Experimental and Numerical Study of Temperature Developments in PIR Core

Sandwich Panels with Joint

Y C Wang, A Foster, University of Manchester, UK

Abstract

This paper presents the results of an experimental and numerical investigation of

temperature developments in sandwich panels consisting of steel sheeting and

polyisocyanurate (PIR) core. Fire experiments were carried out on individual PIR

sandwich panels and PIR sandwich panels with joint. The fire test results were used

to validate a temperature dependant themal conductivity model for PIR, through

numerical heat transfer modelling using the general finite element package

ABAQUS. The fire test results indicate that the temperatures at the joint on the

unexposed side of a sandwich panel is initially lower than that on the panel.

However, at high temperatures, the ablation of PIR core creates large gaps up to

25mm. Due to high radiation within the gap, the joint temperature becomes much

higher than the panel temperature. The results of a numerical parametric study

indicate that if the joint gap can be controlled to be no greater than 5mm, the joint

and the panel temperatures on the unexposed surface would be similar. Joint gaps of

10mm or greater would result in joint temperatures much higher than panel

temperatures and would reduce the sandwich panel system insulation performance of

less than 60 minutes even though the panel may be able to reach much longer

standard fire resistance rating.

Keywords: sandwich panel, joints, fire resistance, fire test, PIR, thermal

conductivity

1. Introduction

Sandwich panels (also known as ‘composite’ or ‘insulated panels’) consist of two

high strength, relatively thin facings that enclose a thicker, low density core material,

see Figure 1. The facings protect the low density core and are the main loadbearing

components. The core material is critical in terms of insulation performance and

should have sufficient shear stiffness in the direction normal to the facings. The

facings of sandwich panels are usually manufactured out of steel. For the core, a

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range of materials can be used, including polyurethane (PUR), polyisocyanurate

(PIR) and expanded polystyrene (EPS).

Sandwich panels are a pre-fabricated, high performance wall-cladding and roofing

systems with superior insulating characteristics. Traditionally, sandwich panels have

been specified for industrial buildings and warehouses (commonly known as ‘big

sheds’) but are now a favourable choice in building construction, where they are

being used in retail, leisure, cold stores, transport and energy sectors, in addition to

uses in schools and hospitals. Sandwich panels may be used as standing alone

components, or as part of Structural Insulated Panel (SIP) system in dwellings and

apartment construction where the sandwich panels are connected to OSB panels,

timber studs, gypsum plasterboards and other components (BRE 2010, Hopkin et al

2011, Lennon and Hopkin 2010). This research is concerned with standing alone

sandwich panels.

Figure 1: Arrangement of a sandwich panel core and facing materials (Olive Container Conquer 2014).

Joints play an important role both in terms of thermal and mechanical performance.

As shown in Figure 2, the joints between sandwich panels are weak points, therefore

a better performing joint is the driver in prolonging fire resistance times. Although

there has been a lot of testing of PIR panels, there is a lack of detailed explanations

on how the joints affect sandwich panel fire resistance and how joint design may be

improved to increase sandwich panel fire resistance time. Understanding how joints

behave and affect sandwich panel performance in fire is fundamental to better design

of sandwich panels for improved fire resistance.

2

Figure 2: Flames penetrating through sandwich panels at the joint (Reed 2004).

The fire resistance of a sandwich panel is measured by its ability to contain a fully

developed fire. In unloaded sandwich panels, fire resistance is quantified by either

insulation or integrity performance, both strongly influenced by heat transfer through

the joints. It is therefore critical that knowledge of heat transfer within a panel joint

is available to develop thorough understanding of sandwich system performance in

fire. Because sandwich panels are proprietary products, there is a general lack of

publically available information reporting detailed research studies. In particular,

there is very little reported research on sandwich panel joint performance in fire

(Davies 2001). In the fire tests reported by Hopkin et al (2011) on structural

insulated panel systems, sandwich panels were connected to structural loadbearing

components including OSB panels, timber studs and gypsum plasterboards. These

structural loadbearing components govern the panel system fire performance.

Furthermore, the emphasis of their study was on whole system performance, rather

than on sandwich panel joint performance. This paper is focused on standing alone

sandwich panels.

Whilst fire testing is indepensible to quantify sandwich panel fire performance, it is

very expensive to conduct. Therefore, it is desirable to be able to use numerical

modelling to predict sandwich panel fire performance. An important requirement of

accurate numerical heat transfer modelling is accuracyc of the input material thermal

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properties. Because PIR is lightweight, the key thermal property is its thermal

conductivity. Sandwich panel manufacturers (FERPFA 2006) typically only provide

thermal conductivity values at ambient temperature . This was also assumed in

numerical modelling by other researchers (BRE 2010). However, as will be

explained in section 3 of this paper, the high temperature thermal conductivity of

PIR may be many times the ambient temperature value. It is important that the

effects of temperature on thermal conductivity are included.

Therefore, the specific objectives of this paper are as follows:

To carry out fire experiments to obtain temperature distributions in standing

alone sandwich panels and their joints.

To validate a temperature dependent thermal conductivity model for PIR.

To carry out a series of numerical parametric studies to investigate the effects

of joint gap on sandwich panel temperature developmennts in fire, so that

methods may be developed to improve insulation performance of standing

alone sandwich panels.

The research will be carried out by carrying out some fire experiments on individual

sandwich panels and on sandwich panels with joint, and then use numerical

modelling to investigate the effects of joint gap on sandwich panel insulation

performance in fire, after comparison of numerical modelling and fire test results for

validation of the temperature dependent thermal conductivity model for PIR.

2. Fire tests

2.1 Fire tests on individual panels

To validate the temperature dependent thermal conductivity model for the PIR core

(see section 3.1), two small scale fire tests were conducted on sandwich panels with

PIR core. The two specimens had two different thicknesses, 80mm and 100mm, and

were subjected to fire exposure from inside a furnace in a setup designed to monitor

through-thickness temperature distributions. The planar dimensions of the samples

were square with a length of 700mm. The two panel specimens were cut from

sandwich panels of lightly profiled steel sheets of thickness 0.5mm with PIR foam

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core. The standard ISO 834 (International Organization for Standardization (ISO)

1999) fire temperature-time curve was followed in the furnace.

Seventeen thermocouples in total were used in each test (T1 to T17); 5 on the

unexposed side (T1, T10-T13), 5 on the fire side (T9, T14-T17) and 7 through the

thickness of the panel (T2-T8). Figure 3 and Figure 4 show the panel dimensions and

details of the thermocouple layout on the surfaces. The surface thermocouples were

attached to the surface steel sheets of the sandwich panel specimens mechanically

with staples. The mechanical fastenings ensured that the thermocouples stayed in

position at high temperatures.

Figure 3: Plan view of thermocouple positions on the surfaces (Left – unexposed surface; Right – fire exposed surface; Dimensions in millimetres).

Figure 4 shows the elevation view of through thickness positions of these

thermocouples for both the 80mm and 100mm thick panels. The internal

thermocouples were uniformly spaced through the thickness at a spacing of 10mm

and 12.5mm, respectively.

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Figure 4 Through-thickness locations of internal thermocouples (Dimensions in millimetres)

Figure 5 and Figure 6 show the exposed surfaces of the samples after testing. The

exposed side steel facing delaminated from the foam core. This happened quite early

in the fire test (about 1.5 minutes as indicated by loud popping noise from inside the

furnace) when the surface temperature was about 100oC. This phenomenon plays a

noticeable role in heat transfer through the panels which will be considered in the

subsequent heat transfer analyses.

Figure 5: View of exposed fire side steel facings after test completion (Left – 80mm thick panel; Right – 100mm Thick Panel).

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Figure 6: 80mm panel after test – picture showing delamination of fire side steel facing (Maximum centre displacement approximately 1cm).

In addition to the above mentioned observations, hot spots were observed on the

steel facings of the unexposed sides of the specimens, as can be seen in Figure 7.

These hot spots were areas of rapid heat transfer through the panel, most likely

where the foam had cracked allowing for quick transfer of heat by radiation to the

unexposed side of the panel. These hot spots appeared at random locations, resulting

in relatively large variations in the recorded temperatures on the unexposed surface.

Figure 7 Unexposed sides of panel specimens after tests showing hot spots (Left – 80mm thick panel; Right – 100mm thick panel).

A section of the charred foam was broken in half to assess the pore size of the chared

core. Figure 8 shows the pore size of the fully charred foam is on average 2.5mm.

7

Figure 8 Broken in half sample of charred foam (Average pore size 2.5mm).

2.2 Fire tests on sandwich panels with joints

The objective of these fire tests was to provide information for the assessment of

sandwich panel joint fire performance. Due to a lack of suitable fire testing

laboratory, the fire tests were carried out at the Lancashire Fire and Rescue Services

facility, Chorley, Lancashire, United Kingdom. This is a very rudimentary fire

testing facility used by the fire brigade for training. Figure 9 shows photos of the

metal container which formed the fire enclosure, from the outside and the inside. The

sandwich panels were placed inside the two apertures on the front face of the

container. The back side of the container was open to provide ventilation to the fire.

(a) External apperance (b) Internal apperance

Figure 9: Fire test container showing the 2 apertures used for the tests.

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The fire condition inside the metal container was generated using wood cribs. Due to

limited support, thorough preparation of the experiments was not possible.

Therefore, there was no characterisation of the wood cribs, and neither were detailed

measurements, e.g. for recordidng the non-uniform fire temperature field, heat

release rate etc. However, in modelling of these experiments, because the recorded

surface temperature was used as input data, this lack of information on fire behaviour

is considered acceptable. In total, eight joint specimens were fire tested. The

specimens consisted of 2 different sandwich panel thicknesses, 95mm and 120mm,

and investigated both vertical and horizontal joint orientations. Each test was

duplicated to provide additional verification of results. shows a list of the test

specimens with a summary of the main characteristics. Figure 10 shows the test

schematic and dimensions of the test panels. The sandwich panel samples consisted

of rigid polyisocyanurate (PIR) foam of density 40kg/m3 encased by steel facings of

0.7mm thickness.

Table 1 List of fire test specimens.

Specimen Number

Joint OrientationPanel Thickness

(mm)

1 Vertical 95

2 Vertical 95

3 Horizontal 95

4 Horizontal 95

5 Vertical 120

6 Vertical 120

7 Horizontal 120

8 Horizontal 120

9

Figure 10: Schematic of 95mm thick panel with horizontal joint (Dimensions in millimetres; Left – Isometric view of tests; Right – Cross section elevation view of

tests).

Figure 11 shows a cross section of the sandwich panel secret fix joint design. The

screw between the panels would be used to fix the panels to the supporting steel

framework for structural safety in practical construction. Because these fire tests

were small scale and were unloaded, there was no risk of structural failure of the

panels and hence the scree was not used, thus leaving the panels to move freely at

the joint.

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Figure 11: Trimapanel joint between sandwich panels (Tata Steel 2014)

In total, 30 thermocouples per test were used. Figure 12 and Figure 13 show

locations of the thermocouples. 18 of the thermocouples were placed within the joint

and 12 thermocouples were placed away from the joints in the panels. All

thermocouples were placed in the centre plane of the panels.

Two displacement transducers were positioned on each test specimen at either side

of the joint. The displacement transducers were used to measure the mid-panel

displacements and movement of the panel on either side of the joint. In addition, a

thermal imaging camera was used to measure the external surface temperature of the

test specimens.

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Figure 12: Numbering of thermocoules in the tests (95mm and 120mm specimens).

Figure 13: Positions of thermocouples located in the 95mm thick panel specimen.

To ensure that the thermocouples stayed in place during the tests, high strength

adhesive Plexus MA300 (ITW Plexus 2006), a two-part methacrylate adhesive, as

well as mechanical fixings in the form of metal staples were used to fix the

thermocouples in place at several locations.

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Because of the elongated shape of the metal container, the combustion gas

temperature inside was highly non-uniform, particularly in the longitudinal direction

of the container. Therefore, no combustion gas temperature was measured. Instead,

the exposed surface temperatures of the sandwich panels were used as the basis of

assessment of the relative performance of different systems.

The two apertures in the metal container (Figure 9) allow two duplicate tests of each

panel arrangement to be carried out simultaneously, as shown in Figure 14.

Figure 14: Completed setup prior to fire test.

The fire load was provided by 20 wood pallets as shown in Figure 15, each weighing

approximately 14.75kg. The inner dimensions of the container measured 2141mm

(wide) by 2830mm (high) and 10631mm (long). The wood pallets were placed from

the front inside face of the container where the specimens were inserted and

occupied a space of 2140 (wide) by 1800 (high) and 1000 (long). As mentioned

earlier, ventilation for the fire was provided by opening the back panel of the

container.

13

Figure 15 Wooden pallets used to generate the fire load.

The test setup was crude due to the difficulty of sourcing a suitable, well controlled

fire test facility which could allow generation of a large quantity of smoke.

Therefore, many of the tests did not generate consistent experimental results, i.e. the

measured temperatures in nominally identically samples side by side in the same

experiment were drastically different. Among the tests, only the temperature results

of the duplicate Specimens 7 and 8 were close, as shown in Figures 16 and 17.

Therefore, only these results used in further in-depth analysis.

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Figure 16: Comparison of joint temperatures, specimen 7 and 8 (Dotted line – specimen 8).

Figure 17: Joint thermocouple comparison, specimen 7 and 8 (Dotted line – specimen 8).

15

An important finding from sandwich joint tests was the ablation of the PIR foam in

the joint areas of the sandwich panels. This is shown in Figure 18 and Figure 19.

This is because ablation of the foam occurs in areas with greater access to oxygen.

This ablation in the joint has significant implications on the panel fire resistance

performance. When the gap in the joint region is small, the air acts as an insulator

because the air has lower thermal conductivity than the foam. However, as more

foam is vaporised from the joint, the gap in the joint region increases and heat is

more rapidly transferred through the joint from the exposed to the unexposed side of

the panel system due to cavity radiation. Figure 20 and Figure 21 compare

temperatures in the joint and in the panel on the unexposed side for the two

specimens. The joint temperature is lower than the panel temperature initially (before

30 minutes). Therefore, it is possible that by controlling the gap dimension in the

joints, improved thermal performance of the panel system can be achieved. This will

be further investigated in Section 4 of this paper.

Figure 18: Ablation of foam in panel with horizontally oriented joint (Gap size approximately 10mm).

16

Figure 19: Ablation of foam in panel with vertically oriented joint (Gap size approximately 25mm).

Figure 20: Comparison of unexposed side panel surface and joint temperatures for specimen 7.

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Figure 21: Comparison of unexposed side panel surface and joint temperatures for Specimen 8.

3: Numerical modelling

The heat transfer modelling was performed using the commericial finite element

software ABAQUS (Hibbitt, Karlsson & Sorensen 2012). The two key issues of

accurate modelling of sandwich panel fire resistance are thermal conductivity of the

PIR foam and gaps in the joints.

3.1: Temperature dependent thermal conductivity model for PIR

Many early researchers (Russell 1935; McIntire & Kennedy 1948; Skochdopole

1961; Gorrin & Churchhill 1961) proposed that the overall effective thermal

conductivity of a porous foam, λ*, has the following four contributions:

Conduction through the solid, λs ,

Conduction through the gas, λg ,

Convection within the cells, λc , and

Radiation through the cell walls and across the pores, λr .

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In a porous foam such as PIR, heat is transferred through the solid material by

conduction and through the pores by radiation, conduction and convection. When the

pore size is small, such as in PIR foam, convective heat transfer through the pores is

negligible, i.e. λc =0.

According to Glicksman (1994), the total effective thermal conductivity is given by:

λ¿=δλg+1−δ

3λs ( f s√( a

b )+2(1− f s )( ab )

14 )+λr

(1)

where

a: is the longitudinal pore size

b: is the transverse pore size

fs: is the fraction of solid materials in the foam

δ: is foam porosity

Sinofsky (1984) found that solid polymer conductivity did not vary with different

blowing agents, age formulation or processing history for PIR. Torpey (1987) found

that there was little difference in the thermal conductivity of solid between PIR and

PUR foams. Furthermore, because PIR foam has very high porosity, the thermal

conductivity of solid makes a small contribution to the overall value of thermal

conductivity. Therefore, the following thermal conductivity model of the solid

urethane resin, according to Harding and James (1962), can be used:

λS=0.173+0.00004 × ( (1.7 T +32 )−74 ) (2)

Smith and Griffiths (1998) gave the temperature dependent thermal conductivity of

gas as:

λg= λg , 0( TT0 )

0.8

(3)

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In which T is the material temperature in Kelvin, T0 is the ambient temperature in

Kelvin and λg,0 is the thermal conductivity of air at ambient temperature which

equals 0.0264W/mK (Lemmon & Jacobsen 2004).

Glicksman et al (1987) and Glicksman (1994) used the Rooseland equation to

calculate the radiative contribution of cell walls at random orientations. The radiative

contribution was given as:

λr=163 K

σ T 3

(4)

where

K=4 . 10√( f s ρ f

ρs )d

+((1− f s ) ρf

ρs)Kw

in which the additional symbols are:

σ: is the Stefan-Boltzmann constant

ρs: is the solid density

ρf: is the foam density

d: is pore dimension in the direction of heat transfer

Kw: is the cell wall extinction coefficient

Table 2 tabulates the thermal conductivity – temperature relationship. It can be seen

that the thermal conductivity of PIR foam increases exponentially with temperature.

At 600oC the thermal conductivity is an order of magnitude greater than that at

ambient temperature. At 1000oC, the thermal conductivity increases over 25 times

above the ambient temperature value, which shows the importance of including

temperature effects in the thermal conductivity model of porous materials.

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Table 2 Temperature Dependant Thermal Conductivity of PIR foamTemperature,

TEffective Thermal

Conductivity λ*

(oC) (W/mK)1 0.02920 0.03150 0.034100 0.039200 0.051300 0.065400 0.083500 0.104600 0.130700 0.162800 0.199900 0.2421000 0.293

3.2: Heat transfer modelling of sandwich panel without joint

In the fire experiments described in section 2, it was observed that the fire-exposed

side steel facings delaminated from the foam core early on in the tests and the steel

facings were drawn slightly towards the fire side about the middle of the panels. This

observed phenomenon was included in the simulation model geometry, as shown in

Figure 22, where the steel facings were set back by 10mm from the foam as observed

in the experiments.

Heat transfer was assumed to be through the thickness direction of the panels,

therefore, the numerical simulation models were 2D.

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Figure 22: Numerical simulation model geometries and meshes used in the analysis of the 80mm and 100mm thick panels.

The thermal conductivity and specific heat values of steel were according to EN

1993-1-2-2005 (British Standards Institution 2009).

The elements chosen were heat transfer 2D shell elements (DC2D4, 4-noded linear

heat transfer quadrilateral elements).

The fire-exposure and the ambient temperature were defined as radiant and

convective heat transfer boundaries. Emissivities of the fire and the steel surface

were assumed to be 0.9. The convective heat transfer coefficient was assumed to be

10 W/m2K. On the unexposed side, the ambient environmental temperature was

taken as 20oC. The emissivity of the ambient air was 1.0.

The air gap between the steel sheeting and the foam on the fire exposed side was

simulated using cavity radiation. The emissivity of the foam surface was assumed to

be 0.9.

22

Figure 23 and Figure 24 compare the simulation and fire experimental results of

temperatures throughout the thicknesses of the 80mm and 100mm thick panels. On

the fire-exposed surface and inside the panels, because thermocouples at the same

locations recorded very similar temperature values, the average temperatures were

used. On the unexposed surface, the recorded temperatures at different locations

showed relatively large variations due to some thermocouples being near the hot

spots. Therefore, the measured temperatures recorded by all the thermocouples on

the unexposed side are shown. It is not possible to simulate hot spots in the current

numerical model.

Figure 23: Comparisons of numerical simulation and experimental results of temperatures throughout the 80mm thick Panel (Distances measured from unexposed

surface).

23

Figure 24: Comparisons of numerical simulation and experimental results temperature for the 100mm thick Panel (Distances measured from unexposed

surface)

Both Figure 23 and Figure 24 show that except for the hot spots, the numerical

simulation results are in a very good agreement with experimental results throughout

the thickness. In the temperature dependent thermal conductivity model, the highest

uncertainty lies at high tempeatures (>500oC) due to the dominant effect of radiation

within the foam cells. The good agreement in Figures 23 and 24 between the

simulation and measured high temperatures can be considered to validate the high

temperature thermal conductivity values in Table 2.

3.3: Heat transfer modelling of sandwich panel with joint

Figure 25 compares the simulation and test results of temperatures throughout the

different depths of the 95mm sandwich panel. For these comparisons, due to the

difficulty of establishing the fire temperature, the measured temperatures (TC17 in

Figure 12) on the fire exposed surface were used as the thermal boundary condition.

The simulation results are satisfactory.

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This comparison further validates the adopted temperature dependent thermal

conductivity model for PIR (Table 2).

Figure 25: Panel temperatures in Specimens 7 and comparison with ABAQUS predictions.

It was not possible to make detailed comparison for joint temperatures because of the

unquantifiable joint gap size during the fire test. However, the results in the next

section, in Figures 30-32, indicate that the heat transfer model for joint is able to

reproduce the trends of relative temperature change between the joint and the panel.

4: Joint gap effects and methods of improving fire resistance of sandwich panels

The fire test results on sandwich panel joints show that a considerable loss of core

material occurred within the joints of the sandwich panels as the exposed core was

subjected to high temperatures causing core ablation. This gap in the panel joint was

a major contributor to the increased temperatures seen at the joints. By limiting the

development of this gap, reductions in joint temperature rises would be possible. A

numerical parametric study has been carried out to quantify the effects of changing

the gap size. The primary objective of this parametric study was to establish the

tolerable gap size below which the joint temperature rise would not exceed the panel

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temperature rise on the unexposed side. Achieving this would make the joints non-

critical in deciding sandwich panel fire resistance.

To model heat transfer in the joint, cavity radiation was introduced in the joint

between sandwich panels. It is appreciated that hot gases may migrate through the

joint to enhance heat transfer due to convective heat flow. This was not included in

the modelling. Nevertheless, as will be shown in the results in Figures 30-32, the

adopted numerical model is able to reproduce the observed relative changes of

temperature in the joint and in the panel on the unexposed side.

The joint gap size ranges from the initial 1.35mm to the final 10mm after PIR core

ablation. Figure 26 shows the panel arrangement. It should be pointed out that the

gap size was assumed to be unchanged and the actual gap size at high temperatures

may be less than the final gap size after cooling down due to presence of thermal

expansion at high tempreatures. However, there are many other factors affecting the

gap size in addition to thermal expansion, including ablation of PIR, and interaction

between the core and the facing. Any closing in gap would benefit sandwich panel

thermal performance at the joints. However, it would be very difficult to quantify

this benefit because it would be quite difficult to quantify the exact gap size.

The thermal boundary condition on the exposed side of the panel is defined using the

standardised ISO 834 fire curve. All panels are 120mm thick.

Figure 26 Geometry of the sandwich panel joint thermal model, the joint gap size (shown as 5mm in the figure) is varied.

Figure 27 and Figure 28 show the temperature distribution contours of three of the

joint models (gap sizes 1.35mm, 5mm and 10mm) at 5 and 20 minutes into the

analysis.

26

Figure 27: Joint temperature contours for gap sizes of 1.35mm, 5mm and 10mm [Analysis time = 5 minutes; temperatures in oC].

27

Figure 28: Joint temperature contours of the thermal analysis models for gap sizes of 1.35mm, 5mm and 10mm [Analysis time = 20 minutes; temperatures in oC].

The results in Figure 27 and Figure 28 show that at the initial stage of fire exposure,

before heat has transferred to the unexposed side, temperatures in the joint regions

on the exposed side are higher than in the panels. Furthermore, the wider the joint

gap, the bigger the affected joint zone. However, during this stage, the joints on the

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unexposed sides are at low temperature so fire resistances of the panels are not

affected by the joints.

However, deep into the analysis when heat has travelled to the unexposed side, the

joint temperatures on the unexposed side can be higher or lower than the panel

temperatures, depending on the joint gap size. Figure 28 shows the temperature

distribution contours of the three panels at 20 minutes into the analysis. This figure

indicates that if the gap is small, there is a ‘cold zone’ on the unexposed surface of

the joint because heat is dissipated from the joint void space on the unexposed side.

However, when the gap is large (10mm), there is a “hot zone” on the unexposed

surface of the joint because heat is transferred to the unexposed surface directly by

cavity radiation from the hotter joint zone.

Figure 30 to Figure 32 compare temperature developments on the unexposed side of

the panels at the joint (P1) and away from the joint (P2) as shown in Figure 29.

Figure 29: Locations for direct temperature comparison between different gap sizes [In the joint – Position P1; Away from joint – Position P2].

29

Figure 30: Temperature comparisons of temperatures at positions P1 and P2 with no additional gap.

Figure 31: Temperature comparisons of temperatures at positions P1 and P2 with a 5mm gap.

30

Figure 32: Temperature comparisons of temperatures at positions P1 and P2 with a 10mm gap.

The presence of a gap in the panel joint has profound influence on heat transfer

through a joint. The controlling factor for fire resistance time of a sandwich panel in

a fire test is the temperature rise on the unexposed side of the panel near the joint

area. Thus, preventing these joint temperature rises will offer great potential for

improving the panel’s fire resistance. In standard fire resistance test of a sandwich

panel, the panel is considered to have failed its insulation criterion if the temperature

increase on the unexposed side reaches 140oC (British Standards Institution 1999).

Figure 30 shows if there is no additional gap in the joint, the joint zone has slightly

lower temperature than on the panel until about 70 minutes into the analysis. This

follows the trend of the test results shown in Figures 20 and 21 in the early stages of

fire exposure when the joints would have remained intact. Afterwards, the joint

temperature is slightly higher than the panel surface temperature. However, as far as

determining fire resistance is concerned (temperature increase reaching 140oC), the

joint temperature and the surface temperature give very similar results.

If the joint gap is 5mm, Figure 31 shows the joint temperature on the unexposed side

is higher than the panel surface temperature. However, the sandwich panel system

31

still approaches a fire resistance time for insulation of 90 minutes. This level of fire

resistance is highly desirable for sandwich panel manufacturers. Figure 25 shows

that for a joint gap size of 10mm, the joint temperature is much higher than the panel

temperature, reflecting the trend in the fire test results of Figure 20 and 21 during the

later stages of fire exposure when the joint would have opened up big gaps. With a

10mm gap size, insulation failure of the sandwich panel may be less than 60 minutes

due to high temperature increase in the joint region, even though the sandwich panel

can achieve more than 90 minutes of fire resistance.

From the above discussions, the joint temperature temperature on the unexposed

surface would not greatly exceed that in the panel if the joint gap is less than 5mm.

However, in current sandwich panel construction, as shown by the test results in

Figure 19, the joint opening may be as large as 10mm after foam ablation. This gap

size can govern sandwich panel performance if the target standard fire resistance

time is 60 minutes. To reduce the final gap size, intumescent strips may be used.

Intumescent strips expand when exposed to heating and the expansion can partially

fill the increasing gap due to PIR core ablation. However, at present, no test data is

available to confirm validity of this suggestion.

5 Conclusions

This paper has presented the experimental results of a few fire tests and ABAQUS

heat transfer simulation results for PIR cored sandwich panels with and without

joints. The fire test results were used to validate a temperature dependent thermal

conductivity model for the PIR core and the heat transfer simulation methodology

for sandwich panels including joints. A parametric study has also been conducted to

demonstrate how modifications to key aspects of sandwich panel design could

potentially improve the fire resistance times of PIR foam sandwich panel systems.

It has been highlighted in this research that a large gap forms in the joints of

sandwich panels due to ablation of the exposed PIR core at high temperatures,

resulting in temperature increases seen on the unexposed sides of the joints of the

panel assemblies. If the size of the gap in a panel joint can be minimised, then

significant reduction in temperature rise on the unexposed face of the panel system at

the joint can be achieved. The numerical modelling results of this paper indicate that

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if the joint gap size can be controlled to be within 5mm, then it would be possible for

a 120mm thick PIR sandwich panel assembly to achieve a standard fire resistance

period for insulation performance of more than 90 minutes. If the gap size is 10mm,

which can be reached as demonstrated in the fire experiments, the temperature

increase in the joint region can limit the sandwich panel system fire resistance to less

than 60 minutes.

Acknowledgements

The research reported in this paper was supported by an EPSRC Industrial CASE

Studentship made available to the second author by Tata Steel Ltd (Voucher no.

1000049X). The authors are grateful to Lancashire Fire and Rescue Services for their

help and assistance in conducting the fire tests.

REFERENCES

British Standards Institution, 1999. BS EN 1364-1:1999 - Fire resistance tests for non-loadbearing elements - Part 1:Walls, London.

British Standards Institution, 2009. BS EN 1993-1-2:2005 Eurocode 3 : Design of steel structures - Part 1-2: General rules - Structural fire design, London.

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