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10th International Conference on Short and Medium Span Bridges
Quebec City, Quebec, Canada, July 31 – August 3, 2018
EXPERIMENTAL STUDY ON REINFORCED CONCRETE BEAMS AFTER FIRE REPAIRED BY BOLTED SIDE-PLATING
Li, Ling-zhi 1,2, Bai, Yang 1, Su, Lei 1, Yu, Jiang-tao 1, Lu, Zhoudao1
1 College of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China2 [email protected]
Abstract: The structural performance of reinforced concrete (RC) beams degrades seriously after exposed to fire, thus the retrofitting techniques for fire-damaged RC beams are of great importance. The bolted-side plating (BSP) is an innovative technique where the steel plates are attached to the beams’ side faces using anchor bolts. It can enhance RC beams in both tensile and compressive reinforcement thus immune from reduction of ductility and the peeling failure on the plate-RC interface. Considering this background, the bending and shear tests of post-fire beams strengthened by BSP were conducted. The influence of the bolt spacing, the arrangement of steel-angle stiffeners, and the plate depth and thickness on the strength, stiffness and ductility was investigated. The results indicated that the bearing capacity and the stiffness of all the fire-damaged specimens were significantly improved by the BSP technique, but the variation of ductility was dependent on the arrangement of strengthening. The local buckling can be restrained effectively by adding stiffeners along the compressive edge. The digital image correlation method can be used to investigate the development of displacement and strain fields.
1 INTRODUCTION
Reinforced concrete (RC) beams after fire might fail to achieve the prescribed performance due to the degradation of material properties caused by elevated temperature (El-Hawary
et al. 1996; Kumar et al. 2003; Lee et al. 2008), thus the recovery and retrofitting of the fire-damaged RC beams is of great importance for the subsequent serviceability and safety of
RC buildings.
Numerous strengthening techniques were proposed by researchers to retrofit RC beams (Triantafillou 1998; Li et al. 2013; Yang et al. 2018; Yu et al. 2018). Among which the bolted-
side plating (BSP), i.e. anchoring steel plates to the side faces of RC beams with anchor bolts (Li et al. 2017; Su et al. 2014), was an innovative method that can avoid the brittle
debonding and over-reinforced failure, if compared with the method of gluing carbon fiber reinforced polymers (CFRPs) to the tensile soffit (Deniaud et al. 2001; Park et al. 2003; Alam
et al. 2009; Zhu et al. 2017). It also has the advantages such as easy for construction and less space occupancy if compared to the conventional method of increasing cross-section
by newly cast concrete and cementitious composite (Wei et al. 2018; Yu et al. 2018a, 2018b).
Since the BSP technique has been accepted worldwide, studies on BSP beams have drawn comprehensive attention. Barnes et al. (2001) compared the retrofitting effect of two
methods of plate attachment, namely adhesive bonding and bolting. Oehlers, Ahmed, Nguyen and Smith (Ahmed et al. 2000; Nguyen et al. 2001; Oehlers et al. 1997a, 1997b; Smith
et al. 1999) conducted comprehensive experimental and theoretical studies to investigate the strengthening effect, partial-interaction and local buckling of BSP beams. Su, Zhu, Siu,
Lo and Li (Su et al. 2008, 2013; Zhu et al. 2010; Siu et al. 2011; Lo et al. 2014; Li et al. 2016) devoted themselves to optimizing the strengthening strategy of BSP beams, restraining
the local buckling of steel plates, and simplifying the design procedure. Kolsek et al. (2013) proposed a theoretical model to analyze the stress-strain state of linear-elastic BSP beams
cracked in flexure. Li, Jiang and Xu (Li et al. 2016; Jiang et al. 2017; Xu et al. 2017) conducted experimental and numerical studies on BSP beams and proposed a simplified design
method that can take the local buckling into account.
Although systematic researches on the BSP technique have been carried out, most of them were related to its implementation under room temperature, those focusing on fire-damage
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Anchor bolt
250
275
100
Steel plate 2600×100×4
250
75
300×7=2100Steel plate 2600×100×4
75
250
300
200
100×25=2500
300×7=2100
75
250
Steel plate 2600×100×4
250
300
300×7=2100
7575
250
200
Steel plate 2600×100×4
200
250
75
300×7=2100 250
75
300
Steel plate 2600×100×6
300×7=2100
200
75
250
300
75
250
P
T1
T2
150 1502600
900500900
350
L3L2 L1
P
P
T1
T2
150 1502600
900500900
350
L3L2 L1
P
2600
P P 350
(a) Reinforcement(b) FP40B03
(c) FP43B13 (d) FP43B33
(e) FP43B33A (f) FP63B33
RC beams have yet been sufficient. Therefore, this paper will report an experimental study, in which RC beams were exposed to fire and retrofitted with the BSP technique, and then
their flexural and shear performance was investigated.
2 EXPERIMENTAL PROGRAM
2.1 Specimen details
A total of eleven RC beams were fabricated as listed in Test set-up and instrumentation. The former six beams were used to investigate the flexural strengthening effect of the BSP
technique for fire-damaged RC beams as shown in Figure 1, the cross section is 200 mm × 350 mm, the stirrup is D10–100, the compressive and tensile reinforcements are 2D12 and
2D20, respectively. The latter six were employed to investigate the shear strengthening effect as shown in Figure 2, the cross section is 200 mm × 400 mm, the stirrup is D6–200, the
compressive and tensile reinforcements are 2D22 and 4D22, respectively.. All the RC beams was first exposed to fire, and then two of the fire-damaged RC beams, namely FCTRL
and SCTRL, remained unstrengthened. The rest were retrofitted with two steel plates anchored to their side faces, whose main control parameters were the plate depth, plate
thickness, bolt spacing and stiffeners at the compressive edge to limit buckling, and there strengthening details can be found in Figure 1 and Figure 2.
2.2 Test set-up and instrumentation
The RC beams were first exposed to fire in accordance with the international standard ISO834 temperature curve for 2 hours, and a constant load was imposed on the specimens to
simulate the service load, as shown in Figure 3. Thermocouples were installed in the beams to measure the temperature under fire.
For the flexural test, the clear span between the two roller supports was 2300 mm, and the shear span between the support and the nearest loading point was 900 mm, as illustrated
in Figure 1(a). While for the shear test, the clear span was 2400 mm, and the shear span was 495 mm, thus the shear span-to-depth ratio was 1.5, as illustrated in Figure 2(a). The
digital image correlation (DIC) method was employed to record the deformation development of both the RC beam and the steel plate. Two layers of white and black random paint
spots were sprayed onto the specimen surfaces, whose shapes before and after deformation were captured by digital single lens reflex cameras.
Table 1: Strengthening Parameters of specimens
Specimen Plate thickness (mm)
Plate depth (mm)
Rows of bolts
Upper bolt spacing (mm)
Lower bolt spacing (mm)
Steel angle (mm)
1 FCTRL — — –– — — —2 FP41B03 4 100 1 — 300 —3 FP43B13 4 300 2 100 300 —4 FP43B33 4 300 2 300 300 —5 FP43B33A 4 300 2 300 300 L63×40×56 FP63B33 6 300 2 300 300 —7 SCTRL –– –– –– –– –– ––8 SP42B22 4 200 2 200 200 ––9 SP42B11 4 200 3 100 100 ––
10 SP43B22 4 300 2 200 200 ––11 SP43B11 4 300 3 100 100 ––
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P
T1
T2
150 1502600
900500900
350
L3L2 L1
P
P
T1
T2
150 1502600
900500900
350
L3L2 L1
P
2600
P P
350
(a) Reinforcement(b) SP42B11
(e) SP43B11
(c) SP43B22 (d) SP42B22
4900
2600 2600Furnace
Reaction frame
Hydraulic jack
Specimen
1700
Figure 1: Details of specimens for flexural testing
Figure 2: Details of specimens for shear testing
Figure 3: General view of specimens under fire exposure
3 RESULT DISCUSSION
3.1 Thermal response
The temperature development of the RC beams under fire was shown in Figure 4(a), where d represents the distance of the thermocouple from the beam’s bottom surface. The
temperature of the furnace and the international standard ISO834 curve were also plotted for comparison. It is evident that the furnace temperature was slightly lower than the ISO834
curve, and the temperature of concrete surface was even lower. The temperature of concrete decreased with its embedment depth. In addition, the temperature of concrete inside the
beam maintained at 100 °C for about 50 min, due to the evaporation of moisture in concrete. It is also observed that the maximum temperature of inside concrete appeared 50
minutes after flameout, hence the fire failure might appear during the cooling phase after the flameout.
The development of the mid-span deflection of the RC beams was shown in Figure 4(b), and the variation of external load was plotted for comparison. It is shown that when the
external load was imposed on the RC beams, a deflection of about 4 mm occurred instantly. After the fire ignition, the beams’ deflection increased steadily with the temperature. When
the external load was unloaded at the flameout, the deflection decreased 4 mm instantly, which means the initial deflection caused by external load was recovered. Thereafter, the
deflection caused by fire was gradually recovered, and the residue deformation after 24 hours was estimated to be less than 5 mm, thus its influence on the subsequent strengthening
and loading test could be ignored.
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(a) Temperature–time curves of RC beams (b) Time‒deflection curves of RC beams
Figure 4: Development of temperature and deflection
3.2 Structural response of flexural test
3.2.1 Failure mode and flexural capacity
Figure 5 shows that the failure modes of all the specimens under flexural testing, which were similar and bending failure occurred. The first vertical flexural crack appeared in the mid-
span firstly, then new flexural cracks appeared one after another with the increase of load. When the load reached a certain value, no new vertical cracks appeared, but the cracks
gradually expanded and widened. Eventually, concrete in the compression zone of the mid-span crushed, and the compression rebars yielded. Then, the beams continued to be
loaded using displacement-controlled loading. When the load dropped to 0.85 Pu, the test was terminated. Due to the restriction of steel plates to concrete, the BSP beams showed
better deformability than the unstrengthened beam FCTRL during the test.
(a) FCTRL
(b) FP41B03
(c) FP43B13
(d) FP43B33
(e) FP43B33A
(f) FP63B33
Figure 5: Failure modes of specimens
It is shown from Table 2 that the specimens can be sorted in order of peak load (Pu) as following: FCTRL < FP41B03 < FP43B33 < FP63B33 < FP43B13 < FP43B33A. After
strengthened, the loading capacity of the fire-damaged beams was significantly improved. Compared with other strengthened beams, the peak load of FP41B03 was the minimum,
indicating that anchoring steel plates to the lower part of beam’s side faces will lead to over-reinforcement failure. Compared with FP43B33, the peak loads of FP63B33, FP43B13 and
FP43B33A were all much greater, which shows that the reinforcement effect of BSP beams could be improved by increasing the thickness of steel plates, the number of anchor bolts,
or adding angle stiffeners. Compared with FP43B13 and FP43B33A, the increase rate of FP63B33 is the lowest, which means increasing plate thickness without corresponding
increase in anchor bolts will lead to a reduction of cooperation degree between steel plate and concrete thus an unsatisfactory strengthening effect.
Table 2: Flexural capacity, stiffness and ductility of specimens
SpecimenFlexural capacity Stiffness Ductility
Pu (kN) Change (%) Ke (kN/mm) Change (%) Ut (kN.mm) Change (%)
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1 FCTRL 236.0 — 19.7 — 33797.0 —2 FP41B03 327.6 39 % 24.7 25 % 26422.4 −22 %3 FP43B13 422.2 79 % 31.0 57 % 66779.5 98 %4 FP43B33 384.4 63 % 29.3 49 % 32388.0 −4 %5 FP43B33A 431.3 83 % 32.7 66 % 51414.2 52 %6 FP63B33 418.6 77 % 35.5 80 % 31202.4 −8 %
Figure 6: Comparison of load–vertical displacement curves
The load-deflection curves in the mid-span of the beams are plotted in Figure 6, where the curves are generally divided into three phases: the straight ascending line, the ascending
curve, and the descending curve. That is, all the specimens experienced the initial elastic stage, the elasto-plastic stage after cracking and the post failure stage beyond the peak
load. The trend of the load-deflection curve was in accord with the characteristics of flexible failure.
3.2.2 Stiffness and ductility
It can be seen from Table 2 that the specimens can be sorted by the stiffness (Ke) as following: FCTRL < FP41B03 < FP43B33 <FP43B13 < FP43B33A < FP63B33. This indicates
that the stiffness of the fire-damaged beams can be effectively increased by the BSP technique and increasing the plate thickness resulted in a significant improvement, while
anchoring narrow steel plates in the tension zones only provides a negligible improvement.
It can also be seen from Table 2 that the specimens can be sorted by the ductility (Ut) as following: FP41B03 < FP63B33 ≈ FP43B33 ≈ FCTRL < FP43B33A < FP43B13. The ductility
of FP41B03 even decreased compared with FCTRL, which indicates that anchoring steel plates in the tension zones will lead to over-reinforcement shortcoming and the ductility will
decrease. Compared with FP43B33, the Ut of FP63B33 slightly decreased, while the Ut of FP43B33A and FP43B13 significantly increased. This shows that increasing plate thickness
cannot increase ductility unless more bolts are added concurrently, while adding steel-angle stiffeners can effectively increase the ductility. Compared with FP43B33A, the Ut of
FP43B13 is larger, which means adding more anchor bolts not only limits the buckling of steel plates but also increases the overall degree of cooperation between steel plates and
concrete.
3.3 Structural response of shear test
3.3.1 Failure mode and shear capacity
The failure modes of the specimens for shear test are shown in Figure 7. For the unstrengthened specimen SCTRL, the flexural cracks occurred first at the mid span of the beam,
then the first diagonal crack occurred, and after that more diagonal cracks appeared and the failure of the beam was triggered by the formation of a main diagonal crack as shown in
Figure 7(a). Thus, the beam SCTRL reached its shear capacity.
(a) SCTRL(d) SP43B22
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(b) SP42B22
(c) SP42B11
(e) SP43B11
Figure 7: Failure modes of specimens
The shear capacities of all the specimens are listed in The shear capacities of all the specimens are listed in The shear capacities of all the specimens are listed in Table 3. After being
strengthened, the occurrence of cracks in SP42B22 and SP42B11 was delayed and the shear capacity was significantly increased. This is because with the development of diagonal
cracks and mid-span displacement, the bolted steel plates will work together with the original fire-damaged RC beam gradually to resist external load. The main diagonal crack in
SP42B11 was obvious, the steel plates deformed and contributed to the resistant capacity significantly. However, because the bolt spacing was too large in SP42B22, the end
anchorage of the bolted steel plates was weak, the failure was induced by the crush of the concrete near the left support, thus the strengthening effect was not desirable.. After being
strengthened, the occurrence of cracks in SP42B22 and SP42B11 was delayed and the shear capacity was significantly increased. This is because with the development of diagonal
cracks and mid-span displacement, the bolted steel plates will work together with the original fire-damaged RC beam gradually to resist external load. The main diagonal crack in
SP42B11 was obvious, the steel plates deformed and contributed to the resistant capacity significantly. However, because the bolt spacing was too large in SP42B22, the end
anchorage of the bolted steel plates was weak, the failure was induced by the crush of the concrete near the left support, thus the strengthening effect was not desirable.. After being
strengthened, the occurrence of cracks in SP42B22 and SP42B11 was delayed and the shear capacity was significantly increased. This is because with the development of diagonal
cracks and mid-span displacement, the bolted steel plates will work together with the original fire-damaged RC beam gradually to resist external load. The main diagonal crack in
SP42B11 was obvious, the steel plates deformed and contributed to the resistant capacity significantly. However, because the bolt spacing was too large in SP42B22, the end
anchorage of the bolted steel plates was weak, the failure was induced by the crush of the concrete near the left support, thus the strengthening effect was not desirable.
For the depth of the bolted steel plates was 300 mm in specimens SP43B22 and SP43B11, the side faces of the RC beams were covered thus the occurrence of the flexural and
shear cracking was not observed. The shear capacity of SP43B22 was only slightly greater than that of SP42B22, and even much less than SP42B11, which was caused by the poor
end anchorage of the bolted steel plates due to the large bolt spacing (i.e. 200 mm). On the other hand, the enhancement of SP43B11 was significant due to the great plate depth and
the small bolt spacing. The failure modes of specimens SP43B22 and SP43B11 were illustrated in Figure 7(d) & (e).
Table 3: Shear capacity, stiffness and ductility of specimens
Specimen Shear capacity Stiffness DuctilityPu (kN) Change (%) Ke (kN/mm) Change (%) Ut (kN.mm) Change (%)
1 SCTRL 580 -- 79.4 -- 2580 --2 SP42B22 800 38% 90.2 14% 5812 125%3 SP42B11 930 60% 88.6 12% 18927 634%4 SP43B22 840 45% 82.1 3% 15359 495%5 SP43B11 1030 78% 113.5 43% 20458 693%
3.3.2 Stiffness and ductility
The load‒deflection curves of all the specimens are shown in Figure 8 and the parameters Ke and Ut were compared in The shear capacities of all the specimens are listed in The
shear capacities of all the specimens are listed in Table 3. After being strengthened, the occurrence of cracks in SP42B22 and SP42B11 was delayed and the shear capacity was
significantly increased. This is because with the development of diagonal cracks and mid-span displacement, the bolted steel plates will work together with the original fire-damaged
RC beam gradually to resist external load. The main diagonal crack in SP42B11 was obvious, the steel plates deformed and contributed to the resistant capacity significantly.
However, because the bolt spacing was too large in SP42B22, the end anchorage of the bolted steel plates was weak, the failure was induced by the crush of the concrete near the
left support, thus the strengthening effect was not desirable.. After being strengthened, the occurrence of cracks in SP42B22 and SP42B11 was delayed and the shear capacity was
significantly increased. This is because with the development of diagonal cracks and mid-span displacement, the bolted steel plates will work together with the original fire-damaged
RC beam gradually to resist external load. The main diagonal crack in SP42B11 was obvious, the steel plates deformed and contributed to the resistant capacity significantly.
However, because the bolt spacing was too large in SP42B22, the end anchorage of the bolted steel plates was weak, the failure was induced by the crush of the concrete near the
left support, thus the strengthening effect was not desirable.. It can be found that the ascending branch of the load‒deflection curve of SCTRL were substantially straight, which
indicated a shear failure occurred for SCTRL.
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Figure 8: Comparison of load-vertical displacement curves
Figure 9: Load–displacement curves based on two measurement methods
After being strengthened, the stiffness and ductility were enhanced. Due to the poor end anchorage of the bolted steel plates in SP42B22, there was no evident post-peak descending
branch and the enhancement was the least among all the BSP specimens. On the other hand, SP42B11 presented improved shear performance thanks to its halved bolt space.
Although the plate depth was much greater in SP43B22, the strengthening effect was only slightly greater than SP42B22 and even not so significant compared to SP42B11. This is
because the large bolt spacing could not guarantee a satisfactory collaboration between the steel plates and the RC beam. When both greater plate depth and less bolt spacing were
employed, the stiffness and ductility of SP43B11 was greater than those of the others.
3.4 Analysis of BSP beams’ behavior based on DIC
The digital image correlation (DIC) was a non-contact optical technique for measuring strain and displacement. In order to verify its accuracy, the data obtained from DIC analysis was
compared with the data measured by LDTs, as shown in Figure 9. It can be seen that the load-displacement curves obtained from the DIC analysis was very close to the curves
measured by LDTs.
As shown in Figure 10 – Figure 11, the fields of both the horizontal and vertical displacements present are symmetrically distributed. For the BSP beams for flexural test, the local
buckling area of the bolted steel plates can be visually identified in the vertical displacement field. It is seen from Figure 12 – Figure 13 that the fields of principal strains are also
symmetrically distributed. It is evident from Figure 12 that the local buckling of steel plates appeared first at the loading points rather than the mid-span, which indicates that the plate
buckling was a result of not only bending but also shearing. There was obvious stress concentrated zones on specimen surface, and the main crack path agreed with strain
concentration zones.
(a) Horizontal displacement field (P = Pu) (b) Vertical displacement field (P = Pu)
Figure 10: Displacement fields of FP43B33 derived for the DIC technique
(a) Horizontal displacement field (P = Pu) (b) Vertical displacement field (P = Pu)
Figure 11: Displacement fields of SP42B22 derived for the DIC technique
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(a) P = 0.75 Pu (b) P = Pu
Figure 12: Principal tensile-strain fields of FP43B33 derived for the DIC technique
(a) Principal strain field (e1) (b) Principal strain field (e2)
Figure 13: Principal strain fields of SP42B22 derived for the DIC technique
4 CONCLUSIONS
The flexural and shear behaviors of several fire damaged RC beams strengthened with the BSP technique was investigated and the following conclusions can be summarized:
(1) Obvious temperature gradient was observed in the RC beams under fire, delay of peak temperature was also found after flameout due to the poor heat conductivity of concrete.
The post-fire residual deformation was negligible for the subsequent strengthening and loading tests.
(2) After being retrofitted using the BSP technique, both the flexural and shear capacity, the stiffness and the ductility of the fire-damaged RC beams were recovered, and the
occurrence of the concrete cracks was delayed slightly.
(3) The enhancement increases as the increase of the plate depth and the decrease of the bolt spacing.
(4) Local buckling occurred in the bolted steel plates under a combination of bending and shear stress in the later stage of testing, which was restrained effectively by adding
stiffeners or using more anchor bolts.
(5) The DIC technique could be employed to record the development of displacement and strain fields, thus analyze the failure mode of BSP beams reliably.
Acknowledgements
The research described here received financial support from the National Natural Science Foundation of China (Project No. 51778496 and No. 51778497).
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