CHOSEN PROPERTIES OF BUILDING MATERIALS IICHOSEN PROPERTIES OF BUILDING MATERIALS II

Lecture abstract• Thermal properties of building materialsMechanisms of heat transport, standard quantities, specific thermal conductivity, specific heat capacity, linear thermal expansion, heat storage capacity, thermal resistance, thermal

transmittance, heat transfer coefficient, measurement methods, examples of material properties

• Acoustic properties of building materialsWave resistance, , acoustic hardness, sound absorption exponent, sound-reduction index, sound propagation, typical material properties

• Alternative

materialssheep

wool

Literature•

Stavební

hmoty, L. Svoboda a kolektiv, JAGA Group

s.r.o.,

Bratislava, 2004.

• ČSN 73 0540 Tepelná

ochrana budov (2005)

•

Nizkoenergetické

domy –

principy a příklady, Jan Tywoniak

a kol., Grada

Publishing, a.s., 2005.

•

Fyzika stavebního inženýra –

J. Binko, I. Kašpar, SNTL/ALFA, 1983.

•

Meranie

termofyzikálních

veličín

–

J. Krempaský, SAV Bratislava, 1969.

• Praktická

fyzika –

Z. Horák, SNTL, Praha 1958.

•

Fyzikální

a mechanická

zkoušení

stavebních materiálů

- Michalko O., Mikš

A., Semerák

P., Klečka T., ČVUT 1998.

Thermal properties of building substances

•

Heat transfer

–

according to the physical fundamental principle, three different ways of heat transfer can be recognised:

-

conduction

in substances-

convection

of substances

-

radiation

Conduction

–

heat transfer on conduction principles is realised in continuum

-

particular particles of substances share the kinetic energy of kinetic energy of disarranges thermal movementsdisarranges thermal movements

–

the heat transport is realised

from the places having higher temperature to the place of lower temperature

-

conduction takes place in solid, liquid and gaseous substances

Convection-

the heat transport by convection is also related to the continuum

-

it takes place only in liquids (liquids, gases)-

spontaneous convection is caused by the effect of density decreases by means of heating

-

the temperature changes in liquids evoke the transport of heated liquid, whereas the cold liquid is advected

by this movement

-

in liquids, especially in gaseous phase the convection transport of heat is dominant in comparison with conduction

Radiation-

heat transport by radiation is not limitted

by continuum

-

heat is transported by electromagnetic radiation-

energy exchange between surfaces having different temperature

-

In case of infrared radiation (wave length 760 nm –

1 mm), the heat transport is denoted heat emissionheat emission

Building materials -

mainly porous and lacunary

-

except

the conduction,

convection and radiation take parts in heat transport-

especially in the case of bigger pores, the convection of

gases and water vapour must be taken into account-

on the

opposite

sides

of

pores, the

radiation

must

be

considered

as well

The way of heat transport in material is dependent on the following properties:o porosity (pores dimension) and bulk density o structure

of materials

o temperatureo type of material (metallic, non-metallic)o moisture content

Effect of heat on materials

•

due to the heat energy that comes into the material, the temperature changes of material occur•, these temperature changes are acompanied

by the changes of

material dimensions (volume changes), strength changes, která

je doprovázena

změnami

rozměrů

materiálu

(objemu), změnou

pevnosti,

hardness and ductility changes

Thermal

dependence is

characteristic

for

all

material

properties!!!

-

length (volumetric) changes caused by the temperature changes can lead (in dependence on strength characteristics of particular material) to the crack formationcrack formation-

the most sensitive to this changes are first of all layers formed from

different type of material having different thermal expansion coefficient (from this point of view, the synergic performance of steal and concrete in reinforced concrete is highly

advantageous)

-

thermal energy can evoke also other serious harmful changes that can lead to the thermal decomposition of materialdecomposition of material

(e.g. decomposition

of gypsum after high temperature drying)

Thermal material parameters

parameters that are important especially for material of structures separating two environments of diffetent

thermal,

moisture and pressure conditions

thermal properties can be devided

to:

o thermal physical quantities

–

specific thermal conductivity, specific heat capacity, linear thermal expansion, volumetric expansion

o thermal technical quantities–

heat storage capacity, thermal resistance of material layer

having specific thickenss, thermal transmittance

Thermal material parameters

o heat storage parametersheat storage parameters

(heat capacity –

specific, volumetric)

o heat transport parametersheat transport parameters

(thermal conductivity, thermal diffusivity)

o mechanical parametersmechanical parameters

(linear thermal expansion, volumetric changes, shrinkage)

normy definující

tepelné

vlastnosti stavebních materiálů

a požadavky na tepelně

izolační

funkci

stavebních konstrukcí

ČSN 73 0540-1 Tepelná

ochrana budov. Část 1: Termíny, definice a veličiny pro navrhování

a ověřování. (2005),

nahrazení

normy z roku 1994.

ČSN 73 0540-2 Tepelná

ochrana budov. Část 2: Termíny, požadavky. (2002) –

nahrazení

stávající

normy z roku 1994.

ČSN 73 0540-3 Tepelná

ochrana budov. Část 3: Výpočtové hodnoty veličin pro navrhování

a ověřování. (1994)

ČSN EN 12524 Stavební

materiály a výrobky –

Tepelné

a vlhkostní

vlastnosti –

Tabulkové

návrhové

hodnoty (2001).

ČESKÁ

TECHNICKÁ

NORMA ICS 91.100.01; 91.120.10

(Září

2001)

Stavební

materiály

a výrobky

-

Tepelně

vlhkostní

vlastnosti

-Tabulkové

návrhové

hodnoty

ČSN

EN 12524

(73 0576)

Building materials and products -

Hygrothermal

properties - Tabulated design values

Matériaux

et produits

pour le bâtiment

-

Propriétés

hygrothermiques -

Valeurs

utiles

tabulées

Baustoffe und -produkte -

Wärme-

und feuchteschutztechnische Eigenschaften -

Tabellierte Bemessungswerte

This standard is the Czech version of the European Standard EN 12524:2000. The European Standard EN 12524:2000 has the status of a Czech Standard.

Thermal-technical standards introduce three types of thermal-physical quantities:

o standard values

–

the numerical value of specific

quantity determined by standartised approach

o characteristic values

–

the numerical value of specific quantity statistically evaluated from measured data

o calculating values

–

determined on the basis of

performed calculation according to the standards on the basis of standard or characteristic value of specific quantity (eventually tabulated value from the standard) –

consideration of safety loadings, coefficients etc.

Thermal-physical quantities

Specific thermal conductivity

-

basic thermal-physical property of homogeneous building materials (volumetric uniformity)

-

it describes the capability of particular material to transport

the heat, in case of temperature gradient

- characterized by thermal conductivity coefficient

λ [Wm-1K-1]

-

it has numerical value as the

unit density of heat flux at temperature gradient 1 K m-1

in the specific substance

The heat transport can be described e.g. by Fourier‘s relation

gradTq λ−=where q is the density of heat flux, T temperature

The velocity of heat transport is expressed by heat flux

or heat rate

where Q represents amount of transported heat and τ corresponding time of this transport.

Surface densitiy

of thermal flux JQ

is defined as

The driving force of heat transport is temperature change expressed by temperature gradienttemperature gradient.

τddQI q =

n

Q

dSdI

q =

• Thermal conductivity coefficient is not constant quantity.

•

It depends on the material structure, porosity, temperature, pressure, moisture content, rate of compression, powder density etc.

•

High thermal conductivity is characteristic for metals (e.g. Cu -402 W m-1K-1), lower thermal conductivity have liquids (e.g. water 0,56 W m-1K-1), the lowest thermal conductivity exhibits gases (e.g. dry air 0,0258 W m-1K1).

•

The thermal conductivity coefficient is used in thermal technical calculations within the design of buildings, building envelopes and insulation systems –

calculation of thermal

transmittance, thermal resistance, energy consumption of buidling

etc.

Pursuant to thermal conductivity coefficient, the building materials can be divided into the following groups:

• highly thermal insulation materials λ =

0,03 –

0,10 Wm-1K-1

(bulk density <

500 kgm-3)

• materials having good thermal insulation properties λ = 0,10 –

0,30 Wm-1K-1

(bulk density <

800 kgm-3)

• materials having medium thermal insulation properties λ = 0,30 –

0,60 Wm-1K-1

(bulk density <

1600 kgm-3)

• materials having common thermal insulation propertiesλ = 0,60 –

1,25 Wm-1K-1

(bulk density <

2400 kgm-3)

• high density inorganic materialsλ = 1,25–

3,5 Wm-1K-1

(bulk density > 2400 kgm-3)

• other high density orthotropic materialsλ > 3,5 Wm-1K-1

• metals having thermal conductivity λ

> 50 Wm-1K-1

Dependence of thermal conductivity on density.

porosity

bulk

density

concrete

expanded

plastics

AAC

lime

stone

glass

wool

wood

particle

board

Dependence of thermal conductivity on bulk density 1 –

light-weight concrete from expanded perlite, 2 –

AAC,

3 –

CS, 4 –

ceramsite

concrete, 5 –

ceramic brick

Effect of powder density of Liapor

on its thermal performance.

Powder

density

(kg/m3)

Thermal

conductivity

coefficient

(W/mK)

-

thermal conductivity coefficient is strictly dependent on moisture content in material (moisture rising negatively affects

the thermal insulation properties of materials)

- effect of thermal conductivity of liquid water(cca

0,58 Wm-1K-1), cca

25x > thermal conductivity of dry air

(cca

0,025 Wm-1K-1), also the convection way of heat transport plays an important role

-

in case of water freeze, another rising of thermal conductivity can be expected ( λ

= 2,3 Wm-1K-1

at -10°C)

Temperature

(°C)

Water

Air

-

the substantial increase of thermal conductivity coefficient due to the moisture rising has serious consequences in practical realisation of building structures (especially of thermal insulations and building envelope layers)

the water absorptive materials must be during their storage, application,

as well as after their inbuilt in structure

protected against the moisture (some materials have not reversible properties)

-

within

the design of building structures and constructive details there is necessary to take into account the value of thermal conductivity coefficient that corresponds

to the

practical moisture of material (thermal conductivity of dry material should not be used in calculations, attention should be payed also on sorption properties of materials)

-

in Czech technical standard ČSN 73 0540-1 the effect of moisture on the change of thermal conductivity is introduced using Zu [-] (Zw ) coefficient

- moisture coefficient of material Zu [-]

-

a2

coefficient of regression of linear dependence of thermal conductivity on moisture rising (slope of dependence)- λk

characteristic value of thermal conductivity

2u

k

aZλ

=

Dependence of thermal conductivity on moisture rising.

AAC

Light

weight

perlite

concrete

Ceramic

brick

Vol. moisture

(%)

Dependence of thermal conductivity of EPS –

S boards, (measured on specimens having bulk density16 kg/m3).

Vol. moisture

(%)

Ther

mal

con

duct

ivity

(W/m

K)

-

for improvement of thermal insulation properties, there is advantageous to have more of smaller pores in comparison with bigger pores where the radiation takes place

-

anisotropic materials exhibit different values of thermal conductivity in dependence on direction of heat transport (mineral wool, glass wool, laminates with glass carriers, etc.)

-

Wood

type

PineOak

Bulk

density

Thermal conductivity

Perpendicular

to fibres

Parallel

to fibres

-

porous materials are characteristic by intensive radiation in pores in dependence on temperature rising

rising of thermal conductivity coefficient

-

for informative determination of temperature dependent thermal conductivity following relation can be used:

- λ0 thermal conductivity coefficient at 0°C [W/m-1K-1]-

t temperature for which thermal conductivity is determined

[°C]

0 0,0025t tλ λ= +

Dependence of thermal conductivity of EPS boards (measured for the specimens having bulk density 20 kg/m3).

Temperature (°C)

Ther

mal

con

duct

ivity

Measurement of thermal conductivity• direct methods• indirect methods

The principles of all measurement methods are based on measurement of temperature field distribution in the studied specimen of material.

In respect to the achievement of temperature field, the measurement methods can be divided to stationary methods (constat heat rate) and non-stationary methods (the heat rate changes within the measurement).

Stationary methods are exact, more simple, reliable and control able. Their disadvantage consists in setting time of stable temperature field, what is highly time consuming even within the measurement of small samples.Within the measurement of wet samples, the moisture can be redistributed –

changes in the measured data.

Methods of thermal conductivity measurement can be divided also according to other aspects:

•

shape of heat source

–

point source, line source (circular, linear), surface, volumetric and combined

•

shape of measured sample

–

samples of not defined shape, samples having defined geometrical shape (vzorky nedefinovaného tvaru, definovaného geometrického tvaru (speheres, boards, cylindres)

• time propagation of heat input of source

Devices for thermal conductivity measurement•

apparatus Shotherm Showa Denko –

measurement in non-

stationary state• the measurement is based on hot-wire method –-

measurement of temperature gradient in defined distance

from linear source of heat energy of constat heat rate-

trough the hot wire, the heat is transported into the

measured material-

hot wire is placed between the sample and material of

known λ

that is impermeable for heat- in time, exponential temperature rising is observe-

measurement takes several seconds (possible to measure

thermal conductivity of wet specimens)

2 1

2 1

ln( )4 ( )q t t

T Tλ

π⋅ −

=−

• Device ISOMET 104 (Applied Precision)- based on non-stationary measurement-

into the studied material, the thermal impulses are emitted

and time dependent thermal response of material is measured- temperature changes are measured as a function of time

•

Stationary methods

-

Gaurded hot plate -

Metoda Poensgen‘s methods, Poensgen-Erikson‘s method, Bock‘s methods-

measurement is based on transport of steady state heat flux

from the heated measurement board through the studeid specimen to the cooled board of the device

Most common measurement configuration of guarded hot plate method

λ

= W/A [1/(ΔT/d)]

where W is the electrical power input to the main heater, A is the main heater surface area, ΔT is the temperature difference across the sample, and d is the sample thickness

• Indirect methods

I-

based on measurement of another physical quantity

(dynamic method of thermal conductivity measurement – determination of thermal diffusivity a)

Equation of heat transport:

ρλ

ca =

)(xT

xtTc

∂∂

∂∂

=∂∂ λρ )( tλλ =

(inverse analysis of measured temperature profiles –

similar with

determination of moisture dependent moisture

diffusivity)

• Indirect methods

IILaser flash method-

based on measurement of the temperature rise on the back face

of a thin disc sample caused by a short energy pulse on the front surface-

the specimen is placed in a furnace and heated to a uniform

temperature-

short (1 ms or less) pulse coming from a laser or a flash lamp

irradiates one surface of the specimen

-

temperature rise on the rear surface- the thermal diffusivity is calculated from -this temperature versus time curve- and the thickness of the sample

Heat capacity c- specific

–

related to kg of substance [J kg-1

K-1]

- volumetric

–

related to m3

of substance [J m-3

K-1]

-

defined as a amount of heat that must be supplied to 1 kg of material so that it is heat up of 1 K

-

index x is the type of thermodynamical change at which the heat is transported into the material (constant pressure, volume), in case that no volume change is observed within the material heating, its interior energy is rising as well as its temperature (in case of volume changes, the material performs within its expansion work that must be supploied by other addition of heat)-

solid and liquid substances are characteristic by small

thermal expansion (cp

≈

cv

)

xx dT

dQm

c ⎟⎠⎞

⎜⎝⎛=

1

-

highly dependent on temperature and moisture-

in dependence on moisture rising, the specific heat capacity

rises as well-

additive quantity dependence of heat capacity

on moisture content can be simply described by the following relation

- where c is specific heat capacity of wet material- cw specific heat capacity of water (cca 4182 J/kgK při 20°C)- moisture content by mass [kg/kg]- c0 specific heat capacity of dry material

-

dependence of specific heat capacity on temperature can not be described by simple generally valid relation. It is highly individual for specific type of materials

)1/()( 0 uuccc w ++=

Values of specific capacity of materials in the dry state:

• inorganic materials cca 840 –

1500 Jkg-1K-1

•

artifitial organic materials and mixture of inorganic-organic materials 1000 –

2500 Jkg-1K-1

• natural organic materials cca 2500 Jkg-1K-1

Measurement of specific heat capacity

•

calorimetric measurement –

vessel equipped with measurement devices for heat changes monitoring•

the principle of the measurement is based on the heat heat

conservation lowconservation low

-

in the closed thermally-insulated system are the heats received by colder solids equal to heats supplied by warm solids

–

providing that the materials do not change their

phases, no chemical reactions takes place within the heat transport and no mechanical work is performed.

TmcQ x Δ= ∑∑==

=n

iii

n

iiii cmttcm

11

Measurement of specific heat capacity–

adiabatic calorimeter I/II–

its walls are perfectly thermally insulated from the ambient

environment–

the supplied heat evokes the temperature rising inside the

calorimeter –

mixing calorimeter

TTTT

Mvcmc kvv

−−+

=2

1

mv

… liquid mass

cv

… specific heat capacity of liquid (water)

vk

… water value of calorimeter

T …

final temperature of calorimeter

T1

… beginning temperature of calorimeter

T2

… specimen temperature before its immersion into the calorimeter

Measurement of specific heat capacity–

adiabatic calorimeter I/II

Mk

… mass of dry calorimeter

M1

… mass of calorimeter partially filled by water (½ of volume)

T1

… temperature in calorimeter at the beginning of measurement

T2

… temperature of heated water

)()(1868,41

112

12 kk MMT

TTTMMv −−Δ

−−−=