Influence of recycled polyethylene terephthalate fibres on plastic shrinkage and mechanical properties of concrete

Abstract

Polyethylene terephthalate bottles production has drastically increased year after year due to high versatility of polyethylene terephthalate plastics and considerable consumption of beverages. In tandem with that increase, the major concern of society has been the improper disposal of this non-biodegradable material to the environment. To deal with this concern, recycled polyethylene terephthalate bottles were incorporated in concrete as fibre reinforcements in this study. The objective of this research is to evaluate the mechanical properties of recycled polyethylene terephthalate fibre reinforced concrete (RPFRC) in comparison with control concrete without fibres. polyethylene terephthalate fibres with three different diameters (0.45, 0.65, and 1.0 mm) and two lengths (20 and 30 mm) were added at various proportions (0.5%, 1.0%, 1.5% and 2.0%) by volume of concrete in order to determine the effect of fibres initially on compressive, flexural and splitting tensile strengths of concrete. The results revealed that none of the fibres have detrimental effects up to 1% volume fraction, however further addition caused slight reductions on mechanical properties in some conditions. Plastic shrinkage resistance and impact resistance tests were also performed according to related standards. Polyethylene terephthalate fibres were observed to have marked improvements on those properties. Such a good performance could be attributed primarily to the bridging effect of fibres.

Graphical abstract

Keywords

recycled PET / fibre-reinforced concrete / mechanical properties / plastic shrinkage / impact energy

1 Introduction

Plastic is a very distinctive material due to its advantageous properties such as low cost, lightweight, durability, and versatility. Amount of commercial plastic waste around the world is increasing day by day because of high consumption products made of plastics. In 2019, the global plastic production was around 368 million metric tons (348 in 2017 [1]) and very high portion of this amount ended up landfilled or incinerated [2]. Polyethylene terephthalate (PET) bottles were initially used in water and beverage packaging, then started gaining interests in other industries such as medicine, cleaning products and cosmetics. PET is one of the most used plastics and its production by 2020 is predicted to be approximately 73 million metric tons which makes PET waste to make up the biggest proportion amongst all plastic wastes [3]. Consequently, utilization of recycled PET waste in concrete applications can lead to diminishing of this waste and improved concrete properties. The PET recycling sector in North Cyprus is in a developing stage and demand for this waste is very low as there is very little usage area of this waste in the industry. Thus, the major proportion of the PET waste is sent to landfill or incinerated whereas the minor proportion is collected, shredded, and sent to other countries with very low profits. Therefore, utilization of recycled PET waste in concrete applications may result in a more developed recycling sector, less waste to landfills or incinerated in North Cyprus and most importantly more sustainable concrete applications can be attained.

Nowadays, literature studies involve incorporation of recycled PET as aggregates or fibre reinforcements, with or without mechanical treatments in concrete or mortar [4–7]. Although concrete has satisfactory properties, there are some intrinsic drawbacks such as low tensile strength, low resistance against plastic shrinkage cracks and brittle feature (low ductility). Influence of recycled PET fibres on mechanical properties of concrete without replacing any other constituent in concrete has been studied by various researchers [8–10]. Incorporation of PET fibres in concrete provide crack control and enhance ductility, and diminish the waste disposed by incineration and landfill that are causing environmental problems.

Wide range of microcracks tend to occur in the concrete matrix due to tensile stresses generated by various loading modes and shrinkage mechanisms. On the other hand, ‘bridging effect’ is the main function of recycled PET fibres that are used as reinforcements in concrete applications. Thus, fibres have ability of preventing propagation and diminishing the amplitude of cracks. Furthermore, this particular property of fibres offers distinct benefits such as improved post-peak ductility and energy absorption [11,12].

Contradictory results were obtained in the literature about the effect of recycled PET fibres on the compressive strength of RPFRC. Fraternali et al. [8] reported a high increase of compressive strength up to 35% at 1.0% volume fraction. It was also observed that the effect of fibres with lower aspect ratio (length/diameter ratio) was more significant on improving compressive strength. On the other hand, Kim et al. [11] observed a moderate decrease from 1% to 9% in compressive strength with increasing fibre volume from 0.5% to 1.0%. According to a study conducted by Ochi et al. [13], moderate increase was observed in compressive strength compared to control concrete up to 1.0% fibre content, further addition of fibres diminished the increase or even turned the result into a decrease for different water to cement (w/c) ratios. The reason for these variations in results can be ascribed to different geometries and surface characteristics of recycled PET fibres.

In the study by Ochi et al. [13], recycled PET fibres with 0.7 mm diameter and 30 mm length were manufactured from recycled PET bottles. There was a moderate increase in compressive strength up to 1.0% volume fractions, followed by a decrease upon further addition. However, PET fibres were observed to improve the bending strength significantly (25%–36% depending on water to cement ratio) over control concrete at 1.5% fibre fraction. According to Pelisser et al. [9], fibres with diameter of 25 µm and length of 20 mm were observed to have no significant effect on compressive strength at 0.05%, 0.18%, and 0.30%. On the other hand, flexural strength was improved between 13.6% and 19.2% compared to plain concrete at the same volume fractions.

Plastic shrinkage cracks are one of the main reasons for low performance in concrete applications. Pavements, bridge slabs and car parking floors are prone to plastic shrinkage cracks which occur before complete hardening of concrete, since those expansive surfaces are restrained and exposed to high moisture evaporation rates and high temperatures during placement. If the surface cracks that occur by virtue of plastic shrinkage remain undetected, these cracks act as channels for passage of external deteriorating agents, hence long term durability of concrete gets worse [14]. According to a study done by Borg et al. [10] recycled PET fibres were found to have a favourable effect on plastic shrinkage crack reduction in concrete. Furthermore, longer PET fibres were observed to have higher performance than short PET fibres with same diameter at all fibre volume fractions. Kim et al. [15] examined the effect of straight, crimped, and embossed PET fibres on plastic shrinkage resistance of cement-based composites at various fibre fractions. Crimped and embossed fibres which were found to have better bond strength than straight fibres exhibited better performance in controlling cracks up to 0.5% volume fraction. However, once the fibre volume fraction was further increased, fibre geometry was observed to have no further effect on controlling cracks as the cracks were controlled by an adequate number of fibres.

Study conducted by Pelisser et al. [9], revealed that, addition of recycled PET fibres with 25 µm diameter and 20 mm length at very low volume fractions (0.05%, 0.18%, and 0.30%) is observed to enhance the impact energy up to 2.3 times compared to plain concrete. Nevertheless, no effect of fibres on the first crack was observed.

However, the effect of PET waste on concrete properties was examined in many studies in the literature; this waste was mostly used in shredded form as an addition to reinforcement or as a replacement of aggregates. On the other hand, the influence of mechanically extruded recycled PET fibres with different dimensions were seldomly studied. Therefore, in this present study, manufactured recycled PET fibres with three diameters (0.45, 0.65, and 1.0 mm), two lengths (20 and 30 mm) and four volume fractions (0.5%, 1.0%, 1.5%, 2.0%) were used in order to evaluate the influence of recycled PET fibres on the mechanical properties of concrete compared to conventional concrete. The main aim of this project is to undertake a comprehensive investigation and fill the gaps in the literature by assessing the function of fibres with regards to dimension and proportion on compressive and indirect tensile strengths of concrete. Moreover, the effect of fibres on plastic shrinkage resistance and impact resistance of RPFRCs governed by fibre dimensions and volume fractions were investigated according to related standards.

2 Methodology

2.1 PET FRC sample preparation

Plain and PET fibre reinforced concrete specimens were mixed, placed and cured according to ASTM C192/192M-16a [16]. All constituents were weighed and then mixed using a conventional rotary pan mixer. Control concrete and recycled PET fibre reinforced concrete (RPFRC) specimens were prepared for analysis of fresh properties, mechanical strengths, plastic shrinkage resistance and impact resistance by using the mix design formulation parameters given in Tab.1. Water to cement ratio (w/c) of 0.49 was provided for all mixes. Recycled PET monofilaments which were manufactured as brush hair from recycled PET bottles, with various diameters (0.45, 0.65, and 1.0 mm) were procured from a broom company in Turkey. Fibres with densities in range of 1.41–1.53 gr/cm³ were then cut from the monofilaments into desired lengths of 20 and 30 mm and added into concrete mixes on a volume basis (see Fig.1). Aspect ratio of fibres mentioned in the study implies to length/diameter ratio of each fibre type. Fibre volume percentages were chosen to be 0.5%, 1.0%, 1.5%, and 2.0%. Fibres were added gradually at the last stage of mixing to avoid bunching of fibres and non-uniform fibre distribution. Vibrating table was applied to all samples to eliminate air bubbles and achieve a homogeneous mix. All specimens prepared were cured at 23 °C and 95% relative humidity until the day of testing.

2.2 Experimental procedure

2.2.1 Slump

Although the fresh state of concrete is temporary, its characteristic and condition indirectly affect the properties of hardened concrete. Workability is the primary property of fresh concrete which is defined as ease of placement without segregation. Therefore, slump tests were conducted prior to casting in order to see the effect of recycled PET fibres with various diameters and lengths on the slump of concrete at different volume percentages according to ASTM C143/143M-20.

2.2.2 Compressive strength

Compressive strength is one of the major criteria of evaluating the quality of concrete. Cube specimens with target strength of 40 MPa and dimensions of 150 mm × 150 mm × 150 mm were casted for evaluation of compressive strength as shown in Fig.2(a). Four cubes for each sample were tested and average value was taken. Tests were carried out at a hydraulic compressive testing machine with a capacity of 3000 kN (Fig.2(b)).

2.2.3 Flexural strength and splitting tensile strength

Flexural and splitting tensile strength tests are methods of determining indirect tensile strength of concrete. Prismatic specimens (100 mm × 100 mm × 500 mm) and cylindrical specimens (100 mm × 200 mm) of control and recycled PET fibre reinforced concrete were casted for flexural strength and splitting tensile strength tests according to ASTM C293/C293M-16 and ASTM C496/C496M-17 standards, respectively. Specimens were kept in a moist curing room at a controlled temperature and relative humidity for 28 d. On the day of testing, all specimens were removed from curing room and excess water was wiped from the surfaces. Dimensions and masses of all specimens were also measured prior to each testing. Moreover, each sample was visually inspected after failure and any unexpected crack pattern or failure mechanism was marked down. Flexural and splitting tensile strength tests were carried out at a hydraulic testing machine with a capacity of 200 and 3000 kN, respectively (Fig.3(a) and 3(b)).

2.2.4 Plastic shrinkage

ASTM C1579-13 was followed to evaluate plastic shrinkage resistance of recycled PET fibre reinforced concrete specimens [17]. Environmental chamber containing the plastic shrinkage moulds with installed stress riser and internal restraints were prepared as shown in Fig.4(a) and 4(b). Environmental chamber consisted of heaters and dehumidifiers to maintain temperature at (36 ± 3) °C and relative humidity at 30% ± 10%. Evaporation rates over 0.5 kg·m⁻²·h causes compressive strains which creates tensile strength sufficient to cause cracks in early stages [15]. In order to achieve an average evaporation rate of 1 kg·m⁻²·h and to validate the experiment, an industrial high velocity fan was located among heaters and moulds to provide 5 m/s wind speed.

Plastic shrinkage tests were conducted for various fibre dimensions and volume fractions. For every PET-added concrete sample, a plain concrete sample was also prepared as a reference. Freshly prepared concrete was placed into the metal shrinkage moulds (Fig.5). Hygrometer was installed 100 mm above moulds to monitor temperature and relative humidity continuously and a handheld anemometer was used to measure wind speed above specimens during tests. The setting time of both control and fibre-added specimens were regularly monitored using the procedures described in ASTM C403/403M (2008) [18].

Heaters and fans were switched off when final setting of both control and RPFR concrete samples attained. Moulds were then covered with plastic sheets to prevent evaporation and kept at environmental conditions for other (8 ± 2) h before crack width measurement. Optical handheld microscope (Fig.4(c)) capable of measuring crack width to nearest 0.02 mm was used during analyses. Average crack width of each sample was determined by taking the average of 20 values along the crack line over stress riser and Crack Reduction Ratio (CRR) was calculated by using Eq. (1):

(1)CRR=1 -\left[\frac{Av.CrackWidthofRPFRC}{Av.CrackWidthofControlConcrete}\times 100\mathrm{\%}\right].

2.2.5 Drop weight impact resistance

ACI Committee 544 (1988) [19] report was followed for the determination of impact resistance of recycled PET fibre reinforced concrete. Drop weight impact test was performed in order to evaluate energy absorption capacity of PFRC specimens until a specific level of distress. Samples of concretes which were reinforced with 30 mm long PET fibres were cast into cylindrical moulds (300 mm × 150 mm). The samples were then cut into four smaller cylindrical specimens with (63.5 ± 3) mm lengths to be adapted for testing equipment, using a table saw with diamond blade.

Test setup consists of a 4.54 kg compaction hammer which is released from a distance of 457 mm onto a steel ball. Steel ball was placed in a positing bracket on top of the specimen as shown in Fig.6. Compaction hammer was dropped repeatedly, and the number of blows required to cause the first visible crack and ultimate crack for each sample were recorded. Four positioning lugs were attached to the base plate at 4.76 mm distance from the circumference of specimens. Ultimate failure crack was defined as the point when concrete pieces were in contact with three of the four positioning lugs due to deformation of specimens. Finally, impact energy was calculated using Eq. (2), where E is the impact energy (N·m), M is the mass of steel ball (kg), V is the velocity of drop hammer (calculated as 2.12 m/s) and N is the number of blows needed for ultimate crack [20].

(2)E=\frac{1}{2}M{V}^{2}N.

3 Results and discussion

Results obtained from the experimental investigations are given below.

3.1 Slump

Fig.7(a) and 7(b) represent slump values for 30 and 20 mm fibre-added concrete, respectively. It can be seen that the addition of recycled PET fibres has reduced the slump of the concrete in all conditions. This could be due to an increase in the surface area to be covered by cement matrix and inter-particle friction as a result of fibre addition [21]. The fibres are suspect to agglomerate and form bunches of fibres, which in turn, reduce the followability of concrete [22]. Moreover, utilization of fibres prevents the flow of concrete matrix by causing a formation of network structure [23]. This was an expected behaviour and this trend was commonly observed in various literature studies [9–24]. Fibres with 30 mm lengths were observed to have minimal effect on slump up to 0.5% volume fraction, where further increase of fibre volume resulted in consistent decrease of slump. At 2.0% fibre fraction, reductions in slump were 76%, 61%, and 38% for 0.45, 0.65, and 1.0 mm fibres with 30 mm lengths, respectively. On the other hand, short fibres (20 mm) exhibited a sudden decrease from 0.5% to 2.0% and lower slump was obtained at all fibre fractions. At 2.0% fibre fraction, reductions in slump were 80%, 70%, and 49% for 0.45, 0.65, and 1.0 mm fibres with 20 mm lengths, respectively. The results also revealed that longer fibres exhibited higher slump values at all volume fractions. Nevertheless, for length comparison, longer fibres are expected to have a higher detrimental effect on the workability due to higher potential of entanglement. However, at a fixed total fibre volume, the number of 20 mm fibres is higher than that of 30 mm fibres, which increases potential fibre to fibre interactions and indirectly reduces the slump. Although fibres caused up to five times slump reduction compared to plain concrete, RPFRCs still provided sufficient workability and did not require excessive external vibration for compaction.

3.2 Compressive strength

The influence of fibre dimensions and fibre volume fractions on compressive strength are illustrated in Fig.8. It can be seen from results that 0.45 mm diameter fibres with 30 and 20 mm lengths enhanced compressive strength compared to that of control concrete up to 1.5% and 1.0% introduction of fibres respectively. The highest improvement of 14% compared to control concrete was achieved by 0.45 mm diameter and 30 mm length fibres, which has the highest aspect ratio amongst all available fibres. Fibres have an ability to restrict extension of cracks and alter crack direction thus delaying crack growing rate [25] and increasing the energy required to cause failure [22]. Furthermore, distributed PET fibres in the concrete mixture enhance homogeneity and reduce the voids, which as a result, increases the cohesiveness of concrete [26]. Results also revealed that 0.65 mm fibres exhibited an increase at 0.5% volume fraction, conversely further introduction of fibres in concrete resulted in a slight decrease in compressive strength. The compressive strength decreases in most cases with further addition of PET fibres which can be attributed to change of adhesion between fibres and cement paste. Moreover, 1.0 mm fibres exhibited a slight decrease at all fibre contents. This slight reduction in compressive strength could be attributed to trapped air voids caused by fibre entanglement, which reduced the load-bearing area within the sample cross-section. This phenomenon was observed to be valid for all fibre types. The results are qualitatively contradicting with the study conducted by Fraternali et al. [8]. Regarding to aspect ratio, the fibres with higher aspect ratios exhibited lower compressive strength values. This behaviour of recycled PET fibres depends on the geometry, dispersion, and the adherence to the cement matrix. The effect of fibre length on compressive strength was negligible except for 0.45 mm diameter fibres. However, the control concrete specimens were failed disastrously, the fibre incorporated specimens were failed with several minor cracks on the surface while the concrete specimens were still held together by the fibres after failure.

3.3 Flexural strength and splitting tensile strength

Fig.9 and Fig.10 represent results for flexural and splitting tensile strengths, respectively. According to the results, fibres with 0.45 mm diameter and 30 mm length exhibited the highest improvement in flexural strength up to a fibre content of 1.5%. By contrast at 2.0% volume fraction, flexural strength drastically decreased to a strength of concrete without fibres. The reason for this drastic decrease could be attributed to formation of fibre entanglement during mixing, as this characteristic is the main drawback of fibre reinforcements. For the rest of the fibres, moderate increase up to 10% was observed compared to control concrete at different dosage of fibre additions by virtue of the crack arresting mechanism of fibres. Although, the control specimen cracks and collapses instantly at the first crack without any deformation and prior warning. The failure progresses with bending without any instant collapse in the PET fibre introduced specimens. During concrete failure, the load is transferred to the plastic fibres, which in turn, prevent the spreading of cracks [23,27]. When fibres with 1.0 mm diameter and 30 mm length were added at 1.5% and 2.0% fibre volume fractions, flexural strength decreased by 4% and 15% respectively. The highest improvement of 20% was attained by fibres with 0.45 mm diameter and 30 mm length, which have the highest aspect ratio. Meanwhile, slight variations were observed for fibres with diameter of 1.0 mm and lengths of 30 and 20 mm, which have the lowest aspect ratios.

Like flexural strength, splitting tensile strength was also improved by 20% compared to concrete without fibres with the inclusion of 0.45 mm diameter and 30 mm length fibres at 0.5% volume fraction. This was followed by a slight decrease in progress up to 1.5% volume fraction. On the other hand, at 2.0% volume fraction, the tensile strength declined by 9%. Apart from the fibres with 1.0 mm diameter and 20 mm length, increments in splitting tensile strengths were observed up to 1.0% fibre fraction. However, further addition of fibres led to similar or even slightly lower values compared to control concrete. This trend is qualitatively coherent with a similar study in the literature [25]. Fig.11 shows the correlation between the compressive strength and flexural strength of 0.45 mm diameter PET fibre incorporated concretes. The graph highlights a direct relationship for 30 mm fibres, which means by increasing compressive strength, the rate of gaining in flexural strength increases. The established R² value of 0.91 indicates a high linear correlation between the two variables. On the other hand, inverse relationship was observed for 20 mm fibres while the established R² value of 0.60 indicates a very weak relationship between the two variables. This inverse relationship between two different fibre lengths could be attributed to the influence of aspect ratio of fibres on the bonding strength of concrete as explained before in this section.

The results in Fig.11 also revealed that splitting tensile strengths of RPFRC specimens decreased with respect to increase in fibre volume fractions. As in flexural strength, fibres with higher aspect ratios were seen to perform better than fibres with lower aspect ratios, due to greater crack arrest ability. The reason for reduced flexural and splitting tensile strengths could be the entanglement of fibres that are creating air voids which result in stress concentrations within the samples. Moreover, the smooth surface of recycled PET fibres creates poor bonding between cement matrix and fibres, which leads to formation of voids and microcracks resulting in slightly poorer strength characteristics [21]. Any potential improvement of flexural strength and splitting tensile strength due to the sewing effect of fibres could be dominated by the reduction of strength due to aforementioned air voids. Concrete without fibres instantly split once the specimen cracked, while PET fibre reinforced concrete specimens exhibited little deformations without splitting (see Fig.12). This represents that the fibre reinforced concrete has the ability to absorb energy in the post cracking phase. The flexural and splitting tensile strengths were expected to increase due to the ability of fibres to carry applied loads through interfacial bonds, which would prevent the propagation and sudden failure of concrete [12]. However, other variables such as fibre geometry, consolidation and bonding strength between fibre surface and cement paste could have an opposite effect on the results. Furthermore, increasing the fibre volume fraction beyond a point increases the interaction between fibres, which possibly reduces the adhesion between fibres and cement paste. Nevertheless, the fibre length of 0.45 mm fibres had a visible effect on mechanical properties, and the effect of length for other fibre diameters was not significant.

3.4 Plastic shrinkage resistance

Plastic shrinkage resistance of the samples has been investigated in accordance to ASTM C1579-13 [17]. Crack widths of control and RPFRC samples were measured using a hand-held optical microscope. Fig.13 shows the relationship between volume percentages and CRR for fibres with varying diameters and lengths. With reference to the effects of fibre length, longer fibres showed higher performance than short fibres at all fibre fractions. Poor performance of shorter fibres was attributed to poorer bonding of fibres with the cement matrix, due to lower surface area per single fibre. In general, fibres with higher aspect ratios (l/d) performed better in the reduction of plastic shrinkage cracks because of greater bond with cement matrix that enhances the energy absorbed by sewing effect of fibres. Moreover, increasing volume of fibres was observed to increase CRR for all fibre types. Even at 0.5% volume fraction, 0.45, 0.65, and 1.0 mm fibres with 30 mm lengths reduced crack widths by 68%, 65%, and 44%, respectively. Besides, fibres with 0.45 and 0.65 mm diameters and 30 mm lengths were found to reduce crack widths by roughly 100% at 2.0% fibre addition compared to conventional concrete. Shorter fibres with diameters of 0.45, 0.65, and 1.0 mm at 1.5% fibre fraction reduced crack widths by 75%, 75%, and 62%, respectively. This behaviour could be due to the bridging effect of PET fibres caused by a mechanism of load transfer from cement matrix to fibres. Fig.14 illustrates the crack paths formed upon stress risers for concrete specimens with and without fibres during plastic shrinkage resistance tests. The results were qualitatively coherent with previous investigations by Borg et al. [10] and Kim et al. [15] with reference to fibre volume fraction and fibre aspect ratio. However, various literature studies revealed that deformed PET fibres performed better than straight and smooth fibres due to higher mechanical bond strength [23], it was also mentioned that once the fraction of fibre volume exceeds 0.5%, an adequate number of fibres were included to control cracks due to plastic shrinkage, thus the fibre geometry had no further effect [15].

3.5 Drop weight impact resistance

Average number of blows required for ultimate crack of 150 mm × 63.5 mm specimens gathered from drop-weight impact test to determine the impact resistance of RPFRC specimens are shown in Tab.2. Although there was no apparent effect of fibre addition on impact resistance to the first crack of concrete, their influence on impact resistance to the ultimate crack was found to be significant. Different fibre types were mentioned in the literature that had a positive effect on the impact resistance to initial crack of concrete unlike PET fibres [28,29]. Nevertheless, these fibres were observed to have less improvements on the ultimate crack resistance than PET fibres.

Fig.15 clearly indicates that at all fibre diameters, increasing fibre contents enhances impact energy (E), especially at over 1.0% fibre addition. Dashed line represents the impact energy calculated with regards to number of blows for control concrete. At 2.0% fibre addition, 0.45, 0.65, and 1.0 mm fibres improved impact energy by 10, 5.7, and 4.5 times compared to control concrete, respectively. Influence of fibres on impact energy increased with respect to increase in aspect ratio. Increasing fibre length results in either more uniform cracks or higher number of cracks. Therefore, for the same length fibres, decreasing fibre diameter increases the energy absorbed. This phenomenon was attributed to the bridging effect of fibres on concrete, while at an equal fibre volume, higher aspect ratio represents larger bonding surface area between fibres and cement paste.

Several micro-cracks were developed across RPFRC specimens instead of a single macro-crack and thus higher surface energy was dissipated [30]. Fig.16 shows crack patterns after ultimate failure of control and 0.45 mm, 2% fibre added concrete specimens subjected to drop-weight impact test. While control specimens (without fibre) were broken down and split into two pieces, PET fibre reinforced concrete specimens were exposed to external cracks without disintegration. A clear anchorage of recycled PET fibres was apparent in specimens with fibres [29]. Even at the ultimate failure, the PET fibres were observed to keep the lumps of concrete together and this characteristic of concrete prevents sudden collapse [29]. Therefore, utilization of PET fibres as reinforcements in concrete enhanced ductility of concrete specimens by providing better resistance against deformations induced by impact loads and shocks.

4 Conclusions

In this study, a comprehensive and comparative laboratory analyses were performed in order to investigate the effect of recycled PET fibres of different geometries on mechanical properties of concrete. Fibres with 0.45, 0.65, and 1.0 mm diameters and lengths of 20 and 30 mm were incorporated into concrete at 0.5%, 1.0%, 1.5%, and 2.0% in volume basis. PET added and control concrete specimens were prepared to investigate the behaviour of recycled PET fibre reinforced concrete compared to conventional concrete during compression, flexural and splitting tensile strength tests and plastic shrinkage and impact resistance tests. The environmental advantage of effective utilization of this non-biodegradable material is another preliminary motivation for the present study. The following conclusions are drawn based on results presented above.

1) Slump cone test results revealed that for all diameter fibres, reduction in consistency of fresh concrete samples were observed with respect to increase in the volume fraction. Up to 76% and 80% reduction in slump was observed for 30 and 20 mm fibres, respectively. This was an expected behaviour due to increased surface area and inter particle friction due to the addition of fibres.

2) Addition of PET fibres with diameters of 0.45 and 0.65 mm typically exhibited around 14.5% higher compressive strength compared to control concrete up to 1.0% volume fractions. However, further introduction of those fibres resulted in reduction of compressive strength. On the other hand, 1.0 mm fibres exhibited a decrease in strength at all fibre volume fractions. The highest reduction of 15% was observed at 2.0% fibre addition.

3) Incorporation of PET fibres generally does not have any detrimental effect on flexural strength up to 1.5% and on splitting tensile strength up to 1.0% volume fractions. Results also brought out that, at higher fibre dosages, the bridging effect of fibres was hindered by formation of air voids due to entanglement. Maximum improvements in flexural and splitting tensile strengths were 10% and 27% which were obtained by 0.45 mm diameter fibres with 30 mm lengths.

4) The addition of PET fibres improved the resistance of concrete against plastic shrinkage cracking substantially up to 1.5% volume fraction. Reduction in crack widths between 62%–100% with reference to control concrete was achieved by all fibre types.

5) Even though PET fibres had no distinct influence on impact resistance to the first crack, excellent improvements of the impact energies (up to 4.5–10 times compared to control concrete) was achieved by all types of fibres.

References

[1] Associationof Plastic Manufacturers. An analysis of European plastics production, demand and waste data. 2018 (available at the website of Plasticseurope)

[2] PlasticsEurope (PEMRG). Production of plastics worldwide from 1950 to 2019 (in million metric tons). 2020 (available at the website of Statista)

[3] BusinessWire. Polyethylene terephthalate (PET) production worldwide in 2014 and 2020 (in million metric tons). 2015 (available at the website of Statista)

[4] Pereira de OliveiraL A, Castro-GomesJ P. Physical and mechanical behaviour of recycled PET fibre reinforced mortar. Construction & Building Materials, 2011, 25( 4): 1712– 1717

[5] AzhdarpourA M, NikoudelM R, TaheriM. The effect of using polyethylene terephthalate particles on physical and strength-related properties of concrete: A laboratory evaluation. Construction & Building Materials, 2016, 109 : 55– 62

[6] BuiN K, SatomiT, TakahashiH. Recycling woven plastic sack waste and PET bottle waste as fiber in recycled aggregate concrete: An experimental study. Waste Management (New York, N.Y.), 2018, 78 : 79– 93

[7] WonJ P, JangC I, LeeS W, LeeS J, KimH Y. Long-term performance of recycled PET fibre-reinforced cement composites. Construction & Building Materials, 2010, 24( 5): 660– 665

[8] FraternaliF, CianciaV, ChechileR, RizzanoG, FeoL, Incarnato L. Experimental study of the thermo-mechanical properties of recycled PET fiber-reinforced concrete. Composite Structures, 2011, 93( 9): 2368– 2374

[9] PelisserF, MontedoO R K, GleizeP J P, RomanH R. Mechanical properties of recycled PET fibers in concrete. Materials Research, 2012, 15( 4): 679– 686

[10] BorgR P, BaldacchinoO, FerraraL. Early age performance and mechanical characteristics of recycled PET fibre reinforced concrete. Construction & Building Materials, 2016, 108 : 29– 47

[11] KimS B, YiN H, KimH Y, KimJ H J, SongY C. Material and structural performance evaluation of recycled PET fiber reinforced concrete. Cement and Concrete Composites, 2010, 32( 3): 232– 240

[12] WangJ Y, ChiaK S, LiewJ Y R, ZhangM H. Flexural performance of fiber-reinforced ultra lightweight cement composites with low fiber content. Cement and Concrete Composites, 2013, 43 : 39– 47

[13] OchiT, OkuboS, FukuiK. Development of recycled PET fiber and its application as concrete-reinforcing fiber. Cement and Concrete Composites, 2007, 29( 6): 448– 455

[14] SivakumarA, SanthanamM. A quantitative study on the plastic shrinkage cracking in high strength hybrid fibre reinforced concrete. Cement and Concrete Composites, 2007, 29( 7): 575– 581

[15] KimJ H J, ParkC G, LeeS W, LeeS W, WonJ P. Effects of the geometry of recycled PET fiber reinforcement on shrinkage cracking of cement-based composites. Composites Part B, Engineering, 2008, 39( 3): 442– 450

[16] ASTMStandard C192/192M-16a. Making and Curing Concrete Test Specimens in the Laboratory. West Conshohocken, PA: ASTM, 2016

[17] ASTMStandards C1579-13. Standard Test Method for Evaluating Plastic Shrinkage Cracking of Restrained Fiber Reinforced Concrete (Using a Steel Form Insert). West Conshohocken, PA: ASTM, 2013

[18] ASTMStandard C403/403M. Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance. West Conshohocken, PA: ASTM, 2008

[19] ACICommittee 544. Measurement of Properties of Fiber Reinforced Concrete. Farmington Hills: American Concrete Institute, 1988

[20] MararK, ErenÖ, ÇelikT. Relationship between impact energy and compression toughness energy of high-strength fiber-reinforced concrete. Materials Letters, 2001, 47( 4−5): 297– 304

[21] MezaA, SiddiqueS. Effect of aspect ratio and dosage on the flexural response of FRC with recycled fiber. Construction & Building Materials, 2019, 213 : 286– 291

[22] Al-HadithiA I, NoamanA T, MoslehW K. Mechanical properties and impact behavior of PET fiber reinforced self-compacting concrete (SCC). Composite Structures, 2019, 224( March): 111021

[23] YinS, Tuladhar R, ShiF, CombeM, CollisterT, SivakuganN. Use of macro plastic fibres in concrete: A review. Construction & Building Materials, 2015, 93 : 180– 188

[24] De SilvaS, PrasanthanT. Application of recycled PET fibers for concrete floors. Engineer, 2019, 52( 1): 21– 27

[25] ShahidanS, RanleN A, ZukiS S M, KhalidF S, RidzuanA R M, NazriF M. Concrete incorporated with optimum percentages of recycled polyethylene terephthalate (PET) bottle fiber. International Journal of Integrated Engineering, 2018, 10( 1): 1– 8

[26] GuL, Ozbakkaloglu T. Use of recycled plastics in concrete: A critical review. Waste Management (New York, N.Y.), 2016, 51 : 19– 42

[27] AlaniA H, BunnoriN M, NoamanA T, MajidT A. Mechanical characteristics of PET fibre-reinforced green ultra-high performance composite concrete. European Journal of Environmental and Civil Engineering, 2022, 26( 7): 2797– 2818

[28] MastaliM, DalvandA, SattarifardA. The impact resistance and mechanical properties of the reinforced self-compacting concrete incorporating recycled CFRP fiber with different lengths and dosages. Composites. Part B, Engineering, 2017, 112 : 74– 92

[29] BhogayataA C, AroraN K. Impact strength, permeability and chemical resistance of concrete reinforced with metalized plastic waste fibers. Construction & Building Materials, 2018, 161 : 254– 266

[30] MuraliG, VenkateshJ, LokeshN, NavaT R, KarthikeyanK. Comparative experimental and analytical modeling of impact energy dissipation of ultra-high performance fibre reinforced concrete. KSCE Journal of Civil Engineering, 2018, 22( 8): 3112– 3119

RIGHTS & PERMISSIONS

Higher Education Press