Effective strengthening schemes for heat damaged reinforced concrete beams

Journal Pre-proofs Original article Effective strengthening schemes for heat damaged Reinforced concrete beams M. Jamal Shannag, Abdulhafiz Alshenawy PII: DOI: Reference:

S1018-3639(19)30310-1 https://doi.org/10.1016/j.jksues.2019.10.003 JKSUES 364

To appear in:

Journal of King Saud University - Engineering Sciences

Received Date: Accepted Date:

18 April 2019 22 October 2019

Please cite this article as: Jamal Shannag, M., Alshenawy, A., Effective strengthening schemes for heat damaged Reinforced concrete beams, Journal of King Saud University - Engineering Sciences (2019), doi: https://doi.org/ 10.1016/j.jksues.2019.10.003

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Production and hosting by Elsevier B.V. on behalf of King Saud University.

Effective strengthening schemes for heat damaged Reinforced concrete beams Authors: Corresponding Author: Prof. M. Jamal Shannag, PhD Professor-Structural Engineering King Saud University Riyadh 11421-Saudi Arabia Off. 966-1-4676928 Fax: 96614677008 e.mail: [email protected] or [email protected]

Co-author: Dr. Abdulhafiz Alshenawy, PhD Associate Professor King Saud University Riyadh 11421-Saudi Arabia Off. 966-1-4677023 Fax: 96614677008 e.mail: [email protected]

M. Jamal Alshannag is a professor of Civil Engineering at King Saud University at Riyadh-Kingdom of Saudi Arabia. He received his PhD in Civil Engineering from the University of Michigan, Ann Arbor, in 1995, USA. His research interests include fiber reinforced cement-based composites, concrete materials, structural repair, durability of concrete, lightweight concrete and ferrocement.

Abdulhafiz Alshenawy is an associate professor of Civil Engineering at King Saud University at Riyadh-Kingdom of Saudi Arabia. He received his PhD in Civil Engineering from the University of Minnesota, Minnesota, U.S.A., 1992. His research interests include soil mechanics and foundation, constitutive modeling for soils and rocks, and laboratory testing.

Effective strengthening schemes for heat damaged Reinforced concrete beams

Abstract Several effective strengthening schemes for enhancing the flexural strength of heat-damaged reinforced concrete (RC), beams are proposed. A series of twenty-eight RC rectangular beams were cast, subject to a temperature of 600°C for 3 hours, strengthened, and then tested in flexure to determine the effect of externally applied carbon and glass fiber reinforced polymeric sheets (CFRP) and (GFRP) respectively, as a method for regaining the beams’ flexural capacity. The load-deflection behavior and the modes of failure of the beams tested are described. The beams strengthened in shear and flexure with CFRP and GFRP sheets applied at the sides and the bottom of the beams in U-shaped jackets or strips showed a significant increase in their flexural strength compared to control and heat damaged ones. They achieved a high percentage of the control beams load carrying capacity, and exhibited a ductile failure characterized by the initiation of typical flexural cracks in the moment zone followed by the occurrence of flexure-shear cracks in the constant shear span. Optimizing the CFRP and GFRP strengthening schemes proposed could provide promising and cost effective repair alternatives for enhancing the flexural capacity of fire damaged RC members.

Keywords: Concrete; Fiber Reinforced Polymers; Temperature; strengthening.

1. Introduction The exposure of RC members to high temperatures during a fire induces an extensive degree of damage that could lead to significant losses in load carrying capacity, compressive and bond strengths of concrete, and plastic deformation of steel bars, (Jadooe, Al-Mahaidi, and Abdouka 2018), (Jadooe, Al-Mahaidi, and Abdouka 2017), (Arioz 2007), (Zega and DiMaio 2006), (Luccioni et al., 2003). Fires in tunnels and buildings have shown that the behavior of concrete under elevated temperatures can be unstable, Aydin, 2008), and (Aydin and Baradan, 2007). This instability is caused as a result of the vapor pressure build up mechanism and restrained thermal dilatation mechanism. Therefore, there is a growing need to understand the effect of elevated temperatures on such members and develop appropriate post-fire repair and rehabilitation techniques. Moreover, time is identified as the critical factor for repair in these situations, as any delays

would cause consequences.

serious

financial

Many researchers have conducted huge work on repair and retrofit of deteriorated concrete structures using fiber reinforced polymers FRP, (Lin and Tang, 1995), (Yaqub and Bailey, 2013), (Haddad et al., 2008, 2007), (Al-Salloum et al., 2016), (Nayak et al., 2018). However, literature studies have revealed inconsistent data on the repair of heat damaged concrete structures using FRP, because of the uncertainties about the performance of polymer composites in any fire following repair. (Jadooe, Al-Mahaida, and Abdouka, 2018) described in an experimental and

numerical study, the flexural behavior of RC beams damaged by heating and then strengthened using NSM laminates. They found that the repair with NSM CFRP laminates using epoxy adhesive increases the load capacity and stiffness of the beams. Furthermore, they were able to predict the experimental behavior of the beams fairly well using finite element simulation. Haddad, Shannag, and Moh’d, 2008), studied the response of RC shallow beams under the effect of 600οC for a period of 3 hrs and then repaired using concrete jackets reinforced with fibers FRC, ferrocement and fiber glass composites GFRP. Their test results indicated that beams repaired with GFRP sheets and SFRC jackets achieved 127% and 106% respectively, of the load carrying capacity of the control beam specimens. (Haddad, Shannag, and Moh’d, 2007), studied also the effect of high temperature on reinforced concrete T-beams. The beams were subjected to 600οC for a period of 2.5 hours and then repaired using Fiber reinforced concrete jackets FRC. They concluded that, the beams repaired with steel FRC jackets showed a considerable increase in their flexural capacity. The main objective of this research is to enhance the flexural capacity of reinforced concrete beams exposed to high temperatures by proposing effective strengthening schemes using fiber reinforced polymeric FRP sheets. Two types of polymeric sheets are used: carbon fiber reinforced polymer (CFRP), and glass fiber reinforced polymer, (GFRP) sheets; three strengthening schemes: flexural, shear, and flexural shear together.

2.Experimental Work Twenty-eight beams were prepared and tested in flexure. Test program was divided into two sets of beams (B1 & B2) as shown in Figs. 1 and 2. The main difference between these two sets was the longitudinal reinforcement provided for

the beam specimens. Specimen set B1 was provided with a reinforcement ratio satisfying the minimum requirements of the (ACI 318 code, 2014), ρmin, whereas beam set B2 was provided with a reinforcement ratio of 0.3 ρbal.

2 Ø6

200

Ø 8 s t ir r u p s @ 7 5 m m c / c

2 Ø8

150

B 1 : B e a m S e c tio n Ø 8 S t ir r u p s @ 7 5 m c / c

375

2 Ø6

150

375

75

50

900

50

2 Ø8 R o ll e r s u p p o r t

B 1 : B e a m E le v a tio n

Figure 1: Details for specimens of beam set B1

2 Ø6

200

Ø8 stirrups@ 75 mm c/c

3 Ø10

150

B2: Beam Section Ø8 Stirrups @ 75 m c/c

375

2 Ø6

150

375

75

900

50

50

3 Ø10 Roller support

B2: Beam Elevation

Figure 2: Details for specimens of beam set B2 2.1 Test Matrix The details of the test matrix are provided in Table 1. All beams except the control room temperature beam specimens were subject to a temperature of 600C for a duration of 3 hours and then strengthened using different flexural and shear strengthening schemes and then tested up to failure. As described in Table 1, 2 strengthening schemes were used for the beam set B1 and 3 different strengthening schemes were used for the beam set B2. Details of the strengthening schemes have been provided in Table 1, and Figs. 3 through 6 describe the elevation and sections for the different strengthening schemes used in this study.

Beam specimens were duplicated to enhance the reliability of the experimental test results. As seen from the figures, the strengthening schemes were decided upon after testing the control specimens at both room and elevated temperatures. Scheme 1 involves strengthening specimens of beam set B1 in flexure alone and strengthening the specimens of beam set B2 in shear alone as shown in figures 3 and 4 respectively. Scheme 2 involves strengthening beam specimens of both sets B1 and B2 in both shear as well as flexure as shown in Fig. 5. Figure 6 shows the strengthening scheme 3 for the beam set B2.

Table 1: Details of specimens to be tested under flexure Designation

Description

Bottom steel

Top steel

Stirrups

Additional

B1

Control (room temperature & 600o C)

2Ø8

2Ø6

Ø 8 @ 75 mm

none(no strengthening)

B2

Control (room temperature & 600o C)

3 Ø 10

2Ø6

Ø 8 @ 75 mm

none(no strengthening) Number of layers =1

B1-S1-CF

CFRP Flexural strengthening

2Ø8

2Ø6

Ø 8 @ 75 mm

width of sheet= 150 mm thickness of sheet= 1 mm

B1-S1-GF

GFRP Flexural strengthening

2Ø8

2Ø6

Ø 8 @ 75 mm

Number of layers = 2 Width of sheet= 150 mm thickness= 2.6 mm

Number of layers = 1 B2-S1-CF

CFRP Shear strengthening

3 Ø 10

2Ø6

Ø 8 @ 75 mm

Number of Strips = 3 width of strip = 75 mm spacing between strips = 50 mm

Number of layers = 2 B2-S1-GF

GFRP Shear strengthening

3 Ø 10

2Ø6

Ø 8 @ 75 mm

Number of Strips = 3 Width of strip = 75 mm Spacing between strips = 50 mm

B1-S2-CF & B2S2-CF

CFRP Flexural & Shear strengthening

2Ø8

2Ø6

Ø 8 @ 75 mm

Flexural:

Number of layers =1 width of sheet= 150 mm thickness of sheet= 1 mm 3 Ø 10

2Ø6

Ø 8 @ 75 mm

Shear: Number of layers = 1 Number of Strips = 1 width of strip = 200 mm

2Ø8

2Ø6

Ø 8 @ 75 mm

Flexural: Number of layers =2 width of sheet= 150 mm

B1-S2-GF & B2S2-GF

thickness= 2.6 mm

GFRP Flexural & Shear strengthening 3 Ø 10

2Ø6

Ø 8 @ 75 mm

Shear: Number of layers = 2 Number of Strips = 1 width of strip = 200 mm Flexural: Number of layers =1 Width of sheet= 150 mm thickness= 1 mm

B2-S3-CF

CFRP Flexural & Shear strengthening

3 Ø 10

2Ø6

Ø 8 @ 75 mm

Shear: Number of layers = 1 Number of Strips = 3 width of strip = 75 mm spacing between strips = 50 mm Flexural:

B2-S3-GF

CFRP Flexural & Shear strengthening

3 Ø 10

2Ø6

Ø 8 @ 75 mm

Number of layers =1 width of sheet= 150 mm thickness= 1 mm

Shear: Number of layers = 1 Number of Strips = 3 width of strip = 75 mm Spacing between strips = 50 mm

150

375

375

1 Layer of 0° CFRP (L = 1000 & W = 150) OR 2 Layers of 0° GFRP (L = 1000 & W = 150)

900

50

50 Roller support

2 Ø6

200

Ø8 stirrups@ 75 mm c/c

150

2 Ø8 1 Layer of 0° CFRP (L = 1000 & W = 150) OR 2 Layers of 0° GFRP (L = 1000 & W = 150)

Scheme 1: B1 CFRP & GFRP

Figure 3: Details for strengthening scheme S1 for the beam set B1

75

150

375

375

50 1 Layer of 90° CFRP U WRAP 2 Layers of 90° GFRP U WRAP

900

50

50 Roller support

2 Ø6

200

Ø8 stirrups@ 75 mm c/c 1 Layer of 90° CFRP U WRAP OR 2 Layers of 90° GFRP U WRAP

3 Ø10

150

Scheme 1: B2 CFRP & GFRP Figure 4: Details for strengthening scheme S1 for the beam set B2

2.2 Material Properties

Concrete Mix: An ordinary concrete strength mix was designed following ACI 211 procedure (Mindess et al., 1996), to

achieve a 28-day strength of 35 MPa and 120 mm slump. The mix proportions in kg/m3 consist of 400 kg cement, 230 kg water, 980 kg crushed

limestone, 380 kg fine limestone, and 270 kg silica sand. The maximum size of aggregate was restricted to 10 mm. Uniaxial compression tests were carried out on three standard (ASTM C39, 2013), cylinders after 28 days to determine the compressive strength.

Reinforcing steel: Deformed steel bars of 10 mm were used as main reinforcement, and 8 mm diameters were used as shear reinforcement. Steel bars of 6 mm diameters were also used

as top reinforcement in some beams. The measured tensile yield strength as per (ASTM E8/E8M, 2013), was found to be 593 MPa, 570 MPa, and 301 MPa for the 10 mm, 8 mm, and 6 mm diameter bars respectively.

375

375

200

200

150 Flexural Strengthening

1 Layer of 0° CFRP (L = 1000 & W = 200) OR 2 Layers of 0° GFRP (L = 1000 & W = 200) Shear Strengthening 1 Layer of 90° CFRP U WRAP 2 Layers of 90° GFRP U WRAP

900

50

50 Roller support

2 Ø6

200

2 Ø6

Ø8 stirrups@ 75 mm c/c 1 Layer of 90° CFRP U WRAP 2 Layers of 90° GFRP U WRAP

150

2 Ø8 1 Layer of 0° CFRP (L = 1000 & W = 150) OR 2 Layers of 0° GFRP (L = 1000 & W = 150)

B1: Beam Section

200

Ø8 stirrups@ 75 mm c/c 1 Layer of 90° CFRP U WRAP OR 2 Layers of 90° GFRP U WRAP

150

3 Ø10 1 Layer of 0° CFRP (L = 1000 & W = 150) OR 2 Layers of 0° GFRP (L = 1000 & W = 150)

B2: Beam Section

Scheme 2: B1 & B2 CFRP & GFRP Figure 5: Details for strengthening scheme S2 for both the beam sets B1 & B2

75

150

375 50

375

S hear S trengthening 1 Layer of 90° C FR P U W R AP 2 Layers of 90° G FR P U W R AP F lexural S trengthening 1 Layer of 0° C F R P (L = 1000 & W = 200) O R 2 Layers of 0° G F R P (L = 1000 & W = 200)

900

50

50 R oller support

2 Ø6

200

Ø 8 stirrups@ 75 m m c/c 1 Layer of 90° C FR P U W R AP O R 2 Layers of 90° G FR P U W R AP

3 Ø 10 1 Layer of 0° C FR P (L = 1000 & W = 150) O R 2 Layers of 0° G F R P (L = 1000 & W = 150)

150

S chem e 3: B 2 C FR P & G FR P Figure 6: Details for strengthening scheme S3 for the beam set B2

FRP Sheets: The CFRP sheet used in this study was uni-directional carbon fabric Tyfo SCH-41. The GFRP laminate used was Tyfo SEH 51 which is also a custom weave uni-directional glass fiber. The properties

of CFRP and GFRP sheets are listed in Table 2. A two component Tyfo S epoxy was used for both the CFRP as well as GFRP sheets; the epoxy properties are listed in Table 3.

Table 2: Properties of FRP systems used in this study Property

CFRP system

GFRP system

Thickness per layer (mm)

1

1.3

Ultimate tensile strength (MPa)

986

552

Ultimate tensile strain

1.00%

1.90%

Tensile modulus of elasticity (GPa)

95.8

27.6

Based

on manufacturer provided data

Table 3: Properties of epoxy adhesive (Tyfo S) used in this study Adhesive property Tensile strength (MPa) Tensile modulus of elasticity (GPa) Tensile strain at break Glass transition temperature, Tg (°C) Based

on manufacturer’s data sheet

Value 72.4 3.18 5.00% 82

2.3 Specimen Preparation Reinforcement cages were placed within the wooden formwork which was used to cast the concrete. To minimize in between batch variations, same concrete mix was used for casting all the beams. Standard ASTM cylinders 150 × 300 mm were also cast along with the beams which would be used to test the concrete properties. The concrete was compacted using an electrical vibrator. After the casting process was complete, the specimens were then cured for a period of two months. After this period, the specimens would be subjected to sand blasting in order to prepare the surface for strengthening using the appropriate strengthening schemes. The specimens were then subjected to the designated heating regime, after which they were taken out of the oven and allowed to cool for a period of 3 days.

Before the FRP sheets could be applied, the beam specimens were thoroughly cleaned for any dust, laitance or any unstable material using an acetone cleaner. A thin layer of resin based epoxy was then applied on the concrete surface. The FRP sheets were cut in suitable sizes and completely saturated by the epoxy using the approved saturation methods of the manufacturer. The epoxy was distributed evenly over the entire FRP sheet and once the desired saturation was obtained the sheet was bonded to the concrete beam substrate. All voids between the substrate and sheet were carefully removed and the process was repeated for schemes which had two layers of sheets. The manufacturer recommended curing time for the epoxy was 72 hours during which time the beams were allowed to cure in laboratory conditions.

2.4 Heating Regime For Beam Specimens

decided upon. A group of three beams were placed in the oven each time and heated to a temperature of 600C for a duration of 3 hours. Individual timetemperature curve used in the study and the ISO 834 fire curve are shown in Figure 7; the heating rate of the oven varied between 5–15 °C/min.

The heating of beam specimens was carried out in an electric oven after a curing period of 2 months in the laboratory at a temperature of 23o C and 60% relative humidity. The electric furnace used in this study had a base of 1 m × 1 m, on which the beam sizes were

1000 900

Temperature (°C)

800 700

Exposure Time = 3 hrs

600 500

Cooling Starts here

400 300 200 100

Room Temperature (26 °C) Oven (600 °C) ISO 834 Fire Curve

0 Time (mins)

Figure 7: Time-temperature curve used in the study and the ISO 834 fire curve

2.5 Test Procedure All beams were tested under four points bending tests using a 2000 kN capacity AMSLER flexural testing machine. The beams were tested under displacement control mode at a rate of 2.0 mm/min. The load applied on the beams was measured using a load cell and the deflection of the beams was measured using a linear 3. Discussion of Test Results The flexural behavior of beams subjected to a temperature of 600C for a duration of 3 hours is evaluated in this section. Results

3.1 Effect of Strengthening Schemes Beam set B1: Scheme 1: Figure 8 shows the loaddeflection comparison for all beams at room and elevated temperatures along with the beam specimens strengthened by Scheme 1 in flexure using CFRP and GFRP sheets. It was found that the ultimate flexural load carrying capacity for the beams strengthened using CFRP sheets,

variable displacement transducer connected to a computer. Strain gauges were placed on flexural and shear reinforcement for control beam specimens which were not subjected to heating. FRP strain gauges were also applied to measure the strains in FRP surface during the tests. The data was recorded using a data logger at intervals of 1 sec.

for the beams are presented in terms of their peak load carrying capacities, midspan deflection at peak loads and failure modes. B1-S1-CF-600 was 101.61 kN which is an increase of 99% compared with the unstrengthened heated beam specimen B1CON-600 (50.96 kN). A comparison of the heated specimen B1-CON-600 with the GFRP strengthened

specimen B1-S1-GF-600 showed an increase of 131.2% in the flexural capacity. A considerable increase in the stiffness of the beam was also observed for both the

strengthened beams in comparison with the heated specimen. Comparing the heated strengthened specimens to the control specimen at room temperature B1CON-R, it was found that, the load carrying

capacity increased by 76.9 and 105.2% respectively as a result of CFRP and GFRP strengthening. The stiffness of the strengthened beams was close to that of the control beams at room temperature.

Figure 8: Load-mid-span deflection curves for all beams of beam set B1 Scheme 2: Figure 8 shows also the loaddeflection comparison for control beams at room and elevated temperatures along with the beam specimens strengthened by Scheme 2, using CFRP and GFRP sheets. As described in earlier sections, scheme 2 incorporated strengthening of beam specimens in shear as well as flexure. Because of providing more shear reinforcement to the section, the load carrying capacities of scheme 2 strengthened beams B1-S2-CF-600 & B1-

S2-GF-600 were 123.05 & 144.9 kN respectively. This was an increase of 141.5 and 184.3% in flexural capacity respectively for the CFRP and GFRP beams compared with the heated beam specimen B1-CON600. In comparison with the non-heated specimen B1-CON-R, the flexural load carrying capacity of the strengthened beams was 115 and 152.3% higher. Once again stiffness of the strengthened beams was comparable to that of the un-heated control specimen B1-CON-R.

Beam Set B2: Scheme 1: Scheme 1 was designed in order to enhance the shear capacity of beam specimens of set B2 since shear failure was observed in the un-strengthened heated specimen B2-CON-600. Figure 9 shows the load-deflection comparison for all beams at room and elevated temperature along with the beam specimens strengthened by Scheme 1 in shear only using CFRP and GFRP laminates. It was found that, the

ultimate flexural load carrying capacity for the beams strengthened using CFRP laminates, B2-S1-CF-600 was 112.5 kN which is an increase of 5.6% compared with the un-strengthened heated beam specimen B2-CON-600 (106.75 kN). A comparison of the heated specimen B2CON-600 with the GFRP strengthened specimen B2-S1-GF-600 showed an increase of 19.4% in the flexural capacity

from 106.74 kN to 127.5 kN, as a result of strengthening the beam specimens. Comparing the heated strengthened specimens to the control specimen at room temperature B2-CON-R, it was found that, the load carrying capacity could not reach the control un-heated specimen levels

which is to be expected as no additional strengthening was provided. However, an enhancement in ductility was noticed within the strengthened specimens and the failure mode compared to the heated specimen was shifted from a shear failure to flexural ductile failure mode.

Beam B2-Comparisons

160 140

Load (kN)

120

B2-CON-R B2-CON-600 B2-S1-CF-600 B2-S1-GF-600 B2-S2-CF-600 B2-S2-GF-600 B2-S3-CF-600 B2-S3-GF-600

100 80 60 40 20 0 0

10

20

30

40

50

Mid-span Deflection (mm)

Figure 9: Load-mid-span deflection curves for all beams of set B2

Scheme 2: Scheme 2 for the beam set B2 was designed to enhance the flexural strength as well as provide end anchorage to the FRP flexural laminates. Figure 9 shows also the load-deflection curves for control as well as beams strengthened using the scheme 2 of the study. It is observed that the flexural capacity of beam strengthened using CFRP (B2-S2-CF-600)

and GFRP (B2-S2-GF-600) sheets was 146.9 kN and 150.95 kN respectively indicating an increase of 37.6 and 41.4% compared with the heated control specimen B2-CON-600. The flexural capacity was almost similar to that of the control unheated specimen B2CON-R, however the stiffness of the strengthened beam specimens was lower compared with the unheated control beam.

Scheme 3: Figure 9 shows also the load versus mid-span deflection for beams strengthened using scheme 3 of this study. It is observed that the flexural load carrying capacity of beam B2-S3-CF-600 and B2-S3GF-600 was 143.7 and 146.7 kN

respectively. The scheme was designed to enhance the shear strength of the beam which was reduced as a result of the heating regime. The scheme did succeed in enhancing the flexural capacity of the beams compared with the heated un-

strengthened specimen B2-CON-600 by 34.6 and 37.5 % respectively for CFRP and GFRP beams, and also succeeded in reaching the flexural capacity of the un-

heated control beam specimen B2-CON-R. However, the ductility of the unheated control beam specimen could not be replicated by the strengthened beams.

3.2 Failure Modes

but also enhanced the shear capacity of the beam specimens. The FRP U-jackets prevented complete end-delamination of the FRP sheets allowing for the concrete flexural cracks to propagate towards the compression zone causing concrete crushing within the compression zone. Once again intermediate crack debonding was observed as a result of flexural cracks developing in the beam and causing the onset of FRP sheet delamination at crack locations. The end anchorages could only result in avoiding any kind of end delamination. Once the CFRP sheet was debonded, the concrete beam behaved exactly similar to the control beam specimen B1-CON-600 and the final failure was due to steel rebar yielding in tension and causing typical flexural compression failure.

Beam set B1: The final failure mode at which crack debonding of the FRP sheets begins to debond from regions of concrete cracks caused during the flexural tests is shown in Figs. 10 through 13. As the flexural cracks propagate, FRP sheets get debonded along with pieces of concrete. This type of failure is common in FRP strengthened beams and results in the beam failing prior to reaching its maximum possible capacity. Before the final failure occurred, flexural cracks and normal shear/flexure cracks were observed in the beam. In order to avoid end delamination of FRP sheets, the second scheme for the beam set B1 was designed to anchor the FRP sheets using FRP U-jackets at beam ends. This U-jacketing not only provided anchorage for the flexural reinforcement Beam set B2: The un-strengthened heated beam specimen B2-CON-600 failed in shear. Scheme 1 was designed to increase the shear strength of the beam thereby resulting in a flexural failure and also providing an exact estimate as to the reduction in the flexural capacity due to the heating process. Failure mode for beams B2-S1-CF-600 and B2-S1-GF-600 was similar and was a typical flexural failure resulting in vertical cracks in the flexural zone and crushing of concrete in the compression zone. No de-bonding or delamination of the shear strips was noticed. Figures 10 through 13 show typical failure modes for selected beam specimens

strengthened using schemes 2 and 3 of the beam set B2. It is observed that, all beams failed in a shear failure mode as a result of enhancement of flexural capacity due to the FRP reinforcement. This resulted in the flexural capacity of the strengthened beams being similar to that of the unheated control beam specimen B2-CON-R. This type of behavior although undesirable demonstrated the substantial contribution of FRP reinforcement in enhancing the flexural strength of heat damaged beams. Better optimization of the FRP reinforcement design would result in overcoming this type of failure mode.

Figure 10: Typical failure mode for CFRP strengthened beam using Scheme 2 (B2-S2-CF-600)

Figure 11: Typical failure mode for GFRP strengthened beam using Scheme 2 (B2-S2-GF-600)

Figure 12: Typical failure mode for CFRP strengthened beam using Scheme 3 (B2-S3-CF-600).

Figure 13: Typical failure mode for GFRP strengthened beam using Scheme 3 (B2-S3-GF-600)

4. Conclusions

The following conclusions could be drawn from this investigation: 1. Heating rectangular concrete beams reinforced with minimum reinforcement ratio, and 0.3 of the balanced reinforcement ratio, at 600°C for three hours induced a considerable loss in their flexural capacity, up to 11.3% and 26.6% respectively compared to unheated control beams.

2. The beams reinforced with

minimum reinforcement ratio, heated at 600°C for three hours, and strengthened in flexure with CFRP and GFRP sheets showed a significant increase in their flexural capacity, up to 131.2% and 184.3% respectively compared to unstrengthened heated beams. 3. The beams reinforced with a reinforcement ratio of 0.3 of the balanced

reinforcement ratio, heated at 600°C for three hours, and strengthened in shear with CFRP and GFRP sheets showed an increase in their flexural capacity, up to 5.6% and 19.4% respectively compared to un-strengthened heated beams, whereas the beams strengthened in flexure and shear showed an increase in their flexural capacity, up to 37.5% and 41.4% respectively.

Acknowledgements The authors would like to acknowledge the support provided by the Deanship of Scientific Research at King

4. Most of the beams tested in this

investigation including heated and strengthened beams exhibited tension controlled failure characterized by the initiation of typical flexural cracks in the constant moment zone followed by the occurrence of flexure-shear cracks in the constant shear span.

5. Among all the effective

strengthening schemes suggested, the full flexural capacity of heat damaged beams can be regained by optimizing the application of U-shaped FRP jackets or strips on the beams.

Saud University, through the Research Centre at the College of Engineering.

Conflict of Interest The authors declare that they have no conflict of interest.

References ACI COMMITTEE 318, 2014. Building Code Requirements for Structural Concrete, (ACI 318M-14). American Concrete Institute, Farmington Hills, Mich., USA. Arioz, O., (2007). Effects of elevated temperatures on properties of concrete. Fire Safety Journal, 42: 516– 522. ASTM C 39/C39M, 2013. Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, Vol. 04.03, West Conshohocken, PA, USA.

ASTM E8/E8M, 2013. Standard Test Methods for Tension Testing of Metallic Materials, Vol. 04.03, West Conshohocken, PA, USA. Aydın, S. 2008. Development of a high temperature-resistant mortar by using slag and pumice. Fire Safety Journal 43: 610–617. Aydın, S., Baradan, B. 2007. Effect of pumice and fly ash incorporation on high temperature resistance of cement-based mortars. Cement and Concrete Research 37(6); 988–995.

Jadooe, Awad ; Al-Mahaidi, Riadh; Abdouka, Kamiran (2018) “Performance of heat-damaged partially-insulated RC beams strengthened with NSM CFRP strips and epoxy adhesive,” Construction and Building Materials, Vol. 159, pp. 617-634. Jadooe, Awad ; Al-Mahaidi, Riadh; Abdouka, Kamiran (2018) “Behaviour of heat-damaged partially-insulated RC beams using NSM systems,” Construction and Building Materials, Vol. 180 (Aug 2018), pp. 211-228. Jadooe, Awad ; Al-Mahaidi, Riadh; Abdouka, Kamiran; 2017. Bond behavior between NSM CFRP strips and concrete exposed to elevated temperature using cement-based and epoxy adhesives, Journal of Composites for Construction, Vol. 21, no. 5 (Oct 2017), article no. 04017033. Haddad, R.H., Shannag, M.J., Moh’d, A. 2008. Repair of heat-damaged RC shallow beams using advanced composites. Materials and Structures 41(2): 287-299. Haddad, R.H., Shannag, M.J., Hamad, R.J., 2007. Repair of heat-damaged reinforced concrete T-beams using FRC jackets. Magazine of Concrete Research 59(3): 223–231. Lin, C.H, Chen, S.T., Tang, C.A. 1995. Repair of fire damaged reinforced concrete columns. ACI Structural Journal 92, 4006-4011. Luccioni, B.M., Figueroa M.I., Danesi R.F. 2003. Thermo-mechanic model for concrete exposed to elevated temperatures. Engineering Structures 25:729–42.

Mindess, S.; Young, J.F.; Darwin, D. 1996. Concrete, 2nd edition, Prentice-Hall, New Jersey. Nayak, A.N., Kumari, A., Swain, R.B., 2018, Strengthening of RC Beams Using Externally Bonded Fiber Reinforced Polymer Composites, Structures 14 (2018) 137-152. Salloum, Y.A., Almusallam, T.H., Elsanadedy, H.M., Iqbal, R.A. 2016. Effect of elevated temperature environments on the residual axial capacity of RC columns strengthened with different techniques. Construction and Building Materials 115: 345-361. Yaqub, M., Bailey, C.G, Nedwell, P., Khan, Q.U., Javed, I. 2013. Strength and stiffness of post-heated columns repaired with ferrocement and fiber reinforced polymer jackets. Composites: Part B, 44: 200–211. Zega, C.J., DiMaio, A.A. 2006. Recycled concrete exposed to high temperatures. Magazine of Concrete Research 58 (10): 675–682.