Fourth Asia-Pacific Conference on FRP in Structures (APFIS 2013) 11-13 December 2013, Melbourne, Australia

© 2013 International Institute for FRP in Construction

COMPRESSIVE STRENGTH OF CONCRETE DAMAGED BY ELEVATED TEMPERATURE AND CONFINED BY CFRP FABRICS

Yaman S. S. Al-Kamaki 1,2,* and Riadh Al-Mahaidi 1 1 Faculty of Engineering and Industrial Sciences, Swinburne University of Technology,

Hawthorn, VIC 3122, Australia 2 University of Duhok (UoD), Duhok, Kurdistan Region- Iraq

* Corresponding author: Swinburne University of Technology, Room202, AV Building, Hawthorn Campus, Burwood Road, VIC 3122, Australia. Tel: +61 413427161. Email: yalkamaki@swin.edu.au

Emails: (Riadh Al-Mahaidi) ralmahaidi@swin.edu.au

ABSTRACT In general, concrete structures have high fire-resistance. When exposed to fire, however, the strength and stiffness of the concrete and reinforcing steel deteriorate significantly. Fibre-reinforced polymer (FRP) wraps are an excellent material for strengthening concrete to increase its axial load capacity. This paper describes an experimental study on fourteen concrete cylinders after exposure to 500oC for one hour and cooling to room temperature which were then wrapped and tested under axial compression to failure to determine the ultimate axial strength. The purpose of the study was to extend the Carbon fiber reinforced polymer (CFRP) confinement technique to investigate the residual concrete strength of post-heated RC cylinders strengthened/confined with CFRP fabrics. The experimental parameters included type of reinforcement and number of CFRP fabric layers. It was found that repairing heat-damaged cylinders with 1, 2 and 3 layers of unidirectional CFRP can be highly effective for enhancing the compressive strength of concrete damaged by high temperature. The residual concrete strength of post-heated cylinders can be restored to the original level or higher than that of unwrapped cylinders. The failure mode of the confined concrete was predominantly rupture of the CFRP sheets. KEYWORDS Residual concrete strength, compression, confined concrete, fibre reinforced polymer (FRP), repairing, heating. INTRODUCTION In general, concrete has very good fire-resistance. In the early 1920s, (Lea 1920, Lea and Stradling 1922), pioneering studies were conducted on factors influencing concrete strength at high temperature. Later, further studies were reported on the effect of high temperature/fire on normal strength concrete using three basic methods (Castillo 1987, Castillo and Durrani 1990, Phan 1996, Phan and Carino 1998, Husem 2006, Biolzi et al. 2008). The three methods were; (1) unstressed tests, where the specimens are heated under no initial stress and loaded to failure at elevated temperature, (2) stressed tests, where a fraction of the ultimate compressive strength at room temperature is applied and sustained during heating and when the target temperature is reached the specimens are loaded to failure; and (3) residual unstressed tests, where the specimens are heated with no load applied, cooled down to room temperature and then loaded to failure. Greater emphasis has been given to the unstressed residual properties of concrete after exposure to elevated temperatures (Bazant and Kaplan 1996). When concrete is a subjected to elevated temperature, surface cracks starts to form (Ali et al. 2004). However, when the temperature surpasses 500°C, most changes experienced by concrete are considered irreversible (Luccioni et al. 2003). Arioz (2007) has noticed that the surface cracks became visible when the temperature reached 600°C. According to Chan et al. (1999), the range between 400 and 800°C is critical to the strength loss. When the temperature reaches 900°C, the outer layers of concrete members become drastically hot, the inner layers remaining cooler (Nassif et al. 1995).

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In a typical fire, the temperature reaches 500°C in about 10 min and 950°C in 1 h (Nassif et al. 1995). Concrete loses 25% of its unfired compressive strength when heated to 300°C, and 75 % at 600°C (Concrete Society 1990). However, after strength loss, it is possible to repair concrete members after adequate rehabilitation. Therefore, the properties of concrete retained after a fire are of importance for determining the load-carrying capacity and for reinstating fire-damaged constructions (Hertz 2005). It is also has been shown that the remaining strength after a fire is influenced by different parameters (Chan et al. 1999, Annerel and Taerwe 2009). These include material properties (aggregate, cement paste, aggregate-cement paste bond and their thermal compatibility with each other, on the resistance of concrete) and environmental factors (heating rate, duration of exposure to maximum temperature, cooling rate, loading conditions and moisture regime). Previous experimental studies on concrete under high temperatures/fire have shown that (1) the concrete strength heated under load is either equal to or higher than the strength of concrete heated without load (both for hot and residual cold strength); (2) longer exposure to elevated temperatures reduces strength, and (3) compressive strength is reduced more if the concrete is heated in sealed conditions (Petkovski 2010). In relation to concrete columns and cylinders, many studies (Bisby et al. 2004, Kodur et al. 2004, Bisby et al. 2005, Kodur et al. 2006, Chowdhury et al. 2007, Khalifa et al. 2009, Chowdhury et al. 2012) deal with the effect of supplemental fire protection (i.e using insulation materials with FRP) thickness, configuration, length and method of adhesion on the fire performance of concrete specimens strengthened in compression with externally bonded FRP materials. On the other hand, there have also been some studies of the behaviour of fire-damaged concrete specimens after they have been strengthened using FPR materials. For example, Saafi and Romine (2002) studied the effect of fire on concrete cylinders confined with Glass fiber reinforced polymer (GFRP) after exposure to elevated temperatures of 90°C, 180°C, and 360°C for 0.5, 1, and 3 hour durations. The temperatures were chosen to be 0.5 Tg, 1 Tg and 2 Tg. The authors demonstrated that the axial compressive strength reduced approximately 15%, 26%, and 47% for the FRP-wrapped cylinders after 3-hour exposure to temperatures of 90ºC, 180ºC, and 360ºC respectively. Cleary et al. (2003) investigated the residual strength of GFRP and Aramid fiber reinforced polymer (AFRP)-confined concrete cylinders in axial compression after elevated temperatures of about 120ºC, 135ºC, 150ºC and 180ºC, respectively. They observed reductions in ultimate strength of the wrapped cylinders in the range of 4%, 13% and 18% after exposure for 90 minutes at temperatures of 135ºC, 150ºC and 180ºC respectively, but essentially no reduction in strength at 120ºC. Al-Salloum (2008) examined experimentally the effect of high temperature on the performance of plain concrete cylinders and cylinders externally confined with CFRP and GFRP sheets under axial compression load after exposure to temperatures of 100oC and 200oC for periods of 1, 2, and 3h. They reported that after 3 h at 100oC the loss of strength of CFRP and GFRP was observed to be about 5% and 11 % respectively, and about 27% and 17 % at 200oC respectively. Ji et al. (2008) presented test results of residual strength of FRP tube-encased concrete cylinders under uniaxial compression load after they were exposed to a jet fire of 982oC for 4 min, 8 min, and 12 min, respectively. Test results showed that fire exposure has a significant effect on reducing the load-carrying capacity of FRP tube-encased concrete cylinders, but the burned cylinders have a higher ductility than the control cylinders. Bisby et al. (2011) extended the FRP confinement technique to strengthening fire-damaged plain circular concrete columns using a single layer of unidirectional CFRP sheet. The plain cylinders were heated to a temperature of 300oC, 500oC, or 686oC for durations of total heating of 120 or 240 min. The results showed that cylinders exposed to temperatures of 500oC and 686oC for 120 min experienced reductions in compressive strength in the order of 29% and 50% respectively. Specimens exposed to a temperature of 686oC for 240 min experienced a compressive strength reduction in the order of 58%. Yaqub and Bailey (2011) conducted an experimental study to investigate the axial capacity and ductility of post-heated circular RC columns repaired with epoxy resin mortar, GFRP and CFRP jackets. The columns were heated in an electric furnace to 500oC at a rate of 2.5oC/min. They concluded that after heating RC circular columns to 500oC, their strength was reduced by up to 42%. The strength of post-heated columns repaired with FRP jackets was increased by 29% more than the control column’s strength and 122% higher than post-heated columns. To date, there have been no studies of the restoration of the residual strength of concrete after fire by using strengthening materials. This subject needs more investigation that will be beneficial in engineering practice and can help engineers to know the degree of deterioration of concrete structure after exposure to high temperatures and assist them to decide how a structure can be repaired. This paper addresses the lack of experimental data on the residual compressive strength of concrete cylinders after heating to temperatures up to 500°C and reports on the performance of CFRP fabric for the repair of fire-damaged concrete in order to restore lost strength.

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EXPERIMENTAL PROGRAM

Materials One concrete mixture was prepared using ordinary Portland cement Type (I), natural, normal weight fine aggregate (clean and well-graded) and natural normal river gravel with maximum size of 9.5mm. The chemical composition, mineralogical composition, and physical properties of Portland cement Type (I) are given in Table (1), tested at Duhok construction laboratory. The fine aggregate was found to have a fineness modulus of 2.48. Sieve analysis was carried out and the results indicate that the fine aggregate conformed to ASTM/C33 (2003), as shown in Table (2). Coarse aggregate had a specific gravity of 2.68. Potable water was used for both mixing and curing the specimens. Three types of steel reinforcement with the properties shown in Table (3) were used throughout the study. A CYBERTRONIC universal tensile/compression machine was used for testing purposes, as shown in Figure (1a) and the stress-strain relationships for the steel specimens are shown in Figure (1b). SikaWrap-230C, a unidirectional woven carbon fibre fabric with the properties shown in Table (4), was used for the purpose of wrapping specimens. The epoxy material used for bonding was a two-component epoxy matrix material (A: white & B: grey) Sikadur-330. The system properties are summarized in Table (5).

Table 1. Properties of the Portland cement Type (I) Chemical component Projects cement %CaO 63.17SiO2 19.56Ai2O3 5.31Fe2O3 3.56SO3 1.83MgO 2.37Loss of ignition 2.55Insoluble residue 0.39L.S.F. 0.97Mineralogical composition C3S 62.46C2S 9.04C3A 8.05C4AF 10.82Physical Test ResultsFineness (Blaine Air Permeability) 3124 cm2/gInitial setting time 2.46 hrFinal setting time 3.53 hrCompressive strength (3 days) 404 kg / cm2

Compressive strength (7 days) 474 kg / cm2

Soundness (Le Chatelier) 1.5 mmSpecific gravity 3.15

Table 2. Sieve analysis of fine aggregate

Sieve size Limit of percentage passing ASTM-C33-03

Total percentage passing (by weight)

No. 8 (2.36 mm) 80 – 100 100No. 16 (1.18 mm) 50 – 85 72No. 30 (0.60 mm) 25 – 60 47No. 50 (0.30 mm) 5 – 30 22No. 100 (0.15 mm) 0 – 10 5

Table 3. Properties of Steel Reinforcement

Type of Reinforcement fy (MPa)

fu (MPa)

Elongation, % (at ultimate)

Ø 2.48 mm smooth 393 455.5 - Ø 6 mm deformed (Normal) 636 707.8 10 Ø 12 mm deformed (Normal) 548.6 665.4 16.6

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Figure 1. (a) CYBERTRONIC universal tensile/compression machine (b) Stress- strain relationship steel

reinforcement

Table 4. Material properties of CFRP

Type SikaWrap-230 C is a unidirectional woven carbon fiber fabric

Material specification Fiber Areal Weigh 230 g / cm2 ± 10 g / cm2 Young’s modulus 238000 MPaTensile strength 4300 MPaThickness 0.131 mm (based on fiber content)Elongation at break 1.8 %

Table 5. Epoxy material properties

Description Sikadur-330 Solvent-free, thixotropic 2-component impregnation resin on epoxy resin base

Mix ratio (A:B) 4 : 1 by weightDensity 1.31 Kg / l (comp. A + B mixed)Tensile Strength 30 MPa Flexural E-Modulus 3800 MPa

Details of Specimens Fourteen 150 × 250 mm cylinders were prepared from 150 × 300 mm standard cylinders after omitting a spacing of 50 mm using a wooden spacer in order to fit the electric furnace size. Two types of specimens were prepared for testing plain and RC cylinder specimens, divided into four groups. Groups 1 and 2 consisted of plain concrete specimens and Groups 3 and 4 consisted of RC specimens. The specimens of each group had identical properties. Group 1 (control) consisted of three specimens wrapped with 0, 1 and 2 layers of CFRP. Other groups were exposed to 500oC for 1 hr before wrapping with different CFRP layers. The difference between the last two groups was in the type of ring reinforcement. The arrangement of the main and ring reinforcement for the specimens and details of the wrapping can be found in Table (6). Preparation of Specimens An electric mixer was used to prepare a homogenous mixture of concrete at the ratios of 1: 1.5: 3: 0.45 (cement: sand: gravel: w/c ratio by weight). The mixture was poured in the mould in three layers, each layer being compacted by 25 strikes using a 16 mm diameter standard steel rod, as recommended for casting plain concrete in the ASTM/C470 (2002) and ASTM/C192 (2002) specifications. The top surface of the concrete was then

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finished using a trowel and the specimens were left in the mould for 24 hours. After demoulding, the specimens were put in a curing tank in the laboratory for curing for a further 28 days. With each group, three 150 × 300 mm cylinders were cast to measure the control plain compressive strength. The concrete cylinders were left in the moulds for one day, before being marked and removed from the moulds and immersed in water for 28 days. Later, the specimens were removed from the water basin and stored at room temperature for one day, and then all specimens were dried in an oven at 110 ± 5ºC for 24 hours. Immediately after drying, the cylinders were placed in the electric Muffle furnace shown in Figure (2a) and subjected to an elevated temperature rise of 8.3oC/min. After reaching 500oC, heating was continued for one hour. After heating, the specimens were allowed to cool to room temperature for 24 hours prior to wrapping process. As a result of heating, hairline cracks were formed as illustrated in Figure (2b).

To achieve a good bond between the concrete and the CFRP layers when using epoxy for bonding, the surfaces of all cylinders were carefully cleaned using a steel brush to remove any dirt and dust as demonstrated in Figure (2c). The CFRP sheet was then cut and prepared according to the surface area to be wrapped with a 150mm overlap. For all wrapped specimens, a constant overlap length of the CFRP sheet equal to 50 mm was provided to avoid debonding and to provide full confinement. The epoxy resin was then prepared by mixing the two components A and B at a ratio of 4:1. The preparation of the CFRP sheets was followed by carefully painting the cylinder surfaces with epoxy using a soft paint brush. CFRP sheets were then carefully pasted on each specimen according to the requirements for each specimen. A steel roller was used in order to distribute the epoxy on the CFRP layer to ensure good impregnation and the removal of all entrapped air bubbles. The entire process of applying the CFRP sheets was completed within 30 minutes. Before testing commenced, the specimens were left to cure for 7 days according to the manufacturer’s recommendations. To ensure a plane surface to distribute the load uniformly before testing, all specimens were capped using dry gypsum using a No.16 sieve according to the recommendation of ASTM/C617 (2003). The steel base for the capping device was filled with gypsum paste and the specimen was inverted and left for 30 minutes. After hardening of the gypsum the base sides were released and the excess gypsum removed. The specimens were then ready for testing. Specimens prepared for testing are illustrated in Figure (2d).

Figure 2. (a) Electric muffle furnace (b) Hairline cracking resulting from heating (c) Specimen surface cleaning (d) Specimens ready for testing after wrapping and capping processes

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Testing Technique A Walter + Bai AG, Switzerland, universal computerized testing machine was used for testing the concrete specimens. A general view of the testing machine is shown in Figure (3). The rate of loading was kept constant at 0.3 MPa / sec. The load data were collected using the computerized testing machine every 10 sec for loading of the tested specimens until failure was reached. The details of the specimens cured normally (unheated) and the specimens tested after exposure to heating, in addition to the results of load and deformations are summarized in Table (6).

Figure 3. View of the testing machine

RESULTS AND DISCUSSION The test results of confined and unconfined ultimate compressive strength and ultimate load capacity for the control specimens and specimens damaged by elevated temperature then wrapped by CFRP fabrics are summarized in Table (6). The concrete compressive stress values shown in the table are calculated according to the equations given in the footnotes of Table (6). The test results are presented and discussed in the following sections. Effect of Elevated Temperature on Compressive Strength The strength of the post-heated plain and RC circular cylinders was reduced after heating to 500oC for one hour. The strength was reduced up to 67.4%, 55.2% and 23.9% for plain specimens, specimens with main reinforcement and specimens with main and secondary reinforcement respectively, as shown in Figure (4). Effect of CFRP Layers on Concrete Circular Cylinders The full CFRP wrapping system led to enhancement in the ultimate concrete compressive strength relative to that of the unwrapped cylinders, as shown in Figure (5). Overall the post-heated cylinders wrapped with one, two or three layers of CFRP fabric regained more strength than the un-heated columns. This indicates that it is possible to repair circular plain and RC cylinders damaged by high temperature using CFRP sheets. For example, the strength of the control group was increased by 43.2% and 60.4% after confinement by 1 and 2 layers of CFRP respectively. The strength of post-heated plain cylinders was reduced to 9.4 MPa then when repaired with 1, 2 or 3 layers of CFRP sheets, it increased by 28.5%, 51.8% and 63% more than the original strength of unheated cylinders and 76.7%, 84.3% and 87.9% more than post-heated plain cylinders. Effect of CFRP Layers and Main Reinforcement on Post-Heated Circular Cylinders When post-heated concrete cylinder is confined by CFRP and contains only vertical steel bars (Group 3), its compressive strength is higher than those without vertical steel bars (Group 2) and the compressive strength is

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fairly dependent on the types of steel reinforcement, as shown in Figure (6). It was found that when the same number of CFRP layers was used for Groups 2 and 3, concrete with only vertical bars with no steel ties produces almost the same strength enhancement of about 18%. Effect of CFRP Layers, Main and Secondary Reinforcement on Post-Heated Cylinders When post-heated concrete cylinders are confined by steel reinforcement ties and CFRP together (Group 4), their compressive strengths are very close to those without steel reinforcement ties (Group 3) as shown in Figure (7). This indicates that the compressive strength of concrete cylinders confined by CFRP is irrelevant to the type of steel reinforcement and depends mainly on the CFRP. The reason is that when CFRP reaches its ultimate strain, the strains of steel reinforcement of Groups 3 and 4 are still within yielding and ultimate strains.

Table 6. Details of specimens and test results

Gro

up

Cyl

inde

r Sy

mbo

l

Bur

ning

C

ondi

tion

CFR

P La

yers

Arrangement of Reinforcement f′c

(MPa)

Ultimate Compression

Load (kN)

Compressive Strength (MPa) Main Secondary

1

CP0

-

-

- 23.5

508.5 28.8

CP1 1 - 896.6 50.7

CP2 2 1256.6 71.1

2

CPB0

500o C

fo

r 1 h

r

-

- - 25

166.3 9.4

CPB1 1 711.4 40.3

CPB2 2 1055.9 59.8

CPB3 3 1374.2 77.8

3

CRBM0

500o C

fo

r 1 h

r

- 4Ø 12 mm

Ø 2

.48m

m

26.5

288.2 12.9*

CRBM1 1 4Ø 12 mm 919.4 48.9*

CRBM2 2 4Ø 12 mm 1296.5 70.4*

CRBM3 3 4Ø 12 mm 1738.0 95.6*

4

CRBMS0

500o C

fo

r 1 h

r

- 4Ø 12 mm

4 Ø

6m

m @

6cm

c/c

27

445.3 21.9*

CRBMS1 1 4Ø 12 mm 961.8 51.4*

CRBMS2 2 4Ø 12 mm 1347.8 73.3*

C=Cylinder, P=Plain, R=Reinforced, B=Burning, M=Main Reinforcement & S=Secondary Reinforcement(ties) *Compressive Strength= (Pc/A-As), Pc=Pu-Ps , Ps= Asfy ,Pc= Force in concrete (N), A= Area of specimen (mm2), As= Total area of longitudinal steel reinforcement (mm2), Pu= Ultimate load (N), Ps= Force carried by longitudinal steel (N), fy= Yield stress of longitudinal steel (MPa).

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Figure 4. Effect of 500oC of heating for 1 hour on concrete compressive strength

Figure 5. Effect of number of CFRP layers on concrete compressive strength

28.8

9.4

12.9

21.9

0

5

10

15

20

25

30

Ulti

mat

e C

ompr

essi

ve S

tren

gth

(MPa

)

Effect of Heating

CP 0°C

CPB 500°C

CRBM 500°C

CRBMS 500°C

28.8

50.7

71.1

9.4

40.3

59.8

77.8

12.9

48.9

70.4

95.6

21.9

51.4

73.3

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3

Ulti

mat

e C

ompr

essi

ve S

tren

gth

(MPa

)

No. of CFRP Layers

Group (1) CP

Group (2) CPB

Group (3) CRBM

Group (4) CRBMS

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Figure 6. Effect of number of CFRP layers and main reinforcement on concrete compressive strength

Figure 7. Effect of number of CFRP layers, main reinforcement and steel ties

on concrete compressive strength

Test Observations and Failure Mode After testing, photographs were taken to show the pattern of cracks, the rupture of CFRP sheets and the failure modes of specimens and the results are shown in Figure (8). The damage zone appears to be concentrated at the middle third of the cylindrical specimens. For specimens wrapped with CFRP sheets, severe fractures can be observed associated with obvious buckling of the longitudinal bars and deformation of the hoop ties. From the results of damaged specimens after testing it is clear that in general the failure patterns and the fracture of CFRP sheets are the same for heated and unheated specimens. This observation can be added to those related to the strength and deformation of damaged reinforced concrete repaired with CFRP sheets to show that there is a similarity between heated and true concrete strengthened with CFRP sheets.

40.3

59.8

77.8

48.9

70.4

95.6

0

10

20

30

40

50

60

70

80

90

100

1 2 3

Ulti

mat

e C

ompr

essi

ve S

tren

gth

(MPa

)

Effect of CFRP Layers and Main reinforcement

Group (2) CPB

Group (3) CRBM

40.3

59.8

48.9

70.4

51.4

73.3

0

10

20

30

40

50

60

70

80

1 2

Ulti

mat

e C

ompr

essi

ve S

tren

gth

(MPa

)

Effect pf CFRP Layers, Main Reinforcement and Secondary Rienforcement

Group (2) CPB

Group (3) CRBM

Group (4) CRBMS

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Figure 8. Failure patterns of specimens and CFRP rupture CONCLUSIONS From the above findings, it can be concluded that:

1. The carbon fibre composite is very effective for retrofitting concrete cylinders damaged by elevated temperature. 2. The use of CFRP is an efficient technique and means of providing external confinement of concrete for

reinstating and enhancing the strength of fire-damaged concrete compressive cylinders. 3. Maximum strength of a confined concrete cylinder increases proportionally with an increase in the number of

CFRP sheets. 4. The ultimate compressive strength failure mode of confined concrete cylinders is governed by the rupture of core

concrete and the rupture of CFRP strengthened on the exterior. ACKNOWLEDGEMENTS The tests described in this paper were carried out at the University of Duhok (UoD), Iraqi Kurdistan Region, Faculty of Engineering and Applied Science, School of Engineering, Department of Civil Engineering. The collaboration, continuous support and useful suggestions of Professor Riadh Al-Mahaidi are gratefully acknowledged. Thanks are due to Dr. Azad Abdulkadir Mohammed for the initial idea for the research. Thanks are also due to all demonstrators in the Civil Engineering Department at UoD for their role in collecting the experimental data.

CPB CPBCPB CPB

CP2 CP1 CP0

CRBM CRBMCRBMCRBM

CRBMS CRBMSCRBMS

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