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ENHANCING IMPACT RESISTANCE OF CONCRETE SLABSSTRENGTHENED WITH FRPS AND STEEL FIBERS

Doo-Yeol YooPhD candidateKorea University1, 5-ga, Anam-dong, Seongbuk-gu, Seoul, 136-713, South Koreadooyoul@korea.ac.kr

Kyung-Hwan MinPhD candidateKorea University1, 5-ga, Anam-dong, Seongbuk-gu, Seoul, 136-713, South Koreaalskh@korea.ac.kr

Jin-Young LeePhD candidateKorea University1, 5-ga, Anam-dong, Seongbuk-gu, Seoul, 136-713, South Koreatyrael21@korea.ac.kr

Young-Soo YoonProfessorKorea University1, 5-ga, Anam-dong, Seongbuk-gu, Seoul, 136-713, South Koreaysyoon@korea.ac.kr

AbstractConcrete has been extensively used as a protective structure to resist blast and impactloads for many years. Recently, to increase the impact resistance of concrete, researchrelated to the development of various strengthening methods increasing the use of steelfibers and fiber reinforced polymers (FRPs) has been actively performed. In this study,the compressive and flexural behaviors of normal strength concrete (NSC) and steelfiber reinforced concrete (SFRC), which includes 30 mm long hooked steel fibers involume fractions of 0.5, 0.75, 1.0, 1.25 and 1.5%, were evaluated under static loadingconditions. In order to investigate the flexural strengthening effect of externally bondedFRP sheets and steel fibers on one-way slabs in a high strain rate range conditions, theimpact tests were performed using a drop-weight impact testing machine. Test resultsindicated that the flexural resistance of concrete is significantly improved bystrengthening with FRP sheets and steel fibers.

Keywords: impact load, steel fiber reinforced concrete, fiber reinforced polymer, strainrate

1. IntroductionConcrete has excellent impact resistance in comparison with other constructionmaterials. Nevertheless, existing concrete structures designed without consideration ofimpact or blast load can be vulnerable under unexpected extreme loads. Accordingly, toimprove the resistance of concrete structures under the extreme loads, additionalstrengthening methods are required (Krauthammer, 2008; Malvar et al., 2007).

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Research relevant to the usage of steel fibers and FRPs to improve the structuralperformance of concrete has been conducted. Especially, recently the FRP strengtheningmethod has been in the limelight, this is because it has many advantages under highstrain rate loadings (Malvar et al., 2007; Chen et al., 2005; Buchan et al., 2007).

Generally, the damage forms of concrete by impact loads include spalling, penetration,back-face scabbing, perforation or shear failure, flexural failure etc. (Fig. 1) (Kennedy,1976; Hughes, 1984). In this case, to prevent the local and global failure of concretestructures by impact loads, impact resistance of concrete with various strengtheningmethods should be evaluated by performing experiments.

In this study, therefore, static and impact behaviors of normal strength concrete (NSC)and steel fiber reinforced concrete (SFRC), which includes steel fibers with variousvolume fractions of 0.5, 0.75, 1.0, 1.25 and 1.5%, were evaluated. Furthermore, in orderto investigate the flexural strengthening effect of externally bonded FRP sheets underimpact loads, two different types of FRP sheets (AFRP, CFRP) were applied to one-wayslabs and they were tested using a drop-weight impact testing machine.

(a) Penetration and spalling (b) Scabbing (c) Perforation (d) Overall target response

Figure 1. Damage patterns of concrete beam (Kennedy, 1976)

2. Experimental Program2.1 Materials and mix proportions

In this investigation, Type 1 Portland cement was used as the cementitious material.Crushed gravels with a maximum size of 13 mm and sea sands were used as aggregates.A liquid type superplasticizer (SP) was applied for workability. The details of the mixproportions investigated are presented in Table 1.

Table 1. Mix proportions of concreteW/C S/a Unit weight (kg/m3)(%) (%) Water Cement Fine agg. Coarse agg. SP35 45 236 675 592 733 2.36

In order to assess the static and impact behaviors of concrete including steel fibers,0%~1.5% (by volume) of 30 mm long hooked steel fibers were added. Table 2summarizes the material properties of the steel fiber used in the test specimens. Theflexural strengthening effect of two different FRPs (AFRP and CFRP) externallybonded with epoxy resin was also investigated under impact loading. The mechanicalproperties of the fibers and resin are summarized in Table 3.

Table 2. Properties of end-hooked steel fiber

Type fl(mm)

Diameter(mm)

Aspect ratio( ff dl / )

Density(kg/m3)

tf(MPa)

tE(GPa)

Ultimateelongation (%)

Hooked fiber 30.0 0.5 60.0 7.9 1,196 200 0.6where, fl =fiber length, fd =fiber diameter, tf =tensile strength, and tE =elastic modulus

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Table 3. Properties of fibers and resin (FRP sheet)Fiber & Resin Aramid Carbon EpoxyTensile strength (MPa) 2,880 4,900 54Elastic modulus (GPa) 120 250 3.0Ultimate strain (%) 2.9 2.1 8.0Thickness (mm) 0.194 0.111 -Density (kg/m3) - - 1200

2.2 Test procedure

2.2.1 Static loading tests (compressive and flexural strengths)

Cylindrical specimens of 100 mm diameter and 200 mm length were produced tomeasure compressive strength (ASTM C 39) by using a universal testing machine(UTM) with a maximum load capacity of 2,500 kN. The flexural strength (ASTM C1609) was estimated by a 4-point loading test. It was performed under displacementcontrol at a loading rate of 0.01 mm/s.

2.2.2 Impact loading test

A series of prismatic specimens with dimension of 50×100×350 mm3 were made for theimpact flexural test. Various volume fractions of steel fibers were used ranging from0.5% to 1.5% and two different unidirectional FRPs (AFRP and CFRP) were used tostrengthen the longitudinal direction of the specimens in order to evaluate impactresistance. Especially, in order to prevent overestimation of the benefit of FRPstrengthening, the FRPs were only bonded on the surface of the specimens in clear-span(=300 mm) (Fig. 2). The concrete was stored in water at 20±3°C immediately afterdemoulding, and after 14 days the FRPs were adhered and then cured at 50% relativehumidity and at a temperature of 20°C until testing.

An impact test was carried out using a drop-weight test machine with a maximumcapacity of 800 Joules. Impact load and velocity were measured by a load cell andspeedometer affixed to the drop weight tup, respectively. The striking face of the dropweight was spherical with a radius of curvature of 25 mm. A single impact load wasapplied to the mid-span of the specimens by dropping a free-falling 12.965 kg dropweight from a drop height of 1,045 mm. The potential energy and average impactvelocity were about 133 J(kg·m2/s2) and 4.5 m/s, respectively. All specimens weresupported and fixed by a steel frame at a point 25 mm inside the ends as shown in Fig. 3.

Non AFRP CFRP

Cle

ar s

pan

= 30

0mm

Support

300 mm

100 mm

P

50 mm(thickness)

One way SlabSpecimen

SteelFrame

Figure 2. Test specimens Figure 3. Test set up for one-way slab specimens

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3. Results and Discussions3.1 Static loading tests (compressive and flexural strengths)

The strength test results are summarized in Table 4. Compressive strength was slightlyreduced by about 1.5%~13% due to the addition of fibers. This results from theinhomogeneous distribution of the steel fibers within the concrete. On the contrary,flexural strength linearly increased by about 0.2%~24% caused by the bridging effect offibers against the further development of cracks.

Table 4. Mechanical properties of NSC and SFRC

Series Volume of fiber (%) Compressive strength (MPa) Flexural strength (MPa)NSC 0.00 53.20 6.02

0.50 52.43 6.030.75 50.41 6.33

SFRC 1.00 49.59 6.381.25 46.49 7.031.50 50.00 7.49

3.2 Impact loading test

Fig. 4 shows a comparison of the load-time relationships of the concrete slabsstrengthened with AFRP (AS) and CFRP (CS) with the non-strengthened slab (NS). Inthe case of the NS specimen, the impact load sharply dropped after the peak load due toplastic hinge formation at the mid-span of the slab. The impact load bearing capacity ofthe CS and AS specimens was increased by about 30% after FRP strengthening and theimpact load gradually declined after peak load.

As shown in Fig. 5(a), the peak loads of the AS and CS specimens were about 19%higher than that of the NS specimen, this was increased as a higher volume of steelfibers were used. The maximum deflection measured by the drop weight tup decreasedby about 34% due to the FRP strengthening effect (Fig. 5(b)). It also sharply declinedwith the addition of steel fibers from 0.5% to 1.0% (by volume) and graduallydecreased afterward (Vf=1.0%~1.5%).

-10

0

10

20

30

40

0 1 2 3 4

Load

(kN

)

Time (ms)

NS-0.75%

AS-0.75%

CS-0.75%

Figure 4. Tup load-time curve of SFRC slabs (with a volume fraction of 0.75%)with and without FRP sheets

5

0

10

20

30

40

50

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Peak

load

(kN

)

Volume fraction of steel fiber (%)

NS AS CS

y = -6.377x + 28.14R² = 0.987

y = -1.405x + 23.27R² = 0.999

0

5

10

15

20

25

30

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Max

. def

lect

ion

(mm

)

Volume fraction of steel fiber (%)

NS AS CS

(a) Peak load (b) Maximum deflection by tupFigure 5. Influence of steel fibers and FRP sheets to impact resistance

All specimens, except the NS-0.5%, 0.75% and 1.0%, were failed by second strike ofthe drop weight. The average measured impact energy was 132.4 J, which is verysimilar to the potential energy (≈133 J). For the AS and CS specimens the dissipatedenergy, which is equal to the area of the hysteresis loop in the load–deflection curve,were about 2.3~2.7 times higher than that of the NS specimen due to the FRPstrengthening effect (Fig. 6). It should be noticed that the externally bonded FRP sheetsresulted in ductile behavior of concrete slabs after a blow of the drop weight and, assuch, a considerable portion of the impact energy was adsorbed. The dissipated energyduring the second blow was substantially reduced by the flexural cracks, FRPdebonding, spalling, and so on.

0

20

40

60

80

100

120

140

NS AS CS

Dis

sipa

ted

ener

gy (

J)

Specimen

0.50%0.75%1.00%1.25%1.50%

0

20

40

60

80

100

120

140

NS AS CS

Dis

sipa

ted

ener

gy (

J)

Specimen

0.50%0.75%1.00%1.25%1.50%

(a) 1st blow (b) 2nd blowFigure 6. Dissipated energy

Fig. 7 shows failure patterns of the NS, AS and CS specimens. In the case of the NSspecimen, it was failed due to the formation of plastic hinges at the center and both ends.The reason why plastic hinges were formed at both ends is that the steel framerestrained the rigid body rotation. Even though the AS and CS specimens also hadplastic hinge formation at both ends, they are more likely to fail by a flexural crack atthe center with FRP debonding.

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(a) NS specimen

(b) AS specimen

(c) CS specimenFigure 7. Failure patterns of SFRC specimens strengthened with FRP sheets on top surface (Left)

and side surface (Right)

In order to prevent the distortion of the mid-span deflection by detachment of the LVDTfrom the specimen, it was fixed underneath the center of the specimen using epoxy. Thedeflection of the NS specimens could not be measured because most of the specimenscompletely failed after the first blow. As shown in Fig. 8, the AS and CS specimenshave similar deflection behaviors. The deflection was almost linearly increased up to amaximum value and thereafter was decreased by the negative moment from the endrestraints. A negative deflection indicating an upward movement, however, did notoccur due to wide flexural cracks and FRP debonding.

0

5

10

15

20

0 25 50 75 100 125 150

Def

lect

ion

(mm

)

Time (ms)

S-1.50%

S-0.50%

S-1.00%

S-0.75%

S-1.25%

0

5

10

15

20

0 25 50 75 100 125 150

Def

lect

ion

(mm

)

Time (ms)

S-1.50%

S-0.50%

S-1.00%

S-0.75%

S-1.25%

(a) AS specimen (b) CS specimenFigure 8. Mid-span deflection-time curve from center LVDT

The specimen with a higher volume of steel fibers had a lower maximum deflection(Fig. 9(a)). Over 20% lower maximum deflections were observed in the AS specimencompared to the CS specimen. Fig. 9(b) shows the residual deflections of the AS and CSspecimens. Although these have wide variations, the residual deflections were reducedas the volume of steel fiber increases. The average residual deflections of the AS and CSspecimens were about 6.60 mm and 7.87 mm, respectively.

7

0

5

10

15

20

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Max

. def

lect

ion

(mm

)

Volume fraction of steel fiber (%)

AS CS0

5

10

15

20

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Res

idua

l d

efle

ctio

n (m

m)

Volume fraction of steel fiber (%)

AS CS

Ave.=7.87mm

Ave.=6.60mm

(a) Maximum deflection (b) Residual deflectionFigure 9. Comparison of mid-span deflection behaviors of AS and CS specimens

Fig. 10 shows the bending load versus center deflection curve. All specimens have anegative (-) load after the peak load due to the end restraint from steel frame. However,the negative (-) load was very small due to the FRP debonding and the wide flexuralcracks. The residual load measured in the drop weight tup was, on average, about 4 kN.

From the test result, the deflections measured in the drop weight tup and LVDT showeddifferent behaviors (Fig. 11). When measured by using the LVDT, the maximumdeflection was lower than that of the tup deflection, and the time to reach maximumdeflection was delayed. This was caused by the penetration of the drop weight withspalling.

-10

0

10

20

30

40

50

0 1 2 3 4

Load

(kN

)

Deflection (mm)

S-1.50%

S-0.50%

S-1.00%S-0.75%

S-1.25%-10

0

10

20

30

40

50

0 1 2 3 4

Load

(kN

)

Deflection (mm)

S-1.00%

S-0.50%

S-1.25%S-0.75%

S-1.50%

(a) AS spcimen (b) CS spcimenFigure 10. Bending load-deflection curve for concrete slab with various FRP strengthenings

-5

0

5

10

15

20

0 5 10 15 20

Def

lect

ion

(mm

)

Time (ms)

Tup deflection

Maximum deflections

Center deflectionby LVDT

Time delay

Figure 11. Comparison of deflection-time curve by tup and LVDT(AS specimen with a volume fraction of 1.25%)

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The strength of the concrete increased as a higher strain rate was applied. Normally, theincrease of strength is calculated using a dynamic increasing factor (DIF). Therefore, inthis study, the maximum compressive strain rate was measured by a strain gage attachedon the top surface of the specimen and determined from the strain versus time history.

The maximum observed strain rates are shown in Table 5. The maximum strain rates atthe top extreme fiber of the slabs were approximately dε/dt= 0.19 s-1~0.33 s-1 and theseincreased as a higher volume of steel fiber was added.

Table 5. Maximum observed strain rate (dε/dt)

Series Volume of fiber (%) Blow Max. dε/dt (s-1)

NS

0.50

1

0.1900.75 0.1991.00 0.1901.25 0.3321.50 0.246

4. ConclusionsThe following conclusions were drawn from the experimental test results of the staticand impact tests of NSC and SFRC strengthened with and without FRP sheets:

1) The compressive strength of concrete was reduced by the addition of steel fibers,whereas the flexural strength was linearly increased.

2) In the case of the AS and CS specimens, the peak impact loads were about 19%higher than that of the NS specimen and the maximum deflections of the tup weredecreased by about 34% due to the strengthening effect of AFRP and CFRP sheets.

3) About 2.3~2.7 times higher impact energy was dissipated by strengthening withAFRP and CFRP sheets compared to the NS specimen.

4) The maximum mid-span deflection and residual deflection of the CS specimen wereabout 1.3 times higher than those of the AS specimen. This indicates that AFRP sheetgives better impact resistance performance with SFRC than CFRP sheet.

AcknowledgementsThis work was supported by the National Research Foundation of Korea (NRF) grantfunded by the Korea government (MEST) (No. 2007-0056796).

References[1] Krauthammer, T., Modern Protective Structures, CRC Press, New York, 2008.[2] Malvar, L. J., Crawford, J. E., and Morrill, K. B., Use of Composites to Resist Blast, Journal of

Composites for Construction, Vol. 11, No. 6, 2007, pp. 601~610.[3] Chen, C. C. and Li, C. Y., Punching Shear Strength of Reinforced Concrete Slabs Strengthened

with Glass Fiber Reinforced Polymer Laminates, ACI Structural Journal, Vol. 102, No. 4, 2005, pp.535~542.

[4] Buchan, P. A. and Chen, J. F., Blast Resistance of FRP Composites and Polymer StrengthenedConcrete and Masonry Structures–A State-of-the-art Review, Composite Part B: Engineering, Vol.38, Nos. 5-6, 2007, pp. 509~522.

[5] Kennedy, R. P., A Review of Procedures for the Analysis and Design of Concrete Structures toResist Missile Impact Effects, Nuclear Engineering and Design, Vol. 37, No. 2, 1976, pp.183~203.

[6] Hughes, G., Hard Missile Impact on Reinforced Concrete, Nuclear Engineering and Design, Vol.77, No. 1, 1984, pp.23~35.