Common Questions About Shotcrete
Understanding 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.
Q1: 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.
Q2: 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)
Q3: 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 4000 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.
Have more shotcrete-specific questions? Request a quote or contact our team of experts for help with your next concrete project.
Tags: Shotcrete, Concrete Repair, Fly Ash
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