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 – 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
1
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
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.
2
2,5
2,5
La=20 cm
2,5
La=20 cm
U25 cm
25-0-25
Fire behavior of reinforced concrete beams flexurally strengthened2,5with CFRP
laminates
2,5
La=20 cm
25 cm
La=20 cm
2,5
La=20 cm
25-0-25
2,5
La=20 cm
2.5 cm
25 cm
2.5 cm
La=20
0cm
2.5 cm
25 cm
25-25-25
2,5
2,5
2,5
La=20
cm cm
La=20
25-25-25
La=20 cm
2,5
La=20 cm
5
75-25-75
75-25-75
5
7,5
7.5 cm2,5
5
2,5
Figure 1. Geometry of fire protection systems.
5
La=20 cm
30 cm
5
La=20 cm
30 cm
7.5 cm
32,5 cm
La=20 cm
7,5
2,5
5
2,5
32,5 cm
32,5 cm
5 cm
2,5
La=20 cm
5
5 cm
5
La=20 cm 7.5 cm 7,5
2.5 cm
2,5
La=20 cm
30 cm
5
5
75-50-75
7.5 cm
5
La=20 cm
La=20 cm
75-50-75
5
La=20 cm
5 cm
5 cm
2,5
La=20 cm
La=20 cm
30 cm
30 cm
25 cm
5
5 cm
5
25 cm
7,5
2,5
2,5
50-25-50
La=20 cm
2,5
2,5
La=20 cm
La=20 cm
2.5 cm
5 2,5
2.5 cm
5
50-25-50
2,5
La=20 cm
5 cm2,5
2,5
32,5 cm
5
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
30 cm
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
7,5 calcium silicate plates were added to the beams lateral panels.
5 ToLa=20
cm extra
insulation.
fix it,
2,5
7,5
2,5
32,5 cm
Table 1. Characteristics of all the beams tested and its nomenclature.
Nomenclature
Strengthening
technique
Resin
-
-
Thickness of fire protection
system
Anchorage
zone
Current zone
-
-
EBR
-
-
EBR-25-0-25
25 mm
-
25 mm
25 mm
50 mm
25 mm
75 mm
25 mm
75 mm
50 mm
RC
EBR-25-25-25
Epoxy S&P 220
EBR-50-25-50
EBR-75-25-75-L
EBR
EBR-75-50-75-L
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
-
-
25 mm
-
25 mm
25 mm
NSM-E-50-25-50
50 mm
25 mm
NSM-C
-
-
25 mm
-
25 mm
25 mm
50 mm
25 mm
NSM-E
NSM-E-25-0-25
NSM-E-25-25-25
NSM-C-25-0-25
NSM-C-25-25-25
NSM-C-50-25-50
NSM
NSM
Epoxy S&P 220
Cement base Adicrete ER
3
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
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
3
4
Bulk density [Kg/m ]
500
Thermal conductivity [W/mK]
0.09
Thermal expansion [m/mK]
7.0 x 10
-6
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
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.
0.10 m
b)
a)
2Ø6
2Ø6 mm
2Ø6 mm
0.2
CFRP
laminate
Laminado de CFRP
0.2
0.12 m
Ø6 mm // 0.06 m
Ø6//0.06
2Ø6
1.10 m
0.12m 0.08 m
0.08m 0.12m
CFRP
Laminate
Laminado
20x1.4
)
(20mmmm
x 1,4mm)
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-50-25-50Tg+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.
5
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
a)
b)
c)
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
6
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
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 timetemperature 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.
7
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
Campânula
Campanula
(exaustão)
(exaustion)
Pórtico de
reacção
Reaction frame
transmission
Viga deLoad
transmissão
beam
de carga
0.42 m
0.42 m
0.42 m
Viga
Tested
beam
ensaiada
Apoio
Support
Laminado
Laminate
Oven
Forno
T(t)
ISO 834
Pesos
Weights
Pesos
Weights
0.95 m
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.
8
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
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.
T7 (Air temperature)
La
10 cm
10 cm
T10 T11
h/3
T9
La
10 cm
T12
10 cm
T13 T14
T6
T5
h/3
T1
T2
T4
T3
T15
T8
Figure 6. Location of thermocouples outside all EBR beams.
T7 (Air temperature)
La
10 cm
La
10 cm
10 cm
h/3
T10 T11 T12
T6
T13
10 cm
T14 T15
T5
T8 T1 T9
T16’
h/3
T16
T2
T4
T3
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
9
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
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 all
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
10
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
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.
500
700
Temperature (˚C)
400
350
300
250
200
150
100
50
T1
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T13
T14
Laminate rupture
Tg
600
Temperature (˚C)
T1
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T13
T14
Laminate rupture
Tg
450
500
400
300
200
100
0
0
0
5
10
15
20
0
5
10
15
Time(min)
Figure 8. Time-temperature curve of EBR beam.
25
30
35
Figure 9. Time-temperature curve of EBR-25-0-25
beam.
250
T1
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T15
Laminate rupture
Tg
150
100
50
0
0
5
10
15
20
25
30
35
Time(min)
Figure 10. Time-temperature curve of EBR-25-25-25
beam.
T1
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T15
Laminate rupture
Tg
200
Temperature (˚C)
200
Temperature (˚C)
20
Time(min)
150
100
50
0
0
10
20
30
40
50
Time(min)
Figure 11. Time-temperature curve of EBR-50-25-50
beam.
11
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
200
150
Temperature (˚C)
150
100
50
0
0
20
40
60
T1
T2
T4
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T15
Laminate rupture
Tg
125
Temperature (˚C)
T1
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T14
T15
Laminate rupture
Tg
100
75
50
25
0
80
0
20
40
Time(min)
Figure 12. Time-temperature curve of EBR-75-2575-L beam.
80
Figure 13. Time-temperature curve of EBR-75-5075-L beam.
250
250
150
100
50
0
T1
T2
T3
T4
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T15
Laminate rupture
Tg- S&P
Tg- Araldite 2014
200
Temperature (˚C)
T1
T3
T4
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T15
Laminate rupture
Tg
200
Temperature (˚C)
60
Time(min)
150
100
50
0
0
20
40
60
80
Time(min)
Figure 14. Time-temperature curve of EBR-50-2550-Tg-L beam.
0
10
20
30
40
50
Time(min)
Figure 15. Time-temperature curve of EBR-50-2550-Tg+S&P-L beam.
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.
12
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
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.
500
600
Temperature (˚C)
400
350
300
250
200
150
100
50
T1
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T15
Laminate rupture
Tg
500
Temperature (˚C)
T2
T3
T4
T5
T ar
T8
T9
T10
T11
T12
T13
T14
T15
Laminate rupture
Tg
450
400
300
200
100
0
0
0
5
10
15
20
25
30
0
5
10
Time(min)
15
20
25
30
Time(min)
Figure 16. Time-temperature curve of NSM-E beam.
Figure 17. Time-temperature curve of NSM-C
beam.
700
Temperature (˚C)
700
600
500
400
300
200
100
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T15
Laminate rupture
Tg
600
Temperature (˚C)
T1
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T15
Laminate rupture
Tg
800
500
400
300
200
100
0
0
0
10
20
30
40
50
Time(min)
Figure 18. Time-temperature curve of NSM-E-25-025 beam.
0
10
20
30
40
50
Time(min)
Figure 19. Time-temperature curve of NSM-C-25-025 beam.
13
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
450
600
Temperature (˚C)
500
400
300
200
100
T1
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T15
T16
Laminate rupture
Tg
400
350
Temperature (˚C)
T1
T2
T3
T4
T5
T6
T ar
T8
T10
T11
T12
T13
T14
T15
T16
Laminate rupture
Tg
300
250
200
150
100
50
0
0
0
20
40
60
80
0
100
20
40
T1
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T16
T16'
Laminate rupture
Tg
350
Temperature (˚C)
300
250
200
150
100
50
0
40
60
80
100
100
120
140
Time(min)
Figure 22. Time-temperature curve of NSM-E-5025-50 beam.
700
T1
T2
T3
T4
T5
T6
T ar
T8
T9
T10
T11
T12
T13
T14
T15
Laminate rupture
Tg
600
Temperature (˚C)
400
20
80
Figure 21. Time-temperature curve NSM-C-25-2525 beam.
Figure 20. Time-temperature curve of NSM-E-2525-25 beam.
0
60
Time(min)
Time(min)
500
400
300
200
100
0
0
20
40
60
80
100
120
Time(min)
Figure 23. Time-temperature curve of NSM-C-5025-50 beam.
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.
14
12
Midspan deflection variation (mm)
Midspan deflection variation (mm)
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
10
8
6
4
2
0
0
20
40
Time (min)
EBR
EBR -25-25-25
EBR -50-25-50-Tg-L
EBR-75-25-75-L
60
80
20
18
16
14
12
10
8
6
4
2
0
0
50
100
150
Time (min)
EBR -25-0-25
EBR -50-25-50
EBR -50-25-50-Tg+S&P-L
EBR-75-50-75-L
Figure 24. Midspan deflection variation of EBR
beams.
NSM-E
NSM-E-25-0-25
NSM-E-25-25-25
NSM-E-50-25-50
NSM-C
NSM-C-25-0-25
NSM-C-25-25-25
NSM-C-50-25-50
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
15
Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
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
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Systems”, Master in Civil Engineering Dissertation, Instituto Superior Técnico, Lisboa, 2010.
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composite laminates”, Proceedings of the third Ph.D. Symposium, Vienna, 2000.
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Fire behavior of reinforced concrete beams flexurally strengthened with CFRP laminates
[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
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[14] A. Palmieri, S. Matthys, L. Taerwe, ”Fire Endurance and Residual Strength of Insulated Concrete
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[16]
Araldite
2014
Structural
Adhesive
technical
http://www.freemansupply.com/datasheets/Araldite/2014.pdf
data
sheet
available
at
[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
17