Concrete structures such as bridges, dams, tunnels, and buildings are susceptible to various types of damages all over the world. Cracking has been identified as the most damaging as it significantly injures the service life of reinforced concrete structures. Although various researchers across the world have focused on materials development, it is still challenging to protect structural elements from concrete deteriorations.
Deterioration in concrete has been confirmed to increase with age, hence the cost of maintenance and repairs. However, it has also been found that concrete has a natural ability to heal the inherent cracks by itself. This self-healing process is necessary to minimize the ingression of aggressive ions into concrete and hence lower deteriorations. The self-healing process occurs in the early stages of cement hydration owing to precipitation of calcium carbonate within the crack walls when fresh concrete is exposed to atmospheric conditions.
Engineered cementitious composites is a unique form of high performance fiber reinforced concrete, which is highly ductile and tolerate to damage under mechanical loading, shear, tension. Engineered cementitious composites control crack width and allow cracks to form carefully. Therefore, these materials can undergo tensile strain hardening response together with non-catastrophic damage in the form of multiple crack formation. This ability to restrict the crack width to less than 60µm even at ultimate loading will lead to autogenous crack healing.
Ryerson University research team, Mohamed Sherir, Khandaker Hossain and Mohamed Lachemi reported the development of an effective self-healing concrete system made of engineered cementitious composites and MgO-type expansive agent. They implemented new measures in investigating the performance of engineered cementitious composite-MgO self-healing system pegged on accelerated autoclave test to model experimental results practical to field conditions. Their research work is published in journal, Construction and Building Materials.
The authors successfully presented a comprehensive experimental investigation analyzing the effectiveness of Magnesium oxide as a self-healing agent in engineered cementitious composites taking into account the effects of calcination temperature, high volume of fly ash of different types, and magnesium oxide particle size. They analyzed the hydration activity of the magnesium oxide system through microstructural analysis implementing scanning electron microscopy.
The authors observed from the test results that the best calcination system was 900 °C with 2 h of holding time implementing 45µm particle size based on the high hydration activity of magnesium oxide expansive agent in the form of powder. They found that low calcium class-F fly ash was the best supplementary cementing materials in the production of engineered cementitious materials-magnesium system based on low expansion feature of magnesium oxide.
The high-volume fly ash was used in a bid to realize extremely low expansion within the crack walls just to heal hairline microcracks in the proposed self-healing system without injuring its durability. This similar behavior was noticeable in magnesium oxide powder state with regards to loss of carbon dioxide content as well as retained hydration activity.
The high flexural strength recovery observed in the case of pre-packed engineered cementitious composite-MgO specimen cured under the accelerated autoclaved conditions when compared to their counterparts without MgO confirmed the self-healing potential of the self-healing system.
Mohamed A.A. Sherir, Khandaker M.A. Hossain, Mohamed Lachemi. The influence of MgO-type expansive agent incorporated in self-healing system of engineered cementitious composites. Construction and Building Materials, volume 149 (2017), pages 164–185.
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