ENHANCING IMPACT RESISTANCE OF CONCRETE SLABS STRENGTHENED WITH FRPS AND STEEL FIBERS

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1 ENHANCING IMPACT RESISTANCE OF CONCRETE SLABS STRENGTHENED WITH FRPS AND STEEL FIBERS Doo-Yeol Yoo PhD candidate Korea University 1, -ga, Anam-dong, Seongbuk-gu, Seoul, , South Korea Kyung-Hwan Min PhD candidate Korea University 1, -ga, Anam-dong, Seongbuk-gu, Seoul, , South Korea Jin-Young Lee PhD candidate Korea University 1, -ga, Anam-dong, Seongbuk-gu, Seoul, , South Korea Young-Soo Yoon Proessor Korea University 1, -ga, Anam-dong, Seongbuk-gu, Seoul, , South Korea Abstract Concrete has been extensively used as a protective structure to resist blast and impact loads or many years. Recently, to increase the impact resistance o concrete, research related to the development o various strengthening methods increasing the use o steel ibers and iber reinorced polymers (FRPs) has been actively perormed. In this study, the compressive and lexural behaviors o normal strength concrete (NSC) and steel iber reinorced concrete (SFRC), which includes 3 mm long hooked steel ib ers in volume ractions o.,.7, 1., 1.2 and 1.%, were evaluated under static loading conditions. In order to investigate the lexural strengthening eect o externally bonded FRP sheets and steel ibers on one-way slabs in a high strain rate range conditions, the impact tests were perormed using a drop-weight impact testing machine. Test results indicated that the lexural resistance o concrete is signiicantly improved by strengthening with FRP sheets and steel ibers. Keywords: impact load, steel iber reinorced concrete, iber reinorced polymer, strain rate 1. Introduction Concrete has excellent impact resistance in comparison with other construction materials. Nevertheless, existing concrete structures designed without consideration o impact or blast load can be vulnerable under unexpected extreme loads. Accordingly, to improve the resistance o concrete structures under the extreme loads, additional strengthening methods are required (Krauthammer, 8; Malvar et al., 7). 1

2 Research relevant to the usage o steel ibers and FRPs to improve the structural perormance o concrete has been conducted. Especially, recently the FRP strengthening method has been in the limelight, this is because it has many advantages under high strain rate loadings (Malvar et al., 7; Chen et al., ; Buchan et al., 7). Generally, the damage orms o concrete by impact loads include spalling, penetration, back-ace scabbing, peroration or shear ailure, lexural ailure etc. (Fig. 1) (Kennedy, 1976; Hughes, 1984). In this case, to prevent the local and global ailure o concrete structures by impact loads, impact resistance o concrete with various strengthening methods should be evaluated by perorming experiments. In this study, thereore, static and impact behaviors o normal strength concrete (NSC) and steel iber reinorced concrete (SFRC), which include s steel ibers with various volume ractions o.,.7, 1., 1.2 and 1.%, were evaluated. Furthermore, in order to investigate the lexural strengthening eect o externally bonded FRP sheets under impact loads, two dierent types o FRP sheets (AFRP, CFRP) were applied to one-way slabs and they were tested using a drop-weight impact testing machine. (a) Penetration and spalling (b) Scabbing (c) Peroration (d) Overall target response 2. Experimental Program Figure 1. Damage patterns o concrete beam (Kennedy, 1976) 2.1 Materials and mix proportions In this investigation, Type 1 Portland cement was used as the cementitious material. Crushed gravels with a maximum size o 13 mm and sea sands were used as aggregates. A liquid type superplasticizer (SP) was applied or workability. The details o the mix proportions investigated are presented in Table 1. Table 1. Mix proportions o concrete W/C S/a Unit weight (kg/m 3 ) (%) (%) Water Cement Fine agg. Coarse agg. SP In order to assess the static and impact behaviors o concrete including steel ibers, %~1.% (by volume) o 3 mm long hooked steel ibers were added. Table 2 summarizes the material properties o the steel iber used in the test specimens. The lexural strengthening eect o two dierent FRPs (AFRP and CFRP) externally bonded with epoxy resin was also investigated under impact loading. The mechanical properties o the ibers and resin are summarized in Table 3. Table 2. Properties o end-hooked steel iber Type l Diameter Aspect ratio Density (mm) (mm) ( l / d ) (kg/m 3 t E t Ultimate ) (MPa) (GPa) elongation (%) Hooked iber ,196.6 where, l =iber length, d =iber diameter, =tensile strength, and E =elastic modulus t t 2

3 Table 3. Properties o ibers and resin (FRP sheet) Fiber & Resin Aramid Carbon Epoxy Tensile strength (MPa) 2,88 4,9 4 Elastic modulus (GPa) Ultimate strain (%) Thickness (mm) Density (kg/m 3 ) Test procedure Static loading tests (compressive and lexural strengths) Cylindrical specimens o 1 mm diameter and mm length were produced to measure compressive strength (ASTM C 39) by using a universal testing machine (UTM) with a maximum load capacity o 2, kn. The lexural strength (ASTM C 169) was estimated by a 4-point loading test. It was perormed under displacement control at a loading rate o.1 mm/s Impact loading test A series o prismatic specimens with dimension o 1 3 mm 3 were made or the impact lexural test. Various volume ractions o steel ibers were used ranging rom.% to 1.% and two dierent unidirectional FRPs ( AFRP and CFRP) were used to strengthen the longitudinal direction o the specimens in order to evaluate impact resistance. Especially, in order to prevent overestimation o the beneit o FRP strengthening, the FRPs were only bonded on the surace o the specimens in clear-span (=3 mm) (Fig. 2). The concrete was stored in water at ±3 C immediately ater demoulding, and ater 14 days the FRPs were adhered and then cured at % relative humidity and at a temperature o C until testing. An impact test was carried out using a drop-weight test machine with a maximum capacity o 8 Joules. Impact load and velocity were measured by a load cell and speedometer aixed to the drop weight tup, respectively. The striking ace o the drop weight was spherical with a radius o curvature o 2 mm. A single impact load was applied to the mid-span o the specimens by dropping a ree-alling kg drop weight rom a drop height o 1,4 mm. The potential energy and average impact velocity were about 133 J(kg m 2 /s 2 ) and 4. m/s, respectively. All specimens were supported and ixed by a steel rame at a point 2 mm inside the ends as shown in Fig. 3. Support P Non AFRP CFRP Clear span = 3mm One way Slab Specimen Steel Frame mm (thickness) 1 mm 3 mm Figure 2. Test specimens Figure 3. Test set up or one-way slab specimens 3

4 3. Results and Discussions 3.1 Static loading tests (compressive and lexural strengths) The strength test results are summarized in Table 4. Compressive strength was slightly reduced by about 1.%~13% due to the addition o ibers. This results rom the inhomogeneous distribution o the steel ibers within the concrete. On the contrary, lexural strength linearly increased by about.2%~24% caused by the bridging eect o ibers against the urther development o cracks. Table 4. Mechanical properties o NSC and SFRC Series Volume o iber (%) Compressive strength (MPa) Flexural strength (MPa) NSC SFRC Impact loading test Fig. 4 shows a comparison o the load-time relationships o the concrete slabs strengthened with AFRP (AS) and CFRP (CS) with the non-strengthened slab (NS). In the case o the NS specimen, the impact load sharply dropped ater the peak load due to plastic hinge ormation at the mid-span o the slab. The impact load bearing capacity o the CS and AS specimens was increased by about 3% ater FRP strengthening and the impact load gradually declined ater peak load. As shown in Fig. (a), the peak loads o the AS and CS specimens were about 19% higher than that o the NS specimen, this was increased as a higher volume o steel ibers were used. The maximum delection measured by the drop weight tup decreased by about 34% due to the FRP strengthening eect (Fig. (b)). It also sharply declined with the addition o steel ibers rom.% to 1.% (by volume) and gradually decreased aterward (V =1.%~1.%). 4 Load (kn) 3 1 CS-.7% AS-.7% -1 NS-.7% Time (ms) Figure 4. Tup load-time curve o SFRC slabs (with a volume raction o.7%) with and without FRP sheets 4

5 Peak load (kn) Volume raction o steel iber (%) NS AS CS Max. delection (mm ) y = x R² =.987 y = -1.4x R² = Volume raction o steel iber (%) NS AS CS (a) Peak load (b) Maximum delection by tup Figure. Inluence o steel ibers and FRP sheets to impact resistance All specimens, except the NS-.%,.7% and 1.%, were ailed by second strike o the drop weight. The average measured impact energy was J, which is very similar to the potential energy ( 133 J). For the AS and CS specimens the dissipated energy, which is equal to the area o the hysteresis loop in the load delection curve, were about 2.3~2.7 times higher than that o the NS specimen due to the FRP strengthening eect (Fig. 6). It should be noticed that the externally bonded FRP sheets resulted in ductile behavior o concrete slabs ater a blow o the drop weight and, as such, a considerable portion o the impact energy was adsorbed. The dissipated energy during the second blow was substantially reduced by the lexural cracks, FRP debonding, spalling, and so on. Dissipated energy (J) %.7% 1.% 1.2% 1.% Dissipated energy (J) %.7% 1.% 1.2% 1.% NS AS CS NS AS CS Specimen Specimen (a) 1 st blow (b) 2 nd blow Figure 6. Dissipated energy Fig. 7 shows ailure patterns o the NS, AS and CS specimens. In the case o the NS specimen, it was ailed due to the ormation o plastic hinges at the center and both ends. The reason why plastic hinges were ormed at both ends is that the steel rame restrained the rigid body rotation. Even though the AS and CS specimens also had plastic hinge ormation at both ends, they are more likely to ail by a lexural crack at the center with FRP debonding.

6 (a) NS specimen (b) AS specimen (c) CS specimen Figure 7. Failure patterns o SFRC specimens strengthened with FRP sheets on top surace (Let) and side surace (Right) In order to prevent the distortion o the mid-span delection by detachment o the LVDT rom the specimen, it was ixed underneath the center o the specimen using epoxy. The delection o the NS specimens could not be measured because most o the specimens completely ailed ater the irst blow. As shown in Fig. 8, the AS and CS specimens have similar delection behaviors. The delection was almost linearly increased up to a maximum value and thereater was decreased by the negative moment rom the end restraints. A negative delection indicating an upward movement, however, did not occur due to wide lexural cracks and FRP debonding. Delection (mm ) 1 1 S-.7% S-.% S-1.2% S-1.% S-1.% Delection (mm ) 1 1 S-.% S-.7% S-1.2% S-1.% S-1.% Time (ms) Time (ms) (a) AS specimen (b) CS specimen Figure 8. Mid-span delection-time curve rom center LVDT The specimen with a higher volume o steel ibers had a lower maximum delection (Fig. 9(a)). Over % lower maximum delections were observed in the AS specimen compared to the CS specimen. Fig. 9(b) shows the residual delections o the AS and CS specimens. Although these have wide variations, the residual delections were reduced as the volume o steel iber increases. The average residual delections o the AS and CS specimens were about 6.6 mm and 7.87 mm, respectively. 6

7 Max. delection (mm) 1 1 AS CS Residual delection (mm) 1 1 Ave.=7.87mm Ave.=6.6mm AS CS Volume raction o steel iber (%) Volume raction o steel iber (%) (a) Maximum delection (b) Residual delection Figure 9. Comparison o mid-span delection behaviors o AS and CS specimens Fig. 1 shows the bending load versus center delection curve. All specimens have a negative (-) load ater the peak load due to the end restraint rom steel rame. However, the negative ( -) load was very small due to the FRP debonding and the wide lexural cracks. The residual load measured in the drop weight tup was, on average, about 4 kn. From the test result, the delections measured in the drop weight tup and LVDT showed dierent behaviors (Fig. 11). When measured by using the LVDT, the maximum delection was lower than that o the tup delection, and the time to reach maximum delection was delayed. This was caused by the penetration o the drop weight with spalling. 4 4 Load (kn) 3 1 S-1.% S-.% S-.7% S-1.% Load (kn) 3 1 S-.% S-.7% S-1.2% -1 S-1.2% S-1.% S-1.% Delection (mm) Delection (mm) (a) AS spcimen (b) CS spcimen Figure 1. Bending load-delection curve or concrete slab with various FRP strengthenings Delection (mm ) 1 1 Tup delection Maximum delections Time delay Center delection by LVDT Time (ms) Figure 11. Comparison o delection-time curve by tup and LVDT (AS specimen with a volume raction o 1.2%) 7

8 The strength o the concrete increased as a higher strain rate was applied. Normally, the increase o strength is calculated using a dynamic increasing actor (DIF). Thereore, in this study, the maximum compressive strain rate was measured by a strain gage attached on the top surace o the specimen and determined rom the strain versus time history. The maximum observed strain rates are shown in Table. The maximum strain rates at the top extreme iber o the slabs were approximately dε/dt=.19 s -1 ~.33 s -1 and these increased as a higher volume o steel iber was added. 4. Conclusions Table. Maximum observed strain rate (dε/dt) Series Volume o iber (%) Blow Max. dε/dt (s -1 ) NS The ollowing conclusions were drawn rom the experimental test results o the static and impact tests o NSC and SFRC strengthened with and without FRP sheets: 1) The compressive strength o concrete was reduced by the addition o steel ibers, whereas the lexural strength was linearly increased. 2) In the case o the AS and CS specimens, the peak impact loads were about 19% higher than that o the NS specimen and the maximum delections o the tup were decreased by about 34% due to the strengthening eect o AFRP and CFRP sheets. 3) About 2.3~2.7 times higher impact energy was dissipated by strengthening with AFRP and CFRP sheets compared to the NS specimen. 4) The maximum mid-span delection and residual delection o the CS specimen were about 1.3 times higher than those o the AS specimen. This indicates that AFRP sheet gives better impact resistance perormance with SFRC than CFRP sheet. Acknowledgements This work was supported by the National Research Foundation o Korea (NRF) grant unded by the Korea government (MEST) (No ). Reerences [1] Krauthammer, T., Modern Protective Structures, CRC Press, New York, 8. [2] Malvar, L. J., Craword, J. E., and Morrill, K. B., Use o Composites to Resist Blast, Journal o Composites or Construction, Vol. 11, No. 6, 7, pp. 61~61. [3] Chen, C. C. and Li, C. Y., Punching Shear Strength o Reinorced Concrete Slabs Strengthened with Glass Fiber Reinorced Polymer Laminates, ACI Structural Journal, Vol. 12, No. 4,, pp. 3~42. [4] Buchan, P. A. and Chen, J. F., Blast Resistance o FRP Composites and Polymer Strengthened Concrete and Masonry Structures A State-o-the-art Review, Composite Part B: Engineering, Vol. 38, Nos. -6, 7, pp. 9~22. [] Kennedy, R. P., A Review o Procedures or the Analysis and Design o Concrete Structures to Resist Missile Impact Eects, Nuclear Engineering and Design, Vol. 37, No. 2, 1976, pp.183~3. [6] Hughes, G., Hard Missile Impact on Reinorced Concrete, Nuclear Engineering and Design, Vol. 77, No. 1, 1984, pp.23~3. 8