STRUCTURAL PERFORMANCE CAPACITY EVALUATION OF RECYCLED PET

FIBER ADDED CONCRETE

J.H.J. Kim 1*, H.Y. Kim 1, S.B. Kim 1, N.H. Yi 1 and K.S. Lee 1 1 School of the Civil and Environmental Engineering,

Yonsei University, Korea *Email: jjhkim@yonsei.ac.kr

ABSTRACT This study was performed to prove the possibility of utilizing short plastic fibers made for recycled polyethylene terephthalate (RPET) as a structural material. In order to verify the performance capacity of RPET fiber added concrete as a structural material, it was compared with the performance capacity of polypropylene (PP) fiber added concrete for fiber volume fractions of 0.5%, 0.75%, and 1.0%. To measure the strength and ductility capacities of reinforced concrete (RC) member casted with RPET fiber added concrete, flexural test was performed on RC beams. For structural member performance, ultimate strength, relative ductility and energy absorption of RPET added RC beam are significantly larger than OPC specimen. Also, the results showed that ultimate flexural strength and ductility both increased, as fiber volume fraction increased. These trends are similarly observed in the tests of PP fiber added concrete specimens. The study results indicate that RPET fiber can be used as an effective additional reinforcing material in concrete members. KEYWORDS RPET fiber, PP fiber, fiber volume fraction, flexural strength, ductility, energy absorption. INTRODUCTION Concrete is the most widely used construction material in the world due to its high compressive strength, long service life, and low cost. However, concrete has inherent disadvantages of low tensile strength and crack resistance. In order to improve weaknesses of the material, numerous studies on fiber reinforced concrete have been performed. The research results showed that addition of fibers drastically improves the performance of concrete and negates its disadvantages (Banthia and Sheng 1996). Polyethylene terephthalate (PET) is one of the most important and extensively used plastics in the world, especially for manufacturing beverage container. The current production of PET exceeds 6.7million tons/year in the world and shows a skyrocketing increase in Asian region recently due to the increasing demands in China and India (JVEC 2008). In Korea, the production of PET bottle has grown to 130 thousand tons/year (KPCA 2008). However, most of PET bottles used for beverage container are thrown away after single usage and disposed PET bottles are treated by landfill and incineration, which are causing serious environmental problems (Korean Institute of Resources Recycling 2008). In order to recycle PET wastes, additional expenses are required for reprocessing. Also, color change and purity degradation of the recycled PET bottle limit the usage of recycled PET (RPET) plastics for manufacturing new products (Rebeiz et al. 1993). Thus, a more effective solution, requiring less expense is needed for PET bottle wastes. One possible solution is using RPET as reinforcing short fibers in structural concrete. It can provide greater crack control and ductility enhancement capacities for quasi-brittle concrete as well as mass consumption alternative, which is a very important issue in the merit of recycling wasted materials. The current applications of RPET in construction industry is using them as resin for polymer concrete and synthetic coarse aggregate for lightweight concrete (Rebeiz et al. 1993). These types of application, however, have usage limitation in construction industry due to a difficulty of mass consumption. In order to overcome the limitation, numerous studies have been undertaken (Kim et al. 2008). One type of solution to this limitation is using RPET as reinforcing short fibers in concrete to improve structural performance. In this study, experimental studies on flexural ductility and ultimate flexural strength capacities, as well as failure mode of RPET fiber added RC beam have been performed. Based on the study results, the possibility of using RPET as a structural material is evaluated.

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MATERIAL CONTENTS AND MIX PROPORTION

Cement and aggregates Ordinary Portland cement with a density of 3.15g/cm2 and Blaine fineness of 3,488cm2/g was used in the study. Crushed gravel with maximum size of 25mm was used for coarse aggregate, river sand for fine aggregate, and AE water reducing agent for admixture. Recycled PET fiber In this study, RPET plastic sheet produced from waste PET bottles is used. RPET plastic sheet is slited into thin strands, producing straight type RPET continuous fibers using slitting machine. However, straight plastic fiber type has low bond strength with concrete matrix, reducing intended crack resisting ability of fiber concrete. In order to improve bond strength between RPET fibers and concrete matrix, surface configuration is applied on RPET fibers using deforming machine. Various surface configurations can be applied to RPET fiber using different gear types in deforming machine. Generally, the manufactured types of fiber are crimped, twisted, and embossed types. For this study, embossed type was used over other types for its superior bond properties (Kim el al. 2008). After RPET strands are applied with a desired surface configuration, the surface of RPET fiber is coated with maleic anhydride grafted polypropylene. The extra surface coating allows extra bonding strength of RPET fiber (Kim el al. 2008). Afterward, the strands are chopped into desired fiber length using chopping machine. Figure 1 shows the manufacturing system of RPET fibers.

(a) RPET fiber

(b) PP fiber

Figure 1. Manufacturing system of recycled PET fibers Figure 2. Geometry of the synthetic fiber

To verify the properties of RPET fiber, RPET fiber is compared with similar size PP fiber. It is important to note that PP fiber used in the study is sold in the market and the surface configuration is crimped type. Figure 2 show photos of manufactured RPET and purchased PP fibers used in the study, respectively. The basic properties of both types of fibers used for the study are shown in Table 1.

Table 1. Properties of synthetic fibers

Fibers Type Dimension (mm)

Length (mm)

Density (g/cm3)

Elastic modulus (MPa)

Tensile strength (MPa)

Ultimate elongation

(%) RPET Embossed 0.2ⅹ1.3 50 1.38 10,175.4 420.7 11.2

PP Crimped 0.38ⅹ0.9 50 0.91 6,000.0 550.0 15.0 Mix proportions of recycled PET fiber added concrete Mixture proportion of concrete with 0.41 water/binder ratio (W/B) and 4.5±1.5% air content was used. PP and RPET fibers were added to the concrete. The primary variables in this study are (a) synthetic fiber type and (b) fiber volume fraction. Also, from our previous trial tests as mentioned before, the optimal fiber volume fraction of RPET ranges from 0.5 % to 1%. According to many research reports, fiber volume fraction of short synthetic fibers usually ranges from 0.25 % to 2% (Hannant 1978). Hence, fiber volume fractions for this study were chosen as 0.5%, 0.75% and 1.0%. The concrete mix proportion used in this experiment is tabulated in Table 2.

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Table 2. Mix proportion of concrete

Specimens W/B (%)

S/a (%)

Unit weight (kg/m3) Fiber volume fraction (%) C FA W S G AE RPET PP

OPC

0.41 43.8 355 40 161 775 994 2.37

- - RPET 0.5 0.5 - RPET 0.75 0.75 - RPET 1.0 1.0 -

PP 0.5 - 0.5 PP 0.75 - 0.75 PP 1.0 - 1.0

STRUCTURAL MEMBER CAPACITY TEST In order to simulate the most realistic structural stress conditions, the specimen was designed to apply both shear and flexure stresses using length to depth ratio of 2.6. Also, the specimen was tested with 4 point loadings to enforce this condition. However, the specimen was designed to fail by flexural failure rather than shear failure to calculate the overall ductility and energy absorbing capacities. Flexural strength test A total of 7 specimens are casted for the experiment. All specimens were tested after 28 days from casting. Specimen was reinforced with three D13 bars (diameter of 13mm) as tensile reinforcements and 10mm diameter shear stirrups with 150mm spacing. Figure 3 shows the dimensions and details of the beam specimens. Figure 4 shows the test set-up. RC beam specimens with hinge-roller supports were tested by UTM with a maximum load capacity of 2000kN. The load was applied as stroke control loading. The rate of vertical displacement at the mid-span was 0.025 mm/sec. Before loading, a strain gauge was attached at the bottom surface of the concrete beam to measure the tensile strain. To obtain an accurate deflection reading, a Linear Variable Differential Transducer (LVDT) was mounted at the mid-span. Crack initiation and propagation were monitored by visual inspection during testing, and the crack patterns were marked

Figure 3. Dimensions and details of test beam (unit: mm) Figure 4. Set-up of test specimen

Experimental results and discussion

Load-deflection results The measured load-deflection relations from RC flexural tests are shown in Figure 5. The results showed that elastic behavior of all of the specimens before cracking were similar even though the fiber volume fractions were different. Before the yielding of tensile reinforcements, initiation of cracks occurred earlier in the fiber added concrete specimens than in the OPC concrete specimen. However, once the rebar yields, the fiber added concrete showed better crack resistance and strain-hardening capacities than the OPC concrete specimen. The reason for these types of pre- and post-yielding behaviors of the RC member is due to the fiber-matrix bonding characteristic. In uncracked specimen, the softer material characteristic of fiber concrete compare to that of regular concrete causes cracks to initiate easier, causing more cracks to form in fiber concrete than regular concrete. However, once the cracks form, the fiber concrete matrix bonding capacity resists the opening of tensile cracks, making the cracked member’s tension toughness to increase. Due to the increase in crack resisting and controlling capacities, the cover concrete crack spalling and reinforcement debonding failure due to macro-crack propagation are delayed, which stabilizes the overall behavior of the structural RC member. In turn, this delay increases the overall flexural strength and ductility (equivalent to energy absorbing capacity) of the RC member. This trend has been report by Lin et al. (1999), where fiber added concrete resists tensile stresses by crack bridging and fiber pull-out resistance using fiber concrete bonding (Lin et al. 1999).

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(a) RPET fiber (b) PP fiber

Figure 5. Comparison of the load-deflection curves for OPC, RPET and PP at the age of 28days The calculated flexural strength from the test data is tabulated in Table 3. As shown in Table 3, the RPET specimens with fiber volume fraction of 0.5%, 0.75%, and 1.0% had 25%, 31%, and 32% of ultimate strength increase, respectively, compare to the OPC concrete specimen. Also, the PP fiber added concrete specimens showed similar trends observed in RPET fiber added concrete specimens.

Table 3. Summarized flexural strength test results

Specimens Pcr (kN)

Δcr (mm)

Py (kN)

Δy (mm

Pu (kN)

Increased strength (%)

Δu (mm)

OPC 32.6 1.01 101.4 4.80 121.6 100 16.94 RPET 0.5 24.8 0.43 109.0 4.52 152.6 125.5 165.0 RPET 0.75 22.0 0.51 108.8 4..67 159.8 131.4 141.36 RPET 1.0 32.4 0.62 107.8 5.31 160.4 131.9 143.36

PP 0.5 32.8 0.75 108.2 4.93 154.0 127.0 140.07 PP 0.75 28.0 0.69 106.3 5.21 150.4 123.7 149.16 PP 1.0 25.6 0.61 106.6 4.71 156.6 128.6 144.22

Cracking modes for the beams Crack distributions at failure for all of the tested specimens are show in Figure 6. In Figure 6, solid and dotted crack lines represent cracks that are formed during pre- and post yielding stages of rebars, respectively. The control specimen of OPC specimen showed a general RC flexural failure behavior as expected. A general flexural failure process of ordinary RC beam with shear stirrups is as follows. (1) Initial flexural tensile cracks form on the bottom surface and

propagates upward; (2) Upward crack propagations make neutral axis to rise and

effective cross-section to be smaller; (3) Eventually, the member fails by either concrete compression

failure (brittle failure) or rebar tension failure (ductile failure). In this study, the OPC specimen was designed to fail by rebar tension failure, which is proved by the crack distributions shown in Figure 6. However, fiber added RC beam failure mode is rather different than ordinary RC beam in that they are expected to fail after using up large ultimate tensile capacity originated from both short fiber bridging and rebar tensile capacity, which is indicative by finite plastic rotation of the member. In Figure 6, both RPET and PP fiber added specimens ultimately failed by both concrete compression and rebar tension failure. This type of failure can only occur when the effective load resisting cross-section remains approximately equivalent to the original effective cross-section and that bending between rebar and concrete remains sufficiently intact to produce uniformity. And, this behavior can only occur when the crack formed during the initial stages of loading are abridged by the short fibers randomly distributed in the member. This behavior was explained by Ronald when he suggested that

OPC

PP 0.75

RPET 0.75

PP 0.5

RPET 0.5

PP 1.0

RPET 1.0

OPCOPC

PP 0.75PP 0.75PP 0.75

RPET 0.75RPET 0.75RPET 0.75

PP 0.5PP 0.5PP 0.5

RPET 0.5RPET 0.5RPET 0.5

PP 1.0PP 1.0PP 1.0

RPET 1.0RPET 1.0RPET 1.0

Figure 6. Crack patterns of each specimen

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reinforcing fibers in concrete fails by the process of bridging and pull-out. Obviously, the degree of bonding is also an important issue, but this variable was eliminated from our study by using embossed type of surface configuration, giving the optimal bond characteristic in RPET fibers. The load-deflection diagram shows that the maximum mid-span deflection is approximately 400% larger in fiber added specimen than OPC specimen as shown in Figure 5. As the load-deflection diagrams indicate, the loading resisting capacity of OPC and RPET specimens are incomparable and RPET concrete member is a totally different type of structural member than OPC member. This behavior can also be explained by the microscopic failure observed in RPET specimen. Generally, the damage of fiber added concrete follows the procedure of ① matrix damage, ② fiber/matrix debonding, ③ fiber bridging, ④ fiber failure, and ⑤ fiber pull-out as shown in Figure 7(a). The magnified crack photo of RPET RC specimen shown in Figure 7(b) proves that the specimen also resisted crack growth by fiber bridging, failure, and pull-out. This type of crack controlling characteristic allows delay in crack opening, concrete-rebar debonding, and ultimate member failure of the member.

① Damage of the matrix② Fiber/matrix debonding③ Fiber bridging ④ Fiber failure⑤ Fiber pull-out

⑤

④

③② ①⑤

④

③② ①

Concrete matrixRecycled PET fiber

① Damage of the matrix② Fiber/matrix debonding③ Fiber bridging ④ Fiber failure⑤ Fiber pull-out

⑤

④

③② ①⑤

④

③② ①

Concrete matrixRecycled PET fiber

⑤

③

④

Magnification

⑤

③

④

⑤

③

④

⑤

③

④

Magnification

(a) Fracture mechanism of synthetic fiber in concrete (b) Bridging effect of RPET fiber-matrix in beam

Figure 7. Fracture mechanism of RPET fiber in concrete Ductility and ultimate failure energy capacity In order to quantify the load-deflection capacity of the tested specimens, the quantification ratio of ductility index and ultimate failure energy capacity are used. Ductility index is defined as the ratio of yield to ultimate failure of a member. Deflection ductility index can be defined as

y

u

∆∆

=∆µ (1)

where, µ is ductility index of a member; ∆ is the defection of a member; u and y are ultimate and yield failure, respectively. Ultimate failure energy capacity of a member is defined as the area under load-deflection curve as shown in Figure 8. The ductility index and energy capacity for tested beams are tabulated in Table 4. As shown Table 4, RPET fiber added concrete specimen has a relative ductility index of 7.65 to 10.34, which is approximately 7 to 10 times greater than that of OPC specimen. This reveals that RPET fiber is effective in increasing ductility of concrete. The significant increase in relative ductility of fiber added concrete compare to that of OPC concrete comes from fiber bridging and bond effect. Especially, RPET 0.5 specimen showed a relative ductility index of 10.34, which is approximately 10 and 1.3 times higher than that of OPC and PP 0.5 specimens, respectively. The calculated ultimate failure energy capacity of fiber reinforced concrete members are approximately 4.0~4.8 times greater than that of OPC member. Especially, RPET 0.5 specimen had the most superior energy capacity, equivalent to the ductility index case. However, the ductility index and energy capacity values calculated for the members showed slight difference, where some specimens had higher values in ductility index compare to energy capacity. This difference can be attributed to the failure mode of each specimen where ductility index reflects the actual deflection ratio rather than the failure process such as multiple crack formations and yielding process elongation. However, both ductility index and energy capacity calculations prove that RPET fiber can be used as an effective reinforcing

∆1 ∆u

P1

P2

Pu

Ultimate failure energy capacity

∆2

P

∆

Figure 8. Energy capacity calculation

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fiber in concrete to improve ductility of RC member. Also, the test result indicates that RPET 0.5 has the best ductility and energy absorbing capacities among 3 types of concrete used in the test. However, ductility index and energy capacity decrease as RPET fiber volume fraction increases, therefore an appropriate fiber volume fraction should be used, considering optimal ratio between cost to member performance.

Table 4. Calculated ductility index and energy capacity of test specimens

Specimens ∆y (mm)

∆u (mm)

Ductility index (△u /△y)

Relative ductility index

Energy capacity (kN·m)

Relative energy capacity

OPC 4.80 16.94 3.53 1 4.64 1 RPET 0.5 4.52 165.0 36.50 10.34 22.50 4.85

RPET 0.75 4.67 141.36 30.27 8.58 20.87 4.30 RPET 1.0 5.31 143.36 27.00 7.65 21.05 4.34

PP 0.5 4.93 140.07 28.41 8.05 18.74 4.04 PP 0.75 5.21 149.16 28.63 8.11 21.77 4.69 PP 1.0 4.71 144.22 30.62 8.67 19.64 4.23

CONCLUSION The conclusions from the study of RPET concrete member capacities are as follows 1) RPET fiber added concrete specimen increased ductility and ultimate strength of concrete beam capacities observed in flexural tests. The comparison of OPC and RPET specimens showed that RPET concrete specimens increased by maximum of 1000% and 30% in ductility and ultimate load capacities, respectively. The improvements of member capacities come from cracking bridging and pull-out resistance effect of fiber added concrete. Due to better crack resisting characteristics of fiber concrete, the cover concrete crack spalling and reinforcement debonding failure due to macro-crack propagation are delayed. This delay increases the overall flexural strength and ductility (equivalent to energy absorbing capacity) of the RC member. 2) RPET fiber added concrete specimen failed by both concrete compression and rebar tension failures. This behavior can only occur when the effective load resisting cross-section remains approximately equivalent to the original effective cross-section, bonding between rebar and concrete remains sufficiently intact to produce uniformity, and cracks formed during the initial stages of loading are abridged by the short fibers randomly distributed in the member. The maximum mid-span deflection is approximately 400% larger in fiber added specimen than in OPC specimen. 3) RPET fiber added concrete specimen had relative ductility index and ultimate failure energy capacity of approximately 7 to 10 times greater than that of OPC specimen. Because of the fibers bridging effect in concrete matrix, the relative ductility and energy absorbing capacity of fiber added concrete were significantly higher than that of OPC concrete. However, ductility index and energy capacity decrease as RPET fiber volume fraction increases. As the study results indicate, RPET fibers can improve performance capacity of concrete member. It can also improve crack controlling capacity, transforming member failure mode form brittle to ductile. Therefore, if an optimal fiber volume fraction and other material parameters are properly used, RPET fiber can be used as a very beneficial structural material in the view point of eco-friendliness and cost saving. ACKNOWLEDGMENTS The authors acknowledge Korean Ministry of Education, Science and Technology (Bio-housing Research Institute) and Korea Science and Engineering Foundation (R01-2008-000-11176-0) for financial supports. REFERENCES Banthia, N. and Sheng, J. (1996). “Fracture toughness of micro-fiber reinforced cement composites”, Cement

and Concrete Composites, 18(4), 251-269. Hannant, D.J. (1978). Fibre Cement and Fibre Concretes, John Wiley & Sons Publication, 52-61. JVEC. Vinyl Environmental Council of Japan. (2008). http://www.vec.gr.jp/enbi/seisan.htm. Kim, J.H.J., Park, C.G., Lee, S.W., Lee, S.W. and Won, J.P. (2008) “Effects of the geometry of recycled PET

fiber reinforcement on shrinkage cracking of cement-based composites”, Composites Part B: Engineering, 39(3), 441-450.

Korean Institute of Resources Recycling. (2008). Recycling white paper, Cheong Moon Gak, Korea. KPCA. Korea PET Container Association. (2008). http://www.kpcaa.or.kr. Lin, Z., Kanada, T. and Li, V.C. (1999). “On interface property characterization and performance of fiber

reinforced cementitious composites”, Journal of Concrete Science and Engineering, RILEM, 1, 173-184. Rebeiz, K.S., Fowler, D.W. and Paul, D.R. (1993). “Recycling Plastics in Polymer Concrete for Construction

Applications”, Journal of materials in civil engineering, ASCE, 5(2), 237-248.

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