Self-healing cementitious composites incorporating innovative materials
Discovering new cement based materials characterized by higher durability and longer service life is crucial for sustainable infrastructure. Engineered Cementitious Composite (ECC) with high potential of micro-crack healing can enhance ductility/durability of concrete structures. However, low water-to-cement ratio in ECC matrix in addition to the lack of cementitious properties of newly formed healing-products in micro-cracks could be an obstacle to implementing continuous hydration to stimulate self-healing and stop re-opening of old healed micro-cracks during re-loading. MgO-type expansive agent (MEA), widely used to compensate effectively autogenous shrinkage in mass concrete, can be used to eliminate such obstacles and to induce self-healing. This research studied self-healing capability of MEA in ECC through an extensive experimental investigation and developed/proposed an ECC-MgO system. Test results indicated “900°C-2 hours of holding time-45 μm particle size” as the best calcination system based on higher MEA hydration in powder state. Additionally, 5% lightly burnt MgO combined with low-calcium Class-F-fly ash (as 55% cement replacement) was found to be a better choice in designing self-healing-ECC-MgO system in terms of lower expansion effect of MEA.
Further, self-healing property of ECC-MgO system under different environmental exposures (laboratory/water/natural(field)/autoclave curing) was investigated based on mechanical/durability properties of control/pre-cracked specimens compared with their ECC counterparts. The ECC-MgO system exhibited remarkable self-healing property in multiple-cracked/damaged specimens when test results were analyzed based on development/recovery of compressive/flexural strength/load resistance, ultrasonic pulse velocity, heat of hydration, expansion/drying shrinkage, rapid chloride permeability, sorptivity/water absorption and freeze/thaw resistance in addition to crack healing/cracking characteristics and microstructural characterizations through Thermogravimetric Analysis (TGA)/Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM). Finally, the self-healing ability of ECC-MgO system was evaluated through structural performance (regarding strength/stiffness/energy absorbing/crack-healing capacity recovery) of damaged and subsequently healed water/field cured link slabs used for joint-free bridge deck construction. Superior self-healing ability of ECC-MgO system was attributed to low MEA water demand coupled with its delayed hydration characteristics which lead to the formation of more cohesive/strong cementitious MgO crystals within crack walls (at later ages) supplementing CaCO3 precipitations (formed earlier) leading to more effective crack-healing and developing post-healed cracks at new locations upon reloading