SHOTCRETE ENGINEERING KNOWLEDGE LIBRARY

1. Shotcrete fundamentals

(Full Article – Shotcrete Fundamentals)

Shotcrete is concrete projected at high velocity and compacted by impact energy rather than vibration.

• Typical projection velocity exceeds 20 m/s
Practical example: reducing air volume lowers velocity and causes aggregate bounce rather than embedment.

• Early structural support is achieved rapidly
Practical example: with correct accelerator dosage, early strength of approximately 0.5 MPa can be reached within 1 to 2 hours.

• Shotcrete works as part of a support system
Practical example: 75 mm fibre reinforced shotcrete combined with 2.4 m rock bolts at 1.2 m spacing.

2. Wet shotcrete process and pumping system

(Full Article – Equipment Systems and Pumping Mechanics)

Wet shotcrete is mixed with water before pumping and transported hydraulically to the nozzle.

• Typical pump outputs range from 8 to 30 m³ per hour
Practical example: a piston pump delivering 20 m³ per hour requires stable rheology to avoid surging at the nozzle.

• Practical pumping distances range from 120 to 250 m
Practical example: at distances beyond 200 m, pressure increases and segregation risk becomes significant.

• Accelerator is introduced at the nozzle
Practical example: 400 kg cement per m³ requires approximately 16 to 32 kg accelerator at 4 to 8 percent dosage.

3. Shotcrete mix design and material behaviour

(Full Article – Mix Design and Material Interaction)

Shotcrete mixes must remain pumpable while adhering to vertical and overhead surfaces.

• Cement content typically 380 to 450 kg per m³
Practical example: 400 kg per m³ provides sufficient paste to embed fibres.

• Aggregate grading normally limited to 0 to 8 mm
Practical example: oversized aggregate increases rebound at the crown.

• Water cement ratio typically 0.40 to 0.45
Practical example: increasing to 0.55 may reduce 28-day strength by more than 20 percent.

• Silica fume typically 6 to 10 percent of cement
Practical example: 32 kg per m³ improves cohesion and reduces fibre rebound.

4. Substrate preparation and rock interaction

(Full Article – Substrate Preparation and Ground Conditions)

Shotcrete performance depends on bond to competent rock.

• Loose rock must be removed prior to spraying
Practical example: a 20 mm loose slab behind shotcrete can cause full lining delamination.

• Surfaces must be washed down before spraying
Practical example: drilling dust creates a slip plane between rock and lining.

• Rock surface must be damp, not dry or flowing
Practical example: dry rock absorbs water from the paste, weakening bond.

5. Application mechanics and nozzle technique

(Full Article – Application Mechanics and Nozzle Technique)

Application technique governs compaction and rebound.

• Nozzle angle should remain close to 90 degrees
Practical example: spraying at 70 degrees can increase rebound by over 30 percent.

• Stand-off distance typically 1.5 to 2.0 m
Practical example: spraying at 3.0 m reduces impact energy and causes sloughing.

• Spraying must begin at the bottom and work upward
Practical example: crown-first spraying leads to rebound contamination in lower layers.

6. Layer build-up and thickness control

(Full Articles – Quality Control Systems, Testing Methods, And Performance Verification)

Shotcrete thickness must be developed progressively.

• Typical layer thickness 40 to 60 mm
Practical example: attempting 120 mm in one pass causes sagging.

• Design thickness typically 50 to 150 mm
Practical example: 75 mm fibre shotcrete commonly used in development ends.

• Thickness verified by continuous probing
Practical example: probe every 1 to 2 m during spraying.

7. Rebound behaviour and performance indicators

(Full Article – Rebound Engineering, Measurement Methods, And EFNARC Monitoring Requirements)

Rebound is a key indicator of application efficiency.

• Typical rebound values
Sidewalls 5 to 10 percent
Overhead areas 10 to 20 percent

• Increasing rebound indicates loss of control
Practical example: increase from 10 to 18 percent suggests nozzle angle deviation.

• Visual estimation is unreliable
Practical example: measured rebound often differs by more than 8 percent from visual judgement.

8. EFNARC rebound measurement methodology

(Full Article – Rebound Engineering, Measurement Methods, And EFNARC Monitoring Requirements)

EFNARC requires rebound to be measured using the weighing method.

• Plastic sheeting must extend 10 percent beyond test area
Practical example: 10 m² sprayed area requires at least 11 m² sheeting.

• Minimum sprayed thickness for testing is 50 mm
Practical example: thinner layers give inaccurate volume calculation.

• Rebound calculation example
Sprayed volume 1.5 m³
Wet density 2300 kg per m³
Total sprayed mass 3450 kg
Rebound collected 500 kg
Rebound result 14.5 percent.

9. Quality control testing and verification

(Full Article 7 – Quality Control Systems)

Testing verifies structural performance.

• Flow test target typically 500 to 600 mm
Practical example: flow below 450 mm increases blockage risk.

• Compressive strength typically 40 MPa at 28 days
Practical example: lower results indicate excess water or poor curing.

• Energy absorption commonly ≥ 700 joules
Practical example: achieved using 40 kg per m³ steel fibres.

• Thickness verification during spraying
Practical example: real-time probing prevents under-application.

10. Equipment inspection and reliability

(Full Article 8 – Equipment Inspection and Maintenance)

Equipment stability directly affects lining quality.

• Delivery hoses typically limited to 10 m lengths
Practical example: long hoses increase pressure fluctuation.

• Clamp safety pins mandatory at every joint
Practical example: missing pins allow clamp opening under pressure.

• Greasing required at start of each shift
Practical example: two grease strokes per nipple prevents hinge seizure.

11. Blockage control and pressure safety

(Full Article – Shotcrete Safety Systems, Risk Control, and Underground Work Practices)

Blockages represent high stored energy risk.

• Pump pressure commonly 60 to 75 bar
Practical example: uncontrolled hose rupture can cause fatal injury.

• Blockages must never be pushed forward
Practical example: pump reversal releases pressure safely.

• Air must be isolated before intervention
Practical example: residual air can eject concrete violently.

12. Shotcrete safety systems

(Full Article – Shotcrete Safety Systems, Risk Control, and Underground Work Practices)

Shotcrete requires strict safety controls.

• Exclusion zones during spraying mandatory
Practical example: no personnel beneath fresh shotcrete for minimum 2 hours.

• PPE required at all times
Practical example: goggles prevent accelerator splash injuries.

• Ventilation required
Practical example: poor airflow increases chemical exposure risk.

13. Training, competence, and performance control

(Full Article 10 – Training and Competence)

Shotcrete performance depends on people.

• Operator skill can change rebound by more than 10 percent
Practical example: trained operator achieves 8 percent, untrained 18 percent.

• Training improves productivity
Practical example: output increases from 12 to 20 m³ per hour.

• Certified operators improve consistency
Practical example: fibre dosage remains within design tolerance.

14. Engineering outcome when EFNARC principles are applied

When shotcrete is executed according to EFNARC guidance:

• Rebound typically reduces by 30 to 50 percent
• Thickness compliance improves
• Material waste decreases
• Safety incidents reduce
• Structural reliability increases

This knowledge hub provides the technical foundation for consistent, safe, and durable sprayed concrete support systems.