Fire Insulation Schemes for FRP-strengthened Concrete Slabs by Brea Williams

8/11/2019 Fire Insulation Schemes for FRP-strengthened Concrete Slabs by Brea Williams

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Fire insulation schemes for FRP-strengthened concrete slabs

Brea Williamsa, Luke Bisbyb,*, Venkatesh Kodurc, Mark Greenb, Ershad Chowdhuryb

aHalsall Associates Ltd, 210 Gladstone Ave., Suite 3001 Ottawa, Ontario, Canada K2P 0Y6

bDepartment of Civil Engineering, Ellis Hall, Queens University, Kingston, Ontario, Canada K7L 3N6

cNational Research Council Canada, Bldg M-59, 1200 Montreal Road, Ottawa, Ontario, Canada K1A 0R6

Received 27 April 2005; accepted 13 May 2005

Abstract

In recent years, widespread deterioration of civil infrastructure has been a catalyst for the application of externally bonded fiber reinforced

polymer (FRP) sheets for reinforcement or strengthening of concrete structures. However, the performance of these FRP strengthening

systems in fire is a serious concern, and this represents a critical obstacle to the widespread implementation of FRP repair techniques in

buildings. This paper presents the results of an experimental and numerical study conducted to investigate the performance in fire of insulated

FRP-strengthened concrete slabs. Four different supplemental fire insulation systems are examined through standard fire tests, and a

numerical model to predict member behavior in fire is presented. Model predictions are shown to satisfactorily agree with test data. The

results of this study indicate that appropriately designed and insulated FRP-strengthened concrete slabs are capable of achieving satisfactory

fire endurances.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Polymer-matrix composites (PMCs) (A); High-temperature properties (B); Computational modeling (C); Thermal analysis (D)

1. Introduction and background

Research initiatives around the world during the past two

decades have documented the behavior of externally bonded

fiber reinforced polymers (FRPs) for strengthening

reinforced concrete (RC) structures. In these applications,

FRPs are bonded to the exterior of RC structures, typically

using an epoxy resin saturant/adhesive, to provide

additional tensile or confining reinforcement, which

supplements that provided by the internal reinforcing

steel. Sufficient research and implementation has now

been conducted for the development of various design

codes and guidelines for the application of FRPs inconjunction with concrete structures [14]. However, the

majority of applications to date have been on bridges and

parking structures, where fire is not a primary concern. For

externally bonded FRP systems to access the full range of

potential applications, including strengthening and repair of

interior building components, the issue of the fire resistance

of FRP materials and externally-bonded systems must beaddressed.

Various concerns are associated with the behavior of

FRPs during fire. Most FRPs are susceptible to combustion

of their polymer matrix, potentially resulting in increased

flame spread and toxic smoke evolution. In addition,

commonly used polymer matrices and adhesives rapidly

lose strength and stiffness above their glass transition

temperature (Tg). The critical Tg threshold, which depends

on the specific polymer matrix constituents, among other

factors, typically varies from 65 to 82 8C for externally

bonded systems[1]. Thus, if left unprotected in fire FRPs

may ignite, supporting flame spread and toxic smokeevolution [5], and may rapidly lose mechanical and/or

bond properties[6].Based on a detailed review of literature,

studying the variation in mechanical properties of various

FRPs at high-temperature, Bisby [6] suggested a series of

semi-empirical relationships to describe the variation in

strength and stiffness of unidirectional infrastructure

composites with temperature. These relationships were

derived by fitting a sigmoid function, using a least-squares

regression analysis, to a database of results from tests on

unidirectional epoxy-matrix composites of glass, carbon,

and aramid fibers that were available in the literature.

Composites: Part A 37 (2006) 11511160

www.elsevier.com/locate/compositesa

1359-835X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesa.2005.05.028

*Corresponding author. Tel.:C1 613 533 3086; fax:C1 613 533 2128.

E-mail address: [email protected] (L. Bisby).
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As an example, Fig. 1shows the resulting variation in

tensile strength for concrete, steel, and carbon or glass/

epoxy FRPs with increasing temperature. It is evident that

FRP strength is more sensitive to elevated temperatures than

either steel or concrete. In addition to reductions in FRP

materials strength and stiffness, the bond between FRPs

and concrete, which is critical to maintain FRPs effective-

ness in most externally-bonded concrete repair applications,

is likely to be severely reduced at temperatures above Tg.

Little research has been performed in this area with respect

to externally-bonded FRP systems, although results from

bond tests on FRP reinforcing bars at high-temperature haveconfirmed the almost complete loss of bond strength at

temperatures above Tg[7].

Real building fires are unique, and their true behavior is

somewhat difficult to predict with accuracy. Hence,

standard fire tests have been developed by the research

community to represent typical building fires. For instance,

ASTM E119 [8] specifies a standard timetemperature

curve to be followed in standard fire resistance tests. This

curve reaches temperatures in excess of 1000 8C after 2 h. In

describing fire performance of a structural member or

assembly, a range of factors should be considered including:

smoke evolution, smoke toxicity, flame spread, fire

separation characteristics, and load-bearing capacity. How-

ever, the research program described herein is concerned

primarily with the structural fire endurance and fire

separation functions, which, for a floor slab assembly, are

defined by ASTM E119 as the length of time during which

each of the following three criteria are satisfied:

1. The structural member is capable of withstanding its

applied service load (the load which might reasonably be

expected to be supported by the member during a fire);

2. The reinforcing steel maintains a temperature of less

than 593 8C; and

3. The average temperature of the unexposed surface does

not rise more than 140 8C, and no individual point on the

unexposed face rises more than 180 8C, above initial

room temperature levels[8].

A limited number of studies exist documenting the

behavior of FRP-strengthened concrete members under fireconditions. Deuring[9]conducted a fire test program which

demonstrated that rectangular RC beams strengthened in

flexure with externally bonded carbon FRP strips, and

without supplemental fire insulation, experienced loss of

interaction between the concrete and FRP as early as 20 min

into the ISO 834 Standard [10] fire test, while FRP-

strengthened beams protected with supplemental fire

insulation schemes (consisting of mechanically fastened

insulating boards) displayed lower temperatures at the

concrete/adhesive interface and lost interaction only after

about 1 h of fire exposure. A second test program conducted

at Ghent University, Belgium[11]studied the effect of the

supplemental fire protections thickness, configuration,length, and method of adhesion (adhesive only or

mechanical fastening plus adhesive) on the fire performance

concrete beams strengthened in flexure with externally

bonded carbon FRP strips. Mechanical anchorage was

shown to provide superior maintenance of the insulations

bond to the concrete beams, and a U-shaped fire protection

configuration (applied to both the base and sides of the

beams) provided more effective insulation capacity,

reducing temperatures in the carbon FRP strip, and resulted

in lower overall deflections and greater time to loss of

composite interaction.

The only available study on the fire performance of full-scale RC members strengthened with externally-bonded wet

lay-up FRP sheets was conducted as an initial phase of the

ongoing study reported herein, where three 3.81 m-long

400 mm-diameter circular concrete columns, strengthened

(confined) with carbon FRP sheets, were insulated, loaded

to service load levels, and exposed to ASTM E119 standard

fire conditions for more than 4 h without failing[6,12]. The

insulated FRP-wrapped columns achieved 4 h fire ratings

according to ASTM requirements based on axial load

capacity, and the authors concluded that, while FRP

materials are highly sensitive to the effects of elevated

temperatures, appropriately designed, and in most cases

insulated, FRP-strengthened RC columns are capable of

achieving satisfactory fire endurances.

The current paper presents the results of an experimental

and numerical investigation conducted on four intermedi-

ate-scale insulated FRP-strengthened RC slabs exposed to

standard fire conditions. The research was performed at

Queens University, Canada, and the National Research

Council Canada (NRC), in collaboration with industrial

partners Fyfe Co. LLC and Degussa Building Systems. Two

different wet lay-up externally bonded FRP strengthening

systems and three supplemental fire insulation systems have

been studied to date. Temperature data from the fire tests are

Fig. 1. Approximate variation in tensile strength with temperature for

concrete and reinforcing steel (based on approximate equations presented

by Lie [14]) and for unidirectional carbon and glass fibre/epoxy matrix

FRPs (based on semi-empirical analytical relationships developed by Bisby

[6]).

B. Williams et al. / Composites: Part A 37 (2006) 115111601152


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presented and compared with predictions of a numerical

finite difference model, developed by the authors.

2. Experimental program

The purpose of the tests discussed herein was to evaluatethe performance in fire and thermal effectiveness of various

supplemental fire insulation systems for externally bonded

FRP reinforcing systems for concrete, such that the

insulation schemes could be optimized for application on

full-scale FRP-strengthened RC beam-slab assemblies (to

be fire tested at a later date). In addition, the data from the

tests was used to validate numerical heat transfer models,

which describe insulation performance and enable para-

metric studies. These slab tests thus represent a preliminary

investigation of insulations effectiveness. The reader

should note that no load, other than self-weight, was applied

to the slabs during their fire exposures.

2.1. Slab specimens

The slabs were designed with dimensions (954!

1331 mm) that would allow two specimens to be tested

concurrently in the intermediate-scale furnace at NRC. A

slab thickness of 150 mm was selected as being representa-

tive of typical RC building slabs that are encountered in

practice in North America. Minimal internal steel reinforce-

ment was provided, consisting of 315 mm diameter

deformed reinforcing steel bars spanning the long direction,

and 310 mm diameter reinforcing bars spanning the short

slab direction. The reinforcement was designed with a clearcover of 25 mm, which is typical of cover values used in

practice [13]. The concrete mix had a specified 28-day

strength of 28 MPa, and incorporated pure crushed lime-

stone (carbonate) aggregate. The volumetric moisture

content in the concrete was determined to be approximately

4.5% at the time of testing.

2.2. Strengthening and insulation

Twoofthefourslabswerestrengthenedandprotectedwith

FRP and insulation systems provided by Fyfe Co. LLC, and

two were strengthened and protected with systems provided

by Degussa Building Systems. Fig.2 shows schematicsof the

completed slab cross sections, including the FRP and

insulation schemes used on each. FRP was applied to the

tension faces of the slabs, with fibers running in the longer

dimension, using a wet lay-up procedure in a manner

representative of a typical field installation.

Slabs 1 and 2 were protected with different thicknesses of

a two-component fire protection system developed specifi-

cally for the current application by Fyfe Co. LLC. This

system consisted of a layer of Tyfow VG insulation (VG),

applied to the exterior of the carbon FRP wraps, followed

by Tyfow EI coating (EI), applied to the outside surface of

the VG. VG is a spray-applied fire-resistant plaster that was

installed in thicknesses of 19 and 38 mm for slabs 1 and 2,

respectively. EI is an intumescent epoxy surface-hardening

coating, which was trowel-applied to a thickness of

0.25 mm on the exterior of the VG insulation on each

slab. Intumescent coatings are essentially specialized paints

that, when exposed to temperatures in excess of their

Fig. 2. Through-thickness details of the four insulated FRP-strengthened

reinforced concrete slab specimens tested.

B. Williams et al. / Composites: Part A 37 (2006) 11511160 1153


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activation temperature expand to many times their original

thickness and form an insulating char that protects the

underlying material from the thermal insult of the fire.

Slab 3 was protected with 38 mm of MBracew Insulation

1, and slab 4 with 38 mm of MBracew

Insulation 2insulation, both of which were supplied by Watson Bowman

Acme Corporation and are proprietary Portland cement

based mortars incorporating lightweight fillers which were

trowelled onto the exterior surface of the FRP sheets. The

fire protection systems for slabs 3 and 4 did not include any

secondary surface coatings.Table 1provides details of the

slab specimens, FRP and insulation systems used, and

parameters that were varied.

A total of 12 thermocouples were installed in (or on) each

slab at various locations throughout the concrete depth and

within the FRP and insulation layers. This allowed

temperatures to be recorded at various interfaces, and

within the concrete, and allowed for qualitative comparison

of the various insulation systems that were being evaluated.

2.3. Test setup

Two slabs were tested, two at a time, in the intermediate-

scale furnace at NRC, as shown in Fig. 3. Two layers of

ceramic fiber blanket insulation were placed between the

slabs to allow them to react independently to the fire. Each

slab was supported on an insulated ledge along three sides of

the furnace. The slabs were exposed to fire in accordance

with the ASTM E119 [8] standard fire curve, with no

additional applied load. The standard fire curve is includedinFig. 4.

3. Fire test results

3.1. Slabs 1 and 2

Slabs 1 and 2 were both protected with a two-part

passive/intumescent fire protection system and were

identical except for the thickness of passive VG insulation

applied to the exterior of the FRP. During fire exposure,

the EI coating activated (i.e. it expanded and charred) within

5 min of fire exposure when it reached a temperature of

about 235 8C. Within 10 min of fire exposure the intumes-

cent reaction was completed and the EI layer delaminated

from the underside of the slabs, falling into the test furnace.At 132 min, the insulation on slab 1 debonded from the

underside of the slab and exposing the FRP directly to the

fire. Within 5 min of the insulation debonding the FRP had

completely delaminated from the slab, followed shortly

thereafter by extensive spalling of the concrete cover which

exposed the reinforcing steel directly to the fire. Cracks,

approximately 5 mm in width, subsequently developed at

the unexposed surface of slab 1. Slab 2, which had double

the thickness of insulation, performed extremely well during

fire exposure and showed little apparent damage during the

full 4 h of the fire test. The insulation system on slab 2

remained intact, with only very minor cracking observed.

3.2. Slabs 3 and 4

Slabs 3 and 4 were both protected with Portland cement-

based passive fire insulation mortars; neither of these

insulation schemes incorporated an intumescent coating.

Both slabs performed extremely well and were exposed to

fire for 4 h without failing; however, development of minor

cracks in the insulation of both slabs was observed within

Table 1

Details of slab specimens tested to date

No. FRP type No. layers

FRP

Insulation

system

Insulation

thickness

(mm)

Fire resistance (min)

Criterion 1 Criterion 2 Criterion 3 Tgcriterion

1 Tyfow SCHa 2 Tyfow VG/EIa 19 147 O240 42

2 Tyfow SCHa 2 Tyfow VG/EIa 38 O240 O240 104

3 MBracew

CF130b1 MBracew

Insulation

1350c

38 O240 O240 46

4 MBracew

CF130b1 MBracew

Insulation 2

38 O240 O240 52

a Additional information available fromhttp://www.fyfeco.com.b Additional information available fromhttp://www.mbrace.com.c Additional information available fromhttp://www.degussa.com.

Fig. 3. The intermediate-scale slab furnace at NRC with two slab specimens

installed and ready for fire testing.

http://www.elsevier.com/locate/compositesahttp://www.elsevier.com/locate/compositesahttp://www.mbrace.com/http://www.mbrace.com/http://www.degussa.com/http://www.degussa.com/http://www.mbrace.com/http://www.elsevier.com/locate/compositesa


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the first 2 h of the tests. These cracks appeared to graduallywiden as the test progressed, likely due to thermally induced

drying shrinkage of the insulation. Nonetheless, the fire

insulation on slabs 3 and 4 remained intact for the full

duration of the test.

3.3. Temperatures

Fig. 4 shows the temperatures measured within the

concrete, FRP, and insulation layers for all four slabs tested

to date, and Fig. 5 shows a comparison of temperatures

recorded at the same location in all four slabs. For all of the

slabs, the temperature at the fire/insulation interface rosemore slowly than the fire temperature, and remained at

temperatures slightly lower than the furnace temperature

throughout the test. This is due to a combination of: the

protective capacity of the EI layer in slabs 1 and 2; the

thermocouples being slightly embedded in the insulation in

all four slabs; and a mild heat-sink effect wherein the

insulation draws heat away from the thermocouples at its

surface. It is interesting to note the similar performance of all

four insulation systems at this location, indicating that the EI

coating on slabs 1 and 2 had little direct beneficial effect on

the thermal insulating performance of the system (although

the EI coating does play a significant role as a surfacehardening agent for the VG layer under ambient conditions).

At the insulation/FRP interface, the temperature initially

increased until a plateau was reached at about 100 8C. The

duration of this plateau appeared to be dependent both on

the thickness of the insulation and on the type of passive

insulation used. For instance, for slabs 1 and 2, which

incorporated gypsum-based passive insulation of different

thicknesses, a doubling of the insulation thickness from 19

to 38 mm for slabs 1 and 2, respectively, resulted in an

increase in the 100 8C temperature plateau from about

34 min into the test to 176 min. Slabs 3 and 4, both of which

were insulated with Portland cement-based insulation

materials with a thickness of 38 mm, both displayed

plateaus that were somewhat less pronounced and that

lasted only until 65 and 70 min of fire exposure,

respectively. It is thought that the observed temperature

plateaus at this location in all slabs occurred due to the

evaporation of free and chemically bound water from the

insulation/FRP interface at temperatures close to 100 8C.

During this time, most of the thermal energy penetrating the

insulation was consumed through the latent heat of

evaporation of water, and only minimal temperature

increases were observed. Once the water had evaporated,

the temperature again began to rise. The gypsum-based

Fig. 4. Temperatures recorded at various locations in slabs 1 through 4 during fire testing.



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insulation on slab 2, also at a thickness of 38 mm, had a

longer plateau due to the greater volume of water fordehydration of hydrated gypsum as compared with hydrated

Portland cement.

In slab 1, temperature increased sharply beyond the

100 8C plateau to approximately 400 8C at 55 min, at which

point the rate of temperature rise decreased somewhat,

likely due to decomposition of the FRPs epoxy matrix/

adhesive; an endothermic reaction which is known to occur

at a temperature of about 400 8C. At approximately 132 min

of fire exposure the temperature at this interface in slab 1

increased dramatically due to delamination of the insulation

layer. This is clearly evident inFigs. 4 and 5. In slabs 2, 3

and 4, the temperature at the insulation/FRP interface rose at

a more gentle rate, and at 4 h the temperature at this

interface was 272, 409 and 405 8C, respectively.

Because the FRP/concrete interface was slightly farther

from the fire and was, to a very minor extent, insulated by

the low thermal conductivity FRP layer, the temperature at

this location rose more slowly than at the insulation/FRP

interface in all slabs. The temperature leveled off at

temperatures slightly less than 100 8C due to the moisture

evaporation plateau at the adjacent insulation/FRP interface.

The temperature then increased again in all slabs until the

end of the test. Delamination of the insulation and FRP on

slab 1 at 132 min of fire exposure created a large

temperature spike, resulting from sudden combustion of

the fire-exposed FRP. At the end of the test, thetemperatures at the FRP/concrete interface in slabs 2, 3

and 4 were 206, 246 and 284 8C, respectively. It is worth

noting that these temperatures are, in all cases, significantly

greater than the Tgvalues for the adhesive/matrices used.

3.4. ASTM E119 fire endurance criteria

As stated earlier, ASTM E119 specifies three criteria,

two of which are based purely on thermal requirements, to

determine the fire endurance rating for slabs and floor

assemblies.Figs. 6 and 7, respectively, show the tempera-

tures recorded at the bottom of the internal tensile steel

reinforcement (Criterion 2) and at the slabs unexposed

surfaces (Criterion 3) in comparison with the allowable

ASTM E119[8] temperature limits.

Fig. 6 shows that slabs 24 maintained reinforcement

temperatures of less than 593 8C for the full 4-h duration of

the tests. In fact, the maximum observed reinforcement

temperatures in slabs 2, 3 and 4 were only 104, 131 and

158 8C, respectively. Not surprisingly, the Criterion 2

temperature limit was exceeded in slab 1 at 147 min, shortly

after the insulation debonded from the FRP, resulting in

spalling and direct exposure of the reinforcement to the fire.

Fig. 7shows that the average temperature of the unexposed

Fig. 5. Comparison plots showing temperatures recorded at the same location in each of the four slabs.



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face of all four slabs remained less than the limiting

temperature for the full 4 h duration of the fire exposure.Thus, according to the thermal requirements of ASTM E119

[8], slab 1 had a fire endurance rating of 147 min, whereas

slabs 24 all had fire endurances in excess of 4 h. It is

interesting to note that concrete slabs are typically required to

achieve 2 or 3 h fire endurance ratings in North America.

Since the slabs in the current study were not subjected to load

during fire exposure it is not possible to determine their fire

rating according to the first of the ASTM E119 criteria. Tests

on loaded slabs will be required to achieve this goal.

3.5. FRP effectiveness during fire

The ASTM E119 requirements are concerned only with

the overall performance of structural assemblies during fire,

and they do not specifically address questions regarding the

effectiveness of the externally bonded FRP systems. It is

interesting, however, to consider what the likely effects of

the observed temperatures might be on the FRP wraps. It is

well known that the structural performance of an FRP

material decreases rapidly beyond the Tg of the polymer

matrix. While some research studies have indicated that

unidirectional FRPs used in infrastructure applications can

retain much of their longitudinal strength and stiffness at

temperatures well above theirTg, for practical purposes it is

almost certain that the bond between the FRP and the

concrete would be lost at, or slightly above, thistemperature. If it is assumed that the Tg of the matrices

used in the current study is 82 8C, a value that is at the upper

end of the likely range for systems currently used in

concrete repair applications [1], then the FRP would

probably be ineffective as early as 42, 104, 46, and 52 min

for slabs 1 through 4, respectively.

It is important to recognize that the above concept of

maintenance of FRP effectiveness during fire is not

currently enforced by applicable fire testing standards in

North America, and given the overall goals of fire-safety

engineering it does not appear that it necessarily should be.

Slab or floor assemblies are required to retain sufficient

strength during fire to support their full service load for the

required duration of fire. Thus, even if the FRP loses its

effectiveness relatively early on in the fire exposure, the

insulation will protect the underlying concrete slab so that

the overall fire endurance of the assembly proves

satisfactory. The important design consideration, then, is

to ensure that the existing (unstrengthened but insulated)

slab retains sufficient strength during fire to resist full

service loads for the required duration. In most cases this

criterion is relatively easy to ensure, provided that the

increase in strength due to FRP wrapping is kept within the

range of 2550%, depending on the live-to-dead load ratio,

and assuming that the slab is provided with some form ofsupplemental fire insulation.

4. Numerical model

A numerical heat transfer model was developed in

conjunction with the slab testing described above in an

attempt to predict the temperatures at various points

throughout the cross section of an insulated FRP-

strengthened RC slab. The model was programmed by

Bisby[6]and Williams[14]using a modified version of a

one-dimensional explicit finite difference heat transfer

procedure that has been presented previously by Lie [15].

The effects the intumescent coating on slabs 1 and 2 are not

included in the analysis, due to its relatively insignificant

effect on the heat transfer behavior and because of the

complexities associated with modeling these types of

coatings [16]. The model discretizes the insulation, FRP,

and concrete into a series of elemental layers and

successively applies simple thermal equilibrium equations

to each. The variation in the thermal properties (thermal

conductivity and heat capacity) of all materials with

increasing temperature is taken into account using relation-

ships suggested by Lie[15]for concrete, Griffis et al.[17]

Fig. 6. Temperatures recorded at the level of the internal reinforcing steel in

all four slabs.

Fig. 7. Average of temperatures recorded at the unexposed face for all four

slabs (average of five temperature readings).



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for carbon FRP, and Bisby [6] for the spray-applied

insulation. It is assumed that fire exposure occurs from

below, that heat transfer to the slab occurs due to radiation

only, and that heat loss from the top of the slab is by

convection only using empirical equations for convective

heat transfer above a horizontal surface as suggested by

Spiers [18]. The effects of moisture evaporation from theconcrete are included by assuming that when the tempera-

ture in a layer reaches 100 8C, all of the heat transferred to

that layer is used to evaporate water. During evaporation,

the temperature in the layer is assumed to remain at 100 8C.

Temperatures begin to rise again only when all of the

moisture in the layer has completely evaporated. The effects

of moisture migrating away from the source of heat, a

phenomenon which has been observed in testing and which

is thought to play a role in the models inability to

accurately predict temperatures near 100 8C, are not

included at this time. Because the overall theory and

application of this type of explicit finite difference heat

transfer model is not novel and has been presented in detail

elsewhere [6,14,15,19], it is not necessary to completely

describe the equations used in the current analysis.

The reader should note that several commercially

available models exist to simulate heat transfer in reinforced

concrete structural members[20,21]. The available models

typically account for the latent heat of moisture evaporation

from the concrete, but most do not attempt to treat moisture

migration. Attempts to include moisture migration in

concrete under fire exposure have been presented in the

literature[22], although the required equations significantly

increase the computational effort required and have not yet

been included in the current analysis.Relatively straightforward numerical models have also

been developed and presented previously to treat heat

transfer in thick FRP composites exposed to fire. A review

of the state of the art in this area is presented by Davies et al.

[23]. When thick FRPs are exposed to fire, pyrolysis of the

polymer matrix near the fire exposed surface will occur,

leading to the formation of a protective char. The thickness

of the char layer increases with increasing fire exposure and

forms a thermal barrier that insulates the interior of the FRP

component. The char eventually degrades and heat transfer

beyond this point is governed by the thermal properties of

the fibers that remain. Thus, accurate heat transfer analysis

in thick FRPs requires consideration of the effects of matrix

pyrolysis, off gassing, char formation, and char erosion.

However, for the case of FRP-strengthened reinforced

concrete slabs subjected to fire, the FRP strengthening

materials are typically very thin (less than 12 mm), and

their contribution to the overall heat transfer in the member

is not significant. This point is evidenced by referring to

Fig. 8, where the observed temperature drop across the FRP

sheet is seen to be insignificant for at least the first 3 h of the

fire test. Hence, the additional computational effort required

to treat pyrolysis and charring of the FRP is, in the opinion

of the authors, not warranted for the current illustrative

analysis. The observed increase in temperature drop across

the FRP at later stages of the fire exposure may indeed be

due to polymer matrix pyrolysis at elevated temperatures,

and attempts will be made to account for these affects in

future analyses. In cases where FRP materials are

sufficiently thick to significantly affect heat transfer in the

overall structural member, the reader in encouraged to

consult the work of Davies et al.[23].

As an example of the models output,Fig. 8provides a

comparison of the predicted and measured temperatures in

the insulation, FRP, and concrete for slab 2 during

exposure to the standard fire. The temperature at the fire/

insulation interface is over-predicted in this case, likely

because of the heat sink behavior experienced by the

thermocouple located at the surface of the insulation. Inaddition, the EI coating has not been included in the

model. Finally, it is likely that the fire/insulation interface

thermocouple was very slightly embedded in the

insulation, thus reducing the observed temperatures at

that location. At the insulation/FRP and FRP/concrete

interfaces, the model adequately captures the slow rise in

temperature to around 100 8C and appropriately predicts a

decreased rate of temperature increase due to moisture

evaporation from the insulation. However, the temperature

drop across the FRP layer, which is observed to increase

above 100 8C, is not well captured by the model. The

increasing drop in temperature across the FRP is likelydue to a change in thermal properties of the epoxy matrix

above Tg, a behavior that remains incompletely under-

stood and is that not currently accounted for in the model.

Overall, it appears that the model satisfactorily

captures the thermal behavior at each interface, such

that it could be used for preliminary parametric studies.

The model is currently being further refined to allow it to

more accurately describe the variation in thermal proper-

ties of the various materials involved. The effects of

moisture migration and evaporation form the insulation

are also being incorporated.

Fig. 8. Comparison of model predictions and observed temperatures in slab

2 during fire testing.



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5. Preliminary parametric studies

While the numerical model described above should be

regarded as preliminary, simple parametric studies were

conducted and have led to some interesting insights with

respect to the fire performance of externally bonded FRP

strengthening systems for RC members. As stated pre-viously, there is some question as to whether FRP materials

used for externally strengthening RC members can maintain

their effectiveness during fire, and what thicknesses of

insulation might be required to ensure that this occurs. Two

insulation parameters are critical in any such discussion:

thermal conductivity and insulation thickness.

Preliminary parametric studies was conducted based on a

number of assumptions: (1) that the insulation system was

that used on slabs 1 and 2; (2) that fire endurance is defined

as the time at which the FRP bondline temperature exceeds

the adhesives Tg; and (3) that Tgof the FRP is 82 8C. It is

important to remember that the second assumption is usedhere only for the purposes illustration, and the authors do

not wish to promote the use of Tg limits in defining fire

resistance for FRP-strengthened concrete members. It has

also been assumed in this discussion that the slab specimens

and FRP wrapping schemes are as outlined earlier for slabs

1 and 2 of the experimental program (i.e. carbonate

aggregate slabs, 150 mm thick with two layers of

externally-bonded carbon/epoxy FRP wraps).

Using the above assumptions, the effect of insulation

thickness on fire resistance as predicted by the numerical

model is shown in Fig. 9. As expected, fire resistance

increases with greater insulation thickness, reaching about

90 min with 50 mm of insulation. The gain in fire resistance

with increases in insulation thickness is roughly exponen-

tial. It is also apparent that, even with a substantial thickness

of insulation, it appears that it will be difficult to maintain

the FRP temperature below its glass transition temperature

for a prolonged period of fire exposure.

By assuming that the thermal conductivity of the

insulation remains constant with increasing temperature(an assumption which does not hold true for most insulation

systems in fire) it is possible to examine the influence of

thermal conductivity on the fire resistance of FRP-

strengthened concrete slabs using the assumptions

mentioned previously. It is also assumed that the insulation

is 25 mm thick. Fig. 10 shows that fire resistance is

maximized at lower thermal conductivity values, and that

fire resistance decreases rapidly within the 0.10.5 W/m-K

range. At thermal conductivity values above 0.5 W/m-K,

negligible reductions in fire endurance are observed. It

should be noted that the best fire insulation materials that are

currently practical for use in buildings have thermalconductivities of about 0.1 W/m-K.

The above discussion points to the fact that, given the

thermal conductivities of currently available, cost-effective

insulation materials, and given also the fact that insulation

thickness greater than about 50 mm rapidly become imprac-

tical in many field applications, it appears that it will be very

difficult to maintain the effectiveness of externally-bonded

FRP reinforcement during fire. However, as stated previously,

maintaining effectiveness of the FRP wrap is in no way an

essential criterion for achieving adequate fire resistance.

6. Conclusions

This paper presents the results of an experimental and

numerical investigation into the fire performance of

unloaded, intermediate-scale, insulated FRP-strengthened

RC slabs. Four slab specimens were strengthened and

insulated, and their internal temperatures were monitored

during exposure to the ASTM E119 standard fire. Two

different FRP strengthening systems and three different fire

protection schemes were considered. Based on the results of

both experimental and numerical studies, the following

conclusions can be drawn:

Fig. 9. Predicted variation in fire resistance with varying insulation

thickness (fire resistance defined in terms of the matrix/adhesive glass

transition temperature).

Fig. 10. Predicted variation in fire resistance with varying insulation

thermal conductivity (fire resistance defined in terms of the matrix/adhesive

glass transition temperature).



10/10

According to ASTM E119 fireendurance criteria, a 4-hfire

endurance rating (based on thermal criteria only) can be

achieved with 38 mm of any of thefour insulation schemes

examinedherein. A smaller thickness of Insulation System

1 (19 mm) provided approximately 2 h of fire protection

for a 150 mm thick reinforced concrete slab.

Observations from the fire tests indicate that providingsufficient insulation thickness is important in minimizing

cracking and preventing possible delamination of the fire

protection layer and concrete cover.

A simple heat transfer model has been developed that can

provide reasonable estimates of the temperature within

insulated FRP-strengthened RC slabs during fire exposure.

Further refinement of material thermal properties, and

incorporation of moisture migration, is necessary to

improve correlation with measured data.

While it appears that it will likely be difficult to maintain

the effectiveness of externally-bonded FRP strengthening

systemsduring fire, it is possibleto achievesatisfactoryfire

performance for FRP-strengthenedRC members, provided

they are appropriately designed and adequately insulated.

Tests on full-scale insulated FRP-strengthened RC slabs

under load are required to confirm this conclusion.

7. Ongoing research

The testing and analysis presented in the current paper

represent one phase of a larger ongoing research study. In

addition to the testing and analysis presented herein, the

overall research project includes full-scale fire tests and

numerical modeling of insulated and uninsulated circular

and square FRP-wrapped RC columns [12] and insulated

FRP-strengthened reinforced concrete beam-slab

assemblies, both under load. More recently, bench-scale

tests on the high-temperature and residual mechanical and

bond properties of FRP materials are being conducted.

Acknowledgements

The authors wish to acknowledge the funding provided

by the Natural Sciences and Engineering Research Council

of Canada (NSERC) and the Intelligent Sensing for

Innovative Structures (ISIS) Network of the Canadian

Network of Centres of Excellence. The authors would also

like to acknowledge the support of the National Research

Council of Canada, Fyfe Company LLC, Degussa Building

Systems, and Queens University, Canada.

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Fire Insulation Schemes for FRP-strengthened Concrete Slabs by Brea Williams

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