behavior of concrete damaged by high temperature exposureand confined with cfrp fabrics

8/10/2019 Behavior of Concrete Damaged by High Temperature Exposureand Confined With Cfrp Fabrics

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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 6308 (Print),ISSN 0976 6316(Online), Volume 5, Issue 8, August (2014), pp. 148-162 IAEME

148

BEHAVIOR OF CONCRETE DAMAGED BY HIGH TEMPERATUREEXPOSUREAND CONFINED WITH CFRP FABRICS

Yaman S.S. Al-Kamaki 1, Riadh Al-Mahaidi 2, Azad A. Mohammed 3

1(PhD candidate, Faculty of Science, Engineering and Technology, Swinburne University ofTechnology, Australia& University of Duhok (UoD), Duhok, Kurdistan Region- Iraq)

2(Professor of Structural Engineering, Faculty of Science, Engineering and Technology, SwinburneUniversity of Technology, Australia)

3(Professor of Structural Engineering, University of Sulaimani, Sulaimani, Kurdistan Region- Iraq)

ABSTRACT

This paper describes an experimental study of the axial compression behavior of plain and

reinforced concrete (RC) cylinders. 28 identical cylinders were fabricated, of which 22 weresubjected to high temperature then confined with CFRP sheets after air cooling. The variablesconsidered in these tests included the presence and type of reinforcement and the number of CFRPsheet layers. It was found that it is possible to repair heat damaged concrete even to the extent ofachieving the original compressive strength prior to heating through the use of appropriate CFRPmaterials. The use of CFRP materials was found to not only increase the compressive strength butalso the ductility of both un-heated and heat-damaged concrete. Test results also show that the axialtoughness increases with number of carbon fiber reinforced polymer (CFRP) layers. The lateraltoughness of damaged concrete not wrapped with CFRP sheets is less than that of concreteundamaged by exposure to high temperature and of that achieved by cylinders repaired by CFRP.The compressive failure mode of all cylinders wrapped with CFRP sheets correlated with a suddenrupture of the sheets approximately at the mid-height of cylinders.

Keywords: FRP-Confined Concrete Cylinders, Heating, Stress, Strain, Toughness.

1. INTRODUCTION

In recent years, the construction industry has shown significant interest in the use of fiberreinforced polymer (FRP) materials for the repair and strengthening of concrete structures. This canbe attributed to the numerous advantages of FRPs, including their extremely high strength-to-weight

INTERNATIONAL JOURNAL OF CIVIL ENGINEERINGAND TECHNOLOGY (IJCIET)

ISSN 0976 6308 (Print)ISSN 0976 6316(Online)Volume 5, Issue 8, August (2014), pp. 148-162 IAEME: www.iaeme.com/ijciet.aspJournal Impact Factor (2014): 7.9290 (Calculated by GISI)www.jifactor.com

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ratios, versatility, and resistance to electrochemical corrosion, which give FRP materials superiorityover conventional materials such as steel. A particularly attractive use of carbon fiber reinforcedpolymer (CFRP) in structural engineering applications involves the strengthening of existingreinforced concrete (RC) columns by bonding CFRP wraps to the external surface of the member.Indeed, the use of such wrapping of RC columns is now considered the method of choice for

strengthening these types of members to resist static and dynamic loads. Various studies have shownthat this form of circumferential reinforcement can significantly increase both the strength andductility of RC members. Hence, CFRP applications have been used widely in the repair andrestoration of RC columns(Ilki et al., 2002, Green et al., 2006, Chowdhury et al., 2007). This is alsothe case where concrete has been damaged by fire.

(Lea, 1920) and, (Lea and Stradling, 1922) studied the effect of high temperatures on thestrength of concrete. Since that time many researchers (Malhotra, 1956, Mohamedbhai, 1982,Schneider, 1988, Khoury, 1992, Khoury, 2000, Xiao and Knig, 2004, Annerel and Taerwe, 2009)have studied various aspects of the effect of high temperature / fire exposure on concrete strengthincluding the strength at room temperature and the strength following exposure to heating (andcooling). The strength after exposure to heating and air cooling is referred to in the literature as theresidual strength and is relevant to an understanding of the strength of a concrete structure after a fire(Bazant and Kaplan, 1996).

In a standard fire (ISO 834, 2012), the temperature reaches 500C in less than 5 minutes and950C in around 60 minutes. The temperature reached by the concrete depends on the thermalproperties of the concrete and duration of exposure such that with practical members a temperaturegradient is likely to exist throughout the member cross-section. The effect of heating on concretestarts in the form of surface cracking (Ali et al., 2004) which becomes visible when the temperaturereaches 600C (Arioz, 2007). According to (Chan et al., 1999), the range between 400C and 800Cis most critical to strength loss at elevated temperature.

The effect of fire on the mechanical properties of concrete after the member has cooled (i.e.the residual strength) is well documented and it has been shown that exposure to high temperaturesleads to reduced modulus of elasticity and compressive strength (i.e. residual properties) and thecompressive strength and ductility of concrete cylinders have been found to be enhancedsignificantly when they are wrapped with CFRP sheets (Saafi, 2002, Lau and Anson, 2006, Youssefand Moftah, 2007, Ji et al., 2008). Test results (Poon et al., 2004) have shown that after exposure to600C and 800C, plain concrete retains 45% and 23% of its compressive strength, on average,respectively. The results also show that after concrete is exposed to elevated temperatures, the loss ofstiffness is much quicker than the loss in compressive strength, but the loss of energy absorptioncapacity is relatively slower.

(Bisby et al., 2011) used FRP confinement to strengthen fire-damaged plain concrete circularcolumns using a single layer of unidirectional CFRP sheets after they had been heated to atemperature of 300C, 500C, or 686C for durations of total heating ranging from 120 to 240 min.The results showed that cylinders (before wrapping) exposed to temperatures of 500C and 686Cfor 120 min experienced reductions in compressive strength in the order of 29% and 50%

respectively. Specimens exposed to a temperature of 686C for 240 min experienced a compressivestrength reduction in the order of 58%. The ultimate axial strains of the FRP-confined concrete wereincreased by more than 100% in all cases when compared with the unconfined concrete. (Yaqub andBailey, 2011) conducted experiments to investigate the strength and ductility of post-heated circularRC columns repaired with epoxy resin mortar, glass fiber reinforced polymer (GFRP) and CFRP

jackets. The columns were heated in an electric furnace to 500C. They concluded that after heatingRC circular columns to 500C, their strength was reduced by up to 42%. The strength of post-heated


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column repaired with FRP jackets increased by 29% more than that of the control columns(unheated) and 122% higher than that of post-heated (but unwrapped) columns.

There is a need for an improved understanding of the effect of fire on practical concretemembers and the subsequent use of CFRP fabric for restoring the residual strength of concrete afterfire exposure. Such knowledge can contribute to the development of an engineering methodology to

assess the residual strength of a concrete member and the enhancement in mechanical properties thatcan be achieved through the use of CFRP. The purposes of this paper are (1) to provide experimentaldata on the residual compressive strength of concrete after heating to temperatures up to 500C; (2)to provide test data on the effectiveness of using CFRP fabric for repairing such fire- damagedconcrete; (3) to study the deformation behavior of damaged concrete cylinders and the modes offailure as affected by the variables studied in the research.

2. EXPERIMENTAL PROGRAM

2.1 MaterialsOrdinary Portland cement (Type I) produced by the Kurtlen company in Turkey was used

throughout the experimental program. Both the chemical composition and the physical propertiesindicate that the cement conforms to the (Iraqi Standard Specification No.5, 1984). Natural, normalweight fine aggregate (clean, and well-graded) taken from Pishabeer pit in the Zakho region wasused and was found to have a fineness modulus of 2.48. Sieve analysis was carried out and theresults indicated that the fine aggregate conformed to (ASTM/C33, 2003). Natural river gravel fromthe Pishabeer pit was used throughout the experimental program as coarse aggregate. The maximumsize was found to be 9.5 mm and the specific gravity 2.68. Potable water was used for both mixingand curing the specimens.

Three types of steel reinforcement with the properties shown in Table 1 were used throughoutthe research. A CYBERTRONIC universal tensile/compression machine was used for testingpurposes. The results for yield strength, ultimate strength and elongation at ultimate are shown inTable 1.

SikaWrap-230C, a unidirectional woven carbon fiber fabric with the properties shown inTable 2, was used for the purpose of wrapping specimens. The epoxy material used for bonding wasa two-component epoxy matrix material (A: white & B: grey) Sikadur-330. The system propertiesare summarized in Table 3.

Table 1: Properties of steel reinforcement

Type of Reinforcement f y (MPa)f u

(MPa)Elongation, %(at ultimate)

2.48 mm smooth 393 455.5 - 6 mm deformed 636 707.8 10

12 mm deformed 548.6 665.4 16.6

Table 2: Material properties of CFRPType SikaWrap-230C is a unidirectional woven carbon

fiber fabricMaterial specification Fiber Areal Weigh 230 g/cm 2 10 g/cm 2 Youn s modulus 238000 MPaTensile strength 4300 MPaThickness 0.131 mm (based on fiber content)Elongation at break 1.8 %


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Table 3: Epoxy material properties

Description Sikadur-330 Solvent free, thixotropic 2-componentimpregnation resin on epoxy resin baseMix ratio (A:B) 4: 1 by weight

Density 1.31 Kg / l (comp. A + B mixed)Tensile Strength 30 MPa

Flexural E-Modulus 3800

2.2 Details of specimensFour groups, including plain and reinforced concrete 150 250 mm cylinders (total of 28

specimens) were prepared and tested. Each group permitted the testing of nominally identicalspecimens. Groups 1 (3 2 repeats) and 2 (4 2 repeats) consisted of plain concrete cylinders andGroups 3 (4 2 repeats) and 4 (3 2 repeats) consisted of reinforced cylinders with Group 4 havingboth axial and tangential steel reinforcement. Group 1 (control) were wrapped with 0, 1 and 2 layersof CFRP but were not subjected to heating. The other groups (i.e. Groups 2, 3 and 4 were exposed to500C for 1 hour before wrapping. Groups 2 and 3 cylinders were wrapped with 0, 1, 2 and 3 layerswhilst Group 4 was wrapped with 0, 1 and 2 layers. Groups 1 and 2 specimens contained noreinforcement, Group 3 contained longitudinal reinforcements and Group 4 cylinders were reinforcedwith both axial and tangential reinforcements. The arrangement of the main and transversereinforcement for the specimens and details of the wrapping can be found in Table 4.

Table 4: Details and test results of specimens

Group CylinderSymbolf c

(MPa)Heating

ConditionCFRPLayers

Arrangement ofReinforcement Ultimate

CompressionLoad (kN)

CompressiveStrength(MPa)

NotesMain Secondary

1

PC_0

23.5 -

-

- -

508.5 28.8 Unheated unwrappedplain cylinder

PC_1 1 896.6 50.7Unheated wrapped

plain cylinder with 1CFRP layer

PC_2 2 1256.6 71.1Unheated wrapped

plain cylinder with 2CFRP layers

2

HPC_0

25 500oC

for 1 hr

-

- -

166.3 9.4 Heated plaincylinder, unwrapped

HPC_1 1 711.4 40.3Heated plain

cylinder, wrapped

with 1 CFRP layer


cylinder, wrappedwith 2 CFRP layers


cylinder, wrappedwith 3 CFRP layers


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3

HRC_M_0

26.5 500oC

for 1 hr

-

4 12 mm 2.48mm

288.2 12.9*Heated reinforcedcylinder with mainbars, unwrapped

HRC_M_1 1 919.4 48.9*

Heated reinforcedcylinder with main

bars, wrapped with 1CFRP layer

HRC_M_2 2 1296.5 70.4*


bars, wrapped with 2CFRP layers

HRC_M_3 3 1738.0 95.6*


bars, wrapped with 3CFRP layers

4

HRC_MS_0

27 500oC

for 1 hr

-

4 12 mm 4 6mm @6cm c/c

445.3 21.9*


bars and rings,unwrapped

HRC_MS_1 1 961.8 51.4*

Heated reinforced

cylinder with mainbars and ties,wrapped with 1

CFRP layer

HRC_MS_2 2 1347.8 73.3*


bars and ties,wrapped with 2

CFRP layers

C= cylinder, P= plain, R = reinforced, H=heating, M= main reinforcement and S= secondary reinforcement (ties)

*Compressive Strength= (Pc/A-As), Pc= Pu-Ps, Ps= Asfy ,Pc= Force in concrete (N), A= Area of specimen (mm 2), As= Total area of longitudinal steelreinforcement (mm 2), Pu= Ultimate load (N), Ps= Force carried by longitudinal steel (N), fy= Yield stress of longitudinal steel (MPa)

2.3 Preparation of specimens

A concrete mixture of 1: 1.5: 3: 0.45 (cement: sand: gravel: w/c ratio by weight) was used forpreparing the concrete specimens. First, all the materials were accurately weighed and prepared andthen fine and coarse aggregates were placed in an electric mixer. The cement was then added and leftto mix with the fine and coarse materials for two minutes. The water was then added and the mixturewas rotated for another two minutes until a homogenous mixture was obtained.

For reinforced concrete specimens, a small amount of concrete was first put in the mold toform the bottom concrete cover, and then the steel reinforcement cage (longitudinal and tangentialreinforcement) was put in the cylindrical mold. The mixture was then poured in three layers, eachlayer being compacted by 25 strikes using a 16 mm diameter standard steel rod, as recommended forcasting plain concrete in the (ASTM/C470, 2002) and (ASTM/C192, 2002) specifications.Furthermore, for each layer, external compaction was achieved by striking the molds gently with arubber hammer to exclude any remaining air bubbles and to ensure the set of the cover. The top

surface of the concrete was then finished using a trowel and the specimens were left inside the moldfor 24 hours. After demolding, the specimens were put in a water tank in the laboratory for curing for28 days.

After 28 days the specimens were removed from the water tank and stored at roomtemperature for one day, and then all specimens were dried in an oven at 110 5C for 24 hours.

Immediately after drying, the cylinders were placed in the electric Muffle furnace, shown inFig. 1(a) and subjected to an elevated temperature rise of 12C/min, Fig. 1(b). After reaching 500C


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the heating was continued for one hour. After heating the specimens were allowed to cool to roomtemperature.

As a result of heating, hairline cracks were formed. The cracks were marked and an image ofthese cracks can be seen in Fig. 2(a). The surfaces of all specimens to be wrapped with CFRP sheetswere then carefully cleaned using a steel brush to remove any dirt and dust. This process is essential

to provide a coarse surface texture to increase the bond between the concrete surface and the CFRPlayers when using epoxy for bonding. The CFRP sheet was then cut and prepared according to thesurface area to be wrapped. For all wrapped specimens a constant overlap length of the CFRP sheetequal to 100 mm was provided to avoid debonding and to provide full confinement. The epoxy resinwas then prepared by mixing the two components A and B at a ratio of 4:1. The preparation of theCFRP sheets was followed by carefully painting the cylinder surfaces with epoxy using a softpaintbrush. CFRP sheets were then carefully pasted on each specimen according to the requirementsfor each specimen. A steel roller was used in order to distribute the epoxy on the CFRP layer toensure good impregnation and that all entrapped air bubbles were removed. The entire process ofapplying the CFRP sheets was completed within 30 minutes. Before testing commenced, thespecimens were left to cure for 7 days according to the manufacturers recommendations.

Before testing, all specimens were capped according to the recommendation of(ASTM/C617, 2003). The capping process is important to ensure a plane surface in order todistribute the load uniformly. For capping, gypsum was used, and the dry gypsum was sieved using aNo.16 sieve to remove any deleterious substances. The steel base for the capping device was filledwith gypsum paste and the specimen was inverted and left for 30 minutes. After hardening of thegypsum the base sides were released and the excess gypsum removed. The specimens were thenready for testing. Specimens prepared for testing are illustrated in Fig. 2(b).

With each group, three 150 300 mm cylinders were cast to measure the control plaincompressive strength. The concrete cylinders were left inside the mold for one day, before beingmarked and removed from the molds and immersed in water for 28 days.

(a) (b)Fig.1: (a) MATEST electric furnace, (b) Cylinders heating cycle

0

100

200

300

400

500

600

0 50 100 150 200 250 300 350 400 450 500

T e m p e r a t u r e

( C )

Time (Minutes)

500C

42 minutes

102 minutes


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(a) (b)Fig. 2: (a) Hairline cracking resulting from heating, (b) Specimen ready for testing after wrapping

and capping processes

2.4 Testing techniqueA Walter + Bai AG, Switzerland universal computerized testing machine was used for testingthe concrete specimens. A general view of the testing machine is shown in Fig. 3(a). The testing wasundertaken under load control at 0.3 MPa / sec.The lateral deformation of the specimens was measured by using two dial gauges with an accuracyequal to 0.01 mm, placed at 180 apart and located at the mid-height of each specimen. The gaugeswere placed on a specially fabricated metal base with adjustable and moveable metal arms to controlthe required position, as illustrated in Figs. 3(b) and 3(c). The lateral (or radial) deformation wasobtained by taking the average of the readings of the two dial gauges for each load increment. Tomeasure axial displacement, one dial gauge was attached to the cylinder using a speciallymanufactured metal ring located on the top third of the cylinder. The measurement of axialdeformations was carried out using a dial gauge attached to a collar wrapped around the top edge ofthe cylinder. The needle of the gauge made contact with a lever connected to a collar wrappedaround the bottom edge of the cylinder, as shown in Figs. 3(b) and (c). The gauge length for axialdeformation was the center-to-center distance between the plate rings, which was equal to 210 mm. The load and displacement data were collected using four digital video recorders (1 for load and 3for dial gauges) and recorded every 10 sec up until failure.

(a) (b) (c)Fig. 3: (a) View of the testing machine, (b) Schematic view of specimen showing the measurement

units measurement units, (c) View of specimen showing the measurement units


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3. RESULTS AND DISCUSSION

The test results for each of the cylinders are summarized in Table 4. The ultimatecompression loads as obtained directly from the tests are given as are the concrete compressivestrengths (stress) which has been obtained by applying the equations given in the footnotes of Table

4 which permits the effect of longitudinal steel (present in Groups 3 and 4) to be discounted. The testresults are discussed in the following sections. For Figures, the following terminology-PC (plaincylinder-Group 1), HPC (heated plain cylinder-Group 2), HRC_M (heated cylinder with onlylongitudinal reinforcement-Group 3) and HRC_MS (heated cylinder with longitudinal reinforcementand steel ties-Group 4).

3.1 Effect of elevated temperature on compressive strengthThe concrete strength of the post-heated plain and RC circular cylinders was found to have

been reduced after heating to 500C for one hour. The strengths were reduced up to 67.4%, 55.2%and 23.9% for plain specimens, specimens with main reinforcement and specimens with main andsecondary reinforcement, respectively, as shown in Fig. 4(a).

3.2 Effect of CFRP layersThe full CFRP system led to enhancement in the ultimate residual concrete compressive

strength relative to that of the unwrapped cylinders, as shown in Fig. 4(b). The post-heated cylinderswrapped with one, two or three layers of CFRP fabric had more strength than the un-heated columns.This indicates the usefulness of wrapping to restore the strength lost due to high temperature. Forexample, the strength of the control group (Group 1) was increased by 76% and 146.9% afterconfinement by 1 and 2 layers of CFRP respectively. The strength of post-heated plain cylinders(Group 2) was reduced to 9.4 MPa, then when repaired with 1, 2 or 3 layers of CFRP sheets, itincreased by 39.9%, 107.6% and 170.1% more than the original strength of the unheated cylinders.Fig. 4(b) shows the variation of ultimate concrete compressive strength with different numbers ofCFRP layers.

3.3 Effect of CFRP layers and main reinforcement on post heated cylindersWhen post-heated concrete cylinder is confined by CFRP and contains only vertical steel bars

(Group 3), its apparent concrete compressive strength is higher than for cylinders without verticalsteel bars (Group 2). However, this effect is relatively small compared with the effect of CFRPconfinement and the number of layers as shown in Fig. 4(c).

3.4 Effect of CFRP layers, main and secondary reinforcement on post heated cylindersWhen post-heated concrete cylinders have both longitudinal reinforcement and steel ties

(Group 4) their concrete compressive strengths were only slightly increased over those obtained forlongitudinal bars only (Group 3). This was the same for one layer and two layers of CFRP. This isshown in Fig. 4(d). This figure illustrates that the compressive strength of concrete cylinders depends

mainly on the CFRP. The reason for this is that when CFRP reaches its ultimate strain, the strains ofsteel reinforcement of Groups 3 and 4 are still within yielding and ultimate strains.


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Fig. 4: (a) Effect of 500C of heating for 1 hour on residual concrete compressive strength, (b) Effectof number of CFRP layers on concrete compressive strength, (c) Effect of number of CFRP layersand main reinforcement on concrete compressive strength, (d) Effect of number of CFRP layers,

main reinforcement and steel ties on concrete compressive strength

3.5 Stress-strain relationshipThe stress-strain relationships have been derived from the load displacement measurements.

In the case of longitudinal strain, this has been calculated by dividing the measured displacement bythe gauge length. The lateral strain was calculated by dividing the average lateral displacement bythe diameter of the cylinder. These relationships are shown in Figs. 5(a) to (d) for cylinders inGroups 1, 2, 3 and 4. It can be noted from these figures that the shape of the relationships changedepending on the presence or absence of CFRP layers and the number of layers. This is true for bothplain and RC concretes cylinders.

As a result of wrapping, the slope of the last portion of the stress-strain relationships isincreased and failure usually occurs at a larger axial strain value compared with the unwrappedcontrol specimens. This increase in the failure strain is a function of the number of CFRP layersapplied to the concrete. There is also a difference in the behavior of heated and unheated concretes inrelation to lateral strain. In unheated specimens the lateral strain at failure increases as a result of theaddition of CFRP (see Fig. 5(a)). However, in the case of the heated specimens the lateral failurestrain reduces (see Figs. 5(b) to (d)), but not all results change with further addition of CFRP layers.


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(a) (b)

(c) (d)

Fig. 5: (a) Stress-strain relationship of Group 1 specimens, (b) Group 2 specimens,(c) Group 3 specimens, (d) Group 4 specimens

It is helpful to compare the stress-strain curves for specimens taken from each group of tests.The first comparison is for all specimens with no CFRP and includes heated and unheated cylinders.This is given in Fig. 6(a) compares HPC_0 (heated plain cylinder) with the non-heated plaincylinder, PC_0; heated longitudinally reinforced cylinder, HRC_M_0 and heated longitudinalreinforced with ties, HRC_MS_0. This figure shows that the strength of the heated cylinders issignificantly lower than that of the unheated plain specimen, with the greatest reduction beingassociated with the unreinforced heated specimens. In contrast, the lateral strain at failure is muchgreater for the heated specimens.

In a similar manner, Figs. 6(b) to (d) give a comparison of the stress-strain curves forcylinders wrapped with one layer, two layers and 3 layers of CFRP, respectively. As a result ofwrapping with one layer, the strength of all cylinders containing steel reinforcement (HRC_M_1 and

HRC_MS_1) achieves a strength that is slightly higher than the wrapped unheated plain cylinders(PC_1). This demonstrates the significant effect of the presence of steel reinforcement on theresidual strength of the cylinders. In the case of two layers (Fig. 6(c)), only the specimens with bothlongitudinal reinforcement and ties have a strength which is higher than the unheated plain cylinderalso wrapped with two layers. All of the heated cylinders, high load capacity can be obtained,regardless of the presence of steel reinforcement. Fig. 6(d) illustrates the additional benefit of steelties even when cylinders are wrapped with 3 layers.

0

10

20

30

40

50

60

70

80

90

-0.01 -0.005 0 0.005 0.01 0.015 0.02

S t r e s s

( M P a )

Lateral Strain Axial Strain

Unheated unwrapped plain cylinder (PC_0) Unheated wrapped plain cylinder with 1 CFRP layer (PC_1) Unheated wrapped plain cylinder with 2 CFRP layers (PC_2)

0

10

20

30

40

50

60

70

80

90

100

- 0.02 - 0.015 -0 .01 -0 .005 0 0 .005 0 .01 0 .015 0 .02 0 .025 0 .03

S t r e s s

( M P a )


Heated plain cylinder, unwrapped (HPC_0) Heated plain cylinder, wrapped with 1 CFRP layer (HPC_1) Heated plain cylinder, wrapped with 2 CFRP layer (HPC_2) Heated plain cylinder, wrapped with 3 CFRP layer (HPC_3)

0

10

20

30

40

50

60

70

80

90

100

110

120

- 0.02 - 0.015 -0 .0 1 -0 .0 05 0 0 .0 05 0 .01 0 .015 0 .0 2 0 .0 25 0 .0 3

S t r e s s

( M P a )


Heated reinforced cylinder with main bars, unwrapped (HRC_M_0) Heated reinforced cylinder with main bars, wrapped with 1 CFRP layer (HRC_M_1) Heated reinforced cylinder with main bars, wrapped with 2 CFRP layers (HRC_M_2) Heated reinforced cylinder with main bars, wrapped with 3 CFRP layers (HRC_M_3)

0

10

20

30

40

50

60

70

80

- 0.02 - 0.015 -0 .01 -0 .005 0 0 .005 0 .01 0 .015 0 .02 0 .025 0 .03

S t r e s s

( M P a )


Heated reinforced cylinder with main bars and t ies, unwrapped (HRC_MS_0) Heated reinforced cylinder with main bars and t ies, wrapped with 1 CFRP layer (HRC_MS_1) Heated reinforced cylinder with main bars and t ies, wrapped with 2 CFRP layers (HRC_MS_2)


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(a) (b)

(c) (d)

Fig. 6: (a) Stress-strain relationship of specimens as affected by heating, (b) specimens

wrapped with one layer of CFRP sheet, (c) specimens wrapped with two layer of CFRP sheet,(d) specimens wrapped with three layer of CFRP sheet

3.6 ToughnessThe longitudinal and lateral toughness exhibited by a cylinder can be considered to be

reflected by the area under the respective stress versus strain curves. These are shown in Figs. 7(a)and (b) for all tested specimens and were calculated using trapezium method. Fig. 7(a) shows thatthere is a steady increase in axial toughness with increasing CFRP layers. It also illustrates that thepresence of steel reinforcement improves the axial toughness and the axial toughness is notsignificantly affected by preheating the concrete before wrapping with CFRP sheets. This is seenfrom a comparison of the Group 1 (plain and not heated) results with the Group 2 (plain and heated)results.

Fig. 7(b) plots the lateral toughness versus numbers of CFRP layers. This figure illustratesthat the lateral toughness of damaged concrete not wrapped with CFRP sheets is less than that ofconcrete undamaged by exposure to high temperature. For the damaged specimens wrapped with onelayer of CFRP, the lateral toughness is still low but increases with increasing numbers of CFRPlayers. However, in general, the lateral toughness is low for preheated concrete wrapped with onlyone layer of CFRP.

0

5

10

15

20

25

30

-0.02 -0.01 0 0.01 0.02

S t r e s s

( M

P a )


PC_0HPC_0HRC_M_0CRBMS0

0

10

20

30

40

50

60

-0.01 -0.005 0 0.005 0.01 0.015 0.02

S t r e s s

( M

P a )


PC_1HPC_1HRC_M_1HRC_MS_1

0

10

20

30

40

50

60

70

80

-0.01 0 0.01 0.02 0.03 0.04

S t r e s s

( M P a )


PC_2HPC_2HRC_M_2HRC_MS_2

0

10

20

30

40

50

60

70

80

90

100

-0.01 0 0.01 0.02 0.03

S t r e s s

( M P a )


HPC_3HRC_M_3


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(a) (b)

Fig. 7: (a) Variation of axial toughness with CFRP layer,(b)Variation of lateral toughness with CFRP layer

3.7 Failure modesTypical failure modes of the control and wrapped test specimens are shown in Fig. 8. The

failure mode for cylinders not confined by CFRP exhibited, prior to failure, vertical cracking startingfrom the top of the cylinders and propagating downward along the length of the cylinders in thedirection of loading. These cylinders failed by splitting as the result of shear stresses, as shown inFig. 8. The maximum compressive load of these specimens was achieved shortly after cracksdeveloped and propagated with a corresponding rapid reduction in load resistance.

In the case of the cylinders confined by CFRP, the failure mode depended on the lateral

confining pressure. All wrapped concrete specimens failed by tensile rupture of the CFRP wrap inthe column mid-height region in a sudden explosive manner (sudden fracture) with loud acousticemission as the CFRP wrapping experienced excessive tension in the hoop direction, as seen inFig.8.This is the most common failure pattern observed in CFRP/epoxy/concrete systems at normalambient circumstances. None of the confined concrete specimens failed at the lap location of theCFRP wrapping demonstrating adequate load transfer at the lapped joint. The failed CFRP wrappinghad concrete bonded to it after failure, indicating adequate adhesion and proper load transfer betweenthe CFRP and concrete substrate. Confined concrete cylinders resisted load beyond failure of theconcrete core as a result of the CFRP confinement actively provided lateral support.

Specimens confined with one layer of CFRP fabric lost strength suddenly due to rupture ofthe CFRP. On the other hand, the strength loss was gradual, in more than one step, with rupture ofdifferent layers for cylinders wrapped with 2 or 3 layers. After removal of the ruptured wrap post-

test, the severely crushed state of the concrete was evident and the concrete failure plane wasgenerally conical. For the CFRP-confined cylinders, the stirrup steel was found to have fractured dueto buckling of the longitudinal steel bars.

0

5

10

15

20

25

30

0 1 2 3

A x i a l

T o u g h n e s s

( k N

. m m

/ m m )

No. of CFRP Layers

Unheated wrapped plain cylinder (PC)Heated wrapped plain cylinder (HPC)Heated wrapped RC cylinder with main bars (HRC_M)Heated wrapped RC cylinder with main bars and ties (HRC_MS)

0

1

2

3

4

5

6

7

8

0 1 2 3

L a t e r a l

T o u g h n e s s

( k N

. m m

/ m m

)

No. of CFRP Layers

Unheated wrapped plain cylinder (PC)Heated wrapped plain cylinder (HPC)Heated wrapped RC cylinder with main bars (HRC_M)Heated wrapped RC cylinder with main bars and ties (HRC_MS)


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Fig. 8: Failure patterns of specimens and CFRP rupture

4. CONCLUSION

The following conclusions can be drawn based on the results of the present experimental

study:

1. Heating of concrete to elevated temperatures followed by air cooling, as experienced in realfires, can have a very significant effect on the residual properties of the concrete. Heating to500C for one hour resulted in a 67.4% reduction in axial strength for plain concrete but wasless for concrete reinforced with steel either longitudinally or laterally.

2. CFRP sheets can be used to repair concrete damaged by exposure to high temperature.Compared with a non-heated unwrapped cylinder-PC_0 (Group 1), the residual compressivestrength ratios were found to be 76.04% and 146.88% as a result of wrapping with one and twolayers of CFRP, respectively. There is a proportional increase in the percentages of residualcompressive strength with the number of CFRP sheet layers for all other Groups.

3. The presence of axial reinforcement and/or ties reduces the impact of heating on the residualstrength of a compression member, however a significant (although lesser) loss of strengthoccurs. The addition of CFRP layers can enhance the strength and ductility of such temperaturedamaged members.

4. The lateral toughness of damaged concrete not wrapped with CFRP sheets is less than that ofconcrete undamaged by exposure to high temperature. Also the addition of CFRP layersincreases the axial toughness with this increase being proportional to the number of layers.

5. All wrapped concrete specimens failed by tensile rupture of the CFRP wrap almost in thecolumn mid-height region in a sudden explosive manner.


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5. ACKNOWLEDGEMENTS

The tests described in this paper were carried out in the Department of Civil Engineering ofthe School of Engineering in the Faculty of Engineering and Applied Science at the University ofDuhok (UoD) in the Iraqi Kurdistan Region. Thanks are due to all demonstrators in the Civil

Engineering Department at UoD for their role in collecting the experimental data.

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behavior of concrete damaged by high temperature exposureand confined with cfrp fabrics

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