Rehabilitation of post-tensioned flat slab affected by fire

Abstract

This paper explains about the evaluation and rehabilitation of a severely fire-affected post-tensioned flat slab without demolition. The portion under rehabilitation was a post-tensioned slab in the ground floor of a tower with 3 Basements + Ground + 13 floors, to be used as office space. The post-tensioned flat slab was designed with isolated drop panels. The building was designed to resist gravity and lateral loading with a combination of reinforced concrete columns and shear walls. The flat slab had suffered various levels of damages including spalling of concrete with the bottom un-tensioned reinforced and pre-stressing tendon exposed. Columns in the affected area also had significant damages due to exposure to the high temperature. The total affected area of the floor was around 500 m2 including four columns. The damaged face of the flat slab and all faces of the column were chipped to remove the loose and delaminated concrete. The strength of the existing concrete and reinforcement was ascertained with core cuts, TGA and DTA tests and cut samples. Based on the results, a comprehensive retrofit plan was drawn up without the need to pull down the affected slab. The rehabilitation involved change of the design basis from isolated drop panels to formation of a continuous drop connecting the columns in the affected area. The design utilised the residual strength available in the concrete, un-tensioned and pre-tensioned reinforcements. Self-compacting concrete with mineral and chemical admixtures was used for forming the retrofit zone and was placed from above the existing affected slab. Shear connection between existing and new concrete was created with chemical grouted dowel reinforcements. The entire rehabilitated slab and column was grouted with low viscous epoxy grout to seal the micropores and restore normalcy to the slab. The complete rehabilitation of the area was done without any change in the intended design capacity and with the most optimum and economical solution.