Shotcrete and gunite are specialized forms of concrete placement that use high-pressure spraying to apply material onto surfaces often vertical or overhead. These techniques are widely used in tunnels, swimming pools, retaining walls, slope stabilization, and architectural features. While they offer flexibility and speed, estimating for shotcrete and gunite requires a different approach than traditional cast-in-place concrete.
From accounting for rebound loss and nozzle labor to factoring in surface prep and curing, this guide walks you through the essentials of estimating concrete for shotcrete and gunite applications. We’ll break down the differences, provide a comparison table, and share step-by-step tips to help you bid accurately and confidently.
Shotcrete vs. Gunite Estimating Considerations
| Estimating Factor | Shotcrete (Wet-Mix) | Gunite (Dry-Mix) |
|---|---|---|
| Mix Delivery | Pre-mixed concrete delivered via pump | Dry mix delivered via hose, water added at nozzle |
| Rebound Loss | 5–15% depending on surface and skill | 15–30% due to dry mix and higher air pressure |
| Application Speed | Faster for large volumes | Slower, better for detailed or small areas |
| Labor Requirements | Requires pump operator and nozzleman | Requires skilled nozzleman and mixer |
| Surface Preparation | Requires clean, damp surface | Requires clean, dry surface |
| Thickness Control | Easier to control with wet mix | Requires skilled nozzleman for uniform thickness |
| Curing Needs | Standard curing compounds or wet curing | More prone to shrinkage; wet curing recommended |
| Equipment Costs | Pump, hoses, compressor | Gunite machine, air compressor, hoses |
| Best Use Cases | Tunnels, retaining walls, large structural repairs | Pools, decorative walls, small-scale applications |
Sources: ACI 506R Guide to Shotcrete, Pool & Spa News
Step-by-Step Estimating Process
Step 1: Define Scope and Surface Area
Start by identifying:
- Total surface area to be sprayed
- Required thickness (typically 2–6 inches)
- Type of surface (soil, rock, concrete, rebar)
Use drawings or site measurements to calculate square footage and volume.
Step 2: Calculate Concrete Volume
Use the formula: Volume = Surface Area × Thickness
Convert to cubic yards or cubic meters. Add a waste factor for rebound loss:
- Shotcrete: Add 10–15%
- Gunite: Add 20–30%
Step 3: Account for Rebound and Overspray
Rebound is the material that bounces off the surface and cannot be reused. Include:
- Waste factor in material volume
- Labor for cleanup and disposal
- Additional material cost
Step 4: Estimate Labor and Equipment
Include:
- Nozzleman and helper labor hours
- Pump or gunite machine rental
- Compressor and hose setup
- Mobilization and demobilization
Adjust labor rates based on surface complexity and access.
Step 5: Include Surface Preparation and Curing
Surface prep may include:
- Cleaning and dampening (shotcrete)
- Cleaning and drying (gunite)
- Rebar installation or mesh support
Curing may involve:
- Wet burlap or plastic sheeting
- Curing compounds
- Extended labor for monitoring
Step 6: Add Finishing and Detailing Costs
For architectural or exposed surfaces, include:
- Troweling or carving labor
- Color additives or texture molds
- Mockups and client approval process
Shotcrete & Gunite Estimating Checklist
- Calculate surface area and required thickness
- Add waste factor for rebound and overspray
- Include labor for nozzleman, pump operator, and helpers
- Estimate equipment rental and setup time
- Plan for surface preparation and curing methods
- Include finishing labor and materials for exposed surfaces
- Document assumptions and exclusions clearly
- Format estimate using CSI Division 03 standards
- Coordinate with structural and geotechnical teams
- Include contingency for weather and access challenges
Real-World Example: Estimating Shotcrete for a Tunnel Liner
A contractor bidding on a transportation tunnel in Denver:
- Calculated surface area of tunnel walls and arch
- Specified 4-inch shotcrete thickness with 10% rebound loss
- Included pump rental, compressor, and nozzleman labor
- Planned for wet curing with burlap and water spray
- Coordinated with structural engineer for rebar layout
- Submitted a CSI-formatted estimate with full scope coverage
- Won the bid and completed the project ahead of schedule
Why Specialized Estimating Matters
| Outcome | Benefit to Contractors |
|---|---|
| Fewer Change Orders | Accurate volume and waste forecasting reduces surprises |
| Improved Profit Margins | Realistic labor and equipment costs prevent overruns |
| Stronger Client Relationships | Professional estimates build trust with owners and engineers |
| Better Scheduling | Phased planning improves crew and equipment allocation |
| More Competitive Bids | Tailored estimates win more work in specialty concrete applications |
Final Thoughts
Shotcrete and gunite offer powerful solutions for complex concrete applications but they require specialized estimating strategies. By accounting for rebound loss, surface prep, curing, and equipment needs, contractors can produce accurate bids that protect margins and deliver quality results.
At MSB Estimating, we specialize in concrete estimating for specialty applications from tunnel liners and slope stabilization to pools and architectural walls. Our team delivers CSI-formatted takeoffs, labor forecasting, and scope coverage tailored to your project.
Need Help With Shotcrete or Gunite Estimating?
Contact MSB Estimating today to streamline your bids for sprayed concrete projects.
