By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jul 15 2024A recent article published in Materials demonstrated the use of plastic waste in concrete as a solution to the environmental concerns related to plastic waste disposal. Results of different experimental investigations were presented to analyze the plastic’s impact on concrete’s mechanical properties.
Polypropylene fibers, FPP. Image Credit: https://www.mdpi.com/1996-1944/17/14/3408
Background
Plastic waste management has recently gained significant attention due to the visible consequences of plastic pollution. Plastic is widely used across various sectors for its versatility and durability. However, its non-biodegradability leads to extensive landfill usage, threatening marine ecosystems and the food chain.
Numerous alternative approaches have been investigated to address these environmental concerns and manage plastic waste. Among these, the construction sector has emerged as a promising option. Incorporating plastic waste materials into concrete offers economic benefits and provides a sustainable alternative to conventional disposal methods.
Generally, integrating plastic waste into concrete can degrade certain mechanical properties such as compressive and flexural strength, steel-concrete bond, and workability. Despite a lower compressive strength, concrete with plastic waste aggregates is advantageous for constructing nonstructural or lightweight elements. Thus, this study examined 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 distinct ordinary concrete mixes having low (C1) and high (C2) strength. The true volume of plastics in one cubic meter of concrete was ensured to be lower than that used for the mixes themselves.
While P1 was derived from shredded and washed recycled plastics, P2 was obtained by shredding different types of plastic bottles. Alternatively, P3 was produced through grinding, washing, and fused extraction of polyethylene (PE) and polypropylene (PP) plastics.
For comparison, reference previous concrete mixes comprising cement, sand, coarse aggregate, and plastic waste fibers (PC) were considered. In addition, reference concrete with silica fume (PC-SF) and fly ash (PC-FA) as partial cement substitution were studied. PC, PC-SF, and PC-FA were produced using Portland cement and polypropylene fibers (FPP) obtained by crushing, cutting, and washing various packages.
Monotonic compressive tests were carried out on cubic specimens as per standards. Concrete mixes C1 and C2, with and without plastic aggregates, were investigated after 7, 14, and 28 days of curing under controlled environmental conditions of 20 ± 2 °C and 95% relative humidity.
Additionally, tensile strength from a split test and elastic modulus from compression were obtained for all specimens. For comparison, PC, PC-SF, and PC-FA were subjected to compressive tests after 7, 28, and 90 days of curing.
Results and Discussion
The experimental investigation revealed the primary dependence of concrete’s strength on its mass density according to the true mass density of plastic waste. The plastic waste with a higher mass density caused a lower reduction in the mass density of the mix. Consequently, the C1-P2 mix with the lowest mass density exhibited maximum strength variation.
While the low-mass-density concrete (<2100 kg/m3) could accommodate up to 20% (of true volume) plastic aggregate to maintain 40% of the strength of the reference mix, high-mass-density concrete (>2400 kg/m3) could accommodate up to 30% plastic aggregate.
Overall, the concrete compressive strength decreased with increasing percentage of plastic aggregates. This decrease was minor (<20%) for up to 10% plastic aggregates. Moreover, the mixes with a lower mass density exhibited a greater variation in compressive strength. Alternatively, no clear dependence of concrete strength on the size of the plastic aggregates was evident.
The comparisons with some experimental results from the literature revealed inconsistent variation of normalized compressive strength with mass density variation. Moreover, mixes with higher mass densities might experience higher strength variation for the same proportion of added plastic aggregates.
While the inclusion of plastic fibers in PCs exhibited less pronounced variations in compressive strength due to reduced microcrack propagation, higher proportions of the fibers could lead to segregation problems during mixing. Adding FPP can also enhance concrete strength depending on parameters such as the type and length of fibers.
Conclusion
Overall, the researchers comprehensively examined the compressive strength of concrete cubes with and without incorporating plastic waste aggregates. The influence of different concrete formulations, plastic types, and percentages was investigated experimentally and compared with results in the literature.
P2 added to the C1 mix exhibited maximum impact on concrete strength (20% plastic inclusion resulted in 51% strength reduction). Alternatively, the lowest and relatively contained impact was observed for P1 added to the C2 mix.
The researchers suggest further experimental investigations to remove the existing consistencies in the data reported in the literature. Incorporating plastic waste will help determine the mechanisms involved in concrete strength variation and improve the feasibility of using recycled plastic in construction applications.
Journal Reference
Oddo, M. C., Cavaleri, L., La Mendola, L., & Bilal, H. (2024). Integrating 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
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