Shotcrete FAQs: Bonding, Mixes, and Repairs

Shotcrete FAQs: Bonding, Mixes, and RepairsUnderstanding shotcrete involves knowing how it performs in the field, what materials enhance its application, and how to troubleshoot issues during placement. Below are frequently asked questions submitted by engineers and contractors, with insights from the American Shotcrete Association (ASA) on bond strength, mix design, repair techniques, and aggregate selection.How do we evaluate the bond strength between shotcrete layers?When a shotcrete layer is applied over another that has dried or partially cured, it's essential to assess bond strength. Research suggests that tensile bond strength on shotcreted material typically exceeds 150 psi (1 MPa), with bond shear strength estimated at 200 psi (1.4 MPa). Multiplying the tensile bond strength by a factor of 2 to 3 provides a general approximation of bond shear strength. Rough, clean surfaces and proper application practices significantly enhance layer bonding.Is fly ash necessary in a shotcrete mix?Fly ash is not mandatory but is commonly used in wet-mix shotcrete for several benefits:

Reduces CO₂ emissions as a cement replacement

Improves pumpability due to its spherical particle shape

Enhances workability and reduces permeability

Slows setting time, helpful in hot weather

Helps mitigate alkali-silica reactivity (especially Class F fly ash)

Can we apply shotcrete directly over existing concrete pool walls and joints?Yes, shotcrete can be used effectively for pool wall repairs, provided the following conditions are met:

Surface must be roughened (±0.10 in. or 2 mm amplitude) and clean

Achieve a saturated surface dry (SSD) condition

Use high-quality materials and high-velocity application

Shotcrete should reach a minimum 28-day compressive strength of 4,000 psi (28 MPa)

No bonding agents should be used

For thicker repairs, anchors or reinforcing mesh may be required

If the repair crosses existing expansion joints, ensure joints in the shotcrete align accordingly. This allows structural continuity and prevents future cracking.