Effect of Recycled Polyethylene Terephthalate (PET) fibres on fresh and hardened properties of concrete: a review

ABSTRACT

Concrete is poorly tensile in strength and susceptible to cracking. Therefore, researchers have explored the incorporation of PET fibres into concrete as reinforcement. PET is a polymer commonly used in food and beverage packaging, and textile industry. This study reviews the fresh and hardened properties of recycled PET fibre-reinforced concrete (rPET-FRC). The workability, compressive, tensile, and flexural strength are investigated under factors such as rPET fibre volume, dimensions, water–cement ratio, and the use of blended cement. This review includes experimental results of previously conducted rPET-FRC research along with results gained from rPET-FRC where the rPET fibres are obtained from the same recycling plant. Furthermore, the experimental results are broadly elaborated by explaining the reasons behind the obtained results. This review is a valuable reference that offers insights into the effectiveness of rPET fibres in enhancing concrete’s overall mechanical properties. By highlighting the most effective rPET fibre volume fractions, water-cement ratios, and the influence of blended cement, this review enables researchers to optimise the design and application of rPET-FRC, promoting sustainable and innovative solutions in the global infrastructure sector. Further studies to be carried out in the future are recommended and existing research gaps related to rPET-FRC are also highlighted.

KEYWORDS:

• Fibre-reinforced concrete
• Polyethylene Terephthalate
• Recycled PET
• PET fibre
• mechanical strength
• workability

1. Introduction

Concrete is one of the main construction materials in the world as it forms the basis of modern life due to its versatility, strength, durability, and cost-effectiveness (Koo et al. 2014). Even though concrete has many versatile properties, there are some drawbacks in concrete. Compared to other typical building materials, concrete has a relatively low tensile strength and low ductility. As a result, concrete is more prone to cracking (Huang et al. 2022). When the load is applied to unreinforced concrete, these micro-cracks tend to amalgamate and form macro-cracks (Thomas and Moosvi 2020). Upon further loading, the macro-cracks can result in catastrophic concrete failure (Banthia, Zanotti, and Sappakittipakorn 2014). Reinforcement bars in concrete are used as continuous bars to withstand tensile and shear stresses. However, unlike reinforcement bars, the fibres in concrete are discontinuous and more randomly dispersed throughout the cementitious matrix (Veigas, Najimi, and Shafei 2022). This results in concrete achieving better crack control (Mukhopadhyay and Khatana 2015). Hence, fractures from micro- and macro-cracks can be lessened using reinforcement materials in the form of fibres such as nylon, polypropylene (PP), polyvinyl chloride (PVA), polyethylene terephthalate (PET), steel, acrylic, and aramid (Barzin 2011; Mello, Ribellato, and Mohamedelhassan 2014). Therefore, it is evident from past research that the addition of various types of fibres to concrete has caused a significant improvement in the mechanical properties of concrete (Ahmed et al. 2021; Veigas, Najimi, and Shafei 2022). Among commonly used fibres in concrete, PET fibres have become an upcoming research area currently.

PET is a synthetic thermoplastic polymer mostly used in packaging, textile fibres, beverages, and other liquid containers (Nkomo et al. 2022). They are mostly discarded in landfills which creates not only environmental issues but also disposal problems which results in drain blockages and sometimes flooding (Becerril-Arreola and Bucklin 2021; Choudhary, Sangwan, and Goyal 2019). Hence, PET has become a major environmental pollutant due to its heavy and one-time usage. Past studies have been done under the incorporation of PET fibres in concrete in order to improve various mechanical properties because fibre reinforcement has shown positive results in improving crack control of concrete and also as a measure to minimise environmental pollution caused by PET fibres (Choudhary, Sangwan, and Goyal 2019; Pereira De Oliveira and Castro-Gomes 2011; Thomas and Moosvi 2020). PET fibre was proved to be used in various applications such as loadbearing bricks, walls, spraying and lining tunnels, and paving in road construction (Benthyathiar et al. 2022; Ochi, Okubo, and Fukui 2007). Moreover, since cement is one of the major constituents in concrete, which is one of the major environmental pollutants, the modern construction industry is more likely to use secondary materials for the replacement of cement. Therefore, use of blended cement is an effective way of reducing the environmental pollution caused by cement production by enhancing the physical properties as well (Hussien and Mohammed 2022). Therefore, researchers have expanded their focus on enhancing the properties of PET fibre concrete while using secondary cementitious materials for cement replacement.

Recycled PET fibres, also known as rPET fibres, are fibres made by recycling PET plastic. rPET plastic involves a process called depolymerisation, in which the plastic is broken down into its individual monomers (PET) and then reassembled into new plastic products (Gudayu et al. 2021). rPET fibres have numerous benefits over virgin PET fibres, including reduced energy consumption and greenhouse gas emissions during production, and a lower overall environmental impact. They also help to reduce waste by repurposing materials that would otherwise end up in landfills. In addition, the use of rPET fibres can help to conserve resources, as it takes less energy and raw materials to produce rPET fibres compared to virgin fibres (Meawad, and Ibrahim 2019). Moreover, when the performance of rPET fibres is considered, they are stronger and more durable, making them suitable for use in a variety of applications (Taherkhani 2014; Won et al. 2010).

A thorough search has been conducted on several research articles having rPET fibres to overcome environmental impacts caused due to disposal of PET plastic waste in the current world. Reusing waste not only solves the problem of landfills but also provides an attractive way to use these materials in the construction sector. However, there is a variation of the experimental results of mechanical properties such as compressive, tensile and flexural strengths of rPET-FRC in past literature (Corinaldesi and Nardinocchi 2016; Nkomo et al. 2022). As per several past studies it is mentioned that the mechanical strength improves with the inclusion of the rPET fibres in concrete, while some other studies show contrasting results (Payrow et al. 2011). Major concerns for this variation of past results from each other are the difference in the original source of the rPET fibres obtained, difference in the dimensions, geometry and the fibre volume fraction, difference in water–cement (w/c) ratios and the use of blended cement (Ambassah 2019; Barbuta et al. 2017; Ochi, Okubo, and Fukui 2007). Therefore, it is important to compare the effect of such factors on the fresh and hardened properties of rPET-FRC, where the rPET fibres are obtained from the same recycling plant, along with other existing studies on rPET-FRC. Therefore, this paper presents a review of the research work done up to date regarding the mechanical properties of rPET-FRC where rPET fibres have been collected from a local recycling plant in Sri Lanka along with experimental results retrieved from other past studies on rPET-FRC. The fresh and hardened properties of rPET fibre concrete have been evaluated under several factors such as the effect of fibre dimensions, w/c ratio, and the use of blended cement. The authors have carefully discussed the relevant properties of rPET-FRC to determine the effects of fibre dimensions, w/c ratio, and the use of blended cement on the fresh and hardened properties of concrete. This paper provides valuable information for the researchers for further studies.

2. Significance of the review

The interest in rPET-FRC is on the rise in the construction industry, being recognised as an effective solution to the environmental issues posed by rPET plastics and a means to enhance concrete’s tensile strength, ductility capacities, and crack control. Consequently, investigating past studies focusing on the effects of rPET fibre volume fraction, dimensions, and other factors on the fresh and hardened properties of rPET-FRC becomes critical in identifying knowledge gaps for future investigations. Therefore, this paper is dedicated on comprehensively reviewing the most recent studies to understand how fibre dimensions, volume fraction, w/c ratio, and novel insights such as the use of blended cement influence the fresh and mechanical properties of rPET-FRC as well.

This paper includes research studies investigating the fresh and hardened properties of rPET-FRC, where rPET fibres originate from a specific recycling plant in Sri Lanka along with relevant past literature on rPET-FRC as well. Although past studies have explored the mechanical performance of rPET-FRC, comparing their results can be challenging due to variations in the original sources of rPET fibres, impacting both the properties of the fibres and the concrete containing them. Hence, this review paper has gathered studies that utilise the same type of rPET fibres from the same recycling plant as a measure to address the material differences that affect the properties of rPET-FRC and facilitate meaningful comparisons along with previous research. Moreover, no previous review papers were found on elaborating the effect of rPET fibre dimension, w/c ratio, and use of blended cement separately on the mechanical properties of rPET-FRC. Therefore, this review paper can be considered as a valuable source on the effect of critical factors on the mechanical properties of rPET-FRC.

3. Methodology

3.1. Literature search

The literature search for the review was carried out from October 2022 to August 2023 and comprised both searching grey literature and academic literature. The search was done using Web of Science, Scopus, Science Direct, Directory of Open Access Journals and Google (Scholar) search engines. For practical reasons, the academic literature search was conducted in English. No date restrictions were set during the search for the literature, whereas all literature that could be found regarding the scope of the study were used. Following the focus of waste material incorporated cementitious composites, the literature search concentrates on PET fibre reinforced concrete. The methodology used for the literature search is based on the rapid review approach as a less time-consuming alternative to a systematic review. First, all the past literature related to PET fibre reinforced cementitious composites up to date were gathered and categorised under the type of PET fibre used (recycled/virgin), type of the cementitious composite used (concrete/mortar/cement paste). This was done to narrow down and decide on the scope of the review paper. In total, the search resulted in a collection of 70 sources (academic and grey literatures) regarding PET fibre reinforced cementitious composites and after categorisation, 29 papers were listed under the usage of rPET fibre, while 41 papers regarding virgin PET fibres. Furthermore, out of the total of 70 sources, 52 were literature conducted on PET-FRC, whereas 11 sources were on PET fibre reinforced mortar and 7 sources on PET fibre reinforced cement paste. Therefore, the scope of the review paper was decided to be concentrated on fibre reinforced concrete including both recycled and virgin PET fibres. Moreover, the review paper was aimed to focus on the engineering performance of recycled/virgin PET-FRC. As the next step, the engineering performance characteristics that the selected past literature have investigated are summarised. These 52 sources included characteristics such as compressive strength, tensile strength, flexural strength and workability which were selected as the parameters to be elaborated in the review paper. Furthermore, out of the selected sources, three major sources were identified as studies conducted using the same type of recycled PET fibres obtained from the same recycling plant in Sri Lanka. Therefore, based on those three similar papers, this review paper was separated into sections: the effect of fibre dimensions, fibre volume fractions, w/c ratio, and the effect of use of blended cement in rPET-FRC. However, no attempt was made to evaluate the engineering performance of PET fibre reinforced concrete in the past literature by ourselves; only readily available information on the engineering performance of PET-FRC was collected.

4. Effect of fibre dimension on mechanical properties of recycled PET fibre concrete

4.1. Compressive strength

In the study conducted on the enhancement of mechanical strength using short rPET fibres in concrete by (Sandaruwini, Bandara, and De Silva 2012), three volume fractions of rPET fibre (1%, 2%, 3%) with two types of diameters 0.91 mm and 0.56 mm having a length of 55 mm were used. Compressive strength, flexural strength, splitting tensile strength and workability of rPET fibre concrete were assessed in this study. The relevant results obtained by (Sandaruwini, Bandara, and De Silva 2012) in comparison with other recent studies are discussed in the following sections. Test results for compressive strength are shown in Figures 1 and 2 for Plain Concrete and rPET-FRC of both diameters for a w/c ratio of 0.62.

Figure 1. Average compressive strength of plain concrete and rPET-FRC (ϕ=0.91mm) (Sandaruwini, Bandara, and De Silva 2012).

The average compressive strength variation of Plain concrete and rPET-FRC for rPET fibres (diameter = 0.91mm) with the rPET fibre percentage.

Figure 2. Average compressive strength of plain concrete and rPET-FRC (ϕ=0.56mm) (Sandaruwini, Bandara, and De Silva 2012).

The average compressive strength variation of Plain concrete and rPET-FRC for rPET fibres (diameter = 0.56mm) with the rPET fibre percentage.

A decrease in the compressive strength of both types of rPET fibre concrete was observed when compared with plain concrete. When considering 0.91 mm and 0.56 mm diameter fibre mixes, the maximum compressive strength has been achieved for 1% of rPET fibres, but the compressive strength has reduced from 28% to 4%, respectively, when compared to plain concrete (Sandaruwini, Bandara, and De Silva 2012). Furthermore, as the use of the rPET fibre volume fraction increases, the compressive strength also seems to decrease. When considering the 0.91 mm diameter fibre mix, the compressive strength has decreased further by 30% and 31% for 2% and 3% fibre volume fractions, respectively (Sandaruwini, Bandara, and De Silva 2012). Similarly, in the 0.56 mm diameter fibre mix, the compressive strength has decreased up to 14% and 16% for 2% and 3% fibre volume fractions, respectively, when compared with that of plain concrete. According to the study by (De Silva and Prasanthan 2019), the optimum volume fraction for rPET fibres of diameter 0.7 mm and 50 mm long in concrete for compressive strength was identified as 1% with an increment of compressive strength by 15.30%. With the use of rPET fibres of 35 mm length, 5 mm width, and 0.2 mm thickness, a maximum reduction of the compressive strength of 14% occurred for 3% fibre volume fraction because of the smooth texture of the rPET fibres in the mix which affects the bonding properties and adhesion of the concrete ingredient materials (Hidaya, Mutuku, and Mwero 2017). The same variation of decrement of compressive strength with the incorporation of rPET fibres was observed in the study by (Nkomo et al. 2022) where fibres extruded into 12 mm were used. Similarly, by the addition of higher rPET fibre volume fraction in concrete, they can also create additional internal stresses within the concrete, which can weaken the overall structural component (Irwan et al. 2013; Nagarnaik et al. 2013). This is especially true if the rPET fibres are not evenly distributed within the concrete, as this can create local areas of high stress (Fraternali, Spadea, and Berardi 2014; Mwonga, Kabubo, and Gathimba 2022). According to (Borg, Baldacchino, and Ferrara 2016), the addition of rPET fibres leads to a reduction in compressive strength between 0.5% and 8.5% when compared to the control mix, while a better performance was achieved using the 30-mm-long fibres than 50-mm-long fibres. On the contrary, the study by (Marthong 2015) shows that with the addition of rPET fibres of two different dimensions, a better improvement can be obtained for specimens made from a smaller fibre with a length of 50 mm and a width of 0.5 mm as compared to the larger fibre which has a length of 100 mm and a width of 1 mm. However, on further increasing the fibre volume fraction beyond 0.5% a decrease in compressive strength is assured (Marthong 2015; Nagarnaik et al. 2013).

The highest compressive strength with rPET fibres was found when 1% of 0.56 mm diameter rPET fibres were used (Sandaruwini, Bandara, and De Silva 2012). The compressive strength has further decreased when the diameter of the fibres used has increased. Similarly, (Taherkhani 2014) showed that among the 1 cm, 2 cm and 3-cm-long rPET fibres, the best performance was achieved with the 1-cm-long PET fibres. However, the compressive strength was still reduced by 6.5% and 32% when 0.5% and 1% of fibre volume fractions of 1-cm-long PET fibres were added. This indicated that the compressive strength of the PET-FRC mixtures decreases with increasing fibre volume fraction and length (Taherkhani 2014). The same variation was found in the study by (Marthong 2015), where the effect of dimensions of rPET fibres on compressive strength was investigated. (Foti 2011; Foti 2013; Foti 2019) reported a declining trend in compressive strength when incorporating ‘O’-shaped virgin PET fibres at 0.5% and 0.75% volume fractions, resulting in reductions of 37% and 33%, respectively. Similarly, the addition of 0.5% short lamellar virgin PET fibres led to a 32% decrease in compressive strength, reducing it from 59.1 MPa to 40.19 MPa. In contrast, (Khalid et al. 2018) found an increasing trend in compressive strength with the incorporation of ring-shaped virgin PET fibres in concrete. The highest compressive strength, 35.29 MPa (+4%) was observed with a 1.5% content of 5 mm thick ring-shaped virgin PET fibres, compared to the reference concrete mix with 34.01 MPa. Additionally, a 0.75% volume fraction of ring-shaped virgin PET fibres resulted in a compressive strength of 35.23 MPa (Khalid et al. 2018). Notably, among all PET fibre geometries studied, the 10 mm thick ring-shaped fibres exhibited the highest compressive strength of 35.45 MPa (+5%). A study by (Kim et al. 2010) showed that the use of embossed rPET fibres with 50 mm length, 1.3 mm width, and 0.2 mm thickness at 0.5%, 0.75% and 1.0% volume fractions can cause the compressive strength to be decreased within a range of 1–9% because the fibres can interfere with the formation of the ‘cementitious matrix’, which is the structure that gives concrete its strength. When the fibres are beyond the most optimum dimensions and the fibre volume fraction, they can create a cementitious matrix which is less efficient at transferring loads causing overall less mechanical strength (Kim et al. 2010; Meza et al. 2021). Furthermore, adding rPET fibres beyond the optimum amount or rPET fibre exceeding the optimum dimensions can also increase the porosity of the concrete, which can reduce its compressive strength (Irwan et al. 2013; Trejbal et al. 2016). Since porosity is the measure of the voids or empty spaces within the concrete, a higher porosity means that there is less solid material to carry the load. This can lead to a reduction in the compressive strength of the concrete (Marthong 2015). It is worth noting that the effect of rPET fibres on the compressive strength of concrete is not always negative (Pereira, de Oliveira Junior, and Fineza 2017). However, the optimal volume fraction of rPET fibres for a given application will depend on a variety of factors, including the volume fraction of rPET fibres being used and the specific properties that are desired in the final product.

4.2. Splitting tensile and flexural strength

A significant improvement in the flexural strength and splitting tensile strength with 1% and 2% volume fractions were observed by (Sandaruwini, Bandara, and De Silva 2012) when it comes to 0.56 mm diameter rPET fibre mix. When considering the flexural strength of 0.56 mm diameter rPET fibre mix, an improvement of 4% and 2% has been recorded for 1% and 2% fibre volume fractions, respectively, when compared with plain concrete. In addition, for tensile strength, an improvement of 5% and 4% has been recorded for 1% and 2% fibre volume fractions, respectively, when compared with plain concrete. Therefore, from the results, it was revealed that the addition of 1% of 0.56 mm diameter rPET in concrete has better splitting tensile and flexural strength properties (Sandaruwini, Bandara, and De Silva 2012). However, when considering the splitting tensile and the flexural strength in 0.91 mm diameter rPET fibre concrete, it is noticeable that the splitting tensile and flexural strengths have decreased compared to that of plain concrete (Sandaruwini, Bandara, and De Silva 2012). The flexural strength has decreased from 9%, 3%, 5% for rPET fibre volume fractions of 1%, 2%, and 3%, respectively, when compared with plain concrete. Similarly, when considering the tensile strength, it has also reduced from 20%, 19%, and 1% for rPET fibre volume fractions of 1%, 2%, and 3%, respectively, when compared with plain concrete according to the study by (Sandaruwini, Bandara, and De Silva 2012). The average splitting tensile and flexural strengths of rPET fibre concrete with the two different fibre diameters (0.56 mm and 0.91 mm) are given in Figures 3 and 4.

Figure 3. Average flexural and splitting tensile strength of plain concrete and rPET-FRC (ϕ=0.91mm) (Sandaruwini, Bandara, and De Silva 2012).

The flexural and split tensile strength variations of Plain concrete and rPET-FRC for rPET fibres (diameter = 0.91mm) with the rPET fibre percentage.

Figure 4. Average flexural and splitting tensile strength of plain concrete and rPET-FRC (ϕ=0.56mm) (Sandaruwini, Bandara, and De Silva 2012).

The flexural and split tensile strength variations of Plain concrete and rPET-FRC for rPET fibres (diameter = 0.56mm) with the rPET fibre percentage.

According to the study by (De Silva and Prasanthan 2019), the maximum splitting tensile strength was observed when 1% of rPET fibres of 0.7 mm and 50 mm length were added into concrete where the splitting tensile strength was improved by 18.77%. Moreover, the same type of rPET fibres was able to achieve the highest flexural strength at a volume fraction of 1% by improving the flexural strength by 22.44% (De Silva and Prasanthan 2019). Studies by (Salhotra, Khitoliya, and Kumar 2021; Salhotra, Khitoliya, and Kumar Arora 2021) show that both splitting tensile and flexural strengths decrease with the increment of fibre aspect ratio where the splitting tensile strength has decreased from 4.65 MPa to 4.35 MPa when the aspect ratio was increased from 25 to 75, respectively. Similarly, the flexural strength has reduced from 6.20 MPa to 5.43 MPa with the increment of fibre aspect ratio from 25 to 75 for 3% rPET fibre volume fraction (Salhotra, Khitoliya, and Kumar 2021; Salhotra, Khitoliya, and Kumar Arora 2021). Furthermore, it was observed that although 3% of PET fibres with an aspect ratio of 25 achieved the maximum flexural strength of 4.38 MPa, it still resulted in a 5% strength reduction (Salhotra, Khitoliya, and Kumar Arora 2022; Salhotra, Khitoliya, and Kumar Arora 2021). Because of rPET fibres addition, an enhancement in splitting tensile strength in the range of 2–6% and 4–7% for 2.3 and 3.0 aspect ratios, respectively, is shown in the study by (Khatab, Mohammed, and Hameed 2019). According to (Marthong 2015), smaller fibre dimensions with a length of 5 mm and width of 0.5 mm demonstrated an improvement in the tensile strength due to strong short fibre dispersions, which led to enhanced bridging action in the concrete matrix. The tensile strength, however, reduced as the fibre volume fraction increased above 0.5% (Marthong 2015). Similarly, (Mohammed and Rahim 2020) found that the incorporation of 0.75% and 1% of 20-mm-long PET fibres resulted in a decrease in both split tensile and direct tensile strengths of concrete. The split tensile strength dropped from 4.36 MPa to 3.95 MPa and 3.87 MPa when 0.75% and 1% of PET fibres were added, respectively. In contrast to the aforementioned findings (Khalid Ali, Ismail Al-Hadithi, and Tareq Noaman 2022), demonstrated that the inclusion of 0.5% and 1% of 50-mm-long PET fibres led to an increase in split tensile strength by 12% and 5%, respectively. These results were further supported by (Marthong and Marthong 2016), who observed a 50% and 15% enhancement in split tensile strength with the addition of 0.5% and 1% of 50-mm-long PET fibres in concrete. In a separate study by (Khalid et al. 2018), the incorporation of ring-shaped 5-mm-thick PET fibres led to notable increases in split tensile strength. With 0.5%, 1%, and 1.5% PET fibre volume fractions, the strengths were enhanced from 3 MPa to 3.5 MPa, 4.0 MPa, and 3.4 MPa, respectively. Additionally, the split tensile strength values for ring-shaped 10-mm-thick PET-FRC increased to 3.5 MPa, 4.2 MPa, and 3.7 MPa with 0.5%, 1%, and 1.5% of PET fibre volume fractions, respectively (Khalid et al. 2018). It is explained that the rPET fibres tend to bond with concrete components and work towards reinforcing processes that result in a conveyor medium for the stresses generated in the cracked locations (Alshkane et al. 2016; Khatab, Mohammed, and Hameed 2019). Pereira, de Oliveira Junior, and Fineza (2017) showed that the synergistic effect between the factors fiber volume fraction and fiber length both influence the concrete tensile effiiciency, but the fiber length is more influencial for tensile strength. Studies on rPET fibre concrete further state that due to the fibre-bridging action the fibres imparted during cracking, the addition of rPET fibre increases the tensile performance and demonstrates the potential to absorb energy in the post-cracking condition (Fraternali et al. 2013; Marthong and Marthong 2016; Meza and Siddique 2019).

On the other hand, using 0.75% of ‘O’-shaped PET fibres resulted in only a 2% improvement in flexural strength, increasing it from 4.5 MPa to 4.55 MPa (Foti 2011; Foti 2013; Foti 2019). On the contrary, the use of 0.5% of ‘O’-shaped and short lamellar fibres led to reductions in flexural strength, resulting in 3.7 MPa and 3.6 MPa, respectively (Foti 2011; Foti 2013; Foti 2019). Furthermore, as the fibre volume fraction was increased up to 1%, there was a notable reduction up to 9.56 MPa, causing an 11% drop in flexural strength (Marthong 2015). Conversely, incorporating 0.5% of 40 mm long flattened-end PET fibres resulted in an increase in flexural strength up to 14.87 MPa, while a 1% fibre content caused a decrease to 10.15 MPa (Marthong 2015). On the other hand, utilising 0.5% of long (80–100 mm) straight slit and flattened-end PET fibres led to an 8% increment in flexural strength, reaching 11.66 MPa. However, using 1% of the same fibre type resulted in a significant drop of 25%, reducing the flexural strength drastically to 8.23 MPa (Marthong 2015). Similar variations in flexural strength were observed by (Marthong and Marthong 2016) where 50-mm-long PET fibre volume fractions of 0.5% and 1% increased the flexural strength from 3.35 MPa to 4.10 MPa and 3.58 MPa, respectively. Moreover, the study by (Assaad, Khalil, and Khatib 2022) explains that the behaviour of RC beams is mostly controlled by the yielding of tensile steel, hence the flexural strength of RC beams does not diminish even with 3% or 4.5% PET inclusions.

Although the study by (Nkomo et al. 2022) presents that the splitting tensile strength tends to decrease even than conventional concrete for all rPET fibre volume fractions, both Nkomo et al. (2022) and Al-Hadithi, Abdulrahman, & Al-Rawi (2020) showed that the addition of rPET fibres positively impacted on delaying the development and propagation of cracks during testing. The reduction of splitting tensile strength and flexural strength in 0.91 mm diameter rPET fibre concrete could be because of the air voids formed within the concrete due to fibre clumping (Irwan et al. 2013; Meza et al. 2021). It is worth noting that the effect of rPET fibres on the tensile and flexural strength of concrete is not always positive. The fibres can interfere with the bond between the cement paste and the aggregate, which can reduce the overall strength of the material (Mechtcherine 2012; Shahidan et al. 2018; Trejbal et al. 2016). The optimal volume fraction of rPET fibres for a given application will depend on the specific properties that are desired in the final product, as well as the type of cement and other materials used in the mixture (Meza and Siddique 2019). However, it has been investigated that rPET fibres enhance the mechanical properties of concrete at lower rPET fibre volume fractions (Nkomo et al. 2022). When fibres are added to concrete, they act as reinforcement to help resist tensile stresses (Kim et al. 2008). The fibres provide additional paths for the stress to be transmitted through the material, which can help to increase its overall tensile strength (Kim et al. 2010).

4.3. Workability

According to the study by (Sandaruwini, Bandara, and De Silva 2012) the variation of slump with the rPET fibre content for two types of rPET fibre diameters is tabulated in Table 1. With the increment of rPET fibres in the mix, it has been revealed that the workability also decreases. The highest slump for both 0.56 mm and 0.91 mm diameter rPET fibre mixes was obtained for 1% fibre volume fraction which are still 13% and 11% lesser, respectively, when compared with plain concrete. The lowest slump values were achieved when the rPET fibre volume fraction is at 3% for both mixes where the slump has decreased from 60% to 43% when compared with plain concrete (Sandaruwini, Bandara, and De Silva 2012). Similar behaviour of rPET fibre concrete has been investigated in the study by (Shahidan et al. 2018) where the slump has been reduced from 60 mm to 40 mm when the rPET fibre volume fraction was increased from 2% to 5%, respectively. Supporting these findings, (Adnan and Dawood 2020) also concluded that the slump reduced from 200 mm to 175 mm and 120 mm when the PET fibre volume fraction was increased from 0% to 1.5% and 3%, respectively. A study by (Ambassah 2019) concluded that the workability decreased drastically with the addition of PET fibres in the concrete mix, where the reduction of slump was 50%, 75%, and 90% when 0.5%, 1.0%, and 1.5% PET fibre volume fractions were added. Furthermore, the study by (Marthong 2015) showed that with the addition of 0.5% straight slit sheet PET fibres in the concrete mix significantly reduced the workability where the slump was reduced from 80 mm to 56 mm. However, similar slump values were observed when 0.5% of the flattened end and deformed slit sheet PET fibre were mixed with concrete, concluding that the workability does not change significantly when varied geometries of PET fibres with the same volume fraction were used (Marthong 2015). Continuous reduction of the slump with the addition of PET fibres in concrete was noticed in the study by (Hidaya, Mutuku, and Mwero 2017) through observing that the slump reduces by 33%, 48.9% and 62.2% for 1%, 2% and 3% PET fibre addition, respectively, as compared to the control mix. In contradiction, the study by (Jean et al. 2012) showed that the addition of 0.05% of PET fibres in concrete has caused a drastic increment of the slump up to 155 mm where the control mix with no fibres indicated only a slump of 100 mm. However, following the same trend as in other studies, the slump values dropped significantly up to 70 mm and 50 mm when the PET fibre volume fraction was increased up to 0.18% and 0.30%, respectively (Jean et al. 2012). It is important to note that the addition of 1.5% of PET fibres in the concrete mix caused a drastic reduction in workability denoting a slump of only 10 mm, while concrete with no fibre denoted a slump of 100 mm (Kassa, Kanali, and Ambassah 2019).

Table 1. Average slump variation of plain concrete and rPET-FRC (Sandaruwini, Bandara, and De Silva 2012).

	Slump (mm) (Φ=0.91 mm)	Slump (mm) (Φ=0.56 mm)
Plain Concrete	170	150
1% PET-FRC	150	130
2% PET-FRC	110	100
3% PET-FRC	70	85

This reduction in slump of concrete was attributed to the presence of fibres in the mix as they lump on each other, reducing the slump while the mixture is still workable (Mechtcherine 2012). Furthermore, a reduction in the workability of fresh concrete may be caused by an adhesion within the concrete and holding the other ingredients of concrete together impeding easy flow (Hidaya, Mutuku, and Mwero 2017). Moreover, it can be observed better workability can be obtained with rPET fibres with smaller diameter (0.56 mm). This reduction of slump when rPET fibres with a higher diameter (0.91 mm) are being used was attributed to the presence of larger rPET fibres in the mix as they lump each other. Another reason behind this decrease is due to the presence of rPET fibres in concrete which causes more friction between the particles and this leads to less workability in the mixtures (Irwan et al. 2013). In addition, the high volume fraction and large surface area of the fibres can easily absorb the cement paste thereby increasing the viscosity of the concrete mixture (Nkomo et al. 2022). As the rPET volume fraction increases, the plasticity and consistency of fresh concrete will decrease.

5. Effect of water–cement ratio on mechanical properties of rPET fibre concrete

The w/c ratio plays a significant role in determining the workability, strength, durability, shrinkage, and overall performance of concrete. In the context of rPET-FRC, the w/c ratio affects the dispersion and bonding of the rPET fibres within the concrete matrix, which, in turn, influences various properties of the composite material, especially the workability and the strength.

5.1. Compressive strength

The results obtained from the study by (Rathnayaka et al. 2015) are mentioned in Figures 5 and 6. It is noticeable that for a w/c ratio of 0.45 and 1% rPET fibre volume fraction, a maximum reduction of 42.35% occurs, but when the w/c ratio was reduced to 0.3, the maximum reduction was dropped drastically down to only a 11.29% when the compressive strength is considered. Moreover, the reduction of compressive strength compared to plain concrete when the w/c ratio is 0.45 for 2% and 3% fibre volume fractions are 42% and 38%, respectively (Rathnayaka et al. 2015). However, the reduction of compressive strength when the w/c ratio is 0.30 for 2% and 3% fibre volume fractions are only 9% and 11%, respectively. This further explains that when the w/c ratio is increased, the resultant reduction in the compressive strength also increases in rPET-FRC. Therefore, from the experimental results gained from the study it can be concluded that 0.3 w/c ratio can be considered as the optimum w/c ratio for the rPET-FRC (Rathnayaka et al. 2015).

Figure 5. Variation of compressive strength with rPET fibre volume fraction for w/c ratio of 0.45 (Rathnayaka et al. 2015).

The compressive strength variation of Plain concrete and rPET-FRC with the rPET fibre percentage for w/c ratio 0.45. The compressive strength is illustrated for 3, 7, 14, 21, and 28 days.

Figure 6. Variation of compressive strength with rPET fibre volume fraction for w/c ratio of 0.30 (Rathnayaka et al. 2015).

The compressive strength variation of Plain concrete and rPET-FRC with the rPET fibre percentage for w/c ratio 0.30. The compressive strength is illustrated for 3, 7, 14, 21, and 28 days.

According to the study by (Kim et al. 2010), rPET-FRC specimens exhibited compressive strength decrease within a range of 1–9% for a w/c ratio of 0.41. Moreover, it was noticed that the compressive strength varied from 27.9 MPa to 24.6 MPa for 1% rPET fibre volume fraction when the w/c ratio was increased from 0.45 to 0.65, respectively, indicating that the increase of w/c ratio causes reduction of compressive strength of rPET-FRC (Irwan et al. 2013). The decrease in compressive strength may be brought on by either a weak link between rPET fibre and cement pastes or by the low strength of rPET fibre itself (Irwan et al. 2013). Similarly, (Ochi, Okubo, and Fukui 2007) concluded that the increment of the w/c ratio from 0.55 to 0.65 reduces the compressive strength by 37% for 1% rPET fibre volume fraction, which was the identified optimum volume fraction. It was further noticed that the 1% rPET fibre volume fraction with 0.55 w/c ratio even showed a compressive strength higher than plain concrete by 6% (Ochi, Okubo, and Fukui 2007). Supporting the findings of (Fraternali et al. 2011; Ochi, Okubo, and Fukui 2007) also observed an increment of compressive strength by 35% for 1% rPET fibre volume fraction for a w/c ratio of 0.53.

In the study by (Assaad, Khalil, and Khatib 2022), the decrease in w/c from 0.55 to 0.46 has effectively contributed in making up for the compressive strength loss brought on by the inclusion of 4.5% rPET, which is explained by a decreased matrix porosity and improved concrete microstructure. Nevertheless, with a high w/c ratio of 0.65, the study by (Irwan et al. 2013) showed that the compressive strength increased up to 24MPa for a rPET fibre volume fraction of 0.5%, while the control sample only showed a compressive strength of 22MPa. As per (Foti 2011), the compressive strength of rPET-FRC dropped by 31.4% for a w/c ratio of 0.70 and a rPET fibre volume fraction of 0.5%. Furthermore, (Shahidan et al. 2018) proved that the addition of rPET fibres to concrete reduced its compressive strength. The study by (Payrow et al. 2011) shows on how plastic aggregate inclusion affected concrete’s compressive strength and discovered that, for any given level of plastic aggregate content, the compressive strength decreased with the w/c. Therefore, the results of the study by (Rathnayaka et al. 2015) agreed with the studies conducted by (Payrow et al. 2011; Shahidan et al. 2018) showing that the increase of fibre volume fraction and w/c ratio in concrete will affect the loss in compressive strength of concrete.

5.2. Splitting tensile strength

According to the study by (Rathnayaka et al. 2015), the splitting tensile strength of rPET fibre concrete increases with the inclusion of rPET fibres. Figure 7 shows that the samples with a w/c ratio of 0.3 have reached a higher tensile strength than the samples with a w/c ratio of 0.45. In both cases, the splitting tensile strength of rPET fibre concrete is higher than that of the control specimen despite the volume fraction of rPET used. In the case of a w/c ratio of 0.3, the maximum value for tensile strength is achieved at 1% of rPET fibre indicating an improvement of 8% when compared with plain concrete. However, for the case of 0.45 w/c ratio, the maximum tensile strength is achieved for 3% of rPET fibre volume fraction, indicating an improvement of 22% when compared with plain concrete. For both w/c ratios, 2% rPET fibre concrete has shown an improvement of 5% when compared with plain concrete. The reason for the improvement in the splitting tensile strength of concrete with rPET fibre addition would be that the fibres bridge across the micro cracks, and moreover, rPET fibre concrete specimens take longer time and higher load to fail (Ambassah 2019; da Silva Magalhães and Soares Viana Fernandes 2015). Therefore, the incorporation of rPET fibres in concrete can also improve the initial cracking strength and ultimate ductility index (Anandan and Alsubih 2021; Hidaya, Mutuku, and Mwero 2017). In contrast to the study by (Irwan et al. 2013; Rathnayaka et al. 2015) observed that the splitting tensile strength decreases with the addition of rPET fibres into concrete, indicating a loss of 15%, 18.5%, and 22% with 0.5%, 1.0% and 1.5% of rPET fibre volume fractions, respectively.

Figure 7. Variation of splitting tensile strength with rPET fibre volume fraction for different w/c ratios (Rathnayaka et al. 2015).

The split tensile strength variation of Plain concrete and rPET-FRC with the rPET fibre percentage for both w/c ratios 0.30 and 0.45.

However, when the w/c ratio is increased the splitting strength of the rPET fibre concrete is decreased for the optimum amount of fibres (1%) because the more water that is added to the concrete mix, the more the cement paste was diluted (Rathnayaka et al. 2015). As a result, the paste was not able to develop the same amount of strength as it does with less water. A similar variation in splitting tensile strength was observed by (Irwan et al. 2013) when the w/c ratio was increased from 0.45 to 0.65. The respective splitting tensile strength values with the w/c ratio as per the study by (Irwan et al. 2013) are tabulated in Table 2.

Table 2. The variation of the splitting tensile strength with the w/c ratio and PET fibre volume fraction (Irwan et al. 2013).

w/c ratio	PET fibre volume fraction (%)	Splitting tensile strength (MPa)
0.45	0	3.65
	0.5	3.57
	1.0	3.50
	1.5	2.56
0.55	0	3.52
	0.5	2.98
	1.0	2.87
	1.5	2.74
0.65	0	3.43
	0.5	3.24
	1.0	2.60
	1.5	2.37

This is because the water is needed to react with the cement and form the bond that holds the concrete together (Mechtcherine 2012). If there is excess water, the bond is weakened, resulting in a decrease in the strength of the concrete (Irwan et al. 2013). Moreover, the study by (Shahidan et al. 2018) showed that the splitting tensile strength of rPET-FRC increases by 7.7% for a rPET fibre volume fraction of 1% with a w/c ratio of 0.45. However, it was noticed that the split tensile strength was reduced by 3%, 15.4%, and 7.7% for rPET fibre volume fractions of 0.5%, 1.5%, and 2.0%, respectively (Shahidan et al. 2018). Nevertheless, the splitting tensile strength of rPET-FRC increased with a w/c ratio of 0.65 for rPET fibre volume fractions of 0.5%, 1.0%, and 1.5% denoting splitting tensile strengths of 3.24 MPa, 3.43 MPa, and 3.67 MPa, respectively (Irwan et al. 2013), whereas concrete with no rPET fibres showed a splitting tensile strength of only 2.97 MPa (Irwan et al. 2013). Study by (Ambassah 2019) also showed an increment of splitting tensile strength from 2.5 MPa to 3.3 MPa when the PET fibre volume fraction also increased from 0% to 1.5%.

5.3. Workability

As per the study by (Rathnayaka et al. 2015), the slump of rPET fibre concrete has significantly increased with the increment of w/c ratio from 0.3 to 0.45. The slump in rPET-FRC has decreased when compared with plain concrete in all cases. However, the highest slump was attained for both w/c ratios of 0.45 and 0.30 for 1% rPET fibre volume fraction, indicating 6% and 45% loss of slump, respectively, when compared with plain concrete (Rathnayaka et al. 2015). For w/c ratio 0.3, with the inclusion of rPET fibres upto 2% and 3%, the slump has decreased drastically from 50% to 72%, respectively. However, the reduction of slump for 2% and 3% rPET fibre volume fractions for w/c ratio 0.45 was only 12% and 48%, respectively (Rathnayaka et al. 2015). The reason for this phenomenon is the fact that when the amount of rPET fibre mixed was increased, due to the increment of surface area of fibres in the mixture, an extra demand of water was required to increase the friction to coat the fibres (Mwonga, Kabubo, and Gathimba 2022). Therefore, with the increment of w/c ratio in concrete, the slump also increases since the rPET fibres are more capable of dispersing freely within concrete enabling concrete to have a better flow (Hidaya, Mutuku, and Mwero 2017). These results were further validated by (Irwan et al. 2013), where the slump of rPET-FRC has reduced within the ranges of 20–80%, 3–29%, and 5–38% for w/c ratios of 0.45, 0.55 and 0.65, respectively. However, the slump continued to decrease with the increment of rPET fibre volume fraction in concrete.

Similarly, according to the study by (Shahidan et al. 2018), plain concrete with no PET fibres showed a slump of 90 mm with a w/c ratio of 0.45 and for the same w/c the slump drastically dropped up to 60 mm for a PET fibre volume fraction of 0.5% and it continued to decrease with the increment of PET fibre volume fraction until the slump reached 40 mm for 2% of PET fibres. As per the study by (Rasheed, Alyhya, and Kadhim 2021), the workability decreased from 80 mm to 60 mm while the w/c (0.47) remained unchanged as the volume of PET fibres was added from 0% to 1.5%. The high surface area of the concrete components, which necessitates a large volume of water to maintain the same value of slump, may be the cause of this behaviour (Rasheed, Alyhya, and Kadhim 2021). Another factor that could contribute to the decline in concrete’s workability is the shape of the fibres, which might constrain the mobility of the material’s components (Rasheed, Alyhya, and Kadhim 2021). Furthermore, (Assaad, Khalil, and Khatib 2022) showed that the slump decreased from 200 mm to 190 mm for 4.5% rPET fibre concrete when the w/c ratio was reduced from 0.55 to 0.46. According to the study by (Thomas and Moosvi 2020), the slump for concrete with binary cement (metakaolin) decreased from 89 mm to 85 mm with the addition of 0.2% of rPET fibres denoting a loss of 4.5%. Moreover, the slump continued to decrease up to 74 mm with the increment of rPET fibre volume fraction until 0.8% (Thomas and Moosvi 2020). Nevertheless, (Taherkhani 2014) showed that the addition of 0.5% volume fraction of PET fibres into concrete increases the workability twice the time of plain concrete. The observed phenomena can be attributed to the limited reinforcing effect of the fibres at a 0.5% inclusion level. PET fibres do not absorb water, leading to an excess of water remaining in the mixture, which contributes to increased workability. Conversely, mixtures containing 1% of the PET fibre exhibit a lower slump compared to the control mixture, indicating reduced workability (Taherkhani 2014). This can be attributed to the reinforcing effect of the higher fibre content (Taherkhani 2014).

Moreover, a similar variation of the slump was obtained from the study by (Sandaruwini, Bandara, and De Silva 2012), where the slump considerably decreased with the addition of rPET fibres into concrete. As shown in Table 3, the slump significantly decreases with the inclusion of rPET fibres when compared to that of plain concrete. Apart from the presence of rPET fibres in the mix lumping each other, reduction in the workability of fresh concrete may also be caused by adhesion within the concrete and holding the other ingredients of concrete together impeding easy flow (Nibudey, Nagarnaik, and Parbat 2014). For a higher w/c ratio of 0.6 (Ambassah 2019) noticed that the slump decreased by 75% for 1% of PET fibre volume fraction indicating a slump of 25 mm, while plain concrete indicated a slump of 100 mm. Similarly, the slump further decreased up to 10 mm when the PET fibre volume fraction increased to 1.5% (Ambassah 2019). In contradiction, a w/c ratio of 0.6 and PET fibre volume fraction of 1.5% has shown a slump of 30 mm, while 2.5% PET fibres have shown a slump of 40 mm indicating an increment in the workability when the PET fibre volume fraction is increased (Awoyera, Olalusi, and Iweriebo 2021).

Table 3. Average slump variation of plain concrete and rPET-FRC (Rathnayaka et al. 2015).

	Slump (mm) (w/c=0.45)	Slump (mm) (w/c=0.30)
Plain Concrete	240	160
1% PET-FRC	225	87
2% PET-FRC	212	80
3% PET-FRC	125	45

6. Effect of blended cement in mechanical properties of rPET Fibre concrete

The use of blended cement has become a trend in the current construction industry as a measure of being environmentally friendly by partially replacing cement in concrete (Mehta, and Ashish 2020). Apart from the environmental benefits, using blended cement in concrete enhances fresh and hardened properties of concrete as well as durability (Ambassah 2019). As aforementioned, with the inclusion of rPET fibres mechanical properties of concrete get affected negatively even though the crack control is improved. Therefore, the use of blended cement in rPET fibre concrete is considered a method of further improving the mechanical properties of rPET fibre concrete (Mehta and Ashish 2020). Moreover, in terms of rPET fibre-reinforced concrete, using blended cement adds up to another benefit in the durability of the rPET fibres. Due to the high alkalinity of the cement matrix, the rPET fibres can deteriorate which will further result in reduction of mechanical and durability properties of rPET fibre concrete with the increment of porosity (Adhikary, Ashish, and Rudzionis 2021; Silva et al. 2005). But partial replacement of cement using pozzolans causes lowering of the alkalinity of concrete (Adhikary, Ashish, and Rudzionis 2021). Therefore, researchers have found it is important to investigate the effect of the use of blended cement on the mechanical properties of rPET fibre concrete.

6.1. Compressive strength

Study by (Saumyasiri 2020) has proven that for a Portland blended cement rPET fibre concrete mix of w/c ratio of 0.30, a significant improvement in compressive strength can be achieved when compared to that of Ordinary Portland Cement rPET-FRC. However, in the study by (Saumyasiri 2020), the compressive strength decreased with the addition of rPET fibres into Portland cement concrete, which was contrary for Portland blended cement concrete. The variation of compressive strength in Portland blended cement concrete with respect to the rPET fibre volume fraction is illustrated in Figure 8. Accordingly, it is noticeable that the compressive strength of 0.5% and 1% of rPET fibre volume fractions are higher by 14% and 15% than that of the control mix where no rPET fibres were mixed in the blended cement concrete. However, a further increase in the rPET fibre volume fraction (3%) resulted in a reduction in the compressive strength by 14% compared to plain blended concrete.

Figure 8. Variation of compressive strength of rPET-FRC with blended cement (Saumyasiri 2020).

The compressive strength variation of Plain Blended concrete and rPET-blended-FRC with the rPET fibre percentage. The compressive strength is illustrated for 7, 14, and 28 days.

From the study by (Saumyasiri 2020), it can be concluded that compressive strength of rPET fibre concrete significantly increases when rPET fibres are incorporated into blended cement instead of Portland cement. As illustrated in the Figure 9, both 0.5% and 1% of rPET fibres in concrete have performed well in terms of compressive strength when compared with the control mix. The reduction of the degradation of rPET fibres because of diminishing alkaline medium in the cement matrix as well as the reduction of porosity due to usage of blended cement can be stated as causes for the above results (Rostami et al. 2020). Furthermore, fly ash extends the hydration reaction process by up to six months due to its larger and less reactive particles than Portland cement. Fly ash concrete gradually gains compressive strength, even though it does not initially reach the appropriate level of strength. This is a result of fly ash particles continuing to hydrate after 28 days (Ambassah 2019). This is further revealed from the study by (Ambassah 2019), where the compressive strength of rPET-FRC with fly ash has increased from 23.3MPa to 25.9MPa for a rPET fibre volume fraction of 1.5%, when the cement replaced by fly ash increased from 20% to 30%. Study by (De Silva et al. 2020) also showed that the compressive strength of blended cement PET-FRC improves with the addition of PET fibres. Furthermore, the study concluded that 1% PET fibre volume fraction indicated the best mechanical performance indicating an increment of 21.75% in compressive strength when compared with the blended concrete with no fibres (De Silva et al. 2020). Moreover, the study by (Nibudey, Nagarnaik, and Parbat 2014) showed that, as the fibre volume fraction increased from 0.0% to 1.0% in fly ash-based Portland Pozzolana Cement concrete, the compressive strength increased from 42MPa to 45MPa. Thereafter, a decline in compressive strength was seen for PET fibre volume fractions from 1.5% to 3.0% where the compressive strength decreased from 42MPa to 33MPa (Nibudey, Nagarnaik, and Parbat 2014). The largest improvement in compressive strength was noted at 1.0% of fibre volume fraction of aspect ratio 50 (Nibudey, Nagarnaik, and Parbat 2014). It was further explained that, during the compression test, the ductility of normal concrete was found to be higher in PET-FRC because normal concrete cubes failed rapidly and shattered into pieces whereas PET-FRC cubes did not (Nibudey, Nagarnaik, and Parbat 2014). However (Barbuta et al. 2017), stated that when the fly ash percentage in concrete is increased and the highest compressive strength (31.76MPa) was obtained when 10% fly ash is used with 0.25% rPET fibre even though the compressive strength was still less than that of rPET-FRC without fly ash (29.68MPa). When considering usage of blended cement in rPET-FRC (Thomas and Moosvi 2020), has stated that inclusion of 10% Metakaolin increases the compressive strength of rPET-FRC for a fibre volume fraction until 0.4%. Furthermore, using cement blended with fly ash and ground granulated blast furnace slag (GGBS) has caused an increment in compressive strength by 3–6% although the rPET fibre volume fraction was increased from 2% to 4% (Singh and Vikas Khandelwal 2020). However, when silica fume mixed blended cement was used, the compressive strength has reduced as a result of 20 mm length rPET fibre addition by 0.75% reaching 10.2% (Mohammed and Rahim 2020). According to the study by (Alani et al. 2022), the highest compressive strength of 153MPa was obtained at 25% Ultra-fine palm oil fuel ash (UPOFA) and 20% silica fume with 1% rPET fibre when compared with other contents of UPOFA and silica fume. This is because of the effectiveness of UPOFA and silica fume binary binders improved the bonding between rPET fibres and other concrete components. The characteristics of UPOFA and silica fume fine particle size and pozzolanic reactivity reduced the number of voids and created more C-S-H gels, which strengthened the interfacial link between the aggregate and paste and improved the interface’s bonding abilities with rPET fibres (Alani et al. 2022). When a higher rPET fibre volume fraction is used, fibres are more likely to bunch together which hinders the ability to obtain a homogeneous spread of fibres within the concrete mixture which leads to decrement of compressive strength. Moreover, the smooth texture of rPET fibres in the mix could affect the bonding properties and adhesion of the rPET fibres to the cement matrix where it could further cause the reduction of compressive strength (Ambassah 2019).

Figure 9. Variation of flexural and splitting tensile strength with the rPET fibre volume fraction with blended cement (Saumyasiri 2020).

The flexural and split tensile strength variations of Plain Blended concrete and rPET-blended-FRC with the rPET fibre percentage. The flexural and split tensile strengths are illustrated for 28 and 56 days.

6.2. Splitting tensile and flexural strength in the blended cement rPET-FRC

According to the study by (Saumyasiri 2020), both splitting tensile strength and flexural strength have significantly increased with the increment of rPET fibre volume fraction in the blended concrete mix and the variation is shown in Figure 9. When considering the splitting tensile strength, it has increased by 37%, 44%, and 49% with increment of rPET fibre volume fraction in the mix as 0.5%, 1.0%, and 1.5%, respectively. Similarly, when the rPET fibre volume fraction increases as 0.5%, 1.0%, and 1.5%, the flexural strength has improved by 41%, 60%, and 104%, respectively. This indicates that together with the increment of rPET fibre volume fraction in blended concrete, the splitting tensile and flexural strengths increase drastically. Furthermore, the results of (De Silva et al. 2020) showed 25.24% of increase in splitting tensile strength and 42.70% of increase in flexural strength for the addition of 1.0% PET fibres to the high-strength concrete after 28 days. A possible reason for the increment of strength of rPET-FRC with blended cement is the reduction of portlandites that occur when pozzolanic materials react with calcium hydroxide, which is liberated during the process of Portland cement hydration (Fernández et al. 2017; Yu et al. 2018). Due to the reduction of portlandites, the alkalinity of the concrete mix decreases which further results in minimising the breakage of bonds and bridges within the rPET fibres (Ambassah 2019; Yu et al. 2018).

However, as per the results of (Ambassah 2019) and (Kassa, Kanali, and Ambassah 2019), the split tensile strength for the combination of 1.5% PET fibres and 27.5% cement replaced by fly ash was the same as plain concrete that denoted a split tensile strength of 3.3 MPa, while 20%, 22.5%, 25%, and 30% replacement of cement using fly ash showed lower tensile strengths. Supporting the findings of (Saumyasiri 2020), the study by (Alani et al. 2022) also showed that the addition of and UPOFA and silica fume have given superior splitting tensile and flexural strengths to 1% rPET fibre concrete, and the best performance in splitting tensile and flexural strengths were shown by 25% UPOFA and 20% silica fume indicating 9.31 MPa and 29.03 MPa, respectively. Besides the bridging action of rPET fibres, by increasing the interfacial bonding between the cement matrix and the fibres and the aggregates through an enhanced reaction via pozzolanic activity from the increased volume of pozzolanic materials in the concrete mixtures, the UPOFA and SF inclusion in the fiberised concrete has beneficial effects on splitting tensile strength (Alani et al. 2022). (Barbuta et al. 2017) also indicated that superior splitting tensile and flexural strengths have been noted with 10% fly ash and 0.25% of rPET fibre.

7. Discussion

7.1. Effect of fibre dimension on mechanical properties of recycled PET fibre concrete

The effect of fibre dimensions on the mechanical properties of rPET-FRC is a significant aspect that directly influences the overall performance and behaviour of the composite material. Throughout this review study, it has become evident that varying the length, diameter, aspect ratio, and volume fraction of rPET fibres in concrete mixes can have profound impacts on the material’s tensile strength, flexural strength, and crack resistance. Generally, longer fibres tend to enhance the post-cracking behaviour of the concrete by providing bridging effects across cracks, thus increasing the ductility and overall toughness of the composite. On the other hand, shorter fibres tend to improve the early-age mechanical properties and workability of the concrete, due to their better dispersion and ease of mixing. Regarding diameter, larger diameter rPET fibres can contribute to increase the tensile and flexural strengths, as they can carry higher loads and provide more substantial resistance to crack propagation. Smaller diameter rPET fibres, while not as effective in load-carrying capacity, still contribute to reducing crack widths and promoting crack control. Moreover, adding rPET fibres is more likely to significantly decrease the compressive strength and workability of rPET-FRC when compared to that of plain concrete. Higher fibre volume fraction in rPET-FRC has caused voids resulting in decreased compressive strength and workability. However, it should be noted that there is a trade-off when selecting the rPET fibre dimensions. Excessive rPET fibre volume fraction or improper selection of dimensions can lead to issues such as fibre balling, segregation, and difficulty in mixing, which may compromise the overall workability and homogeneity of the concrete.

Through the analysis of the above-mentioned studies done for rPET-FRC, the length of rPET fibres can typically range from 12 mm to 50 mm. rPET fibres within the range of 12 mm to 50 mm tend to provide better reinforcement, as they can bridge cracks and distribute stress more effectively than those of rPET fibres shorter than 12 mm. The diameter of rPET fibres can range from 0.2 mm to 1.0 mm. rPET fibres having diameters within this range are usually more effective in terms of strength enhancement, as they can resist higher stresses and distribute them more evenly. Moreover, it is concluded that the rPET fibre volume fraction ranges from 0.1% to 1.0% for optimum results in mechanical strength and workability.

To optimise the mechanical properties of rPET-FRC, it is essential to carefully select the appropriate rPET fibre dimensions based on the specific application and design requirements. Additionally, further research and experimentation should be carried out to better understand the complex interactions between rPET fibre dimensions, concrete mix proportions and curing conditions to create rPET-FRC that are more reliable and efficient to use in specific applications.

7.2. Effect of water–cement ratio on mechanical properties of rPET fibre concrete

The w/c ratio has a profound effect on the performance and mechanical properties of rPET-FRC such as workability, compressive, tensile and flexural strength. Throughout this review study, it has become evident that the w/c ratio significantly influences the workability, strength, and overall behaviour of the composite material. A lower w/c ratio generally leads to higher strength and improved mechanical properties due to a more compact and densely packed cementitious matrix. This is crucial for rPET-FRC as it enhances the bond between the fibres and the cement paste, resulting in better load transfer and crack resistance. Moreover, a lower w/c ratio reduces the porosity and permeability of the concrete, which is important when considering the sustainability and durability of the construction materials.

Conversely, a higher w/c ratio may result in increased workability and ease of concrete placement. However, this comes at the cost of reduced strength and durability, as the excess water creates more voids and weakens the overall structure. Higher w/c ratios can also promote early-age cracking and adversely affect long-term performance.

To achieve optimal results, engineers and concrete practitioners must strike a balance between workability and strength by carefully selecting the appropriate w/c ratio for rPET-FRC. This involves considering the specific requirements of the construction project, environmental conditions, and the desired performance of the final structure. However, it can be concluded that for effective workability and strength, the w/c ratio could range from 0.35 to 0.55.

Continued research and exploration of optimal water-cement ratios for rPET-FRC are essential to advance the field of construction materials and promote sustainable infrastructure development.

7.3. Effect of blended cement in mechanical properties of rPET Fibre concrete

The incorporation of blended cement in rPET-FRC has demonstrated significant impact on its mechanical properties, providing a promising avenue for enhancing the overall performance of the composite material. Throughout this study, it has become evident that blended cement, which combines Portland cement with supplementary cementitious materials (SCMs) such as fly ash, GGBS, or silica fume, can increase the mechanical performance of concrete when used in conjunction with rPET fibres. Blended cement concretes have shown improved compressive strength, flexural strength, and tensile strength compared to plain concrete mixes with Ordinary Portland cement alone. The pozzolanic reactions of SCMs contribute to a denser and more refined microstructure, reducing the porosity and enhancing the interfacial bond between the cement paste and rPET fibres. This improved bond enhances the load transfer mechanism, resulting in enhanced crack resistance, and mechanical performance of the concrete.

However, it is important to note that the effectiveness of blended cement depends on various factors, including the type and proportion of SCMs used, their reactivity, and the specific application of the concrete. Optimal mix design and careful selection of SCMs are essential to achieve the desired mechanical properties while considering the specific construction project requirements. Therefore, continued research and experimentation in this area are crucial to further refine concrete mix designs and promote the widespread adoption of blended cement in the construction industry, fostering the development of more resilient and eco-conscious infrastructure.

8. Conclusion

rPET-FRC is an upcoming study area for researchers with environmental friendliness as well as because of its improved performance. This paper presents a review on the effect of rPET fibre dimensions, rPET fibre volume fraction, w/c ratio, and the use of blended cement on the fresh and hardened properties of rPET-FRC. Along with presenting experimental data obtained from rPET-FRC where the rPET fibres are obtained from the same recycling plant, experimental data of other past studies in the same context were also included in the paper. While presenting the experimental data, this review paper has included the reasons behind the given results as explained through previous studies as well. This study has highlighted the most effective rPET fibre volume fractions, w/c ratios and the use of blended cement, enabling international researchers to optimise the design and application of rPET-FRC.

This study concludes that the mechanical properties of rPET-FRC are significantly influenced by several key factors, including PET fibre dimension, fibre volume fraction, w/c ratio, and the use of blended cement. The dimension of rPET fibres, including their length, diameter, and aspect ratio, directly impacts the load-carrying capacity of the concrete. Longer fibres (12 mm to 50 mm) tend to enhance crack resistance and strength by providing bridging effects across cracks, while shorter fibres improve early-age mechanical properties and workability. The fibre volume fraction in rPET-FRC influences the overall compressive, tensile, and flexural strength of the material. Increasing the rPET fibre volume fraction generally leads to improved crack control and load-carrying capacity, but excessive amounts can cause fibre balling and segregation issues. Therefore, the identified rPET fibre volume fraction ranging from 0.1% to 1.0% shows most effective results in mechanical strength and workability. The w/c ratio significantly affects the strength, durability, and permeability of rPET-FRC. Lower w/c ratios (0.35 to 0.55) result in higher strength and improved performance due to a denser cementitious matrix and enhanced bonding between fibres and the cement paste. Blended cement, when used in conjunction with rPET fibres, can lead to enhanced mechanical properties and sustainability of the concrete. The incorporation of SCMs reduces the carbon footprint and enhances the microstructure, resulting in improved strength and crack resistance.

It is crucial to recognise that these factors are interconnected, and their combined effects must be carefully considered during concrete mix design and production. A holistic approach that optimises the rPET fibre dimension, fibre volume fraction, w/c ratio, and the use of blended cement is necessary to achieve the desired mechanical properties while meeting the specific requirements of the construction project.

By tailoring these factors to suit the project’s needs, engineers and concrete practitioners can design more resilient, durable, and sustainable concrete structures capable of withstanding various environmental and loading conditions. Continued research and innovation in this area will further advance the field of rPET-FRC and contribute to the ongoing development of modern construction materials. The publication of relevant codes and standards that will address all issues pertaining to the rPET-FRC design is deemed necessary.

Statements and declarations

The authors have no relevant financial or non-financial interests to disclose. No funding was received to assist with the preparation of this paper.

Authors contributions

S.M.D.V. Suraweera: Formal analysis, Writing – original draft, Writing – review and editing

Sudhira De Silva: Conceptualisation, Data curation, Writing – review and editing

All authors reviewed the results and approved the final version of the manuscript.

Acknowledgments

The authors of this paper would like to acknowledge greatly to all the authors who have published valuable research articles and books that have been used for this review paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

S.M.D.V. Suraweera

S.M.D.V. Suraweera is a cotutelle PhD student under the Department of Civil and Environmental Engineering at University of Ruhuna, Sri Lanka and the Department of Civil and Infrastructure Engineering at RMIT University, Australia. She received her BSc (Engineering) degree from the Faculty of Engineering, University of Sri Jayewardenepura, Sri Lanka with a minor in Structural Engineering.

Sudhira De Silva

SudhiraDe Silva is a Chair Professor at the Department of Civil and Environmental Engineering, Faculty of Engineering, University of Ruhuna, Sri Lanka. He received his BSc (Engineering) degree from the University of Moratuwa, Sri Lanka, and PhD in Civil Engineering from Saitama University, Japan. His interests include the development of application of eco-friendly and sustainable materials, seismic vulnerability assessment and retrofitting of existing masonry buildings, and the application of circular economy concepts to the construction industry. He is a Chartered Engineer, a Fellow of the Institution of Engineers Sri Lanka and a member of the Society of Structural Engineers Sri Lanka.