RepairQ We have a client with a 6 1/2 in. (165 mm) thick reinforced concrete roof slab, the underside of which is in need of repair. There are places where the concrete has spalled, exposing reinforcing bar that has a 3/4 in. (19 mm) cover. There is efflorescence, and there is spalling that does not expose reinforcing bar and some at the steel supporting the concrete slab. In addition, there are hairline cracks and rust spots. Is shotcrete a feasible overhead repair for this situation? What holds the shotcrete to the slab? What is the minimum thickness of shotcrete we should specify? Should we specify shotcrete to be used only at the spalls, cracks, and efflorescence or the whole underside of the slab? Do you have a shotcrete repair procedure that we can put in our specification?
This type of repair is commonly done using the shotcrete process. The extent of the repair is an engineering issue, not a shotcrete issue—shotcrete can and is used for patches and overlays. The shotcrete will adhere to the properly prepared existing concrete. It is installed such that the weight of the plastic shotcrete does not;exceed the adhesion to the existing surfaces; if additional material is needed, it is added at the initial layer or layer set up. The minimum thickness is related to the material used for the repair and the need to establish cover on the existing or added reinforcing. Some repair mortars can be placed as thin as 1/2 in. (13 mm).
Please find a link to a paper on “Concrete Repair by Shotcrete Application”.
The success of the shotcrete repair will be highly dependent upon using a qualified shotcrete contractor and doing an excellent job of preparing the surfaces. Where the reinforcing is exposed, you should require that it be chipped out the entire perimeter allowing for a space of 3/4 in. (19 mm) behind the reinforcing bar so that the repair material can completely encase the reinforcing.
