Shotcrete: Sprayed Concrete Explained

What Shotcrete Actually Is

Shotcrete isn't a special type of concrete—it's a placement method. You take regular concrete (or mortar) and blast it through a hose at 60-80 mph onto a surface. The force of impact compacts the material, eliminating the need for forms and vibration.

The term "shotcrete" covers both concrete (with coarse aggregate) and mortar (sand and cement only). "Gunite" is an older trade name for dry-mix shotcrete that's still used, especially in pool construction. They're essentially the same process.

Shotcrete was invented in 1907 by Carl Akeley, a taxidermist who needed a way to apply plaster to animal sculptures. The construction industry adopted it for tunnel linings, and it's been standard practice for pools, retaining walls, and slope stabilization ever since.

Dry-Mix vs Wet-Mix

Two fundamentally different processes, same end result:

Dry-Mix (Gunite)

Dry cement and aggregate travel through the hose. Water is added at the nozzle by the operator. This gives the nozzleman control over the water content—critical for vertical and overhead applications where too much water causes slump and falloff.

Pros: Better for stop-and-start work, longer hose runs possible, operator controls consistency, equipment is simpler to clean.

Cons: More dust, higher rebound (wasted material), requires skilled nozzleman, slower application rate.

Wet-Mix

Concrete is fully mixed with water before entering the pump. The complete mix travels through the hose and is projected by compressed air at the nozzle.

Pros: Faster application, less rebound, less dust, more consistent mix, less operator skill required.

Cons: Must use material quickly before it sets, harder to clean equipment, not ideal for intermittent work.

Which Is Better?

Wet-mix dominates commercial work—it's faster and wastes less material. Dry-mix (gunite) remains popular for pools and repair work where flexibility matters more than speed. Both achieve similar final strengths when applied correctly.

Common Applications

Swimming Pools

The classic shotcrete application. The curved, organic shapes of modern pools would be nearly impossible with traditional forms. Shotcrete builds up pool shells 6-8 inches thick, following any contour. Most in-ground pools in the US are shotcrete construction.

Retaining Walls

Shotcrete over soil nails or rock anchors creates strong retaining structures without excavating for forms. The material bonds to irregular rock faces and can follow natural terrain.

Tunnel Linings

Miners spray shotcrete immediately after excavation to stabilize freshly exposed rock. It's faster and safer than installing forms underground. Modern tunneling would be impractical without shotcrete.

Slope Stabilization

Highway cuts, hillsides, and embankments get shotcrete to prevent erosion and rockfall. The material conforms to irregular surfaces and can incorporate drainage features.

Structural Repairs

Deteriorated concrete on bridges, parking structures, and buildings gets cut out and replaced with shotcrete. It bonds well to existing concrete and reaches areas where forms can't go.

Cost Comparison

Application	Shotcrete	Formed Concrete
Retaining wall (per SF)	$8-15	$20-40
Pool shell	$5,000-15,000	N/A (not practical)
Tunnel lining (per LF)	$200-500	$400-1,000+
Slope stabilization	$10-20/SF	Often not feasible

Shotcrete saves money on forming costs but has higher material waste (rebound) and requires specialized crews. For curved or irregular surfaces, it's almost always cheaper than alternatives. For flat walls, formed concrete may win on cost.

Strength and Quality

Properly applied shotcrete achieves the same strength as conventional concrete—3,000 to 5,000+ PSI depending on the mix design. The key word is "properly." Bad shotcrete application creates voids, laminations, and weak spots.

What Can Go Wrong

• Rebound inclusion: Material that bounces off gets trapped in the fresh shotcrete, creating weak layers
• Shadow zones: Areas behind rebar or obstructions that don't get fully filled
• Laminations: Layers that don't bond properly, usually from shooting over material that's started to set
• Sand pockets: In dry-mix, areas where water didn't fully penetrate

Quality depends heavily on the nozzleman's skill. Certification programs (ACI) exist specifically for shotcrete application because the technique matters so much.

Reinforcement

Shotcrete typically incorporates:

• Rebar: Installed before shooting, the shotcrete encases it. Requires careful technique to avoid shadow zones.
• Wire mesh: Common in pools and thin sections. Easier to shoot around than rebar.
• Fiber reinforcement: Steel or synthetic fibers mixed into the shotcrete. Reduces cracking and improves toughness.
• Soil nails/rock bolts: For retaining walls, anchors extend into the earth and shotcrete covers the face.

Limitations

• Finish quality: Shotcrete surfaces are rough. If you want smooth, you need to trowel or apply a finish coat.
• Weather sensitivity: Can't shoot in rain, extreme cold, or high winds.
• Dust and mess: Dry-mix especially creates significant dust. Not neighbor-friendly.
• Rebound waste: 10-30% of material bounces off and can't be reused. Factor this into material estimates.
• Skilled labor: Good nozzlemen are scarce. Bad application creates problems that are hard to fix.

Is Shotcrete the Same as Gunite?

Gunite is a trade name for dry-mix shotcrete. In practice, pool contractors use "gunite" and "shotcrete" interchangeably. Technically, gunite refers specifically to the dry-mix process, while shotcrete covers both wet and dry methods.

How Thick Should Shotcrete Be?

Depends on the application. Pool shells: 6-8 inches. Retaining walls: 4-8 inches. Tunnel linings: 4-12 inches. Slope stabilization: 3-6 inches. Structural engineers specify thickness based on loads and conditions.

Can Shotcrete Be Applied Overhead?

Yes, but it's challenging. Dry-mix works better overhead because the nozzleman can adjust water content to prevent sag. Wet-mix overhead requires accelerators and experienced technique. Expect higher rebound rates overhead.

How Long Does Shotcrete Take to Cure?

Same as regular concrete: initial set in 24-48 hours, full strength at 28 days. Pools typically wait 7-14 days before filling. Keep shotcrete moist during curing for best results.

Is Shotcrete Waterproof?

No more than regular concrete—which means it's not. Pools require waterproof coatings (plaster, pebble finish, tile). Retaining walls often need drainage and waterproofing behind them. Shotcrete alone is not a waterproofing solution.

Key Takeaways

• Shotcrete is a placement method, not a special concrete type
• Two methods: dry-mix (gunite) adds water at nozzle; wet-mix is pre-blended
• Best for curved surfaces, pools, tunnels, retaining walls, slope stabilization
• Costs $8-15/SF for walls—cheaper than formed concrete for irregular shapes
• Achieves same strength as conventional concrete when properly applied
• Quality depends heavily on nozzleman skill—certification matters
• 10-30% material waste from rebound is normal
• Not waterproof by itself—additional coatings required for pools and below-grade