Recycled Concrete Aggregates – Properties and Uses

Recycled Concrete Aggregates, also known as Crushed Concrete Aggregates, are derived from demolished or rebuilt concrete structures. These fragments are cleaned and broken into smaller pieces to create recycled aggregate. This aggregate serves as a replacement for conventional aggregate in the production of new concrete or for various other applications.

The utilization of crushed concrete aggregate reduces the need for disposal and makes efficient use of existing concrete resources. The specifications of ACI 555R-01, British Standard, and the Building Contractors Society of Japan outline the uses and guidelines for crushed concrete in different applications.

Why Use Recycled Concrete Aggregates?

Recycled concrete offers a cost-effective alternative to crushing natural raw materials, making it an attractive choice. Its utilization promotes sustainability in construction projects, as it reduces the depletion of natural aggregates and has a smaller carbon footprint compared to concrete made with traditional materials. Moreover, using crushed concrete requires less energy compared to mining and processing new aggregates.

Read Also: High-Strength Concrete vs Normal Concrete Properties and Difference

Recycled Concrete Aggregates Properties

1. Size of Aggregate

Crushed or Recycled concrete is required to undergo processing in order to achieve appropriate sizes. The method used for crushing the concrete, whether mechanical or manual, plays a significant role in determining the strength of the resulting coarse aggregate, which is used to produce new concrete. It has been reported that coarse aggregates that meet the standards can be obtained through this process.

However, the fine aggregates produced are larger and have a more angular shape compared to those typically used in concrete production. As a result, concrete made with crushed concrete aggregate tends to have a harsh and unworkable consistency. To address this issue, a portion of natural sand can be added.

It is also worth noting that recycled aggregate can be used without the need for washing. Additionally, both the crushing procedure and the size of the recycled aggregate have an impact on the amount of adhered mortar.

2. Aggregate Grading

The grading of aggregate particles has a significant impact on the amount of mixing water needed to achieve proper workability in concrete. When the proportion of fine particles in the aggregate increases, it results in an increased requirement for water.

This, in turn, can lead to a decrease in concrete strength unless the cement content is also increased. It is not advisable to use recycled sand in concrete due to its high absorption capacity, which can cause shrinkage effects.

3. Density 

The density of crushed concrete aggregate is lower compared to that of natural aggregate. When concrete is crushed, it undergoes a reduction in density due to the breaking down of its particles. This process results in a more porous and less compacted material.

In contrast, natural aggregate, which is sourced directly from quarries or rivers, tends to have a higher density as it has not undergone any crushing or alteration. The difference in density between crushed concrete aggregate and natural aggregate can have implications for various applications and engineering purposes.

4. Water Absorption

Water absorption is a crucial property that sets recycled aggregate apart from natural aggregate. Unlike raw aggregates, recycled aggregate exhibits a higher water absorption capacity. This disparity in water absorption significantly impacts the characteristics of both fresh and hardened concrete.

Consequently, the absorption capacity of recycled aggregate plays a significant role in determining the workability of recycled aggregate concretes. It is important to avoid the use of recycled sand because of its high absorption capacity, which can lead to a shrinkage effect in the concrete.

5. Los Angeles Abrasion Loss

Crushed concrete aggregate meets the standards set by ASTM International in terms of Los Angeles abrasion loss.

6. Contaminants

The presence of contaminants in recycled aggregate has a detrimental effect on the strength of concrete produced using this aggregate.

Read Also: Rice Husk Ash Concrete Characteristics – Workability, Strength and Admixtures

Influence of Recycled Aggregate on Concrete Properties

1. Concrete Strength Reductions

The strength of concrete made from crushed concrete aggregate is generally lower compared to concrete made with natural aggregate. The reduction in strength for concrete produced using recycled coarse aggregate and natural fine aggregate can range from 5 to 24%.

This decrease in strength is influenced by factors such as the strength of the crushed concrete, the water-to-cement ratio of both the crushed and new concrete. In the case of concrete produced from both recycled coarse and fine aggregate, the strength reduction falls within the range of 15 to 40%.

2. Strength Variations

Concrete produced using recycled aggregate exhibits higher strength variation compared to concrete made from natural aggregate. When using recycled aggregate in concrete, it becomes imperative to increase the cement content in order to attain equivalent workability and compressive strength as conventional concrete.

Any alterations in the production process or properties of the constituents utilized lead to variations in the strength of the resulting concrete.

3. Flexural and Tensile Strength

ACI 55.1R has reported that concrete made from recycled coarse aggregates exhibits a decrease in flexural and tensile strength compared to conventional concrete, with a reduction of 10%. In the worst-case scenario, this reduction can increase to 20%. However, when coarse recycled aggregate is combined with natural sand, the resulting concrete shows similar flexural and tensile strength as concrete made solely from natural aggregate.

For flexural tests, the standard size of beams, as specified by BS 1881, is 150mm x 150mm x 700mm. On the other hand, according to ASTM C790, the size of the flexural test specimen is 3.2mm x 12.7mm x 125mm.

4. Modulus of Elasticity

The modulus of elasticity of concrete, when produced using both recycled coarse and fine aggregate, is found to be lower compared to concrete that is made solely with recycled coarse aggregate.

5. Creep

The creep of concrete fabricated with recycled concrete aggregate surpasses that of concrete produced using natural aggregate by a significant margin, ranging from 30% to 60% higher.

6. Drying Shrinkage

Concrete made from recycled aggregate typically exhibits a higher degree of drying shrinkage compared to concrete composed of natural aggregate. This means that the potential for shrinkage, resulting from moisture evaporation, is greater in recycled aggregate concrete.

Moreover, when both coarse and fine aggregates are derived from recycled sources, the percentage of drying shrinkage tends to escalate further. The utilization of recycled materials in the production of concrete can therefore contribute to an increased susceptibility to drying shrinkage, necessitating careful considerations and appropriate measures to mitigate its effects.

7. Permeability

Concrete made from recycled aggregate typically exhibits a higher degree of drying shrinkage compared to concrete composed of natural aggregate. This means that the potential for shrinkage, resulting from moisture evaporation, is greater in recycled aggregate concrete.

Moreover, when both coarse and fine aggregates are derived from recycled sources, the percentage of drying shrinkage tends to escalate further. The utilization of recycled materials in the production of concrete can therefore contribute to an increased susceptibility to drying shrinkage, necessitating careful considerations and appropriate measures to mitigate its effects.

8. Freezing and Thawing Resistance

Research has demonstrated that the freezing and thawing resistance of concrete constructed using crushed concrete aggregate is equivalent to that of concrete manufactured with natural aggregate.

9. Carbonation, Chloride Penetration, and Reinforcement Corrosion

In comparison to concrete made with natural aggregate, concrete incorporating crushed concrete aggregate tends to exhibit higher levels of carbonation, particularly when the recycled aggregate has previously undergone carbonation. Likewise, the rate of corrosion in reinforcing elements within concrete constructed using recycled aggregate is generally greater than that observed in conventional concrete.

10. Shear Strength

The shear strength of recycled aggregate concrete is typically lower compared to normal concrete. One of the main reasons for this is that the recycled aggregate obtained from field-demolished concrete, for instance, can be relatively weaker than the natural aggregate typically used in concrete production.

As a result, when recycled coarse aggregates are utilized, there is generally a decrease in strength of approximately 10%. This reduction in shear strength can be attributed to the inherent characteristics of recycled aggregates, which may not possess the same level of durability and robustness as their natural counterparts.

Recycled Concrete Aggregates Uses

Crushed concrete is a highly versatile material that finds numerous applications in the civil construction industry, making it an essential component of many major projects. The uses of crushed concrete can be broadly categorized into two main groups:

1. Structural Applications: Crushed concrete can be used as a base material for roads, highways, and airport runways. Its durable nature and excellent load-bearing capacity make it suitable for providing a solid foundation for these infrastructure projects. Additionally, crushed concrete can be used as a sub-base layer beneath asphalt or concrete pavements, enhancing their stability and longevity.
2. Non-Structural Applications: Crushed concrete is also utilized in various non-structural applications. For instance, it can be used as a fill material in land reclamation projects or as a backfill material for trenches and excavations. Crushed concrete can be employed for creating temporary construction access roads, reducing the need for traditional construction materials. It can also be utilized as a drainage material in stormwater management systems, allowing water to permeate through while preventing soil erosion.

By recycling concrete waste and transforming it into a valuable resource, the use of crushed concrete contributes to sustainable construction practices. It helps reduce the demand for virgin aggregates, conserves natural resources, and minimizes the environmental impact associated with traditional construction materials. Overall, crushed concrete offers a cost-effective and environmentally friendly alternative for various construction needs.

1. Uses of Crushed Concrete (Unprocessed)

Applications that do not require any additional processing typically involve various forms of general bulk fills. These include their usage in bank protection, serving as a base or fill for drainage structures, and supporting road construction projects. Additionally, they can be utilized for the construction of noise barriers and embankments.

2. Uses of Processed Crushed Concrete (Crushed Concrete Aggregate)

Crushed concrete aggregate, once processed, finds application in various construction projects. It is commonly used in bridge foundations, structural grade concrete, lean-concrete, bituminous concrete, soil-cement pavement bases, and pavements. Additionally, it is suitable for constructing shoulders, median barriers, sidewalks, curbs, and gutters. Its versatility makes it an ideal choice for a range of infrastructure developments, including freeways, airport runways, and oil & gas civil construction projects.

Read Also: How Water Impurities Effects on Concrete Strength, Durability and Other Properties

FAQs about Recycled Concrete Aggregates