Fire Resistive Materials: THERMAL PERFORMANCE

TESTING

Performance Assessment and Optimization of Fire Resistive Materials

NIST

July 14, 2005

MicrostructureExperimental

3-D Tomography2-D optical, SEM

Confocal microscopy

Modeling3-D Reconstruction

ParametersPorosity

Pore SizesContact Areas

Properties(all as a function of T)

ThermalHeat CapacityConductivity

DensityHeats of Reaction

AdhesionPull-off strength

Peel strengthAdhesion energy

Fracture toughness

EquipmentTGA/DSC/STASlug calorimeter

DilatometerBlister apparatus

Materials Science-Based Studies of Fire Resistive Materials

EnvironmentalInterior

Temperature, RH, load

ExteriorTemperature, RH, UV, load

Performance PredictionLab scale testingASTM E119 Test

Real structures (WTC)

Importance of Thermal Properties

• The major role of the FRM is to slow down the temperature rise of the structural steel during a fire exposure

• Its effectiveness will be largely determined by its thermal properties

• While heat capacity, density, and heats of reaction and phase changes do play a role, it is mainly the effective thermal conductivity of the material as a function of temperature that will dominate the performance

Fourier’s Law of Heat Conduction

q=-kA(∂T/∂x)• Heat transfer is proportional to temperature

gradient, exposed area, and thermal conductivity k

• k and thickness (x) are the variables “under our control”

• k is really an effective thermal conductivity as it includes contributions of conduction, radiation, and convection

But, what about endothermic reactions?

• Endothermic reactions and phase changes (evaporation) do absorb energy, but a couple of additional points merit consideration– 1) Energy absorption will maintain a higher temperature

gradient across the FRM specimen which will in turn increase the driving force for heat transfer (∂T/∂x) --- as noted to me by Prof. Fred Mowrer of Univ. of MD.

– 2) Steam and hot gases generated during these reactions may be transferred inward towards the steel surface, actually temporarily but significantly increasing the effective thermal conductivity

– 3) If the material has a low permeability, these reactions may result in (explosive) spalling (Example- high performance concrete)

A brief aside on “liquid” water in FRMs

• Thermal conductivity of liquid water is about 0.6 W/m•K; air or water vapor is about 0.03 W/m•K (a factor of 20)

• Materials with “liquid” water also have a higher propensity for spalling, etc.

• Make sure that your materials are really dry before E119 (or other) testing

Thermal Conductivity at High Temperatures

• How to measure it?– ASTM C1113: Hot wire method– High-temperature guarded hot plate (world-

class facility under construction in BFRL)– Transient plane source method (Hot Disk®-

will be delivered to BFRL in August 2005)– Slug calorimeter (built at BFRL in 2004 and

used extensively since then)• Similar in principle to the Cenco-Fitch Apparatus

used in ASTM D2214 for estimating the thermal conductivity of leather (first published by Fitch in 1935)

Hot Disk® System

• Manufactured in Sweden• Measures k in range of

0.005 to 500 W/m•K• With high temperature

attachments, temperature range of room temperature to 700 oC

• Also provides volumetric heat capacity values

• http://www.hotdisk.se

Slug Calorimeter Technique• Sandwich specimen of two “slabs” of FRM

covering two sides of a steel slug of known mass and heat capacity

• Monitor slug temperature change as entire sandwich is exposed to a heating/cooling cycle

• Calculate effective thermal conductivity during multiple heating/cooling cycles

• For detailed information see: Bentz, D.P., Flynn, D.R., Kim, J.H., and Zarr, R.R., “A Slug Calorimeter for Evaluating the Thermal Performance of Fire Resistive Materials,” accepted by Fire and Materials, 2005, available at http://ciks.cbt.nist.gov/garbocz/slugpaper1 .

Slug calorim

eter

Specim

en

Specim

en

Guardinsulation

Guardinsulation

Q directionheating

cooling

Retainingplate

6 “3/16” inch holes at 2”, 3”, and 4”

½”

2”

4”

6”

Slug Calorimeter

Experimentalsetup

Details of slug calorimeter

304 stainless steel

Raw Data for Exposure of Intumescents

0

150

300

450

600

750

900

0 60 120 180 240 300 360

Time (min)

Tem

per

atu

re (

C)

Furnace - Int. A Furnace - Int. B

Slug - Int. A Slug - Int. B

Time for central slug to reach 538 oC varied from 52 minutes to 93 minutes for the two different materials

Determination of Effective k∂2T/ ∂z2=(1/α)(∂T/∂t)

With B.C.: T(0,t)=Ft, k(∂T/∂z) +H(∂T/∂t)=0α is the thermal diffusivity

H is the thermal capacity of one half of the slug plateF is the rate of temperature increase/decrease of the slug

Solution: T(z,t)=F[t-(H+lC)z/k+Cz2/(2k)]l is the specimen thickness, C its volumetric heat capacity

ΔT = (Fl/k)*[H+lC/2]ΔT is the temperature difference across the specimen

k=Fl(H+lC/2)/ΔT=Fl(MScpS+MFRMcpFRM)/2AΔT

Slug Calorimeter Results: Fumed Silica Board

-Results for (non-reactive) fumed silica board in good agreement with previous NIST (1988) measurements

0.00

0.01

0.02

0.03

0.04

0.05

0 50 100 150 200 250 300 350 400 450 500 550

Mean Specimen Temperature (oC)

k [W

/(m

•K)]

NIST data heating (1)

cooling (1) heating (2)

cooling (2) heating (3)

cooling (3)

Slug Calorimeter Results: FRM A

-Good agreement with previously measured values-Differences between 1st and 2nd heating cycle provide valuable information on influences of endothermic and exothermic events, including reactions and phase changes- Good repeatability in cooling curves for different runs

0.0

0.1

0.2

0.3

0.4

0 100 200 300 400 500 600 700 800

Mean Specimen Temperature (OC)

k [

W/(

m•K

)]

heating (1) cooling (1)

heating (2) cooling (2)

heating (5-16h) cooling (5)

hot wire TPS

Slug Calorimeter Results: FRM C

-Reasonable agreement with previously measured values-Differences between 1st and 2nd heating cycle provide valuable information on influences of endothermic and exothermic events - Good repeatability in cooling curves for 3 different runs

0.0

0.1

0.2

0.3

0.4

0 100 200 300 400 500 600 700 800

Mean Specimen Temperature (oC)

k [W

/(m

•K)]

hot wire method TPS methodheating (1) cooling (1)heating (2) cooling (2)heating (3) cooling (3)

Slug Calorimeter Results: Comparison

0.0

0.1

0.2

0.3

0.4

0.5

0 150 300 450 600 750

Mean Specimen Temperature (oC)

k [W

/(m

•K)]

- High variability between different FRMs especially at higher T- High T insulation boards (two lower curves) have effective k

values significantly lower than current FRMs and exhibit minimal increases at higher temperatures

k from 2nd heating curves

Comparison of Time for Steel Slug to Reach 538 oC

40

50

60

70

80

90

100

110

120

A B C D E F G H I J K L M N O P

"Fai

lure

" ti

me

(min

)

1st heating

2nd heating

Determination of Heat Capacity• Volumetric heat capacity provided

directly with Hot Disk® thermal conductivity measurements

• For small specimens, can measure using differential scanning calorimetry (DSC) or simultaneous thermal analysis (DSC/STA)– Both units available in BFRL at NIST– Equipment for larger sample volumes (12

mL) and temperatures up to 1000 oC is available from Setaram ($$$)

Heat Capacity via DSC (ASTM E1269)

0.4

0.8

1.2

1.6

2

2.4

2.8

3.2

3.6

4

4.4

0 100 200 300 400 500 600 700

Temperature (oC)

Hea

t Cap

aci

ty (

J/g

•K)

.Sapphire reference

Sapphire calculated

FRM (no mass corr.)

FRM (mass corr.)

Need to run a sapphire reference scan and applicationof a correction (fudge) factor

Summary• Thermal conductivity at high temperatures

is a critical performance parameter for FRMs to properly protect structural steel

• BFRL/NIST labs are uniquely equipped for measuring this property and are continually upgrading our capabilities

• Measurement of effective thermal conductivity provides product differentiation that can be utilized both to improve existing materials and to develop new ones