The world has continued to witness a surge in large-scale constructions made of reinforced concrete materials. However, these materials are still susceptible to severe environmental conditions that can potentially deteriorate their performance, leading to accidents, property damage, and loss of human lives. Repairing and strengthening have been chosen over replacement as economically and environmentally feasible methods for maintaining the deteriorated reinforced concrete structures. Numerous analytical and experimental have been conducted to developed suitable strengthening techniques to realize the full serviceability of deteriorated structures. Nevertheless, most of these methods exhibit several drawbacks, particularly debonding issues at the interface of the repair materials and the concrete, that hinder their worldwide applications as strengthening methods.
Previous research efforts have shown that the polymer cement mortar (PCM) overlying method is a promising solution for strengthening damaged reinforced concrete structures owing to its higher flexural strength and ductility properties. Unfortunately, for PCM-strengthened reinforced concrete beams, premature debonding is the main failure mode, in which the bond between the concrete and the PCM results in a weak link that deteriorates the composite structure. Overall, developing highly effective and efficient methods to prevent the debonding failure mode is desirable. The success of this particular intervention depends on the interfacial bonding strength, that is, its capacity to transmit stresses and ensure monolithic behavior. Additionally, the bonding between the concrete substrate and repair materials is influenced by various factors such as surface roughness, moisture conditions, constituent materials’ compressive strengths and environmental conditions like temperature and freeze-thaw cycles.
To address the challenges above, Professor Tamon Ueda from Shenzhen University in collaboration with Mr. Mahmudul Hasan Mizan (Ph.D. student) and Dr. Koji Matsumoto from Hokkaido University conducted a study where they enhanced the interface bonding between the existing concrete and PCM using silica fume and surface penetrant. The work is currently published in the journal, Constructions and Building Materials.
In their approach, the research team aimed to prevent the premature debonding failure mode in PCM overlay strengthening techniques and increase their load-carrying capacity via bi-surface shear stress tests. The experimental parameters included three levels of surface roughness: highest level with sandblasting, and medium and lowest levels with steel wire brushing, as well as low and normal concrete compressive strengths of 16.73 MPa and 29.59 MPa, respectively. The bonding mechanisms and performance were also evaluated and compared to elucidate the effects of surface roughness and compressive strengths.
The authors found that mixing silica fume with PCM significantly strengthens the interface bonding strength. The specimen with 5% silica PCM exhibited a remarkably higher increase in the interfacial strength: 36.84% and 35.05% respectively for low strength and normal strength for surfaces roughened by sandblasting and 135.35% and 181.12% for surfaces roughened by high and low steel wire brushing, respectively. Moreover, compressive strength of substrate concrete only increased the bonding strength after exceeding the optimum level of surface roughness. The introduction of silica fume into the PCM shifted the pure interface failure closer to concrete cohesion failure mode for low strength type concrete and composite fracture mode for normal strength type concrete.
In summary, the study developed a new method for enhancing the concrete-PCM interfacial bonding strength and preventing premature bonding failure using silica fume. It is considered that pozzolanic reaction of silica fume fills micro pores and consolidates the interface. Compared with the normal PCM, 5% silica PCM demonstrated a higher increase in the interfacial shear strength with an increase in the surface roughening level. Additionally, application of the surface penetrant at the interface decreased the interfacial shear strength leading to pure interfacial fracture mode (I) regardless of the surface roughness level. In a statement to Advances in Engineering, the authors explained that silica fume significantly increased concrete-PCM interfacial bonding strength and is a good potential candidate for restoration of deteriorated concrete structures.
Mizan, M. H., Ueda, T., & Matsumoto, K. (2020). Enhancement of the concrete-PCM interfacial bonding strength using silica fume. Construction and Building Materials, 259, 119774.