What is a soffit in concrete construction?

The soffit is the underside of any horizontal or sloping construction element — the underside of a concrete floor slab, a beam, a staircase flight, a balcony, a bridge deck or a car park deck seen from the level below. In concrete repair the soffit is one of the most safety-critical surfaces on a structure, because when concrete spalls from a soffit it does not stay put: the cover concrete delaminates and falls, dropping debris onto the people, vehicles or plant directly beneath. Soffit spalling is usually driven by the same mechanisms as any other spalling — carbonation and chloride attack corroding the reinforcement, which expands and forces the cover off — but it shows up first on soffits because that is where cover is often thinnest, where water tracks to and drips from, and where the reinforcement sits closest to an aggressive environment (car park decks and bridge soffits exposed to de-icing salts are classic examples). Repairing a soffit is harder than repairing a wall or floor because everything works against gravity. Loose material must first be hammer-sounded and broken out to sound concrete, exposed steel cleaned and primed, and the section then reinstated with a thixotropic (non-sag) repair mortar specifically formulated to hold its shape overhead while it cures, or by a form-and-pour technique where shuttering is fixed to the underside and a flowable micro-concrete is poured into the void, or by sprayed concrete for large areas. Access almost always means MEWPs, scaffold or crash decks, and where fragments are already falling the first job is to make the area safe with debris netting and propping before any permanent repair begins. MPS Concrete Solutions repairs spalled and delaminating concrete soffits on car park decks, balconies, bridges and building slabs across the UK, working overhead to BS EN 1504 with non-sag mortars and form-and-pour reinstatement.