Using Upcycled Plastic Waste to Build Low-Maintenance Rural Roads

The design and build of all-weather rural roads carry substantial cost implications. Premium construction materials can be expensive and may not be available locally. Allowance should be made as well for post-disaster remedial works especially for disaster-prone areas.

Road construction is one of the most conservative sectors, and technology can sometimes be outdated. Upscaling is warranted to improve the quality of finished roads and maintain their serviceability.

Moreover, rural road projects usually adopt generic specifications that have been historically sufficient in providing long-lasting roads. However, they are often subject to value engineering, mostly because of financial restrictions.

Using recycled plastic waste as an alternative material for road construction can reduce construction costs, conserve natural resources, and maximize the integration of locally available marginal materials and environment-friendly technologies.  

Plastic waste can be dry-processed as aggregate coating in asphalt for flexible pavement or post-processed as interconnected cells to contain cement concrete mix for rigid pavements.

Dry-Processed Plastic Waste in Asphalt Mix

In the dry-processed approach, the plastic element is added to the bitumen before it is mixed into the aggregate. It is an effective way to add plastic waste to asphalt mix. The 1.67-kilometer road from Sehore to Thuna in Madhya Pradesh is one of the projects under the PMGSY program that implemented this approach. The road surfacing comprised a seal coat, overlaying a 20 millimeter (mm) thick open-graded premix carpet layer (OGPC), which incorporates plastic waste. The road was opened for traffic in December 2013. After the end of the 5-year warranty period, no defects had been reported on this road, in contrast to the experience with conventional materials, where around 25% of the work would have typically required some repairs within the same period.

Incorporating plastic waste into a road asphalt mixture using the dry process in Sehore. Photo credit: ADB.

To make this approach effective, the following aspects must be met during production:

• A consistent and uniform distribution through the aggregate must be ensured to control the performance of the asphalt mixes.
• The effectiveness of plastic waste to coat the aggregate must be verified to ascertain the claimed benefit, as well as to assess the required bitumen replacement. The quality of the coating determines the durability of an asphalt mix.
• The plastic shreds should meet standard guidelines, which include the source of plastic material, acceptable dimensions, maximum impurity, and melt-flow value. At a minimum, these control requirements should clearly be stated in the project specifications to ensure that they are properly implemented.
• Better controls should be required to ensure that the correct dosage of plastic has been added. For this, the use of calibrated containers is recommended.

Plastic Cell-Filled Concrete Block Pavement

Originally developed in South Africa, the concept involves filling interconnected diamond-shaped plastic cells with granular materials. These plastic cells can be made of recycled low-density polyethylene or high-density polyethylene and are typically 0.2 mm–0.5 mm thick. These cells are then manually welded together using a paddle sealer.

Pre-construction processing of plastic cells. (Left) Processed low-density polyethylene ready for welding into plastic cells. (Right) A paddle sealer welds polyethylene sheets to create the plastic cells. Photo credit: ADB.

For the rigid pavements of PMGSY roads, the granular materials are replaced with cement concrete materials, as shown in the pictures below.

Application of plastic cell during road construction. (Left) Plastic cells filled with granular materials. (Right) Plastic cells filled with cement concrete. Photo credit: ADB.

The diamond-shaped heat-welded plastic cells are used to encase concrete blocks, each of which measures 150 mm by 150 mm with a depth of 100 mm to 150 mm. During compaction, cell walls get deformed and provide interlocking among the blocks. Some practices use plastic pipe as formwork to assist in the installation of the cementitious material. After compaction, the curing process starts as per regular concrete cement.

The use of plastic cells allows small movement and flexibility in the pavement layer. For this reason, this type of pavement is also called a “flexible–rigid” pavement.

The process for installing plastic cell-filled concrete block pavement (PCCBP) generally takes a longer time and is more labor-intensive. There must be a continuous feed of fresh concrete to ensure a good quality finish at the end of each day shift. If the cementitious grout cures before the filling process is completed, the grout will set and harden, and it will be difficult to correct the finished profile. The irregular finished profile can lead to poor surface quality and may accelerate damage to both the pavement and vehicles because of the rough surface.

The quality control requirements of PCCBP are similar to conventional concrete, such as the use of the vertical slump test for the concrete mix and the compression strength test for cube specimens cast on site during each day of production. When used as surface course, a suite of assessments should be performed to determine texture depth, skid resistance, and regularity of the pavement.

Interlocking between concrete blocks results in good bearing capacity of the pavement. Concrete compressive strength of 15 megapascal can be adequate for traffic operation. The road can likewise be opened within 24 hours for light traffic, while for heavier traffic, there must be a wait of at least 14 days to ensure that the materials have reached sufficient strength.

PCCBP construction and maintenance represent an economical solution compared to conventional concrete and flexible pavements. The reduction in layer thickness from 150 mm in regular pavement quality concrete to 100 mm in PCCBP contributes to reduced production costs. The life cycle cost of PCCBP is also comparatively less than conventional flexible and rigid pavement constructions.

This approach is more economical for regions where the cost of labor is more affordable than investing in construction plants. Furthermore, the labor skill sets required for PCCBP installation are similar to those needed for conventional cement concrete works.

Overall, PCCBP is considered suitable for low-volume village roads and is not recommended for very heavy traffic.

Hypothetically, a combination of asphalt and concrete layers that incorporate plastic waste (i.e., OGPC layer and PCCBP) can maximize further the impact of reduced plastic waste. This also offers multiple benefits, such as:

• roads with good serviceability, durability, and riding quality provided by the asphalt surfacing;
• improved sustainability as fewer plastic waste end up in landfills;
• an economic advantage from the reduction of bitumen content, improved durability, and reduced layer thickness; and
• positive socioeconomic impact from the opening of new markets for plastic waste and opportunities for small business units.

Figure 1. Hypothetical Flexible-Rigid Pavement Structure

DLC = dry lean concrete, GSB = granular subbase, mm = millimeter, OGPC = open-graded premix course, WBM = water-bound macadam.
Source: Asian Development Bank.

Generally, road serviceability depends on robust design, well-executed installation, and sustained maintenance activities. An asphalt surface course provides good riding quality and protection of the substrate from vertical moisture or rainwater ingress. A conventional asphalt surface course, such as an OGPC layer plus seal coat, would typically require resurfacing after the initial 5 years of service. Anecdotal information suggests that around 25% of rural roads required localized repairs during the first 5 years. On the other hand, using plastic waste in the OGPC layer seems to have improved the performance of the surface course during the same period.

A cement concrete surface course, including one that uses PCCBP, can be a durable long-lasting layer. The predicted service life of a well-installed cement concrete surface course can be 3 to 5 times longer than the asphalt alternative (i.e., can be in excess of 25 years); however, the riding quality tends to be poorer.

A base course or subbase materials—such as water-bound macadam, a stabilized layer, and granular subbase—are expected to last up to 10 years if these materials are well protected from moisture and excessive loading. The life expectancy of these layers might be shorter when they are overlaid with asphalt layers (such as OGPC layer and seal coat) than with cement concrete (such as PCCBP).

Considering the above, it is possible that the hypothetical pavement structure as shown in Figure 1 can offer good riding quality with longer service life and less maintenance requirement. This said, there should always be available and adequate drainage to protect both the embankment and the unbound or weakly bound substrates.

The hypothetical flexible–rigid pavement balances cost, performance, and environmental benefits.

Nonetheless, more tests are required to further validate this hypothesis. This should include the installation and monitoring of the service performance of trial sections in low-volume traffic rural roads.

A. Heriawan. 2020. Upcycling Plastic Waste for Rural Road Construction in India. ADB South Asia Working Paper Series 69.  Manila: Asian Development Bank.