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Construction Engineering Research and Innovation Infrastructure and the Built Environment DRSR – Research team specialized in Development and Research on Structures and Rehabilitation

Understanding the Role of Carbon Fibers in Strengthening Concrete Beams

Concrete beam strengthened with carbon fibers

Abstract

Shear failure in reinforced concrete (RC) beams presents a significant risk in construction, often leading to sudden and catastrophic ruptures without warning. This issue has sparked ongoing research to improve the resilience of concrete structures. A recent study by researchers at École de technologie supérieure (ÉTS) examines the interactions between traditional steel stirrups and modern externally bonded carbon fiber-reinforced polymers (EB-CFRPs). The study aims to refine how we predict their combined effects on shear strength.

Introduction: The Challenge of Shear Failure in Concrete

This study investigates the shear failure in reinforced concrete (RC) beams, focusing on the contribution of externally bonded carbon fiber-reinforced polymers (EB-CFRPs) and their interaction with steel stirrups. Current design guidelines do not adequately address the inverse interaction between steel stirrups and EB-CFRPs, leading to inaccuracies in predicting shear resistance. Through experimental and numerical tests, this research develops analytical and numerical models to account for this interaction, showing that the shear contribution of EB-CFRPs decreases as their ratio to steel stirrups increases. A finite-element model was developed in this study, which proposes an analytical model for effective CFRP strain, validated by experimental data. The results highlight the impact of this interaction on shear crack patterns, load-deflection behavior and strain profiles, providing a more accurate prediction of shear resistance in strengthened RC beams.

The Problem of Shear Strength in RC Beams

Concrete beams, while resistant to compression, can fail unexpectedly when subjected to shear forces. This brittle failure can be particularly dangerous, prompting engineers to seek more reliable methods for enhancing the shear capacity of these structures. The study highlights a critical oversight in existing design guidelines: they often fail to account for the complex interactions between different reinforcement materials, particularly the inverse relationship between steel stirrups and EB-CFRPs.

Exploring the Inverse Interaction

Our study found that as the quantity of EB-CFRP used in a beam increases, its contribution to shear strength can actually diminish compared to steel stirrups. This counterintuitive finding suggests that simply adding more carbon fiber composites may not always lead to a better performance. Instead, the researchers propose a new model that takes into account this inverse interaction, leading thereby to more accurate predictions of shear strength. Details of the geometry, steel reinforcement position and EB-CFRP configuration are provided in Figure 1.

Beam cross-section
beam elevation view
Figure 1. Beam details: (a) Cross-sections of a geometrically similar beam (mm); (b) Typical elevation and internal instrumentation

Suggested Finite-Element (FE) Modeling

This section explains how the study used computer simulations to analyze the behavior of concrete beams strengthened with CFRP sheets. The goal was to accurately model the interaction between the concrete, steel reinforcement and CFRP, especially when cracks form and materials bond together. To make the simulation more efficient, a two-dimensional (2D) model was used, focusing on key areas like the concrete, the support structure, and the layers between the concrete and CFRP or steel. In the model, simple 2D elements were used to represent the CFRP sheets and steel bars. To make sure that the simulation worked smoothly, the method used was designed to handle issues like concrete cracking and how materials slip apart after failure. The analysis also carefully considered factors such as how the load is applied, the loading speed and other settings to ensure accurate results. A schematic of the simulated beam is shown in Figure 2.

Simulated beam using ABAQUS 2D
Figure 2. ABAQUS 2D simulation of the strengthened RC beams and their defined elements

Innovative Approaches to Model Development

The researchers conducted extensive tests and analyses to develop analytical and numerical models that consider this inverse interaction. By reviewing over 100 previous studies, they were able to identify patterns and relationships to more effectively predict the behavior of RC beams under shear loads. Their findings emphasize the importance of understanding how different materials work together, rather than treating them as isolated entities.

Validation of the Proposed FE Model

To test the accuracy of the simulation, two beams from a previous study were used: one regular beam and one strengthened with CFRP. The simulation results were compared with experimental data. Then, after confirming the model’s accuracy, a further study explored the interaction between the internal and external reinforcements in concrete beams when strengthened with U-shaped CFRP sheets. Both beams failed because they lost their ability to resist shear forces. The strengthened beam showed signs of debonding, where the CFRP separated from the concrete substrate. The simulation predicted the strain and shear load in the beams to be very close to the experimental results. For example, the predicted shear force in the regular beam was 4.2% higher than the real test, and the shear force in the strengthened beam was 10% higher in the simulation. As the ratio of CFRP to steel stirrups increased, shear cracks became more widespread, as shown in the simulated model images (Figure 3), which display the crack patterns in both beams. To ensure the simulation's reliability, different mesh sizes were tested, and a 10 mm mesh size was found to yield the most accurate results. This size has been validated by previous studies, confirming the simulation’s robustness.

Propagation of cracks in a beam
Figure 3. Propagation of shear cracks in RC beam strengthened with EB-FRP, as a function of increasing ratio of EB-FRP to steel-stirrups

Key Findings and Their Implications

The research indicates that increasing the EB-CFRP to steel stirrups ratio could lead to a decrease in the contribution of carbon fibers to shear strength. This insight is crucial for engineers designing reinforced concrete structures, as it challenges the conventional perception that more reinforcement is always better. By providing a clearer understanding of how these materials interact, the study aims to improve safety and performance in real-world applications.

Visual Elements

To further engage readers, incorporating relevant images, such as shear failure diagrams, photos of EB-CFRP applications or flowcharts illustrating the research process will enhance comprehension and interest. These visual aids can clarify complex concepts and break up text for easier reading.

Conclusion: A Step Towards Safer Structures

This research contributes significantly to the field of construction engineering by shedding light on the complex interactions between traditional and modern reinforcement methods. By refining the models used to predict shear strength, engineers can design safer, more efficient concrete structures. As we continue to innovate in materials science, understanding these interactions will be key to developing robust solutions that stand the test of time.

Additional Information

For more information on this research, please read the following articles: Abbasi A, Chaallal O, El-Saikaly G. Inverse interaction of shear between steel-stirrups and externally bonded carbon fiber-reinforced polymers in shear-strengthened reinforced concrete beams: Analytical and numerical models. Journal of Reinforced Plastics and Composites. 2024 June 5:07316844241254571.

References

Mofidi, Amir, et Omar Chaallal. 2010. « Shear strengthening of RC beams with EB FRP: Influencing factors and conceptual debonding model ». Journal of Composites for Construction, vol. 15, no 1, p. 62-74.

Header Image Reference

Provided by the authors. CC License.

Image References

All images were provided by the authors. CC License.