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Composites Communications

The effect of recycled PET bottles on the fracture toughness of polymer concrete

Abstract

In this study, recycled polyethylene terephthalate (PET) bottles are shredded in different sizes and added to the polymer concrete composition to investigate the effect on fracture toughness and fracture energy as well as an idea to reduce environmental pollution. In order to measure the mode I fracture toughness, center cracked Brazilian disk (CBD) specimens are manufactured with 20 wt% epoxy resin and 80 wt% quartz aggregate in the size of 2.4–4.75 mm. Recycled PET fillers in two different sizes, i.e., fine (less than 2.4 mm) and coarse (2.4–4.75 mm) with 4 wt% are added to the polymer concrete. Experimental results show that the addition of the fine and coarse PET filler materials to the polymer concrete mixture can increase the fracture toughness relative to the control material. However, the coarse PET filler can improve much more significantly the fracture toughness and fracture energy compared to the polymer concrete containing the fine PET filler. This is due to resistance made by the coarse PET filler via two energy dissipation mechanisms: a) crack growth through the matrix/PET interface by rounding the coarse PET filler; b) PET bridging. In addition, a linear relationship between the fracture toughness and fracture energy of tested polymer concrete mixtures was observed.

Introduction

Concrete made of cement, water and aggregates is the most common material for the construction of various structures. It has good mechanical properties and it is widely used in buildings, bridges, airports, dams and marine structures [1]. Unfortunately, construction of concrete structures due to the weak properties of cement concrete such as low wear resistance, high permeability, low chemical resistance and phenomena such as cavitation and chlorine-induced corrosion, and low frost resistance are difficult and time consuming [2]. The solution used in recent years to address these problems is the use of polymer concretes (PCs). If the cement and water in cement concretes is replaced with a polymer, it is called a PC material [3]. Therefore, the components of a PC are: aggregates, polymers and additives. In general, over 75–80% of a PC is occupied by fine and coarse aggregates. Epoxy and polyester resins are two common polymers used as matrix. In addition to aggregates and matrix, micro fillers are sometimes added to the composition to fill the air void [4,5]. The advantages of PC are: a) easy application in thin cross sections, b) saving concrete from carbonation, c) loss of alkalinity, d) very good resistance against corrosion and chemical reactivity and weathering effect, e) set very quickly, f) useful for repairing of existing structures. The disadvantages of PC are: a) it is very expensive than a conventional concrete, needing high skill and precise work, c) The chemicals or resins used in the polymer concrete can be risky [6]. Experimental studies show that the tensile strength and fracture toughness of PC are 3.5–4.5 times the common plain cement concrete materials [[7], [8], [9]] and hence it can be a good replacement for cement concretes.

In general, there are three main ways to improve the properties of a PC: 1) Change the type of resin: This is not an economical way due to the high cost of resins with good mechanical properties; 2) Change the percentage of concrete ingredients: To do this, if the percentage of resin is reduced, the concrete becomes dry and may results in lack of sufficient cohesion with the aggregates, and if the percentage of resin increases, the cost will increase and some mechanical properties decrease. For example, some investigations are done on finding the optimum composition of PC to reach high mechanical properties [[10], [11], [12]]. 3) Adding fillers: This method depends on the type of filler (i.e., waste materials) may be simpler and cheaper than other methods. It should be noted that some fillers such as nano particles are not cost-effective for this application. Of course, the filler percentage is limited and often very small weight percentage of total PC volume is considered for the fillers. In this study, the third method (i.e., adding the fillers) is applied for improving the fracture toughness and fracture energy. Up to now, researchers have tried to apply different waste to the concrete composition to improve the properties [13]. For example, Son and Yeon [14] studied the effect of adding methacrylic acid on the mechanical properties of the PC material at low temperatures. The results showed that the PC containing methacrylic acid has higher modulus of elasticity, strength, and Poisson's ratio. Shi-Cong and Chi-Sun [15] used glass waste as aggregate and fly ash as fillers in the PC mixture. Their results showed that chemical resistance of this concrete has increased up to 20% with better flexibility, low porosity. D'Alessandro et al. [16] investigated the experimental thermodynamic conductivity of concrete containing polymers derived from the recycling of electrical wire sheets. Measuring the dynamic stiffness and thermal conductivity showed that the produced concrete could be used successfully for the thermal and acoustic coatings of light weight insulations in concrete constructions. Khalid et al. [17] investigated the potential use of agricultural waste as filler in PC composition. Palm oil fuel ash was added as a micro-filler to the PC composition. Microstructural examination, particle size distribution, nitrogen adsorption test, thermal analysis, flow-ability and compression test were performed on the samples. The results showed that this PC has better physical properties and compressive strength and lower porosity. Teixeira et al. [18] investigated the effects of burning wood ash on the mechanical properties of PC material. Their results showed that the addition of wood ash has a negative effect on the properties of PC compared to the conventional concrete, but they are suitable for durability. Alperen and Şahin [19] investigated the effect of adding electronic plastic waste materials such as monitors on the mechanical properties of PC materials. Resin/filling material ratio has been determined as 10–90%, 15–85% and 20–80%, e-plastic/filling material ratio has been decided to be 0%, 5%, 15% and 25%. They observed that the addition of these wastes increases the ductility and reduced the flexural strength and compressive strength. However, this reduction is very small by adding 5 wt% of the electronic wastes. Also, experimental study shows that ideal resin ratio and ideal e-plastic ratio values are 15% and 5%, respectively. Jasim [20] mixed high density polyethylene waste with Portland cement to investigate the possibility to produce plastic cement, and studied the effect of replacing sand by fine polyethylene waste with different percentage on the properties of product. The experiments were done by using the waste of polyethylene packages including bottle and food crates in the range of 10%–80% by volume as a short reinforcement structure. The results show that there is a possibility to produce plastic cement from polyethylene waste and Portland cement by using 60% and 40%, respectively. In addition, their density was decreased, ductility increased, and the workability improved, which lead to produce lightweight materials.

In this study, we have focused on waste plastics because they are used too much in everyday life due to their many capabilities as well as their cheapness. But plastics are non-degradable and it is not surprising if plastic waste is recognized as one of the biggest environmental problems in the world today. One of the plastic wastes is polyethylene terephthalate (PET) which is used in drinking bottles. PET is a general-purpose thermoplastic polymer which belongs to the polyester family of polymers. PET is highly flexible, colorless and semi-crystalline resin in its natural state. Depending upon how it is processed, it can be semi-rigid to rigid. It shows good dimensional stability, resistance to impact, moisture, alcohols and solvents. Since PET is an excellent water and moisture barrier material, plastic bottles made from PET are widely used for mineral water and carbonated soft drinks. Recycled PET can be converted to fibers, fabrics, sheets for packaging and manufacturing automotive parts [21]. Few studies can be found in the literature to add shredded PET filler to the cement concrete or PC composition. For example, Sosoi et al. [22] found that by adding sawdust and PET bottles to the PC and by conducting experiments to determine the density, performance, and compressive strength of reinforced concrete, both types of waste reduced the density. The sawdust reduced the performance of PC but the PET increased the performance and also the PET showed better 1.5% compressive behavior than sawdust.

The above literature review shows that adding different type of fillers to the PC is a common work in order to improve the mechanical properties. But fracture and cracking behavior of PC materials made of waste materials, especially recycled polyethylene terephthalate (PET) has rarely been investigated in the previous works. Also, the effect of shredded PET size is not investigated on the fracture toughness of this material. Therefore, the purpose of this study is to investigate the influence of adding different size of shredded PET on the fracture toughness and fracture energy of PC.

Section snippets

Polymer

There are two types of resins used for making polymer concretes, i.e., polyester and epoxy. Since the mechanical properties of polyester resin is low and controlling the gel time is more difficult than epoxy, epoxy resins are usually preferred in structural applications [[24], [25]]. The matrix of the PC material is a two-part epoxy resin, namely EPL-1012 and EPH-112 as a hardener with mix ratio by weight of 100:12. EPL 1012 is a Bisphenol F type epoxy resin. Its low viscosity allows for

Results and discussion

Fig. 3 shows the load-displacement curves of BD specimens with central crack. Table 3 summarizes the information of each sample including specimen's code, maximum failure load, diameter and thickness as well as the percentages of aggregate, resin and PET.

Results in Table 3 show that mode I fracture toughness increases by adding PET filler to the PC. It has increased up to 8.5% and 16.3% when 4 wt% fine and coarse fillers are added, respectively. Therefore, it can be concluded that the impact of

Conclusion

The main objective of this study was to investigate the effect of adding recycled PET bottles as the filler on the mode I fracture toughness and fracture energy of PC material. For this purpose, center cracked Brazilian disks are made from PC without PET and reinforced by fine and coarse PET fillers. Specimens are tested under quasi-static compression. Experimental results showed that mode I fracture toughness of PC has increased up to 8.5% and 16.3% by adding of 4 wt% fine and coarse PET

CRediT authorship contribution statement

M. Asdollah-Tabar: Data curation, Writing - original draft, Investigation, Validation. M. Heidari-Rarani: Supervision, Methodology, Writing - review & editing, Project administration. M.R.M. Aliha: Conceptualization, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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