WASTE PLASTICS AS ARTIFICIAL AGGREGATES IN CONCRETE: IS IT POSSIBLE?
During the last decades plastic has been object of a sort of bias: on the one hand, it has undoubtedly simplified our life. Think about how heavy our bags would be if we could drink water only from glass bottles! And what about our picnics without light HDPE food containers or plastic cutlery: plastic now is everywhere, in furnitures, medical devices, electronic equipments. On the other hand, plastic has a very long biodegradation period due to its high thermal, mechanical and chemical resistance, so its disposal has harmful effects on the environment. As example of this, PET bottles can contaminate natural water streams, killing aquatic animals and can clog up urban drainage systems if they are not properly addressed. Similarly, incineration of plastic waste produces secondary pollutants such as toxic gases, dioxins and carbon monoxide: hence, neither landfilling nor incineration of waste plastic are attractive options. Now, the question is: could the incorporation of plastic waste as aggregate in concrete solve a part of the problem associated with plastic waste disposal? Yes, it can!
Could partial replacement of aggregates by plastic waste be helpful in solving the shortages of aggregates on construction site? Yes, it can! Reportedly, the most treated material by researchers has been polyethylene terephthalate (PET), but also polyvinylchloride (PVC), high density polyethylene (HDPE), plastic derived from electronic devices have been considered by the scientific community as possible artificial aggregate in concretes and mortars’ formulations. So, let’s have a look in scientific literature about this topic…
Independently from the polymer type plastic, the first operation needed is a cleaning process from metals, wood, ceramics, glass, papers, labels, and other contaminants that could compromise mechanical performances. Secondly, a mechanical grinding process is required to reduce dimensions and to give plastic aggregates a shape suitable for their use in concrete mixes (either powder, granules, pellets or flakes). Think about post-consumer PET bottles, coming from urban and industrial collection sites, that are compressed in bales: the bales of PET-waste mostly consist of dirty PET-bottles, usually contaminated with other materials and with some non-PET containers that should be removed. A particular pre-treatment of PET wastes is reported by N. Saikia and J. de Brito  who, to obtain a flaky waste PET aggregate and a pellet-shaped product, introduced a cleaning and a separation phase of waste plastic by physico-chemical methods after the mechanical grinding of PET wastes. The pellet was produced from plastic flakes, at first by extrusion after a heating and melting process in vacuum, then by granulation in a rotary cutter in water.
The mixture of water and grains of polymer was subsequently separated by vibration and then centrifuged to remove excess water. A.M. Mustafa Al Bakri et al.  used a HDPE plastic waste aggregate modified by heat treatment above its melting point, and after that cooled at room temperature: the heating process induced changes in the physical characteristics of the plastic wastes and in its microstructure.
It can be inferred that the main reason behind all the changes in the properties of concrete is the weak bonding between plastic aggregates and cement paste. Some researchers have suggested improving this bond by surface modification of the plastic aggregate using chemical treatments. A successful study performed by T.R. Naik et al.  demonstrated that treating plastic with an oxidizing agent could strengthen the bond between plastic and cement, thus creating reactive chemical species (polar and hydrophilic) on the plastic surface that could take part in cementitious reaction to produce concrete with higher strength: this study investigated plastic treatment using bleach (hypochlorite) and bleach with sodium hydroxide (alkaline bleach). Z. H. Lee et al.  investigated the effects of treating PET wastes using both hydrogen peroxide and calcium hypochlorite before incorporating them in concrete as coarse aggregates. Treating the plastic with calcium hypochlorite may modify the initially smooth surface to be more angular and rougher, resulting in more contact and higher friction between the particles, therefore causing a lower slump and a loss in workability.
The incorporation of plastic wastes affects the quantity of free water available in the concrete matrix as well as its workability. The workability of concrete could rise or decrease as the amount of fine recycled waste plastic aggregate increases depending on the physical properties (especially particle shape, size, roughness, surface texture), chemical properties of plastic materials, incorporation ratios, water-cement ratio and amount of cement paste. It means that concrete mixtures with bigger size, angular, sharped edged, and non-uniform aggregate particles show a lower slump value compared to the mixtures containing small size, smooth, round shape, and uniform aggregate particles. On the one hand, due to the increased surface area and irregular shape, workability of concrete mixtures that contain waste plastic aggregates has been decreased as the amount of plastic wastes rises (Z.Z. Ismail et al. , C. Albano et al. , K. S. Kumar et al. ). On the other hand, some researchers reported an enhancement in concrete workability due to a smoother outer surface of plastic particles compared to natural aggregates, which cannot absorb the water. Therefore, more free water is available to decrease the inner friction between the cement paste and plastic aggregate. That’s why Y-W.Choi et al. experimented a double slump value with 75% of plastic aggregates compared to that of the normal concrete (at w/c of 0.53). A small amount of added plastic do not affect the mixture’s workability: M. Frigione , incorporating 5% of PET aggregates (derived from waste un-washed PET bottles) instead of fine aggregate, experimented neither a remarkable workability loss of concrete mixtures nor segregation in any mixtures. Also expanding this replacement percentage up to 50%, the reduction in slump could be considered insignificant (M. Frigione et al. ). Similar improvements in concrete workability were experimented by A.M.K. Najjar et al. , using PVC derived plastic aggregate in concrete mixtures, reaching a maximum at 25% replacement level.
Fresh and dry densities
As waste plastic materials have lower bulk density (HDPE plastic: 930–970 kg/m3, PET: 1270 kg/m3) than natural aggregates (2450-2570 kg/m3), increase in the content of plastic aggregate reduces both fresh and dry density of concrete: the reduction is greater with bigger and flakier particles of plastic aggregate.
Most authors reported a gradual reduction in compressive, flexural and splitting tensile strength with increasing waste plastic content (both fine and coarse). The chemical composition of plastic wastes differs largely from that of natural aggregates, since plastic is composed of organic compounds that have much lower polarity and therefore cannot generate hydrogen bonds with cement. Among the different mechanisms influencing the decrease in compressive strength, the most prominent are:
Decrease in adhesive strength between the surface of the waste plastic and the cement paste, as well the particles size of the waste plastic increase. Additionally, waste plastic is a hydrophobic material which may restrict the water requirement for the hydration of cement (Z.Z. Ismail et al. ).
Low strength of the interfacial transition zone between the waste plastic aggregate and cement paste, so PET aggregates cannot interact with cement paste (N. Saikia et al. ). This reduction is more pronounced in concrete containing fine and coarse flaky PET aggregate rather than pelletized PET aggregate, directly proportional to the loss of workability (the increased water to cement ratio necessary to maintain slump leads to a higher water absorption and higher porosity of the concrete mix).
Higher content of bleeding water (located mostly around the aggregate plastic particles) in concrete mixes containing waste PET aggregates, that weakens the bond between them and the cement matrix (M. Frigione ).
Some studies, however, show that the loss in compressive strength can be minimized at small plastic size and low percentage replacement. S. Osubor et al.  obtained, at 5% and 3 mm flaky plastic, a compressive strength not significantly different from the compressive strength of the concrete without PET. M. Frigione  obtainedirrelevant differences in compressive and splitting tensile strengths at 28 days and 1 year between reference concretes and mixes containing 5% of PET aggregates at low w/c ratio (0.45). B. Balasubramanian et al. , replacing coarse aggregate by E-Waste obtained from crushed Printed Circuit Boards at various sizes, observed a compressive strength of concrete 27% higher than reference conventional concrete at 15% of E-Waste used as coarse aggregate.In printed circuit boards the chemical composition is usually silica 63.55% and copper 36.44%, elements helpful to increase the strength.
Tamrin and Nurdiana J.  tested lamellar HDPE plastic samples with different sizes at 5% replacement level to produce 10 MPa concrete suitable for non-structural walls, base concrete in the rigid pavement on highways, paving blocks for parking lots with low loads, wall panels and concrete footpaths. For the use of precast concrete walls especially, concrete mixtures containing HDPE could reduce the building’s structural load and energy consumption within the building by lowering the inside temperature. Together with fillers (e.g., sand, quarry fine), this type of concrete mix could help prevent heat transfer within a structure. After being treated with chemicals, oxidation of plastic surface (particularly with hypochlorite) allows a stronger bond to be formed between plastic and cement paste, due to a reduced hydrophobicity and the improved polarity of the plastic surface after treatment: among the surface modification methods, Ca(ClO)2 treatment is the most effective in increasing the strength of concrete as compared to the normal concrete strength (Z.H. Lee et al. ).
The ductility of concrete is significantly increased with the addition of plastic aggregate, up to
50%. However, the ductility of plastic concrete is related to the type of plastics used. Reportedly, it is suggested that if a small fraction of the waste plastics had a shape similar to short fibers, it is able to bridge the crack to a certain extent, providing the material with some post-peak toughening. Furthermore, the fracture energy reduces with the increase in plastic aggregate content. M. Frigione et al.  noted how PET granules’ (of sub-rounded shape and size less than 4.75 mm) flexibility influences failure modes of concrete specimens: they appear to retain their shape even after reaching peak load, so they have lower chance to collapse while the control sample with no PET granules is brittle. In fact, examining the failure pattern of the concrete beams tested for flexure, it is evident that specimens containing PET granules don’t split in half after reaching ultimate strength as conventional concrete, but appear with only minor cracks retaining their shape. PET granules exert a bridging effect, with a consequent significant post-crack performance of concrete. Therefore, their prospective usage where the concrete is subjected to the dynamic and recurring load, such as pavements, may be considered.The specimens containing plastic aggregates could be able to carry load for a few minutes after failure without full disintegration (K. Hannawi et al. ).
The use of recycled plastic aggregates in civil engineering applications, such as pavement and infrastructure, can be an alternative to disposing of them in landfill sites, or to incineration. Recycled plastic aggregates can also be used for producing concrete bricks (for general applications), blocks, façade elements, non-structural concrete panels, and temporary shelters. For structural concrete applications, in structures with lower imposed loads and where the durability is less important, a certain amount of plastic aggregates may be used. Through the literature search, it is postulated that up to 20% of plastic aggregates in concrete may be acceptable without compromising its engineering properties. Altering the shape, size, and surface of plastic aggregates can also play an important role in improving the concrete properties.
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