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Recycled waste PET for sustainable fiber-reinforced concrete
Recycled waste PET for sustainable fiber-reinforced concrete
18
Dora Foti Department of Civil Engineering Sciences and Architecture, Polytechnic University of Bari, Bari, Italy
18.1
Introduction
Concrete is a building material generally made with a mixture of water, cement, and aggregates. It can be considered an artificial conglomerate stone that possesses characteristics similar to those of a rock, including good compression strength and a poor strength to tensile stresses. The main defect of concrete is, in fact, its very low tensile strength, so low that it is often completely neglected in the calculus models. If concrete is today by far the most widely used building material in the world, it is thanks to reinforcements that make up for the poor tensile strength and brittle behavior of the concrete. Among these, steel reinforcement is often utilized, though fibers of different materials are also added to the cement matrix. Typically, they are metal fibers, in polymer, carbon, glass, or natural material. Such a concrete is called fiber-reinforced concrete (FRC). The properties of the composite concrete depend on the characteristics of the two matrices (cement and fibers), on their dosages, and in particular, on the geometry; the volumetric/weight percentage; and the mechanical characteristics of the fiber, the adherence between the fiber and the matrix of concrete; the mechanical characteristics of the matrices. The fibers may help to reduce the phenomenon of cracking and/ or significantly increase the energy absorbed in the process of fracture (toughness). The latter depends on several factors, including, for example, the aspect ratio (i.e., the ratio length/equivalent diameter of the fibers), the amount of fibers, their orientation and their spread within the cement matrix, the physical-mechanical characteristics of the latter. At a constant composition and dosage, the fibers with a greater aspect ratio are more effective. They are characterized, in fact, by a greater surface area that allows the strengthening of the bond with the concrete, reducing the risk of slipping. At the same time, however, the use of slender fibers, which are those with a higher aspect ratio, has also negative effects: it relates especially to the dispersion of fibers within the cement matrix, for which it is not possible to obtain a good dispersion (that instead would be desirable), and the workability of the fresh mix. These side effects generally lead to a limitation of the dose of slender fibers.
Use of Recycled Plastics in Eco-efficient Concrete. https://doi.org/10.1016/B978-0-08-102676-2.00018-9 Copyright © 2019 Elsevier Ltd. All rights reserved.
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A further important aspect is the shape of the surface: if uneven or wavy it ensures a better adherence to the cement matrix and thus a greater strength to the pull out of the fibers subject to tensile stresses. In recent decades the research is focusing, in particular, on the reuse of waste plastics, thus trying to combine the advantages in terms of a better behavior of the concrete mixture with those derived from the recycling of large quantities of waste that would otherwise be destined to solve problems such as landfill, incineration. In many recent studies efforts have been made to analyze the possible utilizations for postconsumer and recycled plastic in the production of concrete to gain both economic and environmental benefits (Batayneh et al., 2007; Siddique et al., 2008). Among the plastic waste, polyethylene represents the largest fraction, followed by polyethylene terephthalate, most known as PET. It is obtained in large quantity from plastic bottles utilized as containers of beverages and mineral water. As result of the drastic increase in beverage consumption, the production of polyethylene terephthalate (PET) bottles has increased exponentially, also due to the favorable properties of this plastic, including low density, high resistance, weight ratio, high durability, ease of conception/fabrication, and low cost. Particular interest is growing, at present, in the reuse of fibers obtained from waste PET bottles. With the aim to reduce the waste and take profit from this material, some preliminary studies can be found in Silva et al. (2004). The good results in terms of mechanical characteristics push the research toward the utilization of this kind of fibers and the consequent performance of the new concrete mixture. The present study, in fact, mainly concerns the reuse of PET intended for many different applications and, especially, to make up the common bottles of water. Its good characteristics of adhesion with concrete make it more suitable to be used in the mix design of concrete compared to other polymers. In this chapter the state-of-the-art on the use of PET fibers for the reinforcement of concrete is presented. In previous studies such material has been used in different ways with or without manufacturing, the latter employing PET directly cut from waste bottles (Batayneh et al., 2007; Siddique et al., 2008; Soroushian et al., 2003; Silva et al., 2004). PET has been utilized as a binder, in the form of a resin, or as discrete fibers immersed in the mixture, and as continuous long strips arranged as grids to reinforce a concrete element. Discrete fibers have been considered with different shapes (strips or circular fibers) and different length (short and long strips) (Pereora de Oliveira et al., 2011; Foti, 2011, 2013b; Fraternali et al., 2013). In all the cases the goal is to describe how the use of PET fibers, added to concrete, can prevent, or at least limit, the presence of cracks. In particular, the adhesion between concrete and fibrous reinforcement and the global behavior of this fiber-reinforced concrete are observed in order to evaluate the possibility of investigation to deepen in the future. The last part of the present chapter describes the laboratory tests carried out on specimens of concrete reinforced with fibers obtained from waste water bottles made of PET, simply by cutting the bottles without any chemical process or manufacturing. The tests are part of extensive research on the use of PET as a reinforcing material in concrete structures and/or masonry (Foti, 2011, 2013b; Fraternali et al., 2013; Foti et al., 2012, 2013). The tests provide interesting results concerning the strength
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to static and dynamic loads (even impacting) of concrete reinforced with PET fibers, suggesting a possible use of this material especially for those applications frequently subject to shocks and impact forces, such as new jersey parapets, road and airport pavements, wharfs. The reinforcement with PET has the advantage of being less corrosive and less expensive than the reinforcement with metallic nets and grids of carbon steel or glass.
18.2
Use of PET in concrete
In the context of composite materials and in particular of those that make use of polymeric materials, in recent years the research on the use of PET has had a remarkable development. At the base of this choice there are various factors: the wide availability of this material at low cost, considering the large use made thereof, especially in the food industry; the physical and mechanical characteristics, which allow its use in place of the common steel, carbon, or glass fibers; the clear advantages in terms of durability resulting therefrom. To give a major impulse to the development of this solution there was also the problem of the disposal of waste products. PET, in fact, is one of the major components of the waste stream, thinking also of the countless uses that it has, not only in industry but also in our daily lives. For all these reasons, in the last 20 years there have been numerous studies on this topic, with the aim of highlighting the advantages and problems associated with the use of PET, and they provided the basis for further future research.
18.2.1 Mechanical properties of PET PET is a thermoplastic resin composed of phthalates forming part of the family of polyesters. It has a good tensile strength: the tensile stress at break is lower if compared to common steels bars for reinforced concrete but clearly higher than the very low tensile strength of concrete. For example, for a C25/30 concrete the characteristic tensile strength is 2.56 MPa, while for PET it can assume values around 85 MPa. Table 18.1 compares the mechanical characteristics of C25/30 and PET. The elongation at break is very high, like, in general, for many plastics; PET can in fact reach deformations up to 50%. The tensile modulus of elasticity of PET assumes values between 2800 and 3100 MPa lower than concrete. Table 18.1 Mechanical characteristics of C25/30 concrete and PET Tensile Strength
Elongation at Break
C25/ 30
2.56 MPa
e
PET
85 MPa
50%
Tensile Modulus of Elasticity Et
2800e3100 MPa
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PET could be utilized for many applications: it is used for the production of films, tubes, labels, and it is also widely exploited by the food industry for the production of bottles and containers, thanks to the compatibility for contact with food sanctioned by Directive 2000/72/EC of the European Commission. The numbers regarding the production of PET give an idea of how much it has spread, not only in industry but also in our daily lives; in 2006, the estimated annual production amounted to about 12.3 million tons. At the base of this enormous diffusion there is the wide range of uses for which this material is suitable and relatively cheap compared to other materials, even when compared with other plastics (Gu and Ozbakkaloglu, 2016). The same polyethylene (PE), which, for example, is used to make the caps of common bottles for water, has a higher cost than PET (which is used instead for the body of the bottle). This is the reason why there is a high quantity of waste PET for disposal that it would be better to find solutions for its recycling.
18.2.2
PET fibers
In recent years, numerous authors have focused on the use of PET fibers mostly dispersed within the concrete mix, giving rise to a wide range of studies concerning types of fibers that considerably vary in shape, size, surface roughness, and other characteristics. It has been found that in all the cases the percent volume of fiber added to the concrete has a direct influence on both the compressive and tensile strength of ecological concrete and that the fiber length has a direct influence on the tensile strength of ecological concrete (Lopes Pereira et al., 2017). As for the shape and the dimensions, most of these studies involve little fibers, generally with a width of a few millimeters and a length of some centimeters, uniformly distributed into the concrete matrix. The advantages in using discrete PET fibers reinforcement can be summarized as follows: they enable a restraint of the shrinkage microcracks, a delay of the propagation of these cracks within concrete, and a control of the crack widths. However, the main function carried out by the reinforcing fibers in PET resides in the so-called “sewing effect” that the fibers play against numerous microcracks, which naturally tend to form in the concrete also in the early stages of life of the structure. It is a result of the hygrometric shrinkage that tends to propagate and expand under the effect of the tensile stresses. The fibers thus reduce the amplitude of the cracks bridging the two edges, and this produces significant benefits: in particular, the strength and durability of the structure are improved. The structure, in fact, can take advantage of an additional contribution in the postcracking phase due to the increased ductility of FRC compared to an ordinary concrete; durability is improved because most of the phenomena that involve the degradation of concrete are favored and accentuated by the presence of cracks of a certain amplitude, and vice versa the cracks are noticeably less intense and wide when this amplitude is kept under control by the fibers. As anticipated, fibers may vary in many ways; an important element of variability is the slenderness, which directly influences both the adhesion between the fiber and the
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concrete and the workability of the mixture. Adhesion, in particular, is one of the fundamental problems of FRC with synthetic fibers, especially if it is compared with the good adherence that is normally afforded by common steel reinforcement. To increase this adhesion, it is a common practice to act on the roughness of the surface of the fibers: with this aim it is advantageous to use long strips of PET obtained from the common bottles for food use, the surface of which is often characterized by ripples that promote adhesion to the concrete matrix. As mentioned, however, also the conformation of the fibers, through their slenderness, can affect such adhesion. Slenderness is an important characteristic, since it affects the PET-concrete adherence and the fresh mixture workability. Highly slender fibers have high surface areas, which means that fibers adhere to concrete very well, with the effect of great pullout strength. This goes, however, to the detriment of workability, which is instead favored by the use of fibers with low slenderness that could be incorporated into the concrete mix more easily. In Chang et al. (2013), the PET fiber surface was modified by ambient-temperature plasma activation in order to increase the hydrophilicity of PET fibers. This treatment was followed by acrylic acid monomer grafting. The aim was to modify the surface of PET fibers in order to absorb heavy metals in wastewater. The material has been treated by changing some parameters of discharge, such as the power and the time of the treatment to optimize the properties of the surface. After this use in wastewaters, such material could be utilized in constructions. Another possible use of plasma could be to modify the surface of PET to improve the adherence of this material to concrete. One series of tests utilized 2.6% by volume of waste polypropylene fibers that had undergone a plasma treatment process. The composite exhibited lower flexural strength and toughness in comparison with the composite containing untreated fibers. This was possibly due to a reduction of a frictional bond when the fiber surfaces were cleaned by the plasma treatment. This finding was different from that on polyethylene fibers in which plasma treatment was found to enhance the bonding properties (Wu and Li, 1997). To maximize the economic benefits that accompany the use of a material destined for disposal in replacement of expensive steel bars, in most of the studies carried out on this subject the fibers used have been obtained with the material receiving no further treatment. In particular, a favorable solution is the use of the bottles commonly used commercially for mineral water. By cutting these bottles simply by hand it is possible to obtain fibers or strips of various shapes and sizes, exploiting also the surface roughness of such bottles, as previously said, to improve their adherence. In Pereora de Oliveira et al. (2011), Foti (2011, 2013b), and Fraternali et al. (2013) PET fibers to add to concrete have been obtained directly by cutting waste bottles; an increase of toughness of concrete was noticed, while the compressive strength did not change significantly. The workability of the concrete mix is still good if the fiber content is less than 1.5% in volume. The use of such fibers, directly derived from waste material, does not involve particular differences from the mechanical point of view compared to the fibers subjected to treatments of various types. PET fibers to add to concrete, in fact, could be used after a long and expensive extruding process from waste bottles, obtaining PET monofilaments that were cut in short fibers (Ochi et al., 2007). A series of tests
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were performed on these fibers to get the characteristics of mixibility, toughness, and adherence with the concrete mix, showing good results despite the high costs of manufacturing. In Kim et al. (2010), another kind of manufacturing is proposed, coating the surfaces of PET fibers with maleic anhydride grafted polypropylene. Also, an oxygen plasma treatment on the surface of PET fibers is also proposed in order to act as a microreinforcement of the cementitious mix. The result is a concrete mix with a stronger interfacial bonding due to the improvement in adhesion of the fibers’ surface (Trejbal et al., 2016). This improvement ends in a reduced crack opening with an increase in the durability of concrete too. Polypropylene and polyethylene fibers have also been added to concrete after a manufacturing process with the aim to get regular fibers of different length to evaluate the effects of their geometry and shape on the mechanical characteristics of the reinforced concrete (Silva et al., 2013). The influence of shape and dimension of PET fibers on the mechanical properties of concrete has been also investigated in Marthong and Sarma (2016) (Fig. 18.1). In any case an increase in the tensile strength is obtained. With regard to the chemical behavior, in a recent study (Fraternali et al., 2013) the alkali resistance of such PET fibers was evaluated, and the results obtained showed that this resistance only slightly differs from that of polypropylene fibers formed for extrusion from recycled PET. Later prismatic specimens of mortar (40 40 160 mm) were made. They were reinforced with fibers of 2 mm width and with a length that varies in the different specimens (11.3, 22.6, 35 mm). The analysis of the flexural behavior highlighted some interesting aspects. Firstly, the fiber-reinforced specimens were characterized by a reduction of the effort at the first crack with respect to the nonreinforced specimens (this effect reduced, however, becoming almost negligible for
(d)
1.20
1.50 °
45
°
1.13
3.36
Figure 18.1 Different shapes of PET fibers (Marthong and Sarma, 2016).
1.17
60
2.70 1.
0.70
1.13
8.00
1.00 1.30
50
1.50
1.20
0.
1.00
5.10
8.00
0.8
0
°
39
1.13
34
1.30
0.80 1.131.17
(c)
1.50
1.00
(b) 1.20
(a) 0.90
Recycled waste PET for sustainable fiber-reinforced concrete
3.5
UNR
Load (kN)
3
R-PET 3.50
393
R-PET 1.13
R-PET 2.26
2.5 2 1.5 1 0.5 0
0
1
2 Deflection (mm)
3
4
Figure 18.2 Load-deflection curves of tested mortars at 28 days (Fraternali et al., 2013).
fibers with greater length); but the most relevant result was the increase, in specimens with reinforcement in PET, of the ductility and, at the same time, the toughness, measured by the area under the load-deflection diagram (Fig. 18.2). Toughness is an important mechanical characteristic, as it is linked to the ability of a structure to absorb energy. In most researches on the reuse of the waste PET, fibers have been utilized as fiber reinforcement for concrete. Different kinds of fibers have been considered in these studies; different in shape, dimension, and slenderness (Fig. 18.3). The effect of their shape on concrete has been explored for short strips in Pereora de Oliveira et al. (2011) and in Silva et al. (2013); particularly in Silva et al. (2013), the fibers prevented the complete failure of the composite by holding the cement matrix together and transmitting the load previously supported with a slow crack process; instead during a bending test a sudden failure happened with a complete breaking of the specimen.
Figure 18.3 (a) Sample of short lamellar fibers (Foti, 2011); (b) Hand cutting of R-PET strips from postconsumer bottles. Top: exemplary of the examined bottles; center: macrostrips obtained through longitudinal cutting of the bottle; bottom final “R-PET 1.13” (left), “R-PET 2.26” (center), and “R-PET 3.50” (right) strips (Fraternali et al., 2011, 2013).
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Use of Recycled Plastics in Eco-efficient Concrete
This study demonstrated an improvement of mechanical strength, or anyway a nonsignificant change associated with a strong mechanical interlocking between the fibers/cement matrix interfaces promoted by the section area, roughness, and fibrillation ability of fibers and the crack and microcrack distribution system provided by the uniform distribution and orientation of fibers in the direction opposite to the load stresses (Fig. 18.4). In this way the transfer and spread of the load stresses along fibers prevent cracks diffusion and lead to more resistant and ductile composites. So, that volume fraction of fibers, fiber orientation, and fiber length are critical factors that affect the performance of composites and their ductility. Greater fiber length and volume fractions promote better crack resistance in composites but decrease the workability. In particular, in Pereora de Oliveira et al. (2011) the PET fibers were obtained by mechanical cutting of the lateral sides of waste bottles; the fibers so obtained had dimensions approximately of 2 mm in width, 0.5 mm of thickness, and 35 mm of length. The fibers were added to the concrete mix with a volume percentage equal to 0%, 0.5%, 1.0%, and 1.5%; the best performance of concrete was reached with the last percentage especially regarding the toughness. Instead in Silva et al. (2013), the fiber volume fraction was equal to 2.9% higher than the previous case and the fibers were equal to 12, 24, and 30 mm in length. A different shape of strips, the circular ones, generates an increase of toughness higher than the short lamellar ones and the advantage of an increasing ductility of the composite by a higher dosage of fibers (Foti, 2011, 2013b) (Fig. 18.5). The closed form of the fiber, in fact, produces a better bonding inside the concrete of the two sides of the crack, and it determines a higher adherence if compared to short lamellar strips. In Fraternali et al. (2013) the effects of the presence of longer filaments were compared for cement-based mortar and cement-lime mortars. In the case of cementbased mortars, slight decreases of the first-crack strength due to the R-PET reinforcement was observed; on the contrary, in cement-lime mortar it increased, but the flexural toughness indexes and the residual strength were more effective than the
Figure 18.4 Orientation of fibers in cement matrix: 1dfibers perpendicular to load direction (bridging effect); 2dfibers in the same direction as the applied load (Silva et al., 2013).
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Figure 18.5 Circular PET fibers (Foti, 2013b).
Figure 18.6 Cracking for bending at the centerline of the beam and detail of a roller support (Foti, 2013b).
cement-lime mortar. In (Foti, 2013b) longer strips composed of four overlapping layers of PET were considered. The results showed that the stresses are predominantly absorbed by concrete before the cracking; only in the postpeak phase the strips of PET absorbed the tensile stress, showing a ductile behavior (Fig. 18.6). In the same research 40-cm length specimens have been reinforced with half PET bottles having a ‘‘C’’ section directly cut, dividing by two parts along the length. It was noted that the specimen did not break completely thanks to the presence of PET half bottle and after a first vertical crack in the middle, the cracks propagated inclined of about 45 degrees, with a similar behavior of a beam subjected to bending test (Fig. 18.7) (Foti, 2013b). This kind of research, which utilizes long strips of PET from waste bottles was carried out also on structural element as plates (Foti and Paparella, 2014). The concrete
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Use of Recycled Plastics in Eco-efficient Concrete
Figure 18.7 Loading test on specimens with half-bottle fibers (Foti, 2013b).
Figure 18.8 Grid of PET bottle strips.
specimens were reinforced with a sort of grid of PET (Fig. 18.8). The test setup for the impact tests was inspired by those of Beckmann et al. (2012): the plate was positioned on a metallic frame and a load was dropped from a constant height. The results showed that the specimens reinforced with PET grids did not totally brake, as can been seen from the superficial cracks produced by the impact (Fig. 18.9). These grids, in fact, increased significantly the ductility of the plates. Another interesting aspect is the thermal conductivity of concrete reinforced with PET fibers. It was noticed that there is an increment in the thermal insulation of the concrete both adding pieces of different shapes of waste rubber of tires and/or plastic
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Figure 18.9 Upper side of reinforced plate n. 1 after the impact (Foti and Paparella, 2014).
fibers (strip, square, and irregular) (Fraternali et al., 2011; Yesilata et al., 2009). The insulation performance increases by the 18.52% with the addition of square rubber or by a range of 10.27%e18.16% with PET (Rahmani et al., 2013). The latter data coincide more or less with those shown in Fraternali et al. (2011) where the decrease of thermal conductivity (k) is equal to 18%. Possible models for materials with no tensile strengthdlike concretedreinforced with PET fibers have been proposed in Foti (2013a) or derived from Schmidt et al. (2009) and Fraternali et al. (2010). The behavior of PET-reinforced concrete qualitatively reflects what was in general observed for most of fiber-reinforced concretes and consists essentially on an improvement of the postpeak behavior and impact strength. The impact strength was investigated as an influence on concrete of polypropylene fibers (Bayasi and Zeng, 1997), where it was demonstrated that these fibers have a small favorable effect on compressive strength of concrete when 13-mm long fibers were used. In case of recycled plastics, Soroushian et al. (2003) got a higher impact strength of concrete; in particular, the milled mixed plastic particles and the melt-processed plastic fibers, when used at properly selected dosages, yielded important gains in strength of concrete to impact and restrain shrinkage cracking. There are some studies that compare the virgin and recycling materials. The effectiveness of the reinforcement with recycled fibers, in fact, is not equal to those from the virgin material, but calibrating the right dosage rate (generally higher for recycled materials) it is possible to obtain a similar performance for both types of material (Rebeiz, 1995). For this reason, fibers for concrete reinforcement generally need to be durable in the cementitious environment, to be easily dispersed in the concrete mix, to have good mechanical properties, and to be of appropriate geometric configuration in order to be effective.
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Figure 18.10 Specimen of a beam-column joint to be cast with a PET fiberereinforced concrete (Marthong and Marthong, 2016).
Finally, in Marthong and Marthong (2016) the use of concrete reinforced with PET fibers was applied to the reinforcement of a beam-column connection (Fig. 18.10). Then the specimens have been subjected to reverse cycling loading and compared with the behavior of specimens without any waste PET fibers.
18.3
Tests (summary) and results
Since the mid-90s a series of tests on concrete specimens reinforced with PET fibers were carried out at the Testing Laboratory of the Department of Civil Engineering and Architecture at the Polytechnic University of Bari. The numerous studies were focused on concrete elements different from each other; these specimens included fibers in PET with different shapes and sizes: 1. 2. 3. 4. 5. 6. 7. 8. 9.
Cubic specimens (100 mm 100 mm 100 mm) reinforced with circular or “O” fibers. Cubic specimens (100 mm 100 mm 100 mm) reinforced with short lamellar fibers. Prismatic specimens (100 mm 100 mm 400 mm) without reinforcements (1S, 2S). Prismatic specimens (100 mm 100 mm 400 mm) reinforced with short lamellar fibers (1FC, 2FC). Prismatic specimens (100 mm 100 mm 400 mm) reinforced with circular or “O” fibers (1C, 2C), for 0.50% and 0.75% by weight of concrete. Prismatic specimens (100 mm 100 mm 400 mm) reinforced with circular or “O” fibers for 1% by weight of concrete, with superplasticizers. Prismatic specimens (100 mm 100 mm 400 mm) reinforced with half-bottles of PET. Beam-specimens (100 mm 200 mm 1100 mm) with a bigger dimension and reinforced with PET strips Square-shaped slab specimens (800 mm 800 mm 58 mm) reinforced with a grid of PET strips.
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For comparison purposes the following tests were also carried out: 10. Beam-specimens (100 mm 200 mm 1100 mm) without reinforcements. 11. Square-shaped slab specimens (800 mm 800 mm 58 mm) without reinforcements.
The first comparison, drawn between ordinary concrete specimens and specimens 1 and 2, had the aim to checking the behavior of these kinds of fiber-reinforced concretes. The prescriptions followed for compression tests are those of the European code (UNI EN 12390:2009), so the loads were applied with a uniaxial testing on both sides of specimens through two plates (Fig. 18.11). In specimens with fibers a reduction of resistance occurred but probably these results are partly distorted by the difficulties encountered during the tests. Also tests on short lamellar fibers of PET were carried out to characterize them. In addition, tensile tests on fibers were performed furnishing an average value of the tensile strength equal to 150 N/mm2. In Fig. 18.12 the test set-up on a short lamellar fiber is shown. Then a comparison was done between specimens 4, 5, and 6 (Fig. 18.13), also with different dosages of PET Fibers (0.50% and 0.75% in weight). To reduce the costs and simplify the FRCs production, the fibers utilized were obtained from ordinary “plastic” bottles through cuts perpendicular to their longitudinal axes. Two types of fibers were utilized, the lamellar ones with a section of 2 mm 0.1 mm and a length of 32 mm and the circular fibers with a width variable around 5 mm and a diameter of 30e50 mm. The concrete mixture utilized was Composite Portland concrete of Type II/A-LL (SN EN 197-1:2000).
Figure 18.11 Typical compressive failure of a specimen reinforced with PET fibers (Marthong and Sarma, 2016).
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Figure 18.12 Direct tensile test on a PET fiber (Foti, 2011).
Figure 18.13 Specimens for the first series of preliminary tests (Foti, 2011).
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The prismatic specimens were subjected to bending tests; an Instron electromechanical testing equipment of 50 KN maximum loads performed tests using the Wave maker software Instron Ltd. To verify their behavior, bending tests involved a concentrated load at the centerline to read at each instant the values of load and displacement. The positive influence that the fibers have on the post-peak behavior on concrete elements was confirmed. Both lamellar and “O”-fibers greatly improve the ductility and toughness of the specimens. The increase in toughness is more evident for the “O”-fibers because their special shape helps to bind the concrete on each side of a cracked section, increasing the adherence. The crack pattern in case of bending tests is extremely interesting: a clear single vertical cracking concentrated in the middle, sewn by the fibers (see for e.g., Fig. 18.14 (Fraternali et al., 2011)). It is important to note the deviation of the crack, which did not show the typical V-shape of ordinary concrete failure. Table 18.2 shows the values of the tensile strength obtained during the bending tests. As a consequence of the better behavior of the specimens with circular fibers, it was decided to determine the best dosage of “O”-fibers by weight of the concrete, through the comparison of specimens of test 5 and test 6. An industrial readymade mix concrete was utilized for specimens of test 6 differently from specimens of test 5. For this reason, it was only possible to make comparisons regarding the recovery of the load (Fig. 18.15) and not the peak values.
Figure 18.14 (a) Sewing effect of PET fibers after cracking and (b) detail (Fraternali et al., 2011). Table 18.2 Strength values from the bending tests Specimen
1S
2S
1C (0.75%)
2C (0.75%)
1C (0.50%)
2C (0.50%)
1FC
2FC
fct (N/ mm2)
4.7
4.2
4.8
4.3
3.7
3.6
3.7
3.7
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Specimens reinforced with circular fibers 10
PET 1% - test 1 PET 1% - test 2 PET 0.75% - test 1 PET 0.75% - test 2 PET 0.5% - test 1 PET 0.5% - test 2
Applied load (kN)
8
6
4
2
0 0
2 4 6 8 10 12 14 Maximum deflection at the centerline (mm)
16
18
20
Figure 18.15 Load-deformation plots on specimens with circular fibers (1%, 0.75%, and 0.5%) (Foti, 2013b).
It can be noticed that the recovery of the load, in percentage respect to the peak, is faster for the mix with 1% of PET. The drop of the load is about 70% with respect to the cracking load for PET 0.50%, while 55% for PET 0.75%, and about 35% for PET 1%. Clearly in the case of PET 1% of the fibers are more present in the concrete, so it is easier for the loads to be transferred to the fibers that are able to sew the fracture and to deform in the plastic range. It can be concluded that it is important to increase the percentage of fibers to get an improvement in the concrete behavior but the percentage cannot be high because even after adding superplasticizers the concrete can become less workable. After having analyzed and tested the specimens with a uniform PET fiber reinforcement distributed in the concrete mix, the mechanical characteristics and the behavior of FRC with reinforcement concentrated in the position where high levels of tensile stresses are expected were checked. It does not pursue the prescriptions and the parameters of CNR-DT 204/2006 on the use of fibers. This is true for specimens of test 7, which use strips with C-shaped section obtained from half bottles with a base equal to about 8 cm and two side elevations of about 4 cm and assembled, after an appropriate overlap, with staples (Fig. 18.16). They were placed in the lower part of the specimens, subjected to traction during the bending tests. To verify the behavior of this kind of FRC, it is important to evaluate the adhesion between PET and concrete and the attitude of the strips of PET as a concentrate reinforcement. Therefore, on the specimens a standard test up to a maximum deflection of 20 mm was performed. The crack pattern is extremely interesting: the specimen did not break completely after a first vertical crack concentrated in the mid-section, while inclined shear cracks appeared (Fig. 18.17). Even if the detachment of the bottom area occurred, the concrete and the strips kept a good bond between them.
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Figure 18.16 Half bottle reinforcement (Foti, 2013b).
Figure 18.17 Loading tests on specimens with half-bottle fibers (Foti, 2013b). (a) Testing setup; (b) detail of the cracked section.
Fig. 18.18 shows the load-deflection plot; in particular, the crack in correspondence of the peak and a hardening trend with a subsequent recovery of the load and increment of the deformation are noticed. Given the encouraging results, it was decided to test the behavior of larger elements. So, similar analyses on the behavior between PET and concrete were performed on little beams with localized fiber reinforcement. The reinforcing strips in the specimens had dimensions 45 mm 0.2 mm 300 mm, and were superimposed, assembled, and placed along the entire length of the 1m-specimens, as shown in Fig. 18.19. The results of the bending tests showed that only after the cracking of the concrete, which occurred suddenly, the PET strips started to absorb the tensile stress through a ductile behavior and in particular a large deformation before the total failure of the specimen.
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Load-deflection. specimen n°2
Applied load (kN)
4
3
2
1
0 0
3
6 9 12 15 18 Maximum deflection at the centerline (mm)
21
Figure 18.18 Load-deflection plot for the second specimen (Foti, 2013b).
Figure 18.19 (a) PET strips utilized and (b) their position in the cracking section (Foti, 2013b).
Fig. 18.20 shows the load-deflection plots and highlights the ductile behavior in the postcracking phase of the beams reinforced with PET. Regrettably, the value of the recovery of resistance after cracking is low. This is probably due to the reduced resisting area of PET fibers only equal to 36 mm2. Another important positive result to highlight is that the failure occurred for all the fibers without any sliding from the concrete beam. To complete the research, it was decided to test the behavior of another concrete structural element, a slab. The reinforcement consisted in a sort of grid of long discrete 5 cm wide fiber strips with a 0.2 mm thickness and obtained from PET bottles simply cut along the longitudinal axis and positioned in place of steel (Fig. 18.21). Each strip is assembled to obtain a total length equal to 77 cm and left 1.5 cm from each side of the slab. Ten strips per side were assembled to create a grid that guaranteed the bidirectionality of the reinforcement; two grids were placed in each reinforced slab, not directly in contact
Recycled waste PET for sustainable fiber-reinforced concrete Beam specimens reinforced with PET sheets
12
Beam specimen without fibers Beam specimen 1 Beam specimen 2 Beam specimen 3 Beam specimen 4
10 Applied load (kN)
405
8 6 4 2
0 0
2 4 6 8 Maximum deflection at the centerline (mm)
10
12
Figure 18.20 Load-deflection plot for little beams reinforced with PET strips (Foti, 2013b).
Figure 18.21 Grid of PET bottle (Foti and Paparella, 2014).
but spaced by a layer of concrete to avoid empty spaces between them due to the ripples of the strips themselves. There are no codified prescriptions for impact tests on concrete, so, in this case the impact load tests (Fig. 18.22) consisted in dropping a steel cylinder, with a weight of 290 N, from a height of 1.0 m and to assess the kind of failure and the crack patterns on the slabs (Figs. 18.22e18.24). As Figs. 18.22e18.24 show, after the tests the non-reinforced plate was completely broken, while the reinforced plate was only subject to superficial cracks that did not lead to a complete failure. So, the presence of the two grids of PET reinforcement ensured the correct response to shocks and impact forces thanks to the more ductile behavior of the concrete plates, thus confirming the improvement of the impact
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Figure 18.22 (a) Impact instant for the nonreinforced plate n. 1; (b) Impact instant for the reinforced plate n. 1 (Foti and Paparella, 2014).
Figure 18.23 Upper side of nonreinforced plate n. 1 after the impact (Foti and Paparella, 2014).
strength and the ability to withstand very high bidirectional deformations without reaching the failure. Fig. 18.25 shows the average trend of deformation in the tested slabs. The nonreinforced slabs are characterized by attainment of peak in correspondence of the impact and a subsequent sudden fall in correspondence of the break; in the reinforced plates, instead, it is possible to notice a peak at the instant of impact quantitatively not much different from that of nonreinforced plates. After the impact, for the nonreinforced slabs a stable value of deformation occurred, even if the section had no more strength capacity; for the reinforced slabs, instead, after the peak there is a strong decrease of the deformation followed by an increase with a fluctuating trend around a value much higher than zero; this indicates that the slab continues to resist and maintains its structural integrity except for generated fractures which remain superficial. This behavior confirms what was expected due to the higher ductility of the reinforcement provided by the grids, meaning that the plate still has the possibility to transmit a deformation.
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Figure 18.24 Detail of the bottom side of the reinforced plate after the impact (Foti and Paparella, 2014). Deformation (%) 0,14
Non-reinforced slab 1
0,12
Reinforced slab 2
Reinforced slab 1
Non-reinforced slab 2
0,10 0,08 0,06 0,04 0,02
10,017
10,013
10,009
10,005
9,996
9,992
9,988
9,984
9,980
–0,02
10,000
Time (s)
0,00
Figure 18.25 Time-history of the average deformations for slabs.
In addition, the results showed a good adhesion between concrete and strips, cooperating to absorb the tensile stresses even after the onset of cracks due to bending and shear stresses. In fact, there were no sliding phenomena of any kind and the collapse affected all the strips at the same time. As expected, an interesting postpeak behavior was also found, with a lift of the load, albeit modest, due to the reduced resistant area of the PET strips.
408
18.4
Use of Recycled Plastics in Eco-efficient Concrete
Conclusions
The results obtained confirm, in general, two of the major effects of the use of PET fibers: the greater ductility and the reduction of the workability for dosages of fibers greater than 1% by weight of concrete. This is especially true both for lamellar and “O”-shaped fibers that greatly improve the toughness of the specimens. The enhancement of the toughness is especially evident for the circular fibers; their special shape, in fact, helps to bind the concrete on each side of a cracked section. But the most interesting result concerns the reinforcements made with strips, which represent the most innovative aspect of this research. The tests confirmed the possibility of using strips of PET as reinforcement localized in areas that are expected to be the most stressed in traction. The strips can also be arranged in a grid for concrete plates, so as to give them a very ductile behavior and to avoid the complete failure, thus confirming the improvement of the impact strength. For all these reasons the use of PET strips can be proposed as concentrate reinforcement, in substitution of steel, for structures or secondary structural elements, even if more detailed studies and test campaigns are needed. Different possible uses for this reinforced concrete are proposed, such as, for example, in the production of industrial floors, docks, new jersey barriers, and wharfs in concrete, without forgetting some material properties that include the high efficiency and the resistance to attack by chemicals. Of course, some aspects should be improved in the production of grids and in their characteristics. Finally, it is important to emphasize that it is possible to combine all these benefits with the reuse and the recycle of plastic waste but also with the reduction of the production costs. Future studies will aim at improving the behavior and manufacturing of this reinforcement also improving the adherence of PET with concrete by mean, for example, of coating the surface of the fibers with plasma.
Acknowledgment Francesco Paparella is gratefully acknowledged for his help during the tests at the Laboratory of Testing and Materials “M. Salvati” of the Polytechnic University of Bari, Italy.
References Batayneh, M., Marie, I., Asi, I., 2007. Use of selected waste materials in concrete mixes. Waste Management 27 (12), 1870e1876. Bayasi, Z., Zeng, J., 1997. Properties of polypropylene fiber reinforced concrete. ACI Materials Journal 90 (6), 605e610. Beckmann, B., Hummeltenberg, A., Weber, T., Curbach, M., 2012. Concrete slabs under impact load: drop tower experiments. In: Sudies and Researches, Graduate School in Concrete Structures, Fratelli Pesenti, Politecnico di Milano, Italy, vol. 31, pp. 135e153.
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Chang, J.-E., Chang, Y.-K., Leu, M.-H., Chen, Y.-L., Huang, J.-H., 2013. Heavy metal removal by ambient-temperature argon plasma modified polyethylene terephthalate (PET) fibers with surface acrylic acid grafting. International Journal of Physical Sciences 8 (6), 244e253. Foti, D., Paparella, F., 2014. Impact behavior of structural elements in concrete reinforced with PET fibers. Mechanics Research Communications 57, 57e66. https://doi.org/10.1016/ j.mechrescom.2014.02.007. ISSN: 0093-6413. Foti, D., Paparella, V., Paparella, F., 2012. Sull’impiego del PET riciclato come materiale di rinforzo del calcestruzzo. L’Edilizia, Structural 174, 54e63. ISSN: 1593-3970. (In Italian). Foti, D., Paparella, V., Paparella, F., 2013. Calcestruzzi fibrorinforzati con fibre di PET. Studio della resistenza all’impatto. L’Edilizia, Structural 176, 1e14. ISSN: 1593-3970. (In Italian). Foti, D., 2011. Preliminary analysis of concrete reinforced with waste bottles PET fibers. Construction and Building Materials 25 (4), 1906e1915. https://doi.org/10.1016/ j.conbuildmat.2010.11.066. Foti, D., 2013a. On the numerical and experimental strengthening assessment of tufa masonry with FRP. Mechanics of Advanced Materials and Structures 20 (02), 163e175. Foti, D., 2013b. Use of recycled waste pet bottles fibers for the reinforcement of concrete. Composite Structures 96, 396e404. Fraternali, F., Negri, M., Ortiz, M., 2010. On the Convergence of 3D free discontinuity models in variational Fracture. International Journal of Fracture 166 (1e2), 3e11. Fraternali, F., Ciancia, V., Chechile, R., Rizzano, G., Feo, L., Incarnato, L., 2011. Experimental study of the thermo-mechanical properties of recycled PET fiber-reinforced concrete. Composite Structures 93, 2368e2374. Fraternali, F., Farina, I., Polzone, C., Pagliuca, E., Feo, L., 2013. On the use of R-PET strips for the reinforcement of cement mortars. Journal of Composites Part B: Engineering 46, 207e210. Gu, L., Ozbakkaloglu, T., 2016. Use of recycled plastics in concrete: a critical review. Waste Management 51, 19e42. Kim, S.B., Yi, N.H., Kim, H.Y., Kim, J.H.J., Song, Y.-C., 2010. Material and structural performance evaluation of recycled PET fiber reinforced concrete. Cement and Concrete Composites 32, 232e240. Lopes Pereira, E., Luis de Oliveira Jr., A., Gomes Fineza, A., 2017. Optimization of mechanical properties in concrete reinforced with fibers from solid urban wastes (PET bottles) for the production of ecological concrete. Construction and Building Materials 149, 837e848. Marthong, C., Marthong, S., 2016. An experimental study on the effect of PET fibers on the behavior of the exterior RC beam-column connection subjected to reversed cyclic loading. Structure 5, 175e185. Marthong, C., Sarma, D.K., 2016. Influence of PET fiber geometry on the mechanical properties of concrete: an experimental investigation. European Journal of Environmental and Civil Engineering 20 (7), 771e784. Ochi, T., Okubo, S., Fukui, K., 2007. Development of recycled PET fiber and its application as concrete-reinforcing fiber. Cement and Concrete Composites 29, 448e455. Pereora de Oliveira, L.P., Castro-Gomez, J.P., 2011. Physical and Mechanical Behaviour of recycled PET fibre reinforced mortar. Construction and Building Materials 25, 1712e1717. Rahmani, E., Dehestani, M., Beygi, M.H.A., Allahyari, H., Nikbin, I.M., 2013. On the mechanical properties of concrete containing waste PET particles. Construction and Building Materials 47, 1302e1308.
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Rebeiz, K.S., 1995. Timeetemperature properties of polymer concrete using recycled PET. Cement and Concrete Composites 17, 119e124. Schmidt, B., Fraternali, F., Ortiz, M., 2009. Eigen fracture: an eigen deformation approach to variational fracture. Multiscale Model Simulation 7 (3), 1237e1266. Siddique, R., Khatib, J., Kaur, I., 2008. Use of recycled plastic in concrete: a review. Waste Management 28, 1835e1852. Silva, D.A., Betioli, A.M., Gleize, P.J.P., Roman, H.R., Gomez, L.A., Ribeiro, J.L.D., 2004. Degradation of recycled PET fibers in Portland cement-based materials. Cement and Concrete Research 35, 1741e1746. Silva, E.R., Coelho, J.F.J., Bordado, J.C., 2013. Strength improvement of mortar composites reinforced with newly hybrid-blended fibers: influence of fibers geometry and morphology. Construction and Building Materials 40, 473e480. Soroushian, P., Plasencia, J., Ravanbakhsh, S., 2003. Assessment of reinforcing effects of recycled plastic and paper in concrete. ACI Materials Journal 100 (3), 203e207. Trejbal, J., Kopeckỳ, L., Tesarek, P., Fladr, J., Antos, J., Somr, M., Nezerka, V., 2016. Impact of surface plasma treatment on the performance of PET fiber reinforcement in cementitious composites. Cement and Concrete Research 89, 276e287. Wu, H.C., Li, V.C., 1997. Basic interfacial characteristic of polyethylene fiber/cement composites and its modification by plasma. In: Brandt, A.M., Li, V.C., Marshall, I.H. (Eds.), Brittle Matrix Composites 5. Woodhead Publishing, Cambridge, UK, pp. 14e23. Yesilata, B., Isiker, Y., Turgut, P., 2009. Thermal insulation enhacement in concretes by adding waste PET and rubber pieces. Construction and Building Materials 23, 1878e1882.
Further reading Donatone, G., Foti, D., Paparella, F., 2010. Soluzioni innovative nel campo dei calcestruzzi fibrorinforzati: analisi preliminare su un calcestruzzo rinforzato con fibre di PET. In Concreto. 95, lug/ago 36e44 (In Italian). Plastics Europe, E., 2013. Plastics-the Facts 2013. An Analysis of European Latest Plastics Production, Demand and Waste Data.
18
Dora Foti Department of Civil Engineering Sciences and Architecture, Polytechnic University of Bari, Bari, Italy
18.1
Introduction
Concrete is a building material generally made with a mixture of water, cement, and aggregates. It can be considered an artificial conglomerate stone that possesses characteristics similar to those of a rock, including good compression strength and a poor strength to tensile stresses. The main defect of concrete is, in fact, its very low tensile strength, so low that it is often completely neglected in the calculus models. If concrete is today by far the most widely used building material in the world, it is thanks to reinforcements that make up for the poor tensile strength and brittle behavior of the concrete. Among these, steel reinforcement is often utilized, though fibers of different materials are also added to the cement matrix. Typically, they are metal fibers, in polymer, carbon, glass, or natural material. Such a concrete is called fiber-reinforced concrete (FRC). The properties of the composite concrete depend on the characteristics of the two matrices (cement and fibers), on their dosages, and in particular, on the geometry; the volumetric/weight percentage; and the mechanical characteristics of the fiber, the adherence between the fiber and the matrix of concrete; the mechanical characteristics of the matrices. The fibers may help to reduce the phenomenon of cracking and/ or significantly increase the energy absorbed in the process of fracture (toughness). The latter depends on several factors, including, for example, the aspect ratio (i.e., the ratio length/equivalent diameter of the fibers), the amount of fibers, their orientation and their spread within the cement matrix, the physical-mechanical characteristics of the latter. At a constant composition and dosage, the fibers with a greater aspect ratio are more effective. They are characterized, in fact, by a greater surface area that allows the strengthening of the bond with the concrete, reducing the risk of slipping. At the same time, however, the use of slender fibers, which are those with a higher aspect ratio, has also negative effects: it relates especially to the dispersion of fibers within the cement matrix, for which it is not possible to obtain a good dispersion (that instead would be desirable), and the workability of the fresh mix. These side effects generally lead to a limitation of the dose of slender fibers.
Use of Recycled Plastics in Eco-efficient Concrete. https://doi.org/10.1016/B978-0-08-102676-2.00018-9 Copyright © 2019 Elsevier Ltd. All rights reserved.
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A further important aspect is the shape of the surface: if uneven or wavy it ensures a better adherence to the cement matrix and thus a greater strength to the pull out of the fibers subject to tensile stresses. In recent decades the research is focusing, in particular, on the reuse of waste plastics, thus trying to combine the advantages in terms of a better behavior of the concrete mixture with those derived from the recycling of large quantities of waste that would otherwise be destined to solve problems such as landfill, incineration. In many recent studies efforts have been made to analyze the possible utilizations for postconsumer and recycled plastic in the production of concrete to gain both economic and environmental benefits (Batayneh et al., 2007; Siddique et al., 2008). Among the plastic waste, polyethylene represents the largest fraction, followed by polyethylene terephthalate, most known as PET. It is obtained in large quantity from plastic bottles utilized as containers of beverages and mineral water. As result of the drastic increase in beverage consumption, the production of polyethylene terephthalate (PET) bottles has increased exponentially, also due to the favorable properties of this plastic, including low density, high resistance, weight ratio, high durability, ease of conception/fabrication, and low cost. Particular interest is growing, at present, in the reuse of fibers obtained from waste PET bottles. With the aim to reduce the waste and take profit from this material, some preliminary studies can be found in Silva et al. (2004). The good results in terms of mechanical characteristics push the research toward the utilization of this kind of fibers and the consequent performance of the new concrete mixture. The present study, in fact, mainly concerns the reuse of PET intended for many different applications and, especially, to make up the common bottles of water. Its good characteristics of adhesion with concrete make it more suitable to be used in the mix design of concrete compared to other polymers. In this chapter the state-of-the-art on the use of PET fibers for the reinforcement of concrete is presented. In previous studies such material has been used in different ways with or without manufacturing, the latter employing PET directly cut from waste bottles (Batayneh et al., 2007; Siddique et al., 2008; Soroushian et al., 2003; Silva et al., 2004). PET has been utilized as a binder, in the form of a resin, or as discrete fibers immersed in the mixture, and as continuous long strips arranged as grids to reinforce a concrete element. Discrete fibers have been considered with different shapes (strips or circular fibers) and different length (short and long strips) (Pereora de Oliveira et al., 2011; Foti, 2011, 2013b; Fraternali et al., 2013). In all the cases the goal is to describe how the use of PET fibers, added to concrete, can prevent, or at least limit, the presence of cracks. In particular, the adhesion between concrete and fibrous reinforcement and the global behavior of this fiber-reinforced concrete are observed in order to evaluate the possibility of investigation to deepen in the future. The last part of the present chapter describes the laboratory tests carried out on specimens of concrete reinforced with fibers obtained from waste water bottles made of PET, simply by cutting the bottles without any chemical process or manufacturing. The tests are part of extensive research on the use of PET as a reinforcing material in concrete structures and/or masonry (Foti, 2011, 2013b; Fraternali et al., 2013; Foti et al., 2012, 2013). The tests provide interesting results concerning the strength
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to static and dynamic loads (even impacting) of concrete reinforced with PET fibers, suggesting a possible use of this material especially for those applications frequently subject to shocks and impact forces, such as new jersey parapets, road and airport pavements, wharfs. The reinforcement with PET has the advantage of being less corrosive and less expensive than the reinforcement with metallic nets and grids of carbon steel or glass.
18.2
Use of PET in concrete
In the context of composite materials and in particular of those that make use of polymeric materials, in recent years the research on the use of PET has had a remarkable development. At the base of this choice there are various factors: the wide availability of this material at low cost, considering the large use made thereof, especially in the food industry; the physical and mechanical characteristics, which allow its use in place of the common steel, carbon, or glass fibers; the clear advantages in terms of durability resulting therefrom. To give a major impulse to the development of this solution there was also the problem of the disposal of waste products. PET, in fact, is one of the major components of the waste stream, thinking also of the countless uses that it has, not only in industry but also in our daily lives. For all these reasons, in the last 20 years there have been numerous studies on this topic, with the aim of highlighting the advantages and problems associated with the use of PET, and they provided the basis for further future research.
18.2.1 Mechanical properties of PET PET is a thermoplastic resin composed of phthalates forming part of the family of polyesters. It has a good tensile strength: the tensile stress at break is lower if compared to common steels bars for reinforced concrete but clearly higher than the very low tensile strength of concrete. For example, for a C25/30 concrete the characteristic tensile strength is 2.56 MPa, while for PET it can assume values around 85 MPa. Table 18.1 compares the mechanical characteristics of C25/30 and PET. The elongation at break is very high, like, in general, for many plastics; PET can in fact reach deformations up to 50%. The tensile modulus of elasticity of PET assumes values between 2800 and 3100 MPa lower than concrete. Table 18.1 Mechanical characteristics of C25/30 concrete and PET Tensile Strength
Elongation at Break
C25/ 30
2.56 MPa
e
PET
85 MPa
50%
Tensile Modulus of Elasticity Et
2800e3100 MPa
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Use of Recycled Plastics in Eco-efficient Concrete
PET could be utilized for many applications: it is used for the production of films, tubes, labels, and it is also widely exploited by the food industry for the production of bottles and containers, thanks to the compatibility for contact with food sanctioned by Directive 2000/72/EC of the European Commission. The numbers regarding the production of PET give an idea of how much it has spread, not only in industry but also in our daily lives; in 2006, the estimated annual production amounted to about 12.3 million tons. At the base of this enormous diffusion there is the wide range of uses for which this material is suitable and relatively cheap compared to other materials, even when compared with other plastics (Gu and Ozbakkaloglu, 2016). The same polyethylene (PE), which, for example, is used to make the caps of common bottles for water, has a higher cost than PET (which is used instead for the body of the bottle). This is the reason why there is a high quantity of waste PET for disposal that it would be better to find solutions for its recycling.
18.2.2
PET fibers
In recent years, numerous authors have focused on the use of PET fibers mostly dispersed within the concrete mix, giving rise to a wide range of studies concerning types of fibers that considerably vary in shape, size, surface roughness, and other characteristics. It has been found that in all the cases the percent volume of fiber added to the concrete has a direct influence on both the compressive and tensile strength of ecological concrete and that the fiber length has a direct influence on the tensile strength of ecological concrete (Lopes Pereira et al., 2017). As for the shape and the dimensions, most of these studies involve little fibers, generally with a width of a few millimeters and a length of some centimeters, uniformly distributed into the concrete matrix. The advantages in using discrete PET fibers reinforcement can be summarized as follows: they enable a restraint of the shrinkage microcracks, a delay of the propagation of these cracks within concrete, and a control of the crack widths. However, the main function carried out by the reinforcing fibers in PET resides in the so-called “sewing effect” that the fibers play against numerous microcracks, which naturally tend to form in the concrete also in the early stages of life of the structure. It is a result of the hygrometric shrinkage that tends to propagate and expand under the effect of the tensile stresses. The fibers thus reduce the amplitude of the cracks bridging the two edges, and this produces significant benefits: in particular, the strength and durability of the structure are improved. The structure, in fact, can take advantage of an additional contribution in the postcracking phase due to the increased ductility of FRC compared to an ordinary concrete; durability is improved because most of the phenomena that involve the degradation of concrete are favored and accentuated by the presence of cracks of a certain amplitude, and vice versa the cracks are noticeably less intense and wide when this amplitude is kept under control by the fibers. As anticipated, fibers may vary in many ways; an important element of variability is the slenderness, which directly influences both the adhesion between the fiber and the
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concrete and the workability of the mixture. Adhesion, in particular, is one of the fundamental problems of FRC with synthetic fibers, especially if it is compared with the good adherence that is normally afforded by common steel reinforcement. To increase this adhesion, it is a common practice to act on the roughness of the surface of the fibers: with this aim it is advantageous to use long strips of PET obtained from the common bottles for food use, the surface of which is often characterized by ripples that promote adhesion to the concrete matrix. As mentioned, however, also the conformation of the fibers, through their slenderness, can affect such adhesion. Slenderness is an important characteristic, since it affects the PET-concrete adherence and the fresh mixture workability. Highly slender fibers have high surface areas, which means that fibers adhere to concrete very well, with the effect of great pullout strength. This goes, however, to the detriment of workability, which is instead favored by the use of fibers with low slenderness that could be incorporated into the concrete mix more easily. In Chang et al. (2013), the PET fiber surface was modified by ambient-temperature plasma activation in order to increase the hydrophilicity of PET fibers. This treatment was followed by acrylic acid monomer grafting. The aim was to modify the surface of PET fibers in order to absorb heavy metals in wastewater. The material has been treated by changing some parameters of discharge, such as the power and the time of the treatment to optimize the properties of the surface. After this use in wastewaters, such material could be utilized in constructions. Another possible use of plasma could be to modify the surface of PET to improve the adherence of this material to concrete. One series of tests utilized 2.6% by volume of waste polypropylene fibers that had undergone a plasma treatment process. The composite exhibited lower flexural strength and toughness in comparison with the composite containing untreated fibers. This was possibly due to a reduction of a frictional bond when the fiber surfaces were cleaned by the plasma treatment. This finding was different from that on polyethylene fibers in which plasma treatment was found to enhance the bonding properties (Wu and Li, 1997). To maximize the economic benefits that accompany the use of a material destined for disposal in replacement of expensive steel bars, in most of the studies carried out on this subject the fibers used have been obtained with the material receiving no further treatment. In particular, a favorable solution is the use of the bottles commonly used commercially for mineral water. By cutting these bottles simply by hand it is possible to obtain fibers or strips of various shapes and sizes, exploiting also the surface roughness of such bottles, as previously said, to improve their adherence. In Pereora de Oliveira et al. (2011), Foti (2011, 2013b), and Fraternali et al. (2013) PET fibers to add to concrete have been obtained directly by cutting waste bottles; an increase of toughness of concrete was noticed, while the compressive strength did not change significantly. The workability of the concrete mix is still good if the fiber content is less than 1.5% in volume. The use of such fibers, directly derived from waste material, does not involve particular differences from the mechanical point of view compared to the fibers subjected to treatments of various types. PET fibers to add to concrete, in fact, could be used after a long and expensive extruding process from waste bottles, obtaining PET monofilaments that were cut in short fibers (Ochi et al., 2007). A series of tests
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were performed on these fibers to get the characteristics of mixibility, toughness, and adherence with the concrete mix, showing good results despite the high costs of manufacturing. In Kim et al. (2010), another kind of manufacturing is proposed, coating the surfaces of PET fibers with maleic anhydride grafted polypropylene. Also, an oxygen plasma treatment on the surface of PET fibers is also proposed in order to act as a microreinforcement of the cementitious mix. The result is a concrete mix with a stronger interfacial bonding due to the improvement in adhesion of the fibers’ surface (Trejbal et al., 2016). This improvement ends in a reduced crack opening with an increase in the durability of concrete too. Polypropylene and polyethylene fibers have also been added to concrete after a manufacturing process with the aim to get regular fibers of different length to evaluate the effects of their geometry and shape on the mechanical characteristics of the reinforced concrete (Silva et al., 2013). The influence of shape and dimension of PET fibers on the mechanical properties of concrete has been also investigated in Marthong and Sarma (2016) (Fig. 18.1). In any case an increase in the tensile strength is obtained. With regard to the chemical behavior, in a recent study (Fraternali et al., 2013) the alkali resistance of such PET fibers was evaluated, and the results obtained showed that this resistance only slightly differs from that of polypropylene fibers formed for extrusion from recycled PET. Later prismatic specimens of mortar (40 40 160 mm) were made. They were reinforced with fibers of 2 mm width and with a length that varies in the different specimens (11.3, 22.6, 35 mm). The analysis of the flexural behavior highlighted some interesting aspects. Firstly, the fiber-reinforced specimens were characterized by a reduction of the effort at the first crack with respect to the nonreinforced specimens (this effect reduced, however, becoming almost negligible for
(d)
1.20
1.50 °
45
°
1.13
3.36
Figure 18.1 Different shapes of PET fibers (Marthong and Sarma, 2016).
1.17
60
2.70 1.
0.70
1.13
8.00
1.00 1.30
50
1.50
1.20
0.
1.00
5.10
8.00
0.8
0
°
39
1.13
34
1.30
0.80 1.131.17
(c)
1.50
1.00
(b) 1.20
(a) 0.90
Recycled waste PET for sustainable fiber-reinforced concrete
3.5
UNR
Load (kN)
3
R-PET 3.50
393
R-PET 1.13
R-PET 2.26
2.5 2 1.5 1 0.5 0
0
1
2 Deflection (mm)
3
4
Figure 18.2 Load-deflection curves of tested mortars at 28 days (Fraternali et al., 2013).
fibers with greater length); but the most relevant result was the increase, in specimens with reinforcement in PET, of the ductility and, at the same time, the toughness, measured by the area under the load-deflection diagram (Fig. 18.2). Toughness is an important mechanical characteristic, as it is linked to the ability of a structure to absorb energy. In most researches on the reuse of the waste PET, fibers have been utilized as fiber reinforcement for concrete. Different kinds of fibers have been considered in these studies; different in shape, dimension, and slenderness (Fig. 18.3). The effect of their shape on concrete has been explored for short strips in Pereora de Oliveira et al. (2011) and in Silva et al. (2013); particularly in Silva et al. (2013), the fibers prevented the complete failure of the composite by holding the cement matrix together and transmitting the load previously supported with a slow crack process; instead during a bending test a sudden failure happened with a complete breaking of the specimen.
Figure 18.3 (a) Sample of short lamellar fibers (Foti, 2011); (b) Hand cutting of R-PET strips from postconsumer bottles. Top: exemplary of the examined bottles; center: macrostrips obtained through longitudinal cutting of the bottle; bottom final “R-PET 1.13” (left), “R-PET 2.26” (center), and “R-PET 3.50” (right) strips (Fraternali et al., 2011, 2013).
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This study demonstrated an improvement of mechanical strength, or anyway a nonsignificant change associated with a strong mechanical interlocking between the fibers/cement matrix interfaces promoted by the section area, roughness, and fibrillation ability of fibers and the crack and microcrack distribution system provided by the uniform distribution and orientation of fibers in the direction opposite to the load stresses (Fig. 18.4). In this way the transfer and spread of the load stresses along fibers prevent cracks diffusion and lead to more resistant and ductile composites. So, that volume fraction of fibers, fiber orientation, and fiber length are critical factors that affect the performance of composites and their ductility. Greater fiber length and volume fractions promote better crack resistance in composites but decrease the workability. In particular, in Pereora de Oliveira et al. (2011) the PET fibers were obtained by mechanical cutting of the lateral sides of waste bottles; the fibers so obtained had dimensions approximately of 2 mm in width, 0.5 mm of thickness, and 35 mm of length. The fibers were added to the concrete mix with a volume percentage equal to 0%, 0.5%, 1.0%, and 1.5%; the best performance of concrete was reached with the last percentage especially regarding the toughness. Instead in Silva et al. (2013), the fiber volume fraction was equal to 2.9% higher than the previous case and the fibers were equal to 12, 24, and 30 mm in length. A different shape of strips, the circular ones, generates an increase of toughness higher than the short lamellar ones and the advantage of an increasing ductility of the composite by a higher dosage of fibers (Foti, 2011, 2013b) (Fig. 18.5). The closed form of the fiber, in fact, produces a better bonding inside the concrete of the two sides of the crack, and it determines a higher adherence if compared to short lamellar strips. In Fraternali et al. (2013) the effects of the presence of longer filaments were compared for cement-based mortar and cement-lime mortars. In the case of cementbased mortars, slight decreases of the first-crack strength due to the R-PET reinforcement was observed; on the contrary, in cement-lime mortar it increased, but the flexural toughness indexes and the residual strength were more effective than the
Figure 18.4 Orientation of fibers in cement matrix: 1dfibers perpendicular to load direction (bridging effect); 2dfibers in the same direction as the applied load (Silva et al., 2013).
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Figure 18.5 Circular PET fibers (Foti, 2013b).
Figure 18.6 Cracking for bending at the centerline of the beam and detail of a roller support (Foti, 2013b).
cement-lime mortar. In (Foti, 2013b) longer strips composed of four overlapping layers of PET were considered. The results showed that the stresses are predominantly absorbed by concrete before the cracking; only in the postpeak phase the strips of PET absorbed the tensile stress, showing a ductile behavior (Fig. 18.6). In the same research 40-cm length specimens have been reinforced with half PET bottles having a ‘‘C’’ section directly cut, dividing by two parts along the length. It was noted that the specimen did not break completely thanks to the presence of PET half bottle and after a first vertical crack in the middle, the cracks propagated inclined of about 45 degrees, with a similar behavior of a beam subjected to bending test (Fig. 18.7) (Foti, 2013b). This kind of research, which utilizes long strips of PET from waste bottles was carried out also on structural element as plates (Foti and Paparella, 2014). The concrete
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Figure 18.7 Loading test on specimens with half-bottle fibers (Foti, 2013b).
Figure 18.8 Grid of PET bottle strips.
specimens were reinforced with a sort of grid of PET (Fig. 18.8). The test setup for the impact tests was inspired by those of Beckmann et al. (2012): the plate was positioned on a metallic frame and a load was dropped from a constant height. The results showed that the specimens reinforced with PET grids did not totally brake, as can been seen from the superficial cracks produced by the impact (Fig. 18.9). These grids, in fact, increased significantly the ductility of the plates. Another interesting aspect is the thermal conductivity of concrete reinforced with PET fibers. It was noticed that there is an increment in the thermal insulation of the concrete both adding pieces of different shapes of waste rubber of tires and/or plastic
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Figure 18.9 Upper side of reinforced plate n. 1 after the impact (Foti and Paparella, 2014).
fibers (strip, square, and irregular) (Fraternali et al., 2011; Yesilata et al., 2009). The insulation performance increases by the 18.52% with the addition of square rubber or by a range of 10.27%e18.16% with PET (Rahmani et al., 2013). The latter data coincide more or less with those shown in Fraternali et al. (2011) where the decrease of thermal conductivity (k) is equal to 18%. Possible models for materials with no tensile strengthdlike concretedreinforced with PET fibers have been proposed in Foti (2013a) or derived from Schmidt et al. (2009) and Fraternali et al. (2010). The behavior of PET-reinforced concrete qualitatively reflects what was in general observed for most of fiber-reinforced concretes and consists essentially on an improvement of the postpeak behavior and impact strength. The impact strength was investigated as an influence on concrete of polypropylene fibers (Bayasi and Zeng, 1997), where it was demonstrated that these fibers have a small favorable effect on compressive strength of concrete when 13-mm long fibers were used. In case of recycled plastics, Soroushian et al. (2003) got a higher impact strength of concrete; in particular, the milled mixed plastic particles and the melt-processed plastic fibers, when used at properly selected dosages, yielded important gains in strength of concrete to impact and restrain shrinkage cracking. There are some studies that compare the virgin and recycling materials. The effectiveness of the reinforcement with recycled fibers, in fact, is not equal to those from the virgin material, but calibrating the right dosage rate (generally higher for recycled materials) it is possible to obtain a similar performance for both types of material (Rebeiz, 1995). For this reason, fibers for concrete reinforcement generally need to be durable in the cementitious environment, to be easily dispersed in the concrete mix, to have good mechanical properties, and to be of appropriate geometric configuration in order to be effective.
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Figure 18.10 Specimen of a beam-column joint to be cast with a PET fiberereinforced concrete (Marthong and Marthong, 2016).
Finally, in Marthong and Marthong (2016) the use of concrete reinforced with PET fibers was applied to the reinforcement of a beam-column connection (Fig. 18.10). Then the specimens have been subjected to reverse cycling loading and compared with the behavior of specimens without any waste PET fibers.
18.3
Tests (summary) and results
Since the mid-90s a series of tests on concrete specimens reinforced with PET fibers were carried out at the Testing Laboratory of the Department of Civil Engineering and Architecture at the Polytechnic University of Bari. The numerous studies were focused on concrete elements different from each other; these specimens included fibers in PET with different shapes and sizes: 1. 2. 3. 4. 5. 6. 7. 8. 9.
Cubic specimens (100 mm 100 mm 100 mm) reinforced with circular or “O” fibers. Cubic specimens (100 mm 100 mm 100 mm) reinforced with short lamellar fibers. Prismatic specimens (100 mm 100 mm 400 mm) without reinforcements (1S, 2S). Prismatic specimens (100 mm 100 mm 400 mm) reinforced with short lamellar fibers (1FC, 2FC). Prismatic specimens (100 mm 100 mm 400 mm) reinforced with circular or “O” fibers (1C, 2C), for 0.50% and 0.75% by weight of concrete. Prismatic specimens (100 mm 100 mm 400 mm) reinforced with circular or “O” fibers for 1% by weight of concrete, with superplasticizers. Prismatic specimens (100 mm 100 mm 400 mm) reinforced with half-bottles of PET. Beam-specimens (100 mm 200 mm 1100 mm) with a bigger dimension and reinforced with PET strips Square-shaped slab specimens (800 mm 800 mm 58 mm) reinforced with a grid of PET strips.
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For comparison purposes the following tests were also carried out: 10. Beam-specimens (100 mm 200 mm 1100 mm) without reinforcements. 11. Square-shaped slab specimens (800 mm 800 mm 58 mm) without reinforcements.
The first comparison, drawn between ordinary concrete specimens and specimens 1 and 2, had the aim to checking the behavior of these kinds of fiber-reinforced concretes. The prescriptions followed for compression tests are those of the European code (UNI EN 12390:2009), so the loads were applied with a uniaxial testing on both sides of specimens through two plates (Fig. 18.11). In specimens with fibers a reduction of resistance occurred but probably these results are partly distorted by the difficulties encountered during the tests. Also tests on short lamellar fibers of PET were carried out to characterize them. In addition, tensile tests on fibers were performed furnishing an average value of the tensile strength equal to 150 N/mm2. In Fig. 18.12 the test set-up on a short lamellar fiber is shown. Then a comparison was done between specimens 4, 5, and 6 (Fig. 18.13), also with different dosages of PET Fibers (0.50% and 0.75% in weight). To reduce the costs and simplify the FRCs production, the fibers utilized were obtained from ordinary “plastic” bottles through cuts perpendicular to their longitudinal axes. Two types of fibers were utilized, the lamellar ones with a section of 2 mm 0.1 mm and a length of 32 mm and the circular fibers with a width variable around 5 mm and a diameter of 30e50 mm. The concrete mixture utilized was Composite Portland concrete of Type II/A-LL (SN EN 197-1:2000).
Figure 18.11 Typical compressive failure of a specimen reinforced with PET fibers (Marthong and Sarma, 2016).
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Figure 18.12 Direct tensile test on a PET fiber (Foti, 2011).
Figure 18.13 Specimens for the first series of preliminary tests (Foti, 2011).
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The prismatic specimens were subjected to bending tests; an Instron electromechanical testing equipment of 50 KN maximum loads performed tests using the Wave maker software Instron Ltd. To verify their behavior, bending tests involved a concentrated load at the centerline to read at each instant the values of load and displacement. The positive influence that the fibers have on the post-peak behavior on concrete elements was confirmed. Both lamellar and “O”-fibers greatly improve the ductility and toughness of the specimens. The increase in toughness is more evident for the “O”-fibers because their special shape helps to bind the concrete on each side of a cracked section, increasing the adherence. The crack pattern in case of bending tests is extremely interesting: a clear single vertical cracking concentrated in the middle, sewn by the fibers (see for e.g., Fig. 18.14 (Fraternali et al., 2011)). It is important to note the deviation of the crack, which did not show the typical V-shape of ordinary concrete failure. Table 18.2 shows the values of the tensile strength obtained during the bending tests. As a consequence of the better behavior of the specimens with circular fibers, it was decided to determine the best dosage of “O”-fibers by weight of the concrete, through the comparison of specimens of test 5 and test 6. An industrial readymade mix concrete was utilized for specimens of test 6 differently from specimens of test 5. For this reason, it was only possible to make comparisons regarding the recovery of the load (Fig. 18.15) and not the peak values.
Figure 18.14 (a) Sewing effect of PET fibers after cracking and (b) detail (Fraternali et al., 2011). Table 18.2 Strength values from the bending tests Specimen
1S
2S
1C (0.75%)
2C (0.75%)
1C (0.50%)
2C (0.50%)
1FC
2FC
fct (N/ mm2)
4.7
4.2
4.8
4.3
3.7
3.6
3.7
3.7
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Specimens reinforced with circular fibers 10
PET 1% - test 1 PET 1% - test 2 PET 0.75% - test 1 PET 0.75% - test 2 PET 0.5% - test 1 PET 0.5% - test 2
Applied load (kN)
8
6
4
2
0 0
2 4 6 8 10 12 14 Maximum deflection at the centerline (mm)
16
18
20
Figure 18.15 Load-deformation plots on specimens with circular fibers (1%, 0.75%, and 0.5%) (Foti, 2013b).
It can be noticed that the recovery of the load, in percentage respect to the peak, is faster for the mix with 1% of PET. The drop of the load is about 70% with respect to the cracking load for PET 0.50%, while 55% for PET 0.75%, and about 35% for PET 1%. Clearly in the case of PET 1% of the fibers are more present in the concrete, so it is easier for the loads to be transferred to the fibers that are able to sew the fracture and to deform in the plastic range. It can be concluded that it is important to increase the percentage of fibers to get an improvement in the concrete behavior but the percentage cannot be high because even after adding superplasticizers the concrete can become less workable. After having analyzed and tested the specimens with a uniform PET fiber reinforcement distributed in the concrete mix, the mechanical characteristics and the behavior of FRC with reinforcement concentrated in the position where high levels of tensile stresses are expected were checked. It does not pursue the prescriptions and the parameters of CNR-DT 204/2006 on the use of fibers. This is true for specimens of test 7, which use strips with C-shaped section obtained from half bottles with a base equal to about 8 cm and two side elevations of about 4 cm and assembled, after an appropriate overlap, with staples (Fig. 18.16). They were placed in the lower part of the specimens, subjected to traction during the bending tests. To verify the behavior of this kind of FRC, it is important to evaluate the adhesion between PET and concrete and the attitude of the strips of PET as a concentrate reinforcement. Therefore, on the specimens a standard test up to a maximum deflection of 20 mm was performed. The crack pattern is extremely interesting: the specimen did not break completely after a first vertical crack concentrated in the mid-section, while inclined shear cracks appeared (Fig. 18.17). Even if the detachment of the bottom area occurred, the concrete and the strips kept a good bond between them.
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Figure 18.16 Half bottle reinforcement (Foti, 2013b).
Figure 18.17 Loading tests on specimens with half-bottle fibers (Foti, 2013b). (a) Testing setup; (b) detail of the cracked section.
Fig. 18.18 shows the load-deflection plot; in particular, the crack in correspondence of the peak and a hardening trend with a subsequent recovery of the load and increment of the deformation are noticed. Given the encouraging results, it was decided to test the behavior of larger elements. So, similar analyses on the behavior between PET and concrete were performed on little beams with localized fiber reinforcement. The reinforcing strips in the specimens had dimensions 45 mm 0.2 mm 300 mm, and were superimposed, assembled, and placed along the entire length of the 1m-specimens, as shown in Fig. 18.19. The results of the bending tests showed that only after the cracking of the concrete, which occurred suddenly, the PET strips started to absorb the tensile stress through a ductile behavior and in particular a large deformation before the total failure of the specimen.
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Load-deflection. specimen n°2
Applied load (kN)
4
3
2
1
0 0
3
6 9 12 15 18 Maximum deflection at the centerline (mm)
21
Figure 18.18 Load-deflection plot for the second specimen (Foti, 2013b).
Figure 18.19 (a) PET strips utilized and (b) their position in the cracking section (Foti, 2013b).
Fig. 18.20 shows the load-deflection plots and highlights the ductile behavior in the postcracking phase of the beams reinforced with PET. Regrettably, the value of the recovery of resistance after cracking is low. This is probably due to the reduced resisting area of PET fibers only equal to 36 mm2. Another important positive result to highlight is that the failure occurred for all the fibers without any sliding from the concrete beam. To complete the research, it was decided to test the behavior of another concrete structural element, a slab. The reinforcement consisted in a sort of grid of long discrete 5 cm wide fiber strips with a 0.2 mm thickness and obtained from PET bottles simply cut along the longitudinal axis and positioned in place of steel (Fig. 18.21). Each strip is assembled to obtain a total length equal to 77 cm and left 1.5 cm from each side of the slab. Ten strips per side were assembled to create a grid that guaranteed the bidirectionality of the reinforcement; two grids were placed in each reinforced slab, not directly in contact
Recycled waste PET for sustainable fiber-reinforced concrete Beam specimens reinforced with PET sheets
12
Beam specimen without fibers Beam specimen 1 Beam specimen 2 Beam specimen 3 Beam specimen 4
10 Applied load (kN)
405
8 6 4 2
0 0
2 4 6 8 Maximum deflection at the centerline (mm)
10
12
Figure 18.20 Load-deflection plot for little beams reinforced with PET strips (Foti, 2013b).
Figure 18.21 Grid of PET bottle (Foti and Paparella, 2014).
but spaced by a layer of concrete to avoid empty spaces between them due to the ripples of the strips themselves. There are no codified prescriptions for impact tests on concrete, so, in this case the impact load tests (Fig. 18.22) consisted in dropping a steel cylinder, with a weight of 290 N, from a height of 1.0 m and to assess the kind of failure and the crack patterns on the slabs (Figs. 18.22e18.24). As Figs. 18.22e18.24 show, after the tests the non-reinforced plate was completely broken, while the reinforced plate was only subject to superficial cracks that did not lead to a complete failure. So, the presence of the two grids of PET reinforcement ensured the correct response to shocks and impact forces thanks to the more ductile behavior of the concrete plates, thus confirming the improvement of the impact
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Figure 18.22 (a) Impact instant for the nonreinforced plate n. 1; (b) Impact instant for the reinforced plate n. 1 (Foti and Paparella, 2014).
Figure 18.23 Upper side of nonreinforced plate n. 1 after the impact (Foti and Paparella, 2014).
strength and the ability to withstand very high bidirectional deformations without reaching the failure. Fig. 18.25 shows the average trend of deformation in the tested slabs. The nonreinforced slabs are characterized by attainment of peak in correspondence of the impact and a subsequent sudden fall in correspondence of the break; in the reinforced plates, instead, it is possible to notice a peak at the instant of impact quantitatively not much different from that of nonreinforced plates. After the impact, for the nonreinforced slabs a stable value of deformation occurred, even if the section had no more strength capacity; for the reinforced slabs, instead, after the peak there is a strong decrease of the deformation followed by an increase with a fluctuating trend around a value much higher than zero; this indicates that the slab continues to resist and maintains its structural integrity except for generated fractures which remain superficial. This behavior confirms what was expected due to the higher ductility of the reinforcement provided by the grids, meaning that the plate still has the possibility to transmit a deformation.
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Figure 18.24 Detail of the bottom side of the reinforced plate after the impact (Foti and Paparella, 2014). Deformation (%) 0,14
Non-reinforced slab 1
0,12
Reinforced slab 2
Reinforced slab 1
Non-reinforced slab 2
0,10 0,08 0,06 0,04 0,02
10,017
10,013
10,009
10,005
9,996
9,992
9,988
9,984
9,980
–0,02
10,000
Time (s)
0,00
Figure 18.25 Time-history of the average deformations for slabs.
In addition, the results showed a good adhesion between concrete and strips, cooperating to absorb the tensile stresses even after the onset of cracks due to bending and shear stresses. In fact, there were no sliding phenomena of any kind and the collapse affected all the strips at the same time. As expected, an interesting postpeak behavior was also found, with a lift of the load, albeit modest, due to the reduced resistant area of the PET strips.
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Use of Recycled Plastics in Eco-efficient Concrete
Conclusions
The results obtained confirm, in general, two of the major effects of the use of PET fibers: the greater ductility and the reduction of the workability for dosages of fibers greater than 1% by weight of concrete. This is especially true both for lamellar and “O”-shaped fibers that greatly improve the toughness of the specimens. The enhancement of the toughness is especially evident for the circular fibers; their special shape, in fact, helps to bind the concrete on each side of a cracked section. But the most interesting result concerns the reinforcements made with strips, which represent the most innovative aspect of this research. The tests confirmed the possibility of using strips of PET as reinforcement localized in areas that are expected to be the most stressed in traction. The strips can also be arranged in a grid for concrete plates, so as to give them a very ductile behavior and to avoid the complete failure, thus confirming the improvement of the impact strength. For all these reasons the use of PET strips can be proposed as concentrate reinforcement, in substitution of steel, for structures or secondary structural elements, even if more detailed studies and test campaigns are needed. Different possible uses for this reinforced concrete are proposed, such as, for example, in the production of industrial floors, docks, new jersey barriers, and wharfs in concrete, without forgetting some material properties that include the high efficiency and the resistance to attack by chemicals. Of course, some aspects should be improved in the production of grids and in their characteristics. Finally, it is important to emphasize that it is possible to combine all these benefits with the reuse and the recycle of plastic waste but also with the reduction of the production costs. Future studies will aim at improving the behavior and manufacturing of this reinforcement also improving the adherence of PET with concrete by mean, for example, of coating the surface of the fibers with plasma.
Acknowledgment Francesco Paparella is gratefully acknowledged for his help during the tests at the Laboratory of Testing and Materials “M. Salvati” of the Polytechnic University of Bari, Italy.
References Batayneh, M., Marie, I., Asi, I., 2007. Use of selected waste materials in concrete mixes. Waste Management 27 (12), 1870e1876. Bayasi, Z., Zeng, J., 1997. Properties of polypropylene fiber reinforced concrete. ACI Materials Journal 90 (6), 605e610. Beckmann, B., Hummeltenberg, A., Weber, T., Curbach, M., 2012. Concrete slabs under impact load: drop tower experiments. In: Sudies and Researches, Graduate School in Concrete Structures, Fratelli Pesenti, Politecnico di Milano, Italy, vol. 31, pp. 135e153.
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Chang, J.-E., Chang, Y.-K., Leu, M.-H., Chen, Y.-L., Huang, J.-H., 2013. Heavy metal removal by ambient-temperature argon plasma modified polyethylene terephthalate (PET) fibers with surface acrylic acid grafting. International Journal of Physical Sciences 8 (6), 244e253. Foti, D., Paparella, F., 2014. Impact behavior of structural elements in concrete reinforced with PET fibers. Mechanics Research Communications 57, 57e66. https://doi.org/10.1016/ j.mechrescom.2014.02.007. ISSN: 0093-6413. Foti, D., Paparella, V., Paparella, F., 2012. Sull’impiego del PET riciclato come materiale di rinforzo del calcestruzzo. L’Edilizia, Structural 174, 54e63. ISSN: 1593-3970. (In Italian). Foti, D., Paparella, V., Paparella, F., 2013. Calcestruzzi fibrorinforzati con fibre di PET. Studio della resistenza all’impatto. L’Edilizia, Structural 176, 1e14. ISSN: 1593-3970. (In Italian). Foti, D., 2011. Preliminary analysis of concrete reinforced with waste bottles PET fibers. Construction and Building Materials 25 (4), 1906e1915. https://doi.org/10.1016/ j.conbuildmat.2010.11.066. Foti, D., 2013a. On the numerical and experimental strengthening assessment of tufa masonry with FRP. Mechanics of Advanced Materials and Structures 20 (02), 163e175. Foti, D., 2013b. Use of recycled waste pet bottles fibers for the reinforcement of concrete. Composite Structures 96, 396e404. Fraternali, F., Negri, M., Ortiz, M., 2010. On the Convergence of 3D free discontinuity models in variational Fracture. International Journal of Fracture 166 (1e2), 3e11. Fraternali, F., Ciancia, V., Chechile, R., Rizzano, G., Feo, L., Incarnato, L., 2011. Experimental study of the thermo-mechanical properties of recycled PET fiber-reinforced concrete. Composite Structures 93, 2368e2374. Fraternali, F., Farina, I., Polzone, C., Pagliuca, E., Feo, L., 2013. On the use of R-PET strips for the reinforcement of cement mortars. Journal of Composites Part B: Engineering 46, 207e210. Gu, L., Ozbakkaloglu, T., 2016. Use of recycled plastics in concrete: a critical review. Waste Management 51, 19e42. Kim, S.B., Yi, N.H., Kim, H.Y., Kim, J.H.J., Song, Y.-C., 2010. Material and structural performance evaluation of recycled PET fiber reinforced concrete. Cement and Concrete Composites 32, 232e240. Lopes Pereira, E., Luis de Oliveira Jr., A., Gomes Fineza, A., 2017. Optimization of mechanical properties in concrete reinforced with fibers from solid urban wastes (PET bottles) for the production of ecological concrete. Construction and Building Materials 149, 837e848. Marthong, C., Marthong, S., 2016. An experimental study on the effect of PET fibers on the behavior of the exterior RC beam-column connection subjected to reversed cyclic loading. Structure 5, 175e185. Marthong, C., Sarma, D.K., 2016. Influence of PET fiber geometry on the mechanical properties of concrete: an experimental investigation. European Journal of Environmental and Civil Engineering 20 (7), 771e784. Ochi, T., Okubo, S., Fukui, K., 2007. Development of recycled PET fiber and its application as concrete-reinforcing fiber. Cement and Concrete Composites 29, 448e455. Pereora de Oliveira, L.P., Castro-Gomez, J.P., 2011. Physical and Mechanical Behaviour of recycled PET fibre reinforced mortar. Construction and Building Materials 25, 1712e1717. Rahmani, E., Dehestani, M., Beygi, M.H.A., Allahyari, H., Nikbin, I.M., 2013. On the mechanical properties of concrete containing waste PET particles. Construction and Building Materials 47, 1302e1308.
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Rebeiz, K.S., 1995. Timeetemperature properties of polymer concrete using recycled PET. Cement and Concrete Composites 17, 119e124. Schmidt, B., Fraternali, F., Ortiz, M., 2009. Eigen fracture: an eigen deformation approach to variational fracture. Multiscale Model Simulation 7 (3), 1237e1266. Siddique, R., Khatib, J., Kaur, I., 2008. Use of recycled plastic in concrete: a review. Waste Management 28, 1835e1852. Silva, D.A., Betioli, A.M., Gleize, P.J.P., Roman, H.R., Gomez, L.A., Ribeiro, J.L.D., 2004. Degradation of recycled PET fibers in Portland cement-based materials. Cement and Concrete Research 35, 1741e1746. Silva, E.R., Coelho, J.F.J., Bordado, J.C., 2013. Strength improvement of mortar composites reinforced with newly hybrid-blended fibers: influence of fibers geometry and morphology. Construction and Building Materials 40, 473e480. Soroushian, P., Plasencia, J., Ravanbakhsh, S., 2003. Assessment of reinforcing effects of recycled plastic and paper in concrete. ACI Materials Journal 100 (3), 203e207. Trejbal, J., Kopeckỳ, L., Tesarek, P., Fladr, J., Antos, J., Somr, M., Nezerka, V., 2016. Impact of surface plasma treatment on the performance of PET fiber reinforcement in cementitious composites. Cement and Concrete Research 89, 276e287. Wu, H.C., Li, V.C., 1997. Basic interfacial characteristic of polyethylene fiber/cement composites and its modification by plasma. In: Brandt, A.M., Li, V.C., Marshall, I.H. (Eds.), Brittle Matrix Composites 5. Woodhead Publishing, Cambridge, UK, pp. 14e23. Yesilata, B., Isiker, Y., Turgut, P., 2009. Thermal insulation enhacement in concretes by adding waste PET and rubber pieces. Construction and Building Materials 23, 1878e1882.
Further reading Donatone, G., Foti, D., Paparella, F., 2010. Soluzioni innovative nel campo dei calcestruzzi fibrorinforzati: analisi preliminare su un calcestruzzo rinforzato con fibre di PET. In Concreto. 95, lug/ago 36e44 (In Italian). Plastics Europe, E., 2013. Plastics-the Facts 2013. An Analysis of European Latest Plastics Production, Demand and Waste Data.