Fire Resistive Materials: THERMAL PERFORMANCE TESTING Performance Assessment and Optimization of Fire Resistive Materials NIST July 14, 2005

Materials Science-Based Studies of Fire Resistive Materials Microstructure Experimental 3-D Tomography 2-D optical, SEM Confocal microscopy Modeling 3-D Reconstruction Parameters Porosity Pore Sizes Contact Areas Properties (all as a function of T) Thermal Heat Capacity Conductivity Density Heats of Reaction Adhesion Pull-off strength Peel strength Adhesion energy Fracture toughness Equipment TGA/DSC/STA Slug calorimeter Dilatometer Blister apparatus Environmental Interior Temperature, RH, load Exterior Temperature, RH, UV, load Performance Prediction Lab scale testing ASTM E119 Test Real structures (WTC) Materials Science-Based Studies of Fire Resistive Materials

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 Calorimeter 304 stainless steel Details of slug calorimeter ½” Guard insulation Retaining plate 6 “ Specimen Slug calorimeter Specimen 3/16” inch holes at 2”, 3”, and 4” Details of slug calorimeter Q direction heating 2” cooling 4” 6” Guard insulation 304 stainless steel Experimental setup

Raw Data for Exposure of Intumescents 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)=0 α is the thermal diffusivity H is the thermal capacity of one half of the slug plate F 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

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

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

Slug Calorimeter Results: Comparison k from 2nd heating curves 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

Comparison of Time for Steel Slug to Reach 538 oC

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) Need to run a sapphire reference scan and application of 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