Mechanical Properties And Recovery Of Fiber Reinforced Concrete After High Temperature

Concrete is a non-homogeneous multiphase composite material. A series of physical and chemical processes at high temperatures will result in loss of mass, strength, stiffness and durability of concrete, and the existence of vapor pressure, thermal expansion and thermal stress gives concrete the potential to explosive spall ing. Improving resistance towards explosive spalling to avoid reinforced bar and inner concrete directly exposed to high temperature is the guarantee to high-temperature bearing capacity of the concrete structures. Existing research results showed that there are a lot of unhydrated particles and active dehydrated phases of hydration products in fire-damaged high performance concrete, which will partly regain its strength in an appropriate water or moisture environment. Partly or totally using re-curing concrete when repairing concrete structures according to the extent of damage after different temperatures will significantly reduce the cost to repair concrete structures. In this paper, the influence of different fibers on explosive spalling and recovery of mechanical properties of concrete after elevated temperature is studied, the main work of this paper are summarized as followed:(1) The effects of different fibers, particularly macro-PP fiber, on explosive spalling of ordinary concrete with high water content were studied and the influences of different fibers on compressive and flexural properties of ordinary concrete before and after high temperature were investigated. The results showed that, macro-PP fiber could effectively suppress explosive spalling of concrete, while steel fiber might exacerbate concrete spalling. The addition of fibers had little effect on the compressive and flexural strength of concrete before high temperature, but improve the toughness of concrete. Both compressive and flexural strength of concrete declined after high temperature, and the melting of macro-PP fiber enlarged the strength loss of concrete.(2) The effect of post-fire-curing on compressive and flexural properties recovery of different fiber reinforced high performance concrete after high temperatures was investigated, considering the influence of different temperatures, different fibers and different re-curing ways. At the same time, the crack width of specimen surface was recorded along the re-curing age after natural cooling. The results indicated that after different high temperatures, both compressive and flexural properties of hybrid fiber reinforced high performance concrete (HFHPC) got a certain degree of recovery. In one case, after exposed to 600℃ and water re-curing 60 days, the recovered compressive strength was 90% of its original unfired strength, flexural strength restored to about 70% of the original. Addition of fibers could improve the recovery rate of compressive and flexural strength of high-performance concrete after high temperature, hybrid use of steel fiber and micro-PP fiber showed most obvious effect. Performance restoration of fiber reinforced high performance concrete after high temperature was influenced deeply by the re-curing ways, water re-curing showed most compressive and flexural strength recovery. With increasing water re-curing time, the crack width reduced, initial crack of HFHPC after 400℃ was small, and the phenomenon of crack closure was observed.