A recent article published in Materials The use of plastic waste in concrete has been demonstrated as a solution to the environmental problems related to the disposal of plastic waste. The results of different experimental studies have been presented to analyze the impact of plastic on the mechanical properties of concrete.
Background
Plastic waste management has recently gained significant attention due to the visible consequences of plastic pollution. Plastic is widely used in various sectors due to its versatility and durability. However, its non-biodegradability leads to intensive use of landfills, threatening marine ecosystems and the food chain.
Many alternative approaches have been explored to address these environmental concerns and manage plastic waste. Among these, the construction sector has emerged as a promising option. Incorporating plastic waste into concrete offers economic benefits and is a sustainable alternative to conventional disposal methods.
In general, the integration of plastic waste into concrete can degrade some mechanical properties such as compressive and flexural strength, steel-concrete bond, and workability. Despite lower compressive strength, concrete containing plastic waste aggregates is advantageous for the construction of non-structural or lightweight elements. Thus, this study investigated the properties of concrete comprising plastic waste aggregates of different types, sizes, and proportions.
Methods
Three types of plastic waste aggregates were considered in this study: light blue plastic flakes (P1), plastic flakes (P2) and plastic granules (P3). These were incorporated into two separate ordinary concrete mixes with low (C1) and high (C2) strength. The actual volume of plastic in a cubic meter of concrete was ensured to be less than that used for the mixes themselves.
While P1 is made from shredded and washed recycled plastics, P2 was obtained by shredding different types of plastic bottles. P3 was produced by crushing, washing and melt extraction of polyethylene (PE) and polypropylene (PP) plastics.
For comparison, previous reference concrete mixtures comprising cement, sand, coarse aggregates and plastic waste fibres (PC) were considered. In addition, a reference concrete with silica fume (PC-SF) and fly ash (PC-FA) as partial replacement of cement was studied. PC, PC-SF and PC-FA were produced from Portland cement and polypropylene fibres (PPF) obtained by grinding, cutting and washing of various packages.
Monotonic compression tests were performed on cubic specimens in accordance with the standards. Concrete mixtures C1 and C2, with and without plastic aggregates, were studied after 7, 14 and 28 days of curing under controlled environmental conditions of 20 ± 2 °C and 95% relative humidity.
In addition, the tensile strength obtained by a splitting test and the elastic modulus obtained by compression were obtained for all samples. For comparison, PC, PC-SF and PC-FA were subjected to compression tests after 7, 28 and 90 days of curing.
Results and discussion
The experimental study revealed that the strength of concrete mainly depends on its density based on the actual density of plastic waste. Plastic waste with higher density resulted in lower reduction in the density of the mix. Therefore, the C1-P2 mix with the lowest density showed maximum strength variation.
While low density concrete (< 2100 kg/m3) could accommodate up to 20% (of the actual volume) of plastic aggregates to maintain 40% of the strength of the reference mix, high mass density concrete (> 2400 kg/m3) could accommodate up to 30% of plastic aggregates.
Overall, the compressive strength of concrete decreased with increasing percentage of plastic aggregates. This decrease was minor (<20%) for mixes containing up to 10% plastic aggregates. In addition, mixes with lower bulk density showed a greater variation in compressive strength. Furthermore, no clear dependence of concrete strength on plastic aggregate size was evident.
Comparisons with some experimental results from the literature revealed an inconsistent variation of the normalized compressive strength as a function of the variation of the density. Moreover, mixtures with higher densities could experience a higher variation in strength for the same proportion of added plastic aggregates.
Although the inclusion of plastic fibers in PCs showed less pronounced variations in compressive strength due to reduced microcrack propagation, higher proportions of fibers could lead to segregation problems during mixing. The addition of FPP can also improve the strength of concrete depending on parameters such as fiber type and length.
Conclusion
Overall, the researchers extensively investigated the compressive strength of concrete cubes with and without the incorporation of plastic waste aggregates. The influence of different concrete formulations, different types of plastic and different percentages was experimentally investigated and compared with literature results.
The addition of P2 to mix C1 had a maximum impact on concrete strength (a 20% plastic inclusion resulted in a 51% strength reduction). On the other hand, the lowest and relatively limited impact was observed for the addition of P1 to mix C2.
The researchers suggest further experimental research to eliminate existing inconsistencies in the data reported in the literature. The integration of plastic waste will help determine the mechanisms involved in the variation of concrete strength and improve the feasibility of using recycled plastic in construction applications.
Journal reference
Oddo, MC, Cavaleri, L., La Mendola, L., & Bilal, H. (2024). Integration of plastic waste into concrete: sustainable solutions for the environment. Materials, 17(14), 3408. DOI: 10.3390/ma17143408, https://www.mdpi.com/1996-1944/17/14/3408