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2013SprShotcreteEMag

Shotcrete FAQs As a service to our readers, each issue of Shotcrete will include selected questions and provide answers by the American Shotcrete Association (ASA). Questions can be submitted to info@shotcrete.org. Selected FAQs can also be found on the ASA website, http://shotcrete.org/pages/products-services/technical-questions.htm. Question: What is the standard method for steel fiber­ Question: Is shotcrete a viable option to encase galvanized reinforced shotcrete (SFRS) testing? Are you supposed to core steel beams at a coal unloading facility to protect them from test panels or do you only do that for plain shotcrete? Addition- impact and abrasion? Will the galvanizing on the steel ally, is round panel toughness testing on SFRS standard today inhibit bonding? in the United States? Answer: Yes, shotcrete would be suitable for this application. Answer: SFRS is routinely cored from shotcrete test panels or A well-installed shotcrete lining will be durable and protect the in-place shotcrete linings without difficulty. The shotcrete steel from impact, abrasion, and from the acid attack that occurs should, however, have a minimum compressive strength of from sulfur in the coal. Shotcrete is used to cover both the steel about 10 MPa (1450 psi) at the time of coring. hopper walls and to encase the steel beams. Calcium aluminate Round panel testing of flexural toughness of fiber-reinforced cement is typically recommended for coal bunkers because of shotcrete to ASTM C1550 is often specified and used for quality the mild acid condition that occurs that can attack the steel. control (QC) purposes in tunneling and mining projects in North Whether or not the steel beams are galvanized or not is irrel- America and elsewhere (for example, mines in Australia) vir- evant because the shotcrete will not bond well enough to any tually every day of the year. steel surface without welded studs and mesh to hold it in place. The beams will need to have studs welded and mesh installed Question: We have a client with a 6 1/2 in. (165 mm) thick around the beams for the shotcrete placement. With galvanized reinforced concrete roof slab, the underside of which is in need steel it is often necessary to grind off a spot of the galvanized of repair. There are places where the concrete has spalled, coating at the spot of each stud weld location to properly weld exposing reinforcing bar that has a 3/4 in. (19 mm) cover. There the studs. is efflorescence, and there is spalling that does not expose reinforcing bar and some at the steel supporting the concrete Question: We are working on a geotechnical project in the slab. In addition, there are hairline cracks and rust spots. northwest to repair an existing rockery retaining wall. The wall Is shotcrete a feasible overhead repair for this situation? is around 750 ft (229 m) in length and up to 12 ft (4 m) in height. What holds the shotcrete to the slab? What is the minimum The issue is that some of the basalt boulders within the wall thickness of shotcrete we should specify? Should we specify are weathered soft and falling apart. The total weathered rocks shotcrete to be used only at the spalls, cracks, and efflorescence that are falling apart comprise approximately 7% of the wall. or the whole underside of the slab? Do you have a shotcrete Can we use shotcrete on the weathered rocks to give them more repair procedure that we can put in our specification? stability as a repair process? If not, is there a process we can use with shotcrete to repair the wall without having to rebuild Answer: This type of repair is commonly done using the the entire wall? shotcrete process. The extent of the repair is an engineering issue, not a shotcrete issue—shotcrete can and is used for Answer: Shotcrete has been used in the Northwest to strengthen patches and overlays. The shotcrete will adhere to the properly and overlay existing rockery walls. The need to remove the prepared existing concrete. It is installed such that the weight weathered material is dependent upon the need for the overlay of the plastic shotcrete does not;exceed the adhesion to the to bond with the existing wall, which is an engineering issue existing surfaces; if additional material is needed, it is added and not a shotcrete issue. Shotcrete can and is shot successfully at the initial layer or layer set up. The minimum thickness is against soil and other weathered surfaces. related to the material used for the repair and the need to establish cover on the existing or added reinforcing. Some repair mortars Question: I’m planning to add 6 in. (152 mm) of shotcrete to can be placed as thin as 1/2 in. (13 mm). an existing 12 in. (305 mm) wall of a below-surface concrete Please find a link to a paper on “Concrete Repair by Shotcrete tank to accommodate the removal of an existing middle support Application: http://shotcrete.org/media/Archive/2010Sum_ slab. The soil grade is approximately near the top of the existing ACIRAPBulletin12.pdf. tank wall. I’ve been told that since the existing wall is preloaded The success of the shotcrete repair will be highly dependent with soil, adding shotcrete will not increase the strength of the upon using a qualified shotcrete contractor and doing an thickened wall and that the only way the wall will act as a excellent job of preparing the surfaces. Where the reinforcing whole (based on 18 in. 457 mm thickness) is if the retained is exposed, you should require that it be chipped out the entire soil load is removed, then the shotcrete is added, and then soil perimeter allowing for a space of 3/4 in. (19 mm) behind the is put back in place. Is this assessment accurate? Is there a reinforcing bar so that the repair material can completely encase way make this wall work as 18 in. (457 mm) without removing the reinforcing. the existing soil? 58 Shotcrete • Spring 2013


2013SprShotcreteEMag
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