Fire behavior of reinforced concrete beams …...Fire behavior of reinforced concrete beams...

Fire behavior of reinforced concrete beams

flexurally strengthened with CFRP laminates

João Dinis Pereira da Silva

Extended Abstract

Supervisor:

Prof. João Pedro Ramôa Ribeiro Correia

Jury:

President: Prof. Albano Luís Rebelo da Silva das Neves e Sousa

Examiner: Prof. João Pedro Ramôa Ribeiro Correia

Examiner: Prof. Fernando António Baptista Branco

December 2013

Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates

1

1 – Introduction

To produce a fiber reinforced polymer (FRP) material it is necessary to combine two main materials, the

fibers and the resin. The fibers are responsible for the mechanical response of the composite and the

resin is responsible for the cohesion of the composite and for transferring the stresses to the fibers.

Similarly to concrete, the FRP material can include fillers and additives in its composition. These are

added to the mixture to increase certain properties or improve the manufacturing process. Generally,

the mechanical behavior and response of an FRP material is influenced by its constituents, length and

orientation of the fibers, their distribution on the matrix (resin), the mechanical properties of the matrix

and the bond between fibers and resin [1].

The application of FRP materials has been growing expressively in the last decades with aerospace and

naval industries being their first areas of application. Throughout the years and the evolution of

techniques used to manufacture these materials, like pultrusion, producers were able to manufacture

FRP materials with better properties and characteristics at a lower cost [2]. The resistance of FRP

materials to electrochemical corrosion, their high strength-to-weight ratio, inherent light weight, and

ease of application are advantages that make FRPs attractive for both new construction and

rehabilitation of structural members [3, 4].

With the latest increasing use of FRP materials in construction, it becomes necessary to understand their

characteristics and behavior when used in synergy with the rest of the materials in construction. In one

hand, the behavior of FRP are well known and studied in the last decades, in the other hand its behavior

when subjected to the same actions a building is designed for is practically unknown.

The lack of a comprehensive database on FRP materials makes it difficult for the practicing civil engineer

and designer to use FRP composites on a routine basis. Although a number of reviews have been

published recently related to durability and test, the focus of each has been to summarize the state of

knowledge in general without emphasizing or attempting to prioritize critical areas in which needs are

the greatest for collection, assimilation, and dissemination of data [5].

It is well known that carbon fiber reinforced polymer (CFRP) materials are prone to mechanical

deterioration and ignition/combustion when exposed to elevated temperatures or fire, primarily due to

the thermal susceptibility of their polymer matrixes. As the temperature of the polymer matrix

approaches its glass transition temperature Tg, the matrix transforms to a soft, rubbery material with

reduced strength and stiffness. Common thermosetting polymer matrixes, such as epoxy, typically

exhibit a Tg in the range of 50 to 80°C. Furthermore, when exposed to 300-500ºC, the organic matrix

decomposes, releasing heat, smoke, soot and toxic volatiles [6-9]. In addition to reductions in CFRP

materials’ strength and stiffness, the bond between CFRP’s and concrete, which is critical to maintain

CFRPs’ effectiveness in most externally-bonded concrete strengthening systems, is likely to be severely

reduced at temperatures above Tg [3].

As mentioned, there is practically no data or information about reinforced concrete structures

strengthened with CFRP materials when subjected to the same actions a building is designed for,

especially fire situation. Given this lack of knowledge about the mechanical and thermal response and

performance of these strengthening systems, the present study aims to increase the understanding

about this matter.

Research on the performance of CFRP-strengthened concrete members exposed to fire is rather scarce

and needs further investigation. Only a small amount of studies have been conducted on the


2

performance of FRP-strengthened concrete beams subjected to fire exposure. Among them,

Deuring [10], Blontrock et al. [11] and Burke et al. [12] are the most relevant studies to the present one.

Deuring [10] tested six rectangular beams, 30 mm deep, 400 mm wide and 5 m in length, to fire

situation according to ISO 834 [13]. Four of the beams were strengthened with CFRP strips and two,

from those four, were protected with a 40 mm calcium silicate plate. All the beams were tested loaded

to about 55% of their theoretical load capacity. Deuring [10] determined a 1.8 times improvement to

the fire resistance time when the 40 mm calcium silicate plate was added. In a similar experimental

campaign Blontrock et al. [11] tested a series of 10 CFRP strengthened reinforced concrete beams

protected with various schemes of calcium silicate plates. The U-shape fire protection scheme, applied

to both bottom and sides of the beams, seemed to be the most effective when retarding the increase of

temperatures within the beams. Interestingly, insulation applied only within the anchorage zones of the

FRP, with the middle portion of the FRP left exposed to the fire, preserved the bond and allowed the

CFRP strip to maintain its contribution as tensile reinforcement during the test, similarly to the fully

protected beams [3]. Burke et al. [14] observed similar behavior when testing reinforced concrete slabs

CFRP strengthened through the NSM (near surface mounted) technique.

More recently, Palmieri et al. [14], Kodur et al. [15] and Firmo [3], have conducted similar studies.

Palmieri et al. [14] tests results indicated that, if appropriately insulated, the NSM FRP-strengthened

beams can achieve a satisfactory fire endurance of 1 hour as per fire resistance test specifications.

Numerical results presented by Kodur et al. [15] indicate that a concrete beam strengthened with NSM

FRP reinforcement yields slightly lower fire resistance as compared to a conventional RC beam but

achieves higher fire resistance than that of externally bonded FRP. It is also shown that appropriate

location of NSM FRP reinforcement and insulation scheme can increase the fire resistance of concrete

beams strengthened with NSM FRP reinforcement. Firmo [3] concluded that the protection materials

allow the CFRP strengthening system to be effective during a considerably longer period of fire

exposure, which increases for thicker protections, because temperatures are considerably lower than in

unprotected beams, especially at the concrete-CFRP interface. However, as for the CFRP strengthened

beams, those temperatures at midspan section are still much higher than the Tg of the adhesive.

The main objective of the present paper is to study the fire behavior of reinforced concrete beams

flexurally strengthened with CFRP laminates. To achieve that, several tests were conducted in order to

determine which strengthening technique granted higher fire resistance times (EBR vs. NSM), which

geometry of fire protection system was the most efficient and which resin used (in the NSM technique)

to bond the laminates allowed better mechanical and thermal responses.

2 – Experimental study

2.1 – Test program

To assess the fire behavior of reinforced concrete beams strengthened with CFRP laminates, 17 loaded

beams were tested in a mid-scale oven. These 17 beams are divided as 1 reinforced concrete beam

without CFRP strengthening, 8 reinforced concrete beams strengthened with EBR technique and 8

reinforced concrete beams strengthened with NSM technique. Four beams, the unstrengthened one,

one EBR and two NSM, one of each resin, were tested without fire protection system applied and

become the standard beams. For the other 13 beams various types of fire protection systems

geometries were applied and tested. Figure 1 presents all the geometries of fire protection systems

utilized and their nomenclature.


3

Figure 1. Geometry of fire protection systems.

Table 1 presents all the beams tested to fire. In each beam nomenclature it is possible to understand the

geometry of fire protection system used. In the same table the resin used in each beam is presented as

well as the thickness of the calcium silicate plates used in each zone. In some beams the letter ‘L’ is

added in its nomenclature, it means that in those cases a small defect was noticed in the oven

insulation. To fix it, extra calcium silicate plates were added to the beams lateral panels.

Table 1. Characteristics of all the beams tested and its nomenclature.

25 cm

La=20 cm

2,5

La=20 cm

2,5

30 cm

La=20 cm5

2,5 2,5

5 52,5

La=20 cm

25 cm

La=20 cm

2,5

La=20 cm

2,5

30 cm

La=20 cm

5

2,5 2,5

5 5

2,5

La=20 cm

32,5 cm

La=20 cm

7,5

5 7,5

2,5

25 cm

La=20 cm

2,5

La=20 cm

2,5

30 cm

La=20 cm

5

2,5 2,5

5 5

2,5

La=20 cm

25 cm

La=20 cm

2,5

La=20 cm

2,5

30 cm

La=20 cm

5

2,5 2,5

5 5

2,5

La=20 cm

32,5 cm

La=20 cm

7,5

5 7,5

2,5

25 cm

La=20 cm

2,5

La=20 cm

2,5

30 cm

La=20 cm

5

2,5 2,5

5 5

2,5

La=20 cm

32,5 cm

La=20 cm

7,5

5 7,5

2,5

La=20 cm

2.5

cm

30 cm

La=20 cm

5 c

m

5 cm 5 cm

2.5

cm

32,5 cm

La=20 cm

La=20 cm

25 cm

La=20 cm

2.5

cm

2.5 cm 2.5 cm

75-50-75

U 25-0-25

25-25-25 50-25-50

75-25-75

32,5 cm

La=20 cm

7.5

cm

5 cm 7.5 cm

2.5

cm

7.5

cm

5 cm 7.5 cm

5 cm

25-0-250 25-25-25

50-25-50 75-25-75 75-50-75

Nomenclature Strengthening

technique Resin

Thickness of fire protection

system

Anchorage

zone Current zone

RC - - - -

EBR

EBR

Epoxy S&P 220

- -

EBR-25-0-25 25 mm -

EBR-25-25-25 25 mm 25 mm

EBR-50-25-50 50 mm 25 mm

EBR-75-25-75-L 75 mm 25 mm

EBR-75-50-75-L 75 mm 50 mm

EBR-50-25-50-Tg-L Epoxy Araldite 2014 50 mm 25 mm

EBR-50-25-50-Tg+S&P-L Epoxy Araldite 2014 +

Epoxy S&P 50 mm 25 mm

NSM-E

NSM Epoxy S&P 220

- -

NSM-E-25-0-25 25 mm -

NSM-E-25-25-25 25 mm 25 mm

NSM-E-50-25-50 50 mm 25 mm

NSM-C

NSM Cement base Adicrete ER

- -

NSM-C-25-0-25 25 mm -

NSM-C-25-25-25 25 mm 25 mm

NSM-C-50-25-50 50 mm 25 mm


4

2.2 – Materials

The construction of the reinforced concrete beams involved four major components: (i) concrete, (ii)

CFRP laminate, (iii) resins and (iv) calcium silicate plates.

The concrete strength class and exposure class was C25/30 and XC2, respectively. Maximum limestone

aggregate gravel diameter was set at 22 mm and the cement was Portland CEM II/A-L 42,5R. Various

concrete cubic specimens were tested, in order to confirm the properties given by the manufacturer,

obtaining fcm = 29.48 MPa, fck = 28.25 MPa, fctm = 2.25 MPa and Ecm = 27.39 GPa.

The CFRP laminates were produced by S&P Clever Reinforcement Company with cross section

dimensions of 20 mm wide by 1.4 mm thick for the EBR technique and 10 mm wide by 1.4 mm thick for

the NSM technique. Although its dimensions are different given the technique used, its properties are

the same since it is the same product. The laminates commercial name is S&P Laminates CFK 150 / 2000

with its properties, determined by Firmo [3], being Ef = 170.9 GPa, εfu = 16.0 % and σfu = 2741.7 MPa.

There were three different resins used in the present study, two of them epoxy resins and the other one

a cement based resin. The first epoxy resin used has a commercial name S&P Resin 220 epoxy adhesive,

and its properties, given by the manufacturer, being σtu ≥ 3 MPa, σcu ≥ 90 MPa and σfu ≥ 30 MPa. In his

investigations, Firmo [3] tested some resin specimens obtaining σtu = 17.33 MPa, εtu = 2482 μstrain and

Et = 8.76 GPa. The glass transition temperature of S&P Resin 220 epoxy Tg ≥ 56 ⁰C, given by the

manufacturer.

The second epoxy resin, with the commercial name Araldite 2014, was designed to bond FRP to FRP

materials and there is no information or experiments made about its performance when bonding CFRP

material to concrete. Nevertheless, since this resin has what is considered a high glass transition

temperature (Tg = 85⁰) it was used. It has an elastic modulus of 5 GPa [16].

The third resin used was a cement based resin with a commercial name Adicrete ER, commercialized by

Quimidois company in Portugal. Macedo et al. [17] determined its tensile and compressive strengths

obtaining 16.7 MPa and 66.6 MPa, respectively. About its glass transition temperature, Firmo [18]

through DMA (Dynamic Mechanical Analysis) tests, obtained a value of Tg = 44 ⁰C.

The insulation material used in the fire protection system was calcium silicate. This material was

acquired in the shape of plates with 1.2 m wide, 2.5 cm thick and variable length. Its commercial name is

PROMATECT-L500 and it comes with a honeycomb finishing in one side, being that side the one

supposed to be glued. Table 2 presents its properties given by the manufacturer.

Table 2. Characteristics of the calcium silicate plates [19].

Characteristics [Units] PROMATECT - L500

σcu [MPa] 5.5

σtu [MPa] 1.2

σfu [MPa] 3.0

Bulk density [Kg/m3] 500

Thermal conductivity [W/mK] 0.09

Thermal expansion [m/mK] 7.0 x 10-6


5

2.3 – Test specimens

All the tested beams were 1.5 m long, 0.10 m wide and 0.12 m deep. The flexural reinforcement

consisted of four 6 mm diameter steel bars, with the steel category being A500 NR SD. The shear

reinforcement was made by applying the same 6 mm diameter steel bar with a longitudinal spacing of

0.06 m as seen in figure 2. For the EBR technique the CFRP strengthening system consisted of one

laminate with 1.4 mm thick, 20 mm wide and 1.1 m long bonded to the bottom face of the beams. The

length of the laminate used was such that all the laminate would be inside the inner walls of the oven

and exposed to fire. In the NSM technique two laminates with 1.4 mm thick, 10 mm wide and the same

1.1 m long in the EBR technique were embedded with resin in two grooves made in the bottom face of

the concrete.

Figure 2. a) Geometry of the reinforced concrete beams flexurally strengthened with a CFRP laminate before the installation of the protection system. b) Midspan cross-section of the strengthened EBR beams.

In the EBR beams two types of adhesive were used: S&P Resin 220 epoxy adhesive (Tg ≥ 56 ⁰C) and

Araldite 2014 (Tg = 85 ⁰C). In the EBR-50-25-50-Tg-L only Araldite 2014 was applied, in EBR-50-25-50-

Tg+S&P-L both resins were used, Araldite 2014 in its anchorages zones, 20 cm in from laminate

extremities while its current zone was bonded with S&P resin.

In NSM beams there were also two types of resin applied, S&P Resin 220 epoxy adhesive (Tg ≥ 56 ⁰C) and

cement based Adicrete ER (Tg = 44 ⁰C). The beams in which Adicrete ER was used have a ‘C’ in its

nomenclature, the other NSM beams were all bonded to the laminates with S&P resin.

As mentioned, various geometries of fire protection system were used. Figure 3 presents some

examples. The fire protection system was fixed in two ways, bonded and mechanically. After the calcium

silicate plates were cut to the right dimensions they were bonded to the concrete and each other with

Pattex brand Refractory mastic. The mechanical fixation was applied in the form of U-shape steel. These

plates were screwed to the lateral faces of the beams with small metallic screws. To assist in the lateral

insulation of the oven covers two strips of intumescent tape were glued to each lateral face of the

beam.

2Ø6 mm

Ø6 mm // 0.06 m

1.10 m

Laminado de CFRP

0.08 m 0.08m

2Ø6 mm

0.12m 0.12m

0.2 0.2

Ø6//0.06

0.10 m

0.1

2 m

2Ø6

2Ø6

Laminado

20x1.4 mm )

CFRP laminate

CFRP Laminate (20mm x 1,4mm)

a)b)


6

Figure 3. a) EBR-25-0-25 before fire test. b) NSM-50-25-50 before fire test. c) EBR-75-50-75-L.

2.4 – Test setup

2.4.1 – Oven

The oven where the fire resistance tests were conducted is a vertical oven with external dimensions of

1.35 m wide, 1.20 m deep and 2.10 m tall. The inner walls and the top of the oven are coated with 20

cm thick ceramic wool while the bottom is made with refractory bricks. Taking into account the

presence of the ceramic wool, the oven interior dimensions are approximately 0.95 m wide by 0.80 m

deep.

There are six propane gas burners inside the oven, three on each side, distanced between each other to

optimize its heating process. There are also three thermocouples inside the oven to register

temperatures. Outside the oven but linked to it there is a control unit whose aim is to turn the oven on

and off, read the theoretical and real temperature of the oven and regulate the input of propane gas.

2.4.2 – Beam supports

The tested beams were placed over the opened top of the oven, positioned along its length, with their

axes aligned with the center of the oven. The beams were supported on one roller support and one

fixed support (10cm long x 6 cm wide) placed over metallic plates, on top of the oven’s external walls.

Those metallic plates were suspended with four 24 mm diameter steel rods on a steel reaction frame,

which was erected surrounding the oven. All the beams were installed keeping a vertical distance of at

least 5 cm between their bottom surface and the oven’s lateral walls, in order to allow for the

deformation of the beams during the tests and guarantee that all the length of the laminate was evenly

under the fire action, without touching the oven’s walls, which was accomplished in all tests. In some

a)

b)

c)


7

tests, where the fire protection system had more volume, 2.5 cm thick steel plates were added to the

beams support to guarantee its height.

2.4.3 – Fire loading

To perform the fire resistance tests a fire time-temperature standard curve had to be chosen, with the

choice being to conduct all fire resistance tests under ISO 834 [13] standards. The fire exposure time-

temperature curve is given by the following equation,

where:

- Fire temperature in Celsius;

T0 – Initial oven temperature in Celsius;

– Time since fire beginning in minutes.

2.4.4 – Structural loading

All the beams, with 1.50 m length, were tested in a four point bending system with a 1.26 m span. A

total load of 7.20 kN to the un-strengthened beam, 11.70 kN to all beams strengthened with EBR

technique and 14.77 kN to all beams strengthened with NSM technique, was applied in two sections

(figure 4), positioned at a distance of 0.42 m from the support sections and symmetric relative to

midspan section. The load was applied by two sets of weights, suspended in a load transmission steel

beam with pulley blocks, which facilitated the loading/unloading operations and guaranteed a constant

load speed during the load procedure and constant load during the test. The load transmission beam,

placed over the tested beams, transferred the load to the top face of the concrete beams through two

tubular steel profiles, 5 cm high x 5 cm long, with a final 2 cm diameter steel rod welded to the bottom

of them. In the contact area between the steel rods and the top face of the beam, 5 cm wide steel plates

were added in order to prevent concrete from crushing distributing the loads for a bigger area.


8

Figure 4. Frontal view of test setup (dimensions in meters, not to scale)

2.4.5 – Instrumentation

All beams were instrumented with thermocouples to measure temperature distribution outside and

inside the beams. To measure inside temperatures, 6 type K thermocouples were placed inside the

concrete beams, at the midspan section, following the distributions plotted in figure 5. The same figure

shows all the internal thermocouples nomenclatures.

Figure 5. Location of thermocouples within all beams at midspan section.

0.42 m

0.95 m

Viga

ensaiada

Forno

T(t)

ISO 834

Pesos

Apoio

Pesos

0.42 m 0.42 m

Campânula

(exaustão)

Viga de transmissão

de carga

Pórtico de

reacção

LaminadoLaminate

Oven

WeightsWeights

Support

Reaction frame

Campanula (exaustion)

Tested beam

Load transmission beam


9

To measure the outside temperatures, type K thermocouples were placed longitudinally outside the

beams. In the EBR technique they were placed in the laminate-concrete interface, regarding the NSM

beams the thermocouples were placed inside the grooves at half depth. All outside thermocouples

placements are shown in figure 6 and figure 7.

Figure 6. Location of thermocouples outside all EBR beams.

Figure 7. Location of thermocouples outside all NSM beams.

To measure the deflection at midspan section, an electrical displacement transducer from TML, model

CDP-500 with a 500 mm stroke was tied to a hook screwed to the top face of the beams.

2.5 – Test procedure

The tests were conducted in two different phases. In the first phase, the load transmission beam was

correctly placed over the testing beam, with the steel load distribution plates positioned in its final

10 cm 10 cm

La

10 cm10 cm

La

T1

h/3

T8h/3

T15T2 T3

T4

T5

T6T9 T10 T11 T12 T13 T14

T7 (Air temperature)

10 cm 10 cm

La

10 cm10 cm

La

h/3

h/3

T1

T2 T3

T4

T5

T6

T8 T9

T16 T16’

T10 T11 T12 T13 T14 T15

T7 (Air temperature)


10

place. Then the beams were loaded with both pulley blocks elevating simultaneously, granting a ground

clearance of at least 15 cm to prevent the weights to touch the floor during the test. The beams

remained loaded for a recommended time of 30 min to stabilize its deflections. After several tests it has

been found that the deflections stabilized after a few seconds, therefore the waiting period was reduced

to about 10 min.

In the second phase of the test, the oven propane gas burners were turned on and the beams started to

be subjected to an increasing of temperature. The beams were tested until their strengthening system

ruptured, plus 10 to 15 min, or until the oven maximum time of function was reached, defined at 210

min. After one of these occurrences, the gas burners were turned off, and the front doors of the oven

were open to allow a better and faster cooling of the beams.

3 – Analysis and discussion of fire resistance tests

In this chapter it will only be mentioned the EBR and NSM test beam results. The RC beam,

unstrengthened and unprotected, serves only as a standard beam, not being the main focus of these

experiments.

3.1 – Temperatures analysis

3.1.1 – EBR beams

Figure 8 to figure 15 show the temperatures readings measured as a function of time at different depths

of midspan section and longitudinal profile of EBR beams. The time of rupture of the CFRP strengthening

system is represented by a vertical dotted red line and the glass transition temperature of the resin used

represented with a horizontal dotted black or grey line. In some tests it was impossible to obtain correct

readings (or readings at all) from certain thermocouples, as such those thermocouples were eliminated.

As seen in figure 8 to figure 15, the temperatures read inside the reinforced concrete beams follow the

expected distribution. It is possible to draw an increasing profile of temperatures from thermocouple T1

to thermocouple T6, from the nearest thermocouple to the fire exposed beam face to the furthest one,

vertically. Sometimes thermocouple T4 stagnates its rising temperatures at 100 ⁰C. This can be justified

by the fact that T4 is placed in concrete and, at this temperature, the concrete stays at 100 ⁰C until al l

the constituent water is evaporated. Once all the water at that section has evaporated, all the

heat/energy received by the concrete is put into heating the concrete instead of the water evaporation.

If an average temperature of the anchorage zones is calculated, given by the average temperature at

any given moment of thermocouples T9 to T11 and T12 to T14, it is possible to verify that, at the

moment of CFRP laminate rupture of almost all EBR beams, the glass transition temperature has been

surpassed in at least one anchorage zone. This fact confirms the importance of the glass transition

temperature of the resin used in the design of the strengthening and protecting system. In fact, the

strengthening system ruptures when temperatures at the anchorage zones approach the glass transition

temperature.

As protection calcium silicate plates were added to the current zone of the beams, it is possible to

confirm that the midspan placed thermocouples (T1 to T6) suffer some changes in its readings. As

protection at the current zone is added, the growth rate of temperatures is reduced and becomes more


11

linear unlike the unprotected EBR beams in the current zone which presented growth rate readings very

similar to the oven temperature growth rate, since they are closer and more exposed to the fire.

One identifiable common phenomenon is that some thermocouples readings change severely once the

strengthening system ruptures. Rapidly increases in some thermocouples growth rate are visible and

can be justified by the fact that once the strengthening system ruptures they become exposed and

subjected to more heat. Similar behavior is sometimes identifiable during the test, previously to the

strengthening system rupture. These can be justified by the rupture, cracking and partial collapse of the

protection system. Some facilitated points of heat entering are created exposing the thermocouples

placed in the laminate-concrete interface to more heat. For example, the EBR-50-25-50-Tg-S&P-L and

EBR-75-50-75-L tests show that when the strengthening system ruptures (65 min and 70 min,

respectively), the thermocouples placed in the midspan section and closer to the fire suddenly increase

their temperatures.

When comparing both epoxy resins used, S&P 220 (Tg=54⁰C) and Araldite 2014 (Tg=85⁰C), even though it

is not possible to make a straight comparison because of the geometry of fire protection system used

and all the other properties that are different, it is possible to conclude that Araldite 2014 presents

higher temperatures at the moment of rupture of the strengthening system granting longer fire resisting

times.

Figure 8. Time-temperature curve of EBR beam.

Figure 9. Time-temperature curve of EBR-25-0-25

beam.


beam.


beam.

0

50

100

150

200

250

300

350

400

450

500

0 5 10 15 20

Tem

pe

ratu

re (

˚C)

Time(min)

T1

T2

T3

T4

T5

T6

T ar

T8

T9

T10

T11

T12

T13

T14

Laminate rupture

Tg

0

100

200

300

400

500

600

700

0 5 10 15 20 25 30 35

Tem

pe

ratu

re (

˚C)

Time(min)

T1

T2

T3

T4

T5

T6

T ar

T8

T9

T10

T11

T12

T13

T14

Laminate rupture

Tg

0

50

100

150

200

0 5 10 15 20 25 30 35

Tem

pe

ratu

re (

˚C)

Time(min)

T1

T2

T3

T4

T5

T6

T ar

T8

T9

T10

T11

T12

T13

T14

T15

Laminate rupture

Tg0

50

100

150

200

250

0 10 20 30 40 50

Tem

pe

ratu

re (

˚C)

Time(min)

T1

T2

T3

T4

T5

T6

T ar

T8

T9

T10

T11

T12

T13

T14

T15

Laminate rupture

Tg


12

3.1.2 – NSM beams

Figure 16 to figure 23 show the thermocouples temperatures readings of every NSM beam test. These

readings are measured as a function of time at different depths of midspan section and longitudinal

profile. The moment of strengthening system rupture is represent by a vertical red dotted line and the

glass transition temperature of each resin used in each test as a horizontal black dotted line. Some

thermocouples did not present any readings or presented wrong readings. Those were eliminated and

are not shown in the time-temperature diagrams.

All the inner beam thermocouples readings follow an expected distribution of temperatures. From

thermocouple T1 to thermocouple T6, T1 being the nearest thermocouple to fire action and T6 the

furthest, vertically, all temperature readings are decreasing.

In the unprotected beams (NSM-E and NSM-C), thermocouples T1, T8, T9, T11 and T14 present very

similar time-temperature curves. This phenomenon can be explained by the fact that all these

thermocouples are placed at the same depth and present similar protection conditions, excluding T1

which is placed near concrete surface and presents lightly superior temperatures. With the installation

of fire protective system all thermocouples temperature readings suffer some changes. These tend to

slow their growth rate and become more linear when protective system is applied.

Unlike the EBR beams tested, the NSM thermocouples readings do not suffer any noticeable change

when the strengthening system ruptures. Since the NSM is a technique where the thermocouples are

more protected than in the EBR technique, given the depth at they are placed and the confinement

effect of the concrete, they do not normally tend to become exposed when rupture occurs.

Figure 12. Time-temperature curve of EBR-75-25-

75-L beam.


75-L beam.


50-Tg-L beam.


50-Tg+S&P-L beam.

0

50

100

150

200

0 20 40 60 80

Tem

pe

ratu

re (

˚C)

Time(min)

T1

T2

T3

T4

T5

T6

T ar

T8

T9

T10

T11

T12

T14

T15

Laminate rupture

Tg0

25

50

75

100

125

150

0 20 40 60 80

Tem

pe

ratu

re (

˚C)

Time(min)

T1

T2

T4

T6

T ar

T8

T9

T10

T11

T12

T13

T14

T15

Laminate rupture

Tg

0

50

100

150

200

250

0 20 40 60 80

Tem

pe

ratu

re (

˚C)

Time(min)

T1

T3

T4

T6

T ar

T8

T9

T10

T11

T12

T13

T14

T15

Laminate rupture

Tg0

50

100

150

200

250

0 10 20 30 40 50

Tem

pe

ratu

re (

˚C)

Time(min)

T1T2T3T4T6T arT8T9T10T11T12T13T14T15Laminate ruptureTg- S&PTg- Araldite 2014


13

Given the results of the NSM test, the better performance of the epoxy resin in comparison with the

cement based one is confirmed. With the exception of the unprotected beams, in all NSM tests where

the protection system was the same, the epoxy resin granted higher fire resistance times.

Another visible phenomenon is the fact that when the strengthening system ruptures the average

temperature in the anchorage zones (average of T10-T12 and T13-T15) is much higher than the glass

transition temperature of the resin used. This can be justified by the higher friction forces given by the

concrete confinement and the double layer bond, in comparison to EBR, granting higher resistances in

the NSM technique.

Like in the EBR technique, although the protection of the anchorage zones is very important, the

protection in the current is more important. With the protection system placement it is visible that

when protection is added to the current zone, fire resistance time gains are higher than when the extra

protection is added just to the anchorage zones.

Figure 16. Time-temperature curve of NSM-E beam.

Figure 17. Time-temperature curve of NSM-C

beam.

Figure 18. Time-temperature curve of NSM-E-25-0-

25 beam.

Figure 19. Time-temperature curve of NSM-C-25-0-

25 beam.

0

50

100

150

200

250

300

350

400

450

500

0 5 10 15 20 25 30

Tem

pe

ratu

re (

˚C)

Time(min)

T2

T3

T4

T5

T ar

T8

T9

T10

T11

T12

T13

T14

T15

Laminate rupture

Tg

0

100

200

300

400

500

600

0 5 10 15 20 25 30

Tem

pe

ratu

re (

˚C)

Time(min)

T1T2T3T4T5T6T arT8T9T10T11T12T13T14T15Laminate ruptureTg

0

100

200

300

400

500

600

700

800

0 10 20 30 40 50

Tem

pe

ratu

re (

˚C)

Time(min)


0

100

200

300

400

500

600

700

0 10 20 30 40 50

Tem

pe

ratu

re (

˚C)

Time(min)

T2

T3

T4

T5

T6

T ar

T8

T9

T10

T11

T12

T13

T14

T15

Laminate rupture

Tg


14

3.2 – Mechanical behavior

Figures 24 and 25 present the results of the midspan deflection of every test. They are represented in

the form of a time-deflection diagram and aggregated by strengthening technique. It was chosen to

present the variation of deflection instead of the real deflection given the dispersion of values obtained

for initial deflection. This disparity can be justified by dynamic effects in the electrical displacement

transducer when loading the beams.

There is a common fact of both EBR and NSM technique strengthening system regarding their

mechanical behavior. Both EBR and NSM beams present a gradual and steady growth rate of their

midspan deflection until the strengthening system rupture. At this moment there is an instantaneous

increase of deflection and the growth rate beyond this point increases too. This demonstrates that when

the beams are being heated some of their properties are being affected. While the heating process

proceeds, the beams gradually loose rigidity presenting higher deflections to the same load applied. This

loss of rigidity is justified by the fluidization of the resin when approximating it glass transition

temperature. While more and more resin starts to fluidize, the whole system starts to loose rigidity and

tensions in the cooler parts of the resin build up until its maximum is achieved and the strengthening

system ruptures.

Figure 20. Time-temperature curve of NSM-E-25-

25-25 beam.

Figure 21. Time-temperature curve NSM-C-25-25-

25 beam.

Figure 22. Time-temperature curve of NSM-E-50-

25-50 beam.

Figure 23. Time-temperature curve of NSM-C-50-

25-50 beam.

0

100

200

300

400

500

600

0 20 40 60 80 100

Tem

pe

ratu

re (

˚C)

Time(min)


0

50

100

150

200

250

300

350

400

450

0 20 40 60 80 100

Tem

pe

ratu

re (

˚C)

Time(min)

T1T2T3T4T5T6T arT8T9T10T11T12T13T14T15T16Laminate ruptureTg

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140

Tem

pe

ratu

re (

˚C)

Time(min)

T1T2T3T4T5T6T arT8T9T10T11T12T13T14T16T16'Laminate ruptureTg

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120

Tem

pe

ratu

re (

˚C)

Time(min)



15

Figure 24. Midspan deflection variation of EBR

beams.

Figure 25. Midspan deflection variation of NSM

beams.

In the NSM beams it is possible to identify two ‘ruptures’. In fact, there are two moments where the

deflection increases instantaneously. Since the NSM is a technique where two laminates are introduced

in grooves, each ‘rupture’ can be attributed to each laminate. Associated to these two moments, there

is a correspondent instantaneous increase of midspan deflection and its growth rate, being the second

the representation of instantaneous loss of rigidity.

4 – Conclusions

The main goal of this paper was to study the fire behavior of reinforced concrete beams flexurally

strengthened with CFRP laminates and the influence various types of resin and geometries of fire

protection system used in its performance. With the two sets of EBR and NSM beams tested it was

possible to confirm some theories and gather new information and understanding about the fire

behavior of CFRP strengthened beams.

The use of epoxy resin (S&P 220) vs. cement base resin (Adicrete ER) in the NSM strengthening

technique grants higher fire resistance times when the epoxy one is applied. This can be justified by the

higher glass transition temperature (54 ⁰C vs. 44⁰C) and also by the higher mechanical properties of the

former adhesive.

When comparing both techniques (EBR vs. NSM) in equal conditions (both having the same amount of

laminate, same resin, same geometry of fire protection system and same fire exposure), the better NSM

performance is evident. It grants substantially higher fire resistance times and it can achieve higher resin

temperatures at the moment of rupture of the strengthening system. One main difference, between

strengthening techniques, and major vantage is the fact that in the NSM technique both laminates are

introduced in two grooves made in the concrete. This provides better adhesion between the laminate

and concrete and mobilizes extra friction, thus enhancing its resistance.

In the EBR technique, although some more tests are needed, when evaluating the epoxy resin with

higher glass transition temperature (Araldite 2014 with Tg =85⁰C vs 54⁰C), the test results were very

promising. Even though the Araldite 2014 epoxy resin was not designed to bond CFRP to concrete, it

0

2

4

6

8

10

12

0 20 40 60 80

Mid

span

def

lect

ion

var

iati

on

(m

m)

Time (min)

EBR EBR -25-0-25

EBR -25-25-25 EBR -50-25-50

EBR -50-25-50-Tg-L EBR -50-25-50-Tg+S&P-L

EBR-75-25-75-L EBR-75-50-75-L

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150

Mid

span

def

lect

ion

var

iati

on

(m

m)

Time (min)

NSM-E NSM-C

NSM-E-25-0-25 NSM-C-25-0-25

NSM-E-25-25-25 NSM-C-25-25-25

NSM-E-50-25-50 NSM-C-50-25-50


16

presented higher fire resistance time than the S&P 220 with higher temperatures achieved in the resin

at the moment of strengthening system rupture.

Given the identified ‘cable’ behavior in the laminate when the temperature of the resin approaches its

glass transition temperature, the protection of its extremities (anchorage zones) revealed to be very

important, particularly in the EBR technique. In the NSM technique, the protection of the current zone

proved to be more important than in its extremities, most likely because the NSM techniques obtains a

substantial part of its resistance from the friction between concrete and laminate.

5 – References

[1]- L. C. Bank, “Composites for Construction – Structural Design with FRP Materials”, Wiley, New Jersey,

2006.

[2] - S. Halliwell, T. Reynolds, “Effective Use of Fibre Reinforced Polymers Materials in Construction”, FBE

Report 8, Centre for Composites in Construction, BRE Press, UK, 2004.

[3] – J.P. Firmo, “Fire Behavior of Reinforced Concrete Beams Flexurally Strengthened with CFRP

Systems”, Master in Civil Engineering Dissertation, Instituto Superior Técnico, Lisboa, 2010.

[4] - C.E. Bakis, L.C. Bank, V.L. Brown, E. Cosenza, J.F. Davalos, J.J. Lesko, S.H. Rizkalla, T.C. Triantafillou,

“Fiber-reinforced polymer composites for construction – state-of-the-art review”, Journal of Composites

for Construction, Vol. 6, No. 2, pp. 73-87, 2002.

[5] - V.M. Kharbari, J.W. Chin, D. Hunston, B. Benmokrane, T. Juska, R. Morgan, J. J. Lesko, U. Sorathia, D.

Reynaud, “Durability gap analysis for fiber-reinforced polymer composites in civil infrastructure”,

Journal of Composites for Construction, Vol. 7, No. 3, pp. 238-247, 2003.

[6] A.P. Mouritz, A.G. Gibson, “Fire properties of polymer composite materials”, Dordrecht: Springer,

2006.

[7] N. Dodds, A.G. Gibson, D. Dewhurst, J.M. Davies, “Fire behaviour of composite laminates”,

Composites Part A: Applied Science and Manufacturing, Vol. 31, No. 7, pp. 689-702, 2000.

[8] A.J.B. Tadeu, F.J.F.G. Branco, “Shear tests of steel plates epoxy-bonded to concrete under

temperature”, Journal of Materials in Civil Engineering, Vol. 12, No. 1, pp. 74-80, 2000.

[9] A.P. Mouritz, Z. Mathys, A.G. Gibson, “Heat release of polymer composites in fire”, Composites Part

A: Applied Science and Manufacturing, Vol. 37, No. 7, pp. 1040-1054, 2006.

[10] M. Deuring, "Brandversuche an Nachtraglich Verstarkten Tragern aus Beton“ (in German), Research

report EMPA No. 148.795, Swiss Federal Laboratories for Materials Testing and Research (EMPA),

Dubendorf, Switzerland, 1994.

[11] H. Blontrock, L. Taerwe, P. Vandevelde. “Fire tests on concrete beams strengthened with fibre

composite laminates”, Proceedings of the third Ph.D. Symposium, Vienna, 2000.


17

[12] P.J. Burke, L.A. Bisby, M.F. Green. “Structural performance of near surface mounted FRP

strengthened concrete slabs at elevated temperature”, available at

http://aslanfrp.com/Aslan200/Resources/NSM%20systems%20with%20elevated%20temp-BISBY.pdf

[13] ISO 834, “Fire resistance tests. Elements of building construction”, International Standards

Organization, Genève, 1975.

[14] A. Palmieri, S. Matthys, L. Taerwe, ”Fire Endurance and Residual Strength of Insulated Concrete

Beams Strengthened with Near-Surface Mounted Reinforcement.”, Journal of Composites for

Construction, Vol. 17, No. 4, pp. 454-462, 2013.

[15] V. Kodur, B. Yu, ”Evaluating the Fire Response of Concrete Beams Strengthened with Near-Surface-

Mounted FRP Reinforcement.”, Journal of Composites for Construction, Vol. 17, No. 4, pp. 517–529,

2013.

[16] Araldite 2014 Structural Adhesive technical data sheet available at

http://www.freemansupply.com/datasheets/Araldite/2014.pdf

[17] L. Macedo, I. Costa, J. Barros, “Avaliação da influência das propriedades de adesivos e da geometria

de laminados de fibra de carbono no comportamento de ensaios de arranque“, ISISE - Comunicações a

Conferências Nacionais, Universidade do Minho, 2008.

[18] J.P. Firmo, "Ensaios mecânicos em adesivo cimentício", 2013 (unpublished data).

[19] Promatect-L500 technical data sheet available at http://www.promat-ap.com/pdf/pe.pdf

http://aslanfrp.com/Aslan200/Resources/NSM%20systems%20with%20elevated%20temp-BISBY.pdf

http://www.freemansupply.com/datasheets/Araldite/2014.pdf

http://www.promat-ap.com/pdf/pe.pdf

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