Effects of Polyethylene Terephthalate Fibre Reinforcement on Mechanical Properties of Concrete

Cracked concrete is a problem due to several factors such as poor maintenance, insufficient reinforcement, or steel corrosion leading to crack propagation. There is a need to increase the load-bearing capacity of concrete slabs and increase their life span. The use of waste polyethylene terephthalate fibres in concrete can dramatically alleviate the problem of crack propagation and failure sustainably. Furthermore, the utilization of waste plastic in this manner is environmentally friendly. This study presents the experimental investigation into the mechanical strength properties of concrete with respect to the effect of various mass fractions of polyethylene terephthalate fibre. The polyethylene terephthalate fibres were added at mass fraction of 0.5%, 1.0%, 1.5%, and 2.0%. An experimental investigation was carried out to explore the effect of varying fibre mass fractions on the slump value, rebound number, split tensile strength, flexural strength, and compressive strength. An increase in flexural strength, rebound number and compressive strength was noted with an increase in fibre mass fraction. However, a decrease in split tensile strength was noted. The addition of 0.5% fibre gave the highest compressive and flexural strength of 29.32N/mm 2 and 28N/mm 2 , respectively. However, the addition of fibre lowered the split tensile strength beyond the control specimen at all fibre mass fractions. The experimental results of this study indicate that the addition of polyethylene terephthalate fibre enhances the mechanical strength of concrete at low fibre mass fraction percentages. The PET fibre reinforced concrete is suitable for use in paving and ceiling slabs at a fibre addition of 0.5% for optimum workability and mechanical strength.


Introduction
Concrete is an important material in the construction industry and is generally composed of ne and coarse aggregates, cement, and water. e importance of concrete is due to its high durability, low cost, workability, and strength [1][2][3]. However, concrete has a number of intrinsic aws, which include micro-cracks within the material and at the interfaces. ese defects can originate from strain and stress from external restrains, excess water, bleeding, plastic settlement, and thermal shrinkage. When the load is applied to unreinforced concrete, these micro-cracks tend to amalgamate and form macro-cracks [4]. On further loading, the macro-cracks can result in catastrophic concrete failure. e fracture from micro-and macro-cracks can be lessened by the use of reinforcement material in the form of bres such as nylon, polypropylene, steel, acrylic, and aramid. ese bres assist by stopping the growth of cracks within the concrete [5][6][7]. ere is a need to increase the load-bearing capacity of concrete slabs and increase their life span. e use of waste polyethylene terephthalate (PET) bres and y ash in a hybrid composite slab dramatically alleviates the problem of crack propagation and failure sustainably [8][9][10].
When plain concrete develops tensile stress that exceeds its tensile strength, cracking occurs due to bending or changes in temperature and shrinkage. Concrete has good compressive strength; however, it does not respond adequately under high tensile stresses [11]. Synthetic bres in bre reinforced cementitious composites (FRCCs) can prevent the e ect of excessive tensile stresses by bridging and dispersing cracks and holding the concrete in place. Most synthetic bres reduce the amount of plastic and posthardening crack formation [12]. Synthetic bres assist concrete in developing its optimum long-term integrity by reducing plastic and drying shrinkage crack formation, increasing energy absorption, and improving resistance to impact forces.
Polyethylene terephthalate (PET) is a popular thermoplastic used mainly in textile fibres, beverages, and other liquid containers [13]. Most PET bottles are single use and are disposed of after use, by either burying or incineration. Plastic pollution has been on a steady increase with annual plastic production being approximately 368 million tonnes in the year 2019. ereafter, there was a slight decrease due to the coronavirus pandemic [14]. Plastic wastes such as PET can find use in various applications such as loadbearing bricks, walls, components in asphalts, base, and subbase for road construction [15]. Recycling plastic material is an environmentally friendly method to reduce plastic pollution and also conserve the precious raw petrochemical resources [16,17]. Various synthetic fibres such as polyethylene [18], polyvinyl alcohol (PVA) [19], polyethylene terephthalate (PET), and polyethylene (PP) [20] have been researched as reinforcements in concrete to improve their mechanical properties. Ataei et al. (2017) [21] studied the compressive strength effect of recycled PET particles in concrete. e research reported a decrease in compressive strength after adding PET particles. is occurrence was attributed to the weak cohesion between the particles and the cement resin. Rahmani et al. (2013) [22] gave a similar conclusion of a general decrease in compressive strength with PET particles to concrete. Rahmani et al. (2013) [22] reported that the compressive strength of PET particle reinforced concrete had an increase in compressive strength of 8.86% at a 5% particle mass fraction. However, at 10% PET particle mass fraction, the compressive strength was found to remain the same as that of unreinforced concrete [22]. e compressive strength decreases to 5.14% at a PET particle mass fraction of 15%. Studies that used PET fibre addition reported a marginal increase in compressive strength at 0.5% fibre addition [10]. Sayi et al. (2021) [12] studied the effect of varying PET fibre addition on water and sound permeability and concluded that permeability was reduced by fibre addition. However, research by Sayi et al. (2021) [12] did not consider the mechanical properties of the concrete. Choi et al. (2005) [23] also studied the effects of increasing the mass fraction of PET particles on compressive strength.
e author showed a decline in compressive strength with an increase in the mass fraction of PET particles. At 50% PET particle mass fraction, the loss in compressive strength was found to be 14.52%, while at 75% PET particle mass fraction, the compressive strength loss was 33.06% in reference to unreinforced concrete slabs [8]. e conclusion reached by the work of Choi et al. (2005) [23] was consistent with that of previous studies by Ataei et al. (2017) [21] and Rahmani et al. (2013) [22]. Mukhopadhyay et al (2015) [24] studied the use of hybrid PET and steel fibres to give superior toughness to concrete slabs. e study reported an increase in ultimate tensile strain capacity at peak with an increase in PET fibre loading. However, beyond a certain fibre loading, the ultimate tensile strain starts to decrease. Further, the author noted that an increase in PET fibre length improves strain hardening and multiple cracking behaviours.
is improvement increases the ultimate strain capacity of the concrete slab. However, the study failed to account for the critical length phenomenon, whereby the strength of the concrete starts to reduce at certain fibre lengths. Ismail et al. (2008) [25] studied the flexural strength properties of waste PET particle reinforced concrete. is study concluded that at 20% PET particle mass fraction, there was a decrease of 30.5% in flexural strength.
is decrease in flexural strength was attributed to a reduction in adhesive strength due to the hydrophobic nature of PET. Furthermore, the reduction in flexural strength can also be attributed to the elastic aggregate's elastic nature and nonbrittle loading characteristics. Nonetheless, the flexural strength can be increased using PET fibres, which have a high aspect ratio as shown in study by Alani et al.  [27] who reported an increase of over 18% with PET fibre addition of less than 1%. However, more research needs to be conducted to establish PET fibre fractions that give optimal mechanical strength properties, which form the basis of this study.
e problem of single-use PET plastic is a problem for our environment. e use of waste PET fibres in concrete can alleviate the disposal problem of PET and serve as an efficient and economic reinforcement in concrete preventing crack propagation. Previous research has focused mainly on the use of PET particle and flakes as a replacement for aggregates and tape strips of waste PET. Moreover, limited research has been done on the use of recycled PET fibres, which have undergone the extrusion process commonly used in textile grade fibres. e purpose of this study was to study the use of varying mass fractions of PET fibre on concrete mechanical strength. Various test specimens were fabricated to access the flexural, compressive, and tensile strength and rebound number.

Materials.
Clean tap water from municipality treatment facilities was used in this study. e properties of the cement, fine and coarse aggregate, and polyethylene terephthalate are outlined in the following subsections.

Cement.
e Portland cement used in this study was the Suretech Portland Cement CEM I 52,5N manufactured by PPC.
is cement is of strength class SANS 50197-1 [28]. e properties of the cement used are shown in Table 1.

Polyethylene Terephthalate Fibres.
e PET fibres used in the fibre reinforced concrete were obtained from recycled PET material extruded into 12 mm fibres. e properties of the PET fibres used are shown in Table 2.

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Fine Aggregates.
e fine aggregate used consisted of river sand with the properties shown in Table 3.

Coarse Aggregates.
e coarse aggregate used consisted of 13 mm dolomite with the properties shown in Table 4.

Mix Design.
e mix design used in this study is for M20 concrete as shown in Table 5.
e PET fibre mass fraction was varied over five levels with varying percentages from 0 to 2.0%. e fibre mass fraction was varied in this range due to the strength gain reported by several authors to fall within this range for synthetic fibre reinforced concrete [30]. Furthermore, any further addition above 2.0% drastically compromised the workability of the concrete paste. e dependent variables measured included the slump value, rebound number, compressive strength, flexural strength, and split tensile strength.

Fresh Concrete Mixture.
Concrete was mixed in accordance with SANS 5861-1 [31] using the hand mixing technique. e ambient temperature was recorded and maintained between 22°C and 25°C for storage of the materials. e concrete was hand mixed in a laboratory with the cement and fine aggregate mixed first in accordance with SANS 5861-1 [32].
ereafter, the coarse aggregate and fibres were added and mixed thoroughly until the coarse aggregate was uniformly distributed in the mixture. e fibres were sprinkled over the mixture while mixing to avoid clumping of the fibres. Water was then added slowly until the batch appeared homogeneous and of uniform consistency.
e concrete mix was then placed in a lubricated mould of dimensions consistent with the test to be carried out. e samples were covered with a damp hessian sack for 24 hrs and then demoulded and put into temperaturecontrolled water tanks at 23°C ±/-2°C for 28 days.     [34]. A steel cube mould (150 mm × 150 mm X 150 mm) was used for casting cubes. e samples were cured for 28 days in the water tank and thereafter tested. e ultimate load at failure and stress were recorded, and the compressive strength was calculated.

Flexural Strength Test.
e FRCC exural strength and modulus were determined on a Versa tester beam press machine in accordance with ASTM C78 [35]. is test uses a simple beam with four-point loading. e mould used for this exural test was of dimensions of 150 mm × 150 mm X 510 mm. e modulus of rupture was then calculated.

Split Tensile Strength Test.
e split tensile test was carried out using the universal model number 1887B0001 ELE machine. e test was carried out in accordance with ASTM C496-10 [36]. e maximum load was divided by the geometric dimensions of the test specimen to calculate the splitting tensile strength.
e rate of loading used was 690 kPa/min, and the splitting tensile strength was then calculated.

E ect of PET Fibre on Concrete Workability.
e graph shown in Figure 1 shows the e ect of PET bre addition on the slump value of the fresh concrete. e slump value dropped sharply from 50.30 mm for the control specimen without any reinforcement to 15.00 mm for the FRCC containing 0.5% PET bre. As PET bre content was increased to 1.0%, the slump value dropped from 15.00 mm to 0 mm. Any further increase in bre content yielded a zero-slump value.
is drop could be attributed to the decrease in workability that results from an increase in bre content. e decrease in workability is due to frictional resistance between the PET bres and the concrete particles. Research carried out by Hasan A et al.  [38] reported that increasing the bre content tends to increase the frictional resistance between the bres and the concrete particles resulting in obstruction to the free ow of concrete.
is frictional resistance ultimately reduces the slump value of the concrete. A lower slump value makes it more di cult to pump and place the concrete, although the workability can be increased by the use of superplasticizers as admixtures in the concrete. Further, research is needed to improve the slump value of PET FRCC beyond 1.0% bre addition.

E ect of PET Fibre on Rebound Number.
e graph in Figure 2 shows the average rebound number with an increase in PET bre addition. e addition of PET bres lowered the rebound number as the percentage of bres increased. e addition of 0.5% PET bre resulted in a reduction of 13.27% from the control specimen. Further addition of PET bre to 1.0% resulted in a decrease in rebound number of 19.67% from that of 0.5% bre content. However, the addition of 1.5% bre content resulted in a slight increase in the rebound number. Further addition of 2.0% bre resulted in a drop in rebound hammer number to 12.3. e general decrease in rebound number with an increase in bre loading is related to the compressive strength.
ere is a decrease in the interfacial bond between the PET bres and the cement paste with an increase in bre loading.
is phenomenon results in a progressively decreasing rebound number with an increase in bre content. A study conducted by Baboo et al. (2012) [39] reported a similar trend with the decrease in rebound number with incremental synthetic bre content. e author concluded that the decrease in rebound number was due to the decrease in adhesive strength between the surface of the synthetic bre and the cement paste. Figure 3 shows a comparison between the compressive strength obtained from the destructive cube test and the results of the rebound compressive test. e nondestructive test results follow the same trend as that of the destructive tests, which indicated a decrease in compressive strength with an increase in PET bre content. e trend observed was like that reported by Ede and Ige (2014) [40], who reported a decrease in the rebound number of concretes containing polypropylene bre. However, the rebound number gave signi cantly lower calculated values of compressive strength compared with the destructive test.
is phenomenon could be attributed to the random dispersion and orientation of the PET bres within the concrete giving varied readings of the rebound hammer at di erent points of the concrete specimen.

E ect of PET Fibre on Composite Compressive Strength.
e graph in Figure 4 shows the e ect of bre addition on the 28-day compressive strength of concrete. Figure 4 shows that an increase in compression strength of concrete occurs up to 0.5% PET bre addition. ereafter, there is a 45% decline in compressive strength from 0.5% up to 1% bre addition. e strength of FRCC is signi cantly less than the strength of unreinforced concrete at 1.0% bre addition. ereafter, there is a moderate decline in compressive strength with increased bre content up to 2%. e maximum compressive strength recorded was at 0.5% bre addition, which gave compressive strength of 29.32 N/mm 2 , an increase of 23% in compressive strength over the control specimen. e stress on the FRCC followed a similar trend to the compressive strength, as shown in Figure 5. e compressive stress was reduced with the addition of PET bre in a gradual trend. e failure of the specimens under compressive test for the PET reinforced concrete cubes was not catastrophic as realized for the control specimen. It was a gradual failure. On the other hand, the failure of the control specimen of concrete without any bre reinforcement was a sudden and explosive global failure. e addition of 0.5% PET bres has a modest increase in the compressive strength of the FRCC. However, the addition of PET bres signi cantly alters and increases the ductility of the concrete and impacts the post-cracking ductility of the FRCC.
e increase in the compressive strength between 0% and 0.5% can be attributed to an increase in the bonding that occurs between the concrete mixture contents due to the bre addition. Similar results were observed by Mashrei et al. (2018) [41] and Umasabor et al. [42], who both reported a sharp increase in the compressive strength of concrete reinforced with polypropylene bres and polyethylene   [44] stated that reduced workability led to concrete not being compacted properly reducing its compressive strength.
ere is a sharp decrease in compressive strength between 0.5% and 1.0% of bre addition due to bres having a negative e ect on the hydration of cement. Furthermore, high bre percentages tend to encourage bre clumping during fabrication. Fibre clumping creates nucleus sites for crack formation under a compression load, leading to lower compressive strength. e use of a high percentage of bre in FRCC ˃ 0.5% signi cantly a ects the workability of the concrete. e nish of the concrete is not smooth and may have some voids, which are the origin of the failure cracks of the concrete under compressional load. It was concluded that the best range of PET bre addition is between 0.1% and 0.5% for optimal compressive strength of the FRCC.

E ect of PET Fibre on Composite Flexural Strength.
e graph in Figure 6 shows the e ects of PET bre addition on the exural strength of the FRCC. e addition of 0.5% PET bre increased the exural strength of the FRCC signi cantly from the control specimen of 2.82 N/mm 2 to 3.59 N/mm 2 . Adding a further quantity of PET bre to 1% resulted in a drastic drop in the exural strength to less than that observed for the control specimen giving a exural strength of 1.80 N/mm 2 . Further, the addition of bre gave a steady increase in exural strength. In addition, 1.5% PET bre gave exural strength of 2.25 N/mm 2 . Further, the addition of PET bre to 2.0% gave exural strength of 2.87 N/mm 2 . e highest exural strength was obtained with 0.5% PET bre addition.
e results are consistent with the results of Govindasami (2018) [38], who also reported a signi cant increase in exural strength with the addition of polypropylene bre up to 0.5% and after that a drastic reduction in exural strength with the addition of 2.0% bre. A study by Umasabor (2020) reported a steady decrease in exural strength at the percentage of PET bre above 0.5% bre loading. is nding was consistent with the results observed in this study. e decrease in exural strength at high bre loading was attributed to a decrease in adhesive strength between the surface of the PET bres and the concrete paste by Baboo et al. (2012) [39]. A study by Huang et al. (2021) reported a similar trend with an increase in concrete exural strength at rst at low bre loading percentages and then a decrease in strength with further bre loading.

E ect of PET Fibre on Composite Split Tensile Strength.
e graph in Figure 7 shows the e ect of bre addition on the split tensile strength of FRCC. e addition of bre decreased the split tensile strength of the FRCC. Unreinforced concrete had a split tensile strength of 2.06 N/mm 2 , which dropped marginally to 1.82 N/mm 2 with 0.5% PET bre. However, the further addition of PET bre resulted in a drastic drop in the split tensile strength. e addition of 1.0% PET bre gave a split tensile strength of 1.16 N/mm 2 . is was a drop of 44% from the split tensile strength of unreinforced concrete. After that,  Advances in Materials Science and Engineering there was a slight increase in the split tensile strength with a 1.5% addition of PET bre, giving a strength of 1.21 N/mm 2 . Further addition of PET bre to 2.0% resulted in a reduction in split tensile strength to 0.92 N/mm 2 . e splitting strength of the FRCC was negatively a ected by the addition of bres. However, the bres helped delay the development and propagation of cracks during the testing. e low splitting strength with an increase in bre content could be attributed to the e ect of high air voids due to bre clumping. ese results were consistent with the study carried out by Irwan et al. (2013) [45], who reported a decrease in split tensile strength with PET laments. e author reported that the split tensile strength dropped from 3.65 N/mm 2 , the control specimen, to 3.57 N/mm 2 with 0.5% PET lament reinforced concrete. Irwan et al. (2013) [45] reported a steady reduction in split tensile strength up to 1.5% PET bre addition with various water-to-cement ratios, and the trend remained the same. In contrast, Govindasami et al. (2018) [38] reported a marginal 19% increase in split tensile strength of FRCC containing polypropylene bre up to 1.0% bre addition. e increase in strength, which contrasts with this study, can be attributed to signi cantly higher bre tensile strength properties of polypropylene over PET.

Application of PET Fibre Reinforced Composites in Slabs.
e fabricated PET bre reinforced concrete is suitable for use in paving slabs and ceiling slabs. e concrete pavers can be used in numerous applications such as low volume roads, street roads, and other landscape pavement applications. Furthermore, the use of the PET reinforced paver slabs in these applications is economic in terms of cost of manufacture. ese PET paver slabs will be able to resist crack propagation and will not easily buckle or break. e use of PET reinforced concrete in ceiling slabs has several advantages such as durability, crack control system through the randomly oriented PET bres, and increased ductility. e ductility introduced by the use of the polymeric PET bres increases the toughness of the concrete.
is study has shown that the use of the PET bres in mass fraction of <0.5% gives the best mechanical properties.

Conclusion
e study investigated the e ect of addition of varying mass fraction of PET bre on the mechanical properties of concrete slabs. Based on the experimental results carried out in the study, the following conclusions can be drawn: (i) e slump value was observed to decrease with incremental amounts of PET bre. e use of lower amounts of bres less than 0.5% gave acceptable workability of the FRCC. erefore, it is recommended to maintain less than 0.5% bre content to have acceptable concrete workability. (ii) e addition of 0.5% PET bre to the FRCC increased the compressive strength to 28 N/mm 2 . However, further bre addition exceeding this percentage resulted in a decrease in compressive strength. (iii) e addition of PET bre only decreased the split tensile strength of the FRCC. Unreinforced concrete had a split tensile strength of 2.06 N/mm 2 , which dropped marginally to 1.82 N/mm 2 with the addition of 0.5% PET bre. e addition of 1.0% PET bre gave a split tensile strength of 1.16 N/mm 2 . is was a drop of 44% from the split tensile strength of unreinforced concrete. Further addition of PET bre to 2.0% resulted in a reduction in split tensile strength to 0.92 N/mm 2 . (iv) e addition of 0.5% PET bre alone gave the highest exural strength. However, further addition  Advances in Materials Science and Engineering above 0.5% gave a decreasing trend and unsatisfactory flexural strength.
(v) e addition of PET fibre to the FRCC gave a decreasing rebound number with an increase in fibre content. is was in line with the trend observed for the destructive test. (vi) e developed PET fibre reinforced concrete is suited for use in ceiling slabs and paving slabs at a fibre addition of 0.5% for optimum strength.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this study.