StructuralQ We are building 6 and 8 in. (150 and 200 mm) thick cast-in-place concrete retaining walls with No. 4 (No. 13M) reinforcing bar at 18 in. (450 mm) on center each way. We would like to change to shotcrete as an alternate method of construction. Does the reinforcing bar design have to change for shotcrete application?
Shotcrete would be a great substitute for the retaining wall. Design-wise, the shotcrete is equivalent to concrete because shotcrete is really just a way of placing concrete.
The No. 4 (No. 13M) at 18 in. (450 mm) on-center spacing is not a problem. A No. 4 (No. 13M) bar can be easily encased by a qualified, experienced nozzlemen using either wet- or dry-mix shotcrete. In longer walls, or anywhere where reinforcing bars are lap spliced, the lap splice bars should be spaced apart. ACI 506R-05, “Guide to Shotcrete,” Section 5.4, provides some good guidance on optimizing reinforcing bar layouts for shotcreting. On the issue of lap splices, it says: “If the design allows, direct contact of the reinforcing splices should be avoided. Non-contact lapped bars should have a minimum spacing of at least three times the diameter of the largest bar at the splice.”
Thus, with No. 4 (No. 13M) bars in a lap splice, you should have the reinforcing bars spaced 1.5 in. (38 mm) apart at the splice to allow the shotcrete material to flow around the bar during shooting.
Chapter 8 of ACI 506R-05, “Guide to Shotcrete,” also provides a lot of guidance on proper shooting techniques for a variety of applications, including walls.
