We have a backwash tank on a waste water treatment plant which is made by a secant wall. The lower area for this structure will receive a shotcrete liner approximately 12 ft (3.7 m) tall on average, the interior perimeter of the structure includes 104 lineal ft (32 m) of unreinforced and 440 ft (134 m) reinforced sections which are a 12 in (300 mm) minimum thickness. The drawings call for vertical control joints with waterstop approximately every 30 ft (9 m). We don’t believe the control joints are necessary and could achieve the same desired performance with one monolithic installation of the shotcrete. Are the control joints really necessary when you are installing the shotcrete against a solid secant wall which does not contain any control joints?

By control joints, we assume you mean contraction joints. Shotcrete is a placement method for concrete. All normal concrete experiences drying shrinkage that creates a volume change in the hardened concrete. Though shotcrete has a lower w/cm than most form and pour concrete, it will still undergo shrinkage. In being shot on an existing concrete wall the shotcrete liner will be restrained by the bond to the substrate and the restraint of the horizontal volume change from shrinkage can create internal tensile stresses in the concrete. This is likely the reason the designer has specified contraction joints in the section. 30 ft (9 m) spacing between joints is common in new construction of concrete tanks. The question becomes whether the bond of the shotcrete to the existing substrate is high enough to restrain the volume change and prevent cracking along the hundreds of lineal feet of wall you will be lining. The thickness of the lining, the type and duration of curing, the concrete mix design, the strength of the concrete, the strength of the substrate, the quality of shotcrete application, proper surface preparation and exposure to seasonal temperature changes will impact the effect of the volume change of the lining. With the many variables we’ve pointed out you can see there isn’t a clear answer that covers all situations. We recommend you discuss your opinion with the designer or consult with a professional engineer experienced in shotcrete repairs to fully evaluate the specific structural sections you’re shotcreting.