Study: Evaluation of the mechanical behavior of concrete reinforced with waste tire steel fibers. Image Credit: Bits And Splits/Shutterstock.com
A recent study published in the journal Scientific Reports explored the mechanical behavior of concrete reinforced with waste tire steel fibers (WTSF). The goal was to address the urgent need for sustainable construction materials by assessing WTSF as an alternative to conventional manufactured steel fibers (MSF).
The findings highlight that WTSF can improve the mechanical properties of concrete, but its effectiveness depends on using appropriate fiber content, as excessive amounts may reduce strength.
Addressing Sustainability Challenges in Construction
The construction industry faces a tough challenge when it comes to sustainability, largely due to its dependence on concrete and steel. Steel fibers are often added to concrete to boost its strength and durability, but making those fibers takes a lot of energy and contributes significantly to carbon emissions.
A smarter, more sustainable option is recycled steel fibers from used tires.
Each year, over a billion tires are discarded worldwide. Thus, repurposing the steel from these tires not only helps reduce landfill and incineration waste, but also provides an affordable, eco-friendly way to reinforce concrete.
Concrete reinforced with steel fibers, known as SFRC, is already valued for its improved crack resistance, ductility, and fatigue strength compared to traditional mixes. When WTSF are used and their content carefully optimized, it’s possible to maintain those performance benefits while cutting down the environmental impact.
The Study
For the study in question, the researchers evaluated the mechanical properties of concrete reinforced with WTSF compared to MSF. They tested 84 specimens by dividing them into three groups: control concrete without fibers, concrete with WTSF, and concrete with MSF.
Following the BRE method and the Egyptian Code for Design and Construction of Reinforced Concrete Structures (ECP 203), concrete mixes were designed to achieve a compressive strength of 30 MPa at 28 days. Steel fibers were added at volume fractions of 0.25 %, 0.5 %, and 1.0 %.
A two-stage mixing process was employed to ensure uniform fiber dispersion and prevent agglomeration. The fibers were sourced from a national tire recycling facility and cleaned to remove contaminants. Standard specimens were prepared for mechanical testing: cubes for compressive strength, cylinders for splitting tensile strength, and beams for flexural strength.
Workability was assessed using slump tests, and compressive strength was measured at 7 and 28 days. This approach allowed them to systematically evaluate the effects of fiber type and content on concrete’s fresh and hardened properties.
Key Findings
The outcomes showed important insights into the mechanical behavior of concrete reinforced with WTSF and MSF. MSF led to slight improvements for compressive strength, with increases of 13.5 % and 9.8 % at 0.25 % and 0.5% fiber content, respectively.
In contrast, WTSF significantly reduced compressive strength, decreasing by 6.4 %, 30 %, and 46 % for 0.25 %, 0.5 %, and 1 % content, respectively. This decline depended on the high aspect ratio of WTSF, which promotes clumping and uneven distribution, creating weak points in the concrete.
Both fiber types enhanced tensile strength. MSF achieved a maximum increase of 66–67 % at 1 % content, while WTSF improved tensile strength by 24 % and 38 % at 0.25 % and 0.5 %, respectively, but decreased by 10 % at 1 % due to fiber agglomeration.
Regarding flexural strength, MSF outperformed WTSF, indicating a 40 % increase at 1 %, whereas WTSF reached a 19 % increase at 0.5 %. Workability decreased with the addition of both fibers, with a more pronounced effect for WTSF, demonstrating a zero slump at 1 % due to its tendency to form clusters.
The study also reported that both fiber types improved the modulus of elasticity compared to plain concrete, with WTSF reaching up to 31.9 × 103 MPa and MSF up to 40.4 × 103 MPa.
Overall, the study suggests that while WTSF can enhance tensile properties, it is less effective than MSF in improving compressive and flexural strength, highlighting the need to optimize fiber content for balanced performance.
Practical Applications in Sustainable Construction
This research has significant implications for the construction industry. Using WTSF as reinforcement provides a way to reduce environmental waste while improving certain mechanical properties of concrete. Although WTSF reduces compressive strength compared to MSF, it enhances tensile and flexural strength, making it suitable for non-load-bearing applications such as pavements and partitions. Optimizing fiber content and ensuring uniform distribution are essential to mitigate issues with fiber clumping.
In addition to performance benefits, WTSF provides economic and environmental advantages by recycling waste tires and lowering the carbon footprint of the construction process. While it may not replace MSF in primary load-bearing structures, it is a sustainable alternative for secondary or non-structural components.
Conclusions and Future Directions
In summary, this study provides valuable insights into the mechanical behavior of concrete reinforced with WTSF. While WTSF is a sustainable alternative to MSF, its use reduces compressive strength, limiting its suitability for main load-bearing applications. However, it can enhance tensile and flexural performance, making it promising for non-structural or secondary elements such as pavements, partitions, and non-load-bearing walls.
The findings also confirm that WTSF offers modest improvements in stiffness (modulus of elasticity) at lower fiber ratios, further supporting its viability in sustainable, non-structural concrete applications.
Future work should focus on exploring the long-term durability of WTSF-reinforced concrete under various environmental conditions, refining recycling processes, and investigating other recycled materials to improve both sustainability and mechanical performance.
Overall, this research highlights the potential of WTSF to support environmentally friendly construction practices, balancing material performance with reduced waste and lower carbon emissions.