Tailoring bond between concrete and steel bars with nanotechnology

Reinforced concrete materials are widely used in the building and construction sector. These materials possess remarkable mechanical performance because of the combined high compressive strength of the concrete and the improved tensile performance of the reinforcing elements. Notably, the characteristics of the bond between the reinforcing bars and the concrete are one of the most critical parameters in obtaining the desired composite action. A reliable bond ensures effective stress transfer and prevents corrosion in reinforced concrete structures, thereby enhancing their safety and durability, reducing their environmental footprint, and improving their sustainability.

The most common methods for developing bond strength, such as chemical-physical adhesion, are significantly influenced by reinforcing bar characteristics like surface roughness, concrete performance like mechanical properties and loading state as well as anchorage length. Besides, the debonding of the concrete-reinforcing bars is typically a concrete failure, especially in the interfacial zone of the structure. Based on these facts, enhancing the concrete performance is an effective approach for not only modifying the bond characteristics between the concrete and the bars but also for enhancing the overall performance and durability of the reinforced structure.

The bonding between reactive powder concrete (RPC) and reinforcing bars has appreciable potential applications in reinforced concrete structures because it is higher than that formed by ordinary concrete. This can be attributed to the excellent properties of RPC, including excellent volume stability, high durability and superior strength. At present, much research work in this area has focused on optimizing and improving the performance of RPC through nanotechnology. Nanotechnology uses various nanofillers (e.g., carbon nanotubes, graphene, Nano TiO₂, Nano SiO₂) to tailor the concrete structure at the nanoscale to provide more opportunities for improving the performance and interfacial bonding behaviors. Nevertheless, there is limited research on the bond characteristics of nanofiller-modified RPC-reinforced bars, which hinder effective design and control of the interface between the reinforcing bars and modified RPC.

Recently, Dr. Xinyue Wang, Dr. Sufen Dong and Professor Baoguo Han from Dalian University of Technology in collaboration with Professor Ashraf Ashour from the University of Bradford and Dr. Siqi Ding from The Hong Kong Polytechnic University, investigated the bond behaviors between eight types of nano-engineered RPC and plain steel reinforcing bars. The chemical composition, modifying mechanisms, interfacial microstructures and nanofiller distribution of the resulting reinforced concrete was examined. Finally, a bond-slip relationship model for the interface between RPC modified with nanofillers and steel bars was developed. The work is currently published in the journal, Construction and Building Materials.

The authors findings revealed that incorporating nanofillers is beneficial in reducing the slip and increasing the bond strength between the RPC and the steel bars. Although the modification performance of the nanofillers was influenced by their content and type, their modifying mechanism on the RPC-steel bar interface was ascribed to improvement RPC microstructure, elimination of defects at the interface and intrinsic composition optimization induced by the enrichment effect and the nano-core effects of the nanofillers in the interfacial zone between concrete and steel bars.

Additionally, the bond stress-slip relationship between the nanofiller-modified RPC and steel bars did not only ensure effective stress transfer between the members but also enhanced the corrosion resistance of the steel reinforcement by preventing the entry of harmful substances. It was also beneficial for reducing the thickness of the concrete cover, avoiding brittle failure and enhancing the durability of the reinforced concrete structures. Considering the initial branch, the proposed model accurately described the stress-slip relationship.

In summary, the research team examined the bond characteristics between RPC modified with nanofillers and steel bars and the responsible modifying mechanisms was elucidated. From the results, the presence of nanofillers has numerous advantages that are beneficial in modifying the bond characteristics between RPC and steel bars. The developed model and the revealed mechanisms can be used as references for designing and controlling reinforced concrete structures. In a statement to Advances in Engineering, Professor Baoguo Han, lead and corresponding author will advance the performance of full-size nano-engineered RPC elements or structures for potential new applications.