Sustainable Pavements Using Renolith and Recycled Concrete Aggregates

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Recycled Concrete Aggregate (RCA)

Recycled Concrete Aggregate (RCA) is a material produced from the demolition of concrete structures such as buildings, bridges, and highways. The concrete from these structures is crushed into small pieces, and these pieces can then be used as aggregate in new concrete mixes.

Recycling concrete reduces the amount of waste sent to landfills and can save energy by reducing the need for mining and transportation of new aggregates.

Various researchers and industries have demonstrated the application of stabilised RCA in construction activities, such as pavement base/subbase, road embankment, backfilling of retaining walls, and support structures for pipelines.

One of the most practical ways to reduce the demand for natural aggregates in the construction industry and dispose of the construction and demolition (C&D) wastes in landfills is using recycled concrete aggregate (RCA) for pavement base and subbase applications. 

M. Saberian, J. Li, Effect of freeze–thaw cycles on the resilient moduli and unconfined compressive strength of rubberized recycled concrete aggregate as pavement base/subbase, Transp. Geotech. 27 (2021).

RCA in Soil Cement

In-situ material stabilised with cementitious binders, (colloquially: soil cement) is a composite material that combines soil, cement, and water to create a durable and strong material suitable for a variety of applications including road construction, erosion control, and building foundations. RCA can be used as a partial replacement for the traditional aggregates in soil cement mixes.

Here are some ways RCA can be used in soil cement:

  1. Partial replacement of traditional aggregates: RCA can be used to replace a portion of the traditional aggregates used in soil cement mixes. This can reduce the amount of virgin aggregate required and decrease the environmental impact of the project.
  2. Improved soil stabilization: RCA has been found to improve soil stabilization properties in soil cement mixes. The rough texture and irregular shape of RCA particles can create interlocking bonds with the soil particles, which can improve the overall strength and stability of the soil cement mixture.
  3. Reduced shrinkage: The use of RCA in soil cement can also reduce the shrinkage of the material during the drying process. This is because RCA particles are more porous than traditional aggregates and can absorb moisture from the surrounding soil, reducing the overall shrinkage of the material.
  4. Cost-reduction: The use of RCA in soil cement can be cost-effective compared to using traditional aggregates. RCA is often less expensive than virgin aggregate and can be obtained from local sources, reducing transportation costs.

RCA Limitations

While using RCA in new concrete mixes or soil stabilisation applications can offer environmental and economic benefits, there are also some limitations to consider. Here are a few:

  1. Quality and consistency: The quality and consistency of RCA can vary depending on the source and how it was processed. This means that it may not always meet the same performance standards as new concrete aggregates, and additional testing may be necessary to ensure it’s suitable for the intended use.
  2. Contamination: Recycled concrete may contain contaminants such as metal, wood, or other materials that were part of the original structure. These contaminants can affect the strength and durability of the new concrete, so it’s important to carefully screen and process the RCA to remove any unwanted materials.
  3. Absorption: RCA tends to have higher absorption rates than new concrete aggregates, which can affect the mix design and the overall performance of the concrete. It may be necessary to adjust the mix design or use additional additives to compensate for this.
  4. Cost: While using RCA can be cost-effective in some cases, the cost may be higher in others due to processing and transportation expenses.

Overall, using recycled concrete aggregate in new concrete mixes or soil stabilisation applications requires careful consideration of the specific project requirements and the quality of the RCA being used. Proper processing, testing, and mix design can help overcome many of the limitations associated with using RCA.

Interfacial Transition Zone (ITZ)

The interfacial transition zone (ITZ) in concrete refers to the region where the cement paste and aggregate particles come into contact with each other. The ITZ is the area between the surface of the aggregate and the surrounding cement paste.

The ITZ has a different microstructure than the bulk cement paste, as it is formed by the interaction between the cement paste and the aggregate surface. This region has a higher porosity and a lower strength than the bulk cement paste, which can affect the overall strength and durability of the cemented material.

The ITZ is also a critical zone for the formation of microcracks in concrete and stabilised soils. The differences in properties between the ITZ and the bulk cement paste can lead to stress concentrations, which can cause microcracks to form and propagate through the material. This can ultimately lead to a reduction in strength and durability.

To minimize the negative effects of the ITZ, various measures can be taken, such as optimizing the particle size distribution of the aggregates, using pozzolanic additives to improve the properties of the cement paste, and ensuring proper mixing and curing. By carefully managing the ITZ, it is possible to produce concrete/composites with improved strength, durability, and resistance to cracking.

ITZ Strength: RCA vs Quarried Aggregates

The strength of the ITZ in concrete can vary depending on the type of aggregate used. In general, the ITZ strength of concrete made with RCA is lower than that of concrete made with quarried aggregates. This is because RCA is typically more porous and has a rougher surface than quarried aggregates, which can lead to a weaker bond with the surrounding cement paste in the ITZ. Additionally, RCA may contain contaminants or residual mortar that can further weaken the bond and reduce the strength of the ITZ.

Nano Silica Effects on ITZ

Nano silica (also known as nano-silicon dioxide or nanoscale silica or nano SiO2) is a pozzolanic material that can be added to concrete & soil cement mixes to improve the properties of the ITZ. The addition of nano silica can have the following effects on the ITZ:

  1. Reduced porosity: Nano silica has a very small particle size (typically less than 100 nanometers), which allows it to fill in the pores and voids in the ITZ. This can reduce the porosity of the ITZ and improve the bond between the cement paste and aggregate.
  2. Increased strength: The addition of nano silica can also improve the strength of the ITZ. This is due to the pozzolanic reaction that occurs between the nano silica and the calcium hydroxide in the cement paste, which produces additional calcium silicate hydrate (C-S-H) gel. This additional gel can fill in the gaps between the aggregate and cement paste, resulting in a stronger and more durable ITZ.
  3. Improved durability: The reduced porosity and increased strength of the ITZ can also improve the durability of the concrete. A more compact and stronger ITZ can resist the ingress of moisture and other harmful substances that can degrade the concrete over time.
  4. Reduced cracking: The addition of nano silica can also help reduce cracking in the concrete. The improved bond between the cement paste and aggregate can reduce the formation and propagation of microcracks in the ITZ, which can ultimately lead to a more durable and long-lasting concrete structure.

Overall, the addition of nano silica can be an effective way to improve the properties of the ITZ, resulting in a more durable and high-performing structure.

Renolith and RCA

Renolith 2.0 is a nanopolymer admixture optimised for soil cement applications. A key ingredient in Renolith is Tritonite™, which is a proprietary non-agglomerating nano silica with diameter less than 10nm and very high specific surface area. It is extremely potent in cemented materials.

One application for Renolith 2.0 is to improve the performance of mix designs incorporating RCA. For example, cement stabilised materials from fine grained (silts, clays) and organic (eg. peat) soils have limited utility in civil applications.  Renolith 2.0 admixture enables these problematic soils to be used in a much greater range of applications, such as pavement base layers. However, they often require a high proportion of binder to achieve the required strength, which has cost and sustainability implications. The addition of RCA to these soils can improve the soil grading and reduce the binder demand, resulting in a cost-effective, crack-free, durable, low-carbon road constructed from 100% recycled materials.  The use of a low-carbon binder instead of cement can reduce the greenhouse gas emissions footprint even further.



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