Surreal digital illustration of recycled aggregate concrete with glowing microcracks

Recycled Aggregate Concrete: The Secret to Greener, Stronger Construction?

Avery Sinclair in Tech & Innovation September 2025 • 4 min read.

"Unlocking the Potential of Recycled Materials with Advanced Engineering Techniques"

The construction industry is facing increasing pressure to adopt sustainable practices and reduce its environmental footprint. One promising solution lies in the use of recycled aggregate concrete (RAC), a material made by incorporating recycled concrete aggregates (RCA) from demolished structures into new concrete mixes. This approach reduces waste, conserves natural resources, and potentially lowers construction costs.

However, RAC presents unique challenges compared to traditional concrete made with natural aggregates. The presence of old cement mortar and interfacial transition zones (ITZs) in RCA can affect the overall strength, durability, and failure behavior of the concrete. Understanding these complex interactions is crucial for ensuring the safe and reliable application of RAC in construction projects.

Fortunately, advanced numerical modeling techniques are providing valuable insights into the behavior of RAC at a microscopic level. By simulating the material's structure and response to stress, engineers can identify potential weaknesses, optimize mix designs, and predict long-term performance. One such technique, the discrete-element method (DEM), is proving particularly effective in capturing the complex cracking and failure mechanisms of RAC.

Cracking the Code: How Numerical Modeling Enhances RAC Performance

Surreal digital illustration of recycled aggregate concrete with glowing microcracks

A recent study published in the Journal of Materials in Civil Engineering delved into the failure processes of RAC using a two-dimensional DEM simulation. The researchers created a virtual model of RAC, representing the material as a collection of interconnected rigid blocks with interfaces that mimic the behavior of cement mortar and ITZs. This allowed them to simulate the initiation, propagation, and coalescence of microcracks within the material under compressive loading.

The study focused on understanding how microcracks form and spread in RAC compared to conventional natural aggregate concrete (NAC). The models accounted for the distinct components of RAC, including natural coarse aggregates (NCA), old and new cement mortars, and interfacial transition zones (ITZs) – the boundaries where different materials meet.

Here's a breakdown of the key findings:

ITZs are the Weak Links: Both the new and old ITZs in RAC were identified as zones where microcracks are most likely to start.
Mortar Matters: Most microcracks initiated in and spread through the cement mortar region.
Validation is Key: The numerical models accurately matched experimental results, confirming their reliability in predicting real-world behavior.

These findings highlight the importance of addressing the weaknesses in ITZs to improve the overall performance of RAC. By understanding how cracks initiate and propagate, engineers can develop strategies to enhance the durability and extend the lifespan of RAC structures. One approach involves optimizing the mix design to create stronger bonds between the recycled aggregates and the surrounding cement mortar.

Building a Sustainable Future with Smarter Concrete

Numerical modeling offers a powerful tool for advancing the use of RAC in construction. By simulating material behavior at a microscopic level, engineers can gain valuable insights into failure mechanisms and develop strategies to optimize performance. As the demand for sustainable construction materials continues to grow, RAC, guided by sophisticated modeling techniques, promises to play an increasingly important role in building a greener and more resilient future.

About this Article -

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This article is based on research published under:

DOI-LINK: 10.1061/(asce)mt.1943-5533.0002562, Alternate LINK

Title: Numerical Discrete-Element Method Investigation On Failure Process Of Recycled Aggregate Concrete

Subject: Mechanics of Materials

Journal: Journal of Materials in Civil Engineering

Publisher: American Society of Civil Engineers (ASCE)

Authors: Xin Tan, Wengui Li, Minghua Zhao, Vivian W. Y. Tam

Published: 2019-01-01

Everything You Need To Know

What is Recycled Aggregate Concrete (RAC) and how is it different from traditional concrete?

Recycled Aggregate Concrete (RAC) is a sustainable construction material created by using Recycled Concrete Aggregates (RCA) from demolished structures in new concrete mixes. Unlike traditional concrete that uses natural aggregates, RAC incorporates RCA, reducing waste and conserving natural resources. However, RAC has unique challenges due to the presence of old cement mortar and Interfacial Transition Zones (ITZs) within the RCA, which can affect its strength and durability compared to concrete made with natural aggregates.

What role do Interfacial Transition Zones (ITZs) play in the performance of Recycled Aggregate Concrete (RAC)?

In Recycled Aggregate Concrete (RAC), Interfacial Transition Zones (ITZs) are the boundaries where different materials meet, and they significantly influence the material's performance. Studies using advanced numerical modeling, such as the discrete-element method (DEM), have revealed that both the new and old ITZs are the weak links in RAC, where microcracks are most likely to initiate. The presence of ITZs affects the overall strength, durability, and failure behavior of the concrete, making them a critical area for understanding and improving RAC's performance.

How does numerical modeling, like the discrete-element method (DEM), help improve Recycled Aggregate Concrete (RAC)?

Numerical modeling, particularly the discrete-element method (DEM), is a powerful tool for understanding and improving Recycled Aggregate Concrete (RAC). By creating virtual models that simulate the material's structure at a microscopic level, engineers can analyze how RAC behaves under stress. DEM allows researchers to simulate the initiation, propagation, and coalescence of microcracks within RAC, including components like natural coarse aggregates (NCA), old and new cement mortars, and ITZs. This helps to identify weaknesses, optimize mix designs, and predict the long-term performance of RAC, leading to safer and more reliable construction applications.

What are the key findings from the study on Recycled Aggregate Concrete (RAC) using numerical modeling?

The study utilizing numerical modeling on Recycled Aggregate Concrete (RAC) revealed key insights into its failure behavior. The most important findings indicated that the Interfacial Transition Zones (ITZs), both new and old, are the primary areas where microcracks start. Additionally, the cement mortar region was identified as a critical area where microcracks initiate and spread. The numerical models accurately matched experimental results, thus validating their reliability in predicting real-world behavior. These findings emphasize the importance of addressing the weaknesses in ITZs to enhance RAC's overall performance.

What are the potential benefits of using Recycled Aggregate Concrete (RAC) in construction, and how can its use be optimized?

The use of Recycled Aggregate Concrete (RAC) offers significant benefits, including reducing waste, conserving natural resources, and potentially lowering construction costs, aligning with sustainable construction practices. To optimize RAC, engineers can leverage advanced numerical modeling techniques to understand the material's behavior at a microscopic level. This allows for the identification of weaknesses, such as those in the Interfacial Transition Zones (ITZs), and the development of strategies to improve RAC's durability. Optimizing the mix design, such as creating stronger bonds between the recycled aggregates and the cement mortar, can also enhance performance, ensuring RAC can play an increasing role in building a greener and more resilient future.