Omar Khalid
Plastic is a highly profitable material that has become ubiquitous in our daily lives. However, the harsh reality is that the vast majority of plastic waste ends up in landfills, with a Greenpeace report finding that almost none of the plastic put into recycling bins is actually recycled. This is due to the sheer variety of plastics, the high costs of collection and sorting, and the fact that plastic is not designed to be recycled. As a result, the increase in recycled plastics is negligible compared to the surge in plastic production, with the industry planning to triple output by 2050. This crisis demands drastic action, including reducing plastic production and developing more recyclable products and sustainable alternatives. In the context of this discussion on the challenges of recycling plastics, it is essential to understand the concept of plastic shrinkage, which refers to the volume contraction that occurs when heated plastic solidifies and cools. This phenomenon significantly impacts the final size and quality of plastic products, and proper accounting for shrinkage is crucial to ensure the desired standards and functionality of the end product.
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What You'll Learn
- The use of recycled plastics in concrete can reduce shrinkage and prevent cracking
- Recycled sand mortar increases plastic shrinkage due to higher moisture absorption
- Plastic shrinkage is a major cause of shear strain concentration at the interlayers of 3D printed sections
- The prevention of plastic shrinkage cracking can be achieved by covering the surface of the concrete
- Plastic is not designed to be recycled and most plastic items collected for recycling are not actually recycled
The use of recycled plastics in concrete can reduce shrinkage and prevent cracking
Plastic shrinkage is a well-known issue in the construction industry, causing unsightly cracks in concrete slabs. This phenomenon occurs due to water loss through evaporation or suction during the curing process, resulting in volume reduction and surface cracks. However, the use of recycled plastics in concrete offers a promising solution to mitigate this problem.
Recycled aggregate, when used in its saturated surface dry (SSD) state, acts as an internal curing agent due to its high water content. This additional water content compensates for any water loss during evaporation, thereby reducing the overall shrinkage of the concrete. Consequently, the concrete becomes more resistant to cracking.
The effectiveness of this approach is evident in a study by Etxeberria et al. (2016), who found that concrete made with recycled aggregate exhibited less plastic shrinkage compared to concrete made with natural aggregate. The higher water content in the recycled aggregate played a crucial role in reducing shrinkage and improving the overall performance of the concrete.
Furthermore, the use of fibre reinforcement, a technique often employed with recycled plastics, significantly increases the tensile strain capacity of concrete in its plastic state. This reinforcement mitigates the risk of cracking and enhances the concrete's ability to withstand tensile stresses.
By incorporating recycled plastics, the construction industry can not only reduce plastic waste but also enhance the durability and longevity of concrete structures. This innovative approach to concrete production showcases the potential for sustainable practices to address long-standing issues in construction while promoting the reuse of materials.
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Recycled sand mortar increases plastic shrinkage due to higher moisture absorption
Plastic shrinkage is caused by the loss of water through evaporation or suction during the plastic state of concrete, leading to volume reduction and surface cracks. This phenomenon is particularly significant in 3D-printed recycled sand mortar, where an increase in recycled sand content corresponds to heightened plastic shrinkage due to higher moisture absorption.
Several studies have investigated the impact of recycled sand on the plastic shrinkage of 3D-printed mortar. One study found that recycled fine aggregates, such as sand, contribute to higher shrinkage deformation. This effect becomes more pronounced when both recycled coarse and fine aggregates are used, resulting in increased maximum plastic crack width.
The use of recycled sand in 3D printing alters the capillary water evaporation and restraint conditions during the plastic stage, influencing the plastic shrinkage and cracking properties of the concrete. The research revealed that recycled sand mortar exhibited higher plastic shrinkage values than natural mortar, which can be attributed to its higher moisture absorption capacity.
Additionally, the study by Hou et al. indicated that 3D-printed concrete and mortar with recycled sand tend to crack earlier than their conventional counterparts. This finding underscores the critical role of understanding the plastic shrinkage properties of recycled sand mortar in construction applications.
While recycled sand has proven suitable for 3D-printed mortar in terms of workability and mechanical properties, further investigation is required to fully comprehend the plastic stage shrinkage characteristics. The challenge of early cracking in 3D-printed recycled sand mortar can be mitigated by using building rigid components to restrain shrinkage, covering the concrete surface early, and protecting it from drying winds.
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Plastic shrinkage is a major cause of shear strain concentration at the interlayers of 3D printed sections
Plastic shrinkage is a major concern in 3D printing, as it can lead to shear strain concentration at the interlayers of printed sections. This phenomenon occurs due to moisture loss and the contraction of concrete before it sets, resulting in volume reduction and surface cracks. The impact of plastic shrinkage is more pronounced when using recycled plastics due to their higher absorption of moisture, leading to increased slippage and reduced interlocking at the interlayer region.
The issue of plastic shrinkage is not limited to recycled plastics but is a well-known challenge in concrete printing. As the number of layers increases, both horizontal and vertical shear strain values follow suit, with the length of the layers also playing a significant role. This results in a higher likelihood of cracks, which can compromise the quality of the printed structures and lead to repair costs.
To mitigate the effects of plastic shrinkage, various methods have been proposed. One suggestion is to use building rigid components to physically restrain the printed sections, which has been shown to reduce peak shrinkage values. Additionally, the introduction of fibre reinforcement can increase the tensile strain capacity of concrete, making it more resistant to cracking.
The unique interlayer plane in 3D printing also influences the transfer of shrinkage deformation. This has been observed through experimental methods that evaluate the efficacy of PSC prevention measures and the behaviour of early-age PSC in 3D-printed concrete. By studying the free shrinkage of specimens and introducing controlled sources of restraint, researchers can better understand the mechanisms behind PSC and develop effective mitigation strategies.
Furthermore, the print variables of pass time and print speed have been found to influence the microstructure of 3D-printed concrete. The presence of tri-axial spheroid-shaped air voids, elongated in the print direction, has been observed in printed specimens, highlighting the need for further research into the effects of stress concentrations at micro-cracks or voids. Managing and forecasting negative capillary pressure in freshly deposited concrete filaments are crucial steps in mitigating the adverse effects of plastic shrinkage in 3D printing applications.
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The prevention of plastic shrinkage cracking can be achieved by covering the surface of the concrete
Plastic shrinkage cracking is a common issue in concrete slabs, emerging as parallel cracks that are usually shallow in depth. This phenomenon occurs when the surface moisture of recently placed concrete evaporates faster than it can be replaced by rising bleed water, causing the surface to shrink more than the interior. While these cracks seldom affect the durability and strength of floors or pavement, they are unsightly and can be prevented. The prevention of plastic shrinkage cracking can be achieved by covering the surface of the concrete to reduce evaporation and control moisture loss.
Covering the concrete surface with a polythene sheet is the most effective solution to prevent cracking. On concrete roads and other surfaces where the finished texture is essential, the polythene sheet must be suspended clear of the surface. In addition to using a covering, it is crucial to dampen the subgrade and the concrete forms. This can be done by moistening the formwork and reinforcement to prevent the concrete from drying out too quickly. It is important to note that the subgrade should be dampened without resulting in freestanding water.
Another method to prevent plastic shrinkage cracking is to use a synthetic fibre reinforcement in the concrete design mixture. This reinforcement helps fragile concrete resist tension between the surface layers, reducing the likelihood of cracking. Additionally, using recycled aggregate in its saturated surface dry (SSD) state can be beneficial. The high water quantity of recycled aggregate acts as an internal curing agent, reducing plastic shrinkage compared to non-recycled aggregate concrete.
Furthermore, exposing plastic flakes to small doses of gamma radiation and then mixing the irradiated plastic with cement paste and fly ash can result in stronger concrete. This technique, developed by MIT students, not only strengthens the concrete but also reduces its environmental impact by reusing plastic that would otherwise end up in landfills. By addressing the key causes of plastic shrinkage cracking and implementing these prevention methods, the development of unsightly cracks in concrete surfaces can be minimised.
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Plastic is not designed to be recycled and most plastic items collected for recycling are not actually recycled
Plastic is a highly profitable and convenient material, but it is not inherently designed to be recycled. In fact, most plastic items that are collected for recycling are not actually recycled.
A 2022 Greenpeace report revealed that the vast majority of plastic put into recycling bins is sent to landfills or ends up as waste in other ways. This is because, despite the many types of plastic, none of them can be melted down together. The process of recycling plastic is energy-intensive and costly, and it becomes more expensive as additional steps such as selection and washing are added. The low recycling rates of plastic can be attributed to both technical and economic reasons. For instance, thin plastic bags can clog recycling machinery if not separated properly, and kerbside collections often do not accept this type of mixed plastic. Furthermore, plastic with food residues is usually deemed unfit for recycling, and only good quality, clean plastics can be processed.
The issue is compounded by the fact that plastic is cheap to produce, making recycled plastic significantly more expensive in comparison. As a result, companies often opt for producing new plastic instead of using recycled materials. The plastic industry has also been accused of greenwashing, promoting recycling as a solution to environmental concerns while continuing to increase plastic production.
The scale of the problem is immense, with humans having created around 11 billion metric tons of plastic to date. This amount surpasses the biomass of all terrestrial and marine animals combined, according to a 2020 study. To address plastic pollution, experts advocate for drastic cuts in plastic production, particularly single-use plastics, and the development of better, more recyclable products. While recycling is an important part of the solution, it cannot keep up with the sheer volume and variety of plastic waste being generated.
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Frequently asked questions
Plastic shrinkage is the reduction in volume of plastic as it cools down from a heated, mouldable state.
Plastic shrinkage affects the final size and quality of the product. If shrinkage is not accounted for, the product might end up being smaller than expected, with parts that don't fit together or function well.
Plastic shrinkage can cause shear strain concentration at the interlayers of 3D printed sections. This can lead to slippage and reduced interlocking at the interlayer region, impacting the overall permeability of the printed object.
The use of recycled sand mortar in 3D printing has been found to increase plastic shrinkage values due to the higher absorption of moisture. However, in concrete, the use of recycled aggregate in its saturated surface dry (SSD) state can reduce plastic shrinkage due to its ability to act as a water supply, preventing water deficit.
Despite public efforts to recycle plastic, reports by Greenpeace and other organizations suggest that most plastic ends up in landfills rather than being recycled. The plastic production industry has been criticized for greenwashing and perpetuating misinformation, with a focus on increasing production rather than addressing the environmental impact of plastic waste.
