When shooting onto existing concrete sections the surface must be properly prepared and then shotcreted with proper shotcrete materials, equipment, and placement techniques. This will produce a construction joint that acts monolithically and not be a “cold” joint. Shotcrete placed onto an existing concrete surface will provide an excellent bond IF the following conditions are met:

  1. Make sure the surface is roughened and clean.
    1. The amplitude of roughness should be +/- 1/16th in. (1.6 mm) or more.
    2. If the surface was not roughened when it was chipped out, be sure to have the contractor roughen it.
    3. A high-pressure water blaster (5000 psi [35 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
  2. Bring the concrete surface to saturated surface dry (SSD) condition. This means the surface feels damp, but water is not picked up on a hand.
  3. Make sure the shotcrete placement is properly executed with high velocity placement and quality materials.
    1. The shotcrete should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
  4. Be sure the shotcrete contractor is using an air compressor able to produce at least 185 ft3/min (5.2 m3/min) for wet-mix and 385 ft3/min (11 m3/min) for dry-mix (gunite) of air flow at 120 psi (0.8 MPa).
  5. Use of an ACI-certified shotcrete nozzleman is recommended.
  6. No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.
  7. A minimum thickness of no less than ½ in. (13 mm) is recommended.

This article on the excellent bond between shotcrete provides more detail: www.shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf:

Regarding the additional floor thickness, though it may be shotcreted by an experienced nozzleman, it is difficult to properly shoot horizontal surfaces and control rebound and overspray from the shotcrete placement. We suggest that casting and vibrating for consolidation the horizontal sections is preferred to shotcreting. You should consider a bonding agent since the concrete is cast against the existing concrete floor without any impact velocity. We would also not recommend tapering down to 0 in. thickness. The feather edge will tend to be an area that may easily spall over time. Thus, we recommend cutting an ½ to ¾ in. (13 to 19 mm) deep shoulder so the concrete can have some thickness at its thinnest locations.