic properties of loose materials

• it describes relative distribution of particular grains having specific size

Grain size distribution affects:

• voids• powder density• permeability• compressibility and other mechanical parameters thermal

and acoustic parameters

Specific surface – describes total surfaces area of all the grains of unit amount of matter [m2/kg] – application e.g. for the classification of the milling fineness of cement (ordinary cements 250 – 350 m2/kg)

Hygric properties of building materials

• moisture content of porous building material, water absorption, capillary rising, sorption isotherms, water retention curves, water vapour diffusion, vapour permeability, water vapour resistance factor, equivalent diffusion thickness

highly important parameters that can be in the case of unproper use of materilas in strctures the source of the failures and deffects (effect on hygienic quality of indoors, heating expanses, durability and serivce life of structures)

hygric properties affect other material parameters like bulk density, frost resistance, thermal conductivity, specific heat capacit, mechanical strength and linear and volumetric expansion

clear relation to the porous structure of materials (pore size and distribution), important especially for the following types of materials:

o thermal insulation materialso ceramic materialso concretes, air-aerated concreteso plasters (restoration, thermal insulation)o paintings

Moisture of porous materials

• Completely dry porous materials occur in building practice very rarely, even in the case there are pernament inbuilt in structures

Form of moisture in materials• Free water (fills the porous space)

• Physically bonded water (van der Waals‘s forces)

• Capillary water (in small pores and capillaries)

• Adsorbed water (fills the smallest pores and surface of capillaries)

• Chemically bonded water (part of crystal lattice of materials – crystal water, gypsum – high temperature drying -anhydrite

Type of moisture regarding to its sources:

o production moisture (technological, initial) – related to the wet processes within the material manufacturing

o underground moisture – transported to the material by means of capillary elevation

o sorption moisture – from the environment air

o condensed moisture

o operational moisture – dependent on the type of building utilisation, heating, air conditioning etc. (cooling halls, toilets, industry factories)

Moisture from the point of view of its time propagation –moisture is changed not only during the production process but also during the whole service life of materials and structures

o production moisture – in short time period significantly sinks (especially in the case of wet production processes)

o storage moisture – affect the application and processing of material

o pernament moisture – characteristic for materials pernament inbuilt into the structure – critical moisture –maximal moisture content that can be detected and permitted in material – in case of exceeding this value, the material looses and changes its properties (dangerous application)

Moisture changes during the service life of materials

Moisture – quantities, basic relations

• Gravimetric moisture content (moisture by mass):

• mw mass of wet material [kg, g]• md mass of dry material [kg, g]• mk mass of liquid [kg, g]• wh gravimetric moisture [%hm.]

%100%100 ⋅=⋅−

=d

k

d

dwh m

mm

mmw

• Volumetric moisture content:

• Vw volume of free water [m3]• Vd volume of dry material [m3]

ρw water density [kgm-3]ρd bulk density of dry material [kgm-3]

( )100% . 100% . 100% .w w d h dv

d w d w

V m m ww vol vol volV V

ρρ ρ

−= ⋅ = ⋅ = ⋅

⋅

Moisture transport:

• by sorption of water vapour• by diffusion of water vapour• by moisture diffusivity

Moisture sorption:- sorption of moisture from the water vapour from the ambient

air

• adsorption – intermolecular van der Waals‘s forces that gravitate mutually the molecules of solid phase and water vapour, adsorption leads to the rise of molecular layers on the pore surfaces

• absorption – liquid and gaseous phase of water is absorbed by diffusion and water convection

• chemisorption – chemical bonds between water and solid phase

- equilibrium sorption moisture – there are not observed moisture changes in time

- hygroscopic moisture – it originates in material in the case, when the ambient air is fully saturated by water vapour (maximal equilibrium moisture content)

Assessment of sorption isotherm – storage parameter of gaseous moisture (water vapour)

- dependence between moisture content in materials and relative humidity (ambient)

Sorption process has two phasis:

- surface adsorption at lower values of relative humidity

- capillary condensation – relative humidity more than 40%, for the pores having dimension 2 – 50nm (Thomson-Lord Kelvin)

Capillary saturation

Vacuum saturation

Hygroscopic moistureξ (T, φ)

95%

uvac

ucap

I

II

III

Capillary condensation

Multimolecularadsorption

Monomolecular adsorption

Scheme of the sorption isotherms measurement

Examples of typical solution for the relative humidity simulation

Temperature/Relative humidity Number of references

20°C 23°C 25°C

Silica gel 0.05 0.05 0.05 1

0.113±0.0031 0.113±0.0028 0.113±0.0027 1,3,4

0.111 - 0.111 2

MgCl2.6H2O 0.3307±0.0018 0.329±0.0017 0.3278±0.0016 1,2,3,4

K2CO3 0.441 - 0.440 1

NaNO2 0.654 - 0.643 2,3

0.7547±0.0014 0.7536±0.0013 0.7529±0.0012 1,2,4

- - 0.751 3

NH4Cl 0.7923±0.0044 0.7883±0.0042 0.7857±0.0040 1

0.8511±0.0029 0.8465±0.0027 0.8434±0.0026 1,4

- - 0.842 3

KNO3 0.932 - 0.920 4

K2Cr2O7 0.970 - 0.970 1

0.979 - 0.976 2

- - 0.97 3K2SO4

KCl

NaCl

LiCl

Salt

Sorption isotherm of ceramic brick, air aerated concrete and calcareous marly limestone

0

0,02

0,04

0,06

0,08

0,1

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

f[-]

u[kg

kg-1

]

BRI

AACI

CML

Water retention curve Water retention curve –– storage parameter of water vapourstorage parameter of water vapour

- It describes moisture storage (accumulation) in the over-hygroscopic range (liquid moisture transport is the dominant wayof moisture transport)

- It defines relation between moisture content and capillary pressure

Micropores Macropores

10-10 10-9 10-8 10-7 10-6 10-5 10-4

10-3-

3

Pore diameter [m]

10+4 10+3 10+2 10+1 1 10-1 10-2

Capillary pressure [bar], [1e5 Pa]

0.05 0.60 0.93 0.99

Relative humidity [-]

Sorption isotherm Retention curve

Pore distribution

Hygroscopic moisture Overhygroscopic moisture

ϕ [-] 0.113 0.753 0.969 0.999 0.999 0.999

p [Pa] 2.95E+08 3.84E+07 4.26E+06 4.00E+05 6.00E+04 1.00E+03

⎟⎟⎠

⎞⎜⎜⎝

⎛−==

RTp

pp

l

c

vs

v

ρφ exp

Relation between relative humidity φ and capillary pressure in pores pcdescribes under isothermal conditions Kelvin’s equation

where pv and pvs are pressures of water vapour (pvs – saturated watervapour) [Pa], ρl bulk density of water [kg m-3], R universal gas constant[J mol-1K-1] and T is temperature [K]

0

1

2

3

0,1 1 10 100Suction [bar]

Moi

stur

e co

nten

t [kg

kg

-1]

Retention curve of calcium silicate material

Water diffusion (liquid and gaseous moisture)

- possibility of molecules of water vapour, gases and liquid substances to infiltrate the porous materials

- the water vapour diffusion takes place when the material separates two places of different partial pressures of water vapour

- the diffusion takes place from the higher partial pressure to the place of lower partial pressure of water vapour

- the water vapour diffusion is realised in capillaries having the diameter bigger than 10-7m, because no capillary condensation takes place in these pores

Main quantities usually used for the evaluation of diffusion properties of building materials

• water vapour diffusion permeability

• water vapour diffusion coefficient

• water vapour resistance factor

• equivalent water vapour diffusion coefficient (non-homogeneous materials)

• equivalent water vapour resistance factor (non-homogeneous materials)

• equivalent diffusive thickness of material – capability of material to transport water vapour on the basis of the thickness of material layer

The driving force of water vapour transport is the gradient of partial pressure that is related to the partial density of water vapour flux according to the following equation )

where δ is water vapour diffusion permeabilitywater vapour diffusion permeability [s] that expresses the possibility of particular material to transport the water vapour by diffusion.

It is equal to the density of water vapour flux within the partial water vapour pressure gradient 1 Pa m-1.

gradpJ δ−=

Water vapour diffusion permeabilityWater vapour diffusion permeability [s], [kgm-1s-1Pa-1]

- dependent on temperature (it rises with temperature)- dependent on moisture content (it is reduced with higher moisture

content)- dependent on the ratio of relative humidities- number, shape and dimension of pores (cross-connections of

pores)- for its determination, the cup method is used ČSN 72 7031

where δ is the water vapour diffusion permeability [s], Δm is amount of water vapour diffused through the sample [kg], d is the sample thickness [m], S is the specimen surface [m2], τ is the period of time corresponding to the transport of mass of water vapour Δm [s], and Δpp is the difference between partial water vapour pressure in the air under and above specific specimen surface [Pa].

ppSdm

Δ⋅⋅⋅Δ

=τ

δ

-8

-7

-6

-5

-4

-3

-2

-1

00 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000

Time [s]

Dec

reas

e of

wat

er m

ass[

g]

Example of water vapour transport measurements for calcium silicate material

CSI t[s]

T[K]

P[Pa]

Δmw [kg]

S[m2]

D[m2s-1]

δ[s]

μ[-]

180000 300.3 3656 7.733E-03 5.25E-03 1.10E-05 7.80E-11 2.10

Water vapour diffusion coefficient D Water vapour diffusion coefficient D [m-2s-1]- the most often used parameter describing the water

vapour transport in building practice

D = dRT/Mwhere R is the universal gas constant [8.314 J mol-1K-1], T is the absolute temperature [K] and M is the molar mass of water [0.018 kg mol-1].

Water vapour resistance factor Water vapour resistance factor μμ [-]

- it describes, how many times the material reduced the velocity of water vapour transport in comparison with water vapour transport in air

δ water vapour permeability [s]

μ water vapour resistance factor [-]

N approximate value of diffusion resistance of air 5.45 .109

[s-1] depending on temperature

δμ

⋅=

N1

μ = Da /D

where Da=2,3E-05 [m2 s-1] is the diffusion coefficient of water vapour in the air

Equivalent diffusive thickness of material rd [m]

- dependent on material geometry- it is practically used mainly for the evaluation of diffusive

properties of surface treatments – renovation plasters, coating systems etc.

d material thickness [m]μ water vapour resistance factor [-]

dr dμ= ⋅

Diffusive resistance of material Rd [ms-1]

- it is used in hygrothermal calculations of condensed water vapour (e.g. in the multi-layered system of materials – structure of roofs)

- D – material thickness- N approximate value of diffusion resistance of air

5.45 .109 [s-1] depending on temperature

d dR d N r Nμ= ⋅ ⋅ = ⋅

Material

Bulk densityWater vapour

resistancefactor

Bulk density

Liquid moisture transportLiquid moisture transport

- diffusion, capillary suction, moisture diffusivity- the simplest way how to describe the liquid moisture

transport represents determination water absorption coefficient A [kg m-2s-1/2] from the following equation

I=A t1/2

- I is sorptivity and represents the mass of water on unit material surface [kg m-2], that is in direct contact with water, t time of the contact of material sample with water [s]

square root of time (s1/2)

inflo

w (k

g/m

²)

A (kg/m²s1/2)

first stage

second stage

wcap.h

EvaluationEvaluation ofof waterwater suctionsuction measurementsmeasurements

- water absorption coefficient gives information only on moisture flow, but it does not describe moisture distribution in material - on that account, the moisture transport will be described in the following way:

moisture flow:

− κ is moisture diffusivity [m2 s-1], j moisture flow [kg m-2s-1], ρs matrix density, wm moisture content by mass (gravimetric moisture)

- by the direct application of moisture flow equation we arrive to the relation for the calculation of apparent moisture diffusivity coefficient (Kumaran, 1994)

- where wsat is saturated moisture content (capillary)

hs wj ∇−= κρr

2

⎟⎟⎠

⎞⎜⎜⎝

⎛≈

satwAκ

0

5

10

15

20

25

30

35

40

0 100 200 300 400 500 600

Square root of time [s1/2]

Inflo

w [k

g m

-2]

MUDUsDUh

Water suction curves of different types of mineral wools MU, Dus, Duh (Rockwool Inc.)

Wcap – capillary moisture content

0

10

20

30

40

0 50 100 150 200 250 300 350 400 450

Square root of time [s1/2]

Inflo

w [k

g m

-2]

CSI

CSII

CSIII

CSIV

Results of water suction experiment for calcium silicate materials.

m0 S wsat Α κ

[kg] [m2] [kg m-3] [kg m-2s-1/2] [m2s-1]

1. 9.49E-03 9.146E-03 995.745 0.21 4.45E-08

2. 1.69E-02 1.635E-02 996.553 0.22 4.87E-08

3. 1.60E-02 1.538E-02 994.079 0.21 4.46E-08

- - 995.459 0.21 4.59E-08

Sample

Assessment of water absorption coefficient for water and moisture diffusivity for mineral wool MU.

- in details, the moisture transport can be described by non-linear diffusion equation

- the moisture diffusivity coefficient is introduced as a function of moisture content- determination on the basis of inverse analysis of moisture profiles (suction curves) that can measured in the frame of 1-D suction experiments (Lykov, 1958)- methods of moisture measurement???

))(( wgradwdivtw κ=

∂∂

Suction profiles measured for air aerated concrete.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.05 0.1 0.15 0.2

position [m]

rela

tive

moi

stur

e co

nten

t [kg

/kg]

12900s16500s20100s2370027300s30900s34500s

Moisture diffusivity coefficient determined by the inverse analysisof experimentally measured data – AAC.

1.00E-09

1.00E-08

1.00E-07

1.00E-06

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

relative moisture content [kg/kg]

moi

stur

e di

ffusi

vity

[m2 s-1

]Matano method

Double integrationmethodDif f erence method

Gradient method

Saturation Saturation –– maximal saturation, capillary saturationmaximal saturation, capillary saturation

- the maximum of moisture content that can be present in material

- it can be expressed in mass [kg/kg] or volumetric form [mmass [kg/kg] or volumetric form [m33/m/m33]]- it can be defined after specific time of material immersion into

the water (e.g. 1hr saturation etc.)- it can be defined by its maximal value, where all the open

pores of materials are filled by water (dependent on measurement principles – capillary saturation, vacuum saturation etc.)

- volumetric saturation ranges from 0 to 100%

- saturation by mass can in the case of light materials exceedthe value of 100%

Material Saturation by mass %

Volumetric saturation %

Wood 140 - 170 55 - 70

Steel - 0 - 0

Ceramic brick 20 - 25 36 - 55

Dense concrete 6 - 13 13 - 30

AAC 40 - 90 35 – 40

Expanded polystyrene

70 - 500 < 7

Suction of chosen building materials.

Capillary action (capillary activity)Capillary action (capillary activity)

- property of building materials that is observed in their contactwith water

- characteristic for water wettable materials (most of builsingmaterials)

- within the contact of open pores with water, the water suction starts due to the capillary and sorption forces

- materials having bigger pores are characteristic by fast water suction, however the elevation high is lower

- fine porous materials soak slower, but higher- the capillary rasing of moisture is the most often recognized

reason of moisture presence in buildings exposed to the underground water action

- the capillary activity can be simply described by capillary elevation – characteristic by the difference in the water levels in capillaries in comparison with ambient conditions

it is evoked by capillary forces between water molecules and capillary forces between water molecules and pore surfacepore surface (surface tension of liquid causes the moisture transport in the direction of the resultant of the forces)

Capillarity Capillarity –– capillary moisture transportcapillary moisture transport

- the maximal high of water elevation is described as: σ surface tension of liquid [N/m]θ wetting angle between capillary and the capillary wall [°]r capillary radius [m]ρ bulk density of liquid [kg/m3]g acceleration of gravity [m/s2]

- for the wettable liquids cosθ is close to 1, whereas the surface tension is cca 0.073 N/m

- relation for the calculation of maximal high of capillary actioncan be simplified to the following form

- the average radii of pores of common brick walls is close to 10-5 m – it corresponds to the high of capillary action cca 1.49 m (this value is confirmed also by building practice, since mostof buildings are wetted to the high 1,5 m)

rh 149.0

=

grh

⋅⋅⋅⋅

=ρ

θσ cos2

Surface tension of water in dependence on temperature.

Temperature

Surf

ace

tens

ion

- the wetting angle critically depends on the surface of pores (hydrophobic additives)

- in case of wetting angle θ > 90° we get negative high of capillary action – capillary depression (hydrophobic materials)

a) Wetting

b) No wetting

Capillary action is dynamic phenomenon, whereas not only high of capillary action but also the velocity of water evaporation and the time corresponding to the reach of high of capillary action, h is necessary to take into account.

velocity of capillary action:velocity of capillary action:

η liquid viskosity

time corresponding to high h:time corresponding to high h:

- the moisture transport by capillary action is typical for materials having pore radius in the range from 10-7 to 10-4 m (the highest transport is for the pore radii 10-5 m)

hrv

⋅⋅⋅⋅

=η

θσ4

cos

θση

cos2 2

⋅⋅⋅⋅

=r

ht

bound water

diffusion of water vapour

capillary action

Pores geometry

- water is transported in the capillary, however is not transported through the big pores, since the capillary elevationis lower than the high of capillary

- the moisture transport is not stopped – the water evaporates in the end of capillary and is diffused to the opposite pore wall, where the water condensate and in the liquid phase is transported by capillary action to the next pore

- not only the free water is transported, on the capillary walls the layer of bound water originates as well – this water has completely different properties then free water (it does not freeze at 0°C, it can not be completely evaporated, some of its properties (e.g. dielectrical) are very close to the properties of ice)