Tests Regarding the Effect of Dispersed Reinforcement Made with a Prototype Device from PET Beverage Bottles on the Strength Properties of Concrete

Abstract

Currently, waste generation is a huge problem all over the world. The largest source of generated waste is plastics from plastic packaging made of polyethylene and polypropylene, including PET bottles. Modern ecology aims to reduce the carbon footprint by recycling plastics or by producing biodegradable plastics that are completely broken down by microorganisms into simple particles that occur naturally in nature. Polyethylene terephthalate (PET) has good mechanical properties but is resistant to microorganisms. As a result, it cannot be classified as a biodegradable plastic, but when it is reused for the production of utility products, it becomes a bioplastic. A good way to dispose of PET is to use it for the production of utility products, in which its good mechanical properties can be used. Concrete is a basic material, the consumption of which in the construction industry is enormous. One of the negative features of concrete is its low tensile strength, which can be improved with continuous or dispersed reinforcement. This paper presents the results of compressive and tensile-bending tests of concrete reinforced with dispersed “fibres” of a different length and width, which were produced by a prototype device from PET beverage bottles. The prototype device enables repeatable fibres with a width of 2 mm, and lengths of 38 mm, 62 mm, and 93 mm to be obtained. The highest flexural tensile strength of the concrete was achieved in the case of the PET fibres with a length of 62 mm. It turned out that concrete with such reinforcement has a higher bending tensile strength by 15 % in relation to the tensile bending strength of the concrete without the dispersed reinforcement. The PET fibres also improve compressive strength. PET fibres, in order not to deteriorate in the alkaline environment of concrete, must be secured with an appropriate chemical agent. The effect of concrete reinforcement with the recycled PET fibres was compared to the effect of dispersed reinforcement made of polypropylene and steel fibres. The highest bending tensile strength was obtained in the case of the concretes with the scattered PET reinforcement. However, the differences in the bending tensile strength of concrete are not big and are equal to 0.64 MPa for polypropylene fibres and only 0.09 MPa for steel fibres.

1. Introduction

Concrete is widely used as a building material in the construction industry. Its increased popularity is due to its low production costs and high compressive strength, despite it having a low tensile strength. In order to increase the tensile strength of concrete, reinforcement in the form of steel bars or meshes is commonly used.

Modern concrete technologies are being developed together with scattered reinforcement in the form of fibres made of steel, polymer, glass, carbon and basalt. Such fibres not only have different lengths but also different cross-section areas. Furthermore, concrete reinforcement techniques that involve the combination of various types of fibres are also used. The application of fibres in a hybrid form leads to better efficiency in improving the properties of concrete [1]. When reinforcement fibres are used in a concrete mix, fibre-reinforced concrete, the characteristic features of which include increased tensile strength and a higher resistance to cracking and crevices, is obtained [2,3,4,5]. The test results in publication [6] indicate that fibres can also be used as minimal shearing reinforcement in reinforced concrete beams.

There is currently a strong emphasis on the reuse of plastics in order to reduce carbon footprints. There is a particularly large amount of waste in the form of PET from beverage bottles around the world. Polyethylene terephthalate (PET) has good tensile strength, and due to this mechanical property, such waste from bottles can be used as reinforcement for fibre-reinforced concrete (however, it has to be previously processed). For this purpose, a prototype device was proposed in the article, which enables repeatable dimensions of fibres with a width of 2 mm and lengths of 38 mm, 62 mm, and 93 mm to be obtained. The only disadvantage of PET waste fibres is their poor resistance to hydrolysis in the strongly alkaline environment of concrete, which is reflected in the reduction in the long-term tensile strength of the composite [7,8,9]. However, to prevent alkaline hydrolysis, PET fibres can be coated with ethylene copolymer and vinyl acetate—EVA [8,9].

A large number of plastic bottles are thrown away after being used once, and they are a major cause of environmental pollution. Recycling PET waste requires additional expenses regarding its processing. Plastic waste, including PET, does not decompose quickly, and it circulates in the environment. It can enter water resources and cause health problems for humans and animals. Therefore, finding a cheap way to reuse PET waste would be an effective environmental protection measure. Many researchers from the field of materials science try to use PET waste as fibres to improve the properties of concrete [10,11,12,13,14]. In publications, fibres cut from bottles have irregular shapes/dimensions. The scientific research discussed below focuses on obtaining repeatable dimensions of fibres that are cut from plastic bottles using a prototype device and assesses their influence on the selected mechanical properties of concrete: the compressive strength and the tensile strength. The wider aim of this research is to compare selected mechanical properties of concrete reinforced with PET, polypropylene and steel fibres.

2. Materials and Methods

For the purpose of tests concerning the effect of scattered reinforcement (made of recycled PET fibres from beverage bottles) on the strength properties of concrete, a recipe of a concrete mix (designated as series B) was developed (

Table 1. Recipe of the concrete mix for series B.

Concrete Mix Components	Component Weight Per 1 m3 [kg]
Cement CEM II/B-V 32.5R	300.0
Sand of 0–4 mm	599.5
Gravel of 0–32 mm	1334.3
Water	165.0
Plasticiser REMICRETE SP63 (FM)	1.4

). It contained natural sand aggregate with a grain size of 0–4 mm; gravel with a grain size of 0–32 mm; cement CEM II/B-V 32.5R; water; and plasticiser REMICRETE SP63. The concrete was designed with a water/cement (W/C) ratio equal to 0.55 and a proportion of cement:sand:gravel ratio equal to 1:2:4.45.

By adding the same amount (0.4%) of different dispersed reinforcement to the composition of the concrete mix of series B, the following series of concrete mixes were obtained: Z1, Z2, Z3, Z4, and Z5 (Table 2). For modification of the concrete mix, scattered reinforcement in the form of recycled PET fibres (made in the laboratory from beverage bottles) was used. The fibres had a width of 2 mm and lengths of 38 mm—series Z1; 62 mm—series Z2; and 93 mm—series Z3. Moreover, fibres made of polypropylene with a length of 18 mm (with slenderness ratio l/d = 529.4)—series Z4; and fibres made of 25/0.4 steel (with slenderness ratio l/d = 62.5)—series Z5 were also used.

The recycled PET fibres (Figure 1) were made from used beverage bottles in the Central Laboratory of the Building Industry Institute in the Faculty of Civil Engineering, Mechanics and Petrochemistry at Warsaw University of Technology. The PET beverage bottles were processed into fibres using a specially constructed device.

According to the developed recipe, concrete mixes were made without scattered reinforcement, as well as with scattered reinforcement that was added in a volume fraction of 0.4 % (the quantity of the remaining components did not change).

Table 2. Concrete mix reinforcement for each series.

Concrete Mix Reinforcement	Series
without scattered reinforcement	B
with scattered reinforcement in the form of recycled PET fibres (Figure 2) with a width of 2 mm and a length of 38 mm	Z1
with scattered reinforcement in the form of recycled PET fibres (Figure 2) with a width of 2 mm and a length of 62 mm	Z2
with scattered reinforcement in the form of recycled PET fibres (Figure 2) with a width of 2 mm and a length of 93 mm	Z3
with scattered reinforcement in the form of polypropylene (PP) fibres with a length of 18 mm (Figure 3a)	Z4
with scattered reinforcement in the form of 25/0.4 steel fibres (Figure 3b)	Z5

Table 2. Concrete mix reinforcement for each series.

Concrete Mix Reinforcement	Series
without scattered reinforcement	B
with scattered reinforcement in the form of recycled PET fibres (Figure 2) with a width of 2 mm and a length of 38 mm	Z1
with scattered reinforcement in the form of recycled PET fibres (Figure 2) with a width of 2 mm and a length of 62 mm	Z2
with scattered reinforcement in the form of recycled PET fibres (Figure 2) with a width of 2 mm and a length of 93 mm	Z3
with scattered reinforcement in the form of polypropylene (PP) fibres with a length of 18 mm (Figure 3a)	Z4
with scattered reinforcement in the form of 25/0.4 steel fibres (Figure 3b)	Z5

2.1. Description of the Experiment

After the mixing of the concrete mix components, the consistency class was determined according to standard EN 206 [15]. The tests of the consistency class were conducted using the cone fall method according to standard EN 12350-2 [16] and the flow table method according to standard EN 12350-5 [17].

Afterwards, from each series of concrete mixes, standard cubic samples with dimensions of 150 × 150 × 150 mm were made according to standard EN 12390-1 [18]. In turn, standard samples with dimensions of 100 × 100 × 500 mm were made according to standard EN 12390-1 [18]. The samples were compacted on a vibration table at a frequency of 50 Hz and then matured in water for 28 days at a temperature of 20 ± 2 °C according to standard EN 12390-2 [19]. The compressive strength test (according to standard EN 12390-3 [20]) and the flexural tensile strength test (according to standard EN 12390-5 [21]) were conducted after 28 days from concreting.

The standard cubic samples with dimensions of 150 × 150 × 150 mm were tested for compression according to standard EN 12390-3 [20] on a Toni Technik compression testing machine (the rate of loading 0.5 MPa/s), and the standard samples with dimensions of 100 × 100 × 500 mm were tested for bending tension according to standard EN 12390-5 [21] on a Matest compression testing machine (the rate of loading 0.05 MPa/s).

The compressive strength is given by the equation:

$$f_c=\frac{F}{A_c}$$

(1)

where:

f_c is the compressive strength, in MPa (N/mm²);

F is the maximum load at failure, in N;

A_c is the cross-sectional area of the specimen on which the compressive force acts in mm².

The flexural tensile strength (Figure 4) is given by the equation:

$$f_{cf}=\frac{3\cdot F\cdot l}{2\cdot d_1\cdot d_2^2}$$

(2)

where:

f_cf is the flexural tensile strength, in MPa (N/mm²);

F is the maximum load, in N;

l is the distance between the supporting rollers, in mm;

d₁ and d₂ are the lateral dimensions of the cross-section, in mm.

The mean value of the strength results is given by the equation:

$$f_{cm/cfm}=\frac{1}{n}{\displaystyle \sum _{i=1}^n{f}_{c/cf}}$$

(3)

where:

f_cm/cfm is the mean value of the compressive strength or the mean value of flexural tensile strength, in MPa (N/mm²);

f_c/cf is the value of the compressive strength of the i-th sample or value of the flexural tensile strength of the i-th sample, in MPa (N/mm²);

n is the number of values.

The expanded uncertainty for the mean value of the strength results is given by the equation:

$$U=k\cdot u_c$$

(4)

$$u_c=\sqrt{\frac{1}{n\cdot \left(n-1\right)}{\displaystyle \sum _{i=1}^n{\left(f_{c/cf-i}-f_{cm/cfm}\right)}^2}}$$

(5)

where:

U is the expanded uncertainty for the mean value of the strength results, in MPa (N/mm²);

u_c is the standard uncertainty for the mean value of the strength results, in MPa (N/mm²);

f_cm/cfm is the mean value of the compressive strength or the mean value of flexural tensile strength, in MPa (N/mm²);

f_c/cf−i is the value of the compressive strength of the i-th sample or value of the flexural tensile strength of the i-th sample, in MPa (N/mm²);

k is the coverage factor for the mean values of the test results;

n is the number of values.

2.2. Device and Method of Producing the Recycled PET Fibres

The fibre production procedure began with washing and cleaning an unbent PET bottle in order to remove its label, followed by its bottom being cut off using a knife. The PET bottle was then processed into fibres using a specially constructed device (Figure 5).

The frame (13) of the device (Figure 6) is made of closed steel profiles. An aluminium C-section (12) (with 3 channels (3) that are cut in order to allow 3 different widths of fibres to be prepared) is attached to the frame (13). A knife (9) that enables a bottle (1) to be cut into a strip of desired width is screwed to the C-section (12). A steel bar (2) is also screwed to the C-section (12). This bar (2) is a pivot on which the bottle (1) rotates while being cut into strips. The designed device also consists of 2 rollers (4a, 4b) with the same diameter, which contact each other via rubber-coated operating surfaces. A crank (11) for rotating the entire mechanism is inserted in the axis of one roller (4a). Gearwheels (5) with the same number of teeth are fitted to each of the rubber-coated rollers (4a, 4b). Due to this, there is no slipping in the contact point between the rollers (4a, 4b). A guide rail (7) in the form of a copper tube with an appropriately shaped cross-section is placed behind the rollers (4a, 4b). The rollers (4a, 4b) pull the strip and cause the rotation of the plastic bottle (1) during the cutting process. Strips produced in this way go through the guide rail (7) to a steel disk (6) with knives (8) fitted on its circumference. The disk (6) is connected via a chain drive (10) (with a gear ratio of 1:2) to the roller (4a), in the axis of which the rotating crank (11) is inserted. This disk (6) is divided on its circumference into different sections, which allow for the positioning of 1, 2, 3 or 4 knives (8) that are situated at an appropriate distance from each other. This positioning enables various lengths of PET fibres to be cut from beverage bottles.

2.3. Statistics

In order to determine the expanded uncertainty for the mean values of the test results at the level of 95%, a coverage factor of k = 2 was adopted. This is consistent with international practice. The value of the coverage factor k is chosen on the basis of the desired level of confidence to be associated with the interval defined by U = k·u_c. Typically, k is in the range of 2 to 3. When normal distribution applies, and u_c is a reliable estimate of the standard deviation of measurement, U = 2·u_c (i.e., k = 2) defines an interval with a level of confidence of approximately 95%, and U = 3·u_c (i.e., k = 3) defines an interval with a level of confidence greater than 99%. For statistical data analysis, Excel was used.

3. Results

The consistency class of the concrete mixes was determined according to standard EN 206 [15] and is presented in

Table 3. Consistency classes of the concrete mixes.

Series	Consistency Class Determined according to Standard EN 206 [15]
	Cone Fall Method	Flow Table Method
B	S1	F2
Z1
Z2
Z3
Z4
Z5

.

In order to evaluate the effect of the length of the reinforcement made of recycled PET fibres on the mechanical properties of concrete, compressive and flexural tensile strength tests were performed.

At the first stage of the research, results from testing the mechanical properties of the concrete were used to determine which length of PET fibres caused the biggest growth of compressive strength (Figure 7) and flexural tensile strength (Figure 8) when compared to the compressive and flexural tensile strength of the concrete without the scattered reinforcement.

The addition of PET fibres with the longest length resulted in a reduction in compressive strength when compared to the other concrete series: B, Z1, and Z2. The concrete series Z1, Z2, and Z3 with scattered PET reinforcement showed higher flexural tensile strength than the series B concrete (without the scattered reinforcement).

It can be concluded from the above charts in Figure 7 and Figure 8 that the highest compressive strength and flexural tensile strength were obtained in the case of concrete series Z2 with the scattered reinforcement in the form of recycled PET fibres (length—62 mm). For this reason, the concrete series Z2 was used for further tests of the mechanical properties of concrete. However, during these tests, other types of scattered reinforcement were used. The comparison of compressive and flexural tensile strength for the tested series of concrete with different types of the scattered reinforcement is presented in charts in Figure 9 and Figure 10.

The results presented in Figure 10 show that the highest flexural tensile strength was obtained in the case of the concrete series Z2 with the addition of PET fibres (length—62 mm).

In turn, the highest compressive strength (Figure 5) was obtained in the case of the concretes with the scattered steel reinforcement. However, the difference in the compressive strength of concrete series Z2 and Z5 is not big and is equal to only 4 MPa.

The addition of PET fibres (length—62 mm) caused the biggest increase in flexural tensile strength when compared to the strengths obtained for the other concrete series: B, Z1, Z3, Z4, and Z5. It can be concluded from the charts in Figure 7 and Figure 9 that the concrete series Z5 obtained the biggest compressive strength.

4. Discussion

The conducted tests indicate that all the types of fibres used (polyethylene terephthalate, polypropylene, and steel) affected the tested mechanical properties of the concrete in various ways. Polypropylene fibres added to the concrete in the assumed proportions, when compared to the concrete without the scattered reinforcement, did not improve the flexural tensile strength. However, they did improve the compressive strength. In turn, the concrete reinforced with 25/0.4 steel fibres (slenderness ratio l/d = 62.5), as opposed to the concrete without the scattered reinforcement, had a flexural tensile strength higher by 13% and a much higher compressive strength.

The addition of scattered reinforcement made of the recycled PET fibres to the concrete increased the level of flexural tensile strength but did not significantly affect the level of compressive strength. The highest flexural tensile strength was achieved in the case of the PET fibres with a length of 62 mm. The obtained results do not confirm the results of the authors of works [11,22,23], who stated that compressive strength decreases with an increase in the PET fibre content. The volume fraction of 0.4% of PET fibres significantly increased the bending tensile strength of the concrete by 15% in relation to the tensile bending strength of the concrete without dispersed reinforcement. In publication [10], it can be found that the addition of PET fibres increases the tensile strength of concrete by over 20%, which was not achieved in the authors’ research. The author of publication [13] came to a different conclusion, where the tensile strength of concrete reinforced with PET fibres was lower than that of the reference concrete. The obtained results indicate that concrete containing dispersed reinforcement in the form of PET fibres is an innovative construction material with promising tensile mechanical properties, which is also confirmed by the results of the studies published in [10,11,12,13,14]. It is worth noting that adding PET to concrete not only causes an increase in tensile strength, but also a decrease in thermal conductivity [11], a decrease in sorption [12], and an increase in abrasion resistance [13]. Mohammed A. A. and Mohammed I. I. in paper [14] indicated that the mechanical properties of PET reinforced concrete depend on the fibre index (volumetric content, slenderness ratio l/d). They set useful fibre index boundaries for each property [14]. In paper [24], the behaviour of reinforced concrete beams containing fibres made of waste plastic straws under the three-point bending test is examined. The results show that, in up to 0.5% of waste plastic straws, there is a slight increase in compressive strength and the tensile strength increases with the addition of fibres. The effect of fibre length has not been studied [24]. In publication [25], the coarse aggregate was replaced in various proportions with waste plastic in the form of a plastic bottle cap. The results showed that it is possible to use a certain amount of plastic bottle caps in structural applications without affecting the flexural characteristics of reinforced concrete beams.

5. Conclusions

A prototype device was used to obtain PET fibres—polyethylene terephthalate. The prototype device enables repeatable fibres with a width of 2 mm, and lengths of 38 mm, 62 mm, and 93 mm to be obtained. The volume fraction of 0.4% of PET fibres with a length of 62 mm increased the bending tensile strength of the concrete the most by as much as 15% in relation to the tensile bending strength of the concrete without the dispersed reinforcement, but PET fibres did not have any effect on improving the compressive strength. The results of selected mechanical properties of the obtained fibre-reinforced concrete with recycled PET fibres with a length of 62 mm were compared with the results of the fibre-reinforced concrete with polypropylene and steel fibres. The highest bending tensile strength was obtained in the case of the concretes with scattered PET reinforcement. However, the differences in the bending tensile strength of concrete are not big and are equal to 0.64 MPa for polypropylene fibres and only 0.09 MPa for steel fibres. The fibre-reinforced concrete with steel fibres obtained the biggest compressive strength (as much as 6 MPa).

It is recommended to test concrete reinforced with PET fibres with regards to axial tension (which is the topic of the authors’ next research project), and also to check the effectiveness of transferring the tension caused by concrete shrinkage. Therefore, the use of the prototype device is an effective measure to protect the environment, as it allows the reuse of used PET beverage bottles. Using PET fibres made from beverage bottles on an industrial scale will reduce the cost of manufacturing the fibre-reinforced concrete that can be used, for instance, to make industrial floors. However, for this purpose, it would be most valuable to use PET waste in the form of beverage bottles, the disposal of which is still a problem on a global scale. It can be assumed that obtaining fibres from PET bottles and protecting them from the hydrolysis effect in a strongly alkaline environment has been mastered [8,9].

6. Recommendation

The use of a prototype device on an industrial scale will enable environmental protection, as it will enable the reuse of used PET beverage bottles. The prototype device allows repeatable fibres of the same width and length to be obtained. The volume fraction of 0.4% of PET fibres with a width of 2 mm and a length of 62 mm increases the bending tensile strength of the concrete by 15% in relation to the tensile bending strength of the concrete without the dispersed reinforcement. The PET fibres should be protected from the hydrolysis effect in a strongly alkaline environment [8,9], for example, in the production of industrial floors. The future research works of engineers based on my method of obtaining PET fibres should check the influence of different volume fractions of PET fibres on the mechanical properties of the concrete. The topic of the authors’ next research project is the testing of concrete reinforced with PET fibres with regards to axial tension, and possibly also its impact on strength, and to check the effectiveness of transferring the tension caused by concrete shrinkage.

7. Patents

A patent entitled “The device and method of processing plastic bottles” was filed in the Patent Office of the Republic of Poland. The patent application number is PL435372.