Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 172 (2017) 1176 – 1183
Modern Building Materials, Structures and Techniques, MBMST 2016
Fire Resistance of CFRP-strengthened Reinforced Concrete Beams under Various Load Levels Piotr Turkowskia *, Marek Łukomskia, Paweł Sulika, Paweł Roszkowskia a
Building Research Institute (ITB), Fire Research Department, 1, Filtrowa Str., 00-611 Warsaw, Poland
Abstract The paper presents results of ten fire resistance tests performed on reinforced concrete beams, with and without carbon fibrereinforced polymers (CFRP) strengthening. Part of the beams have been fire protected with fire protective boards of large thicknesses. The tests were performed under various load levels, ranging from 100% to 163% of their design loadbearing capacity in fire situation prior to strengthening. Test results show that structural fire design of CFRP-strengthened elements leads to three possible calculation models of fire protection thickness needed, depending on the load level reduction. ©©2017 Authors. Published by Elsevier Ltd. This 2016The The Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016. Peer-review under responsibility of the organizing committee of MBMST 2016 Keywords: fire resistance, reinforced concrete beams, CFRP, fire protection.
1. Introduction Externally bonded reinforcement, in form of various fibre reinforced polymers (FRP), is more and more commonly used in repair and strengthening of reinforced concrete (RC) structures. Often this is due to change in building exploitation, which often implies additional imposed loads or due to design or erection mistakes. Structural fire design of CFRP-strengthened reinforced concrete structures has been well studied in literature. Many aspects influencing fire resistance of such structures have been determined [1-3]. Besides the bond strength and its glass transition temperature (Tg), one of the most important factor, when dealing with CFRP-strengthened RC beams, is the utilization factor in fire conditions defined as the relation between the loading and design loadbearing capacity
* Corresponding author. Tel.: +485664169; fax: +48228472311. E-mail address:
[email protected]
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016
doi:10.1016/j.proeng.2017.02.137
Piotr Turkowski et al. / Procedia Engineering 172 (2017) 1176 – 1183
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of the element prior to the strengthening. As the load can be significantly reduced in accidental design situation, various element behavior and fire protection application scenarios must be considered. An ongoing research project is currently underway at Building Research Institute (ITB) to demonstrate fire resistance of the above-described elements in full scale fire resistance tests in standardized conditions. The project involves both experimental and numerical studies of the fire performance of externally CFRP-strengthen reinforced concrete beams in bending. This paper presents a recently conducted portion of the experimental part – results of ten full-scale tests. Results of previous tests on performance of fire protection systems designed for CFRP-strengthen reinforced concrete elements, performed within this project, were presented in [4]. 2. Experimental procedure 2.1. Test specimen The tests were performed in Fire Testing Laboratory of Building Research Institute (ITB), between January 2015 and October 2015, on ten reinforced concrete beams, with and without CFRP-strengthening or fire protection material, under various load levels, in accordance with EN 1363-1 [5] and EN 1365-3 [6] European testing standards. Each beam was of the same size: 4700 mm length and 150 mm x 450 mm cross-section. The longitudinal steel reinforcement in the beams consisted of four 12 mm diameter bars (one bar in each corner) made of RB500W steel. The lateral reinforcement consisted of 8 mm diameter deformed steel frames, spaced at 20 cm, made of RB500W steel. For more details, see Fig. 1.
A
A
Fig. 1. Test specimen design, prior to CFRP installment.
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The concrete mixture was made with siliceous aggregate and the concrete class was C 16/20 for beams B-1 to B-6, and C 25/30 for beams B-7 to B-10. Beams B-1 to B-6 conditioned for 120 days in ambient air temperature ranging from 10°C to 30°C and relative air humidity from 20% to 80% with 5% moisture content at the day of test, and beams B-7 to B-10 were conditioned for 200 days in same conditions, with 4% moisture content at the day of test. The topping of each beam was made with 120 mm high and 600 mm wide autoclaved aerated concrete (AAC) blocks, in order to achieve three-sided heating, as it is required in [6]. The CFRP strengthening was made with 4000 mm long strips of 50 mm x 1,2 mm cross-section, externally bonded on the bottom beam surface, in the beams axis, prior to loading, with structural two part adhesive, based on a combination of epoxy resins and special filler, of declared glass transition temperature Tg = 62°C. The CFRP has an ultimate tensile strength of 3100 MPa and tensile elastic modulus of 170 GPa (based on the manufacturer’s declaration). Two fire protective board systems were used as thermal insulation on beams. The first system is based on gypsum of 115 mm equivalent thickness of concrete ε, as defined in EN 13381-3 [7], for 30 mm boards on beams at 120 minutes long standard fire exposure, and the other one on the mixture of calcium silicate and cement of ε = 93 mm in the same conditions. View of the tests specimens, prior to the test, are given in Fig. 2. a
b
c
d
Fig. 2. (a) fire protection material installation; (b) loading setup; (c) beam B-2 prior to the test; (d) beam B-1 prior to the test.
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2.2. Instrumentation and furnace equipment Temperature in each test specimen was measured with 28 thermocouples (type K) – five thermocouples in five bases and additional three thermocouples in the middle of the beam’s span, on the contact surface between concrete and CFRP’s adhesive. Details are given in Fig. 3. Furnace temperature was measured with 8 plate thermometers, in accordance with EN 1363-1 [5]. Furnace pressure was set in such a way, as to achieve 20 Pa on the bottom of the test specimen, as required in EN 1363-1 [5]. The loading was applied with two hydraulic jacks, spaced 130±10 cm , as to achieve uniform bending momentum over at least 25% of the beam’s support length (Lsup). Deflection and rate of deflection was measured in the middle of the span with two wire sensors.
Fig. 3. Thermocouples locations in test specimen.
2.3. Mechanical tests Prior to fire tests, four mechanical tests were performed in order to obtain the value of real loadbearing capacity of beams before and after the strengthening with CFRP. Mechanical parameters calculated in accordance with EN 1992-1-2 [8] or software delivered by FRP’s manufacturer are given in Table 1. Table 1. Mechanical resistance of beams in normal temperature. Calculation paramters
Designation
Value (kNm)
Beam prior to strengthening, design loadbearing capacity in normal conditions
MRd,RC
36,8
Beam prior to strengthening, design loadbearing capacity in fire conditions at time t = 0
MRd,0,fi,RC
43,8
Beam prior to strengthening, real loadbearing capacity measured in strength test
MRd,RC
55,0
Beam after to strengthening, design loadbearing capacity in normal conditions
MRd,CFRP
71,1
Beam after to strengthening, real loadbearing capacity measured in strength test
MR,CFRP
81,0
2.4. Test criteria Fire protection failure time was defined as the time of detachment of the whole underside of the insulation, after which the beam’s bottom concrete surface was revealed and the rate of temperature rise exceeded 50 K/min. Beam failure criteria during fire tests were taken from the EN 1363-1 [5], as the time to reach the limiting deflection Dlim, expressed as (1) or limiting rate of deflection dD/dtlim, expressed as (2), where L is the clear span of the test specimen, in millimeters and d is the distance from the extreme fibre of the cold design compression zone to the extreme fibre of the cold design tension zone of the structural section, in millimeters.
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Dlim
L2 mm 400d
dD dt lim
(1)
L2 mm/min 9000d
(2)
Due to high risk of beam collapse into the furnace chamber, some tests were stopped when only one the abovementioned criterion was exceeded, usually dD/dtlim. This approach is allowed by the classification standard EN 135012 [9], which would normally be used to assess the fire resistance of beams. In practice, with respect to the tested beams, the differences in time in exceeding both criteria are negligible, as exemplified by the beam B-5. After the CFRP detachment, rate of deflection has rapidly accelerated and the difference in time to achive both was less than 15 seconds. 3. Test results Results of all ten tests are given in Table 2, in which the beam characteristics are given, such as CFRP application, fire protection system used. load applied, time of fire protection failure, time of CFRP failure, time of beam failure, as well as the deflection and temperature in the steel reinforcement and CFRP adhesive at the beam failure time. Table 2. Fire resistance test results. Parameter for beam:
B-1
B-2
B-3
B-4
B-5
B-6
B-7
B-8
B-9
B-10
CFRP strengthening (as described above)
yes
–
yes
yes
yes
yes
yes
yes
yes
–
Fire protection system material (“g” – gypsum, “cs” – calcium silicate”)
–
“g”
“g”
–
“g”
“g”
“cs”
“cs”
“cs”
–
Fire protection thickness (mm)
–
50
50
–
50
150
150
50
50
–
Bending moment in the beam (kNm)
44
44
44
71
71
71
71
71
56
46
CFRP failure time (min)
2,25
–
75,5
1,07
57,0
289,5
201,25
74,75
55,25
–
Fire protection failure time (min)
–
168,0
160,0
–
n.r.
n.r.
n.r.
n.r.
n.r.
–
Time to reach dD/dtlim = 5 mm/min (min)
53,25
212,0
211,5
1,07
57,0
289,5
201,25
74,75
242,0
33,75
Time to reach Dlim = 112,5 mm (min)
57,5
n.r.
215,0
n.r.
57,25
n.r.
201,75
n.r.
n.r.
42,50
Beam failure time (min)
53,25
212,0
211,5
1,07
57,0
289,5
201,25
74,75
242,0
33,75
Average temperature of the steel reinforcement at beam failure time (°C)
509,0
514,0
481,0
17,0
66,0
63,0
56,2
70,0
157,9
437,3
Average temperature of the CFRP at CFRP failure time (°C)
196,0
–
83,1
70,4
77,1
68,9
75,6
85,6
81,2
–
Average temperature of CFRP at beam failure time (°C)
909,0
–
1128,0
70,4
77,1
68,9
75,6
85,6
254,4
–
Initial beam deflection after full load application, prior to fire test (mm)
10,5
8,3
8,0
12,0
14,2
9,9
14,4
14,4
14,4
12,6
Deflection of the beam at the fire protection failure time (mm)
–
12,2
6,1
–
n.r.
n.r.
–
–
–
–
Deflection of the beam at the CFRP failure time (mm)
0,2
–
0,5
0,3
4,0
12,5
7,1
3,4
26,8
–
Note: “n.r.” stands for "not reached”.
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The value of the deflection D is zeroed at the commencement of the fire test as the initial deflection from the load applied is not taken into account in the assessment, as required in EN 1363-1 [5]. View of some of the test specimen after the test is given in Fig. 4. Temperature results are given in Fig. 5 and Fig. 6. a
b
c
d
8 Fig. 4. (a) beam B-4 after the test – almost immediate CFRP detachment; (b) beam B-3 after the test – visible fire protection failure; (c) beam B-6 after the test – highest performance in fire no insulation failure; (d) beam B-7 after the test – violent beam collapse into furnace.
1200
B-1
B-2
Temperature (°C)
1000
B-3 800
B-4 B-5
600
B-6 400
B-7 B-8
200
B-9 0 0
30
60
90
120
150
180
210
240
270
300
B-10
Time (min)
Fig. 5. Average temperature of CFRP in tested beams (for beams B-2 and B-10, the graph shows the temperature of the concrete surface).
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160 B-4
Temperature (°C)
140 120
B-5
100
B-6
80 B-7
60
40
B-8
20
62°C
0 0
30
60
90
120
150
180
210
240
270
300
Time (min)
Fig. 6. Average temperature of CFRP in tested beams, under load exceeding beam’s loadbearing capacity prior to strengthening, in comparison with declared glass transition temperature Tg = 62°C.
4. The algorithm Three scenarios of beam behavior can be observed, depending on the load level, which are presented in Fig. 7. The algorithm is based on EN 1990 [10], EN 1992-1-2 [11] and tests results described in section 3. It has been also compared with tests results presented in [12-14], which confirmed its principles. Actions in accidental design scenario (fire) Ed,fi
Design loadbearing capacity of RC beam in fire at time t = 0, prior to CFRP-strengthening Rd,0,fi,RC
Ed,fi ≤ 0,60 Rd,0,fi,RC
YES
Design loadbearing capacity of RC beam in normal temperature, prior to CFRP-strengthening Rd,RC
Case „1" θcr ≥ 500°C
YES
NO 0,60 Rd,0,fi,RC < Ed,fi Ed,fi ≤ Rd,0,fi,RC
NO
YES
Case „2" 350°C ≤ θcr θcr < 500°C
YES
0,70 Rd,RC < Ed,fi Ed,fi ≤ 1,15 Rd,RC NO
NO
Rd,0,fi,RC < Ed,fi
Ed,fi ≤ 0,70 Rd,RC
YES
Case „3" θcr = Tg (50°C to 115°C)
YES
1,15 Rd,RC < Ed,fi
Fig. 7. The algorithm for determining boundary, expressed as θcr conditions for fire resistance insulation selection.
Piotr Turkowski et al. / Procedia Engineering 172 (2017) 1176 – 1183
5. Conclusions From the results of this full-scale fire resistance test on ten reinforced concrete beams, with and without CFRPstrengthening or fire protection, the following conclusions can be drawn: 1. Three scenarios of beam behavior can be observed: scenario 1 – load level in fire lower than resistance of the beam prior to CFRP strengthening, thus no fire protection needed, scenario 2 – load level in fire lower than resistance of the beam prior to CFRP strengthening, but higher than resistance in fire, thus regular fire protection needed, scenario 3 – load level in fire higher than resistance of the beam prior to CFRP strengthening, thus extra fire protection is needed 2. CFRP failure temperature can be taken as the glass transition temperature Tg. In all tested beams, temperature Tg at the CFRP failure time was higher than the Tg declared by the manufacturer. The glass transition temperature value depends on the curing process during application and it can raised. 3. The heating rate of the CFRP’s adhesive used in tests have negligible effect on the value of glass transition temperature Tg, yet is it worth mentioning that the longer the fire exposure the lower the value of Tg. 4. CFRP strengthening does not influence the beam performance in fire if the load applied is smaller than beam’s loadbearing capacity prior to strengthening. 5. Thickness of fire protection material thickness needed to keep the adhesive temperature below glass transition temperature is several times bigger than used for fire protection of regular RC elements. The temperature of the CFRP system remained below its glass transition temperature for about one hour with 50 mm of insulation, which would normally protect RC beam of the same size for 6 hours. With 150 mm of insulation it was possible to achieve about 5 hours with gypsum based fire protection system and about 3 hours with calcium silicate one. It is also necessary to take into account the heat transfer in unprotected concrete areas – additional side lap is needed.
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