Properly prepared surfaces along with proper shotcrete materials, equipment and placement techniques 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:
- Make sure the surface is roughened and clean.
- The amplitude the of roughness should be +/- 1/16th in. (1.6 mm) or more.
- If the surface was not roughened when it was shot be sure to have the contractor roughen it.
- A high-pressure water blaster (5000 psi [34 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
- 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.
- Make sure the shotcrete placement is properly executed with high velocity placement and quality materials.
- The dry-mix shotcrete (Gunite) should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
- Be sure the shotcrete contractor is using an air compressor able to produce at least 385 CFM (11 m3/min) of air flow at 120 psi (0.83 MPa).
- Use of an ACI-certified shotcrete nozzleman is recommended.
- No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.
This article on the excellent bond between shotcrete provides more detail: https://shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf
The pool wall thickness is a part of the pool engineer’s design. Adding 6 to 8 in. to the existing 3 in. (75 mm) would be making the overall thickness 9 to 11 in. (225 to 275 mm). This would require more reinforcing steel, so you should verify with the pool designer the additional reinforcing steel requirements.
You may also find our ASA Position Statements on Pool Construction helpful as a reference for proper pool shotcrete construction. They are freely available at: https://shotcrete.org/products-services-information/resources/