Experimental Study on Flexural Behavior of Bonded Steel-Concrete Composite Beams: A Comprehensive Examination
Steel-concrete composite beams are critical components in modern infrastructure due to their excellent load-bearing capacity, cost-effectiveness, and overall efficiency. The combination of steel and concrete takes advantage of the high strength of steel and the compressive strength of concrete, creating a versatile and durable structure. However, the mechanical shear connectors traditionally used to bond these materials have certain limitations. Over time, mechanical connectors such as studs, bent-up bars, and structural steels can lead to issues like stress concentration, cracking in concrete slabs, and corrosion due to the metal connectors. These challenges not only affect the longevity of the structure but also the structural efficiency.
This study, published in Scientific Reports (April 2025), explores a novel approach to improve the performance of SCCBs by using adhesive bonding instead of mechanical connectors. The focus is on epoxy resin and magnesium phosphate cement (MPC) as adhesives, aiming to explore their effectiveness as shear connectors in the bonding between steel and concrete in composite beams.
Key Findings and Insights:
The experimental research presented in this study involves multiple bending tests comparing the performance of bonded shear connectors with traditional mechanical shear connectors. The results reveal several significant insights:
1. Increased Rigidity of Bonded SCCBs: The use of bonded shear connectors enhances the rigidity of the SCCBs when compared to mechanical shear connectors. The beams bonded with adhesive showed a higher flexural stiffness, indicating that they resisted deformation more effectively under bending loads.
2. Failure Modes of Bonded SCCBs: When subjected to flexural stress, the failure modes of bonded SCCBs predominantly include:
o Yielding of the bottom flange of the steel I-beam, which marks the first stage of failure under excessive stress.
o Cracking of the concrete’s bottom surface, a result of the increasing tensile stress at the concrete-steel interface.
o Local detachment between the bonding surfaces, leading to a loss of structural integrity at the adhesive interface.
o Crushing of the concrete slab at the top surface of the composite beam due to compressive forces.
These failure modes suggest that while bonded SCCBs perform well under load, the adhesive interface remains vulnerable to detachment under extreme stress, necessitating further research to mitigate this issue.
3. Impact of Adhesive Thickness on Ultimate Load Capacity: One of the most critical findings of this study is the relationship between adhesive thickness and the load capacity of the composite beams. As the thickness of the adhesive layer increases, the shear strength improves. The test results indicated that a 6 mm thick epoxy resin adhesive produced the highest ultimate load capacity, approximately 16.8% higher than the mechanical connectors used for comparison. This finding underscores the importance of adhesive thickness in ensuring the performance of bonded SCCBs.
4. Optimal Adhesive Thickness: Based on the findings, it is suggested that the adhesive layer should be at least 4 mm thick, but the optimal thickness is 6 mm. This thickness provides the best balance between structural efficiency and adhesive performance, maximizing the load-bearing capacity without compromising on the beam's overall integrity.
5. Advantages of Bonded Shear Connectors Over Mechanical Ones: The study also emphasizes several advantages of using bonded shear connectors over mechanical connectors:
o Reduced risk of cracking in the concrete slab due to more uniform stress distribution.
o Improved fatigue performance, as adhesives offer better resistance to repetitive loading compared to mechanical connectors.
o Simpler and more convenient construction, with fewer restrictions on the concrete slab thickness. This makes the construction process more flexible and less labor-intensive.
o Corrosion resistance, as adhesives are not susceptible to the same corrosion issues faced by metal shear connectors.
Bonded connectors also eliminate the need for embedded mechanical connectors, which reduces the weight of the structure and increases its overall efficiency.
Bonded vs. Mechanical Shear Connectors: A Comparison
For decades, mechanical shear connectors like studs and bent-up bars have been the standard for joining steel and concrete in composite beams. While these connectors are highly effective in transmitting shear forces, they do come with a range of limitations:
• Stress concentration occurs around the shear connector areas, which can lead to the formation of cracks in the concrete.
• The concrete slab may crush at the connector site due to the local stress concentrations.
• Corrosion can compromise the long-term performance and durability of the beam, especially in environments with high moisture or chemical exposure.
In contrast, bonded shear connectors made from epoxy resin or magnesium phosphate cement provide several benefits:
• They distribute stress more uniformly across the concrete slab, which reduces cracking and damage.
• Fatigue resistance is significantly better, especially under cyclic loading conditions.
• The construction process is more streamlined since there is no need to embed mechanical connectors in the concrete slab, thus reducing assembly time and labor costs.
• Corrosion resistance is a major advantage, as the adhesive connectors do not suffer from the same issues as metal connectors.
These advantages position bonded shear connectors as a superior alternative for many applications, particularly when long-term durability and ease of construction are priorities.
Further Research Directions
While this study provides valuable insights into the performance of bonded shear connectors, it also highlights areas that require further investigation:
• Adhesive Performance: The detachment of adhesive layers under high stress was observed in the experimental tests, suggesting that future research should focus on improving the adhesive’s bond strength to prevent premature failure.
• Alternative Adhesives: Research could explore other types of adhesives, including those that are more resistant to temperature fluctuations, humidity, and chemical exposure.
• Surface Treatments: Further studies could investigate surface treatments for steel and concrete interfaces to enhance the bonding effectiveness of adhesives.
• Long-term Durability: It is essential to assess the long-term durability of bonded SCCBs under various environmental conditions, such as high humidity, temperature extremes, and corrosive environments.
Key Takeaways:
• Bonded steel-concrete composite beams show higher rigidity and flexural stiffness compared to those with mechanical shear connectors.
• The failure modes of bonded SCCBs include yielding of the steel flange, cracking of the concrete, and detachment of the adhesive under stress.
• The ultimate load capacity of bonded SCCBs increases with the adhesive thickness, with 6 mm epoxy resin providing the best performance, yielding a 16.8% increase over mechanical connectors.
• Bonded shear connectors provide several advantages, including reduced concrete cracking, better fatigue performance, easier construction, and superior corrosion resistance.
• The optimal adhesive thickness for bonded SCCBs is 6 mm, providing the best balance between strength and performance.
• Further research is needed to enhance the adhesive bonding and durability, particularly to prevent local detachment during loading.
By advancing the use of bonded shear connectors in SCCBs, this study opens new possibilities for designing more efficient, sustainable, and durable structural components, pushing the boundaries of construction technology.
