I recently had a project that requires a velocity of ±400 psi (2.8 MPa) to be applied to prepared areas of deteriorated concrete. My questions is: What is the velocity of the material being applied if I am using 50 ft x 1-1/2 in. (15 m x 38 mm) shooting hose with 60 psi (0.4 MPa) supplied pressure? I would also like to know how that is calculated.

Shotcrete placement requires high velocity for full consolidation and compaction of the concrete. Standard shotcrete equipment with a properly sized air compressor will produce a velocity of 60 to 80 mph (95 to 130 km/h). The 400 psi you stated is not a velocity, but a pressure. Sixty psi is a very low air pressure and may not create the velocity needed for proper shotcrete placement. Shotcrete generally uses compressors that create 100 to 125 psi of air pressure. Also, depending on the shotcrete process used (wet-mix or dry-mix), ACI 506R-16, “Guide to Shotcrete,” states you need an air compressor that can produce a flow rate of at least 200 ft³/min (5.7 m³/min) for wet-mix to 600 ft³/min (17 m³/min) for dry-mix for your 1-1/2 in. diameter hose.