Mechanism and effect of re-curing on strength recovery of fire-damaged high strength engineered cementitious composite

This study presents novel investigation into the restoration of fire-damaged high strength engineered cementitious composites (HSECC) through re-curing, focusing on how factors like re-curing duration and binder material selection influence the strength recovery process. Five types of HSECC mixes constituting blends of ground granulated blast furnace slag (GGBFS), fly ash, dolomite, silica fume, hybrid polyethylene and steel fibres were first exposed to temperatures ranging from 200°C-800°C, followed by three different types of re-curing including air curing for 28 days or water curing for 7-28 days. A comprehensive testing program was implemented, including detailed compressive stress-strain tests and microstructural analyses using techniques such as SEM, XRD, TG/DSC, and MIP to assess the effects of mix composition and curing methods. Test results showed that post-fire curing in air did not lead to any appreciable strength recovery with no clear evidence of rehydration after 28 days. On the contrary, curing in water resulted in substantial strength recovery, particularly in specimens exposed to 400-600°C, where recovered strength even exceeded the room temperature strength. Microstructural analysis confirmed that water curing promotes the formation of rehydration products, such as ettringite and portlandite, which effectively fill microcracks and voids caused by fibre melting, resulting in a refined pore-size distribution. Although all mixes showed significant recovery after water re-curing, the extent recovery was influenced by the severity of initial thermal damage. These effects were also found to be closely related to the binder type, with GGBFS-based mixes demonstrating superior performance. The study further proposes a possible mechanism for the re-curing process based on these findings.